Germ Cell-Sertoli Cell Junction Dynamics
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Germ Cell-Sertoli Cell Junction Dynamics

Sertoli-Germ (Spermatid)-cell interactions affect spermatogenesis at the molecular, cellular and biochemical levels. Germ cell movement within the epithelium is vital because germ cells, if induced to release into the tubule lumen prematurely, will be unable to fertilize the ovum. On the other hand, if germ cells are forced to remain attached to the seminiferous epithelium for a period of time longer than necessary to complete their development, they will degenerate and eventually be phagocytosed by Sertoli cells and for this, proper regulation of germ cell migration in the seminiferous epithelium and at blood-testes barrier (abbreviated as BTB) is essential (Ref.1). Spermatid/germ cell and sertoli cell interactions in the testis occur through specialized junctions at sites of cell-cell and cell-matrix contact which includes Cadherin-Cadherin junctions; Laminin (LamC3 (Laminin-Gamma-3))-Integrin (Itg-Alpha6 (Integrin-Alpha-6) and Itg-Beta1 (Integrin-Beta-1)) and Nectin junctions. Their involvement in junction restructuring in the seminiferous epithelium, especially at the ectoplasmic specialization (ES), a testis-specific adherens junction (AJ) type contributes to the adhesion between sertoli cells at the blood-testis barrier, as well as between sertoli and developing spermatids at the adluminal compartment (Ref.2 & 3).

The apical ES is constituted by at least three different protein complexes at the Sertoli cell side; Cdh (Cadherin)/Ctnn (Catenin), Nectin2/Afadin, and Integrin/FAK (Focal Adhesion Kinase), which interact correspondingly with Cadherins, Nectin2/3 and LamC3 found on the spermatid surface. The N- and E-Cadherin of the sertoli cells bind to different Catenin members like Ctnn-Alpha (Catenin-Alpha), Ctnn-BetaCtnn-Gamma and p120Ctn (p120-Catenin). These in turn activate the kinases like Fer and c-SrcFer kinase regulates only the N-Cadherin/Catenin complex but not the E-Cadherin/Catenin complex. This is followed by subsequent activation of IQGAP1MTMR2 (Myotubularin-Related Protein-2), the adaptors like Axin and Zyx (Zyxin), cytoskeletal regulators like WASP (Wiskott-Aldrich Syndrome Protein), Fimbrin, Epsin (Epsin), Alpha-Actn (Alpha-Actinin); and the adhesion molecule like ZO1 (Zona Occludens-1). IQGAP1 acts as a negative regulator of cell-cell adhesion which remains associated with E-Cadherin, N-Cadherin, and Ctnn-Beta (but not Itg-Beta1 and Nectin2), as well as with Actin and Vimentin (but not Alpha-Tubulin). Moreover, IQGAP1 localize to the periphery of both sertoli and germ cells in the seminiferous epithelium, at sites of cell-cell contacts. Activation of cell adhesion molecules in turn activates KEAP1 (Kelch-Like ECH-Associated Protein-1) to enhance F-Actin/Myosin7A/Tubulin cross-bridging, Actin polymerization and cell adhesion. Further Cadherin activated MTMR2 and Rab8B modulate Actin polymerization through dephosphorylation inositol lipids and Cfl (Cofilin) disintegration, respectively (Ref.4 & 5). On the spermatid surfaces the Cadherin molecules associate with Rab8B, p120Ctn, Ctnn-Alpha, Ctnn-Beta, Ctnn-Gamma, Axin, Zyx, Fimbrin, EpsinWASP and MTMR2 to increase F-Actin/T-Actin/Myosin/Tubulin cross-bridging leading to cell adhesion. Firm adhesion is also achieved when Nectin2/3 on the spermatid side interacts with Nectin2 of the sertoli cells. Within the germ cells Nectin2/3 remains associated with Afadin, Ponsin, Ctnn-AlphaCtnn-Beta and Ctnn-Gamma, whereas Nectin2 on the sertoli cells binds with Afadin, Ponsin, Ctnn-Alpha, ZO1 and Alpha-Actn . Nectin2/3 and Nectin2 interaction functions as an important regulator of F-Actin/T-Actin/Myosin7A/KEAP1/Tubulin/Epsin complexes to increase Actin polymerization and cross-bridging and in turn cell adhesion (Ref.4).

