CMS: an adapter molecule involved in cytoskeletal rearrangements

The adapter type protein CMS/CD2AP binds to the proto-oncogenic protein c-Cbl through a tyrosine phosphorylation-regulated Src homology 3 domain interaction

CMS/CD2AP is a cytoplasmic protein critical for the integrity of the kidney glomerular filtration and the T cell function. CMS contains domains and motifs characteristic for protein-protein interactions, and it is involved in the regulation of the actin cytoskeleton. We report here that the individual SH3 domains of CMS bind to phosphotyrosine proteins of approximately 80, 90, and 180 kDa in cell lysates stimulated with epidermal growth factor. The second SH3 domain of CMS bound specifically to a tyrosine-phosphorylated protein of 120 kDa, which we identified as the proto-oncoprotein c-Cbl. The c-Cbl-binding site for CMS mapped to the carboxyl terminus of c-Cbl and is different from the proline-rich region known to bind SH3-containing proteins. CMS binding to c-Cbl was markedly attenuated in a tyrosine phosphorylation-defective c-Cbl mutant indicating that this interaction is dependent on the tyrosine phosphorylation of CMS. It also implies that CMS interacts with c-Cbl in an inducible fashion upon stimulation of a variety of cell-surface receptors. Immunofluorescence analysis revealed that both proteins colocalize at lamellipodia and leading edges of cells, and we propose that the interaction of CMS with c-Cbl offers a mechanism by which c-Cbl associates and regulates the actin cytoskeleton.

CD2-associated protein and the kidney

During the past 3 years there have been significant advances in our understanding of the biology of the glomerular podocyte. In particular, two proteins, CD2-associated protein and nephrin, have been identified as critical podocyte proteins that are both required for normal glomerular filtration. In addition to supporting the idea that the slit diaphragm plays a crucial role in glomerular function, these results suggest novel insights into the pathogenesis of glomerular diseases. The present review addresses these recent advances and discusses the implications of the findings.

[Molecular mechanisms of idiopathic nephrotic syndrome]

Steroid-sensitive idiopathic nephrotic syndrome is a T-cell disorder associated with a functional renal impairment. The molecular mechanisms leading from the stimulation of the immune system to the clinical expression of the renal disease can be analyzed according to five biological events: 1) a Th2 activation of T-cells by interleukin-13; 2) a yet unidentified glomerular permeability factor from immune origin; 3) a molecular disorientation of slit diaphragms or glomerular basement membrane responsible for proteinuria; 4) a podocyte cytoskeleton rearrangement responsible for foot process effacement; and 5) renal avidity for sodium and edema formation resulting from a primary stimulation of tubular Na,K-ATPase and an increase of endothelial permeability.

alphavbeta5 integrin recruits the CrkII-Dock180-rac1 complex for phagocytosis of apoptotic cells

Integrin receptors are important for the phagocytosis of apoptotic cells. However, little is known about their function in mediating internalization, as previous studies used blocking antibodies for the inhibition of binding. Here we show that the alphavbeta5 receptor mediates both binding and internalization of apoptotic cells. Internalization is dependent upon signalling through the beta5 cytoplasmic tail, and engagement of the alphavbeta5 heterodimer results in recruitment of the p130cas-CrkII-Dock180 molecular complex, which in turn triggers Rac1 activation and phagosome formation. In addition to defining integrin-receptor signalling as critical for the internalization of apoptotic material, our results also constitute the first evidence in human cells that the CED-2-CED-5-CED-10 complex defined in Caenorhabditis elegans is functionally analagous to the CrkII-Dock180-Rac1 molecular complex in mammalian cells. By linking the alphavbeta 5 receptor to this molecular switch, we reveal an evolutionarily conserved signalling pathway that is responsible for the recognition and internalization of apoptotic cells by both professional and non-professional phagocytes.

Characterization of the CIN85 adaptor protein and identification of components involved in CIN85 complexes

CIN85 is an 85-kDa adaptor protein whose functions in signaling pathways are presently unknown. Using the yeast two-hybrid screen, the B cell linker protein (BLNK) was identified as a binding partner of CIN85. Coimmunoprecipitation experiments using mammalian cells revealed that CIN85 directly bound to BLNK through its SH3 domains. Immunostaining analysis showed that CIN85 and BLNK were colocalized in the cytoplasm. These results indicate a potential role of CIN85 in the B cell receptor-mediated signaling pathway. It was also found that Crk-I, Crk-II, p130(Cas), p85-PI3K, Grb2, and Sos1 were components of CIN85 complexes. CIN85 interacted with itself through its coiled-coil region, resulting in formation of a tetramer. Both the coiled-coil region and SH3 domains of CIN85 were responsible for its subcellular localization. Our data suggest that CIN85 may serve for regulation of various signaling events through formation of its diverse complexes.

