Acclimatory responses of the Daphnia pulex proteome to environmental changes. II. Chronic exposure to different temperatures (10 and 20°C) mainly affects protein metabolism
© Schwerin et al; licensee BioMed Central Ltd. 2009
Received: 15 February 2008
Accepted: 21 April 2009
Published: 21 April 2009
Temperature affects essentially every aspect of the biology of poikilothermic animals including the energy and mass budgets, activity, growth, and reproduction. While thermal effects in ecologically important groups such as daphnids have been intensively studied at the ecosystem level and at least partly at the organismic level, much less is known about the molecular mechanisms underlying the acclimation to different temperatures. By using 2D gel electrophoresis and mass spectrometry, the present study identified the major elements of the temperature-induced subset of the proteome from differently acclimated Daphnia pulex.
Specific sets of proteins were found to be differentially expressed in 10°C or 20°C acclimated D. pulex. Most cold-repressed proteins comprised secretory enzymes which are involved in protein digestion (trypsins, chymotrypsins, astacin, carboxypeptidases). The cold-induced sets of proteins included several vitellogenin and actin isoforms (cytoplasmic and muscle-specific), and an AAA+ ATPase. Carbohydrate-modifying enzymes were constitutively expressed or down-regulated in the cold.
Specific sets of cold-repressed and cold-induced proteins in D. pulex can be related to changes in the cellular demand for amino acids or to the compensatory control of physiological processes. The increase of proteolytic enzyme concentration and the decrease of vitellogenin, actin and total protein concentration between 10°C and 20°C acclimated animals reflect the increased amino-acids demand and the reduced protein reserves in the animal's body. Conversely, the increase of actin concentration in cold-acclimated animals may contribute to a compensatory mechanism which ensures the relative constancy of muscular performance. The sheer number of peptidase genes (serine-peptidase-like: > 200, astacin-like: 36, carboxypeptidase-like: 30) in the D. pulex genome suggests large-scaled gene family expansions that might reflect specific adaptations to the lifestyle of a planktonic filter feeder in a highly variable aquatic environment.
Planktonic crustaceans of the genus Daphnia experience pronounced variations in ambient parameters such as oxygen concentration and temperature in the field and show plastic adaptive responses to these environmental changes. Differential regulation of gene expression provides specific sets of proteins for the maintenance of cellular function under altered ambient conditions. The recent release of the Daphnia pulex genome sequence [1, 2] offers the opportunity to relate proteomic adjustments to the differentially regulated genes.
Temperature affects the performance of poikilothermic animals at all levels of biological organization ranging from biochemical reactions via physiological processes to organismic properties such as fecundity and reproductive success. Acute changes in water temperature, for example, have a strong effect on systemic parameters such as heart and ventilation rate of Daphnia spp. (e.g. ). However, such physiological perturbations can be damped by acclimatory processes. Previous studies [3–6] have shown that the metabolic rates, heart and ventilation rates, and muscular performances of several Daphnia species at 10°C and 20°C are not as different as expected from the Q10 rule, provided the animals have the chance to acclimate to the temperature at which they were tested. Such a type of compensatory control (metabolic cold adaptation) is primarily based on adjustments in enzyme concentration . Nevertheless, a more or less reduced metabolic rate in the cold decreases the nutritive requirements  and causes also a retardation in somatic growth and development [9–11]. To mechanistically explain the role of temperature acclimation for the control of physiological processes, it is essential to know the adjustments which occur at the proteomic level.
The present study analyzed the protein expression patterns of 10°C and 20°C acclimated animals of Daphnia pulex under normoxic conditions. Two-dimensional gel electrophoresis and mass spectrometry were employed to identify the major elements of the temperature-induced subset of the proteome. Based on their putative functions, the probable physiological role of these sets of proteins are discussed.
