Disruption of spermatogenesis in mice lacking A-type lamins

Proteomic analysis of fipronil-induced molecular defects in spermatozoa

The phenylpyrazole insecticide fipronil has wide-ranging applications from agriculture to public health to control undesirable organisms. However, several studies have reported the residual environmental hazards of fipronil and demonstrated its harmful effects even in mammalian reproduction. Therefore, this study was conducted to demonstrate the mode of action of fipronil on mouse spermatozoa. We treated fipronil to spermatozoa and performed comprehensive function evaluations. Moreover, proteomic analyses were conducted to identify the alteration of protein expression levels in spermatozoa. Most of sperm motility and kinematic parameters and intracellular ATP levels were diminished, and the spontaneous acrosome reaction was promoted after treatment with fipronil. Proteomic analyses revealed altered expression levels of 14 proteins after treatment. These proteins have been reported to be associated with sperm-specific pathways, prominently the cytoskeleton of the sperm, "9 + 2" axoneme composition, metabolism, and fertility. Collectively, our results showed that fipronil alters sperm functional-related proteins and therefore influences male fertility. This study elucidates the possible reproductive toxic hazards associated with male infertility through aberrant suppression of sperm proteins.

Exposure to the bromodomain inhibitor N-methyl pyrrolidone blocks spermatogenesis in a hormonal and non-hormonal fashion

N-methyl pyrrolidone (NMP) is an FDA approved molecule used as an excipient in pharmaceutical industry. Besides having a central role in formulation of drugs, the most important function of any excipient is to guarantee the safety of the medicine during and after its administration. Several studies have shown that exposure to NMP and especially in rats produce a gonadotoxic effect leading to infertility. However, the mechanisms underlying the effect of NMP on male reproduction are unknown. The aim of this study was to assess the reproductive toxicity of NMP in male rats and to elucidate the underlying mechanism. Male Sprague Dawley rats were injected intraperitoneally, twice/ week, at a dose of 108 mg/ 100 g of body weight with NMP. Analysis of reproductive parameters revealed testicular atrophy in NMP treated animals compared to control animals. Germ cell composition within the seminiferous tubules was disturbed and manifested in an increase in number of cells with fragmented DNA. A subsequent decrease in number of spermatocytes and spermatids was observed. Alpha screen assay shows that NMP acts at the concentrations we applied in vivo as a low affinity inhibitor for BRDT (testis specific bromodomain protein). BRDT inhibition is mirrored by a significant decrease in the expression of early stage spermatocyte markers (lmna, aurkc and ccna1), during which BRDT expression predominates. A significant decrease in testosterone levels was also observed. Since NMP interferes with spermatogenesis on various levels, its use in humans must be carefully monitored.

Novel targets identified by integrated proteomic and phosphoproteomic analysis in spermatogenesis of swamp buffalo (Bubalus bubalis)

To understand mechanisms of spermatogenesis, the proteome and the phosphoproteome in prepubertal and pubertal swamp buffalo (Bubalus bubalis) testes were analyzed using tandem mass tag (TMT) coupled with liquid chromatography-tandem mass spectrometry (LC–MS/MS). In prepubertal testes, 80 proteins were overexpressed, 148 proteins were underexpressed, and 139 and 142 protein sites had higher and lower phosphorylation, respectively, compared to the levels in pubertal testes. Several of these proteins were associated with reproductive processes such as sexual reproduction, spermatogenesis, fertilization, and spermatid development. In particular, outer dense fiber protein 1 (ODF1), protein maelstrom homolog (MAEL), actin-like protein 7B (ACTL7B), tyrosine-(Y)-phosphorylation regulated (CABYR), and tripartite motif containing 36 (TRIM36) were upregulated with age at both the proteome and phosphoproteome levels. Combining proteome and phosphoproteome analysis can be effectively applied to study the protein/phosphorylation patterns of buffalo testes. These data provide new regulatory candidates and evidence for a complex network in spermatogenesis in buffalo testes, and serve as an important resource for exploring the physiological mechanism of spermatogenesis in mammals.

Cellular and Animal Models of Striated Muscle Laminopathies

The lamin A/C (LMNA) gene codes for nuclear intermediate filaments constitutive of the nuclear lamina. LMNA has 12 exons and alternative splicing of exon 10 results in two major isoforms—lamins A and C. Mutations found throughout the LMNA gene cause a group of diseases collectively known as laminopathies, of which the type, diversity, penetrance and severity of phenotypes can vary from one individual to the other, even between individuals carrying the same mutation. The majority of the laminopathies affect cardiac and/or skeletal muscles. The underlying molecular mechanisms contributing to such tissue-specific phenotypes caused by mutations in a ubiquitously expressed gene are not yet well elucidated. This review will explore the different phenotypes observed in established models of striated muscle laminopathies and their respective contributions to advancing our understanding of cardiac and skeletal muscle-related laminopathies. Potential future directions for developing effective treatments for patients with lamin A/C mutation-associated cardiac and/or skeletal muscle conditions will be discussed.