Actin assembly at membranes controlled by ARF6

The small GTPase, ADP-ribosylation factor-6 (ARF6), has been implicated in regulating membrane traffic and remodeling cortical F-actin. Using real-time video analysis of actin assembly in living cells, we investigated the function and mechanism of ARF6 in control of actin assembly. Expression of an activated form of ARF6 that mimicks the GTP-bound form of the GTPase induced actin assembly resulting in the movement of vesicle-like particles, some of which contain markers for pinosomes. Activated ARF6 also stimulated actin assembly at foci on the ventral surface of the cell and stimulated fluid phase pinocytosis. Particle motility induced by ARF6 involved Arp2/3 complex, tyrosine kinase activity, phospholipase D (PLD) and D3-phosphoinositides, but not phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). We conclude that ARF6 regulates actin assembly for pinosome motility and at foci on the ventral cell surface.

In vivo interaction of the adapter protein CD2-associated protein with the type 2 polycystic kidney disease protein, polycystin-2

We identified a developmentally regulated gene from mouse kidney whose expression is up-regulated in metanephrogenic mesenchyme cells when they are induced to differentiate to epithelial cells during kidney organogenesis. The deduced 70.5-kDa protein, originally named METS-1 (mesenchyme-to-epithelium transition protein with SH3 domains), has since been cloned as a CD2-associated protein (CD2AP). CD2AP is strongly expressed in glomerular podocytes, and the absence of CD2AP in mice results in congenital nephrotic syndrome. We have found that METS-1/CD2AP (hereafter referred to as CD2AP) is expressed at lower levels in renal tubular epithelial cells in the adult kidney, particularly in distal nephron segments. Independent yeast two-hybrid screens using the COOH-terminal region of either CD2AP or polycystin-2 as bait identified the COOH termini of polycystin-2 and CD2AP, respectively, as strong interacting partners. This interaction was confirmed in cultured cells by co-immunoprecipitation of endogenous polycystin-2 with endogenous CD2AP and vice versa. CD2AP shows a diffuse reticular cytoplasmic and perinuclear pattern of distribution, similar to polycystin-2, in cultured cells, and the two proteins co-localize by indirect double immunofluorescence microscopy. CD2AP is an adapter molecule that associates with a variety of membrane proteins to organize the cytoskeleton around a polarized site. Such a function fits well with that hypothesized for the polycystin proteins in renal tubular epithelial cells, and the present findings suggest that CD2AP has a role in polycystin-2 function.

CD2AP is expressed with nephrin in developing podocytes and is found widely in mature kidney and elsewhere

CD2-associated protein (CD2AP) is an adapter molecule that can bind to the cytoplasmic domain of nephrin, a component of the glomerular slit diaphragm. Mice lacking CD2AP exhibit a congenital nephrotic syndrome characterized by extensive foot process effacement, suggesting that CD2AP-nephrin interactions are critical to maintaining slit diaphragm function. We have examined the patterns of expression of both CD2AP and nephrin in developing mouse and human kidney. Both proteins were first detected in developing podocytes at the capillary loop stage of glomerulogenesis and eventually became concentrated near the glomerular basement membrane. CD2AP was also observed diffusely in collecting duct and apically in many cells of proximal and distal tubule. Kidneys from Cd2ap -/- mice initially exhibited normal nephrin localization, but as the mice aged and foot processes became effaced, nephrin disappeared. In laminin-beta(2) mutant mice exhibiting nephrotic syndrome, CD2AP in glomeruli was aberrantly localized in a primarily punctate pattern. Extensive extrarenal expression of CD2AP was observed in endothelial and epithelial cells, in many cases with a specific subcellular localization. Together, these results suggest that CD2AP is not only involved in maintaining the slit diaphragm but may also have a general role in maintaining specialized subcellular architecture. The severity of kidney disease in Cd2ap mutant mice may have eclipsed manifestation of defects in other tissues.