Germ cell adhesion and migration is also dependant on release of hormones (Testosterone) and cytokines (TGF-Beta3 (Transforming Growth Factor-Beta-3) and TNF-Alpha (Tumor Necrosis Factor-Alpha)). The LamC3 on the germ cell surface binds with Itg-Alpha6/Itg-Beta1 leading to Actin reorganization and ES stabilization, which is in part controlled by Testosterone levels in the testis. High Testosterone levels favor ES reassembly and preserve its integrity. LamC3 and Testosterone induced activation of Integrins phosphorylates FAK (Focal Adhesion Kinase) at Tyr397 (Tyrosine-397) and enhances binding of FAK with kinases like PI3K (Phosphatidylinositde-3-Kinase), c-Src and ILK (Integrin-Linked Kinase). FAK binds to the regulatory p85 subunit of PI3K and subsequently recruits its p110 catalytic subunit to the plasma membrane, where it phosphorylates PIP2 (Phosphatidylinositol-4,5-Bisphosphate) at position-3 of the inositol ring, producing a second messenger, PIP3 (Phosphatidylinositol-3,4,5-Trisphosphate). The accumulated PIP3 acts as a membrane anchor that recruits and activates Akt1 (v-Akt Murine Thymoma Viral Oncogene Homolog-1), which is a PI3K effector. The membrane-localized Akt1 is then activated via phosphorylation at Thr308 (Threonine-308) by PDK-1 (Phosphoinositide-Dependent Kinase-1). Both c-Src and phosphorylated Akt1 activates PAK (p21-Activated Kinase), which in turn activate ERK (Extracellular Signal-Regulated Kinase) and LIMK (LIM Kinase) (Ref.2 & 4). The Integrin>Rho GTPases>Rac is another vital route for PAK activation. ERK is also activated by the upstream kinases like FAK and Akt1. Upon activation these kinases, PAK and ERK bring about Cfl disintegration and Actin filament stabilization. Integrins and FAK also associates and activates proteins like p130CASPxn  (Paxillin), Gsn (Gelsolin) and Vcl (Vinculin), which in turn activate Fimbrin/Alpha-Actn /Epsin/F-Actin/Tubulin/KEAP1/Myosin7A complexes to increase Actin polymerization and cell adhesion (Ref.2).

However presence of cytokines like TGF-Beta3 and TNF-Alpha disrupts junction dynamics and helps in the timely release of spermatids by promoting activation of p38, ERK and JNK (c-Jun Kinase) pathways. TGF-Beta3/TGF-BetaR3 (Transforming Growth Factor-Beta Receptor) activates Ras>MEK (MAPK/ERK Kinase)>ERK and MEKK (MAP/ERK Kinase Kinase)> MKK (Mitogen-Activated Protein Kinase Kinase)>p38 routes that leads to Actin depolarization, apical ES disruption and BTB disruption resulting in sprematid/germ cell migration (Ref.6). Similarly, TNF-Alpha/TNF-AlphaR (Tumor Necrosis Factor-Alpha-Receptor) activates CDC42 (Cell Division Cycle-42)/Rac>MKK>JNK signaling route to inhibit F-Actin polymerization. JNK cascade also limits proteolysis of Actin and other cytoskeletal modulators by regulating the production of protease inhibitor, Alpha2M (Alpha-2-Macroglobulin). Alpha2M binds to the extracelluar TGF-Beta3 and TNF-Alpha and inhibits subsequent ligand/receptor interaction. Alpha2M acts as a possible activator that promotes cell adhesion (Ref.7). As such, Sertoli cells are obliged to synthesize, secrete, and efficiently deliver products that are essential for the growth and differentiation of developing germ cells. The blood-testis barrier contributes directly to the complex structural organization of the testis by creating a specialized environment necessary for germ cell development and movement. Further advances in molecular mechanisms that regulate ES and BTB dynamics may decipher precise roles of different junction molecules, cytokines and hormones pertinent to spermatogenesis and fertility (Ref.8).