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REVIEW |
The Mary M Wohlford Laboratory for Male Contraceptive Research, Center for Biomedical Research, Population Council, 1230 York Avenue, New York, New York 10065, USA
1 School of Biological Sciences, University of Hong Kong, Hong Kong, People's Republic of China
(Correspondence should be addressed to C Y Cheng; Email: y-cheng{at}popcbr.rockefeller.edu)
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Abstract |
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 Top Abstract IntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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Introduction |
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TopAbstractIntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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-, β-, -catenins, and ZO-1 that tether integral membrane proteins, such as N-cadherin, occludin, JAMs, and nectins, to the cytoskeletal proteins; catenins and ZO-1 also recruit signaling molecules, such as protein kinases and/or phosphatases, to the same site; for reviews, see Lee & Cheng 2004, Mruk & Cheng 2004, Zhang et al. 2005). The net results of these interactions determine whether the integral membrane proteins are structurally linked to the cytoskeletal network, thus affecting the status of cell adhesion in the epithelium. Consequently, it is of interest to identify and investigate proteins that are crucial to protein-protein interactions at the cell-cell interface in the seminiferous epithelium. In this minireview, we focus on a protein family named 14-3-3, which has recently been shown to be a crucial regulator of protein-protein interactions in various epithelial and endothelial cells in mammals. While many of the studies were done in organs other than the testes, except for a few reports (Wine & Chapin 1999, Chapin et al. 2001, W P Wong & C Y Cheng, unpublished observations), we thought it pertinent to critically evaluate recent data in the field to provide a framework with which functional studies can be performed to tackle the role of 14-3-3 in spermatogenesis. |
Background and physico-chemical properties of 14-3-3 |
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TopAbstractIntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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The seven isoforms identified in mammals (β-beta, -gamma, -epsilon, -zeta, -eta, -theta, and -sigma; Table 1) share about 50% amino acid identity and, consequently, highly similar protein conformations to form either homodimers or heterodimers that serve as the functional protein units (Jones et al. 1995, Chaudhri et al. 2003). Each 14-3-3 monomer is composed of nine -helixes and relatively unconserved carboxyl-terminal and amino-terminal regions (Aitken 2006). Theoretically, the rigid helical structure may provide the functional basis for target binding, whereas the flexible carboxyl terminal allows for alternative binding activities (Bridges & Moorhead 2004). The amino-terminal region forms the interface for dimerization, thus determining the specific combinations of isoforms (Bridges & Moorhead 2004, Aitken 2006).
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To date, more than 200 binding partners of 14-3-3 have been reported, most of which were identified based on proteomic studies. These 14-3-3 binding proteins are involved in a wide range of cellular activities, such as transcription, protein synthesis, metabolic pathways, cell cycle, cell signaling, cytoskeletal organization, and cellular trafficking (Dougherty & Morrison 2004, Jin et al. 2004, Kjarland et al. 2006, Shikano et al. 2006). Notably, such 14-3-3 interactome studies were performed in different cell types, with different methodologies (such as co-immunoprecipitation or affinity chromatography), and with the interacting proteins targeting different 14-3-3 isoforms (Kjarland et al. 2006). Thus, upon comparison, only a small proportion of these parameters (e.g. binding partners of 14-3-3) overlap between different 14-3-3 isoforms (Bridges & Moorhead 2005). However, in a study utilizing the same method but targeting different isoforms, a small degree of overlapping was still reported, which seems to suggest isoform specificity even though there is functional redundancy between different 14-3-3 isoforms (Benzinger et al. 2005, Bridges & Moorhead 2005, Kjarland et al. 2006). This isoform specificity has also been substantiated in some functional studies, which are summarized in Table 1. These observations thus illustrate that 14-3-3 is a crucial molecule in conferring protein-protein interactions in different cells. Since it is found abundantly in the testes (Table 1), it is not surprising that this protein may be crucial to Sertoli-Sertoli and/or Sertoli-germ cell interactions during spermatogenesis. In fact, recent studies have demonstrated the presence of several 14-3-3 members in the testis, such as 14-3-3, 14-3-3β, and 14-3-3; and 14-3-3 was found in both Sertoli and germ cells (W P Wong & C Y Cheng, unpublished observations). In a study using immunohistochemistry and fluorescent microscopy (Fig. 1), it was shown that 14-3-3 was localized prominently at the elongating spermatid-Sertoli cell interface, which is consistent with its localization at the apical ectoplasmic specialization (apical ES, a testis-specific atypical adherens junction, AJ, type (for review, see Wong et al. 2008b)). Meanwhile, some 14-3-3 staining was detected in the basal compartment, consistent with its presence at the BTB (Fig. 1; W P Wong & C Y Cheng, unpublished observations).
