SEMG1 Human

What is SEMG1 and what are its primary functions in human reproduction?

SEMG1 (Semenogelin 1) is one of the main components of human seminal plasma and is classified as an autosomal cancer-testis antigen (CTA) . This multifunctional protein plays several critical roles in human reproduction:

• Regulates sperm motility through dose-dependent inhibition

• Enhances sperm survival in the female reproductive tract

• Inhibits premature sperm capacitation by suppressing reactive oxygen species generation

• Provides antibacterial protection to sperm

• Contributes to the activation of testicular hyaluronidase, facilitating gamete fusion

SEMG1 primarily functions by binding to the surface of spermatozoa, particularly through interaction with EPPIN (Epididymal Protease Inhibitor), which modulates sperm progressive motility and provides antimicrobial protection upon ejaculation .

How does SEMG1 affect sperm motility and survival?

SEMG1 exhibits a dual role in sperm function that appears contradictory but serves a clear biological purpose:

• Motility inhibition: SEMG1 causes a dose-dependent decrease in sperm motility. At high concentrations (e.g., 1 mM), it significantly decreases the percentage of rapid sperm after 3 hours of incubation . This temporary suppression of motility helps conserve sperm energy.

• Survival enhancement: Despite reducing motility, SEMG1 significantly increases sperm survival in the female reproductive tract. In mouse studies, the presence of SEMG1 during intrauterine insemination (IUI) significantly increased the survival rate of intrauterine sperm .

• Protection mechanism: SEMG1 protects sperm against spermicidal attacks in the uterus, similar to how mouse SVS2 (Seminal Vesicle Secretion 2, the ortholog of human SEMG1) functions .

This seemingly paradoxical effect—temporarily reducing motility while enhancing survival—represents an evolutionary strategy to ensure sperm can successfully navigate the female reproductive tract and reach the egg with sufficient energy reserves.

What is known about SEMG1 expression in male infertility conditions?

Research has identified altered SEMG1 expression in certain male infertility conditions:

• Asthenozoospermia (AZS): SEMG1 is significantly upregulated in men with asthenozoospermia compared to fertile controls (p=0.03) . This condition is characterized by reduced sperm motility, and the increased SEMG1 levels may contribute to this motility impairment.

• Correlation with other fertility factors: In the same study that found SEMG1 upregulation, CRISP2 (Cysteine-Rich Secretory Protein 2) was significantly downregulated in men with asthenozoospermia . This suggests a potential regulatory relationship between these proteins in sperm function.

These findings align with SEMG1's known role in reducing sperm motility and suggest that abnormal expression levels could contribute to fertility challenges in men.

What is the evolutionary significance of SEMG1 in primates?

SEMG1 exhibits fascinating evolutionary patterns that provide insights into reproductive adaptations:

• Rapid adaptive evolution: SEMG1 has undergone rapid adaptive evolutionary changes within the WFDC (Whey Acidic Protein Four-Disulfide Core) locus, suggesting strong selective pressures related to reproductive fitness .

• Species-specific variations: While SEMG1 is orthologous to mouse SVS2, there is significant sequence divergence between these proteins, with only partial homology in the 5' region containing a signal polypeptide and Semg domain .

• Positive selection sites: Bayes Empirical Bayes (BEB) analysis revealed eleven positively selected codons in SEMG1, with Cys239 showing particularly strong evidence of positive selection in humans . This residue is crucial for SEMG1 binding to EPPIN.

• Key residue variations across species:

  • Humans and most Hominoidea: Cys239

  • Gorilla gorilla: His239

  • Macaca mulatta and Papio anubis: Arg239

This evolutionary pattern suggests that the SEMG1-EPPIN interaction has been critical for reproductive success in primates, with selective pressures driving changes that optimize sperm protection and motility regulation.

How do SEMG1 and EPPIN co-evolve and interact at the molecular level?

The co-evolution of SEMG1 and EPPIN represents a fascinating example of molecular adaptation in reproductive proteins:

• Conservation of interface residues: Specific amino acid residues of SEMG1 (Trp167, His169, Ser193, and Gln215) and EPPIN (Lys35, Lys50, Asp51, Gln55, Lys81, Tyr89, Leu91, Met104, and Lys120) show remarkable conservation among hominoid primates .

• Human-specific adaptations: EPPIN shows two distinct changes in humans compared to other Hominoidea: residues His92 and Asp99 .

• Multiple interaction regions: SEMG1 appears to have multiple EPPIN interaction sites within repeats IIIa and IIIb of the molecule. The sequence G26-E164 (part of repeat IIIa) provides a critical docking surface for SEMG1-EPPIN interaction in addition to the Cys239 residue .

• Binding mechanism: The entire G26-R281 sequence in SEMG1 appears necessary for optimal SEMG1/EPPIN interaction, as this construct had the lowest EC50 and IC50 values among tested SEMG1 fragments .

This co-evolutionary pattern suggests that the SEMG1-EPPIN interaction has been fine-tuned through evolution to optimize reproductive fitness, with human-specific adaptations potentially contributing to our species' reproductive biology.

What role does the Cys239 residue play in SEMG1's function?

The Cys239 residue of human SEMG1 plays a pivotal role in its function:

• Critical for EPPIN binding: Blocking Cys239 either by reduction and carboxymethylation or by point mutation to glycine inhibits SEMG1's capacity to bind EPPIN and inhibit sperm motility .

• Impact of mutations: Experimental studies of SEMG1 mutants show that changing Cys239 to other residues significantly affects function:

• Evolutionary significance: Cys239 underwent positive selection in humans, likely because it increases the affinity of SEMG1 for EPPIN, allowing effective binding at lower concentrations .

