Skip to content

Advertisement

BMC Physiology

What do you think about BMC? Take part in

Open Access

Variation in branchial expression among insulin-like growth-factor binding proteins (igfbps) during Atlantic salmon smoltification and seawater exposure

  • Jason P. Breves1Email author,
  • Chelsea K. Fujimoto1,
  • Silas K. Phipps-Costin1,
  • Ingibjörg E. Einarsdottir2,
  • Björn Thrandur Björnsson2 and
  • Stephen D. McCormick3
BMC PhysiologyBMC series – open, inclusive and trusted201717:2

https://doi.org/10.1186/s12899-017-0028-5

Received: 5 October 2016

Accepted: 10 January 2017

Published: 18 January 2017

Abstract

Background

In preparation for migration from freshwater to marine habitats, Atlantic salmon (Salmo salar L.) undergo smoltification, a transformation that includes the acquisition of hyposmoregulatory capacity. The growth hormone (Gh)/insulin-like growth-factor (Igf) axis promotes the development of branchial ionoregulatory functions that underlie ion secretion. Igfs interact with a suite of Igf binding proteins (Igfbps) that modulate hormone activity. In Atlantic salmon smolts, igfbp4,−5a,−5b1,−5b2,−6b1 and−6b2 transcripts are highly expressed in gill. We measured mRNA levels of branchial and hepatic igfbps during smoltification (March, April, and May), desmoltification (July) and following seawater (SW) exposure in March and May. We also characterized parallel changes in a broad suite of osmoregulatory (branchial Na+/K+-ATPase (Nka) activity, Na + /K + /2Cl cotransporter 1 (nkcc1) and cystic fibrosis transmembrane regulator 1 (cftr1) transcription) and endocrine (plasma Gh and Igf1) parameters.

Results

Indicative of smoltification, we observed increased branchial Nka activity, nkcc1 and cftr1 transcription in May. Branchial igfbp6b1 and -6b2 expression increased coincidentally with smoltification. Following a SW challenge in March, igfbp6b1 showed increased expression while igfbp6b2 exhibited diminished expression. igfbp5a,−5b1 and−5b2 mRNA levels did not change during smolting, but each had lower levels following a SW exposure in March.

Conclusions

Salmonids express an especially large suite of igfbps. Our data suggest that dynamic expression of particular igfbps accompanies smoltification and SW challenges; thus, transcriptional control of igfbps may provide a mechanism for the local modulation of Igf activity in salmon gill.

Keywords

Insulin-like growth-factorBinding proteinsGrowth hormoneAtlantic salmonSmoltificationOsmoregulationGillLiverSalinity

Background

Anadromous fishes such as Atlantic salmon (Salmo salar L.) exhibit a life history strategy that includes an initial phase in fresh water (FW) followed by migration to marine environments [1]. The transformation of stream-dwelling ‘parr’ to seawater (SW) tolerant ‘smolts’ entails the orchestrated development of physiological, morphological and behavioral traits that support migration to, and subsequent survival in, pelagic marine environments. While dependent upon reaching a necessary size, the timing of this transformation in Atlantic salmon is initiated by environmental cues such as photoperiod and temperature [2, 3]. Depending upon latitude, this transformation typically occurs at 1–4 years of age in wild Atlantic salmon [4, 5]. The months preceding migration are termed ‘smoltification’ and this stage remains incomplete prior to downstream migration; animals of this stage are termed ‘pre-smolts’. At the peak of smoltification, salmon smolts will move downstream into estuaries and then quickly enter marine environments. Upon reaching sexual maturity in the ocean, adults use olfactory cues to return to their natal FW streams to spawn [6]. Smolts that do not gain entry into marine environments will reverse some of the acquired phenotypes such as salinity tolerance, and revert to pre-smolt phenotypes that are better suited for FW environments.

In order to sustain hydromineral balance upon entry into marine environments the parr-smolt transformation is inextricably linked with the acquisition of SW tolerance. As is the paradigm for strictly marine teleosts, the ability of smolts to inhabit SW is sustained by an array of solute- and water-transporting activities in the gill, gut, kidney and urinary bladder [7]. Inasmuch as the gill is the primary tissue for the active transport of monovalent ions, the recruitment of branchial ionocytes (SW-type ionocytes) that extrude Na+ and Cl is essential for gaining SW tolerance. SW-type ionocytes employ ion pumps, cotransporters and channels such as Na+/K+-ATPase (Nka), Na+/K+/2Cl cotransporter 1 (Nkcc1) and cystic fibrosis transmembrane regulator 1 (Cftr1) [8, 9]. Accordingly, branchial Nka activity peaks concomitantly with nkcc1 and cftr1 mRNA levels when salmon achieve maximal SW tolerance [1012]. Thus, the seasonal patterns of these three parameters reliably predict whether juvenile salmon will be able to sustain hydromineral balance upon exposure to SW [11, 12].

In Atlantic salmon, several endocrine systems synchronize the ontogeny of osmoregulatory systems with downstream migration [1, 12]. In particular, the growth hormone (Gh)/insulin-like growth factor (Igf) axis exhibits enhanced activity at the peak of smoltification [1, 13]. From the perspective of whole-organism performance, a connection between the somatotropic axis and salmonid osmoregulation is supported by numerous findings of improved salinity tolerance following exogenous Gh and/or Igf1 treatment [1315]. The hyposmoregulatory actions of Gh are seemingly mediated by multiple molecular pathways, including: 1) branchial Gh receptors (Ghr1), 2) the synthesis and secretion of Igf1 from the liver, 3) the local production of Igf1 in the gill, and/or 4) enhanced responsiveness to cortisol [1621]. Irrespective of the route of action, Gh and Igf1 promote salinity tolerance by regulating branchial Nka activity [22], the gene and protein expression of ionoregulatory factors [8, 23] and ionocyte density [24, 25].

Igfs interact with cognate binding proteins termed Igf binding proteins (Igfbps). The coordinated production, both spatially and temporally, of Igfbps permits modulation of Igf bioavailability in both positive and negative fashions [26]. Igfbps may also exert ligand-independent activities [27]. Studies on teleost Igfbps have primarily focused upon how they mediate growth responses to stressors such as food restriction, temperature, hypoxia, and handling [28], while relatively few studies have investigated Igfbp responses to ionoregulatory challenges [2932]. Attributed to multiple whole-genome duplication events, Atlantic salmon express an expansive set of 19 igfbp genes [33]. Among these igfbps, igfbp4,−5a,−5b1,−5b2,−6b1 and−6b2 are highly expressed in the gill [33]. There is currently no understating of how branchial igfbp expression is modulated in preparation for ionoregulatory challenges faced by developing salmon.

Given that parr-smolt transformation encompasses numerous physiological preparations underlying marine survival, and consequently recruitment, knowledge of its physiological control informs efforts aimed at restoring endangered populations [34, 35]. Thus, the physiology of Atlantic salmon smoltification presents an important physiological context for how Igfbps underlie Gh/Igf-mediated life-history transitions. In turn, our first objective was to assess whether igfbp mRNA levels change during smoltification. We further investigated whether igfbps respond to abrupt transfer to SW, and whether such responses varied with the degree of SW tolerance. Because the gill is a key tissue underlying the development of SW adaptability, we primarily focused on igfbp transcripts that exhibit significant branchial expression.

