GH Rainbow Trout
Description
Genetic and Molecular Basis of GH in Rainbow Trout
Rainbow trout possess a unique GH gene structure resulting from a salmonid-specific genome duplication event 25–100 million years ago . Key findings include:
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Gene Structure: The trout GH gene spans ~4 kilobases and contains six exons, unlike mammalian GH genes (five exons) . This additional intron disrupts translated regions analogous to the last exon in mammals.
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Receptor Diversity: Two GH receptors (GHR1 and GHR2) exist due to tetraploidization. GHR1 expression increases at higher temperatures (12°C vs. 4°C) during embryonic development, while GHR2 remains unaffected by temperature .
Growth and Muscle Development
GH directly stimulates skeletal muscle growth by upregulating:
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Proliferation genes (myf5)
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Differentiation genes (myog)
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GH receptor 1 (ghr1) in muscle cells .
Fasting reduces expression of pcna, igf1, and igf2 but increases proteolysis genes (mstn1b, fbxo32) .
Metabolic Regulation
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Fasting Response: Most rainbow trout develop GH resistance after 4 weeks of fasting, characterized by reduced hepatic ghr2a expression and altered plasma GH-binding protein (GHBP) levels .
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Energy Reserves: Fish with high muscle adiposity resist GH resistance during fasting, suggesting GH’s role depends on energy availability .
Temperature Effects
| Parameter | GHR1 Expression (12°C vs. 4°C) | GHR2 Expression |
|---|---|---|
| Embryonic stages | ↑ (up to hatching) | No change |
| Juvenile liver | ↓ GH-binding capacity at 16°C | - |
| Data from |
Photoperiod Influence
Extended photoperiods partially restore GH-IGF axis gene expression (gh, igf1, ghr1) in submerged trout, mitigating growth suppression caused by prolonged submergence .
Behavioral and Ecological Implications
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Predation Risk: GH-treated trout resume feeding faster after simulated predator attacks, increasing exposure to aerial predators .
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Aquaculture Concerns: Elevated GH levels may alter behavioral patterns, posing ecological risks if transgenic fish interact with wild populations .
Plasma GHBP Dynamics
| Condition | Plasma GHBP Levels (ng/ml) | Hepatic ghr2a Expression |
|---|---|---|
| Fasting (4 weeks) | 5–25 (high variability) | ↓ |
| Refeeding (72 hours) | Fluctuates with prior diet | ↔ |
| Data from |
GHBP levels inversely correlate with free GH availability, complicating interpretations of GH bioavailability .
Product Specs
AIENQRLFNIAVSRVQHLHLLAQKMFNDFDGTLLPDERRQLNKIFLLDFCNSDSIVSPVD
KHETQKSSVLKLLHISFRLIESWEYPSQTLIISNSLMVRNANQISEKLSDLKVGINLLIT
GSQDGVLSLDDNDSQQLPPYGNYYQNLGGDGNVRRNYELLACFKKDMHKVETYLTVAKCR
KSLEANCTL.
Q&A
What is the structure of the growth hormone gene in rainbow trout?
The rainbow trout growth hormone gene spans approximately 4 kilobases, nearly twice the size of mammalian GH genes. Unlike mammalian GH genes which contain five exons, the rainbow trout GH gene comprises six exons. The additional intron in the fish gene interrupts translated regions that are analogous to the last exon of its mammalian counterpart. Furthermore, the rainbow trout GH gene lacks the direct repeats that flank exons I, III, and V in mammalian GH genes, indicating significant structural differences between fish and mammalian GH genes .
How does rainbow trout GH structure differ from mammalian GH?
The rainbow trout GH gene structure exhibits several key differences from mammalian counterparts. Most notably, the internally repeating sequence commonly observed in mammalian growth hormone, prolactin, or placental lactogen is not detected in the predicted polypeptide sequence of fish GH. This finding suggests that the rainbow trout GH gene structure does not support the current hypothesis that internally repeated regions in GH, prolactin, and placental lactogen arose from a small primordial gene, indicating distinct evolutionary pathways .
What are Growth Hormone Binding Proteins (GHBPs) in rainbow trout?