Identified proteins from Daphnia pulex acclimated to 10°C or 20°C
Specificity N10: N20
Matched peptide sequences
pI gel/pI predicted
Putative function (symbolic name)
Endoribonuclease-like protein (ERNA)
Endoribonuclease-like protein (ERNB)
Carboxylesterase, type B
Carboxypeptidase A (CPA1A)
Carboxypeptidase A (CPA1B)
Egg yolk proteins & precursors
see Figure 2
Vitellogenin (VTG1, VTG2, VTG4)
see Figure 3
see Figure 2
see Figure 2
see Figure 2
see Figure 3
see Figure 2
Vitellogenin (VTG1, VTG2)
see Figure 2
Vitellogenin (VTG1, VTG2, VTG4)
see Figure 2
see Figure 2
Vitellogenin (VTG1, VTG2)
Cytoskeleton & muscle proteins
see Figure 2
see Figure 3
Myosin heavy chain (MHC-1)
see Figure 3
List of referred proteins and gene models
Carboxylesterase, type B
It is conspicuous, that a separation of cold-induced and cold-repressed proteins by Mr/pI leads to protein groups of similar classification. Almost all of the identified proteins with a reduced expression in the cold (expression reduction by 40–80%) were secretory enzymes involved in protein digestion (spots 31–32 and 36–41, Table 1). These include three trypsins (TRY4B, TRY5F, TRY5L), two chymotrypsins (CHY1A, CHY1C), one astacin (ACN2), and two carboxypeptidases (CPA1A, CPA1B). All these proteins are synthesized as pro-enzymes (zymogens), which are activated by the removal of an N-terminal propeptide (3–11 kDa). Owing to the similarities in their Mr/pI values, these proteins were multiply identified among the analysed spots. In addition, the multiple occurrence of TRY5F and CHY1C in spots with assigned Mr values of 25 and ≈ 30 kDa may be explained by the possible co-presence of pro-enzymes and enzymes. The only non-proteolytic proteins identified among these spots were two secretory proteins (ERNA, ERNB) carrying the characteristic domain of the EndoU/XendoU family of endoribonucleases [13, 14]. The spot region 28, which was excised and analyzed in a previous study , contained a mixture of enzymes (including peptidases of the family M2 and M13), which made an expression evaluation impossible.
A ten-fold up-regulation in the cold was found for an AAA+ adenosine triphosphatase (ATPase associated with diverse cellular activities; ), a fragment of which was detected in spot 42. Additional identifications comprised proteolytic cleavage fragments of a molecular chaperone (HSP90, spot 47), the heavy chain of myosin (MHC-1) and a pyruvate kinase (both in spot 48). Since the latter three proteins were co-identified with actins and VTGs in the same spots, it was impossible to assess their induction states.
Among the remaining identifications was a group of carbohydrate-modifying enzymes with a constitutive or reduced expression in the cold. Constitutive expressions showed the exo-β-1,3-glucanase EXG5 (spot 34) and the α-amylase AMY (spot 35). The cold-repressed proteins included a cellubiohydrolase (CEL7A, spot 1), an endo-β-1,4-glucanase (CEL9A, co-localized with paramyosin in spot 19), an endo-β-1,4-mannanase (MAN5A, co-localized with a β-1,3-glucan-binding protein in spot 30), and the enolase ENO (spot 19).
As a companion study to a previous investigation of acclimatory adjustments of the Daphnia pulex proteome to hypoxia , the effects of two different acclimation temperatures (10°C and 20°C) on the Daphnia pulex proteome were analyzed by 2D gel electrophoresis and mass spectrometry. Temperature acclimation mostly affected the expression of sets of proteins different from those identified under variable oxygen conditions. Several proteins constitutively expressed or subjected to hypoxic induction were also detected in the 2D gels presented here. The specific sets of proteins up- or down-regulated in the cold (10°C) were identified here for the first time.
