Recombinant Pig Growth hormone receptor (GHR)

Basic Research Questions

• What is the structure and function of porcine Growth Hormone Receptor (GHR)?

The porcine Growth Hormone Receptor is a 638 amino acid homodimeric receptor belonging to the type I cytokine receptor family. It consists of one cytokine receptor homology domain (CRH), a single-pass transmembrane domain, and a cytoplasmic intracellular domain (ICD) . Unlike other members of the class I cytokine receptor family, GHR does not contain a WSXWS motif in the extracellular domain .

Functionally, GHR serves as the receptor for pituitary gland growth hormone (GH1) and is primarily involved in regulating postnatal body growth. Upon binding of GH, the receptor couples to the JAK2/STAT5 pathway . Recent studies have shown that GHR dimers exist on the cell surface before GH binding, and these dimers form in the endoplasmic reticulum . The receptor's extracellular region is not required for maintaining these dimers, although the lower fibronectin type III (FNIII) domain may define receptor-dimer specificity .

• What methodologies are used to produce and characterize recombinant pig GHR protein?

Recombinant pig GHR protein can be produced through multiple expression systems:

a) Bacterial Expression Systems: Recombinant pig GHR fragment protein (typically amino acids 19-264) can be expressed in Escherichia coli with a His tag for purification. This approach yields protein with >85% purity suitable for applications such as SDS-PAGE and binding studies .

b) Mammalian Expression Systems: For functional studies, full-length GHR can be expressed in mammalian cell lines using appropriate expression vectors. This maintains proper post-translational modifications and protein folding .

Characterization methodologies typically include:

• SDS-PAGE and Western Blotting: To confirm protein expression and assess purity

• Spectroscopy: Including circular dichroism to analyze secondary structure

• Binding Assays: To measure interaction with growth hormone using techniques such as surface plasmon resonance

• Functional Assays: To assess signaling pathway activation, particularly JAK2/STAT5 phosphorylation

• What are the key signaling pathways activated by porcine GHR and how do they compare to other species?

The primary signaling pathway activated by porcine GHR is the JAK-STAT pathway. Upon GH binding, JAK2 is activated, which then phosphorylates STAT1, STAT3, and STAT5 transcription factors . Additionally, GHR activates the Src family kinase signaling pathway independent of JAK2 .

A comparison of JAK/STAT activation across different receptors shows:

This signaling pattern is conserved across species, although downstream effects may vary. In pigs, GHR signaling appears to have particularly important effects on liver metabolism, as evidenced by the hepatic steatosis observed in GHR-KO pigs .

Intermediate Research Questions

• How are GHR knockout pig models generated using CRISPR/Cas9, and what are the critical considerations for experimental design?

GHR knockout pig models are typically generated using CRISPR/Cas9 gene editing technology with several critical methodological considerations:

Methodology:

• Guide RNA (gRNA) Design: Multiple gRNAs targeting exon 3 of the GHR gene are designed and tested for efficiency. In one study, five gRNAs were optimized to target the porcine GHR gene .

• Delivery Methods:

  • Electroporation-mediated introduction of CRISPR/Cas9 components into porcine zygotes

  • Transfection of porcine fetal fibroblasts (PFFs) followed by somatic cell nuclear transfer (SCNT)

• Zygote Source Considerations: For increased genetic diversity, some studies use hybrid approaches, such as zygotes derived from domestic porcine oocytes and microminipig spermatozoa .

• Mutation Verification: After embryo transfer to recipient gilts, resulting piglets are screened for mutations using PCR and sequencing. Biallelic mutations can be confirmed at the protein level using Western blotting and immunohistochemistry .

Critical Experimental Considerations:

• Off-target Effects: Careful gRNA design is essential to minimize off-target mutations

• Mosaicism: Founder animals may be mosaic, requiring breeding to establish stable lines

• Breeding Strategy: To avoid inbreeding, establishing different transfected cell lines from fetal tissues is recommended

• Genotyping Protocols: PCR primers (e.g., 5'-AAGCGGTGTCTATGTGCTGATTCTC-3' and 5'-TCAGTGGCTAGAGTATATGATGTTG-3') can be used to distinguish wild-type (530 bp) and targeted alleles (534 bp)

• What are the phenotypic and molecular characteristics of GHR knockout pigs, and how do they compare to wild-type controls?