Growth Hormone Binding Proteins in rainbow trout plasma were first identified by Sohm et al. using GH binding and cross-linking assays along with immunoprecipitation. Research indicates that fish GHBPs stem from the extracellular domain of the membrane-bound GH receptor, similar to non-rodent mammals. Plasma GHBP levels in rainbow trout typically range between 5-25 ng/ml, with significant fluctuations observed during both long-term fasting and short-term refeeding periods. These proteins have dual functions: prolonging the biological half-life of GH while simultaneously decreasing GH availability to target tissues .
How does energy status affect GH resistance in rainbow trout?
The development of GH resistance in rainbow trout is significantly influenced by energy reserves, particularly muscle adiposity. Experimental evidence demonstrates that while most rainbow trout acquire GH resistance within 4 weeks of fasting, fish selectively bred for high muscle adiposity do not develop this resistance. This phenomenon strongly suggests that GH resistance does not manifest while fat reserves remain available for energy metabolism. The mechanism appears to be permissive for protein catabolism only after lipid energy stores are substantially depleted .
What is the relationship between hepatic GH receptor expression and plasma GHBP levels?
Research examining hepatic growth hormone receptor 2a (ghr2a) gene expression and plasma GHBP levels in rainbow trout has revealed a lack of correlation between these parameters. This finding indicates that ghr2a assessment cannot be reliably used as a proxy measure for plasma GHBP levels, despite the fact that circulating GHBPs are derived from the GH receptor molecule. This unexpected dissociation suggests complex regulatory mechanisms governing the production and release of GHBPs independent of hepatic receptor expression .
How do GH-related parameters compare between rainbow trout with different adiposity levels?
Rainbow trout selectively bred for high muscle adiposity (FL line) exhibit distinct GH-related endocrinology compared to lean line (LL) fish. During fasting experiments, FL fish maintain relatively elevated GHBP levels concomitant with low plasma GH levels, suggesting an altered GH-endocrinology profile similar to "obesity" phenotypes observed in humans with low GH and high GHBP plasma levels. These differences indicate that genetic selection for high muscle adiposity has fundamentally altered the GH regulatory system, potentially affecting energy metabolism and growth regulation .
What techniques are available for measuring GHBPs in rainbow trout?
The first immunoassay specifically developed for non-mammalian vertebrate GHBPs was established and validated for rainbow trout and Atlantic salmon. Prior to this development, GHBPs in rainbow trout plasma were detected using GH binding and cross-linking assays combined with immunoprecipitation techniques. Researchers working with GHBPs in rainbow trout should consider the following methodological approaches:
| Methodology | Applications | Limitations | Sensitivity Range |
|---|---|---|---|
| Immunoassay | Quantitative measurement of plasma GHBP levels | Requires species-specific antibodies | 0.5-50 ng/ml |
| GH binding and cross-linking | Detection of binding activity | Semi-quantitative only | Variable |
| GHR-transfected CHO cells | Demonstrating GHBP origin | In vitro system may not reflect in vivo conditions | N/A |
| Immunoprecipitation | Confirmation of GHBP presence | Limited quantification capability | Qualitative |
These methodologies have provided valuable insights into GHBP dynamics, including fluctuations during fasting and refeeding periods, suggesting nutritional regulatory mechanisms .
How should researchers design experiments to study GH resistance in rainbow trout?
When investigating GH resistance in rainbow trout, experimental design should account for several key factors. Based on previous research protocols, the following experimental design elements are recommended:
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Establish clear baseline measurements prior to intervention
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Implement controlled fasting periods of 1, 2, and 4 weeks to observe the progression of GH resistance
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Consider genetic background (e.g., high vs. low muscle adiposity) as an experimental variable
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Measure multiple parameters simultaneously, including:
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Plasma GH levels
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Plasma IGF-I levels
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Plasma GHBP levels
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Hepatic GH receptor gene expression (ghr2a)
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Statistical analysis should incorporate two-way ANOVA to assess main effects (e.g., genetic strain and time) as well as interactions between these factors, with appropriate post-hoc testing when main effects are significant .
How should researchers interpret conflicting GH and IGF-I data in rainbow trout studies?
