Figure 3.

For figure legend, see next page. Figure 3 (See previous page). A hypothetical model that illustrates the involvement of formin 1 in regulating actin microfilament organization at the ES. (A) At the apical ES (left panel), formin 1 is not expressed in any stage of the epithelial cycle such as at stage VI in which bundles of actin microfilaments serve as the attachment sites for adhesion protein complexes, such as laminina-333/α6β1-integrin and nectin 2/nectin 3 adhesion protein complexes. However, formin 1 is robustly expressed at the concave side of spermatid heads at stage VII (right panel) in an ultrastructure known as the apical TBC, representing giant endocytic vesicles (see enlagred view in the boxed area). This endocytic vesicle-mediated protein trafficking event is facilitated by changes in the organization of actin microfilaments via the concerted efforts of the Arp2/3 complex and formin 1 (note: both Arp3 and formin 1 are highly expressed and localized to the same site in stage VII tubules) which, in turn, facilitate the events of endocytosis, transcytosis and recycling so that apical ES proteins can be recycled to assemble “new” apical ES for the newly derived step 8 spermatids that appear in stage VIII of the cycle. (B) Formin 1 is also robustly expressed at the BTB in stages V-VII, likely being used to maintain the actin microfilaments and to facilitate their organization into a bundled configuration to serve as attachment sites for adhesion protein complexes (left panel). At stage VIII, formin 1 expression at the basal/BTB is considerably diminished, and the Arp3 expression is up-regulated at the site. The intrinsic activity of Arp2/3 complex thus induces branched actin polymerization, converting the microfilament bundles into a branched/unbundled configuration to facilitate the events of endocytosis, transcytosis and recycling so that the “old” BTB adhesion proteins can be recycled to assemble a “new” BTB behind the preleptotene spermatocyte being transported across the immunological barrier (right panel). In short, actin microfilaments at the apical ES and the basal ES/BTB can be rapidly re-organized, through the unique and stage-specific spatiotemporal expression of formin 1, Arp3, and possibly other actin bundling proteins (e.g., Eps8, fascin 1, palladin) known to be expressed at the site.