Signaling via LAT (linker for T-cell activation) and Syk/ZAP70 is required for ERK activation and NFAT transcriptional activation following CD2 stimulation

Activation of T cells can be initiated through cell surface molecules in addition to the T-cell receptor-CD3 (TCR-CD3) complex. In human T cells, ligation of the CD2 molecule by mitogenic pairs of anti-CD2 monoclonal antibodies activates T cells via biochemical signaling pathways similar but not identical to those elicited on TCR engagement. This study describes a key role for the p36/38 membrane adapter protein linker for T cell activation (LAT) in CD2-mediated T-cell activation. Following ligation of CD2 on the surface of the Jurkat T-cell line and human purified T cells, LAT was tyrosine phosphorylated and shown to associate in vivo with a number of other tyrosine phosphorylated proteins including PLCγ-1, Grb-2, and SLP-76. Using Jurkat cell lines deficient in ZAP70/Syk (P116) or LAT (ANJ3) expression, CD2-dependent PLCγ-1 and SLP-76 tyrosine phosphorylation required expression both of ZAP70 or Syk and of LAT. As predicted, the absence of either LAT or ZAP70/Syk kinases correlated with a defect in the induction of nuclear factor of activated T cells (NFAT) transcriptional activity, activation of the interleukin-2 promoter, and ERK phosphorylation following CD2 stimulation. These data suggest that LAT is an adapter protein important for the regulation of CD2-mediated T-cell activation.

Signaling via LAT (linker for T-cell activation) and Syk/ZAP70 is required for ERK activation and NFAT transcriptional activation following CD2 stimulation

Activation of T cells can be initiated through cell surface molecules in addition to the T-cell receptor-CD3 (TCR-CD3) complex. In human T cells, ligation of the CD2 molecule by mitogenic pairs of anti-CD2 monoclonal antibodies activates T cells via biochemical signaling pathways similar but not identical to those elicited on TCR engagement. This study describes a key role for the p36/38 membrane adapter protein linker for T cell activation (LAT) in CD2-mediated T-cell activation. Following ligation of CD2 on the surface of the Jurkat T-cell line and human purified T cells, LAT was tyrosine phosphorylated and shown to associate in vivo with a number of other tyrosine phosphorylated proteins including PLCγ-1, Grb-2, and SLP-76. Using Jurkat cell lines deficient in ZAP70/Syk (P116) or LAT (ANJ3) expression, CD2-dependent PLCγ-1 and SLP-76 tyrosine phosphorylation required expression both of ZAP70 or Syk and of LAT. As predicted, the absence of either LAT or ZAP70/Syk kinases correlated with a defect in the induction of nuclear factor of activated T cells (NFAT) transcriptional activity, activation of the interleukin-2 promoter, and ERK phosphorylation following CD2 stimulation. These data suggest that LAT is an adapter protein important for the regulation of CD2-mediated T-cell activation.

Negative regulation of PI 3-kinase by Ruk, a novel adaptor protein

Class I(A) phosphatidylinositol 3-kinase (PI 3-kinase) is a key component of important intracellular signalling cascades. We have identified an adaptor protein, Ruk(l), which forms complexes with the PI 3-kinase holoenzyme in vitro and in vivo. This interaction involves the proline-rich region of Ruk and the SH3 domain of the p85 alpha regulatory subunit of the class I(A) PI 3-kinase. In contrast to many other adaptor proteins that activate PI 3-kinase, interaction with Ruk(l) substantially inhibits the lipid kinase activity of the enzyme. Overexpression of Ruk(l) in cultured primary neurons induces apoptosis, an effect that could be reversed by co-expression of constitutively activated forms of the p110 alpha catalytic subunit of PI 3-kinase or its downstream effector PKB/Akt. Our data provide evidence for the existence of a negative regulator of the PI 3-kinase signalling pathway that is essential for maintaining cellular homeostasis. Structural similarities between Ruk, CIN85 and CD2AP/CMS suggest that these proteins form a novel family of adaptor molecules that are involved in various intracellular signalling pathways.