Methods

Animals

Atlantic salmon (Salmo salar) parr were obtained in October of 2013 from the Kensington National Fish Hatchery, Kensington, CT, and held at the Conte Anadromous Fish Research Center, Turners Falls, MA. Individuals from this cohort were expected to smolt in the spring of 2014 on the basis of their size (>12 cm fork length) in early February [36]. Fish were held in a 1.5 m diameter fiberglass tank supplied with dechlorinated tap water under natural photoperiod. Water temperature was maintained at 9 °C until late June; water was then maintained at 10.5 °C until the conclusion of the experiment. Fish were fed to satiation twice daily with commercial feed (Bio-Oregon, Longview, WA). All experiments were carried out in accordance with US Geological Survey institutional guidelines and an approved IACUC review (SP 9065).

Experimental design

To sample juvenile Atlantic salmon before, during, and after smolting, fish (n = 8) of mixed sex were sacrificed on March 3, April 8, May 1 and July 10 at 09:00 h (Eastern Standard Time), with food withheld for 24 h prior to sampling. In addition, SW challenges were conducted on March 3 and May 1 at 09:00 h. Sixteen smolts were transferred to a tank with recirculating SW (35 ppt) at 9 °C with particle and charcoal filtration and continuous aeration. Food was withheld for the duration of the challenge. Fish were sampled (n = 8) at 09:00 h at 24 and 48 h after transfer to SW.

Sampling

At the time of sampling, fish were netted and anesthetized in buffered MS-222 (100 mg/l; pH 7.0; Sigma, St. Louis, MO). Blood was collected from the caudal vasculature by a needle and syringe treated with ammonium heparin. Blood samples were collected within 5 min from the initial netting. Blood was separated by centrifugation at 4 °C and plasma stored at−80 °C until analyses. Body mass and fork length were measured for calculation of condition factor: (body mass, g)/(fork length, cm)3 × 100. Gill and liver tissues were collected and immediately frozen on dry ice and stored at−80 °C. Four to six additional gill filaments were placed in ice-cold SEI buffer (150 mM sucrose, 10 mM EDTA, 50 mM imidazole, pH 7.3) and stored at−80 °C.

Plasma and gill analyses

Plasma Gh levels were measured by a radioimmunoassay (RIA) validated for Atlantic salmon by Björnsson et al. [37]. Plasma Igf1 levels were measured by a RIA validated for salmonids [38]. Plasma chloride was analyzed by the silver titration method using a Buchler-Cotlove digital chloridometer (Labconco, Kansas City, MO) and external standards. Branchial Nka activity was determined as described by McCormick [39]. Protein concentration of the gill homogenate was determined using a BCA protein assay (Thermo Fisher Scientific, Rockford, IL).

RNA extraction, cDNA synthesis and quantitative real-time PCR (qRT-PCR)

Total RNA was extracted from tissues by the TRI Reagent procedure (MRC, Cincinnati, OH) according to the manufacturer’s protocols. RNA concentration and purity were assessed by spectrophotometric absorbance (Nanodrop 1000, Thermo Scientific, Wilmington, DE). First strand cDNA was synthesized with a High Capacity cDNA Reverse Transcription Kit (Life Technologies, Carlsbad, CA). Relative mRNA levels were determined by qRT-PCR using the StepOnePlus real-time PCR system (Life Technologies). We employed previously described primer pairs for ghr1 [23], igf1, igf2, igf receptor 1a (igfr1a) and elongation factor 1α (ef1α) [40], igfbp1a1,−1b1,−1b2,−2a,−2b1,−2b2,−4,−5a,−5b1,−5b2,−6b1 and−6b2 [33], and nkcc1 and cftr1 [11]. qRT-PCR reactions were setup in a 15 μl final reaction volume with 400 nM of each primer, 1 μl cDNA and 7.5 μl of 2× SYBR Green PCR Master Mix (Life Technologies). The following cycling parameters were employed: 10 min at 95 °C followed by 40 cycles at 95 °C for 15 s, 60 °C for 30 s and 72 °C for 30 s. After verification that levels did not vary across groups, ef1α levels were used to normalize target genes. Reference and target gene levels were calculated by the relative quantification method with PCR efficiency correction [41]. Standard curves were prepared from serial dilutions of gill or liver cDNA and included on each plate to calculate the PCR efficiencies for target and normalization genes (>90%). Relative mRNA levels are reported as a fold-change from the March 3 group (Figs. 13; Table 1) or 0 h groups (Figs. 46; Table 2).
Fig. 1

Seasonal dynamics of condition factor and ionoregulatory parameters. Condition factor (a) and branchial Nka activity (b), nkcc1 (c) and cftr1 (d) mRNA levels in Atlantic salmon maintained in FW from March 3 through July 10. Means ± S.E.M. (n = 8). mRNA levels are presented as a fold-change from the March 3 group. Means not sharing the same letter are significantly different (one-way ANOVA, Tukey’s HSD test, P < 0.05)

Fig. 2

Seasonal dynamics of plasma hormones. Plasma Gh (a) and Igf1 (b) levels in Atlantic salmon maintained in FW from March 3 through July 10. Means ± S.E.M. (n = 8). Means not sharing the same letter are significantly different (one-way ANOVA, Tukey’s HSD test, P < 0.05)

Fig. 3

Seasonal dynamics of branchial gene expression. Branchial ghr1 (a), igf1 (b), igf2 (c), igfr1a (d), igfbp6b1 (e) and -6b2 (f) mRNA levels in Atlantic salmon maintained in FW from March 3 through July 10. Means ± S.E.M. (n = 8). mRNA levels are presented as a fold-change from the March 3 group. Means not sharing the same letter are significantly different (one-way ANOVA, Tukey’s HSD test, P < 0.05)

Table 1

Branchial and hepatic mRNA levels in Atlantic salmon maintained in FW from March 3 through July 10

Target Gene

March 3

April 8

May 1

July 10

Gill

igfbp4

1.00 ± 0.08b

1.06 ± 0.20b

1.49 ± 0.15a,b

1.61 ± 0.10a

igfbp5a

1.00 ± 0.15a,b

0.79 ± 0.14b

1.51 ± 0.18a

0.73 ± 0.05b

igfbp5b1

1.00 ± 0.06a,b

0.47 ± 0.06c

1.17 ± 0.14a

0.70 ± 0.09b,c

igfbp5b2

1.00 ± 0.12a,b

0.57 ± 0.07c

1.13 ± 0.06a

0.76 ± 0.05b,c

Liver

ghr1

1.00 ± 0.30b

2.75 ± 0.33a

1.09 ± 0.16b

0.38 ± 0.08b

igf1

1.00 ± 0.21b

3.04 ± 0.71a

3.49 ± 0.15a

3.25 ± 0.35a

igf2

1.00 ± 0.27

0.91 ± 0.31

0.56 ± 0.11

0.46 ± 0.07

igfbp1a1

1.00 ± 0.45

0.69 ± 0.37

0.10 ± 0.04

0.02 ± 0.004

igfbp1b1

1.00 ± 0.21b

2.50 ± 0.47a

1.05 ± 0.14b

0.36 ± 0.02b

igfbp1b2

1.00 ± 0.16b

5.75 ± 1.26a

2.01 ± 0.27b

0.21 ± 0.04b

igfbp2a

1.00 ± 0.10

0.83 ± 0.07

0.74 ± 0.06

0.73 ± 0.07

igfbp2b1

1.00 ± 0.10a

0.51 ± 0.06b

0.44 ± 0.05b

0.44 ± 0.03b

igfbp2b2

1.00 ± 0.12

1.29 ± 0.15

0.97 ± 0.09

0.90 ± 0.02

Expression levels are presented as a fold-change from the March 3 group. Means ± S.E.M. (n = 8). For a given transcript, means not sharing the same superscript letter are significantly different (one-way ANOVA, Tukey’s HSD test, P < 0.05)

