A kinesin is present at unique sertoli/spermatid adherens junctions in rat and mouse testes

FYN regulates cell adhesion at the blood-testis barrier and the apical ectoplasmic specialization via its effect on Arp3 in the mouse testis

FYN is a non-receptor tyrosine kinase of the SRC family that facilitates virus entry across epithelial tight junctions. However, the role of FYN in mammalian testes in maintaining the blood-testis barrier (BTB) integrity and the adhesion of germ cells to Sertoli cells are not well defined. Here, we show that FYN is a component of the BTB and the apical ectoplasmic specialization (ES) at Sertoli-Sertoli and Sertoli-spermatid interfaces, respectively, and is expressed extensively in mouse testes during postnatal development. FYN was shown to be structurally linked to the actin and microtubule-based cytoskeletons. An in vivo model was used to explore the modulatory effect of FYN on BTB and apical ES dynamics within the testes when adult mice were treated intraperitoneally with CdCl₂ (3 mg/kg body weight). The CdCl₂-induced epithelial restructuring was associated with a transient increase in the interaction between FYN and the actin branching/nucleation protein Arp3, as well as an induction of Arp3 phosphorylation, which possibly lead to actin cytoskeleton remodeling, resulting in BTB damage and germ cell loss in the seminiferous epithelium. Based on the results, we propose a model in which FYN and Arp3 form a protein complex that is responsible for junction reorganization events at the apical ES and the BTB. It is also possible for viruses to break through the BTB and enter the immunoprivileged testicular microenvironment via this mechanism.

Kinesins in Mammalian Spermatogenesis and Germ Cell Transport

In mammalian testes, the apical cytoplasm of each Sertoli cell holds up to several dozens of germ cells, especially spermatids that are transported up and down the seminiferous epithelium. The blood-testis barrier (BTB) established by neighboring Sertoli cells in the basal compartment restructures on a regular basis to allow preleptotene/leptotene spermatocytes to pass through. The timely transfer of germ cells and other cellular organelles such as residual bodies, phagosomes, and lysosomes across the epithelium to facilitate spermatogenesis is important and requires the microtubule-based cytoskeleton in Sertoli cells. Kinesins, a superfamily of the microtubule-dependent motor proteins, are abundantly and preferentially expressed in the testis, but their functions are poorly understood. This review summarizes recent findings on kinesins in mammalian spermatogenesis, highlighting their potential role in germ cell traversing through the BTB and the remodeling of Sertoli cell-spermatid junctions to advance spermatid transport. The possibility of kinesins acting as a mediator and/or synchronizer for cell cycle progression, germ cell transit, and junctional rearrangement and turnover is also discussed. We mostly cover findings in rodents, but we also make special remarks regarding humans. We anticipate that this information will provide a framework for future research in the field.

Sperm Differentiation: The Role of Trafficking of Proteins

Sperm differentiation encompasses a complex sequence of morphological changes that takes place in the seminiferous epithelium. In this process, haploid round spermatids undergo substantial structural and functional alterations, resulting in highly polarized sperm. Hallmark changes during the differentiation process include the formation of new organelles, chromatin condensation and nuclear shaping, elimination of residual cytoplasm, and assembly of the sperm flagella. To achieve these transformations, spermatids have unique mechanisms for protein trafficking that operate in a coordinated fashion. Microtubules and filaments of actin are the main tracks used to facilitate the transport mechanisms, assisted by motor and non-motor proteins, for delivery of vesicular and non-vesicular cargos to specific sites. This review integrates recent findings regarding the role of protein trafficking in sperm differentiation. Although a complete characterization of the interactome of proteins involved in these temporal and spatial processes is not yet known, we propose a model based on the current literature as a framework for future investigations.

Endocrine regulation of sperm release

Spermiation (sperm release) is the culmination of a spermatid’s journey in the seminiferous epithelium. After a long association with the Sertoli cell, spermatids have to finally ‘let go’ of the support from Sertoli cells in order to be transported to the epididymis. Spermiation is a multistep process characterised by removal of excess spermatid cytoplasm, recycling of junctional adhesion molecules by endocytosis, extensive cytoskeletal remodelling and final spermatid disengagement. Successful execution of all these events requires coordinated regulation by endocrine and paracrine factors. This review focuses on the endocrine regulation of spermiation. With the aim of delineating how hormones control the various aspects of spermiation, this review provides an analysis of recent advances in research on the hormonal control of molecules associated with the spermiation machinery. Because spermiation is one of the most sensitive phases of spermatogenesis to variations in hormone levels, understanding their molecular control is imperative to advance our knowledge of the nuances of spermatogenesis and male fertility.