Cold-induced protein sets I: Egg yolk proteins and precursors
The most dominant group among the cold-induced proteins in D. pulex were the vitellogenins (Table 1). Vitellogenin (VTG) is a precursor of the yolk protein vitellin. It is a lipoglycoprotein that is employed as a vehicle to provide the developing embryo with proteins, lipids, carbohydrates, and other essential resources. In many oviparous animals such as insects and non-mammalian vertebrates, VTG is synthesized in extraovarian tissues (e.g. fat body or liver) and is then transported via the blood/hemolymph to the developing oocytes [16, 17]. An exception are the decapod crustaceans which show, in addition to the extraovarian synthesis in the hepatopancreas, an intraovarian synthesis of yolk proteins . Since the VTGs of the branchiopod crustacean Daphnia spp. are more closely related to insect VTGs than to the yolk protein precursors of decapods [19–21], it is reasonable to postulate a vitellogenic tissue that is homologous to the VTG-synthesizing fat body of insects. Although there are some cytological indications for an endogenous synthesis of yolk proteins in amphigonic oocytes , the main site of VTG synthesis in Daphnia appears to be the highly polyploid fat cells, which exhibit periodic variations in lipid and glycogen content, cell size and ultrastructure in relation to the parthenogenetic reproduction cycle [22–24].
The screening of the D. pulex genome database suggests 19 loci with VTG-like coding sequences. Two gene products, VTG1 and VTG4, were identified in the present study (Figure 2). The additional expression of VTG2, however, which shares a high sequence similarity with VTG1, cannot be excluded. VTG1 and VTG2 are homologous to the vitellogenins DmagVTG1 and DmagVTG2 of D.magna . As in D. magna, the VTG1 and VTG2 genes are arranged in a tandem array in a back-to-back orientation, which might enable a coordinated hormonal regulation of their transcription . DmagVTG1 and (probably) DmagVTG2 are the most abundant polypeptides in D.magna parthenogenetic eggs at initial stages of development . At least one of the primary cleavage sites is present in VTG1 and VTG2 of D. pulex (Figure 2, top: 'ARRI'). Given the high sequence identity (88–90%) between the corresponding VTGs of both Daphnia species, it is likely that primary cleavage fragments of similar size occur in the developing eggs of D. pulex as well. However, none of these primary cleavage fragments could be detected in full length (65–155 kDa) among the analyzed spots, which contained only smaller VTG fragments of 18–27 kDa, possibly as a consequence of a residual proteolytic activity during the preparation of whole-animal extracts. Alternatively, smaller-than-expected fragments may have arisen prior to extract preparation by an advanced cleavage of yolk material during embryonic development.
The 4–7-fold up-regulation of VTGs in 10°C acclimated D. pulex (Table 1) was an unexpected finding. About 50–100 adult daphnids were randomly sampled irrespective of their reproductive states for single protein extractions. The protein extracts consequently contained contributions from parthenogenetic eggs and embryos in the brood chamber as well as from maternal tissues. A greater share of vitellogenin in the protein extracts from 10°C acclimated animals may therefore result from a greater amount of eggs in the ovaries and the brood pouch or from an increased vitellogenin concentration in the synthesizing tissues, ovaries, eggs and embryos. An inspection of both acclimation groups did not reveal any differences in clutch size or in the share of animals carrying eggs and embryos. Previous findings on the impact of temperature on clutch sizes in Daphnia are ambiguous: there were reports on lowered , unchanged  or increased  clutch sizes in D. magna at lower temperatures. In this study, the protein concentration in the extracts was quantified and the extracts were appropriately diluted to guarantee the application of identical amounts of protein (142 μg protein) per 2D gel. Compared to the extracts from 20°C acclimated animals, the extracts from 10°C acclimated animals had a 50% higher protein concentration. The slower growth and development of D. pulex in the cold may possibly result in a higher concentration of whole-body protein with the VTGs particularly contributing to this effect.