Generation and characterization of a novel knockin minipig model of Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is an extremely rare genetic disorder for which no cure exists. The disease is characterized by premature aging and inevitable death in adolescence due to cardiovascular complications. Most HGPS patients carry a heterozygous de novo LMNA c.1824C > T mutation, which provokes the expression of a dominant-negative mutant protein called progerin. Therapies proven effective in HGPS-like mouse models have yielded only modest benefit in HGPS clinical trials. To overcome the gap between HGPS mouse models and patients, we have generated by CRISPR-Cas9 gene editing the first large animal model for HGPS, a knockin heterozygous LMNA c.1824C > T Yucatan minipig. Like HGPS patients, HGPS minipigs endogenously co-express progerin and normal lamin A/C, and exhibit severe growth retardation, lipodystrophy, skin and bone alterations, cardiovascular disease, and die around puberty. Remarkably, the HGPS minipigs recapitulate critical cardiovascular alterations seen in patients, such as left ventricular diastolic dysfunction, altered cardiac electrical activity, and loss of vascular smooth muscle cells. Our analysis also revealed reduced myocardial perfusion due to microvascular damage and myocardial interstitial fibrosis, previously undescribed readouts potentially useful for monitoring disease progression in patients. The HGPS minipigs provide an appropriate preclinical model in which to test human-size interventional devices and optimize candidate therapies before advancing to clinical trials, thus accelerating the development of effective applications for HGPS patients.

Nuclear envelope dynamics during mammalian spermatogenesis: new insights on male fertility

The production of highly specialized spermatozoa from undifferentiated spermatogonia is a strictly organized and programmed process requiring extensive restructuring of the entire cell. One of the most remarkable cellular transformations accompanying the various phases of spermatogenesis is the profound remodelling of the nuclear architecture, in which the nuclear envelope (NE) seems to be crucially involved. In recent years, several proteins from the distinct layers forming the NE (i.e. the inner and outer nuclear membranes as well as the nuclear lamina) have been associated with meiosis and/or spermiogenesis in different mammalian species. Among these are A- and B-type lamins, Dpy-19-like protein 2 (DPY19L2), lamin B receptor (LBR), lamina-associated polypeptide 1 (LAP1), LAP2/emerin/MAN1 (LEM) domain-containing proteins, spermatogenesis-associated 46 (SPATA46) and diverse elements of the linker of nucleoskeleton and cytoskeleton (LINC) complex, namely Sad-1/UNC-84 homology (SUN) and Klarsicht/ANC-1/Syne-1 homology (KASH) domain-containing proteins. Herein, we summarize the current state of the art on the cellular and subcellular distribution of NE proteins expressed during mammalian spermatogenesis, and discuss the latest research developments regarding their testis-specific functions. This review provides a comprehensive and innovative overview of the NE network as a regulatory platform and as an essential determinant of efficient meiotic chromosome recombination as well as spermiogenesis-associated nuclear remodelling and differentiation in mammalian male germline cells. Thus, this review provides important novel insights on the biological relevance of NE proteins for male fertility.

The involvement of the nuclear lamina in human and rodent spermiogenesis: a systematic review

The nuclear lamina (NL) is a filamentous protein meshwork, composed essentially of lamins, situated between the inner nuclear membrane and the chromatin. The NL is a component of the nuclear envelope, interacts with a wide range of proteins and is required for normal nuclear structure and physiological development. During spermiogenesis the spermatid nucleus is elongated, and dramatically reduced in size with protamines replacing histones to produce a highly compacted chromatin. There is mounting evidence from studies in human and rodent, that the NL plays an important role in mammalian spermatid differentiation during spermiogenesis. In this review, we summarize and discuss the data available in the literature regarding the involvement of lamins and their direct or indirect partners in normal and abnormal human spermiogenesis.

LEM-domain proteins are lost during human spermiogenesis but BAF and BAF-L persist

During spermiogenesis the spermatid nucleus is elongated, and dramatically reduced in size with protamines replacing histones to produce a highly compacted chromatin. After fertilisation, this process is reversed in the oocyte to form the male pronucleus. Emerging evidence, including the coordinated loss of the nuclear lamina (NL) and the histones, supports the involvement of the NL in spermatid nuclear remodelling, but how the NL links to the chromatin is not known. In somatic cells, interactions between the NL and the chromatin have been demonstrated: LEM-domain proteins and LBR interact with the NL and respectively, the chromatin proteins BAF and HP1. We therefore sought to characterise the lamina-chromatin interface during spermiogenesis, by investigating the localisation of six LEM-domain proteins, two BAF proteins and LBR, in human spermatids and spermatozoa. Using RT-PCR, IF and western blotting, we show that six of the proteins tested are present in spermatids: LEMD1, LEMD2 (a short isoform), ANKLE2, LAP2β, BAF and BAF-L, and three absent: Emerin, LBR and LEMD3. The full-length LEMD2 isoform, required for nuclear integrity in somatic cells, is absent. In spermatids, no protein localised to the nuclear periphery, but five were nucleoplasmic, receding towards the posterior nuclear pole as spermatids matured. Our study therefore establishes that the lamina-chromatin interface in human spermatids is radically distinct from that defined in somatic cells. In ejaculated spermatozoa, we detected only BAF and BAF-L, suggesting that they might contribute to the shaping of the spermatozoon nucleus and, after fertilisation, its transition to the male pronucleus.

