Figure 2. Spatiotemporal expression of Arp3, Eps8 and palladin at the apical ES at stages VII and VIII of the epithelial cycle in adult rat testes.

Localization of barbed end nucleation inducing protein Arp3 (red fluorescence) that effectively “de-bundled” actin microfilaments at the apical ES to form a branched network in the seminiferous epithelium, which also co-localized with F-actin in stage VII tubules. Arp3 is annotated by a “yellow” arrow at the concave (ventral) side of the spermatid head. On the other hand, Eps8, an actin barbed end capping and bundling protein, and palladin, an actin bundling protein, both are being used to confer the bundling of actin microfilaments at the apical ES, were found to localize both at the concave (“yellow” arrow) and convex (dorsal) side (“white” arrow) of the spermatid head, and co-localized, at least in part, with F-actin. In short, conversion of F-actin to an “un-bundled” configuration at the concave side facilitates endocytic vesicle-mediated protein trafficking, such as endocytosis, transcytosis and recycling. But the spatiotemporal expression of Eps8 and palladin surrounding the spermatid head also allow rapid conversion of F-actin between “bundled” and “un-bundled” configuration to facilitate the transport of spermatids across the seminiferous epithelium. At stage VIII, when spermatids are prepared for spermiation, the expression of these proteins are considerably down-regulated. It is of interest to note that since Arp3 is predominantly expressed at the concave side of spermatid heads, this leads one to wonder how can actin microfilaments reorganize at the convex side of spermatid heads? We offer two explanations. First, this can be achieved by inactivation of palladin and/or Eps8 at the site. Second, it may be mediated by spatiotemporal expression of filamin A (an actin cross-linker protein that induces microfilaments to a branched/mesh-like network [85, 115]). Scale bar, 10 μm, which applies to other micrographs in the same panel.