Interpreting growth hormone and IGF-I data in rainbow trout can be challenging due to complex regulatory mechanisms and potential experimental variables. When analyzing potentially conflicting data, researchers should:
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Consider the nutritional and energetic status of experimental subjects
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Examine the relationship between hepatic ghr2a expression and circulating GH levels
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Account for plasma GHBP fluctuations that may affect GH bioavailability
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Evaluate the possibility of tissue-specific GH resistance mechanisms
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Analyze the correlation (or lack thereof) between GH, IGF-I, and GHBP levels
It's important to recognize that the dual opposing functions of GHBPs—prolonging GH half-life while decreasing its availability to target tissues—can complicate data interpretation. Researchers should therefore consider the integrated function of the entire GH-IGF-I axis rather than isolated parameters .
What are the implications of rainbow trout GH gene structure for evolutionary biology?
The structural differences between rainbow trout and mammalian GH genes provide valuable insights into the evolutionary history of this important hormone. The absence of internally repeating sequences in rainbow trout GH, which are present in mammalian growth hormone, prolactin, and placental lactogen, challenges the current hypothesis that these internally repeated regions arose from a small primordial gene. This finding suggests different evolutionary pathways for GH genes in fish and mammals, potentially indicating:
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Independent evolution of gene duplication mechanisms
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Divergent selective pressures on hormone structure and function
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Fundamental differences in the regulation of growth and metabolism
These evolutionary insights have important implications for using rainbow trout as a model organism in comparative endocrinology and for understanding the ancestral state of vertebrate growth regulation systems .
What are promising areas for future GH research in rainbow trout?
Based on current knowledge gaps identified in the literature, several high-priority research directions emerge:
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Elucidation of the molecular mechanisms linking energy reserves and GH resistance
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Development of more sensitive and specific assays for measuring tissue-specific GH signaling
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Investigation of the regulatory mechanisms controlling GHBP production and release
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Comparative genomics approaches to further understand the evolutionary divergence of fish and mammalian GH systems
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Examination of environmental factors (temperature, photoperiod, water quality) on GH signaling efficiency
Research in these areas would significantly advance our understanding of rainbow trout growth hormone physiology and potentially inform broader questions in vertebrate endocrinology .
Product Science Overview
Growth hormone (GH) is a critical regulator of growth and metabolism in vertebrates, including fish. In aquaculture, the use of recombinant growth hormone has been explored to enhance growth rates and improve production efficiency. Rainbow trout (Oncorhynchus mykiss) is a species of significant economic importance in aquaculture, and the development of recombinant growth hormone for this species has been a focus of research.
Growth hormone in rainbow trout is produced by the pituitary gland and plays a vital role in regulating growth, development, and metabolism. The hormone exerts its effects by binding to specific receptors on target tissues, initiating a cascade of signaling pathways that promote growth and protein synthesis. The primary pathways involved include the JAK-STAT, PI3K-Akt, and MAPK pathways .
Recombinant growth hormone is produced using genetic engineering techniques. The gene encoding the growth hormone is inserted into a suitable expression system, such as bacteria or yeast, which then produces the hormone in large quantities. This recombinant hormone is purified and can be used to enhance growth in aquaculture species.
The use of recombinant growth hormone in rainbow trout has shown promising results in terms of increased growth rates and improved feed efficiency. Studies have demonstrated that fish treated with recombinant growth hormone exhibit significantly higher growth rates compared to untreated controls . This can lead to shorter production cycles and increased yields, making aquaculture operations more efficient and profitable.
The mechanisms by which recombinant growth hormone enhances growth in rainbow trout involve several physiological processes. The hormone stimulates the production of insulin-like growth factor 1 (IGF-1) in the liver, which in turn promotes cell proliferation and protein synthesis in various tissues. Additionally, recombinant growth hormone enhances nutrient uptake and utilization, leading to improved growth performance .
While the use of recombinant growth hormone in aquaculture holds great potential, there are several challenges and considerations to address. These include ensuring the safety and efficacy of the hormone, understanding its long-term effects on fish health and the environment, and addressing regulatory and consumer acceptance issues. Ongoing research is focused on optimizing the use of recombinant growth hormone to maximize its benefits while minimizing potential risks .
