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Adherens Junction Dynamics in the Testis and Spermatogenesis

WILL M. LEE

From the ^* Population Council, Center for Biomedical Research, New York, NY 10021; and Department of Zoology, The University of Hong Kong, Hong Kong, China.

Correspondence to: C. Yan Cheng, Population Council, 1230 York Avenue, New York, NY 10021 (e-mail: y-cheng{at}popcbr.rockefeller.edu).

Received for publication August 7, 2002; accepted for publication September 27, 2002.

Structure and Molecular Composition

Four structurally and functionally different forms of anchoring junctions exist: 1) AJs between cells, 2) focal contacts between cells and the extracellular matrix (ECM), 3) desmosomes between cells, and 4) hemidesmosomes between cells and the ECM (for a review, see Alberts et al, 2002). Anchoring junctions are subdivided into two categories based on their connection sites. Both cell-cell AJs and cell-matrix focal contacts are connected to actin filaments, whereas desmosomes and hemidesmosomes are connected to intermediate filaments (for a review, see Alberts et al, 2002). The cell-cell, actin-based AJ is by far the best studied adhering junction type in the testis, and includes ectoplasmic specialization (ES), a testis-specific AJ between Sertoli and germ cells. In the testis, AJs confer adhesion between cells, which is known to be mediated by three AJ-integral membrane protein complexes; namely, cadherin/catenin, nectin/afadin, and integrin/laminin (for reviews, see Taga and Suginami, 1998; Rowlands et al, 2000; Vogl et al, 2000; Cheng and Mruk, 2002) (Figure 1 and Table 1). These three complexes in turn are connected to the actin cytoskeleton (for reviews, see Kemler, 1993; Gumbiner, 1996; Yap et al, 1997; Miyahara et al, 2000; Tachibana et al, 2000) (Figure 1; Table 2). For instance, E- or N-cadherin/ß-catenin complex interacts with the actin network via -catenin, whereas afadin is a putative F-actin-binding protein that also links the nectin/afadin complex to the actin cytoskeleton network (Figure 1). However, recent studies have shown that afadin can also interact with ZO-1, ponsin (an afadin- and vinculin-binding protein), and -catenin in the cytoplasm at the site of AJs (Yokoyama et al, 2001), suggesting that the nectin/afadin complex can also link to the cytoskeleton network via -catenin, a putative actin-binding protein, in addition to its interaction with actin via afadin. Furthermore, -catenin also provides a structural bond that links together the cadherin/catenin and nectin/afadin complexes (Pokutta et al, 2002) indicating that cross-talk exists between the two complexes. In the testis, another AJ complex based on 6ß1 integrin has been shown to be the major cell adhesion constituent protein complex of the ES (Vogl et al, 2000). Because epithelial cells are not likely to use integrin-integrin interaction to mediate cell adhesion (Weitzman et al, 1995), and because 6ß1 integrin receptors are largely restricted to Sertoli cells (for a review, see Vogl et al, 2000) (we also failed to detect integrin ß1 in germ cells isolated from adult rat testes; Siu et al, 2002), it is apparent that laminin, the binding partner of integrin in other epithelia (for a review, see Dym, 1994), is the putative binding partner of 6ß1 integrin and should reside in germ cells. Yet this possibility remains to be studied. Recent studies have shown that laminin 3, a potential nonbasement membrane binding partner of integrin, is localized to AJ sites between round and elongated spermatids, and Sertoli cells in the testis (Koch et al, 1999), which is consistent with its localization at the apical ES. Its pattern of localization is also in sharp contrast to 1/ß1/1 laminin, which is restricted to the basement membrane in the seminiferous epithelium (Koch et al, 1999). These results collectively seem to suggest that the binding partner of 6ß1 integrin in the testis at the site of the ES is composed of at least laminin 3, and possibly may be laminin 12 (Koch et al, 1999).

View larger version (37K): [in this window] [in a new window]   Figure 1. A diagrammatic drawing that illustrates the current structural units of adherens junctions in the testis. Adherens junctions are cell-cell actin-based anchoring junctions and are found between Sertoli cells as well as between Sertoli and germ cells. The cadherin-catenin and nectin-afadin complexes are the basic structural units of adherens junctions between Sertoli cells as well as between Sertoli and germ cells. The integrin-laminin based AJ complex is largely restricted between Sertoli cells and elongated spermatids at the site of ectoplasmic specialization. SG indicates spermatogonia; pSP, preleptotene/leptotene spermatocyte; SP, pachytene spermatocyte; rSp, round spermatid; Sp, elongated spermatid; SC, Sertoli cell; AJ, adherens junction; ILK, integrin-linked kinase; FAK, focal adhesion kinase.