Fig. 4

Effects of SW exposure on ionoregulatory parameters. Plasma chloride (a) and branchial Nka activity (b), nkcc1 (c) and cftr1 (d) mRNA levels in Atlantic salmon subjected to 24 and 48 h SW exposures in March (open bars) and May (shaded bars). Means ± S.E.M. (n = 8). mRNA levels are presented as a fold-change from the 0 h groups. Within a given experiment, denoted by uppercase or lowercase letters, means not sharing the same letter are significantly different (one-way ANOVA, Tukey’s HSD test, P < 0.05)

Fig. 5

Effects of SW exposure on plasma hormones and branchial gene expression. Plasma Gh (a), Igf1 (b), and branchial ghr1 (c), igf1 (d), igf2 (e) and igfr1a (f) mRNA levels in Atlantic salmon exposed to 24 and 48 h SW exposures in March (open bars) and May (shaded bars). Means ± S.E.M. (n = 8). Within a given experiment, denoted by uppercase or lowercase letters, means not sharing the same letter are significantly different (one-way ANOVA, Tukey’s HSD test, P < 0.05)

Fig. 6

Effects of SW exposure on igfbp gene expression. Branchial igfbp4 (a),−5a (b),−5b1 (c),−5b2 (d),−6b1 (e) and−6b2 (f) mRNA levels in Atlantic salmon exposed to 24 and 48 h SW exposures in March (open bars) and May (shaded bars). Means ± S.E.M. (n = 8). Within a given experiment, denoted by uppercase or lowercase letters, means not sharing the same letter are significantly different (one-way ANOVA, Tukey’s HSD test, P < 0.05)

Table 2

Hepatic mRNA levels in Atlantic salmon exposed to 24 and 48 h SW exposures in two separate experiments (March and May)

Target Gene

Experiment

0 h

24 h

48 h

ghr1

March

1.00 ± 0.30a,b

0.36 ± 0.04b

1.95 ± 0.55a

May

1.00 ± 0.14

5.38 ± 4.00

1.79 ± 0.25

igf1

March

1.00 ± 0.21

0.67 ± 0.22

0.84 ± 0.30

May

1.00 ± 0.04b

0.61 ± 0.17b

1.55 ± 0.15a

igf2

March

1.00 ± 0.27

0.63 ± 0.14

0.72 ± 0.20

May

1.00 ± 0.19

4.39 ± 3.35

0.95 ± 0.27

igfbp1a1

March

1.00 ± 0.44b

6.06 ± 2.81a,b

20.15 ± 8.67a

May

1.00 ± 0.39

20.26 ± 10.23

144.97 ± 88.44

igfbp1b1

March

1.00 ± 0.21b

4.45 ± 0.39a

4.55 ± 1.15a

May

1.00 ± 0.13

0.79 ± 0.09

1.38 ± 0.47

igfbp1b2

March

1.00 ± 0.16

2.41 ± 0.31

2.79 ± 0.89

May

1.00 ± 0.14a

0.52 ± 0.04b

1.24 ± 0.21a

igfbp2a

March

1.00 ± 0.10

0.98 ± 0.22

0.84 ± 0.17

May

1.00 ± 0.09

0.85 ± 0.11

1.27 ± 0.23

igfbp2b1

March

1.00 ± 0.10

0.91 ± 0.06

0.65 ± 0.12

May

1.00 ± 0.11

1.02 ± 0.14

1.53 ± 0.32

igfbp2b2

March

1.00 ± 0.12

0.87 ± 0.06

0.97 ± 0.13

May

1.00 ± 0.09

0.82 ± 0.10

0.91 ± 0.19

Expression levels are presented as a fold-change from the 0 h groups. Means ± S.E.M. (n = 8). Within an experiment, means not sharing the same superscript letter are significantly different (one-way ANOVA, Tukey’s HSD test, P < 0.05)

Statistics

Group comparisons were performed by one-way ANOVA followed by Tukey’s HSD test. Significance for all tests was set at P < 0.05. All statistical analyses were performed using GraphPad Prism 6 (San Diego, CA).

Results

Developmental/seasonal patterns

We verified that parr-smolt transformation and subsequent loss of salinity tolerance occurred by profiling multiple morphological and ionoregulatory parameters. Condition factor was significantly reduced on April 8, May 1 and July 10 compared with March 3 (Fig. 1a). We observed progressive silvering of the body and darkening of the fin margins leading to the May 1 sampling (data not shown). Branchial Nka activity was elevated in May compared with pre-smolts (March 3 and April 8) and post-smolts (July 10) (Fig. 1b). Branchial nkcc1 and cftr1 levels were elevated in May compared with all other sampled time points (Fig. 1c, d).

There was a significant effect of season on plasma Gh (one-way ANOVA; P < 0.001), but no significant differences between sampling points were detected by post hoc analysis (Fig. 2a). Plasma Igf1 was elevated in July when compared with April levels (Fig. 2b).

Parr-smolt transformation did not coincide with any differences in branchial ghr1 or igfr1a expression (Fig. 3a, d). Both igf1 and−2 were elevated in post-smolts (July 10) compared with all preceding time points (Fig. 3b, c). Branchial igfbp6b1 and−6b2 (Fig. 3e, f) were significantly elevated near the peak of smoltification (May 1) while there were no clear seasonal effects on igfbp4,−5a, 5b1 and−5b2 (Table 1). In liver, ghr1 expression was elevated in April above all other time points, while igf1 expression was elevated above March 3 levels at all subsequent samplings. There were no clear seasonal effects on hepatic igf2. Hepatic igfbp1b1 and−1b2 were elevated in April above all other time points; igfbp2b1 was reduced from March 3 levels at all subsequent samplings (Table 1).

Seawater exposures in March and May

In both March and May, SW exposures elicited increases in plasma chloride at 24 and 48 h (Fig. 4a). The increase in plasma chloride after SW exposure was substantially greater in March than in May. There were no significant increases in branchial Nka activity or nkcc1 after SW exposure in March or May (Fig. 4b, c). SW exposure induced branchial cftr1 expression in March, but not in May (Fig. 4d).

Plasma Gh levels were elevated by 48 h after SW exposure in March; Gh levels did not respond to SW exposure in May (Fig. 5a). SW exposures did not elicit any changes in plasma Igf1 (Fig. 5b). SW induced branchial ghr1 levels by 48 and 24 h in March and May, respectively (Fig. 5c). Branchial igf1 was not responsive to SW in March or May (Fig. 5d) whereas SW induced igf2 in both March and May (Fig. 5e). In March, igfr1a showed modest increases in response to SW (Fig. 5 f).

Among the igfbps expressed in the gill, igfbp4 and−6b1 were induced by SW exposure in March (Fig. 6A, E), while igfbp5a,−5b1,−5b2 and−6b2 were diminished following SW exposure (Fig. 6B-D, F). As in March, igfbp6b2 was decreased following SW exposure in May (Fig. 6 f). In liver, there were no clear effects of SW exposures on ghr1 and igf2; however, SW induced igf1 in May. igfbp1a1 and−1b1 were similarly induced by SW exposure in March (Table 2).

