How plants LINC the SUN to KASH

Linkers of the nucleoskeleton to the cytoskeleton (LINC) complexes formed by SUN and KASH proteins are conserved eukaryotic protein complexes that bridge the nuclear envelope (NE) via protein-protein interactions in the NE lumen. Revealed by opisthokont studies, LINC complexes are key players in multiple cellular processes, such as nuclear and chromosomal positioning and nuclear shape determination, which in turn influence the generation of gametes and several aspects of development. Although comparable processes have long been known in plants, the first plant nuclear envelope bridging complexes were only recently identified. WPP domain-interacting proteins at the outer NE have little homology to known opisthokont KASH proteins, but form complexes with SUN proteins at the inner NE that have plant-specific properties and functions. In this review, we will address the importance of LINC complex-regulated processes, describe the plant NE bridging complexes and compare them to opisthokont LINC complexes.

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.

Absence of Dpy19l2, a new inner nuclear membrane protein, causes globozoospermia in mice by preventing the anchoring of the acrosome to the nucleus

Sperm-head elongation and acrosome formation, which take place during the last stages of spermatogenesis, are essential to produce competent spermatozoa that are able to cross the oocyte zona pellucida and to achieve fertilization. During acrosome biogenesis, acrosome attachment and spreading over the nucleus are still poorly understood and to date no proteins have been described to link the acrosome to the nucleus. We recently demonstrated that a deletion of DPY19L2, a gene coding for an uncharacterized protein, was responsible for a majority of cases of type I globozoospermia, a rare cause of male infertility that is characterized by the exclusive production of round-headed acrosomeless spermatozoa. Here, using Dpy19l2 knockout mice, we describe the cellular function of the Dpy19l2 protein. We demonstrate that the protein is expressed predominantly in spermatids with a very specific localization restricted to the inner nuclear membrane facing the acrosomal vesicle. We show that the absence of Dpy19l2 leads to the destabilization of both the nuclear dense lamina (NDL) and the junction between the acroplaxome and the nuclear envelope. Consequently, the acrosome and the manchette fail to be linked to the nucleus leading to the disruption of vesicular trafficking, failure of sperm nuclear shaping and eventually to the elimination of the unbound acrosomal vesicle. Finally, we show for the first time that Dpy19l3 proteins are also located in the inner nuclear envelope, therefore implying that the Dpy19 proteins constitute a new family of structural transmembrane proteins of the nuclear envelope.

Mechanical regulation of nuclear structure and function

Mechanical loading induces both nuclear distortion and alterations in gene expression in a variety of cell types. Mechanotransduction is the process by which extracellular mechanical forces can activate a number of well-studied cytoplasmic signaling cascades. Inevitably, such signals are transduced to the nucleus and induce transcription factor-mediated changes in gene expression. However, gene expression also can be regulated through alterations in nuclear architecture, providing direct control of genome function. One putative transduction mechanism for this phenomenon involves alterations in nuclear architecture that result from the mechanical perturbation of the cell. This perturbation is associated with direct mechanical strain or osmotic stress, which is transferred to the nucleus. This review describes the current state of knowledge relating the nuclear architecture and the transfer of mechanical forces to the nucleus mediated by the cytoskeleton, the nucleoskeleton, and the LINC (linker of the nucleoskeleton and cytoskeleton) complex. Moreover, remodeling of the nucleus induces alterations in nuclear stiffness, which may be associated with cell differentiation. These phenomena are discussed in relation to the potential influence of nuclear architecture-mediated mechanoregulation of transcription and cell fate.

The composition, protein genesis and significance of the inner acrosomal membrane of eutherian sperm

As a consequence of the acrosomal reaction during fertilization, the inner acrosomal membrane (IAM) becomes exposed and forms the leading edge of the sperm for adhesive binding to and subsequent penetration of the zona-pellucida (ZP) of the metaphase-II-arrested oocyte. A premise of this review is that the IAM of spermatozoa anchors receptors and enzymes (on its extracellular side) that are required for sperm attachment to and penetration of the ZP. We propose a sperm cell fractionation strategy that allows for direct access to proteins bound to the extracellular side of the IAM. We review the types of integral and peripheral IAM proteins that have been found by this approach and that have been implicated in ZP recognition and lysis. We also propose a scheme for the origin and assembly of these proteins within the developing acrosome during spermiogenesis. During development, the extravesicular side of the membrane of the acrosomic vesicle is coated by peripheral proteins that transport and bind this secretory vesicle to the spermatid nucleus. The part of the membrane that binds to the nucleus becomes the IAM, while its extravesicular protein coat, which is retained between the IAM and the nuclear envelope of spermatozoa becomes the subacrosomal layer of the perinuclear theca (SAL-PT). Another premise of this review is that the IAM of spermatozoa is bound with proteins (on its intracellular side), namely the SAL-PT proteins, which hold the clue to the mechanism of acrosomal-nuclear docking. We propose a sperm cell fractionation strategy that allows for direct access to SAL-PT proteins. We then review the types of SAL-PT proteins that have been found by this approach and that have been implicated in transporting and binding the acrosome to the sperm nucleus.