A striking feature of VTG1-VTG3 is the presence of an N-terminal superoxide dismutase (SOD)-like domain (Figure 2), which is related to the Cu/Zn SODs of prokaryotes . The catalytic activity of this class of SODs depends on Zn2+ and Cu2+ ions, which are coordinated by six histidine residues and one aspartic residue . These residues are still present in VTG3. VTG1 and VTG2 have lost all Zn-binding residues and one of the four histidine residues involved in Cu2+ binding. Functional studies on the purified yolk-protein complexes of D. magna revealed some residual SOD activity per constituent VTG chain (≈ 1%, in comparison to the activity of a bovine Cu/Zn SOD) . Because of the great number of VTG loci in the D. pulex genome and the presence of an apparently intact SOD-like domain in VTG3 (for which EST evidence is available), it is difficult to analyze any (residual) detoxifying capacity of VTG1 and VTG2. Future experimental studies will be necessary to evaluate the suggested implications of the SOD-like domains of the Daphnia VTGs in superoxide detoxification  and copper binding/transportation .
Cold-induced protein sets II: Cytoskeleton and muscle proteins
Actins were the second large set of proteins up-regulated in the cold (Table 1). Although actins were often co-identified with VTGs during the proteomic analysis, the identification of only actin in spot 51 indicates the manifold induction of these proteins. Actin is a highly conserved protein. As major building block of the cytoskeleton and the thin filaments of myofibrils, it is involved in many important cell functions including cell motility, muscle contraction and intracellular transport. Actin generally occurs in multiple isoforms which are expressed in a tissue- and development-specific manner . Compared to the genomes of human, mouse, and fly, which contain six actin loci , seven actin loci are present in the genome of D. pulex (Figure 3). Four of the predicted amino acid sequences (ACT1A, ACT1B, ACT1C, ACT1D) of D. pulex are related to cytoplasmic actin isoforms (5C, 42A) of Drosophila melanogaster [28, 30] The other three D. pulex sequences (ACT2A, ACT2B, ACT2C) are similar to the muscle-specific actin isoforms (57B, 79B, 87E, 88F) of Drosophila. The ACT2C gene is very likely a pseudogene as it lacks about 50% of the actin sequence information. Among the putative cytoplasmic actins of D.pulex, ACT1D possesses less conserved sequence characteristics. The complete lack of EST support for ACT1D and ACT2C suggests that only three cytoplasmic and two muscle-specific actin isoforms are expressed in D. pulex. Because of high sequence identity, a discrimination between these isoforms was not possible in the present study.
In general, a reduction of ambient temperature is immediately responded by a decrease of muscular performance in Daphnia. For example, the limb beating rate decreases which in turn reduces the uptake of oxygen and food. Likewise, the heart rate decreases with the consequence of a reduced hemolymph transport of substrates [3, 5, 8, 11]. However, the heart and limb beating rates were frequently not much different in Daphnia species at identical ambient and acclimation temperatures of 10–12°C or 18–20°C [3, 5]. In addition, maximum swimming activity of 10°C acclimated D. magna was found to be similar to that of 20°C acclimated animals . In poikilothermic animals, the concentration of enzymes involved in cellular metabolism frequently increases with decreasing acclimation temperatures to prevent a too strong depression of metabolic rate (metabolic cold adaptation) . Such a type of long-term compensatory control may apply as well to the cytoskeletal or muscular proteins to maintain a similar level of muscular performance at lower acclimation temperatures.
Cold-repressed protein sets: Proteolytic enzymes
In the cold, different classes of enzymes mainly involved in extracellular digestion were down-regulated. In other words, the capacity for the digestion of proteins increased with acclimation temperature (Table 1). The identification comprised serine peptidases of the chymotrypsin family S1, metallo peptidases of the astacin/adamalysin family M12, and the carboxypeptidase A family M14 (classification according to the MEROPS database) . A screening of the D. pulex genome database revealed more than 200 loci with coding sequences for serine-peptidase domains, 36 loci with astacin-like coding sequences, and 30 loci coding for carboxypeptidase-like domains. However, not all predicted gene products are involved in digestive processes. Serine proteases of the chymotrypsin family, for example, are involved in multiple physiological functions such as digestion, degradation, blood clotting, immunity, and development . Nevertheless, the sheer number of peptidase genes in the D. pulex genome indicates large-scaled gene family expansions that might reflect specific adaptations to the lifestyle of a planktonic filter feeder in a highly variable aquatic environment .