Discussion

The progressive increase in the salt secretory capacity of the gill during smoltification entails developmentally cued patterns of ionocyte differentiation and proliferation, in addition to altered gene transcription within these ionocytes [8, 42, 43]. Knowing that the Gh/Igf axis directs the timing and nature of these cellular behaviors [1315], we hypothesized that Igfbps contribute to smoltification and therefore would exhibit seasonal and SW-responsive patterns of gene expression. We report for the first time that increases in igfbp6b1 and−6b2 expression coincided with parr-smolt transformation and multiple igfbp4,−5 and−6 isoforms were modulated following SW exposures at different stages of smolt development.

Fish in this study underwent smoltification as indicated by multiple parameters. First, we observed the typical decrease in condition factor due to changes in body shape and the utilization of lipid and glycogen stores [36, 4447]. In strong agreement with previous studies, branchial Nka activity and nkcc1 and cftr1 expression concomitantly peaked in May, a hallmark of SW-type ionocyte recruitment [10, 11, 20, 36, 47, 48]. The ability of juvenile salmon to maintain ionoregulatory balance upon direct transfer from FW to SW is readily employed as an operational definition of hyposmoregulatory capacity. In March (pre-smolts) we observed relatively large increases in plasma chloride after SW exposure, whereas in May (smolts), we observed modest increases in plasma chloride after SW exposure. It is interesting that in March, when fish had not yet developed SW tolerance, only cftr1, and not nkcc1, was activated in parallel with chloride perturbations. The branchial epithelium of pre-smolts harbors a population of SW-type ionocytes that presumably employ Cftr1 and Nkcc1 in the apical and basolateral cell membrane, respectively [8, 49, 50]. Within these cells, transcription of cftr1 may be rapidly activated by ionic/osmotic conditions (environmental or internal), a pattern reminiscent of how chloride secretion is activated in opercular epithelia of Fundulus heteroclitus [51]. In any event, the attendant changes in the gill with respect to ionocyte function are consistent with a developmental/seasonal increase in ion-secretion capacity.

Photoperiod-induced increases in plasma Gh levels coincide with Atlantic salmon smoltification [52] and we likewise observed a rise (albeit not significant following post hoc analysis) in plasma Gh levels in April. Nilsen et al. [53] observed increased plasma Gh levels in SW-challenged smolts in May, whereas we observed a Gh response in March. This Gh response was paralleled by increased branchial ghr1 expression. Kiilerich et al. [20] similarly observed increased ghr1 with SW transfer, albeit in smolts transferred to SW in April. While not specifically shown in salmonids yet, Gh release from the pituitary is induced by direct osmosensing in certain euryhaline species such as Mozambique tilapia (Oreochromis mossambicus) [54]. This mode of regulation is compatible with increased plasma Gh levels when blood plasma conditions, such as plasma chloride and presumably osmolality, were perturbed following SW transfer. The osmoregulatory actions of Gh are mediated by its capacity to increase circulating levels and local tissue production of Igfs [14]. Seasonal patterns of circulating Igf1 in Atlantic salmon smolts are variable. In some cases, increases [36, 55], decreases [53], or no well-defined changes [56] have been reported. While we did not observe increased plasma Igf1 in smolts, we did observe increased hepatic igf1 expression, perhaps mediated by increased sensitivity to Gh via upregulation of ghr1. On the other hand, local igf1 and−2 expression in the gill was not elevated in May smolts, patterns important to consider in light of the igfbp responses we subsequently observed.

This is the first time branchial igfbps have been assessed in a salmonid preparing for seaward migration; we assayed igfbp4,−5 and−6 isoforms that exhibit robust branchial expression [33]. igfbp4 exhibited a steady rise in expression throughout spring and summer, with increased expression following SW exposure in March. The function of Igfbp4, at least in mammals, highly depends on the physiological context surrounding its production, and may operate as either a stimulator or inhibitor of Igf1/2 signaling [57, 58]. The activities of a teleost Igfbp4 were first assessed in fugu (Takifugu rubripes) where overexpression delayed embryonic development [59]. Nonetheless, in Atlantic salmon and sea bream (Sparus aurata), igfbp4 expression is implicated in mediating enhanced post-prandial/fasting muscle growth [40, 6062], thereby suggesting a stimulatory effect on Igf activity. The concomitant increases of igfbp4 along with igf2 and igfr1a following SW exposure may reflect a transcriptional program underlying enhanced paracrine signaling in response to ionoregulatory demands.

In contrast with igfbp4, igfbp5a,−5b1 and−5b2 were all reduced following SW exposure in March. Similar to Atlantic salmon igfbp5s, zebrafish (Danio rerio) igfbp5a and−5b are expressed in the gill [63]. igfbp5a is expressed in a sub-population of zebrafish ionocytes termed “NaR cells” specialized for Ca2+ uptake via Trpv5/6 channels. igfbp5a plays an essential role in Ca2+ homeostasis; igfbp5a expression is induced by low environmental [Ca2+] and igfbp5a knockdown inhibits compensatory increases in NaR cell proliferation following reductions in [Ca2+] [31]. While not yet established for Atlantic salmon, Ca2+ uptake across rainbow trout (Oncorhynchus mykiss) gill epithelia similarly employs a Trpv5/6 channel expressed in ionocytes and pavement cells [64]. If Igfbp5a is a conserved regulator of branchial Ca2+ uptake, then the SW-induced reductions in igfbp5a we observed in this study may reflect the increased [Ca2+] of SW compared with FW, and subsequent down regulation of Ca2+ uptake pathways. Interestingly, Dai et al. [63] showed that among the zebrafish Igfbp5 isoforms (−5a and−5b), only Igfbp5b exhibits ligand-independent transactivational activity. Thus, while igfbp5a,−5b1, and−5b2 showed similar responses to SW in the current study, it is likely that they are functionally distinct from one another, but such distinctions are entirely unresolved to date.

Wang et al. [65] described two teleost co-orthologs of human Igfbp6, denoted Igfbp6a and−6b. igfbp6a exhibits low expression in both zebrafish and Atlantic salmon gill, with igfbp6b2 being highly expressed in salmon gill [33, 65]. Among the igfbps we assayed, igfbp6b1 and−6b2 showed seasonal increases in expression with maximum levels in May smolts. Mammalian Igfbp6 exhibits a higher binding affinity for Igf2 versus Igf1, and inhibits Igf actions [66]. Similarly, zebrafish Igfbp6a and−6b attenuate Igf activities and embryonic growth and development [65]. There is currently no information on plasma Igf2 dynamics during smoltification; however, locally produced Igfbp6b1 and/or−6b2 may modulate Igf2 activity in the gill. Moreover, Igfbp6 modulates cell proliferation, migration, and apoptosis in mammalian systems [66, 67], and considering how cell turnover underlies branchial development during smoltification [42], Igfbp6s may similarly contribute to cell-cycle regulation in smolts. Adding further complexity is the disparate regulation of the two igfbp6 isoforms following SW exposures. Nonetheless, the seasonal patterns of both igfbp6s suggest future study of their role(s) in the gill is warranted.