The Cadherin/Catenin Complex

Cadherin—Classical cadherins, such as epithelial cadherin (E-cadherin), neural cadherin (N-cadherin), and placental cadherin (P-cadherin) are AJ-integral membrane proteins (for reviews, see Takeichi, 1990; Takeichi et al, 2000). Each cadherin molecule consists of a highly conserved cytoplasmic domain, followed by a single-pass transmembrane region, and one extracellular domain of approximately 550 residues (for reviews see Takeichi, 1990, 1995; Miyatani et al, 1992; Kemler, 1993; Herrenknecht, 1996; Pötter et al, 1999) (Figure 1). Intracellularly, each cadherin molecule interacts with ß- or -catenin and p120^ctn to form the cadherin/catenin complex, which is the most extensively studied AJ functional unit. p120^ctn binds to the juxtamembrane domain of cadherin (Finnemann et al, 1997; Yap et al, 1998), whereas ß- or -catenins associate with the catenin-binding domain of cadherin (Nagafuchi and Takeichi, 1989; Ozawa et al, 1989; Stappert and Kemler, 1994). Recent studies have confirmed the presence of this complex in the testis (Chung et al, 1998a; Wine and Chapin, 1999; Chapin et al, 2001; Johnson and Boekelheide, 2002a,b; Lee et al, in press).

Studies using immunoprecipitation and immunoblotting techniques with and without cross-linkers have also demonstrated the presence of cadherin and catenin in isolated Sertoli and germ cells in vitro (Lee et al, in press). For instance, germ cells isolated from adult rat testes with negligible contaminations of Sertoli, Leydig, or peritubular myoid cells were shown to express E-cadherin and ß-catenin at an almost equimolar ratio of 1:1 with Sertoli cells (Lee et al, in press). This latest finding thus suggests that both Sertoli and germ cells possess the functional cadherin/catenin complexes and interact with each other in a homotypic fashion, thereby conferring cell adhesion function in much the same way as in other epithelia (see Figure 1). These results are also consistent with a recent immunofluorescent microscopy study that localized N-cadherin to the site between spermatocytes and Sertoli cells (Johnson and Boekelheide, 2002b). These results also strongly illustrate that the testis uses the N-cadherin/catenin complex to regulate cell adhesive function (Lee et al, in press) in addition to the desmosome-like junctions found between Sertoli cells and spermatogonia, spermatocytes, and round spermatids (for reviews, see Russell and Peterson, 1985; Russell, 1993). Furthermore, studies using seminiferous tubules isolated from adult rat testes and Sertoli-germ cell cocultures for cross-linking and immunoprecipitation have shown that this N-cadherin/catenin complex links to the actin network rather than to the intermediate filament-based (eg, vimentin) or microtubule-based (eg, tubulin) cytoskeleton in the testis (Lee et al, in press).

This latest finding via a biochemical approach contrasts somewhat with two earlier studies with immunofluorescent microscopy in which cadherin was found to colocalize with the intermediate filament-based cytoskeleton (Mulholland et al, 2001; Johnson and Boekelheide, 2002b). These apparently conflicting observations can be explained as follows. First, the anti-pancadherin antibodies used in immunoelectron microscopy (Mulholland et al, 2001) cross-react with at least 25 different cadherins that have been identified in the testis (Johnson et al, 2000). Yet a specific anti-N-cadherin antibody was used for the cross-linking and immunoprecipitation experiments. If some other members of the cadherin family other than N-cadherin use the intermediate filament as attachment sites for constructing the desmosome-like junctions in the testis, they would be precluded from the biochemical analysis. Such an argument seems to suggest that the titers of antibodies, or their affinity (or both) used by different laboratories can yield different results by immunohistochemistry or fluorescence microscopy. Second, immunofluorescence microscopy is a highly sensitive technique that can detect a minute amount of antigen-antibody complex in a limited surface area. Yet the cross-linking and immunoprecipitation technique permits an investigator to detect a specific antigen with the use of a much larger quantity of starting material for its subsequent visualization with apparently better resolution and reliability. Third, it is also possible that an intermediate, filament-based cadherin/catenin complex is indeed present in the testis, but it exists only spatially and temporally in the seminiferous epithelium. This postulate is supported by the observation that N-cadherin is found at the apical ES only at stages I-VII (Johnson and Boekelheide, 2002b). In addition, a number of cell adhesion molecules are also spatially and temporally expressed in the seminiferous epithelium (Wine and Chapin, 1999). In this connection, it is worthy to note that earlier morphological studies have shown that desmosome-like junctions (a cell-cell intermediate filament-based adhering junction type) are being used to anchor spermatogonia, spermatocytes, and spermatids onto Sertoli cells (Russell and Clermont, 1976; Russell, 1977a, 1993; Russell and Peterson, 1985) (also see Figure 2). However, it must be cautioned that the constituent proteins of desmosomes in other epithelia are desmocollins, desmogleins, desmoplakin, and plakophilin (for a review, see Alberts et al, 2002), yet none of these proteins have been identified or are being studied biochemically in the testis. It is ironic that a detailed biochemical study to delineate the composition of desmosome-like junctions in the testis is warranted.