GHR knockout pigs exhibit distinctive phenotypic and molecular characteristics:

Growth Parameters:

• Normal birth weight but significant growth retardation beginning around 5 weeks of age

• By 6 months of age, GHR-KO pigs show approximately 60-61% reduced body weight compared to controls

• Body length is also significantly reduced

Organ Development:

• Most organ weights are reduced proportionally to body weight

• Liver, kidneys, and heart are disproportionately reduced

• Relative brain weight is almost doubled

• Mean minimal diameter of cardiomyocytes is reduced by 28%

Molecular and Biochemical Changes:

• Markedly reduced serum IGF1 levels (24 ± 1 ng/mL in GHR-KO vs. 228 ± 24 ng/mL in controls)

• Reduced IGF-binding protein 3 (IGFBP3) activity, but increased IGFBP2 levels

• Significantly elevated serum GH concentrations

• Decreased triglycerides (TGs), total cholesterol (TC), high-density lipoprotein (HDL), and low-density lipoprotein (LDL)

• Elevated serum free fatty acids

• Increased phosphorylation of AKT, suggesting enhanced insulin sensitivity

Histological Features:

• Hepatic steatosis with increased numbers and sizes of intracellular vacuoles

• Oil red O staining confirms increased lipid deposition in liver

• Elevated alanine aminotransferase (ALT) and aspartate aminotransferase (AST), indicating liver damage

• How does GHR deficiency affect metabolic pathway regulation in porcine models?

GHR deficiency leads to significant alterations in metabolic pathway regulation in porcine models:

Altered Signaling Pathways:

• RNAseq analysis reveals that genes related to fatty acid oxidation pathways are significantly altered in GHR-KO pigs

• The transcription factor AHR, which is related to lipid metabolism, is significantly downregulated

• Insulin signaling is affected, with increased phosphorylation of AKT, a key component of the PI3K-AKT pathway

Lipid Metabolism:

• GHR-KO pigs exhibit hepatic steatosis, with increased lipid deposition in the liver

• Serum lipid profile shows decreases in triglycerides, total cholesterol, HDL, and LDL

• Elevated free fatty acid levels suggest impaired fatty acid metabolism

Glucose Metabolism:

• GHR-KO pigs display glucose intolerance

• Increased AKT phosphorylation suggests enhanced insulin sensitivity, which could explain the apparent paradox of glucose intolerance with insulin sensitivity

• The PI3K-AKT pathway, which normally increases glycogen synthesis and inhibits gluconeogenesis upon activation, appears to be altered

Research Methodologies to Study These Effects:

• ChIP Assay: To study transcription factor binding, such as AHR binding to target genes

• Co-Immunoprecipitation (Co-IP): To investigate protein-protein interactions, such as between GHR and AHR

• Western Blotting: To assess protein expression and phosphorylation levels of signaling molecules

• Histological Analysis: H&E staining and Oil Red O staining to assess lipid accumulation in tissues

Advanced Research Questions

• What are the methodological approaches for using transgenic pigs to produce recombinant human growth hormone, and what factors influence expression levels?

The production of recombinant human growth hormone (rhGH) using transgenic pigs involves several methodological steps:

Vector Construction and Transgenic Animal Generation:

• Vector Design: A vector containing the human GH gene under a milk-specific promoter is constructed. For example, the pPBC-hGH-W vector was created by cloning the hGH gene from human cDNA and ligating it to the Woodchuck Hepatitis Virus Posttranscriptional Regulatory Element (WPRE) .

• Cell Transfection: The linearized construct is randomly inserted into porcine fetal fibroblast (PFF) cells, with insertion confirmed by PCR .

• Somatic Cell Nuclear Transfer: Transfected cells serve as donors for SCNT to generate transgenic piglets, with reported cleavage rates of 70.6% and blastocyst rates of 20.3% .

• Germline Transmission: Transgenic founders (F0) are bred to establish stable transgenic lines .

Recombinant Protein Expression and Collection:

• Milk Collection: Milk is collected from lactating transgenic sows, with expression levels varying significantly between individual animals and even within the same animal depending on lactation stage .

• Expression Assessment: rhGH expression is typically assessed using Western blotting and ELISA, with expression levels ranging from 7.8 μg/mL to 1.7 mg/mL .

Factors Influencing Expression Levels:

• Cell Line Variation: Different established cell lines result in varying expression levels despite using identical vectors .

• Integration Site and Copy Number: Random insertion leads to variability in integration sites and copy numbers .

• Lactation Stage: rhGH concentration varies depending on the lactation day, with often higher levels in milk after colostrum .

• Animal Nutrition and Biorhythm: The nutrition and biorhythm of each pig during lactation may influence expression levels .

Enhancement Strategies:

• Crossbreeding transgenic pigs with high rhGH yield and pigs with high milk production capacity

• Selective breeding of pigs with highest expression levels

• Further research on correlations between transgene copy number, integration site, and recombinant protein secretion

• What purification strategies are effective for isolating recombinant proteins from transgenic pig milk, and how can purity and biological activity be assessed?

Purifying recombinant proteins from transgenic pig milk presents unique challenges due to complex milk composition:

Milk Pretreatment:

• Fat Removal: Centrifugation to separate and remove milk fat

• Casein Precipitation: pH adjustment and/or calcium addition to precipitate caseins

• Protein Fractionation: Initial separation of proteins based on size or charge

Chromatography Purification:
A multi-step chromatographic approach is typically required. For rhGH purification from transgenic pig milk, one successful protocol included:

• Three pre-treatments (details not specified in the research)

• Five sequential column chromatography steps

This approach achieved ≥99% purity for rhGH from approximately 15.5 L of transgenic pig milk .