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Unique features of 14-3-3 |
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TopAbstractIntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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Clamping describes the process through which the conformation of the target protein is modified by the binding of 14-3-3. This action is facilitated by the rigid helical structure of 14-3-3, which shows little change after being bound with a ligand (Liu et al. 1995, Yaffe 2002). Thus, 14-3-3 binding helps to reshape a target protein and, consequently, alters its properties. An example of such a clamping target is serotonin-N-acetyltransferase, which is stabilized and kept in an active conformation by 14-3-3 binding (Obsil et al. 2001). Notably, the binding between serotonin-N-acetyltransferase and 14-3-3 is mediated by two phosphorylation sites, which bind to the two separate subunits of a 14-3-3 dimer (Ganguly et al. 2005). Along with the activation of enzymes, 14-3-3 clamping may also inactivate catalytic activities or alter the distal conformation of the target proteins (Moorhead et al. 1996, Athwal et al. 1998, McKinsey et al. 2001, Bridges & Moorhead 2004).
The masking of a specific region in a target protein by 14-3-3 may alter a sequence-specific property of the target protein, such that access by other possible interacting proteins is blocked. This process can be viewed as a competition between 14-3-3 and any other interacting proteins for the target protein (Mackintosh 2004). For instance, 14-3-3 binding masks the nuclear localization signal of Cdc25C protein (a cell-cycle regulator) and sequesters the protein in the cytoplasm (Peng et al. 1997, Kumagai & Dunphy 1999).
14-3-3 also employs a scaffolding mechanism by bringing two target proteins into proximity and co-localizing them at the same cellular site to carry out their cellular effects. This mechanism is plausibly based on the dimeric structure of 14-3-3, which has two independent binding pockets for target proteins. For instance, if a protein kinase and its substrate are bound in the two binding pockets on 14-3-3 simultaneously, the catalytic activity of the kinase is promoted, given the proximity of the enzyme and its substrate. Moreover, the catalytic activity may be tightly regulated owing to the restricted spatial orientation of the two 14-3-3 binding proteins (Bridges & Moorhead 2004). On the other hand, regulation may be carried out to allow for scaffolding, because both the enzyme and the substrate need to be partially phosphorylated before they bind to 14-3-3 (Bridges & Moorhead 2004). Scaffolding is possibly an important function for cellular signaling processes, which involve various regulated kinase activities.
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Functions of 14-3-3 |
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TopAbstractIntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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As discussed above, Cdc25C protein can be docked in the cytoplasm via its binding to 14-3-3. This is one method by which 14-3-3 regulates protein localization. 14-3-3 also regulates cellular protein localization by modulating endoplasmic reticulum (ER) transportation and, thereby, the subsequent cell-surface expression of its binding partners. The membrane-trafficking mechanism involves retrograde transport mediated by coat protein complex I (COPI) and anterograde transport mediated by coat protein complex II (COPII). If a cargo protein has a significant forward trafficking signal, it will interact with COPII and proceed to the Golgi bodies and other intracellular destinations (Barlowe 2003, Bonifacino & Glick 2004). However, if this cargo protein expresses an ER localization signal, it will be recognized by COPI and transported back to the ER (Ma et al. 2001).
14-3-3 has also been found to bind to a number of membrane proteins and modify their cell-surface expression levels (Mrowiec & Schwappach 2006). The first evidence of this important function came from studies of the potassium channel protein KCNK3 (TASK-1). KCNK3 is a multimeric protein with an N-terminal ER localization signal (i.e. COPI interacting motif) and a C-terminal 14-3-3 binding site (O'Kelly et al. 2002, Rajan et al. 2002). Truncation of the C-terminal 14-3-3 binding site of KCNK3 was shown to alter its cell-surface expression (Rajan et al. 2002), thereby modifying the K+-channel function. Moreover, cell-surface expression of TASK-1 was shown to be abolished with the mutation of serine at the -1 site at the N-terminus (Rajan et al. 2002). In another report, the interaction between KCNK3 and 14-3-3 or COPI was found to be mutually exclusive (O'Kelly et al. 2002). All this evidence suggests that unphosphorylated KCNK3 is recognized by COPI and retained on the ER, whereas 14-3-3 binding with phosphorylated KCNK3 can prevent COPI recognition and allow its cell-surface expression (Mrowiec & Schwappach 2006, Shikano et al. 2006). This thus provides a unique mechanism to regulate the K+-channel function by regulating the steady-state level of KCNK3 protein. Several other membrane proteins, such as the MHC II, KATP-channel subunit, and kainate receptor subunit, have been reported to be subjected to similar regulation in their cell-surface expression (Anderson et al. 1999, Yuan et al. 2003, Vivithanaporn et al. 2006). Furthermore, some evidence suggests that 14-3-3 may also regulate the interaction between COPII and its cargo (Shikano et al. 2006; Fig. 2A). Considering the numerous binding partners of 14-3-3, we suggest that 14-3-3 may function in the seminiferous epithelium during spermatogenesis by regulating protein localization.