View larger version (31K): [in this window] [in a new window]   Figure 2. A schematic diagram that illustrates the current concept of AJ dynamic regulation in the testis. The cell adhesion function apparently is regulated by the interplay of phosphatases and kinases that determine the phosphorylation status of the three AJ structural units. Furthermore, intracellular Ca2+ level, proteases, proteases inhibitors, and cytokines also play a role in regulating the opening and closing of AJs, such as ES, between Sertoli and germ cells, as well as between Sertoli cells.

Using reverse transcriptase-polymerase chain reaction with degenerate primer pairs based on cadherins and immunohistochemistry, at least 25 cadherins were detected in the rat testis, including N-, E-, and P-cadherins (Johnson et al, 2000). Furthermore, each cadherin displays unique immunostaining and stage-specific patterns in the seminiferous epithelium. For instance, N-cadherin is localized at basal inter-Sertoli junctions, Sertoli-spermatocyte junctions, and Sertoli-elongated spermatid junctions between stages I and VII (Johnson and Boekelheide, 2002b). However, in vivo studies to evaluate the physiological significance of either E-cadherin and N-cadherin in AJ structures in the testis are difficult if not impossible to perform because targeted mutation of either N- or E- cadherin in mice results in defective development at preimplantation and gastrulation with cadherin^-/- mice dying at early embryonic stages (Larue et al, 1994; Riethmacher et al, 1995; Radice et al, 1997).

     Catenin.— ß-Catenin binds to the cadherin cytoplasmic tail and serves as a linker to -catenin, an actin-binding protein, which in turn conjugates the cadherin/catenin complex to the actin cytoskeleton. ß-Catenin binds to the cadherin cytoplasmic tail via its centrally located armadillo repeats with its N-terminus linking to -catenin (Kemler, 1993; Aberle et al, 1994; Hülsken et al, 1994; Funayama et al, 1995; Jou et al, 1995; Yap et al, 1997) (Figure 1). Although -catenin is highly homologous to ß-catenin, suggesting that both proteins function similarly, deletion studies, however, have shown that ß-catenin cannot be substituted by -catenin or vice versa. For instance, deletion of the -catenin gene disrupts heart development, resulting in embryonic lethality in mice (Bierkamp et al, 1996). Likewise, mouse embryos lacking ß-catenin display epithelial cell adhesive deficiency (Haegel et al, 1995). These results thus demonstrate that whereas both ß- and -catenins share sequence homology, each cannot be functionally substituted by the other. Furthermore, ß-catenin^-/- mice also died at the embryonic stage, making it virtually impossible to assess the role of ß-catenin in spermatogenesis (Haegel et al, 1995).

-Catenin is essential for cadherin adhesive function. For instance, cells lacking -catenin have poor adherence to each other despite the presence of cadherin (Hirano et al, 1992; Watabe et al, 1994). Other studies have shown that -catenin either directly links the cadherin-catenin complex to actin or via -actinin, an actin-binding protein, demonstrating its importance in sustaining stable adhesion (Aberle et al, 1994; Nagafuchi et al, 1994; Nathke et al, 1994; Knudsen et al, 1995; Rimm et al, 1995; Nieset et al, 1997). Recent studies have also demonstrated that -catenin is used by the nectin/afadin adhesion complex in linking afadin to the actin network via a putative afadin-binding site on -catenin at residues 385-651, which also links the nectin/afadin complex to the cadherin/catenin complex (Pokutta et al, 2002), permitting cross-talk between the two complexes.

Using immunohistochemistry, -catenin, ß-catenin, -catenin, and p120^ctn have been detected in the testis (Byers et al, 1994; Wine and Chapin, 1999; Chapin et al, 2001; Janssens et al, 2001; Johnson and Boekelheide, 2002a, Lee et al, in press). For instance, p120^ctn is detected in junctions between Sertoli cells as well as at the contact sites of Sertoli cells-spermatocytes, and Sertoli cells-elongated spermatids (Johnson and Boekelheide, 2002a), consistent with its localization at the ES. Also, p120^ctn colocalizes with ß-catenin and pectin at the contact sites between Sertoli cells and spermatocytes (Johnson and Boekelheide, 2002a). Furthermore, an induction of cadherin and ß-catenin was detected during AJ assembly between Sertoli cells (Chung et al, 1999) as well as between Sertoli cells and germ cells (Lee et al, in press).