Nuclear envelope remodelling during human spermiogenesis involves somatic B-type lamins and a spermatid-specific B3 lamin isoform

The nuclear lamina (NL) is a filamentous protein meshwork, composed essentially of lamins, situated between the inner nuclear membrane and the chromatin. There is mounting evidence that the NL plays a role in spermatid differentiation during spermiogenesis. The mouse spermatid NL is composed of the ubiquitous lamin B1 and the spermatid-specific lamin B3, an N-terminally truncated isoform of lamin B2. However, nothing is known about the NL in human spermatids. We therefore investigated the expression pattern and localization of A-type lamins (A, C and C2) and B-type lamins (B1, B2 and B3) during human spermiogenesis. Here, we show that a lamin B3 transcript is present in human spermatids and that B-type lamins are the only lamins detectable in human spermatids. We determine that, as shown for their mouse counterparts, human lamin B3, but not lamin B2, induces strong nuclear deformation, when ectopically expressed in HeLa cells. Co-immunofluorescence revealed that, in human spermatids, B-type lamins are present at the nuclear periphery, except in the region covered by the acrosome, and that as the spermatid matures the B-type lamins recede towards the posterior pole. Only lamin B1 remains detectable on 33-47% of ejaculated spermatozoa. On spermatozoa selected for normal head density, however, this fell to <6%, suggesting that loss of the NL signal may be linked to complete sperm nucleus compaction. The similarities revealed between lamin expression during human and rodent spermiogenesis, strengthen evidence that the NL and lamin B3 have conserved functions during the intense remodelling of the mammalian spermatid nucleus.

Lamin A/C proteins in the spermatid acroplaxome are essential in mouse spermiogenesis

Spermiogenesis is a complex process of terminal differentiation that is necessary to produce mature sperm. Using protein expression profiles of mouse and human testes generated from our previous studies, we chose to examine the actions of lamin A/C in the current investigation. Lamin A and lamin C are isoforms of the A-type lamins that are encoded by theLMNAgene. Our results showed that lamin A/C was expressed in the mouse testis throughout the different stages of spermatogenesis and in mature sperm. Lamin A/C was also expressed in mouse haploid germ cells and was found to be localized to the acroplaxome in spermiogenesis, from round spermatids until mature spermatozoa. The decreased expression of lamin A/C following injections of siRNA againstLmnacaused a significant increase in caudal sperm head abnormalities when compared with negative controls. These abnormalities were characterized by increased fragmentation of the acrosome and abnormal vesicles, which failed to fuse to the developing acrosome. This fragmentation also caused significant alterations in nuclear elongation and acrosome formation. Furthermore, we found that lamin A/C interacted with the microtubule plus-end-tracking protein CLIP170. These results suggest that lamin A/C is critical for proper structural and functional development of the sperm acrosome and head shape.

Mammalian E-type cyclins control chromosome pairing, telomere stability and CDK2 localization in male meiosis

Loss of function of cyclin E1 or E2, important regulators of the mitotic cell cycle, yields viable mice, but E2-deficient males display reduced fertility. To elucidate the role of E-type cyclins during spermatogenesis, we characterized their expression patterns and produced additional deletions of Ccne1 and Ccne2 alleles in the germline, revealing unexpected meiotic functions. While Ccne2 mRNA and protein are abundantly expressed in spermatocytes, Ccne1 mRNA is present but its protein is detected only at low levels. However, abundant levels of cyclin E1 protein are detected in spermatocytes deficient in cyclin E2 protein. Additional depletion of E-type cyclins in the germline resulted in increasingly enhanced spermatogenic abnormalities and corresponding decreased fertility and loss of germ cells by apoptosis. Profound meiotic defects were observed in spermatocytes, including abnormal pairing and synapsis of homologous chromosomes, heterologous chromosome associations, unrepaired double-strand DNA breaks, disruptions in telomeric structure and defects in cyclin-dependent-kinase 2 localization. These results highlight a new role for E-type cyclins as important regulators of male meiosis.

Understanding the control of meiosis is fundamental to deciphering the origin of male infertility. Although the mechanisms controlling meiosis are poorly understood, key regulators of mitosis, such as cyclins, appear to be critical. In this regard, male mice deficient for cyclin E2 exhibit subfertility and defects in spermatogenesis; however, neither the stages of germ cell differentiation affected nor the responsible mechanisms are known. We investigated how E-type cyclins control male meiosis by examining their expression in spermatogenesis and the consequences that multiple deletions of Ccne1 and Ccne2 alleles produce. Loss of Ccne2 expression increases cyclin E1 levels as a compensatory effect, but there are still meiotic defects and subfertility. Further, loss of one Ccne1 allele in the absence of cyclin E2 results in infertility as does loss of the remaining Ccne1 allele, but with even more severe meiotic abnormalities. We further found that cyclin E1 is involved in sex chromosome synapsis while E2 is involved with homologous pairing and chromosome and telomere integrity. These processes and structures were severely disrupted in absence of both cyclin E1 and E2, uncovering new roles for the E-type cyclins in regulating male meiosis.

Genomic networks of hybrid sterility

Hybrid dysfunction, a common feature of reproductive barriers between species, is often caused by negative epistasis between loci ("Dobzhansky-Muller incompatibilities"). The nature and complexity of hybrid incompatibilities remain poorly understood because identifying interacting loci that affect complex phenotypes is difficult. With subspecies in the early stages of speciation, an array of genetic tools, and detailed knowledge of reproductive biology, house mice (Mus musculus) provide a model system for dissecting hybrid incompatibilities. Male hybrids between M. musculus subspecies often show reduced fertility. Previous studies identified loci and several X chromosome-autosome interactions that contribute to sterility. To characterize the genetic basis of hybrid sterility in detail, we used a systems genetics approach, integrating mapping of gene expression traits with sterility phenotypes and QTL. We measured genome-wide testis expression in 305 male F2s from a cross between wild-derived inbred strains of M. musculus musculus and M. m. domesticus. We identified several thousand cis- and trans-acting QTL contributing to expression variation (eQTL). Many trans eQTL cluster into eleven 'hotspots,' seven of which co-localize with QTL for sterility phenotypes identified in the cross. The number and clustering of trans eQTL-but not cis eQTL-were substantially lower when mapping was restricted to a 'fertile' subset of mice, providing evidence that trans eQTL hotspots are related to sterility. Functional annotation of transcripts with eQTL provides insights into the biological processes disrupted by sterility loci and guides prioritization of candidate genes. Using a conditional mapping approach, we identified eQTL dependent on interactions between loci, revealing a complex system of epistasis. Our results illuminate established patterns, including the role of the X chromosome in hybrid sterility. The integrated mapping approach we employed is applicable in a broad range of organisms and we advocate for widespread adoption of a network-centered approach in speciation genetics.

