The synaptonemal complex (SC) is a proteinaceous, meiosis-specific structure that is highly conserved in evolution. During meiosis, the SC mediates synapsis of homologous chromosomes. It is essential for proper recombination and segregation of homologous chromosomes, and therefore for genome haploidization. Mutations in human SC genes can cause infertility. In order to gain a better understanding of the process of SC assembly in a model system that would be relevant for humans, we are investigating meiosis in mice. Here, we report on a newly identified component of the murine SC, which we named SYCE3. SYCE3 is strongly conserved among mammals and localizes to the central element (CE) of the SC. By generating a Syce3 knockout mouse, we found that SYCE3 is required for fertility in both sexes. Loss of SYCE3 blocks synapsis initiation and results in meiotic arrest. In the absence of SYCE3, initiation of meiotic recombination appears to be normal, but its progression is severely impaired resulting in complete absence of MLH1 foci, which are presumed markers of crossovers in wild-type meiocytes. In the process of SC assembly, SYCE3 is required downstream of transverse filament protein SYCP1, but upstream of the other previously described CE–specific proteins. We conclude that SYCE3 enables chromosome loading of the other CE–specific proteins, which in turn would promote synapsis between homologous chromosomes. Meiosis is a special type of cell division that takes place in the germ line of sexually reproducing diploid organisms. Major events during meiosis are the pairing, recombination, and segregation of homologous chromosomes. As a consequence, daughter cells are haploid and genetically diverse. Therefore, meiosis is of utmost importance for the life of sexually reproducing species as it maintains the species-specific chromosome number and generates genetic diversity within a species. Proper segregation of homologous chromosomes during meiosis requires homolog pairs to be physically linked. The synaptonemal complex (SC), a meiosis-specific structure conserved in evolution, is essential for this process. Defective assembly of the SC has deleterious effects on germ cells and can cause infertility in mice and humans. Here, we report on a newly identified protein component of the mammalian SC that we have named SYCE3. SYCE3 is strongly conserved among mammals. Using the mouse as a model system, we demonstrate that loss of SYCE3 leads to infertility in both sexes. Infertility is caused by disruption of meiosis due to the inability of Syce3^−/− mice to assemble the central element of SCs. Our findings provide new insights into the complexity of SC assembly and its relevance to mammalian fertility.

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.

LINC complexes are nuclear envelope bridging protein structures formed by interaction of SUN and KASH proteins. They physically connect the nucleus with the peripheral cytoskeleton and are critically involved in a variety of dynamic processes, such as nuclear anchorage, movement and positioning and meiotic chromosome dynamics. Moreover, they are shown to be essential for maintaining nuclear shape. Based on detailed expression analysis and biochemical approaches, we show here that during mouse sperm development, a terminal cell differentiation process characterized by profound morphogenic restructuring, two novel distinctive LINC complexes are established. They consist either of spermiogenesis-specific Sun3 and Nesprin1 or Sun1η, a novel non-nuclear Sun1 isoform, and Nesprin3. We could find that these two LINC complexes specifically polarize to opposite spermatid poles likely linking to sperm-specific cytoskeletal structures. Although, as shown in co-transfection/immunoprecipitation experiments, SUN proteins appear to arbitrarily interact with various KASH partners, our study demonstrates that they actually are able to confine their binding to form distinct LINC complexes. Formation of the mammalian sperm head involves assembly and different polarization of two novel spermiogenesis-specific LINC complexes. Together, our findings suggest that theses LINC complexes connect the differentiating spermatid nucleus to surrounding cytoskeletal structures to enable its well-directed shaping and elongation, which in turn is a critical parameter for male fertility.