Is toxicant-induced Sertoli cell injury in vitro a useful model to study molecular mechanisms in spermatogenesis?

Sertoli cells isolated from rodents or humans and cultured in vitro are known to establish a functional tight junction (TJ)-permeability barrier that mimics the blood-testis barrier (BTB) in vivo. This model has been widely used by investigators to study the biology of the TJ and the BTB. Studies have shown that environmental toxicants (e.g., perfluorooctanesulfonate (PFOS), bisphenol A (BPA) and cadmium) that exert their disruptive effects to induce Sertoli cell injury using this in vitro model are reproducible in studies in vivo. Thus, this in vitro system provides a convenient approach to probe the molecular mechanism(s) underlying toxicant-induced testis injury but also to provide new insights in understanding spermatogenesis, such as the biology of cell adhesion, BTB restructuring that supports preleptotene spermatocyte transport, and others. Herein, we provide a brief and critical review based on studies using this in vitro model of Sertoli cell cultures using primary cells isolated from rodent testes vs. humans to monitor environmental toxicant-mediated Sertoli cell injury. In short, recent findings have shown that environmental toxicants exert their effects on Sertoli cells to induce testis injury through their action on Sertoli cell actin- and/or microtubule-based cytoskeleton. These effects are mediated via their disruptive effects on actin- and/or microtubule-binding proteins. Sertoli cells also utilize differential spatiotemporal expression of these actin binding proteins to confer plasticity to the BTB to regulate germ cell transport across the BTB.

Regulation of microtubule (MT)-based cytoskeleton in the seminiferous epithelium during spermatogenesis

In rodents and humans, testicular cells, similar to other mammalian cells, are supported by actin-, microtubule (MT)- and intermediate filament-based cytoskeletons. Although the cytoskeletal network of the testis serves an important role in regulating spermatogenesis during the epithelial cycle, most of the published findings in the literature are limited to studies that only visualize these cytoskeletons in the seminiferous epithelium. Few focus on the underlying molecular mechanism that regulates their organization in the epithelium in response to changes in the stages of the epithelial cycle. Functional studies in the last decade have begun to focus on the role of binding proteins that regulate these cytoskeletons, with some interesting findings rapidly emerging in the field. Since the actin- and intermediate filament-based cytoskeletons have been recently reviewed, herein we focus on the MT-based cytoskeleton for two reasons. First, besides serving as a structural support cytoskeleton, MTs are known to serve as the track to support and facilitate the transport of germ cells, such as preleptotene spermatocytes connected in clones and elongating/elongated spermatids during spermiogenesis, across the blood-testis barrier (BTB) and the adluminal compartment, respectively, during spermatogenesis. While these cellular events are crucial to the completion of spermatogenesis, they have been largely ignored in the past. Second, MT-based cytoskeleton is working in concert with the actin-based cytoskeleton to provide structural support for the transport of intracellular organelles across the cell cytosol, such as endosome-based vesicles, and phagosomes, which contain residual bodies detached from spermatids, to maintain the cellular homeostasis in the seminiferous epithelium. We critically evaluate some recent published findings herein to support a hypothesis regarding the role of MT in conferring germ cell transport in the seminiferous epithelium.

GAR22β regulates cell migration, sperm motility, and axoneme structure

Spatiotemporal cytoskeleton remodeling is crucial for several biological processes. GAR22β interacts with EB1 via a novel noncanonical amino acid sequence and is pivotal for cell motility and focal adhesion turnover. GAR22β is also crucial for generation, motility, and ultrastructural organization of spermatozoa.