The Nectin/Afadin/Ponsin Complex

Nectin/afadin/ponsin is another recently identified cell-cell adhesion system at AJs (Figure 1) that allows both homotypic and heterotypic interactions between cells. Nectin is a Ca²⁺-independent cell-cell integral membrane adhesion molecule belonging to the immunoglobulin superfamily (Takahashi et al, 1999). Nectin has three immunoglobulin-like extracellular domains, a single transmembrane region, and a single cytoplasmic domain. The cytoplasmic domain of nectin has a conserved motif of four amino acids (Glu/Ala-Xaa-Tyr-Val) that interacts with the PDZ domain of afadin, which links directly to the actin cytoskeleton (Mandai et al, 1997; Takahashi et al, 1999) or indirectly via -catenin (Pokutta et al, 2002) (Figure 1). To date, four members of nectin are known: nectins-1, -2, -3, and -4 (Morrison and Racaniello, 1992; Cocchi et al, 1998; Satoh-Horikawa et al, 2000; Mizoguchi et al, 2002). Among them, nectin-3 is the most abundant form in the testis, nectin-2 is modestly expressed in the testis, whereas nectin-1 is expressed largely in the brain (Satoh-Horikawa et al, 2000). In normal mice, nectin-2 is predominantly associated with developing germ cells during the late stages of spermiogenesis when round spermatids transform into spermatids (Bouchard et al, 2000). Also, the localization of nectin-2 is largely restricted between elongated spermatids and Sertoli cells, being highest in stages VI-VIII (Bouchard et al, 2000). Studies of nectin-2^-/- mice have shown that these mice are infertile and have normal tubules containing germ cells, except that spermatids from steps 11 to 16 display abnormal morphology, they have distorted nuclear morphology, and mitochondria are found in the head of spermatids instead of in the midpiece (Bouchard et al, 2000). Whereas nectin-2 and -2 are detected in most testicular cell types such as spermatids, Sertoli cells, and Leydig cells, nectin-3 is almost exclusively expressed by spermatids, suggesting that the nectin/afadin-induced adhesion between Sertoli cells and spermatids may be mediated via heterotypic nectin-based interactions (Ozaki-Kuroda et al, 2002). By using transplantation techniques to introduce nectin-2^-/- and nectin-2^+/- spermatogonia to nectin-2^+/- and nectin-2^-/- testis, respectively, heterotypic interactions of nectins between Sertoli cells and elongate spermatids at the site of ES are only possible via transinteractions between nectin-2 in Sertoli cells and nectin-3 in spermatids (Ozaki-Kuroda et al, 2002).

Two splicing variants of afadin are known to date, including 1-afadin and s-afadin. 1-Afadin is ubiquitously expressed and is also found in the testis, whereas s-afadin is specifically expressed in the brain (Mandai et al, 1997; Ikeda et al, 1999). 1-Afadin is the larger splicing variant, having a PDZ domain and three proline-rich domains, and can connect to the actin cytoskeleton (Mandai et al, 1997; Takahashi et al, 1999). s-Afadin has one PDZ domain but lacks the F-actin-binding domain (Mandai et al, 1997; Ikeda et al, 1999; Yokoyama et al, 2001). Analysis of afadin^-/- mouse embryos has shown that afadin is essential for proper structural organization of cadherin-based AJs and tight junctions in polarized epithelia (Ikeda et al, 1999) because afadin^-/- mice displayed developmental defects during and after gastrulation, such as impaired migration of mesoderm and disorganization of the ectoderm, leading to embryonic lethality (Ikeda et al, 1999).

Ponsin is an afadin- and vinculin-binding protein. When ponsin binds to afadin, it colocalizes with nectin to the cadherin-based AJ structure (Mandai et al, 1999; Tachibana et al, 2000). When ponsin binds to vinculin, it is recruited to both the cadherin-based AJs and focal contact (Mandai et al, 1999). Although ponsin can interact with afadin and vinculin separately, these three molecules do not form a structural complex (Mandai et al, 1999). Studies using Northern blot analysis have detected both afadin and ponsin messenger RNA transcripts in the testis; yet biochemical and functional studies to analyze nectin/afadin/ponsin in the testis are lacking.

Another recent study with fluorescent microscopy and immunogold electron microscopy (Ozaki-Kuroda et al, 2002) has also supported the notion that the nectin/afadin complex is another actin-based cell adhesion functional unit between Sertoli and germ cells in the seminiferous epithelium (see Figure 1) in addition to the cadherin/catenin complex described above. For instance, both nectin-3 and nectin-2 were found residing in spermatids in mice at steps 9-15; namely, both late stage round spermatids, and elongating/elongated spermatids, as well as in Sertoli cells (but not nectin-1 or -4), and were found to interact with each other heterotypically (Ozaki-Kuroda et al, 2002). These data collectively (Bouchard et al, 2000; Ozaki-Kuroda et al, 2002; Lee et al, in press) indicate that it is ironic that functional cell-adhesion complexes exist for the anchoring of germ cells onto Sertoli cells, which are actin-based, and are found in the seminiferous epithelium in a stage-specific manner. For instance, nectin was intensively localized at the interface of spermatids and Sertoli cells (but also with weak staining between spermatocytes and Sertoli cells) at stages IX-IV, yet its staining plummeted rapidly at stage VIII just prior to the release of spermatids at spermiation (Ozaki-Kuroda et al, 2002).