Cell-junction dynamics
14-3-3 has been reported to be involved in regulating cell-junction dynamics. One example of such regulation is its involvement in integrin-mediated cell adhesion (Fig. 2B). p130Cas is a docking protein that can form intracellular signaling complexes through integrin-mediated cell adhesion. In both yeast and mammalian cells, 14-3-3 has been found to be associated with p130Cas in a phosphoserine-dependent manner (Garcia-Guzman et al. 1999). Induced by the attachment of cells to the extracellular matrix, p130Cas and 14-3-3 were found to be significantly co-localized upon integrin-ligand binding (Garcia-Guzman et al. 1999). Thus, 14-3-3 seems to participate in the integrin-activated signaling pathways via p130Cas and to further regulate subsequent cell adhesion responses (Fig. 2B). In the testis, the 6β1-integrin/laminin-333 adhesion complex, wherein 6β1-integrin residing in Sertoli cells and laminin-333 (laminin 3/β3/3) restricted to elongating spermatids (Yan & Cheng 2006), is one of the best studied adhesion complexes at the apical ES (Cheng & Mruk 2002, Mruk et al. 2008, Vogl et al. 2008). Recent studies have shown that the anchoring junction restructuring in the seminiferous epithelium mediated by the integrin/laminin protein complex involves the activation of the integrin downstream adaptors, such as p130Cas, which recruit ERK to the apical ES to elicit junction restructuring (Siu et al. 2003, 2005). It was not known at the time how p130Cas can be docked at the apical ES to carry out its function. Since 14-3-3 has been shown to be abundant at the apical ES (Fig. 1), it is likely that 14-3-3 plays a major role in recruiting p130Cas and other regulatory proteins (e.g. ERK MAP kinase) to the apical ES to facilitate its restructuring during spermatid movement in spermiogenesis and spermiation. This possibility should be carefully evaluated in future studies.
Cell polarity
Cell-polarity proteins, such as Par1, Par3, Par6, and Par5 (i.e. members of 14-3-3 in Caenorhabditis elegans), play critical roles in cell polarization. Notably, the Par3/Par6/aPKC complex is a known binding partner of 14-3-3. During cell-cell contact assembly, components of the Par3/Par6/aPKC complex are brought sequentially into contact with the adhesion junction-associated molecules (Assemat et al. 2007, Ebnet et al. 2008; Fig. 2C). In the testis, Par3, Par6, and aPKC have been identified by corresponding specific antibodies in Sertoli and germ cells in the seminiferous epithelium of adult rats (Wong et al. 2008a). The consequent Par3/Par6/aPKC complex has been shown to serve as a crucial polarity complex at the apical ES to confer spermatid orientation, so that the heads of the developing spermatids in the seminiferous epithelium are properly oriented towards the basement membrane (Wong et al. 2008a). More important, it was shown that this Par3/Par6-based polarity complex is also working in concert with proteins at the apical ES (e.g. JAM-C) and proteins at the BTB (e.g. JAM-A and N-cadherin); as a result, the cellular events of spermiation and BTB restructuring that occur simultaneously at the opposite ends of the Sertoli cell epithelium at stage VIII of the seminiferous epithelial cycle can be precisely coordinated (Wong et al. 2008a). While the precise downstream mechanism(s) that coordinates these two concurrent cellular events involving the Par-based polarity complex remains to be elucidated, it is likely that 14-3-3 and two non-receptor protein tyrosine kinases - c-Src and FAK and their corresponding activated forms p-cSrc and p-FAK - are involved. For instance, it is known that c-Src and FAK are components of the BTB (Siu et al. 2003, Lee & Cheng 2005), whereas their activated p-cSrc and p-FAK forms are found mostly at the apical ES (Siu et al. 2003, Wong et al. 2005, Beardsley et al. 2006). It is possible that 14-3-3 serves as the docking platform for protein kinases (e.g. c-Src and FAK) and their activated forms to mediate the junction restructuring events that facilitate spermiation at the apical ES and the transit of preleptotene spermatocytes at the BTB at stage VIII of the cycle. This concept should be vigorously tested in future experiments.
Par3 can be phosphorylated by Par1 on the Ser-151 and Ser-1085 residues. These two phosphorylated Ser residues are the putative binding sites for Par5/14-3-3 (Benton & Johnston 2003, Hurd et al. 2003), allowing these proteins to form a protein complex (Fig. 2C). The binding of 14-3-3 with the Ser-151 residue blocks the oligomerization and functioning of Par3, and the masking of the Ser-1085 residue inhibits the binding of aPKC (Nagai-Tamai et al. 2002, Benton & Johnston 2003, Brajenovic et al. 2004; Fig. 2C). Consequently, if the cell membrane domain has active Par1 proteins associated, the phosphorylated Par3 will be incapable of forming a functional Par3/Par6/aPKC complex at the cell membrane (Ebnet et al. 2008; Fig. 2C), leading to changes in polarity.