The role of the nuclear lamina in cancer and apoptosis

Not long after the discovery of lamin proteins, it became clear that not all lamin subtypes are ubiquitously expressed in cells and tissues. Especially, A-type lamins showed an inverse correlation with proliferation and were thus initially called statins. Here we compare the findings of both A- and B-type lamin expression in various normal tissues and their neoplastic counterparts. Based on immunocytochemistry it becomes clear that lamin expression patterns are much more complicated than initially assumed: while normally proliferative cells are devoid of A-type lamin expression, many neoplastic tissues do show prominent A-type lamin expression. Conversely, cells that do not proliferate can be devoid of lamin expression. Yet, within the different types of tissues and tumors, lamins can be used to distinguish between tumor subtypes. The link between the appearance of A-type lamins in differentiation and the appearance of A-type lamins in a tumor likely relates the proliferative capacity of the tumor to its differentiation state.While lamins are targets for degradation in the apoptotic process, and accordingly are often used as markers for apoptosis, intriguing studies on an active role of lamins in the initiation or the prevention of apoptosis have been published recently and give rise to a renewed interest in the role of lamins in cancer.

Pushing the (nuclear) envelope into meiosis

A recent study shows that a short isoform of a mammalian nuclear lamin is important for homologous chromosome interactions during meiotic prophase in mice.

The meiotic nuclear lamina regulates chromosome dynamics and promotes efficient homologous recombination in the mouse

The nuclear lamina is the structural scaffold of the nuclear envelope and is well known for its central role in nuclear organization and maintaining nuclear stability and shape. In the past, a number of severe human disorders have been identified to be associated with mutations in lamins. Extensive research on this topic has provided novel important clues about nuclear lamina function. These studies have contributed to the knowledge that the lamina constitutes a complex multifunctional platform combining both structural and regulatory functions. Here, we report that, in addition to the previously demonstrated significance for somatic cell differentiation and maintenance, the nuclear lamina is also an essential determinant for germ cell development. Both male and female mice lacking the short meiosis-specific A-type lamin C2 have a severely defective meiosis, which at least in the male results in infertility. Detailed analysis revealed that lamin C2 is required for telomere-driven dynamic repositioning of meiotic chromosomes. Loss of lamin C2 affects precise synapsis of the homologs and interferes with meiotic double-strand break repair. Taken together, our data explain how the nuclear lamina contributes to meiotic chromosome behaviour and accurate genome haploidization on a mechanistic level.

LINCing the nuclear envelope to gametogenesis

Gametogenesis combines two important features: reduction of the genome content from diploid to haploid by carefully partitioning chromosomes, and the subsequent differentiation into fertilization-competent gametes, which in males is characterized by profound nuclear restructuring. These are quite difficult tasks and require a tight coordination of different cellular mechanisms. Recent studies in the field established a key role for LINC complexes in both meiosis and sperm head formation. LINC complexes comprise SUN and KASH domain proteins that form nuclear envelope (NE) bridges, linking the nucleoskeleton to the cytoskeleton. They are well known for their crucial roles in diverse cellular and developmental processes, such as nuclear positioning and cell polarization. In this review, we highlight key roles ascribed to LINC complexes and to the nucleocytoskeletal connection in gametogenesis. First, we give a short overview about the general features of LINC components and the profound reorganization of the NE in germ cells. We then focus on specific roles of LINC complexes in meiotic chromosome dynamics and their impact on pairing, synapsis, and recombination. Finally, we provide an update of the mechanisms controlling sperm head formation and discuss the role of sperm-specific LINC complexes in nuclear shaping and their relation to specialized cytoskeletal structures that form concurrently with nuclear restructuring and sperm elongation.

Lamine C2 et spermatogenèse

Les lamines A/C sont des filaments intermédiaires présents dans les noyaux des cellules. Leurs rôles sont multiples et des mutations du gène LMNA sont à l’origine de nombreuses maladies appelées laminopathies. Dans les cellules germinales masculines, cette famille de protéines n’est représentée que par la lamine C2. Les données obtenues chez la souris démontrent l’importance de ces filaments dans le déroulement de la méiose masculine et présagent de l’existence d’un nouveau domaine d’infertilité d’origine masculine lié à des mutations de ce filament intermédiaire ou de ses protéines associées.

A truncated lamin A in the Lmna -/- mouse line: implications for the understanding of laminopathies

During recent years a number of severe clinical syndromes, collectively termed laminopathies, turned out to be caused by various, distinct mutations in the human LMNA gene. Arising from this, remarkable progress has been made to unravel the molecular pathophysiology underlying these disorders. A great benefit in this context was the generation of an A-type lamin deficient mouse line (Lmna^−/−) by Sullivan and others,¹ which has become one of the most frequently used models in the field and provided profound insights to many different aspects of A-type lamin function. Here, we report the unexpected finding that these mice express a truncated Lmna gene product on both transcriptional and protein level. Combining different approaches including mass spectrometry, we precisely define this product as a C-terminally truncated lamin A mutant that lacks domains important for protein interactions and post-translational processing. Based on our findings we discuss implications for the interpretation of previous studies using Lmna^−/− mice and the concept of human laminopathies.