Integrin/Laminin Complex

Integrins are transmembrane cell adhesion molecules with putative extracellular, transmembrane, and intracellular domains (Figure 1) (for reviews see Clark and Brugge, 1995; de Melker and Sonnenberg, 1999). Each integrin molecule is composed of an and a ß subunit, which are noncovalently bonded to form a functional receptor. The integrin receptor also mediates signal transduction in mammalian cells. As such, integrin is a dual functional molecule that integrates ECM proteins with the cytoskeleton network and neighboring cells to form an epithelium (for reviews see Aplin et al, 1998; Juliano, 2002; Martin et al, 2002).

The extracellular domain of an integrin receptor contributed by the and ß subunits can determine the types of ligands that can activate the receptor in a process known as outside-in signaling (for reviews see Hynes, 1999; Juliano, 2002). It is interesting that the intracellular domain can also transduce signals within a cell via the integrin receptor, causing a clustering of integrin receptors on the cell surface and can change its ligand-binding affinity in a process known as inside-out signaling (for reviews see Calderwood et al, 2000; Juliano, 2002). Intracellularly, the cytoplasmic domains of both subunits interact with different actin-binding proteins, signaling molecules, and adaptor proteins. In the testis, talin and -actinin (actin-binding proteins), FAK and ILK (signaling molecules), and paxillin (an adaptor protein) are known to interact with the cytoplasmic domains of integrin receptors (Wine and Chapin, 1999; Santoro et al, 2000; Mulholland et al, 2001).

In mammals, 18 and 8 ß subunits are known to exist to date, which can heterodimerize to form at least 24 different integrin receptors (for reviews see Hynes, 1999; Juliano, 2002). In the testis, 1 through 6 integrins, and ß1 and ß3 integrins have been positively identified (Palombi et al, 1992; Schaller et al, 1993; Salanova et al, 1995; Lustig et al, 1998; Mulholland 2001; Merono et al, 2002). Yet the most extensively studied integrin receptor in the testis is 6ß1 (for a review, see Vogl et al, 2000), which has been postulated to be a part of the major protein complex in the apical ES (Mulholland et al, 2001). ß1-Integrin is found both in the basal and the adluminal compartments of the seminiferous epithelium and is a stage-specific receptor, being lowest at stages VII-VIII (Palombi et al, 1992; Mulholland et al, 2001). In mammals, the known binding partner for 6ß1 or 6ß4 integrin is laminin (Shaw and Mercurio, 1993; Rabinovitz and Mercurio, 1997), yet most of the laminin chains identified to date in the testis are confined to the basement membrane (Koch et al, 1999), a modified form of ECM in the testis (Dym, 1994).

A recent study using immunohistochemistry has localized the laminin 3 chain in the testis, showing that it is almost exclusively restricted to the adluminal compartment of the seminiferous epithelium (Koch et al, 1999). These results seemingly suggest that laminin 3 is one of the putative chains that constitutes the binding partner for 6ß1 integrin. Because a functional laminin binding protein is composed of three laminin chains, the remaining chains in the apical ES remain to be identified in the testis.

The significance of 6ß1 integrin chains has been investigated by studies using transgenic mice. For instance, in integrin 6^-/- mice, newborns died soon after birth and displayed severe blistering skin (Georges-Labouesse et al, 1996). In integrin ß1^-/- mice, virtually all mice died at days 5 to 6 postcoitus (Fassler et al, 1995; Stephens et al, 1995). Because none of these transgenic mice reached puberty, their significance in the ES is not known. Furthermore, it is not known how integrin 6ß1 mediates signaling events downstream to regulate the actin-based cytoskeleton network except that integrin-linked kinase (ILK) is implicated in these events (Mulholland et al, 2001). In this connection, it is important to note that 3 and 5 integrin subunits are also localized to the same site of 6ß1 in the testis (Schaller et al, 1993). And both 3 and 6 integrins have been detected in Sertoli cells by immunohistochemistry (Lustig et al, 1998), and a recent study found ß3 integrin in pig Leydig cells (Merono et al, 2002). In summary, the integrin/laminin complex is an emerging regulatory functional unit in the testis, in particular at the site of the ES. Much research is needed to explore how this unit regulates the downstream cytoskeleton network.

