A network of spectrin and plectin surrounds the actin cuffs of apical tubulobulbar complexes in the rat

Long noncoding RNA regulation of spermatogenesis via the spectrin cytoskeleton in Drosophila

The spectrin cytoskeleton has been shown to be critical in diverse processes such as axon development and degeneration, myoblast fusion, and spermatogenesis. Spectrin can be modulated in a tissue specific manner through junctional protein complexes, however, it has not been shown that long noncoding RNAs (lncRNAs) interact with and modulate spectrin. Here, we provide evidence of a lncRNA CR45362 that interacts with α-Spectrin, is required for spermatid nuclear bundling during Drosophila spermatogenesis. We observed that CR45362 showed high expression in the cyst cells at the basal testis, and CRISPR-mediated knockout of CR45362 led to sterile male, unbundled spermatid nuclei, and disrupted actin cones. Through chromatin isolation by RNA precipitation-mass spectrometry (ChIRP-MS), we identified actin-spectrin cytoskeletal components physically interact with the lncRNA CR45362. Genetic screening on identified cytoskeletal factors revealed that cyst cell-specific knockdown of α-Spectrin phenocopied CR45362 mutants and resulted in spermatid nuclear bundle defects. Consistently, CR45362 knockout disrupted the co-localization of α-Spectrin and spermatid nuclear bundles in the head cyst cells at the basal testis. Thus, we uncovered a novel lncRNA CR45362 that interacts with α-Spectrin to stabilize spermatid nuclear bundles during spermatid maturation.

JMY expression by Sertoli cells contributes to mediating spermatogenesis in mice

Sertoli cells are crucial for spermatogenesis in the seminiferous epithelium because their actin cytoskeleton supports vesicular transport, cell junction formation, protein anchoring, and spermiation. Here, we show that a junction-mediating and actin-regulatory protein (JMY) affects the blood-tissue barrier (BTB) function through remodeling of the Sertoli cell junctional integrity and it also contributes to controlling endocytic vesicle trafficking. These functions are critical for the maintenance of sperm fertility since loss of Sertoli cell-specific Jmy function induced male subfertility in mice. Specifically, these mice have (a) impaired BTB integrity and spermatid adhesion in the seminiferous tubules; (b) high incidence of sperm structural deformity; and (c) reduced sperm count and poor sperm motility. Moreover, the cytoskeletal integrity was compromised along with endocytic vesicular trafficking. These effects impaired junctional protein recycling and reduced Sertoli cell BTB junctional integrity. In addition, JMY interaction with actin-binding protein candidates α-actinin1 and sorbin and SH3 domain containing protein 2 was related to JMY activity, and in turn, actin cytoskeletal organization. In summary, JMY affects the control of spermatogenesis through the regulation of actin filament organization and endocytic vesicle trafficking in Sertoli cells.

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 endoplasmic reticulum, calcium signaling and junction turnover in Sertoli cells

The endoplasmic reticulum (ER) forms a continuous network throughout morphologically differentiated Sertoli cells. It is an integral component of intercellular adhesion junctions in this cell type, as well as forming membrane contact sites with the plasma membrane and intracellular organelles. One of the major functions of the ER in cells generally is maintaining calcium homeostasis and generating calcium signals. In this review, we discuss what is currently known about the overall pattern of distribution of the ER in Sertoli cells and the location of calcium regulatory machinery in the various subdomains of the organelle. Current data are consistent with the hypothesis that calcium signaling by the ER of Sertoli cells may play a significant role in events related to junction remodeling that occur in the seminiferous epithelium during spermatogenesis.

An Alternative Model of Tubulobulbar Complex Internalization During Junction Remodeling in the Seminiferous Epithelium of the Rat Testis

Tubulobulbar complexes (TBCs) are elongate subcellular machines responsible for internalizing intercellular junctions during sperm release. Each complex consists of a double-membrane tubular core terminating in a clathrin-coated pit. The core is surrounded by a network of actin filaments, and a distinct swelling or bulb, which lacks an association with actin, develops in the distal third of the structure. The bulb eventually buds from the complex and enters endocytic compartments of the Sertoli cell. The relationship of the actin cuff to the formation and budding of the bulb is not known. To gain insight into this relationship, we perturbed the actin networks of TBCs with cytochalasin D. When isolated testes were perfused with a physiological buffer containing cytochalasin D, apical TBCs at stage VII of spermatogenesis were associated with lower levels of actin compared to controls. At the ultrastructural level, the actin networks in cytochalasin D-treated testes appeared patchy, and ectopic bulbs and swollen tubular regions occurred. When normal untreated samples at early stage VII were analyzed, large elongate bulbs and short tubular sections were observed. Together, these results suggest a new model for TBC vesiculation in which the actin network begins to disassemble and the tubular region begins to swell into a bulb. As actin disassembly continues, the coated pit and most of the tubular region are incorporated into the enlarging bulb. The remaining short neck of the bulb near the base of the complex undergoes scission, and the bulb is internalized.

Novel clathrin/actin-based endocytic machinery associated with junction turnover in the seminiferous epithelium

Tubulobulbar complexes are elaborate clathrin/actin related structures that form at sites of intercellular attachment in the seminiferous epithelium of the mammalian testis. Here we summarize what is currently known about the morphology and molecular composition of these structures and review evidence that the structures internalize intercellular junctions both at apical sites of Sertoli cell attachment to spermatids, and at basal sites where Sertoli cells form the blood-testis barrier. We present updated models of the sperm release and spermatocyte translocation mechanisms that incorporate tubulobulbar complexes into their designs.