We also assayed hepatic igfbp1 and−2 isoforms because their translated products modulate endocrine Igfs [26]. As in mammals, igfbp1 and−2 isoforms are highly expressed in teleost liver [33, 6874]. Igfbp1 inhibits somatic growth, development, and glucose metabolism by restricting Igfs from binding Igf receptors [69, 75, 76]. The only report to date of plasma Igfbp dynamics in smolts (coho salmon; Oncorhychus kisutch) revealed an April peak in plasma Igfbp1 [77]. This elevated Igfbp1 coincided with a drop in condition factor. Here, we observed 2.5- and 5.6-fold increases in igfbp1b1 and -1b2 expression, respectively, in April compared with March. Recall that we also observed a decline in condition factor in early spring, a pattern that routinely occurs when Atlantic salmon smolts, but not parr, are allowed to feed ad libitum [36]. Reduced condition factor is due to both smoltification-related changes in body shape and the utilization of energy reserves such as lipid stores and liver glycogen [44, 45, 47]. Previous work has established that Gh is involved in the lipolysis that occurs during smolting, and likely interacts with cortisol to affect other catabolic changes [78]. When these patterns are further considered with the feeding ecology of migrating smolts [46, 79, 80], smoltification emerges as inherently catabolic. Thus, igfbp1b1 and−1b2 may be further modulating growth and metabolism as part of the metabolic demands of smolt development and in preparation for seaward migration. Interestingly, we detected no seasonal changes in igfbp1a1, an isoform that is sensitive to nutrient conditions [81]. Hepatic igfbp1a1 was, however, induced by SW exposure, a response also seen with a 32-kD Igfbp (putative Igfbp1) in rainbow trout plasma [30, 82]. Collectively, these patterns suggest that duplicated igfbp1s allow for multifactorial control of Igf signaling during development and in response to salinity change. Future work should investigate whether divergent igfbp1 responses are aligned with contrasting sensitivities to hormones such as cortisol, thyroid hormones and insulin, which display seasonal changes and/or mediate stress responses [8385].

With a subset of hepatic and branchial igfbps whose expression patterns parallel parr-smolt transformation now revealed, future study is warranted to resolve how these dynamics relate to changes in circulating levels of actual Igfbp proteins. As the liver is regarded as a major source of circulating Igfbp1 [75, 86], we hypothesize that plasma Igfbp1b1 and−1b2 may be enhanced in early April provided that mRNA levels are suggestive of protein production and secretion. Moreover, with marked changes in branchial igfbp6b1 and−6b2 levels occurring in May, it should be resolved whether their translated products are retained (and act) locally, or whether they enter circulation as endocrine factors. In any event, the development of isoform-specific detection of Igfbps is the next step towards establishing how the complex expression patterns of Atlantic salmon igfbps across varied tissues [33] relate to local and endocrine protein levels.

Conclusions

Salmonids express an especially wide array of igfbps, and our data suggest that multiple Igfbps may contribute to the development of SW tolerance and associated metabolic changes that occur during the parr-smolt transformation. With igfbps such as igfbp6b1 and−6b2 showing increases coincident with smolt development, the challenge is to now identify the specific activities of these isoforms. By comparing the physiologies of anadromous and landlocked salmon populations, investigators have discerned how relaxed selection for SW adaptability affects both endocrine and ionoregulatory systems [11, 53, 87]. We propose that a similar approach, which compares igfbp expression patterns across Atlantic salmon populations, will aid our understanding how Igfbps operate within, and modulate, the hormonal mechanisms that drive smoltification.

Abbreviations

Cftr: 

Cystic fibrosis transmembrane regulator

Ef1α: 

Elongation factor 1α

FW: 

Fresh water

Gh: 

Growth hormone

Ghr: 

Growth hormone receptor

Igf: 

Insulin-like growth-factor

Igfbp: 

Insulin-like growth-factor binding protein

Igfr: 

Insulin-like growth-factor receptor

Nka: 

Na+/K+-ATPase

Nkcc: 

Na+/K+/2Cl cotransporter

qRT-PCR: 

Quantitative real-time PCR

RIA: 

Radioimmunoassay

SW: 

Seawater

Declarations

Acknowledgements

We appreciate the laboratory assistance provided by Andrew Weinstock and Amy Regish during the course of this study. We also acknowledge Spencer Chicoine for assistance with plasma chloride measurements.

Funding

This work was supported by Skidmore College (Start-Up Funds; JPB). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Authors’ contributions

JPB conceived and designed experiments, collected and analyzed qPCR data, and drafted the manuscript. CKF and SKPC collected and analyzed qPCR data. IEE collected and analyzed plasma hormone data. BThB analyzed plasma hormone data and coordinated the study. SDM conceived and designed experiments and coordinated the study. All authors contributed to revising manuscript drafts and approved the final version.

Competing interests

The authors declare that they have no competing interests.

Consent for publication

Not applicable.

Ethics approval and consent to participate

All experiments were carried out in accordance with USGS institutional guidelines and approved by the USGS, Leetown Science Center Institutional Animal Care and Use Committee (Protocol SP9065).

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
Department of Biology, Skidmore College
(2)
Fish Endocrinology Laboratory, Department of Biological and Environmental Sciences, University of Gothenburg
(3)
USGS, Leetown Science Center, S.O. Conte Anadromous Fish Research Center

