Formin 1 regulates ectoplasmic specialization in the rat testis through its actin nucleation and bundling activity

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Article (peer-reviewed)

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During spermatogenesis, developing spermatids and preleptotene spermatocytes are transported across the adluminal compartment and the blood-testis barrier (BTB), respectively, so that spermatids line-up near the luminal edge to prepare for spermiation, while preleptotene spermatocytes enter the adluminal compartment to differentiate into late spermatocytes to prepare for meiosis I/II. These cellular events involve actin microfilament re-organization at the testis-specific/actin-rich Sertoli-spermatid and Sertoli-Sertoli cell junction called apical and basal ectoplasmic specialization (ES). Formin 1, an actin nucleation protein, known to promote actin microfilament elongation and bundling, was expressed at the apical ES but limited to stage VII of the epithelial cycle, whereas its expression at the basal ES/BTB stretched from stage III-VI, diminished in VII and undetectable in VIII tubules. Using an in vitro model of studying Sertoli cell BTB function by RNAi and biochemical assays to monitor actin bundling and polymerization activity, a knockdown of formin 1 in Sertoli cells by ∼70% impeded the tight junction (TJ)-permeability function. This disruptive effect on the TJ-barrier was mediated by a loss of actin microfilament bundling and actin polymerization capability mediated by changes in the localization of branched actin-inducing protein Arp3, and actin bundling proteins Eps8 and palladin, thereby disrupting cell adhesion. Formin 1 knockdown in vivo was found to impede spermatid adhesion, transport and polarity, causing defects in spermiation in which elongated spermatids remained embedded into the epithelium in stage IX tubules, mediated by changes in the spatiotemporal expression of Arp3 and Eps8. In summary, formin 1 is a regulator of ES dynamics.