The synaptonemal complex (SC) is a key structure of meiosis, mediating the stable pairing (synapsis) of homologous chromosomes during prophase I. Its remarkable tripartite structure is evolutionarily well conserved and can be found in almost all sexually reproducing organisms. However, comparison of the different SC protein components in the common meiosis model organisms Saccharomyces cerevisiae, Arabidopsis thaliana, Caenorhabditis elegans, Drosophila melanogaster, and Mus musculus revealed no sequence homology. This discrepancy challenged the hypothesis that the SC arose only once in evolution. To pursue this matter we focused on the evolution of SYCP1 and SYCP3, the two major structural SC proteins of mammals. Remarkably, our comparative bioinformatic and expression studies revealed that SYCP1 and SYCP3 are also components of the SC in the basal metazoan Hydra . In contrast to previous assumptions, we therefore conclude that SYCP1 and SYCP3 form monophyletic groups of orthologous proteins across metazoans.

LINC complexes are evolutionarily conserved nuclear envelope bridges, physically connecting the nucleus to the peripheral cytoskeleton. They are pivotal for dynamic cellular and developmental processes, like nuclear migration, anchoring and positioning, meiotic chromosome movements and maintenance of cell polarity and nuclear shape. Active nuclear reshaping is a hallmark of mammalian sperm development and, by transducing cytoskeletal forces to the nuclear envelope, LINC complexes could be vital for sperm head formation as well. We here analyzed in detail the behavior and function of Sun4, a bona fide testis-specific LINC component. We demonstrate that Sun4 is solely expressed in spermatids and there localizes to the posterior nuclear envelope, likely interacting with Sun3/Nesprin1 LINC components. Our study revealed that Sun4 deficiency severely impacts the nucleocytoplasmic junction, leads to mislocalization of other LINC components and interferes with the formation of the microtubule manchette, which finally culminates in a globozoospermia-like phenotype. Together, our study provides direct evidence for a critical role of LINC complexes in mammalian sperm head formation and male fertility.

LINC complexes are evolutionarily conserved nuclear envelope bridges, composed of SUN (Sad-1/UNC-84) and KASH (Klarsicht/ANC-1/Syne/homology) domain proteins. They are crucial for nuclear positioning and nuclear shape determination, and also mediate nuclear envelope (NE) attachment of meiotic telomeres, essential for driving homolog synapsis and recombination. In mice, SUN1 and SUN2 are the only SUN domain proteins expressed during meiosis, sharing their localization with meiosis-specific KASH5. Recent studies have shown that loss of SUN1 severely interferes with meiotic processes. Absence of SUN1 provokes defective telomere attachment and causes infertility. Here, we report that meiotic telomere attachment is not entirely lost in mice deficient for SUN1, but numerous telomeres are still attached to the NE through SUN2/KASH5-LINC complexes. In Sun1^−/− meiocytes attached telomeres retained the capacity to form bouquet-like clusters. Furthermore, we could detect significant numbers of late meiotic recombination events in Sun1^−/− mice. Together, this indicates that even in the absence of SUN1 telomere attachment and their movement within the nuclear envelope per se can be functional. Correct genome haploidization during meiosis requires tightly regulated chromosome movements that follow a highly conserved choreography during prophase I. Errors in these movements cause subsequent meiotic defects, which typically lead to infertility. At the beginning of meiotic prophase, chromosome ends are tethered to the nuclear envelope (NE). This attachment of telomeres appears to be mediated by well-conserved membrane spanning protein complexes within the NE (LINC complexes). In mouse meiosis, the two main LINC components SUN1 and SUN2 were independently described to localize at the sites of telomere attachment. While SUN1 has been demonstrated to be critical for meiotic telomere attachment, the precise role of SUN2 in this context, however, has been discussed controversially in the field. Our current study was targeted to determine the factual capacity of SUN2 in telomere attachment and chromosome movements in SUN1 deficient mice. Remarkably, although telomere attachment is impaired in the absence of SUN1, we could find a yet undescribed SUN1-independent telomere attachment, which presumably is mediated by SUN2 and KASH5. This SUN2 mediated telomere attachment is stable throughout prophase I and functional in moving telomeres within the NE. Thus, our results clearly indicate that SUN1 and SUN2, at least partially, fulfill redundant meiotic functions.