The MS analysis could identify and assign only those tryptic peptides which were specific for mature proteolytic enzymes. No support was obtained for the N-terminal signal peptides, which direct the nascent proteins to the secretory pathway, and for the pro-peptides, which shield the active sites in the immature trypsinogens or chymotrypsinogens (Figure 5 and 6). Therefore, it can be assumed that the proteases originated from the gut lumen, which (in D. magna) contain the major share of proteases . During the preparation of whole-animal extracts for the present study, intestinal proteins such as proteases are included along with those from other tissues. The presence of high amounts of proteases causes methodical problems , resulting in a contribution of proteolytic fragments to the observed protein spots. On the other hand, the high concentration of proteases being present in the whole-animal extracts documents a high digestive capacity for nutritional protein resources which increases with acclimation temperature. The marked induction of proteases between 10°C and 20°C acclimated animals probably reflects a higher rate of protein turnover at the higher temperature. Between identical ambient and acclimation temperatures of 10 and 20°C, the oxygen consumption rate of D. magna increased by 30%  and that of D. pulex by 60% (unpublished results). Accordingly, the observed induction of proteolytic capacity by a factor of 2–5 (Table 1: trypsin, chymotrypsin) may reflect at least in parts the temperature effect on metabolic rate in acclimated D. pulex. In addition, higher needs for proteins may arise at higher temperatures due to modifications in the allocation and/or requirement of nutrient resources (e.g. enlarged protein needs for growth and reproduction). Previous reports on the impact of temperature on clutch sizes in Daphnia were ambiguous; however, a reduction of vitellogenin and protein concentration was detected in this study between 10°C and 20°C acclimation (see Discussion above). At 20°C acclimation (in comparison to 10°C acclimation), the higher growth rate (and possibly a higher reproduction rate) of D. pulex and/or a faster passage of nutrients through the digestive tract with possibly incomplete nutrient digestion and reduced assimilation efficiency goes hand in hand with a reduced concentration of total protein and vitellogenin in the animals. These relationships at least indicate higher demands for proteins at 20°C acclimation, which may explain the induction of intestinal proteases.
Among the miscellaneous proteins with an unambiguous (one spot-one protein) identification were several carbohydrate-modifying enzymes, which were either down-regulated in the cold (cellubiohydrolase, enolase) or remained constitutively expressed (α-amylase, exo-β-1,3-glucanase), and an AAA+ ATPase, which was strongly up-regulated under cold conditions. AAA+ ATPases are molecular machines that are involved in a variety of cellular functions including vesicle transport, organelle assembly, membrane dynamics and protein unfolding . They contribute to the non-destructive recycling of proteins, play an important role in protein quality control (e.g. chaperone function), and can act as microtubule motor proteins or microtubule-severing enzymes .
Major sets of proteins (egg yolk proteins and precursors, cytoskeleton and muscle proteins, proteolytic enzymes) were differentially expressed in 10°C and 20°C acclimated D. pulex. Compared to 10°C, the acclimation to 20°C was associated with a decrease of vitellogenins, actins and even total protein concentration, as well as with an increase of proteases. The increase of proteolytic enzymes probably reflects a higher cellular demand for amino acids, which may result from higher growth and reproduction rates and/or from a lower efficiency of intestinal protein digestion/assimilation. The decrease of protein reserves (vitellogenins, actins or total protein) also indicates an increasing bottle-neck in the amino acid supply of cells. Conversely, the acclimation to cold conditions induced an increase in protein concentration which may be related to metabolic cold adaptation, a phenomenon for which multiple physiological support exists. Metabolic cold adaptation is a compensatory mechanism which ensures a relative constancy of metabolic rate and muscular performance. Particularly, the increase of actins in the cold maybe related to a compensatory control of muscular proteins to establish a relative constancy of muscular activity and performance.
