Temporal association of protamine 1 with the inner nuclear membrane protein lamin B receptor during spermiogenesis

HANP1/H1T2, a novel histone H1-like protein involved in nuclear formation and sperm fertility

We cloned a testis-specific cDNA from mice that encodes a histone H1-like, haploid germ cell-specific nuclear protein designated HANP1/H1T2. The HANP1/H1T2 protein was specifically localized to the nuclei of murine spermatids during differentiation steps 5 to 13 but not to the nuclei of mature sperm. HANP1/H1T2 contains an arginine-serine-rich domain and an ATP/GTP binding site, and it binds to DNA, ATP, and protamine. To investigate the physiological role of HANP1/H1T2, we generated Hanp1/H1T2-disrupted mutant mice. Homozygous Hanp1/H1T2 mutant males were infertile, but females were fertile. Although a substantial number of sperm were recovered from the epididymides, their shape and function were abnormal. During sperm morphogenesis, the formation of nuclei was disturbed and protamine-1 and -2 were only weakly detectable in the nuclei. The chromatin packaging was aberrant, as demonstrated by electron microscopy and biochemical analysis. The mutant sperm exhibited deficient motility and were not competent to fertilize eggs under in vitro fertilization conditions; however, they were capable of fertilizing eggs via intracytoplasmic sperm injection that resulted in the birth of healthy progeny. Thus, we found that HANP1/H1T2 is essential for nuclear formation in functional spermatozoa and is specifically involved in the replacement of histones with protamines during spermiogenesis. At the time of submission of the manuscript, we found an independent publication by Martianov et al. (I. Martianov, S. Brancorsini, R. Catena, A. Gansmuller, N. Kotaja, M. Parvinen, P. Sassone-Corsi, and I. Davidson, Proc. Natl. Acad. Sci. USA 102:2808-2813, 2005) that reported similar results.

Nuclear envelope remodeling during mouse spermiogenesis: postmeiotic expression and redistribution of germline lamin B3

Lamins are members of a multigene family of structural nuclear envelope (NE) proteins. Differentiated mammalian somatic cells express lamins A, C, B1, and B2. The composition and organization of the nuclear lamina of mammalian spermatogenic cells differ significantly from that of somatic cells as they express lamin B1 as well as two short germ line-specific isoforms, namely lamins B3 and C2. Here we describe in detail the expression pattern and localization of lamin B3 during mouse spermatogenesis. By combining RT-PCR, immunoblotting, and immunofluorescence microscopy, we show that lamin B3 is selectively expressed during spermiogenesis (i.e., postmeiotic stages of spermatogenesis). In round spermatids, lamin B3 is distributed in the nuclear periphery and, notably, also in the nucleoplasm. In the course of spermiogenesis, lamin B3 becomes redistributed as it concentrates progressively to the posterior pole of spermatid nuclei. Our results show that during mammalian spermiogenesis the nuclear lamina is composed of B-type isoforms only, namely the ubiquitous lamin B1 and the germline-specific lamin B3. Lamin B3 is the first example of a mammalian lamin that is selectively expressed during postmeiotic stages of spermatogenesis.

Dynamic properties of germ line-specific lamin B3: the role of the shortened rod domain

The mammalian lamin B2 gene codes for two proteins, the somatic lamin B2 and the germ line-specific lamin B3. Lamin B3 lacks the N-terminus and a part of the alpha-helical rod domain present in lamin B2. These domains are substituted by 84 amino acids unique for lamin B3. When ectopically expressed in somatic cells, lamin B3 causes severe deformation of nuclei which adopt a hook-like configuration. Accordingly, it was proposed that lamin B3 provides the germ line cells with a more flexible nuclear periphery that facilitates spermatogenesis-specific nuclear reorganization events. Here we investigated which protein domains of lamin B3 are responsible for nuclear deformation in transfected cells, and how stable is the nuclear periphery of these cells. Expression of wild-type and mutant lamins evidenced that nuclear deformations are due to the shortened rod domain of lamin B3. Cell fractionation experiments revealed that lamin B3 can be solubilized more easily than lamin B2. Fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) analyses of transfected cells showed that lamin B3 has an increased mobility compared to B2. Our results lead to the conclusion that lamin B3 reduces the stability of the nuclear periphery. They are also consistent with the notion that lamin B3 is relevant to specific properties of the nuclear envelope during spermiogenesis.