Accumulation of the inner nuclear envelope protein Sun1 is pathogenic in progeric and dystrophic laminopathies

Human LMNA gene mutations result in laminopathies that include Emery-Dreifuss muscular dystrophy (AD-EDMD) and Hutchinson-Gilford progeria, the premature aging syndrome (HGPS). The Lmna null (Lmna(-/-)) and progeroid LmnaΔ9 mutant mice are models for AD-EDMD and HGPS, respectively. Both animals develop severe tissue pathologies with abbreviated life spans. Like HGPS cells, Lmna(-/-) and LmnaΔ9 fibroblasts have typically misshapen nuclei. Unexpectedly, Lmna(-/-) or LmnaΔ9 mice that are also deficient for the inner nuclear membrane protein Sun1 show markedly reduced tissue pathologies and enhanced longevity. Concordantly, reduction of SUN1 overaccumulation in LMNA mutant fibroblasts and in cells derived from HGPS patients corrected nuclear defects and cellular senescence. Collectively, these findings implicate Sun1 protein accumulation as a common pathogenic event in Lmna(-/-), LmnaΔ9, and HGPS disorders.

Reproductive aging: insights from model organisms

Aging was once thought to be the result of a general deterioration of tissues as opposed to their being under regulatory control. However, investigations in a number of model organisms have illustrated that aspects of aging are controlled by genetic mechanisms and are potentially manipulable, suggesting the possibility of treatment for age-related disorders. Reproductive decline is one aspect of aging. In model organisms and humans of both sexes, increasing age is associated with both a decline in the number of progeny and an increased incidence of defects. The cellular mechanisms of reproductive aging are not well understood, although a number of factors, both intrinsic and extrinsic to an organism's germline, may contribute to aging phenotypes. Recent work in a variety of organisms suggests that nuclear organization and nuclear envelope proteins may play a role in these processes.

Sperm transcriptome profiling in oligozoospermia

PURPOSE

Investigate in what extent sperm transcriptome of infertile men is different from that of fertile individuals.

METHODS

Semen samples were collected for determination of sperm parameters as well as for RNA isolation. Gene expression profile was investigated in spermatozoa of 8 infertile and 3 fertile men by microarray analysis using the Affymetrix Chip HG-U133 Plus 2.0.

RESULT(S)

We observed up to 33-fold reduction expression of genes involved in spermatogenesis and sperm motility. Furthermore, there is an important decrease in expression of genes involved in DNA repair as well as oxidative stress regulation. In this study, we also show a striking drop in expression of histone modification genes.

CONCLUSION(S)

We found that transcription profile in germ cells of men with idiopathic infertility is different from that of fertile individuals. Interestingly, about 15% of the regulated genes (Eddy Rev Reprod 4:23-30, 1999) play a role in spermatogenesis.

Laminopathies: the molecular background of the disease and the prospects for its treatment

Laminopathies are rare human degenerative disorders with a wide spectrum of clinical phenotypes, associated with defects in the main protein components of the nuclear envelope, mostly in the lamins. They include systemic disorders and tissue-restricted diseases. Scientists have been trying to explain the pathogenesis of laminopathies and find an efficient method for treatment for many years. In this review, we discuss the current state of knowledge about laminopathies, the molecular mechanisms behind the development of particular phenotypes, and the prospects for stem cell and/or gene therapy treatments.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Review: Lamin A/C, caspase-6, and chromatin configuration during meiosis resumption in the mouse oocyte

After in vitro maturation (IVM), isolation of the healthiest oocytes is essential for successful in vitro fertilization. As germinal vesicle (GV) oocytes resume meiosis through healthy or apoptotic pathways without discernable morphological criteria, we checked for an apoptotic element acting at the nucleus level. We hypothesized that caspase-6 with its corresponding substrate, lamin A/C, could be a potential target candidate, because caspase-6 is the only functional caspase for lamin A/C. We used immunohistochemistry methods, Western blots, and a specific caspase-6 inhibitor to determine the presence of lamin A/C and caspase-6 during oogenesis and in isolated oocytes. Our results demonstrated that these proteins were always present and that their distributions were related to oocyte maturity, determined by chromatin configuration and oocyte diameter. Caspase-6 inhibition slowed meiosis resumption suggesting the involvement of caspase-6 in the oocyte apoptotic pathway. Lamin A/C and caspase-6 could be valuable tools in the knowledge of oocyte in vitro destiny.

Cell-extrinsic defective lymphocyte development in Lmna(-/-) mice

Background

Mutations in the LMNA gene, which encodes all A-type lamins, result in a variety of human diseases termed laminopathies. Lmna^-/- mice appear normal at birth but become runted as early as 2 weeks of age and develop multiple tissue defects that mimic some aspects of human laminopathies. Lmna^-/- mice also display smaller spleens and thymuses. In this study, we investigated whether altered lymphoid organ sizes are correlated with specific defects in lymphocyte development.

Principal Findings

Lmna^-/- mice displayed severe age-dependent defects in T and B cell development which coincided with runting. Lmna^-/- bone marrow reconstituted normal T and B cell development in irradiated wild-type recipients, driving generation of functional and self-MHC restricted CD4⁺ and CD8⁺ T cells. Transplantation of Lmna^-/- neonatal thymus lobes into syngeneic wild-type recipients resulted in good engraftment of thymic tissue and normal thymocyte development.