Others

Vezatin is a recently identified AJ-integral membrane protein that is ubiquitously present at the site of AJs in several organs such as brain, lung, kidney, and heart; however, it is now known whether vezatin is present in the testis (Küssel-Andermann et al, 2000). Vezatin interacts with both myosin VIIa and the cadherin-catenin complex (Küssel-Andermann et al, 2000). Vinculin is a cytoskeleton protein found at the site of cell-cell contacts and is capable of binding to actin and -actinin (Wilkins and Lin, 1982; Grove and Vogl, 1989; Menkel et al, 1994). In the testis, vinculin is associated with ES at the adluminal and basal compartment, colocalizing with actin filaments (Grove and Vogl, 1989; Grove et al, 1990; Pfeiffer and Vogl, 1991). Another molecule that is of interest for studying Sertoli-germ cell AJ dynamics is sertolin, because a significant reduction in its expression was detected at the time of AJ assembly in Sertoli-germ cell cocultures (Mruk and Cheng, 1999).

AJ Signaling Molecule Network

Recent studies have identified a number of signaling molecules at the sites of AJs such as Csk, Cas, c-Src, p120^ctn, CK2, and PTP-RL10 (Okada et al, 1991; Brown and Cooper, 1996; Thomas and Brugge, 1997; Tokuchi et al, 1999; Anastasiadis and Reynolds, 2000; Martin, 2001) (see Table 2). For instance, Src, Fer kinase, Csk, CK2, Ctk, and PTP-RL10 have been conclusively demonstrated in rat testis by immunohistochemistry and Northern blot analysis (Pawson et al, 1989; Okada et al, 1991; Kaneko et al, 1995; Guerra et al, 1999; Wine and Chapin, 1999; Chapin et al, 2001). Indeed, the localization of c-Src, Csk, and Fer kinase in the testis and their unusual temporal and spatial localization patterns have implicated their role in the regulation of spermiation (Wine and Chapin, 1999; Craig et al, 2001). For instance, Fer kinase is transiently expressed during spermatogenesis and is exclusively expressed in pachytene spermatocytes (Keshet et al, 1990). Recent studies of Fer kinase^-/- mice have shown that these mice apparently maintain normal spermatogenesis, strongly suggesting that the function of Fer kinase could be superseded by other tyrosine kinases in the testis (Craig et al, 2001) such as c-Src. c-Src is localized in both Sertoli cells and spermatids and is a stage-specific AJ signaling molecule in the testis, being highest at stage VIII prior to spermiation (Chapin et al, 2001). These signaling molecules can alter the cadherin/catenin or the nectin/afadin complexes (or both) via phosphorylation of catenins, nectin, or cadherins, which in turn, alters cell adhesive function at the site of AJs (Brown and Cooper, 1996; Thomas and Brugge, 1997; Anastasiadis and Reynolds, 2000; Martin, 2001). Yet this remains an unexplored area of research in the testis.

Testis-Specific Cell-Cell Actin-Based AJs: Ectoplasmic Specializations and Tubulobulbar Complexes

     Ectoplasmic Specializations— Ectoplasmic specializations are actin-mediated cell adhesion complexes found at the apical region of the seminiferous epithelium in which spermatids anchor onto Sertoli cells before their release into the lumen at spermiation and at the basal region of the seminiferous epithelium between Sertoli cells (Russell, 1977, 1980) (Figure 1; Table 2). Two recent studies using immunofluorescent microscopy techniques, however, suggest that ESs in the testis are classic cadherin-based junction, but they may use vimentin-based intermediate filaments as attachment sites (Mulholland et al, 2001; Johnson and Boekelheide, 2002b). Yet a recent study by cross-linking using dithiobis succinimidyl propionate and immunoprecipitation using antibodies against either N-cadherin, actin, vimentin, or tubulin have unequivocally demonstrated that the cadherin/catenin between Sertoli cells and those between Sertoli and germ cells are indeed actin-based (Lee et al, in press).

It was postulated that the rapid turnover of ESs between Sertoli cells at the basal region of the seminiferous epithelium allows the movement of preleptotene/leptotene spermatocytes across the blood-testis barrier at stages VIII and IX of the cycle (Russell, 1997b), whereas the release of mature spermatids (spermatozoa) at the adluminal compartment is accomplished by the disassembly of ESs in the apical region (Vogl et al, 2000). ESs are morphologically characterized by the plasma membrane of Sertoli cells with a layer of actin filament and a cistern of endoplasmic reticulum (Guttman et al, 2002). 6ß1 Integrin and integrin-linked kinase were recently shown to be the major molecular components of ES (Palombi et al, 1992; Salanova et al, 1995) (Table 2). Actin, vinculin, fimbrin, espin, -actinin, myosin VIIa, gelsolin, and Keap1 are also found at the site of ESs and are likely the putative constituent proteins of ESs (see Table 2) (Franke et al, 1989; Grove and Vogl, 1989; Grove et al, 1990; Bartles et al, 1996; Hasson et al, 1997; Guttman et al, 2002; Velichkova et al, 2002) (Table 2). Still, the function of the molecules that constitute the ES in the testis remains to be elucidated.

