Regulation of Blood-Testis Barrier (BTB) Dynamics, Role of Actin-, and Microtubule-Based Cytoskeletons

Abstract

The blood-testis barrier (BTB) is an important ultrastructure in the testis that supports meiosis and postmeiotic spermatid development since a delay in the establishment of a functional Sertoli cell barrier during postnatal development in rats or mice by 17-20 day postpartum (dpp) would lead to a delay of the first wave of meiosis. Furthermore, irreversible disruption of the BTB by toxicants also induces infertility in rodents. Herein, we summarize recent findings that BTB dynamics (i.e., disassembly, reassembly, and stabilization) are supported by the concerted efforts of the actin- and microtubule (MT)-based cytoskeletons. We focus on the role of two actin nucleation protein complexes, namely, the Arp2/3 (actin-related protein 2/3) complex and formin 1 (or the formin 1/spire 1 complex) known to induce actin nucleation, respectively, by conferring plasticity to actin cytoskeleton. We also focus on the MT plus (+)-end tracking protein (+TIP) EB1 (end-binding protein 1) which is known to confer MT stabilization. Furthermore, we discuss in particular how the interactions of these proteins modulate BTB dynamics during spermatogenesis. These findings also yield a novel hypothetical concept regarding the molecular mechanism that modulates BTB function.

Keywords: Blood-testis barrier; Desmosome; Ectoplasmic specialization; Gap junction; Seminiferous epithelial cycle; Sertoli cell; Spermatogenesis; Testis; Tight junction.

Fig. 1

Illustrations on the relative localization of F-actin-, MT-, and vimentin-based cytoskeletons in the testis and selected regulatory and structural proteins plus their functional domains and organization. (a) The relative localization of F-actin network and the basal ES/BTB proteins ZO-1 and ß-catenin and nucleation proteins Arp3 and formin 1 in cross sections of adult rat testes vs. the MT (α-tubulin)- and intermediate filament (vimentin)-based cytoskeletons and EB1 (a +TIP protein) using corresponding specific antibodies was performed by immunofluorescence analysis as earlier described [15, 17, 18, 81]. The relative location of the BTB is annotated by the yellow arrowheads. It is noted that F-actin, MT, and intermediate filaments also form some track-like structures that lay perpendicular to the basement membrane to support germ cell and organelle transport. Cell nuclei were visualized by DAPI staining. Scale bar, 80 μm, which applies to all other micrographs. (b) This is a schematic drawing illustrating the intact BTB of a stage VII to early stage VIII tubule and the association of different regulatory and nucleation proteins with the corresponding F-actin- and MT-based cytoskeletons. Also shown are the structural adhesion proteins of the BTB. (c) Functional domains of nucleation proteins spire 1 and formin 1 and the simplified models of the two proteins (boxed in rectangles). (d) Functional domains of barbed end nucleation protein complex N-WASP/Arp2/3 and the simplified model of N-WASP (boxed in rectangle). (e) Functional domains of MT regulatory protein EB1, a +TIP protein known to stabilize MTs, and the simplified model of EB1. For (c–e), the illustrations on the right panels illustrate the models by which the corresponding proteins function in tissues including the seminiferous epithelium in the rat testis. Abbreviations: +TIP plus (+)-end microtubule tracking protein, A acidic domain, Arp2/3 complex actin-related protein 2 and 3 complex, B basic domain, C central region, CC coiled-coil domain, CH calponin homology, CID α-catenin interacting domain, CRIB Cdc42/Rac-interactive binding, DAD diaphanous autoregulatory domain, DD dimerization domain, DID diaphanous inhibitory domain, EB1 end-binding protein 1 (a +TIP protein), EBH end-binding protein homology, EEY/F C-terminal EEY/F motif, EF hand calcium-binding motif, FH formin homology, FSI formin-spire interaction motif, FYVE Fab1/YOTB/Vac1/EEA1 zinc-binding domain, GBD GTPase-binding domain, KIND kinase noncatalytic C-lobe domain, MTB microtubule-binding domain, PRD proline-rich domain, N-WASP neuronal Wiskott-Aldrich syndrome protein, SB spire box, WH1 domain WASP homology domain 1, WH2 domain WASP homology domain 2, ZO-1 zonula occludens 1

Fig. 2

A schematic drawing that illustrates the dynamic BTB restructuring is mediated by changes in the spatial expression of actin- and MT-binding/regulatory and nucleation proteins. The left panel depicts a schematic stage VII (or early stage VIII) tubule with an intact BTB supported by proper organized F-actin and MT networks. At stage VIII (right panel), the N-WASP/Arp2/3 complex is activated, causing actin microfilaments to assume a branched configuration, no longer capable of supporting the actin filaments to assume a bundled configuration. EB1 is also moving away from MTs, failing to stabilize the MT network and causing a disruption of the “old” BTB to facilitate the transport of preleptotene spermatocytes across the immunological barrier with a concomitant assembly of a “new” BTB behind the preleptotene spermatocyte. See text for details