Conclusions

Collectively, these data demonstrate that the severe defects in lymphocyte development that characterize Lmna^-/- mice do not result directly from the loss of A-type lamin function in lymphocytes or thymic stroma. Instead, the immune defects in Lmna^-/- mice likely reflect indirect damage, perhaps resulting from prolonged stress due to the striated muscle dystrophies that occur in these mice.

Molecular characterization of Xenopus lamin LIV reveals differences in the lamin composition of sperms in amphibians and mammals

Lamins are nuclear intermediate filament proteins. They are involved in most nuclear activities and are essential for retaining the mechano-elastic properties of the nucleus. Somatic cells of vertebrates express lamins A, B1 and B2 while lamin LIII, a major component of the amphibian oocyte lamina is absent in mammals. The organization of the lamina of germ cells differs significantly from that of somatic cells. Mammalian spermatogenic cells express two short lamins, C2 and B3, that are splice isoforms of lamin A and B2, respectively. Here we identify the previously described Xenopus lamin LIV as splice variant of the lamin LIII gene. LIV contains 40 extra residues in coil 2A of the rod domain, which results in altered assembly properties. Xenopus lamin LIV and mammalian B3 assemble into short structures rather than into long IF-like filaments. Expression of lamin LIV is restricted to male germ cells suggesting that it might be the functional equivalent of mammalian lamin B3. We provide evidence that lamins C2 and B3 are restricted to the mammalian lineage and describe the lamin composition of Xenopus sperm. Our results show that the evolution of germ cell-specific lamins followed separate and distinctly different paths in amphibians and mammals.

Nuclear lamins: key regulators of nuclear structure and activities

The nuclear lamina is a proteinaceous structure located underneath the inner nuclear membrane (INM), where it associates with the peripheral chromatin. It contains lamins and lamin-associated proteins, including many integral proteins of the INM, chromatin modifying proteins, transcriptional repressors and structural proteins. A fraction of lamins is also present in the nucleoplasm, where it forms stable complexes and is associated with specific nucleoplasmic proteins. The lamins and their associated proteins are required for most nuclear activities, mitosis and for linking the nucleoplasm to all major cytoskeletal networks in the cytoplasm. Mutations in nuclear lamins and their associated proteins cause about 20 different diseases that are collectively called laminopathies'. This review concentrates mainly on lamins, their structure and their roles in DNA replication, chromatin organization, adult stem cell differentiation, aging, tumorogenesis and the lamin mutations leading to laminopathic diseases.

Nesprin-2 giant safeguards nuclear envelope architecture in LMNA S143F progeria cells

The S143F lamin A/C point mutation causes a phenotype combining features of myopathy and progeria. We demonstrate here that patient dermal fibroblast cells have dysmorphic nuclei containing numerous blebs and lobulations, which progressively accumulate as cells age in culture. The lamin A/C organization is altered, showing intranuclear and nuclear envelope (NE) aggregates and presenting often a honeycomb appearance. Immunofluorescence microscopy showed that nesprin-2 C-terminal isoforms and LAP2alpha were recovered in the cytoplasm, whereas LAP2beta and emerin were unevenly localized along the NE. In addition, the intranuclear organization of acetylated histones, histone H1 and the active form of RNA polymerase II were markedly different in patient cells. A subpopulation of mutant cells, however, expressing the 800 kDa nesprin-2 giant isoform, did not show an overt nuclear phenotype. Ectopic expression of p.S143F lamin A in fibroblasts recapitulates the patient cell phenotype, whereas no effects were observed in p.S143F LMNA keratinocytes, which highly express nesprin-2 giant. Overexpression of the mutant lamin A protein had a more severe impact on the NE of nesprin-2 giant deficient fibroblasts when compared with wild-type. In summary, our results suggest that the p.S143F lamin A mutation affects NE architecture and composition, chromatin organization, gene expression and transcription. Furthermore, our findings implicate a direct involvement of the nesprins in laminopathies and propose nesprin-2 giant as a structural reinforcer at the NE.

Alterations to nuclear architecture and genome behavior in senescent cells

The organization of the genome within interphase nuclei, and how it interacts with nuclear structures is important for the regulation of nuclear functions. Many of the studies researching the importance of genome organization and nuclear structure are performed in young, proliferating, and often transformed cells. These studies do not reveal anything about the nucleus or genome in nonproliferating cells, which may be relevant for the regulation of both proliferation and replicative senescence. Here, we provide an overview of what is known about the genome and nuclear structure in senescent cells. We review the evidence that nuclear structures, such as the nuclear lamina, nucleoli, the nuclear matrix, nuclear bodies (such as promyelocytic leukemia bodies), and nuclear morphology all become altered within growth-arrested or senescent cells. Specific alterations to the genome in senescent cells, as compared to young proliferating cells, are described, including aneuploidy, chromatin modifications, chromosome positioning, relocation of heterochromatin, and changes to telomeres.