Spatiotemporal cytoskeleton remodeling is pivotal for cell adhesion and migration. Here we investigated the function of Gas2-related protein on chromosome 22 (GAR22β), a poorly characterized protein that interacts with actin and microtubules. Primary and immortalized GAR22β^−/− Sertoli cells moved faster than wild-type cells. In addition, GAR22β^−/− cells showed a more prominent focal adhesion turnover. GAR22β overexpression or its reexpression in GAR22β^−/− cells reduced cell motility and focal adhesion turnover. GAR22β–actin interaction was stronger than GAR22β–microtubule interaction, resulting in GAR22β localization and dynamics that mirrored those of the actin cytoskeleton. Mechanistically, GAR22β interacted with the regulator of microtubule dynamics end-binding protein 1 (EB1) via a novel noncanonical amino acid sequence, and this GAR22β–EB1 interaction was required for the ability of GAR22β to modulate cell motility. We found that GAR22β is highly expressed in mouse testes, and its absence resulted in reduced spermatozoa generation, lower actin levels in testes, and impaired motility and ultrastructural disorganization of spermatozoa. Collectively our findings identify GAR22β as a novel regulator of cell adhesion and migration and provide a foundation for understanding the molecular basis of diverse cytoskeleton-dependent processes.

Testicular histopathology associated with disruption of the Sertoli cell cytoskeleton

Testicular histological alterations following Sertoli cell cytoskeleton disruption are numerous. The Sertoli cell cytoskeleton is comprised of intermediate filaments, microtubules, microfilaments and their direct interacting proteins and performs essential functions including structural support of the seminiferous epithelium, apicobasal movement of elongate spermatids, and release of elongate spermatids from the seminiferous epithelium during spermiation. This review summarizes the histological changes occurring after disruption of the Sertoli cell cytoskeleton, including the signature lesion of seminiferous epithelium sloughing. By presenting examples of histological changes after exposure to toxins or toxicants directly affecting the Sertoli cell cytoskeleton or genetic manipulations of this cytoskeleton, the toxicologist observing similar histological changes associated with exposure to novel compounds can use this information to generate hypotheses about a potential mode of action.

MAP/microtubule affinity-regulating kinases, microtubule dynamics, and spermatogenesis

During spermatogenesis, spermatids derived from meiosis simultaneously undergo extensive morphological transformation, to become highly specialized and metabolically quiescent cells, and transport across the seminiferous epithelium. Spermatids are also transported back-and-forth across the seminiferous epithelium during the epithelial cycle until they line up at the luminal edge of the tubule to prepare for spermiation at stage VIII of the cycle. Spermatid transport thus requires the intricate coordination of the cytoskeletons in Sertoli cells (SCs) as spermatids are nonmotile cells lacking the ultrastructures of lamellipodia and filopodia, as well as the organized components of the cytoskeletons. In the course of preparing this brief review, we were surprised to see that, except for some earlier eminent morphological studies, little is known about the regulation of the microtubule (MT) cytoskeleton and the coordination of MT with the actin-based cytoskeleton to regulate spermatid transport during the epithelia cycle, illustrating that this is a largely neglected area of research in the field. Herein, we summarize recent findings in the field regarding the significance of actin- and tubulin-based cytoskeletons in SCs that support spermatid transport; we also highlight specific areas of research that deserve attention in future studies.

Tubulobulbar complex: cytoskeletal remodeling to release spermatozoa

Tubulobulbar complexes (TBCs) are actin-based structures that help establish close contact between Sertoli-Sertoli cells or Sertoli-mature germ cells (spermatids) in the seminiferous tubules of the testes. They are actin-rich push-through devices that eliminate excess spermatid cytoplasm and prepare mature spermatids for release into the tubular lumen. Just prior to spermiation, the elongated spermatid interacts with the Sertoli cell via an extensive structure comprising various adhesion molecules called the apical ectoplasmic specialization which is partially replaced by the apical TBC, on the concave surface of the spermatid head. The sperm release process involves extensive restructuring, namely the disassembly and reassembly of junctions at the Sertoli-spermatid interface in the seminiferous epithelium. Based on the presence of different classes of molecules in the TBCs or the defects observed in the absence of TBCs, the main functions attributed to TBCs are elimination of excess spermatid cytoplasm, endocytosis and recycling of junctional molecules, shaping of the spermatid acrosome, and forming transient anchoring devices for mature spermatids before they are released. This review summarizes the recent findings that focus on the role of TBCs in cell cytoskeleton restructuring during sperm release in the testes and the molecular mechanism involved.

