High Resolution Localization of Rab5, EEA1, and Nectin-3 to Tubulobulbar Complexes in the Rat Testis

Nectin-3 and shed forms of CSPG4 can serve as epithelial cell receptors for Clostridioides difficile TcdB

IMPORTANCE

Toxin B (TcdB) is a major virulence factor of Clostridioides difficile, a Gram-positive pathogen that is a leading cause of hospital-acquired diarrhea. While previous studies have established that TcdB can engage multiple cell surface receptors in vitro, little is known about how these interactions promote disease and where these receptors localize on colonic tissue. Here, we used immunofluorescence microscopy to visualize Nectin-3 and CSPG4 on tissue, revealing unexpected localization of both receptors on colonic epithelial cells. We show that Nectin-3, which was previously characterized as an adherens junction protein, is also localized to the brush border of colonocytes. Staining for CSPG4 revealed that it is present along epithelial cell junctions, suggesting that it is shed by fibroblasts along the crypt-surface axis. Collectively, our study provides new insights into how TcdB can gain access to the receptors Nectin-3 and CSPG4 to intoxicate colonic epithelial cells.

Nectin stabilization at adherens junctions is counteracted by Rab5a-dependent endocytosis

Cell-cell junctions undergo constant remodeling, which is crucial for the control of vascular integrity. Indeed, transport of junctional components such as cadherins is understood in increasing depth. However, little is known about the respective pathways regulating localization of nectin at cell-cell junctions. Here, we performed an siRNA-based screen of vesicle regulators of the RabGTPase family, leading to the identification of a novel role for Rab5a in the endocytosis nectin-2 at adherens junctions of primary human endothelial cells (HUVEC). Confocal microscopy experiments revealed disordered nectin-2 localization at adherens junctions upon Rab5a depletion. In addition, internalized nectin-2 was shown to prominently localize to Rab5a-positive vesicles in both fixed and living cells. As shown previously, nectin-2 stabilization at junctions is achieved via drebrin-dependent coupling to the subcortical actin cytoskeleton. Consistently, depletion of drebrin in this study leads to enhanced internalization of nectin-2 from junctions. Strikingly, simultaneous silencing of Rab5a and drebrin restored the junctional localization of nectin-2, pointing to Rab5a as counteracting the drebrin-dependent stabilization of nectin-2 at adherens junctions. This mechanism could be further validated by transendothelial resistance measurements. Collectively, our results identify Rab5a as a key player in the endocytosis of nectin-2 and thus in the regulation of adherens junction integrity in primary human endothelial cells.

Knockdown of IP3R1 disrupts tubulobulbar complex-ectoplasmic reticulum contact sites and the morphology of apical processes encapsulating late spermatids†

Tubulobulbar complexes (TBCs) internalize intercellular junctions during sperm release. One of the characteristic features of TBCs is that they form "bulbs" or swollen regions that have well-defined membrane contact sites (MCS) with adjacent cisternae of endoplasmic reticulum. Previously, we have localized the IP3R calcium channel to the TBC bulb-ER contacts and have hypothesized that fluctuations in local calcium levels may facilitate the maturation of TBC bulbs into putative endosomes, or alter local actin networks that cuff adjacent tubular regions of the TBCs. To test this, we injected the testes of Sprague Dawley rats with small interfering RNAs (siRNAs) against IP3R1 and processed the tissues for either western blot, immunofluorescence, or electron microscopy. When compared to control testes injected with nontargeting siRNAs, Sertoli cells in knocked-down testes showed significant morphological alterations to the actin networks including a loss of TBC actin and the appearance of ectopic para-crystalline actin bundles in Sertoli cell stalks. There also was a change in the abundance and distribution of TBC-ER contact sites and large internalized endosomes. This disruption of TBCs resulted in delay of the withdrawal of apical processes away from spermatids and in spermiation. Together, these findings are consistent with the hypothesis that calcium exchange at TBC-ER contacts is involved both in regulating actin dynamics at TBCs and in the maturing of TBC bulbs into endosomes. The results are also consistent with the hypothesis that TBCs are part of the sperm release mechanism.