     Tubulobulbar Complex— Tubulobulbar complex (TBC) is another testis-specific form of actin-based AJs surrounding the head of spermatids at steps 18 and 19, protruding into the invagination of Sertoli cell plasma membrane (Russell and Clermont, 1976; Russell, 1980). TBC consists of two structural elements: a tubular structure and a balloon-like terminal bulbar structure (Russell and Clermont, 1976). The precise molecular constituent of TBC is currently not known. Several hypotheses have been proposed regarding the function of TBC (Russell, 1993). For instance, it was postulated that TBC served as an anchoring device to retain spermatids in the seminiferous epithelium before spermiation. TBC could also be used to remove the linkage between Sertoli cells and spermatids to permit the release of spermatids into the tubular lumen at spermiation. More recent studies have shown that an acrosomal glycoprotein, MN7, is indeed incorporated into the TBC having periodic acid-Schiff reactivity in the core of the TBC, suggesting that the TBC may function as a protein cleavage center at the site of AJs to eliminate excess acrosomal contents before spermiation (Tanii et al, 1999). While this junction type has been described for almost three decades, its molecular and biochemical composition remain to be explored.

Regulation of AJ Dynamics

The disassembly and reassembly of AJs is one of the key events during spermatogenesis because germ cells must translocate from the basal to the adluminal compartment of the seminiferous epithelium to complete spermatogenesis (for reviews see Russell and Peterson, 1985; Russell et al, 1990; Byers et al, 1993; Mruk and Cheng, 2000). However, little attention was given to understand how Sertoli-Sertoli and Sertoli-germ AJs are regulated. Current studies have shown that AJ dynamics are regulated by a wide range of extracellular signaling molecules that include growth factors, kinases, and phosphatases (Figure 2).

     Growth Factors— Cytokines affect AJ function largely by changing the phosphorylation state of the two known AJ functional complexes; namely, cadherin/catenin and nectin/afadin. For instance, epidermal growth factor and hepatocyte growth factor can stimulate tyrosine phosphorylation of ß-catenin, -catenin, and p120^ctn, which in turn, causes dissociation of the cadherin-catenin complex (Shibamoto et al, 1994; Hazan and Norton, 1998), thereby perturbing AJs. Furthermore, vascular endothelial growth factor not only induces tyrosine phosphorylation of cadherin, ß-catenin, and p120^ctn (Esser et al, 1998), it also stimulates the dephosphorylation of p120^ctn and p100^ctn at the specific Ser/Thr sites of these AJ-signaling molecules (Wong et al, 2000). These results thus suggest that reduced adhesive strength between cells can be induced by both tyrosine phosphorylation of cadherin and ß-catenin and serine/threonine dephosphorylation of p120^ctn, which are regulated by growth factors (Esser et al, 1998; Wong et al, 2000). Although recent studies have shown that cytokines such as TGF-ß3 may play a crucial role in the regulation of tight junction dynamics in the testis (Lui et al, 2001, 2002), virtually no report can be found in the literature that has investigated the significance of cytokines in AJ dynamics in the testis.

     Kinases and Phosphatases— The integrity of the cadherin-catenin complex is dependent on the phosphorylation state of cadherin and catenin (Kemler, 1993). Apparently, growth factors modulate cadherin-catenin adhesive function via phosphorylation of AJ proteins. Besides, AJ-associated protein kinases such as Src, Csk, CK2, p120^ctn, and Fer kinase also play an important role in regulating the phosphorylation of the cadherin-catenin complex (see Table 1). For instance, pp60^c-src, a protein tyrosine kinase, can induce phosphorylation of ß-catenin, causing the disruption of -catenin/ß-catenin binding, thus altering cell adhesion function (Shibamoto et al, 1994; Ozawa and Kemler, 1998; Roura et al, 1999). Several protein phosphatases have been shown to interact with the cadherin-catenin complex at the site of AJs, including PTP-1B-like phosphatase, PTP, PTP, and PTPµ (Brady-Kalnay et al, 1995; Balsamo et al, 1996; Fuchs, 1996; Kypta et al, 1996; Cheng et al, 1997). Although a number of signaling pathways can regulate intercellular junctions, the interplay between kinases and phosphatases appears to provide an efficient means for AJ dynamic regulation. Indeed, a putative protein tyrosine phosphatase designated myotubularin has recently been identified in rat testis (Li et al, 2000, 2001a). Subsequent immunohistochemistry study has shown that myotubularin (rMTM) associates with both round and elongated spermatids and Sertoli cells at the site of AJs in the seminiferous epithelium (Li et al, 2000). More important, rMTM was expressed by Sertoli and germ cells (Li et al, 2000, 2001b) and Sertoli cell rMTM messenger RNA can be induced by either germ cell-conditioned medium or Sertoli-germ cell culture (Li et al, 2001a), and seem to suggest that the events of AJ assembly in the testis are regulated at least in part via protein phosphorylation.