Purity Assessment Methods:

• Coomassie Brilliant Blue (CBB) Staining: Though noted as insufficient for detecting rhGH expression in pig milk samples due to the presence of various lipids and proteins

• Western Blotting: More sensitive for detecting specific proteins in complex mixtures

• SDS-PAGE: For analyzing protein purity

• HPLC: For quantitative purity assessment

Biological Activity Assessment:
For rhGH purified from transgenic pig milk, several approaches were used to confirm bioactivity :

• Spectroscopy and Structural Analysis: Comparing the purified protein with commercially available somatropin (Genotropin)

• In Vivo Growth Promotion Assays: Using rat models to assess body weight increase and bone development

• Toxicity Assessments: 4-week continuous administration followed by 2-week recovery period to determine safety profile and no-observed-adverse-effect level (established as 0.6 mg/kg/day)

These assessments confirmed that rhGH purified from transgenic pig milk (CGH942) showed no toxicological differences compared to commercial somatropin and effectively promoted growth .

• How do GHR-knockout pig models compare with other animal models in studying Laron syndrome, and what unique insights have they provided?

GHR-knockout pig models offer several advantages and unique insights compared to other animal models of Laron syndrome:

Comparative Advantages Over Rodent Models:

• Metabolic Phenotype Similarity: Unlike GHR-KO mice, GHR-KO pigs exhibit high levels of free fatty acids and hepatic steatosis, which better recapitulates the abnormal lipid metabolism seen in Laron syndrome patients .

• Physiological Relevance: Pigs have greater similarity to humans in terms of physiology, size, and lifespan compared to rodents, making them more relevant for studying human disorders .

• Detailed Tissue Sampling: The larger size of pigs allows for more comprehensive tissue sampling and longitudinal studies .

Unique Physiological Insights:

• Hepatic Steatosis Mechanism: Studies in GHR-KO pigs have revealed a potential mechanism for fatty liver development involving downregulation of the AHR transcription factor and altered fatty acid oxidation pathways .

• Insulin Sensitivity Paradox: GHR-KO pigs display the seemingly contradictory phenotype of glucose intolerance with increased insulin sensitivity (evidenced by increased AKT phosphorylation), providing insights into the complex relationship between GH signaling and glucose metabolism .

• Differential Organ Growth: The observation that certain organs (liver, kidneys, heart) are disproportionately reduced in size while others (brain) are relatively enlarged provides insights into tissue-specific GH dependence during development .

Immune System Impacts:
Study found significant differences in CD4+CD8α− lymphocyte subpopulations and IFN-α serum levels between wild-type controls and GHR-KO pigs. Proteome analysis of CD4+ and CD4− lymphocyte populations revealed multiple significant protein abundance differences involving pathways related to amino acid metabolism, beta-oxidation of fatty acids, insulin secretion signaling, and oxidative phosphorylation .

• What are the applications and advantages of using GHR-knockout pigs in xenotransplantation research, and what modifications can enhance their utility?

GHR-knockout pigs offer significant advantages in xenotransplantation research, addressing several key challenges:

Advantages for Xenotransplantation:

• Reduced Organ Size: At 6 months of age, GHR-KO pigs show a 61% reduced body weight and a 63% reduced heart weight compared to controls , making their organs more compatible with human recipients, particularly for pediatric applications.

• Cellular Changes: The mean minimal diameter of cardiomyocytes is reduced by 28% , potentially affecting immune responses and organ function post-transplantation.

• Reduced Growth Potential: GHR-KO donor pigs can be used at an age beyond the steepest phase of their growth curve, potentially reducing the problem of xeno-organ overgrowth in recipients .

• Normal Organ Development: Despite their smaller size, proteome study of myocardium samples from GHR-KO pigs did not reveal prominent differences compared to controls , suggesting normal organ development and function.

Methodological Approaches:

• Multi-gene Modification: GHR-KO mutations can be introduced using CRISPR/Cas9 in pigs that already carry other genetic modifications beneficial for xenotransplantation. One study successfully created quadruple-modified (4GM) pigs by introducing GHR-KO in an α1,3-galactosyltransferase (GGTA1)-deficient background that also expressed human cluster of differentiation (hCD46) and human thrombomodulin (hTHBD) .

• Breeding Considerations: GHR-KO sows show normal sexual development and can be mated with genetically multi-modified boars to produce offspring with the expected Mendelian transmission of genetic modifications and consistent expression of transgenes .

• Age Selection: Careful selection of donor age is important. GHR-KO pigs reach a weight of approximately 70-73 kg at 9 months, while control pigs reach this weight range three months earlier .

Monitoring Parameters:

• IGF1 Levels: Serum insulin-like growth factor 1 (IGF1) levels serve as a biomarker of GHR deficiency, with GHR-KO pigs showing dramatically reduced levels (24 ± 1 ng/mL compared to controls at 228 ± 24 ng/mL) .

• Proteome Analysis: Holistic proteome studies of target organs can confirm normal development despite reduced size .