This data demonstrates that the Cys239 residue represents a critical site of adaptive evolution in human reproduction, with direct implications for SEMG1's biological function.

How do SEMG1 fragments and truncations affect its binding to EPPIN and its effect on sperm motility?

Research on SEMG1 fragments has revealed important structure-function relationships:

• Fragment size effects: The full-length SEMG1 sequence (G26-R281, fragment SEMG1 214-42) shows optimal binding to EPPIN, while smaller fragments generally show reduced binding affinity .

• Physiologically relevant fragments: The smaller SEMG1 constructs tested correspond to fragments generated by PSA (Prostate-Specific Antigen) digestion upon ejaculation, making them endogenously present in liquefied semen .

• Binding behavior of specific fragments: Some fragments (e.g., SEMG1 32-8) could inhibit EPPIN-SEMG1 interaction in competition experiments but did not directly bind to EPPIN in concentration-response experiments, suggesting complex interaction mechanisms .

• Correlation with motility inhibition: The ability of SEMG1 fragments to inhibit sperm motility generally correlates with their EPPIN binding affinity .

These findings highlight the complex structure-function relationships in SEMG1 and suggest that different regions of the protein contribute to its various biological activities through both direct and indirect mechanisms.

How can SEMG1 be utilized in assisted reproductive technologies such as intrauterine insemination (IUI)?

Research suggests several potential applications of SEMG1 in assisted reproductive technologies:

• Sperm protection during IUI: Human SEMG1 has been shown to protect intrauterine sperm in mice, similar to the function of mouse SVS2. This protective effect could be leveraged to improve sperm survival during IUI procedures .

• Cross-species utility: Human SEMG1 acts as a protectant for mouse sperm beyond species boundaries, suggesting broad applicability as a sperm-protecting agent .

• Potential therapeutic formulations: SEMG1 or specific fragments containing key functional domains could be developed as sperm-protecting agents for clinical IUI procedures .

• Implications for human IUI success rates: Current IUI treatments are recognized as low-probability therapies. SEMG1-based approaches could provide a basis for developing more effective IUI treatments with improved success rates .

What mechanisms underlie SEMG1's ability to protect sperm in the female reproductive tract?

Several complementary mechanisms contribute to SEMG1's protective effect on sperm:

• Physical binding and coating: SEMG1 binds to the sperm surface through interaction with EPPIN and potentially other factors, forming a protective coating that shields sperm from hostile elements .

• Electrostatic interactions: Both human SEMG1 and mouse SVS2 are basic proteins whose positive charge allows them to interact with negatively charged components on the sperm surface, such as GM1 gangliosides .

• Antimicrobial activity: SEMG1 possesses direct antibacterial activity, providing protection against bacterial challenges in the female reproductive tract .

• Capacitation regulation: SEMG1 inhibits premature capacitation by suppressing reactive oxygen species generation. Once in the female reproductive tract, SEMG1 processing occurs, allowing capacitation to proceed at the appropriate time and location .

These multiple protective mechanisms work in concert to ensure sperm survival during the challenging journey through the female reproductive tract.

What techniques are used to study SEMG1-protein interactions?

Researchers employ several complementary techniques to study SEMG1 interactions:

• Binding assays:

  • Concentration-response experiments using recombinant proteins to determine EC50 values

  • Competition experiments to measure IC50 values for SEMG1 fragment inhibition of EPPIN-SEMG1 binding

• Protein engineering approaches:

  • Production of recombinant SEMG1 fragments to map interaction domains

  • Site-directed mutagenesis to investigate specific residues (e.g., Cys239)

• Evolutionary analysis methods:

  • Sequence analysis across species to identify conserved residues

  • Computational tools like CodeML in PAML4 to detect positive selection

  • Bayes Empirical Bayes analysis for codon-level selection analysis

• Functional assays:

  • Sperm motility measurements to assess functional impacts

  • In vivo studies using mouse models to evaluate effects on intrauterine sperm survival

These complementary approaches provide a comprehensive understanding of SEMG1's interactions and functions in reproductive biology.

What animal models are appropriate for studying SEMG1 function?

The mouse model has proven valuable for SEMG1 research despite important species differences:

• Mouse model advantages:

  • Allows in vivo experiments that would be difficult or impossible with human subjects

  • Mouse epididymal sperm serve as a substitute for human sperm due to ethical limitations

  • Intrauterine insemination in mice enables assessment of SEMG1's effect on sperm survival

• Cross-species considerations:

  • Human SEMG1 affects mouse sperm despite species differences, supporting the model's validity

  • The mouse ortholog of human SEMG1 is SVS2, which has similar functions but limited sequence homology

• Important physiological differences:

  • Fertilization timing: hours in mice versus approximately 80 hours in humans

  • Differences in reproductive tissues, environment, and sexual behavior

Despite these differences, researchers note that "there are likely conserved systems for sperm protection in mice and humans, and mouse sperm may be a good model for human sperm behavior in vivo" . This suggests that the mouse model, while imperfect, provides valuable insights into SEMG1 function.

How is SEMG1 expression quantified in clinical samples?

Based on research with men with asthenozoospermia, SEMG1 expression can be quantified using molecular biology techniques:

• RNA-based quantification protocol:

  • Total RNA extraction from sperm samples

  • cDNA synthesis from the extracted RNA

  • Relative quantification using real-time PCR

  • Normalization with B2M (β2-microglobulin) as reference gene

  • Calculation of fold change using the 2^(-ΔΔCT) method

• Statistical analysis:

  • Mann-Whitney test to compare expression levels between groups

  • Significance threshold typically set at p<0.05

This approach successfully detected significant upregulation of SEMG1 (p=0.03) in men with asthenozoospermia compared to fertile controls , demonstrating its utility for clinical research applications.