Table 3.

Actin binding proteins (ABPs) in the testis ^*

Function al type of actin binding proteins	Actin bindin g protein s	Localization in the seminiferous epithelium	Interactin g partner proteins	Phenotypes after KD or KO in vivo
Actin nucleatio n proteins	Arp3	At the apical ES, it was restricted to the concave side of spermatid heads at stage VII; at the basal ES/BTB, it was not expressed until stage VIII ES[178]	Actin, members of the Arp2/3 complex, N-WASP	Arp3 deficient mice died at blastocysts stage [179]
	Formin 1	At the apical ES, it was restricted to the concave side of spermatid heads at stage VII; at the basal ES/BTB, it was expressed at stages I-VII [180]	Actin, Arp3, α- tubulin-	Formin 1 KO in mice led to a delay in bone formation [181], mice also had 4 digits, a deformed posterior metatarsal, phalangeal soft tissue fusion as well as the absence of a fibula [182]; KD of formin 1 impeded the transport of spermatid and phagosome due to the mis- organization of F-actin at the ES [180].
Actin bundling/ cross- linking proteins	Eps8	At the apical ES, it was restricted to the concave side of spermatid heads at stage VII; at the basal ES/BTB, it was expressed at stages I-VII [118]	Abi-1, IRSp53, Sos1	Eps8 KO mice were less sensitive and more tolerance to ethanol consumption [183]; these Eps8 KO mice appeared to be healthy and fertile [184]; microvilli in cells appeared disorganized and were significantly shorter versus the age-matched WT control mice [185]; Eps8 KD in the testis led to germ cell exfoliation and BTB disruption [118]
	Ezrin	Ezrin was expressed at the basal ES/BTB in all stages of the epithelial cycle except at stage IX when its expression was considerably diminished [88].	Actin, Arp3, FAK, c- Src, JAM- A, laminin- γ3 chain, N- cadherin	Ezrin KD in the testis impeded BTB integrity mediated by changes in the organization of F- actin at the ES [88]; ezrin KO led to death of pups before weaning [186].
	Fascin1	At the apical ES, it was expressed at the concave side of step 19 spermatid heads; at the basal ES/BTB, it was expressed in all stages of the epithelial cycle except VIII [121]	Actin, Arp3, β- catenin, nectin-3, occludin, palladin, Par6	Fascin 1 KO mice had deficient embryonic fibroblasts having shorter and less filopodia and KO mice had shorter life span, but they were viable and fertile [187]; fascin 1 KD in adult rat testes led to dis-organization of F- actin at the apical and basal ES, making the ES incapable of providing proper adhesion support to Sertoli and/or spermatid in which apical ES and TJ/basal ES adhesion proteins were mis-localized [121]
	Palladi n	Highly expressed at the apical ES in all stages, it also expressed at the basal ES/BTB in all stages but considerably down-regulated at VIII [119]	Apr3, ARPC2, c- Src, Eps8	Palladin KO in mice led embryonic lethality due to neural tube closure defects [188]; palladin KD in the testis in vivo perturbed spermatid polarity, leading to defects in spermiation due to spermatid retention in the epithelium [119]
	Plastin 3	At the apical ES, plastin 3 was detected at stages V-VII; at the basal ES/BTB, plastin 3 was found in all stages but considerably diminished at stage VIII [120]	Actin, vimentin	Plastin 3 variants in humans led to higher risk of bone fracture vs. noncarriers [189]; KD of plastin 3 in rat testes in vivo induced structural defects to the F-actin network, leading to premature spermatid release [120]

^*This Table is not intended to be exhaustive, it is based on some recent findings illustrating the role of ABPs in spermatogenesis. Abbreviations used: Abi-1, Abelson interactor 1; Arp3, actin related protein 3; Arpc2, actin-related protein2/3 complex subunit 2; BTB, blood-testis barrier; c-Src, cellular-sarcoma proto-oncogene tyrosine-protein kinase; Eps8, epidermal growth factor receptor pathway substrate 8; ES, ectoplasmic specialization; FAK, focal adhesion kinase; JAM-A, junction adhesion molecule A; KD, knockdown; KO, knockout; N-WASP, neuronal Wiskott-Aldrich Syndrome protein; Par6, partitioning defective 6; TJ, tight junction.