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

Abstract

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.

Figure 1. A schematic drawing illustrating the likely mechanism of spermatid transport across the seminiferous epithelium during spermatogenesis utilizing the microtubule-based cytoskeleton and the intricate relationship between MT and F-actin-rich apical and basal ES

Microtubules are orientated with their plus (+) and minus (−) ends in the Sertoli cell of the seminiferous epithelium and stabilized by MAP, which are being used as the track for spermatids (cargoes) to transport across the epithelium, involving motor proteins (e.g., myosin VIIa), at different stages of the epithelial cycle, moving “up” and “down” the epithelium until fully developed elongated spermatids detach from the epithelium at spermiation. The precise mechanism of spermatid transport remains unknown since many of the crucial molecules involved in this event have yet to be identified and studied in the testis. Abbreviations used:

Figure 2. Functional domains of MARK

MARKs have 6 distinctive functional domains: N-terminal header (N), catalytic kinase domain (KD), common docking domain (CD), ubiquitin-associated domain (UBA), spacer region, and tail domain at the C-terminus.