     Proteases and Protease Inhibitors— The first study in the literature to illustrate that proteases and protease inhibitors are involved in the regulation of Sertoli-germ cell AJ dynamics is the report by Mruk et al (1997). This study demonstrated that the assembly of Sertoli-germ cell AJs in vitro was associated with a transient induction in total serine protease activities as well as a transient induction in the expression of tryptase, uPA, and cathepsin L, at the time germ cells attached to the Sertoli cell epithelium that initiates the assembly of AJs. This study is different from other reported effects of germ cells on Sertoli cell AJ-function because Sertoli cells were first cultured for 4-6 days in vitro, allowing them to form an epithelium with complete AJs and TJs (Mruk, 1997; Chung et al, 1998a, 1999a). Thereafter, freshly isolated germ cells were added to this Sertoli cell epithelium to initiate Sertoli-germ cell AJ assembly and thereby mimicking the in vivo events. Furthermore, such an in vitro system has been characterized in two earlier studies (Enders and Millette, 1988; Cameron and Muffly, 1991), indicating that junctions similar to those found in vivo, such as desmosome-like junctions and ES, indeed exist in this system, which was also partially characterized in our laboratory (Mruk et al, 1997). A more recent study with immunofluorescence microscopy also colocalized N-cadherin and ß-catenin to the same sites between Sertoli cells as well as between Sertoli cells and germ cells when these cells were cultured in vitro (Lee et al, in press), indicating the presence of functional AJs in this culture system. As such, this is the only currently available in vitro model to study the regulation and biology of Sertoli-germ cell AJ dynamics. Whereas these earlier studies using this in vitro model implicate the role of proteases and protease inhibitors on Sertoli-germ AJ dynamics, they are correlative in nature, and the precise physiological significance of proteases and protease inhibitors in AJ dynamics is not known. Using this in vitro model coupled with fluorometric analysis to assess cell adhesion function, it was shown that the presence of ₂-macroglobulin (a putative Sertoli cell secreted protease inhibitor) (Cheng et al, 1990) or aprotinin (a serine protease inhibitor) could indeed facilitate the binding of germ cells onto the Sertoli cell epithelium (Mruk et al, in preparation) (note: cell adhesion is a prerequisite of the subsequent Sertoli-germ cell AJ assembly). And the presence of an antibody against ₂-macroglobulin (but not the preimmune serum) also perturbed germ cell binding to the Sertoli cell epithelium (Mruk et al, in preparation). This recent study thus provides unequivocal proof of the significance of proteases and protease inhibitors in Sertoli-germ cell AJ assembly. In this context, it is of interest to note that Sertoli, germ, and peritubular myoid cells are also known to synthesize a wide range of proteases and protease inhibitors throughout the entire seminiferous cycle (for a review see Fritz et al, 1993), some of which are produced in a stage-specific pattern. For instance, the expression or localization of cathepsin L (Hettle et al, 1986; Chung et al, 1998b), cathepsins D and S (Chung et al, 1998b), and plasminogen activator are stage-specific (Vihko et al, 1989). These studies taken collectively also illustrate the significance of proteases in AJ dynamics, such as spermiation. It is our belief that proteases and protease inhibitors work in a "yin-yang" relationship to regulate the events of junction disassembly and reassembly, which in turn permit the translocation of germ cells from the basal to the adluminal compartment of the seminiferous epithelium (Fritz et al, 1993). Indeed, it was shown that the expression of ₂-macroglobulin and uPA by cocultures of Sertoli-germ cells during AJ assembly are induced differentially (Mruk et al, 1997), further implicating their role in AJ dynamics.

Concluding Remarks

Based on available published reports in the literature, particularly those focused on the testis as reviewed herein, it is apparent the AJ dynamics between Sertoli and germ cells in the testis are regulated by several pathways and different sets of molecules as shown in Figure 2. For instance, the opening and closing of AJ structures are regulated by the phosphorylation status of the cadherin/catenin and nectin/afadin complexes, and possibly the integrin/laminin complex. Also, an increase in overall protease activity is likely to favor the opening of AJ structures, whereas an increase in overall protease inhibitor activity at the site of AJs favor their closing. Furthermore, more resources need to be committed to delineate the biochemical composition of ESs and how the constituent proteins biochemically interact with each other at the site of ESs, both basal and apical, to regulate AJ restructuring and facilitate germ cell movement across the seminiferous epithelium during spermatogenesis.

Footnotes

This work was supported in part by grants from the CONRAD Program (CICCR, CIG-96-05A and CIG-01-72 to C.Y.C., and CIG-01-74 to D.D.M.), the National Institute of Child Health and Human Development (U54-HD-29990, Project 3 to C.Y.C.), the U.S. Agency for International Development (HRN-A-00-99-00010), and the Noopolis Foundation. W.Y.L. was supported in part by a postgraduate research scholarship from the University of Hong Kong.

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