References

  1. Hoar WS. The physiology of smolting salmonids. In: Hoar WS, Randall DJ, editors. Fish Physiology, vol. XIB. New York: Academic; 1988. p. 275–343.Google Scholar
  2. McCormick SD. Ontogeny and evolution of salinity tolerance in anadromous salmonids: hormones and heterochrony. Estuaries. 1994;17:26–33.View ArticleGoogle Scholar
  3. McCormick SD, Shrimpton JM, Moriyama S, Björnsson BT. Effects of an advanced temperature cycle on smolt development and endocrinology indicate that temperature is not a zeitgeber for smolting in Atlantic salmon. J Exp Biol. 2002;205:3553–60.PubMedGoogle Scholar
  4. Boeuf G. Salmonid smolting: a pre-adaptation to the oceanic environment. In: Rankin JC, Jensen FB, editors. Fish ecophysiology. London: Chapman and Hall; 1993. p. 105–35.View ArticleGoogle Scholar
  5. McCormick SD, Hansen LP, Quinn TP, Saunders RL. Movement, migration, and smolting of Atlantic salmon (Salmo salar). Can J Fish Aquat Sci. 1998;55 Suppl 1:77–92.View ArticleGoogle Scholar
  6. Johnstone KA, Lubieniecki KP, Koop BF, Davidson WS. Expression of olfactory receptors in different life stages and life histories of wild Atlantic salmon (Salmo salar). Mol Ecol. 2011;20:4059–69.View ArticlePubMedGoogle Scholar
  7. Marshall WS, Grosell M. Ion transport, osmoregulation, and acid–base balance. In: Evans DH, Claiborne JB, editors. The physiology of fishes. Boca Raton: CRC Press; 2006. p. 177–230.Google Scholar
  8. Pelis RM, McCormick SD. Effects of growth hormone and cortisol on Na+-K+-2Cl cotransporter localization and abundance in the gills of Atlantic salmon. Gen Comp Endocrinol. 2001;124:134–43.View ArticlePubMedGoogle Scholar
  9. Singer TD, Clements KM, Semple JW, Schulte PM, Bystriansky JS, Finstad B, et al. Seawater tolerance and gene expression in two strains of Atlantic salmon smolts. Can J Fish Aquat Sci. 2002;59:125–35.View ArticleGoogle Scholar
  10. Tipsmark CK, Madsen SS, Seidelin M, Christensen AS, Cutler CP, Cramb G. Dynamics of Na+, K+,2Cl − cotransporter and Na+, K + -ATPase expression in the branchial epithelium of brown trout (Salmo trutta) and Atlantic salmon (Salmo salar). J Exp Zool. 2002;293:106–18.View ArticlePubMedGoogle Scholar
  11. Nilsen TO, Ebbesson LOE, Madsen SS, McCormick SD, Andersson E, Björnsson BT, et al. Differential expression of gill Na+/K+-ATPase α- and β-subunits, Na+/K+/2Cl cotransporter and CFTR anion channel in juvenile anadromous and landlocked Atlantic salmon Salmo salar. J Exp Biol. 2007;210:2885–96.View ArticlePubMedGoogle Scholar
  12. McCormick SD, Regish AM, Christensen AK, Björnsson BT. Differential regulation of sodium-potassium pump isoforms during smolt development and seawater exposure of Atlantic salmon. J Exp Biol. 2013;216:1142–51.View ArticlePubMedGoogle Scholar
  13. Björnsson BT. The biology of salmon growth hormone: from daylight to dominance. Fish Physiol Biochem. 1997;17:9–24.View ArticleGoogle Scholar
  14. Sakamoto T, McCormick SD, Hirano T. Osmoregulatory actions of growth hormone and its mode of action in salmonids: A review. Fish Physiol Biochem. 1993;11:155–64.View ArticlePubMedGoogle Scholar
  15. McCormick SD. Endocrine control of osmoregulation in teleost fish. Amer Zool. 2001;41:781–94.Google Scholar
  16. McCormick SD, Dickhoff WW, Duston J, Nishioka RS, Bern HA. Developmental differences in the responsiveness of gill Na+, K + -ATPase to cortisol in salmonids. Gen Comp Endocrinol. 1991;84:308–17.View ArticlePubMedGoogle Scholar
  17. Madsen SS, Bern HA. In-vitro effects of insulin-like growth factor-I on gill Na+,K+-ATPase in coho salmon, Oncorhynchus kisutch. J Endocrinol. 1993;138:23–30.View ArticlePubMedGoogle Scholar
  18. Sakamoto T, Hirano T. Expression of insulin-like growth factor I gene in osmoregulatory organs during seawater adaptation of the salmonid fish: possible mode of osmoregulatory action of growth hormone. Proc Natl Acad Sci U S A. 1993;90:1912–16.View ArticlePubMedPubMed CentralGoogle Scholar
  19. Madsen SS, Jensen MK, Nhr J, Kristiansen K. Expression of Na+-K+-ATPase in the brown trout, Salmo trutta: in vivo modulation by hormones and seawater. Am J Physiol. 1995;269:R1339–45.PubMedGoogle Scholar
  20. Kiilerich P, Kristiansen K, Madsen SS. Hormone receptors in gills of smolting Atlantic salmon, Salmo salar: expression of growth hormone, prolactin, mineralocorticoid and glucocorticoid receptors and 11β-hydroxysteroid dehydrogenase type 2. Gen Comp Endocrinol. 2007;152:295–303.View ArticlePubMedGoogle Scholar
  21. Shrimpton JM, McCormick SD. Regulation of gill cytosolic corticosteroid receptors in juvenile Atlantic salmon: Interaction effects of growth hormone with prolactin and triiodothyronine. Gen Comp Endocrinol. 1998;112:262–74.View ArticlePubMedGoogle Scholar
  22. McCormick SD. Effects of growth hormone and insulin-like growth factor I on salinity tolerance and gill Na+/K+-ATPase in Atlantic salmon (Salmo salar): interaction with cortisol. Gen Comp Endocrinol. 1996;101:3–11.View ArticlePubMedGoogle Scholar
  23. Tipsmark CK, Madsen SS. Distinct hormonal regulation of Na+, K + -atpase genes in the gill of Atlantic salmon (Salmo salar L.). J Endocrinol. 2009;203:301–10.View ArticlePubMedGoogle Scholar
  24. Madsen SS. The role of cortisol and growth hormone in seawater adaptation and development of hypoosmoregulatory mechanisms in sea trout parr (Salmo trutta trutta). Gen Comp Endocrinol. 1990;79:1–11.View ArticlePubMedGoogle Scholar
  25. Prunet P, Pisam M, Claireaux JP, Boeuf G, Rambourg A. Effects of growth hormone on gill chloride cells in juvenile Atlantic salmon (Salmo salar). Am J Physiol. 1994;266:R850–7.PubMedGoogle Scholar
  26. Duan C, Ren H, Gao S. Insulin-like growth factors (IGFs), IGF receptors, and IGF-binding proteins: roles in skeletal muscle growth and differentiation. Gen Comp Endocrinol. 2010;167:344–51.View ArticlePubMedGoogle Scholar
  27. Clemmons DR. Use of mutagenesis to probe IGF-binding protein structure/function relationships. Endocr Rev. 2001;22(6):800–17.View ArticlePubMedGoogle Scholar
  28. Reindl KM, Sheridan MA. Peripheral regulation of the growth hormone-insulin-like growth factor system in fish and other vertebrates. Comp Biochem Physiol A. 2012;163:231–45.View ArticleGoogle Scholar
  29. Johnson J, Silverstein J, Wolters WR, Shimizu M, Dickhoff WW, Shepherd BS. Disparate regulation of insulin-like growth factor-binding proteins in a primitive, ictalurid, teleost (Ictalurus punctatus). Gen Comp Endocrinol. 2003;134(2):122–30.View ArticlePubMedGoogle Scholar
  30. Shepherd BS, Drennon K, Johnson J, Nichols JW, Playle RC, Singer TD, Vijayan MM. Salinity acclimation affects the somatotropic axis in rainbow trout. Am J Physiol Regul Integr Comp Physiol. 2005;288:R1385–95.View ArticlePubMedGoogle Scholar