Nuclear factor kappa B and tumor necrosis factor-alpha modulation of transcription of the mouse testis- and pre-implantation development-specific Rnf33/Trim60 gene

We have previously reported a mouse Rnf33/Trim60 gene that is temporally expressed in the pre-implantation embryo. The Rnf33 structural gene is composed of a short noncoding exon 1 and an intronless coding exon 2. In the present work, Rnf33 was shown to be expressed in the mouse testis and in the testicular cell lines TM3 and TM4. To elucidate Rnf33 transcriptional modulation, a 2.5-kb Rnf33 sequence, inclusive of the upstream regulatory region, exon 1 and the associated intronic sequence, was dissected in transient transfection and luciferase assays. An initiator and an atypical TATA-box were shown to act as the core promoter elements of the gene. Deletion and mutagenesis of the 2.5-kb sequence in luciferase constructs further demonstrated that an intronic and palindromic kappa B (κB) sequence was an important cis element targeted by the nuclear factor-κB (NF-κB) subunits p65/RELA and p50/NFκB1, and also through modulation by tumor necrosis factor α. Transcriptional up-regulation of Rnf33 by NF-κB and tumor necrosis factor-α was directly demonstrated in TM3 and TM4 cells by real-time PCR quantification of the Rnf33 mRNA levels. Small interfering RNA knockdown of p65 and p50 confirmed Rnf33 down-regulation by p65/p50. Spermatogenesis is regulated by a wide range of stimuli, including NF-κB, which, in turn, is regulated by other signals. Hence, demonstration of NF-κB-regulated Rnf33 expression in testicular cells, particularly in Sertoli cells, implicates functional involvement of the putative RNF33 protein in spermatogenesis through association of the RNF33 protein with the microtubule via interaction with kinesin motor proteins, as previously demonstrated [Huang et al., submitted].

KIFC1-like motor protein associates with the cephalopod manchette and participates in sperm nuclear morphogenesis in Octopus tankahkeei

Background

Nuclear morphogenesis is one of the most fundamental cellular transformations taking place during spermatogenesis. In rodents, a microtubule-based perinuclear structure, the manchette, and a C-terminal kinesin motor KIFC1 are believed to play crucial roles in this process. Spermatogenesis in Octopus tankahkeei is a good model system to explore whether evolution has created a cephalopod prototype of mammalian manchette-based and KIFC1-dependent sperm nuclear shaping machinery.

Methodology/Principal Findings

We detected the presence of a KIFC1-like protein in the testis, muscle, and liver of O. tankahkeei by Western Blot. Then we tracked its dynamic localization in spermatic cells at various stages using Immunofluorescence and Immunogold Electron Microscopy. The KIFC1-like protein was not expressed at early stages of spermatogenesis when no significant morphological changes occur, began to be present in early spermatid, localized around and in the nucleus of intermediate and late spermatids where the nucleus was dramatically elongated and compressed, and concentrated at one end of final spermatid. Furthermore, distribution of the motor protein during nuclear elongation and condensation overlapped with that of the cephalopod counterpart of manchette at a significant level.

Conclusions/Significance

The results support the assumption that the protein is actively involved in sperm nuclear morphogenesis in O. tankahkeei possibly through bridging the manchette-like perinuclear microtubules to the nucleus and assisting in the nucleocytoplasmic trafficking of specific cargoes. This study represents the first description of the role of a motor protein in sperm nuclear shaping in cephalopod.

Cytoskeletal dynamics and spermatogenesis

Different cellular events occur during spermatogenesis, and these include (i) mitosis for self-renewal of spermatogonia, (ii) differentiation of type A spermatogonia into type B and commitment of type B spermatogonia to develop into preleptotene primary spermatocytes, (iii) transit of preleptotene/leptotene spermatocytes across the blood-testis barrier in coordination with germ cell cycle progression and meiosis, (iv) spermiogenesis and spermiation. These events also associate with extensive changes in cell shape and size, and germ cell movement. The cytoskeleton, which comprises actin, microtubules and intermediate filaments, is believed to function in these cellular events. However, few studies have been conducted by investigators in the past decades to unfold the role of the cytoskeleton during spermatogenesis. This review summarizes recent advances in the field relating to cytoskeletal dynamics in the testis, and highlights areas of research that require additional emphasis so that new approaches for male contraception, as well as therapeutic approaches to alleviate environmental toxicant-induced reproductive dysfunction in men, can possibly be developed.