Myosin VI maintains the actin-dependent organization of the tubulobulbar complexes required for endocytosis during mouse spermiogenesis†‡

Myosin VI (MYO6) is an actin-based motor that has been implicated in a wide range of cellular processes, including endocytosis and the regulation of actin dynamics. MYO6 is crucial for actin/membrane remodeling during the final step of Drosophila spermatogenesis, and MYO6-deficient males are sterile. This protein also localizes to actin-rich structures involved in mouse spermiogenesis. Although loss of MYO6 in Snell's waltzer knock-out (KO) mice causes several defects and shows reduced male fertility, no studies have been published to address the role of MYO6 in sperm development in mouse. Here we demonstrate that MYO6 and some of its binding partners are present at highly specialized actin-based structures, the apical tubulobulbar complexes (TBCs), which mediate endocytosis of the intercellular junctions at the Sertoli cell-spermatid interface, an essential process for sperm release. Using electron and light microscopy and biochemical approaches, we show that MYO6, GIPC1 and TOM1/L2 form a complex in testis and localize predominantly to an early endocytic APPL1-positive compartment of the TBCs that is distinct from EEA1-positive early endosomes. These proteins also associate with the TBC actin-free bulbular region. Finally, our studies using testis from Snell's waltzer males show that loss of MYO6 causes disruption of the actin cytoskeleton and disorganization of the TBCs and leads to defects in the distribution of the MYO6-positive early APPL1-endosomes. Taken together, we report here for the first time that lack of MYO6 in mouse testis reduces male fertility and disrupts spatial organization of the TBC-related endocytic compartment during the late phase of spermiogenesis.

Diverse functions of myosin VI in spermiogenesis

Spermiogenesis is the final stage of spermatogenesis, a differentiation process during which unpolarized spermatids undergo excessive remodeling that results in the formation of sperm. The actin cytoskeleton and associated actin-binding proteins play crucial roles during this process regulating organelle or vesicle delivery/segregation and forming unique testicular structures involved in spermatid remodeling. In addition, several myosin motor proteins including MYO6 generate force and movement during sperm differentiation. MYO6 is highly unusual as it moves towards the minus end of actin filaments in the opposite direction to other myosin motors. This specialized feature of MYO6 may explain the many proposed functions of this myosin in a wide array of cellular processes in animal cells, including endocytosis, secretion, stabilization of the Golgi complex, and regulation of actin dynamics. These diverse roles of MYO6 are mediated by a range of specialized cargo-adaptor proteins that link this myosin to distinct cellular compartments and processes. During sperm development in a number of different organisms, MYO6 carries out pivotal functions. In Drosophila, the MYO6 ortholog regulates actin reorganization during spermatid individualization and male KO flies are sterile. In C. elegans, the MYO6 ortholog mediates asymmetric segregation of cytosolic material and spermatid budding through cytokinesis, whereas in mice, this myosin regulates assembly of highly specialized actin-rich structures and formation of membrane compartments to allow the formation of fully differentiated sperm. In this review, we will present an overview and compare the diverse function of MYO6 in the specialized adaptations of spermiogenesis in flies, worms, and mammals.

ORP9 knockdown delays the maturation of junction-related endocytic structures in the testis and leads to impaired sperm release†