Regulation of c-SRC activity and function by the adapter protein CAS

SRC family kinases play essential roles in a variety of cellular functions, including proliferation, survival, differentiation, and apoptosis. The activities of these kinases are regulated by intramolecular interactions and by heterologous binding partners that modulate the transition between active and inactive structural conformations. p130(CAS) (CAS) binds directly to both the SH2 and SH3 domains of c-SRC and therefore has the potential to structurally alter and activate this kinase. In this report, we demonstrate that overexpression of full-length CAS in COS-1 cells induces c-SRC-dependent tyrosine phosphorylation of multiple endogenous cellular proteins. A carboxy-terminal fragment of CAS (CAS-CT), which contains the c-SRC binding site, was sufficient to induce c-SRC-dependent protein tyrosine kinase activity, as measured by tyrosine phosphorylation of cortactin, paxillin, and, to a lesser extent, focal adhesion kinase. A single amino acid substitution located in the binding site for the SRC SH3 domain of CAS-CT disrupted CAS-CT's interaction with c-SRC and inhibited its ability to induce tyrosine phosphorylation of cortactin and paxillin. Murine C3H10T1/2 fibroblasts that expressed elevated levels of tyrosine phosphorylated CAS and c-SRC-CAS complexes exhibited an enhanced ability to form colonies in soft agar and to proliferate in the absence of serum or growth factors. CAS-CT fully substituted for CAS in mediating growth in soft agar but was less effective in promoting serum-independent growth. These data suggest that CAS plays an important role in regulating specific signaling pathways governing cell growth and/or survival, in part through its ability to interact with and modulate the activity of c-SRC.

The glioma-associated protein SETA interacts with AIP1/Alix and ALG-2 and modulates apoptosis in astrocytes

Expression of the src homology 3 (SH3) domain-containing expressed in tumorigenic astrocytes (SETA) gene is associated with the tumorigenic state in astrocytes. SETA encodes a variety of adapter proteins containing either one or two SH3 domains, as suggested by the sequence heterogeneity of isolated cDNAs. Using both SH3 domains in a yeast two-hybrid screen of a glial progenitor cell cDNA library, we isolated the rat homolog of the ALG-2-interacting protein 1 or ALG-2-interacting protein X (AIP1/Alix). In vitro confrontation experiments showed that the SH3-N domain of SETA interacted with the proline-rich C terminus of AIP1. In co-immunoprecipitation experiments, SETA and AIP1 interacted and could form a complex with apoptosis-linked gene 2 protein. Endogenous SETA and AIP1 proteins showed similar patterns of staining in primary rat astrocytes. Misexpression of a variety of SETA protein isoforms in these astrocytes revealed that they localized to the actin cytoskeleton. Furthermore, SETA proteins containing the SH3-N domain were able to sensitize astrocytes to apoptosis induced by UV irradiation. Expression of the isolated SH3-N domain had the greatest effect in these experiments, indicating that interference in the interaction between endogenous SETA and AIP1 sensitizes astrocytes to apoptosis in response to DNA damage.

Direct binding of the signaling adapter protein Grb2 to the activation loop tyrosines on the nerve growth factor receptor tyrosine kinase, TrkA

We demonstrate that the signaling adapter, Grb2, binds directly to the neurotrophin receptor tyrosine kinase, TrkA. Grb2 binding to TrkA is independent of Shc, FRS-2, phospholipase Cgamma-1, rAPS, and SH2B and is observed in in vitro binding assays, yeast two-hybrid assays, and in co-immunoprecipitation assays. Grb2 binding to TrkA is mediated by the central SH2 domain, requires a kinase-active TrkA, and is phosphotyrosine-dependent. By analyzing a series of rat TrkA mutants, we demonstrate that Grb2 binds to the carboxyl-terminal residue, Tyr(794), as well as to the activation loop tyrosines, Tyr(683) and Tyr(684). By using acidic amino acid substitutions of the activation loop tyrosines on TrkA, we can stimulate constitutive kinase activity and TrkA-Shc interactions but, importantly, abolish TrkA/Grb2 binding. Thus, in addition to providing the first evidence of direct Grb2 binding to the neurotrophin receptor, TrkA, these data provide the first direct evidence that the activation loop tyrosines of a receptor tyrosine kinase, in addition to their essential role in kinase activation, also serve a direct role in the recruitment of intracellular signaling molecules.

NPHS2, encoding the glomerular protein podocin, is mutated in autosomal recessive steroid-resistant nephrotic syndrome

Familial idiopathic nephrotic syndromes represent a heterogeneous group of kidney disorders, and include autosomal recessive steroid-resistant nephrotic syndrome, which is characterized by early childhood onset of proteinuria, rapid progression to end-stage renal disease and focal segmental glomerulosclerosis. A causative gene for this disease, NPHS2, was mapped to 1q25-31 and we report here its identification by positional cloning. NPHS2 is almost exclusively expressed in the podocytes of fetal and mature kidney glomeruli, and encodes a new integral membrane protein, podocin, belonging to the stomatin protein family. We found ten different NPHS2 mutations, comprising nonsense, frameshift and missense mutations, to segregate with the disease, demonstrating a crucial role for podocin in the function of the glomerular filtration barrier.