  31. Dai W, Bai Y, Hebda L, Zhong X, Liu J, Kao J, Duan C. Calcium deficiency-induced and TRP channel-regulated IGF1R-PI3K-Akt signaling regulates abnormal epithelial cell proliferation. Cell Death Differ. 2014;21(4):568–81.View ArticlePubMedGoogle Scholar
  32. Taniyama N, Kaneko N, Inatani Y, Miyakoshi Y, Shimizu M. Effects of seawater transfer and fasting on the endocrine and biochemical growth indices in juvenile chum salmon (Oncorhynchus keta). Gen Comp Endocrinol. 2016;236:146–56.View ArticlePubMedGoogle Scholar
  33. Macqueen DJ, García de la serrana D, Johnston IA. Evolution of ancient functions in the vertebrate insulin-like growth factor system uncovered by the study of duplicated salmonid fish genomes. Mol Biol Evol. 2013;30:1060–76.View ArticlePubMedPubMed CentralGoogle Scholar
  34. Parrish DL, Behnke RJ, Gephard SR, McCormick SD, Reeves GH. Why aren’t there more Atlantic salmon (Salmo salar)? Can J Fish Aquat Sci. 1998;55 Suppl 1:281–87.View ArticleGoogle Scholar
  35. McCormick SD, Lerner DT, Monette MY, Nieves-Puigdoller K, Kelly JT, Björnsson BT. Taking it with you when you go: how perturbations to the freshwater environment, including temperature, dams, and contaminants, affect marine survival of salmon. Am Fish Soc Symp. 2009;69:195–214.Google Scholar
  36. McCormick SD, Shrimpton JM, Moriyama S, Björnsson BT. Differential hormonal responses of Atlantic salmon parr and smolt to increased daylength: A possible developmental basis for smolting. Aquaculture. 2007;273:337–44.View ArticleGoogle Scholar
  37. Björnsson BT, Taranger GL, Hansen T, Stefansson SO, Haux C. The interrelation between photoperiod, growth hormone, and sexual maturation of adult Atlantic salmon (Salmo salar). Gen Comp Endocrinol. 1994;93:70–81.View ArticlePubMedGoogle Scholar
  38. Moriyama S, Swanson P, Nishii M, Takahashi A, Kawauchi H, Dickhoff WW, Plisetskaya EM. Development of homologous radioimmunoassay for coho salmon insulin-like growth factor-I. Gen Comp Endocrinol. 1994;96:149–61.View ArticlePubMedGoogle Scholar
  39. McCormick SD. Methods for nonlethal gill biopsy and measurement of Na+, K+,-ATPase activity. Can J Fish Aquat Sci. 1993;50:656–58.View ArticleGoogle Scholar
  40. Bower NI, Li X, Taylor R, Johnston IA. Switching to fast growth: the insulin-like growth factor (IGF) system in skeletal muscle of Atlantic salmon. J Exp Biol. 2008;211:3859–70.View ArticlePubMedGoogle Scholar
  41. Pfaffl MW. A new mathematical model for relative quantification in real-time RT-PCR. Nucl Acids Res. 2001;29(9):e45.View ArticlePubMedPubMed CentralGoogle Scholar
  42. Laurent P, Dunel-Erb S, Chevalier C, Lignon J. Gill epithelial cells kinetics in a freshwater teleost, Oncorhynchus mykiss during adaptation to ion-poor water and hormonal treatments. Fish Physiol Biochem. 1994;13(5):353–70.View ArticlePubMedGoogle Scholar
  43. Robertson LS, McCormick SD. Transcriptional profiling of the parr-smolt transformation in Atlantic salmon. Comp Biochem Physiol D. 2012;7:351–60.Google Scholar
  44. Saunders RL, Henderson EB. Influence of photoperiod on smolt development and growth of Atlantic salmon (Salmo salar). J Fish Res Board Can. 1970;27:1295–311.View ArticleGoogle Scholar
  45. Sheridan MA. Alterations in lipid metabolism accompanying smoltification and seawater adaptation of salmonid fish. Aquaculture. 1989;82:191–203.View ArticleGoogle Scholar
  46. Stefansson SO, Björnsson BT, Sundell L, Nyhammer G, McCormick SD. Physiological characteristics of wild Atlantic salmon post-smolts during estuarine and coastal migration. J Fish Biol. 2003;63:942–55.View ArticleGoogle Scholar
  47. Winans GA, Nishioka RS. A multivariate description of change in body shape of coho salmon (Oncorhynchus kisutch) during smoltification. Aquaculture. 1987;66:235–45.View ArticleGoogle Scholar
  48. Mackie PM, Gharbi K, Ballantyne JS, McCormick SD, Wright PA. Na+/K+/2Cl cotransporter and CFTR gill expression after seawater transfer in smolts (0+) of different Atlantic salmon (Salmo salar) families. Aquaculture. 2007;272:625–35.View ArticleGoogle Scholar
  49. Marshall WS, Singer TD. Cystic fibrosis transmembrane conductance regulator in teleost fish. Biochim Biophys Acta. 2002;1566:16–27.View ArticlePubMedGoogle Scholar
  50. McCormick SD, Sundell K, Björnsson BT, Brown CL, Hiroi J. Influence of salinity on the localization of Na+/K+-ATPase, Na+/K+/2Cl cotransporter (NKCC) and CFTR anion channel in chloride cells of the Hawaiian goby (Stenogobius hawaiiensis). J Exp Biol. 2003;206:4575–83.View ArticlePubMedGoogle Scholar
  51. Zadunaisky JA, Cardona S, Au L, Roberts DM, Fisher E, Lowenstein B, et al. Chloride transport activation by plasma osmolarity during rapid adaptation to high salinity of Fundulus heteroclitus. J Membr Biol. 1995;143:207–17.View ArticlePubMedGoogle Scholar
  52. Björnsson BT, Stefansson SO, Hansen T. Photoperiod regulation of plasma growth hormone levels during parr-smolt transformation of Atlantic salmon: implications for hypoosmoregulatory ability and growth. Gen Comp Endocrinol. 1995;100:73–82.View ArticlePubMedGoogle Scholar
  53. Nilsen TO, Ebbesson LO, Kiilerich P, Björnsson BT, Madsen SS, McCormick SD, Stefansson SO. Endocrine systems in juvenile anadromous and landlocked Atlantic salmon (Salmo salar): seasonal development and seawater acclimation. Gen Comp Endocrinol. 2008;155:762–72.View ArticlePubMedGoogle Scholar
  54. Seale AP, Riley LG, Leedom TA, Kajimura S, Dores RM, Hirano T, Grau EG. Effects of environmental osmolality on release of prolactin, growth hormone and ACTH from the tilapia pituitary. Gen Comp Endocrinol. 2002;128:91–101.View ArticlePubMedGoogle Scholar
  55. Ágústsson T, Sundell K, Sakamoto T, Johansson V, Ando M, Björnsson BT. Growth hormone endocrinology of Atlantic salmon (Salmo salar): pituitary gene expression, hormone storage, secretion and plasma levels during parr-smolt transformation. J Endocrinol. 2001;170:227–34.View ArticlePubMedGoogle Scholar
  56. McCormick SD, Sheehan T, Björnsson BT, Lipsky C, Kocik J, Regish AM, O’Dea MF. Physiological and endocrine changes in Atlantic salmon smolts during hatchery rearing, downstream migration and ocean entry. Can J Fish Aquat Sci. 2013;70:105–18.View ArticleGoogle Scholar
  57. Duan C, Clemmons DR. Differential expression and biological effects of insulin-like growth factor-binding protein-4 and−5 in vascular smooth muscle cells. J Biol Chem. 1998;273(27):16836–42.View ArticlePubMedGoogle Scholar
  58. Ning Y, Schuller AG, Conover CA, Pintar JE. Insulin-like growth factor (IGF) binding protein-4 is both a positive and negative regulator of IGF activity in vivo. Mol Endocrinol. 2008;22:1213–25.View ArticlePubMedPubMed CentralGoogle Scholar
  59. Li M, Li Y, Lu L, Wang X, Gong Q, Duan C. Structural, gene expression, and functional analysis of the fugu (Takifugu rubripes) insulin-like growth factor binding protein-4 gene. Am J Physiol Regul Integr Comp Physiol. 2009;296(3):R558–66.View ArticlePubMedGoogle Scholar