Accumulation of multiple forms of lamin A with down-regulation of FACE-1 suppresses growth in senescent human cells

5-Bromodeoxyuridine (BrdU) clearly induces a senescence-like phenomenon in every cell type. Proteome analysis revealed that lamin A and C were most highly increased in the nuclei of HeLa cells upon addition of BrdU. Immunoblot analysis also revealed marked accumulation of nuclear prelamin A. Consistently, farnesylated-proteins converting enzyme 1 (FACE-1) was markedly down-regulated in the same cells. Similar phenomena were also observed in normal human fibroblasts undergoing replicative senescence. Immunochemical analysis confirmed the above results. Lamin A is a major component of lamina and responsible for several genetic diseases. Thus, we ectopically expressed a wild-type, a mature type and a premature type of lamin in HeLa cells. All of these forms similarly inhibited colony formation and delayed cell cycle progression mainly through G2 phase. These results suggest that a change in the amount of lamin A, rather than appearance of its truncated form, is responsible for growth retardation in affected cells.

Ewing sarcoma gene EWS is essential for meiosis and B lymphocyte development

Ewing sarcoma gene EWS encodes a putative RNA-binding protein with proposed roles in transcription and splicing, but its physiological role in vivo remains undefined. Here, we have generated Ews-deficient mice and demonstrated that EWS is required for the completion of B cell development and meiosis. Analysis of Ews(-/-) lymphocytes revealed a cell-autonomous defect in precursor B lymphocyte (pre-B lymphocyte) development. During meiosis, Ews-null spermatocytes were deficient in XY bivalent formation and showed reduced meiotic recombination, resulting in massive apoptosis and complete arrest in gamete maturation. Inactivation of Ews in mouse embryonic fibroblasts resulted in premature cellular senescence, and the mutant animals showed hypersensitivity to ionizing radiation. Finally, we showed that EWS interacts with lamin A/C and that loss of EWS results in a reduced lamin A/C expression. Our findings reveal essential functions for EWS in pre-B cell development and meiosis, with proposed roles in DNA pairing and recombination/repair mechanisms. Furthermore, we demonstrate a novel role of EWS in cellular senescence, possibly through its interaction and modulation of lamin A/C.

Transmembrane protein Sun2 is involved in tethering mammalian meiotic telomeres to the nuclear envelope

Dynamic repositioning of telomeres is a unique feature of meiotic prophase I that is highly conserved among eukaryotes. At least in fission yeast it was shown to be required for proper alignment and recombination of homologous chromosomes. On entry into meiosis telomeres attach to the nuclear envelope and transiently cluster at a limited area to form a chromosomal bouquet. Telomere clustering is thought to promote chromosome recognition and stable pairing of the homologs. However, the molecular basis of telomere attachment and movement is largely unknown. Here we report that mammalian SUN-domain protein Sun2 specifically localizes to the nuclear envelope attachment sites of meiotic telomeres. Sun2-telomere association is maintained throughout the dynamic movement of telomeres. This association does not require the assembly of chromosomal axial elements or the presence of A-type lamins. Detailed EM analysis revealed that Sun2 is part of a membrane-spanning fibrillar complex that interconnects attached telomeres with cytoplasmic structures. Together with recent findings in fission yeast, our study indicates that the molecular mechanisms required for tethering meiotic telomeres and their dynamic movements during bouquet formation are conserved among eukaryotes.

Primary laminopathy fibroblasts display altered genome organization and apoptosis

A number of diseases associated with specific tissue degeneration and premature aging have mutations in the nuclear envelope proteins A-type lamins or emerin. Those diseases with A-type lamin mutation are inclusively termed laminopathies. Due to various hypothetical roles of nuclear envelope proteins in genome function we investigated whether alterations to normal genomic behaviour are apparent in cells with mutations in A-type lamins and emerin. Even though the distributions of these proteins in proliferating laminopathy fibroblasts appear normal, there is abnormal nuclear positioning of both chromosome 18 and 13 territories, from the nuclear periphery to the interior. This genomic organization mimics that found in normal nonproliferating quiescent or senescent cells. This finding is supported by distributions of modified pRb in the laminopathy cells. All laminopathy cell lines tested and an X-linked Emery-Dreifuss muscular dystrophy cell line also demonstrate increased incidences of apoptosis. The most extreme cases of apoptosis occur in cells derived from diseases with mutations in the tail region of the LMNA gene, such as Dunningan-type familial partial lipodystrophy and mandibuloacral dysplasia, and this correlates with a significant level of micronucleation in these cells.

The nuclear lamina

The inner face of the nuclear envelope of metazoan cells is covered by a thin lamina consisting of a one-layered network of intermediate filaments interconnecting with a complex set of transmembrane proteins and chromatin associating factors. The constituent proteins, the lamins, have recently gained tremendous recognition, because mutations in the lamin A gene, LMNA, are the cause of a complex group of at least 10 different diseases in human, including the Hutchinson-Gilford progeria syndrome. The analysis of these disease entities has made it clear that besides cytoskeletal functions, the lamina has an important role in the "behaviour" of the genome and is, probably as a consequence of this function, intimately involved in cell fate decisions. Furthermore, these functions are related to the involvement of lamins in organizing the position and functional state of interphase chromosomes as well as to the occurrence of lamins and lamina-associated proteins within the nucleoplasm. However, the structural features of these lamins and the nature of the factors that assist them in genome organization present an exciting challenge to modern biochemistry and cell biology.