CIZ, a zinc finger protein that interacts with p130(cas) and activates the expression of matrix metalloproteinases

p130(cas) (Cas) is a docking protein that contains an SH3 domain and multiple tyrosine residues. p130(cas) is located at focal adhesions, is tyrosine phosphorylated in response to integrin stimulation, and is thought to transmit signals, via c-Crk and other proteins, for the remodeling of actin stress fibers and cell movement. In a search for the ligands of the SH3 domain of p130(cas) by far-Western screening, we cloned a novel protein named CIZ (for Cas-interacting zinc finger protein). CIZ consists of the following: a putative leucine zipper; a serine/threonine-rich region; a proline-rich sequence; five, six, or eight Krüppel-type C(2)H(2) zinc fingers; and the glutamine-alanine repeat. CIZ binds Cas in cells and is located in the nucleus and at focal adhesions. We showed that CIZ is a nucleocytoplasmic shuttling protein, by using the transient interspecies heterokaryon formation assay. In order to search for the targets of CIZ in nucleus, we determined the DNA binding consensus of CIZ as (G/C)AAAAA(A) by cyclic amplification and selection of targets analysis. The consensus-like sequences are found in several promoters of matrix metalloproteinases (MMPs), which are the enzymes used to degrade the extracellular matrix proteins. CIZ binds to a consensus-like sequence in the MMP-1 (collagenase) promoter. Overexpression of CIZ upregulates the transcriptions from MMP-1, MMP-3 (stromelysin), and MMP-7 (matrilysin) promoters, and this transactivation was enhanced in the presence of Cas. Furthermore, the stable overexpression of CIZ promoted the production of MMP-7 in culture medium. In summary, CIZ, a novel zinc finger protein, binds Cas, is a nucleocytoplasmic shuttling protein, and regulates the expression of MMPs.

Integrin signalling: a new Cas(t) of characters enters the stage

Cellular morphology is determined by the organization of the intracellular actin cytoskeleton, which is influenced by external and internal cues. Focal adhesions are sites at which the actin cytoskeleton is linked to the extracellular matrix by integrin receptor complexes. In addition to providing structural tethering points for cells, integrin receptor complexes transduce signals that influence a broad range of cellular processes, including migration, proliferation, transformation and apoptosis. The Cas proteins (p130Cas, HEF1/Cas-L and Efs/Sin), a family of docking proteins containing multiple interaction domains, are important components of integrin receptor signalling and have been implicated in all of these processes.

Cloning and characterization of a novel adaptor protein, CIN85, that interacts with c-Cbl

The c-Cbl protooncogene product is a prominent substrate of protein tyrosine kinases and is rapidly tyrosine-phosphorylated upon stimulation of a wide variety of cell-surface receptors. We have identified a novel c-Cbl-interacting protein termed CIN85 with a molecular mass of 85 kDa which shows similarity to adaptor proteins, CMS and CD2AP. CIN85 mRNA is expressed ubiquitously in normal human tissues and cancer cell lines analyzed. CIN85 was basally associated with c-Cbl. For interaction of CIN85 with c-Cbl, the second SH3 domain of CIN85 was shown to serve as a central player. The CIN85-c-Cbl association was enhanced shortly after stimulation of 293 cells with epidermal growth factor (EGF) and gradually diminished to a basal level, which correlated with a tyrosine phosphorylation level of c-Cbl. Our results suggest that CIN85 may play a specific role in the EGF receptor-mediated signaling cascade via its interaction with c-Cbl.

Congenital nephrotic syndrome in mice lacking CD2-associated protein

CD2-associated protein (CD2AP) is an 80-kilodalton protein that is critical for stabilizing contacts between T cells and antigen-presenting cells. In CD2AP-deficient mice, immune function was compromised, but the mice died at 6 to 7 weeks of age from renal failure. In the kidney, CD2AP was expressed primarily in glomerular epithelial cells. Knockout mice exhibited defects in epithelial cell foot processes, accompanied by mesangial cell hyperplasia and extracellular matrix deposition. Supporting a role for CD2AP in the specialized cell junction known as the slit diaphragm, CD2AP associated with nephrin, the primary component of the slit diaphragm.