How do transcriptionally inert spermatids differentiate into spermatozoa?Spermatids are spherical cells with centrally located nuclei. How are they transformed into functional spermatozoa? The morphological events during spermiogenesis are discussed in your textbook, and you should read them carefully. We shall focus here on nuclear modifications and their consequences.
The major events in nuclear modification are: chromatin condensation and morphological changes.
Chromatin condensation helps to streamline the cell by reducing volume. It also may serve a protective function, reducing the susceptibility of the DNA to mutation or physical damage.
Condensation is facilitated by formation of specific DNA-protein complexes. Proteins that may be involved include protamines (small, highly basic, arginine-rich proteins), histone-like proteins or other sperm-specific proteins.
Replacement may be gradual, with somatic histones being replaced by sperm-specific proteins, or it may involve discrete steps in which transitional proteins interact with the DNA after the histones are removed and before the protamines are added.
In salmonid fishes, somatic histones become hyperacetylated or undergo similar modifications that reduce the binding between the histones and DNA.
In mammals, somatic histones are removed by protease digestion. Remodeling of the chromatin in mammals is a two-step process. In the first step, the histones are replaced by small, highly basic transition proteins. This eliminates nucleosomes and stops transcription. The replacement of the transition proteins with protamines stabilizes and further compacts the chromatin via the formation of disulfide cross-bridges.
The replacement of histones by sperm-specific proteins results in a termination of transcription. This has important consequences for gene expression during spermiogenesis. It means that all events after this must rely upon post-transcriptional processes. In Drosophila, this occurs in the primary spermatocyte stage. Thus, no transcription occurs in secondary spermatocytes or spermatids. In mammals and birds, some transcription has been detected after meiosis. It is a generalized phenomenon that spermatids lack transcription.
How, then, are proteins synthesized that are needed to complete spermiogenesis? For example, protamine synthesis occurs after transcription has been terminated by replacement of somatic histones with transitional proteins. The protamine is synthesized on transcripts that were themselves synthesized before loss of somatic histones. In fact, these transcripts are stored in the cytoplasm of developing mouse spermatids for 7 days before they are translated. How are they maintained in a repressed state and subsequently de-repressed and translated?
The 3' untranslated region (UTRs) of mouse protamine transcripts appears to determine when they are translated (reviewed by Hecht, 1995). The role of the 3' UTR was demonstrated by fusing the 3' UTR of mouse protamine 1 mRNA to the human growth hormone gene. Mice made transgenic for this transgene transcribed it before histone replacement but did not translate it until late in spermiogenesis when the endogenous protamine messengers were translated. Conversely, replacement of the protamine 3' UTR in the protamine 1 gene with that of human growth hormone allowed translation to commence immediately after transcription (Braun et al., 1989). (At this point, you should review the technique for making transgenic mice (see this resource also as well as Browder et al., p. A-19)
A specific 18 kDa protein binds to a sequence element located in the 3' UTR of the protamine 2 transcripts. This sequence element is also present in the 3' UTR of transition protein 1 and protamine 1. The binding of the RNA-binding protein to the conserved sequences represses translation in vitro. Translational regulation mediated by 3' UTR sequences is a common mechanism of translational control. We shall discuss other examples during the course. It will be essential to learn how the repression is subsequently relieved to facilitate the translation of these repressed transcripts. The RNA-binding protein is a phosphoprotein. Dephosphorylation of the protein prevents its binding to RNA. Hence, one possibility is that a change in its phosphorylation status relieves the inhibitory effect.
Another mechanism that regulates translation during mammalian spermiogenesis may function by controlling the level of polyadenylation of RNA. Several of these transcripts are stored with poly(A) tails of about 160 nucleotides and become partially deadenylated to about 30 nucleotides when they are translated. Polyadenylation is a commonly used strategy in translational regulation during development.
ReferencesBraun, R.E. et al. 1989. Protamine 3'-untranslated sequences regulate temporal translational control and subcellular localization of growth hormone in spermatids of transgenic mice. Genes & Dev. 3: 793-802.
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental Biology. Third edition. Saunders College Pub. Philadelphia.
Hecht, N.B. 1995. The making of a spermatozoon: a molecular perspective. Dev. Genetics 16: 95-103.