The release of late spermatids from the seminiferous epithelium requires the internalization of intercellular junctions by Sertoli cell specific structures called "tubulobulbar complexes" (TBCs). These large, endocytic devices likely evolved from classic clathrin-mediated-endocytosis (CME) machinery, but have several important morphological differences to CME vesicles. Most notable among these differences is that extensive endoplasmic reticulum (ER) membrane contact sites (MCSs) occur with TBCs and not with clathrin-coated pits. One of the well-established functions of ER MCSs is lipid exchange. Previously, we have established that the ORP9 lipid exchange protein is localized to the TBC-ER MCS; however, the function of ORP9 and lipid exchange at the sites is not known. Here we use an in vivo knockdown approach to probe function. The testes of Sprague-Dawley rats were injected with ORP9 targeted siRNA or non-targeted reagents, and the tissues examined by bright field, super-resolution stimulated emission depletion, and electron microscopy. The knockdown of ORP9 was achieved and maintained with daily injections of siRNA for 2-3 day intervals. Compared to controls, sections from ORP9 siRNA-injected testes had longer TBC tubes and fewer fused TBC bulbs. Late spermatids were also abnormally retained in the epithelium of knockdown tissue. These results suggest that ORP9 is necessary for normal TBC bulb vesiculation and fusion, most likely by changing the plasma membrane lipid profile of the TBC. These data also further support the conclusion that TBCs are part of the normal mechanism of sperm release.

Internalization of Intact Intercellular Junctions in the Testis by Clathrin/Actin-Mediated Endocytic Structures: Tubulobulbar Complexes

Sertoli cells of the mammalian seminiferous epithelium form unique subcellular actin-related structures at intercellular junctions. The appearance of these so called "tubulobulbar complexes" (TBCs) precedes both sperm release at the apex of the epithelium and the movement of early spermatogenic cells out of the spermatogonial stem cell niche at the base of the epithelium. TBCs are considered to be part of the mechanism of junction endocytosis by Sertoli cells. The structures contain junction proteins and morphologically identifiable junctions, and are associated with markers of endocytosis. Here we review the current state of knowledge about the structure and function of TBCs. As the complexes form, they morphologically resemble and have the molecular signature of clathrin-coated pits with extremely long necks. As they mature, the actin filament networks around the "necks" of the structures progressively disassemble and the membrane cores expand or swell into distinct "bulbs". These bulbs acquire extensive membrane contact sites with associated cisternae of endoplasmic reticulum. Eventually the bulbs undergo scission and continue through endosomal compartments of the Sertoli cells. The morphology and composition of TBC indicates to us that the structures likely evolved from the basic clathrin-mediated endocytosis mechanism common to cells generally, and along the way they incorporated unique features to accommodate the cyclic turnover of massive and "intact" intercellular junctions that occurs during spermatogenesis. Anat Rec, 301:2080-2085, 2018. © 2018 Wiley Periodicals, Inc.

The organophosphate pesticide methamidophos opens the blood-testis barrier and covalently binds to ZO-2 in mice

Methamidophos (MET) is an organophosphate (OP) pesticide widely used in agriculture in developing countries. MET causes adverse effects in male reproductive function in humans and experimental animals, but the underlying mechanisms remain largely unknown. We explored the effect of MET on mice testes (5 mg/kg/day/4 days), finding that this pesticide opens the blood-testis barrier and perturbs spermatogenesis, generating the appearance of immature germ cells in the epididymis. In the seminiferous tubules, MET treatment changed the level of expression or modified the stage-specific localization of tight junction (TJ) proteins ZO-1, ZO-2, occludin, and claudin-3. In contrast, claudin-11 was barely altered. MET also modified the shape of claudin-11, and ZO-2 at the cell border, from a zigzag to a more linear pattern. In addition, MET diminished the expression of ZO-2 in spermatids present in seminiferous tubules, induced the phosphorylation of ZO-2 and occludin in testes and reduced the interaction between these proteins assessed by co-immunoprecipitation. MET formed covalent bonds with ZO-2 in serine, tyrosine and lysine residues. The covalent modifications formed on ZO-2 at putative phosphorylation sites might interfere with ZO-2 interaction with regulatory molecules and other TJ proteins. MET bonds formed at ZO-2 ubiquitination sites likely interfere with ZO-2 degradation and TJ sealing, based on results obtained in cultured epithelial cells transfected with ZO-2 mutated at a MET target lysine residue. Our results shed light on MET male reproductive toxicity and are important to improve regulations regarding the use of OP pesticides and to protect the health of agricultural workers.