A local autocrine axis in the testes that regulates spermatogenesis

Spermiation--the release of mature spermatozoa from Sertoli cells into the seminiferous tubule lumen--occurs by the disruption of an anchoring device known as the apical ectoplasmic specialization (apical ES). At the same time, the blood-testis barrier (BTB) undergoes extensive restructuring to facilitate the transit of preleptotene spermatocytes. While these two cellular events take place at opposite ends of the Sertoli cell epithelium, the events are in fact tightly coordinated, as any disruption in either process will lead to infertility. A local regulatory axis exists between the apical ES and the BTB in which biologically active laminin fragments produced at the apical ES by the action of matrix metalloproteinase 2 can regulate BTB restructuring directly or indirectly via the hemidesmosome. Equally important, polarity proteins play a crucial part in coordinating cellular events within this apical ES-BTB-hemidesmosome axis. Additionally, testosterone and cytokines work in concert to facilitate BTB restructuring, which enables the transit of spermatocytes while maintaining immunological barrier function. Herein, we will discuss this important autocrine-based cellular axis that parallels the hormonal-based hypothalamic-pituitary-testicular axis that regulates spermatogenesis. This local regulatory axis is the emerging target for male contraception.

KIFC1 participates in acrosomal biogenesis, with discussion of its importance for the perforatorium in the Chinese mitten crab Eriocheir sinensis

Spermatogenesis is a complicated process during which spermatogonia undergo proliferation and divisions leading, after a series of dramatic changes, to the production of mature spermatozoa. Many molecular motors are involved in this process. KIFC1, a C-terminal kinesin motor, participates in acrosome biogenesis and nuclear shaping. We report here the expression profile of KIFC1 during spermatogenesis in the Chinese mitten crab, Eriocheir sinensis. KIFC1 mainly localizes around the nucleus but is also present within the nucleus of the spermatogonium and spermatocyte. At the early spermatid stage, KIFC1 begins to be distributed on the nuclear membrane at the region where the proacrosomal vesicle is located. By the late spermatid stage, KIFC1 is found on the acrosome. Immunocytochemical and ultrastructural analyses have shown that KIFC1 localizes on the perforatorium, which is composed of an apical cap and an acrosomal tubule. We demonstrate that, during spermatogenesis in E. sinensis, KIFC1 probably plays important roles in the biogenesis of the acrosome and in its maintenance. KIFC1 may also be essential for the eversion of the acrosome during fertilization.

Epidermal growth factor receptor pathway substrate 8 (Eps8) is a novel regulator of cell adhesion and the blood-testis barrier integrity in the seminiferous epithelium

In the seminiferous epithelium, Eps8 is localized to actin-based cell junctions at the blood-testis barrier (BTB) and the apical ectoplasmic specialization (ES) in stage V-VI tubules but is considerably diminished in stage VIII tubules. Eps8 down-regulation coincides with the time of BTB restructuring and apical ES disassembly, implicating the role of Eps8 in cell adhesion. Its involvement in Sertoli-germ cell adhesion was substantiated in studies using an in vivo animal model by treating rats with 1-(2,4-dichlorobenzy)-1H-indazole-3-carbohydrazide (adjudin) to induce anchoring junction restructuring, during which Eps8 disappeared at the apical ES before germ cell departure. In Sertoli cell cultures with established permeability barrier mimicking the BTB in vivo, the knockdown of Eps8 by RNAi led to F-actin disorganization and the mislocalization of the tight junction proteins occludin and ZO-1, suggesting the function of Eps8 in maintaining BTB integrity. In vivo knockdown of Eps8 in the testis caused germ cell sloughing and BTB damage, concomitant with occludin mislocalization, further validating that Eps8 is a novel regulator of cell adhesion and BTB integrity in the seminiferous epithelium.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.