Cell polarity and cytoskeletons-Lesson from the testis

Abstract

Cell polarity in the adult mammalian testis refers to the polarized alignment of developing spermatids during spermiogenesis and the polarized organization of organelles (e.g., phagosomes, endocytic vesicles, Sertoli cell nuclei, Golgi apparatus) in Sertoli cells and germ cells to support spermatogenesis. Without these distinctive features of cell polarity in the seminiferous epithelium, it is not possible to support the daily production of millions of sperm in the limited space provided by the seminiferous tubules in either rodent or human males through the adulthood. In short, cell polarity provides a novel mean to align spermatids and the supporting organelles (e.g., phagosomes, Golgi apparatus, endocytic vesicles) in a highly organized fashion spatially in the seminiferous epithelium during the epithelial cycle of spermatogenesis. This is analogous to different assembling units in a manufacturing plant such that as developing spermatids move along the "assembly line" conferred by Sertoli cells, different structural/functional components can be added to (or removed from) the developing spermatids during spermiogenesis, so that functional spermatozoa are produced at the end of the assembly line. Herein, we briefly review findings regarding the regulation of cell polarity in the testis with specific emphasis on developing spermatids, supported by an intriguing network of regulatory proteins along a local functional axis. Emerging evidence has suggested that cell cytoskeletons provide the tracks which in turn confer the unique assembly lines in the seminiferous epithelium. We also provide some thought-provoking concepts based on which functional experiments can be designed in future studies.

Keywords: Actin; Actin nucleation; Blood-testis barrier; Cell polarity; Cytoskeleton; Ectoplasmic specialization; Microtubules; Seminiferous epithelial cycle; Sertoli cells; Spermatids; Spermatogenesis; Testis.

Figure 1. Role of actin- and microtubule (MT)-based cytoskeletons to support cell polarity and spermatogenesis in the mammalian testis

On the left/top panel, the micrographs illustrate the organization of F-actin (green fluorescence, visualized by Alexa Fluor 488 phalloidin) across the seminiferous epithelium in a stage VII tubule with the localization of F-actin at the apical ES and basal ES/BTB enlarged in the yellow and green boxed areas, respectively, and shown below. On the left/bottom panel, the micrographs illustrate the organization of α-tubulin (the building blocks of microtubules (MTs); green fluorescence, visualized by an anti-α-tubulin antibody and an Alexa Fluor 488-conjugated secondary antibody) across the epithelium in a stage VII tubule with the localization of MT at the apical ES and basal ES/BTB enlarged in the yellow and green boxed areas, respectively, and shown below. Cell nuclei were visualized by DAPI. The MT-conferred tracks to support cell and organelle transports are distinctively noted. Scale bar, 80 µm; insets (in yellow or green), 25 µm; which apply to corresponding micrographs/insets. On the right panel, it is the schematic drawing illustrating the organization of actin microfilaments and MTs that support the apical ES and also basal ES which is also an integrated component of the BTB. Also shown as the several known cell adhesion complexes in both sites.

Figure 2. A schematic drawing that illustrates the contrasting roles of the Par- and Crumbs-based polarity complexes vs. the Scribble-based polarity complex to support ES dynamics in the testis

It is noted that both the Par- and Crumbs-based polarity complexes and their component proteins support apical and basal ES integrity whereas the Scribble-based polarity complex (and its component proteins) promotes ES disruption (see text for details). Abbreviations used: aPKC, atypical protein kinase C; Cdc42, cell cycle division 42; CRB, Crumbs; CRB3, Crumbs homolog-3; Dlg, discs large; ES, ectoplasmic specialization; Lgl, lethal giant larvae; Par3, partitioning-defective; Par6, partitioning-defective 6; Pals1, protein associated with Lin-7 1; PatJ, Pals1 associated tight junciton protein.

Figure 3. A schematic drawing that illustrates the role of several autocrine peptides: F5-peptide, NC1 domain peptide and 80 kDa fragment, in the apical ES-basal ES/BTB-basement membrane functional axis to support spermatogenesis

It is noted that F5-peptide derived from the laminin-γ3 chain at the apical ES, NC1 domain peptide from the collagen α3 (IV) chains in the basement membrane, and also the 80 kDa fragment from the laminin-α2 chain in the basement membrane are biologically active autocrine-based peptides in the apical ES-basal ES/BTB-basement membrane functional axis to regulation spermatogenesis. In short, both F5-peptide and NC1 domain peptide induce apical ES degeneration and basal ES remodeling, whereas the 80 kDa fragment derived from the laminin-α2 chain promotes ES integrity. At this writing, the other two laminin chains that constitute the functional ligand with laminin-α2 chain in the basement membrane remain unknown. Also, the identity of the integrin-α chain that form bona fide complex with integrin-β1 chain to serve as the receptor in the basement membrane remains unknown. (see text for details).

Figure 4. A schematic drawing that illustrates the likely biological interactions between the three polarity protein complexes and the autocrine-based peptides in the apical ES-basal ES/BTB-basement membrane to support spermatogenesis in particular spermatid polarity