Water fleas, Daphnia pulex, were raised in the laboratory as described previously . The animals were acclimated at least for three weeks (mostly months) to 10°C or 20°C at normoxic conditions (100% air saturation; oxygen partial pressure: 20 kPa), which was obtained by mild aeration using an aquarium pump. To guarantee an adequate nutrient supply at each acclimation temperature, animals were fed with green algae (Desmodesmus subspicatus) ad libitum (>1 mg C L-1) every second day. Only adult females were used for protein extraction.
Protein extraction, two-dimensional gel electrophoresis and statistical analysis of protein expression were carried out as described previously . Spots showing a sufficient size and staining intensity (relative spot volume, Vrel > 0.1%) and differential expression between 10°C or 20°C acclimation, were excised from representative gels and subjected to in-gel digestion using trypsin and mass spectrometric analysis (nano-HPLC-ESI-MS/MS) . Ratios of relative spot volumes at both temperatures were considered as induction factors. Several spots of high but constitutive expression were also included in the analysis.
Identification and characterization of proteins
Proteins were identified by correlating the ESI-MS/MS spectra with the "Frozen Gene Catalog" of the D. pulex v1.1 gene builds (July, 2007)  using the MOWSE-algorithm as implemented in the MS search engine MASCOT (Matrix Science Ltd., London, UK) . The "Frozen Gene Catalog" contains all manual curations as of July 3, 2007 as well as automatically annotated models chosen from the "Filtered Models" v1.1 set. "Filtered Models" is the filtered set of models representing the best gene model for each locus. The putative function of identified proteins was inferred by sequence homology either from the automated blastp search provided by Joint Genome Institute  or from a manual blastp search provided by NCBI. Derived protein sequences were checked for the presence of N-terminal signal sequences using the SignalP V3.0 server [45–47]. The theoretical molecular weight (Mr) and isolectric point (pI) of mature proteins (without N-terminal signal peptide) was calculated using the ExPASy Proteomics tool "Compute pI/MW" [48–50]. Characteristic domains of protein families were identified using the conserved domain database (CDD) and search engine v2.13 at NCBI [51, 52]. Putative N-glycosylation sites in vitellogenins were predicted using the NetNGlyc 1.0 Server .
Sequence alignments and phylogenetic analysis
Multiple-sequence alignments were performed using the T-Coffee algorithm [54–56]. Phylogenetic trees were constructed using the neighbor-joining algorithm  and a bootstrap analysis with 100 replicates.
- M r :
- V rel :
relative spot volume.
The authors would like to thank Simone König from the Integrated Functional Genomics (University of Münster), and Marco Matthes (at that time at the Technical University Dresden) for providing the animals used for the clonal culture. We are also grateful for support from DECODON GmbH (BioTechnikum Greifswald). The Proteom Centrum Tübingen is supported by the Ministerium für Wissenschaft und Kunst, Landesregierung Baden-Württemberg.
The sequencing and portions of the analyses were performed at the DOE Joint Genome Institute under the auspices of the U.S. Department of Energy's Office of Science, Biological and Environmental Research Program, and by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48, Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231, Los Alamos National Laboratory under Contract No. W-7405-ENG-36 and in collaboration with the Daphnia Genomics Consortium, DGC http://daphnia.cgb.indiana.edu. Additional analyses were performed by wFleaBase, developed at the Genome Informatics Lab of Indiana University with support to Don Gilbert from the National Science Foundation and the National Institutes of Health. Coordination infrastructure for the DGC is provided by The Center for Genomics and Bioinformatics at Indiana University, which is supported in part by the METACyt Initiative of Indiana University, funded in part through a major grant from the Lilly Endowment, Inc. Our work benefits from, and contributes to the Daphnia Genomics Consortium.
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