News | Sperm-Packaging Proteins May Be Key to Male Infertility
Sperm play a vital role in creating life by providing half of the genetic material needed by offspring. Yet how sperm develop the form needed for normal reproductive function remains a difficult biological question.
A University of Michigan team recently examined the molecular mechanisms of sperm formation and how abnormalities may lead to male-factor infertility. Unlike other human cells, sperm package their genetic material with proteins called protamines. This raises a question: Why do sperm use protamines to package DNA rather than the histones used by all other cell types?
Evolution and function of protamines
Protamines occur in organisms including plants, fish, and mammals and have evolved over hundreds of millions of years. By studying their molecular sequences, University of Michigan researchers revealed their important role in sperm formation.
Protamines efficiently package DNA into dense chromatin largely because they are rich in arginine, a positively charged amino acid that binds strongly to negatively charged DNA. The latest research found that protamines also contain species-specific non-arginine amino acids that undergo unexpected post-translational modifications—chemical changes after protein synthesis.
The team believes these modifications are essential to protamine function, especially sperm chromatin packaging. The researchers said, "These DNA-binding molecules are generally positively charged, but we now see that these proteins contain another logic we had not considered, with other amino acids also playing important roles."
Significance of post-translational modifications
Although sperm are not transcriptionally active, modifications on protamines suggest an important role in chromatin packaging. In mouse models, researchers examined a lysine residue specific to mouse protamine. Replacing lysine with alanine, which cannot undergo the modification, caused abnormal sperm morphology, impaired embryonic development, and sharply reduced fertility. Surprisingly, replacing lysine with positively charged arginine also failed to restore normal packaging, showing that the process depends on more than molecular charge.
Implications for male infertility
Male-factor infertility often has no identifiable cause, highlighting the importance of studying these modifications. University of Michigan assistant professor Saher Sue Hammoud said, "We began focusing on protamines because they are present in many animal species yet evolve rapidly, meaning their sequences contain extensive variation." These variants may be potential causes of male infertility.
Hammoud and the team hope to study sperm packaging more deeply and possibly reconstruct the process in vitro, opening new avenues for infertility diagnosis and treatment.
Potential clinical applications
An important finding is that protamine modifications may play a role in early embryonic development, offer a new route for infertility diagnosis, and potentially provide a method for screening early embryos after in vitro fertilization (IVF). University of Michigan assistant professor Samantha Schon said, "These modifications give us a new research direction and may become an important diagnostic tool for infertility, particularly during IVF."
News | Sperm-Packaging Proteins May Be Key to Male Infertility
News | Sperm-Packaging Proteins May Be Key to Male Infertility
Sperm play a vital role in creating life by providing half of the genetic material needed by offspring. Yet how sperm develop the form needed for normal reproductive function remains a difficult biological question.
A University of Michigan team recently examined the molecular mechanisms of sperm formation and how abnormalities may lead to male-factor infertility. Unlike other human cells, sperm package their genetic material with proteins called protamines. This raises a question: Why do sperm use protamines to package DNA rather than the histones used by all other cell types?
Evolution and function of protamines
Protamines occur in organisms including plants, fish, and mammals and have evolved over hundreds of millions of years. By studying their molecular sequences, University of Michigan researchers revealed their important role in sperm formation.
Protamines efficiently package DNA into dense chromatin largely because they are rich in arginine, a positively charged amino acid that binds strongly to negatively charged DNA. The latest research found that protamines also contain species-specific non-arginine amino acids that undergo unexpected post-translational modifications—chemical changes after protein synthesis.
The team believes these modifications are essential to protamine function, especially sperm chromatin packaging. The researchers said, "These DNA-binding molecules are generally positively charged, but we now see that these proteins contain another logic we had not considered, with other amino acids also playing important roles."
Significance of post-translational modifications
Although sperm are not transcriptionally active, modifications on protamines suggest an important role in chromatin packaging. In mouse models, researchers examined a lysine residue specific to mouse protamine. Replacing lysine with alanine, which cannot undergo the modification, caused abnormal sperm morphology, impaired embryonic development, and sharply reduced fertility. Surprisingly, replacing lysine with positively charged arginine also failed to restore normal packaging, showing that the process depends on more than molecular charge.
Implications for male infertility
Male-factor infertility often has no identifiable cause, highlighting the importance of studying these modifications. University of Michigan assistant professor Saher Sue Hammoud said, "We began focusing on protamines because they are present in many animal species yet evolve rapidly, meaning their sequences contain extensive variation." These variants may be potential causes of male infertility.
Hammoud and the team hope to study sperm packaging more deeply and possibly reconstruct the process in vitro, opening new avenues for infertility diagnosis and treatment.
Potential clinical applications
An important finding is that protamine modifications may play a role in early embryonic development, offer a new route for infertility diagnosis, and potentially provide a method for screening early embryos after in vitro fertilization (IVF). University of Michigan assistant professor Samantha Schon said, "These modifications give us a new research direction and may become an important diagnostic tool for infertility, particularly during IVF."
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