Interestingly, Par1 itself can also be phosphorylated by aPKC at Thr-595 and bind to 14-3-3. The binding inhibits the kinase activity of Par1 and releases it from cell membrane (Hurov et al. 2004, Suzuki et al. 2004). Using this mechanism, the functional Par3/Par6/aPKC complex can be localized at the desired cellular site (Fig. 2C). Another study has shown several other phosphorylation sites on Par1 to be important for 14-3-3 binding, and the simultaneous mutation of those phosphorylation sites restricted Par1 at the plasma membrane (Goransson et al. 2006). These results suggest that complex interactions exist among polarity proteins, facilitating the formation and/or dissolution of the polarity protein complex during the epithelial cycle of spermatogenesis, and that 14-3-3 is a crucial regulator mediating protein-protein interactions. For instance, a study has reported that Par6, Pals1, and JAM-C interact at the apical ES in normal rat testes (Wong et al. 2008a). However, at the time of germ cell depletion from the epithelium induced by adjudin, Par6, Src, and Pals1 formed a tighter protein complex, which, in turn, weakened the association of Pals1 and Par6 with JAM-C, destabilizing the JAM-C-based adhesion, and leading to germ cell loss from the epithelium (Wong et al. 2008a). It is plausible that 14-3-3 plays a crucial role in the mediation of protein-protein interactions at the apical ES involving Pals1, Par6, Src, and JAM-C.
In addition to its likely involvement in the apical ES restructuring, 14-3-3 was also implicated in the regulation of BTB restructuring during spermatogenesis (Wong et al. 2008a; Fig. 3). For instance, it has been shown that Par3, Par6, aPKC and JAM-A interact at the BTB to maintain cell adhesion at the site (Wong et al. 2008a). At stage VIII of the epithelial cycle, when primary preleptotene spermatocytes are in transit at the BTB, Par3/Par6/aPKC complexes disassemble, which thus destabilizes JAM-A-based cell adhesion, facilitating the transit of primary spermatocytes at the BTB (Fig. 3). It is likely that 14-3-3 plays a role at the BTB in mediating protein-protein interactions involving Par3, Par6, aPKC, and/or JAM-A (Fig. 3).
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Regulation |
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TopAbstractIntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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Two important regulators involved in 14-3-3 binding are the protein phosphatases PP1 and PP2, which dephosphorylate the desired Ser or Tyr residue(s) at the 14-3-3 binding sites. In the binding between phosphorylated Par3 and 14-3-3 discussed above, PP1 plays an important regulatory role by dephosphorylating multiple sites on Par3, thus controls the binding of 14-3-3 and aPKC to Par3 (Traweger et al. 2008). A similar regulatory pathway exists for Cdc25C, which can be released from 14-3-3 by PP1 and subsequently translocated to the nucleus (Margolis et al. 2003).
The activity of 14-3-3 proteins can also be regulated by phosphorylation. Several protein kinases, such as protein kinase B, protein kinase C, and casein kinase 1, have been shown to be capable of phosphorylating 14-3-3. Moreover, these proteins have been positively identified in the testis and are known to be expressed by Sertoli and/or germ cells (Lee & Cheng 2005, Siu et al. 2005). For instance, since p-PKB-Thr308 and p-PKB-Ser473 were both shown to be induced during anchoring junction disruption at the Sertoli cell-germ cell interface, the phosphorylation of 14-3-3 is likely to occur at the apical ES. It has been reported that 14-3-3 phosphorylation always results in decreased target binding (van Heusden 2005). For 14-3-3, its dimerization and consequent activity can also be regulated by phosphorylation, in which the phosphorylated Ser58 disrupts the dimeric conformation (Woodcock et al. 2003). In short, these findings have demonstrated that the effects of 14-3-3 on protein-protein interactions at the apical ES and BTB in the seminiferous epithelium are regulated by its interactions with many of the recently reported adaptors (e.g. p130Cas), protein kinases (e.g. PKB and Src), polarity proteins (e.g. Par6 and Pals1), and integral membrane proteins (e.g. JAM-A, JAM-C, and integrins).
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Concluding remarks and future perspectives |
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TopAbstractIntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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Declaration of interest |
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TopAbstractIntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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Funding |
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References |
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TopAbstractIntroductionBackground and physico-chemical...Unique features of 14-3-3Functions of 14-3-3RegulationConcluding remarks and future...Declaration of interestReferences |
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Received in final form 2 April 2009
Accepted 14 April 2009
Made available online as an Accepted Preprint 14 April 2009
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