Involvement of classical bipartite/karyopherin nuclear import pathway components in acrosomal trafficking and assembly during bovine and murid spermiogenesis

This study arose from our finding that SubH2Bv, a histone H2B variant residing in the subacrosomal compartment of mammalian spermatozoa, contains a bipartite nuclear localization signal (bNLS) but in spite of this did not enter the spermatid nucleus. Instead, it associated with proacrosomic and acrosomic vesicles, which were targeted to the nuclear surface to form the acrosome. On this basis we proposed that SubH2Bv targets proacrosomic/acrosomic vesicles from the Golgi apparatus to the nuclear envelope by utilizing the classical bipartite/karyopherin alpha (KPNA) nuclear import pathway. To test the protein's nuclear targeting ability, SubH2Bv, with and without targeted mutations of the basic residues of bNLS, as well as bNLS alone, were transfected into mammalian cells as GFP-fusion proteins. Only the intact bNLS conferred nuclear entry. Subsequently, we showed that a KPNA, most likely KPNA6, occupies the same sperm head compartment and follows the same pattern of acrosomal association during spermiogenesis as SubH2Bv. Sperm head fractionation combined with Western blotting located this KPNA to the subacrosomal layer of the perinuclear theca, while immunocytochemistry of testicular sections showed that it associates with the surface of proacrosomic/acrosomic vesicles during acrosomal biogenesis. The identical sperm-localization and testicular-expression patterns between KPNA and SubH2Bv suggested a potential binding interaction between these proteins. This was supported by recombinant SubH2Bv affinity pull-down assays on germ cell extracts. The results of this study provide a compelling argument that these two nuclear homing proteins work in concert to direct the acrosomic vesicle to the nucleus. Their final residence in the subacrosomal layer of the perinuclear theca of spermatozoa indicates a role for SubH2Bv and KPNA in acrosomal-nuclear docking.

Chromatin remodelling initiation during human spermiogenesis

During the last phase of spermatogenesis, spermiogenesis, haploid round spermatids metamorphose towards spermatozoa. Extensive cytoplasmic reduction and chromatin remodelling together allow a dramatic decrease of cellular, notably nuclear volume. DNA packing by a nucleosome based chromatin structure is largely replaced by a protamine based one. At the cytoplasmic level among others the acrosome and perinuclear theca (PNT) are formed. In this study we describe the onset of chromatin remodelling to occur concomitantly with acrosome and PNT development. In spread human round spermatid nuclei, we show development of a DAPI-intense doughnut-like structure co-localizing with the acrosomal sac and sub acrosomal PNT. At this structure we observe the first gradual decrease of nucleosomes and several histones. Histone post-translational modifications linked to chromatin remodelling such as H4K8ac and H4K16ac also delineate the doughnut, that is furthermore marked by H3K9me2. During the capping phase of acrosome development, the size of the doughnut-like chromatin domain increases, and this area often is marked by uniform nucleosome loss and the first appearance of transition protein 2 and protamine 1. In the acrosome phase at nuclear elongation, chromatin remodelling follows the downward movement of the marginal ring of the acrosome. Our results indicate that acrosome development and chromatin remodelling are interacting processes. In the discussion we relate chromatin remodelling to the available data on the nuclear envelope and the linker of nucleoskeleton and cytoskeleton (LINC) complex of spermatids, suggesting a signalling route for triggering chromatin remodelling.

SPAG4L, a novel nuclear envelope protein involved in the meiotic stage of spermatogenesis

SUN domain-containing proteins belong to a novel protein family. To date, several members--SUN1, SUN2, SUN3, and SPAG4--have been identified as nuclear envelope (NE) proteins. In this study, we sought to characterize and define the potential function of SPAG4L, a newly identified SUN protein. Using bioinformatic analysis, we found that SPAG4L contained a conserved SUN domain in the C-terminal. Subcellular localization analysis indicated that the expression of green fluorescent protein-labeled full-length SPAG4L was localized to the NE and the endoplasmic reticulum (ER). Deletion analysis revealed that the transmembrane region and the coiled-coil domain, but not the SUN domain, were required for localization of SPAG4L to the NE and ER. Subsequently, we confirmed that the human testes expressed endogenous SPAG4L as a 43-kDa protein. Further studies revealed that mouse Spag4L colocalized with the NE marker Lamin B1 and the ER marker PDI in isolated mouse spermatocytes. In addition, the expression of Spag4L was observed in meiosis I and II stages, suggesting that Spag4L may be involved in NE reconstitution and nuclear migration occurring during the process of spermatocyte division. Together, the findings indicate that SPAG4L, a new NE protein, may play an important role in the meiotic stage of spermatogenesis.

Subcellular localization of SUN2 is regulated by lamin A and Rab5

SUN2 is an inner nuclear membrane protein with a conserved Sad1/UNC-84 homology SUN-domain at the C-terminus. Intriguingly, SUN2 has also been reported to interact with Rab5, which localizes in early endosomes. To clarify the dual subcellular localization of SUN2, we investigated its localization in lamin A/C deficient cells rescued with lamin A or lamin C isoform, and in HeLa cells transfected with Rab5 or its mutants. We found that expression of lamin A but not lamin C partly restored the nuclear envelope localization of SUN2. SUN2 was redistributed to endosomes upon overexpression of Rab5, but remained on the nuclear envelope when the SUN domain was deleted. To explore the physiological function of SUN2 in vesicle trafficking and endocytosis, we demonstrated the colocalization of endogenous SUN2 and Rab5. Moreover, overexpression of SUN2 stimulated the uptake of transferrin while suppression of SUN2 expression attenuated the process. These findings support a role of SUN2 in endocytosis.