Lamins A and C are present in the nuclei of early porcine embryos, with lamin A being distributed in large intranuclear foci

Gametogenesis and embryogenesis are dynamic developmental stages marked by extensive modifications in the organization of the genome and nuclear architecture. In the literature it is conveyed that only B-type lamins are required in these early stages of development and that A-type lamins are not present or required until differentiation of specific cell types associated with specialized tissue is initiated. To assess the presence of nuclear structures that are putatively involved in genome regulation, we investigated the distribution of lamin proteins throughout the early stages of porcine embryonic development, using testes tissue sections, oocytes and in-vitro fertilized (IVF) porcine embryos and employing anti-lamin antibodies. We have shown that anti-lamin A staining is present at the one-cell, two-cell, four-cell, and six- to eight-cell stages of early porcine embryo development, but diminishes at the morulae and blastocyst stages. Large intranuclear anti-lamin A foci are prominent in the early preimplantation stages. Both anti-lamin A/C and anti-lamin B staining were clearly present in all embryonic stages. Immature porcine oocytes revealed lamin rings using the monoclonal anti-lamin A/C antibody and many immature oocytes exhibited a pale rim staining pattern with anti-lamin A antibody. A-type lamins were not observed in sperm precursor cells. Thus, we have shown that A-type lamins and B-type lamins are present at the nuclear envelope in very early porcine embryos and that lamin A is also found in large intranuclear aggregates in two-cell to eight-cell embryos but is lacking from later embryonic stages.

Mutations that affect meiosis in male mice influence the dynamics of the mid-preleptotene and bouquet stages

Meiosis pairs and segregates homologous chromosomes and thereby forms haploid germ cells to compensate the genome doubling at fertilization. Homologue pairing in many eukaryotic species depends on formation of DNA double strand breaks (DSBs) during early prophase I when telomeres begin to cluster at the nuclear periphery (bouquet stage). By fluorescence in situ hybridization criteria, we observe that mid-preleptotene and bouquet stage frequencies are altered in male mice deficient for proteins required for recombination, ubiquitin conjugation and telomere length control. The generally low frequencies of mid-preleptotene spermatocytes were significantly increased in male mice lacking recombination proteins SPO11, MEI1, MLH1, KU80, ubiquitin conjugating enzyme HR6B, and in mice with only one copy of the telomere length regulator Terf1. The bouquet stage was significantly enriched in Atm(-/-), Spo11(-/-), Mei1(m1Jcs/m1Jcs), Mlh1(-/-), Terf1(+/-) and Hr6b(-/-) spermatogenesis, but not in mice lacking recombination proteins DMC1 and HOP2, the non-homologous end-joining DNA repair factor KU80 and the ATM downstream effector GADD45a. Mice defective in spermiogenesis (Tnp1(-/-), Gmcl1(-/-), Asm(-/-)) showed wild-type mid-preleptotene and bouquet frequencies. A low frequency of bouquet spermatocytes in Spo11(-/-)Atm(-/-) spermatogenesis suggests that DSBs contribute to the Atm(-/-)-correlated bouquet stage exit defect. Insignificant changes of bouquet frequencies in mice with defects in early stages of DSB repair (Dmc1(-/-), Hop2(-/-)) suggest that there is an ATM-specific influence on bouquet stage duration. Altogether, it appears that several pathways influence telomere dynamics in mammalian meiosis.

Barrier-to-autointegration factor-like (BAF-L): a proposed regulator of BAF

Barrier-to-autointegration factor (BAF) is an essential chromatin protein conserved in metazoans. BAF has roles in nuclear assembly, chromatin organization, gene expression, and gonad development and is exploited by retroviruses. BAF forms stable dimers that bind nonspecifically to dsDNA and specifically to LEM-domain proteins (e.g., LAP2beta, emerin, MAN1), homeodomain transcription factors, histones, and lamin A. We characterized a protein named BAF-Like (BAF-L) that in humans is 40% identical to BAF. Overexpression studies in HeLa cells show that BAF-L, like BAF, is a predominantly nuclear protein. Recombinant BAF-L forms stable homodimers and heterodimerizes with BAF in vitro and also interacts with BAF in vivo. BAF-L does not bind significantly to DNA, LAP2beta, or emerin but can form ternary complexes in vitro with BAF plus DNA, or BAF plus LAP2beta. Levels of BAF-L mRNA were high in pancreas and testis, suggesting functions in the germline. BAF-L mRNA was detectable at low levels in eleven other tissues and undetectable in heart and skeletal muscle which are specifically affected by Emery-Dreifuss muscular dystrophy, a disease caused by mutations in either emerin or lamin A. We propose that BAF-L regulates BAF function via heterodimerization and might thereby influence tissue-specific roles of BAF.

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.

Intranuclear membrane structure formations by CaaX-containing nuclear proteins

The nuclear lamina is a protein meshwork lining the nucleoplasmic face of the nuclear envelope. Association of lamins with the inner nuclear membrane is mediated by specific modifications in the CaaX motif at their C-termini. B-type lamins are permanently isoprenylated whereas lamin A loses its modification by a lamin A-specific processing step after incorporation into the lamina. Lamins are differentially expressed during development and tissue differentiation. Here we show that an increased synthesis of lamins B1 and B2 in amphibian oocytes induces the formation of intranuclear membrane structures that form extensive arrays of stacked cisternae. These 'lamin membrane arrays' are attached to the inner nuclear membrane but are not continuous with it. Induction of this membrane proliferation depends on CaaX-specific posttranslational modification. Moreover, in transfected HeLa cells, chimeric GFP containing a nuclear localization signal and a C-terminal CaaX motif of N-Ras induces intranuclear membrane stacks that resemble those induced by lamins and ER-like cisternae that are induced in the cytoplasm upon increased synthesis of integral ER membrane proteins. Implications for the synthesis of CaaX-containing proteins are discussed and the difference from intranuclear fibrous lamina annulate lamellae formations is emphasized.