  60. Bower NI, Johnston IA. Transcriptional regulation of the IGF-signaling pathway by amino acids and insulin-like growth factors during myogenesis in Atlantic salmon. PLoS One. 2010;5(6):e11100.View ArticlePubMedPubMed CentralGoogle Scholar
  61. García de la serrana D, Codina M, Jiménez-Amilburu V, Navarro I, Du SJ, et al. Characterisation and expression of myogenesis regulatory factors during in vitro myoblast development and in vivo fasting in the gilthead sea bream (Sparus aurata). Comp Biochem Physiol A. 2014;167:90–9.View ArticleGoogle Scholar
  62. Valente LMP, Bower NI, Johnston IA. Postprandial expression of growth related genes in Atlantic salmon (Salmo salar L.) juveniles fasted for 1 week and fed a single meal to satiation. Br J Nutr. 2012;2:1–10.Google Scholar
  63. Dai W, Kamei H, Zhao Y, Ding J, Du Z, Duan C. Duplicated zebrafish insulin-like growth factor binding protein-5 genes with split functional domains: evidence for evolutionarily conserved IGF binding, nuclear localization, and transactivation activity. FASEB J. 2010;24(6):2020–9.View ArticlePubMedPubMed CentralGoogle Scholar
  64. Shahsavarani A, McNeill B, Galvez F, Wood CM, Goss GG, Hwang PP, Perry SF. Characterization of a branchial epithelial calcium channel (ECaC) in freshwater rainbow trout (Oncorhynchus mykiss). J Exp Biol. 2006;209:1928–43.View ArticlePubMedGoogle Scholar
  65. Wang X, Lu L, Li Y, Li M, Chen C, Feng Q, et al. Molecular and functional characterization of two distinct IGF binding protein-6 genes in zebrafish. Am J Physiol Regul Integr Comp Physiol. 2009;296:R1348–57.View ArticlePubMedGoogle Scholar
  66. Gallicchio MA, Kneen M, Hall C, Scott AM, Bach LA. Overexpression of insulin-like growth factor binding protein-6 inhibits rhabdomyosarcoma growth in vivo. Int J Cancer. 2001;94(5):645–51.View ArticlePubMedGoogle Scholar
  67. Fu P, Thompson JA, Bach LA. Promotion of cancer cell migration: an insulin-like growth factor (IGF)-independent action of IGF-binding protein-6. J Biol Chem. 2007;282(31):22298–306.View ArticlePubMedGoogle Scholar
  68. Funkenstein B, Tsai W, Maures T, Duan C. Ontogeny, tissue distribution, and hormonal regulation of insulin-like growth factor binding protein-2 (IGFBP-2) in a marine fish, Sparus aurata. Gen Comp Endocrinol. 2002;128:112–22.View ArticlePubMedGoogle Scholar
  69. Kamei H, Lu L, Jiao S, Li Y, Gyrup C, Laursen LS, et al. Duplication and diversification of the hypoxia-inducible IGFBP-1 gene in zebrafish. PLoS One. 2008;3(8):e3091.View ArticlePubMedPubMed CentralGoogle Scholar
  70. Zhou J, Li W, Kamei H, Duan C. Duplication of the IGFBP-2 gene in the teleost fish: protein structure and functionality conservation and gene expression divergence. PLoS One. 2008;3(12):e3926.View ArticlePubMedPubMed CentralGoogle Scholar
  71. Pedroso FL, Fukada H, Masumoto T. Molecular characterization, tissue distribution patterns and nutritional regulation of IGFBP-1,−2,−3 and−5 in yellowtail, Seriola quinqueradiata. Gen Comp Endocrinol. 2009;161:344–53.View ArticlePubMedGoogle Scholar
  72. Shimizu M, Kishimoto K, Yamaguchi T, Nakano Y, Hata A, Dickhoff WW. Circulating salmon 28- and 22-kDa insulin-like growth factor binding protein (IGFBPs) are co-orthologs of IGFBP-1. Gen Comp Endocrinol. 2011;174:97–106.View ArticlePubMedGoogle Scholar
  73. Shimizu M, Suzuki S, Horikoshi M, Hara A, Dickhoff WW. Circulating salmon 41-kDa insulin-like growth factor binding protein (IGFBP) is not IGFBP-3 but an IGFBP-2 subtype. Gen Comp Endocrinol. 2011;171:326–31.View ArticlePubMedGoogle Scholar
  74. Safian D, Fuentes EN, Valdés JA, Molina A. Dynamic transcriptional regulation of autocrine/paracrine igfbp1, 2, 3, 4, 5, and 6 in the skeletal muscle of the fine flounder during different nutritional statuses. J Endocrinol. 2012;214:95–108.View ArticlePubMedGoogle Scholar
  75. Lee PD, Giudice LC, Conover CA, Powell DR. Insulin-like growth factor binding protein-1: recent findings and new directions. Proc Soc Exp Biol Med. 1997;216:319–57.View ArticlePubMedGoogle Scholar
  76. Kajimura S, Aida K, Duan C. Insulin-like growth factor-binding protein-1 (IGFBP-1) mediates hypoxia-induced embryonic growth and developmental retardation. Proc Natl Acad Sci U S A. 2005;102:1240–45.View ArticlePubMedPubMed CentralGoogle Scholar
  77. Shimizu M, Beckman BR, Hara A, Dickhoff WW. Measurement of circulating salmon IGF binding protein-1: assay development, response to feeding ration and temperature, and relation to growth parameters. J Endocrinol. 2006;188:101–10.View ArticlePubMedGoogle Scholar
  78. Sheridan MA. Effects of thyroxine, cortisol, growth hormone, and prolactin on lipid metabolism of coho salmon, Oncorhynchus kisutch, during smoltification. Gen Comp Endocrinol. 1986;64:220–38.View ArticlePubMedGoogle Scholar
  79. Andreassen PMR, Martinussen MB, Hvidsten NA, Stefansson SO. Feeding and prey-selection of wild Atlantic salmon post-smolts. J Fish Biol. 2001;58:1667–79.View ArticleGoogle Scholar
  80. Renkawitz MD, Sheehan TF. Feeding ecology of early marine phase Atlantic salmon Salmo salar post-smolts. J Fish Biol. 2011;79:356–73.PubMedGoogle Scholar
  81. Breves JP, Phipps-Costin SK, Fujimoto CK, Einarsdottir IE, Regish AM, Björnsson BT, McCormick SD. Hepatic insulin-like growth-factor binding protein (igfbp) responses to food restriction in Atlantic salmon smolts. Gen Comp Endocrinol. 2016;233:79–87.View ArticlePubMedGoogle Scholar
  82. Bauchat JR, Busby WH, Garmong A, Swanson P, Moore J, Lin M, Duan C. Biochemical and functional analysis of a conserved IGF-binding protein isolated from rainbow trout (Oncorhynchus mykiss) hepatoma cells. J Endocrinol. 2001;170(3):619–28.View ArticlePubMedGoogle Scholar
  83. Mommsen TP, Plisetskaya EM. Insulin in fishes and agnathans: history, structure, and metabolic regulation. Rev Aquat Sci. 1991;4:225–59.Google Scholar
  84. Pierce AL, Shimizu M, Felli L, Swanson P, Dickhoff WW. Metabolic hormones regulate insulin-like growth factor binding protein-1 mRNA levels in primary cultured salmon hepatocytes; lack of inhibition by insulin. J Endocrinol. 2006;191:379–86.View ArticlePubMedGoogle Scholar
  85. McCormick SD, Regish AM, Christensen AK. Distinct freshwater and seawater isoforms of Na+/K+-ATPase in gill chloride cells of Atlantic salmon. J Exp Biol. 2009;212:3994–4001.View ArticlePubMedGoogle Scholar
  86. Maures TJ, Duan C. Structure, developmental expression, and physiological regulation of zebrafish IGF binding protein-1. Endocrinology. 2002;143:2722–31.View ArticlePubMedGoogle Scholar
  87. Nakajima T, Shimura H, Yamazaki M, Fujioka Y, Ura K, Hara A, Shimizu M. Lack of hormonal stimulation prevents the landlocked Biwa salmon (Oncorhynchus masou subspecies) from adapting to seawater. Am J Physiol Regul Integr Comp Physiol. 2014;307(4):R414–25.View ArticlePubMedGoogle Scholar

Copyright

© The Author(s). 2017

Advertisement