Cellular p32 recruits cytomegalovirus kinase pUL97 to redistribute the nuclear lamina

Replication of human cytomegalovirus is limited at the level of nucleocytoplasmic transport of viral capsids, a process that requires the disassembly of the nuclear lamina. Deletion of the protein kinase gene UL97 from the viral genome showed that the activity of pUL97 plays an important role for viral capsid egress. Here, we report that p32, a novel cellular interactor of the viral kinase pUL97, promotes the accumulation of pUL97 at the nuclear membrane by recruiting the p32-pUL97 complex to the lamin B receptor. Transfection of active pUL97, but not a catalytically inactive mutant, induced a redistribution of lamina components as demonstrated for recombinant lamin B receptor-green fluorescent protein and endogenous lamins A and C. Consistent with this, p32 itself and lamins were phosphorylated by pUL97. Importantly, overexpression of p32 in human cytomegalovirus-infected cells resulted in increased efficiency of viral replication and release of viral particles. Thus, it is highly suggestive that the cellular protein p32 recruits pUL97 to induce a dissolution of the nuclear lamina thereby facilitating the nuclear export of viral capsids.

Non-random chromosome positioning in mammalian sperm nuclei, with migration of the sex chromosomes during late spermatogenesis

Chromosomes are highly organized and compartmentalized in cell nuclei. The analysis of their position is a powerful way to monitor genome organization in different cell types and states. Evidence suggests that the organization of the genome could be functionally important for influencing different cellular and developmental processes, particularly at early stages of development (i.e. fertilization and the consequent entry of the sperm nucleus into the egg). The position of chromosomes in the sperm nucleus might be crucial, because their location could determine the time at which particular chromatin domains are decondensed and remodelled, allowing some epigenetic level of control or influence over subsequent paternal gene expression in the embryo. Here, we analyse genome organization by chromosome position in mammalian sperm nuclei from three breeds of pig, as a model species. We have mapped the preferential position of all chromosomes (bar one) in sperm nuclei in two dimensions and have established that the sex chromosomes are the most internally localized chromosomes in mature sperm. The distribution of two autosomes and chromosomes X and Y in sperm heads was compared in primary and secondary spermatocytes and spermatids in porcine testes. The sex chromosomes were found at the nuclear edge in primary spermatocytes, which correlates with the known position of the XY body and their position in somatic cells, whereas, in spermatids, the sex chromosomes were much more centrally located, mirroring the position of these chromosomes in ejaculated spermatozoa. This study reveals the temporal repositioning of chromosome territories in spermatogenesis.

Transcription in haploid male germ cells

Major modifications in chromatin organization occur in spermatid nuclei, resulting in a high degree of DNA packaging within the spermatozoon head. However, before arrest of transcription during midspermiogenesis, high levels of mRNA are found in round spermatids. Some transcripts are the product of genes expressed ubiquitously, whereas some are generated from male germ cell-specific gene homologs of somatic cell genes. Others are transcript variants derived from genes with expression regulated in a testis-specific fashion. The haploid genome of spermatids also initiates the transcription of testis-specific genes. Various general transcription factors, distinct promoter elements, and specific transcription factors are involved in transcriptional regulation. After meiosis, spermatids are genetically but not phenotypically different, because of transcript and protein sharing through cytoplasmic bridges connecting spermatids of the same generation. Interestingly, different types of mRNAs accumulate in the sperm cell nucleus, raising the question of their origin and of a possible role after fertilization.