The adaptor protein 3BP2 associates with VAV guanine nucleotide exchange factors to regulate NFAT activation by the B-cell antigen receptor

SH3BP2 is an activator of NFAT activity and osteoclastogenesis

Heterozygous activating mutations in exon 9 of SH3BP2 have been found in most patients with cherubism, an unusual genetic syndrome characterized by excessive remodeling of the mandible and maxilla due to spontaneous and excessive osteoclastic bone resorption. Osteoclasts differentiate after binding of sRANKL to RANK induces a number of downstream signaling effects, including activation of the calcineurin/NFAT (nuclear factor of activated T cells) pathway. Here, we have investigated the functional significance of SH3BP2 protein on osteoclastogenesis in the presence of sRANKL. Our results indicate that SH3BP2 both increases nuclear NFATc1 in sRANKL treated RAW 264.7 preosteoclast cells and enhances expression of tartrate resistant acid phosphatase (TRAP), a specific marker of osteoclast differentiation. Moreover, overexpression of SH3BP2 in RAW 264.7 cells potentiates sRANKL-stimulated phosphorylation of PLCgamma1 and 2, thus providing a mechanistic pathway for the rapid translocation of NFATc1 into the nucleus and increased osteoclastogenesis in cherubism.

Phospholipase C-gamma2 and Vav cooperate within signaling microclusters to propagate B cell spreading in response to membrane-bound antigen

B cell receptor (BCR) recognition of membrane-bound antigen initiates a spreading and contraction response, the extent of which is controlled through the formation of signaling-active BCR-antigen microclusters and ultimately affects the outcome of B cell activation. We followed a genetic approach to define the molecular requirements of BCR-induced spreading and microcluster formation. We identify a key role for phospholipase C-γ2 (PLCγ2), Vav, B cell linker, and Bruton's tyrosine kinase in the formation of highly coordinated “microsignalosomes,” the efficient assembly of which is absolutely dependent on Lyn and Syk. Using total internal reflection fluorescence microscopy, we examine at high resolution the recruitment of PLCγ2 and Vav to microsignalosomes, establishing a novel synergistic relationship between the two. Thus, we demonstrate the importance of cooperation between components of the microsignalosome in the amplification of signaling and propagation of B cell spreading, which is critical for appropriate B cell activation.

[The adaptor protein 3BP2 in leukocyte signaling]

Adaptor proteins that do not contain intrinsic enzymatic activity play a critical role in cell biology by regulating the assembly of large multimolecular signaling complexes involved in extracellular signal transduction. The increasing number of diseases associated with aberrant function or expression of adaptor proteins further illustrate their key role in cellular regulation. The adaptor 3BP2 (or SH3BP2) was originally identified more than 10 years ago as an c-Abl binding protein, and next as a partner of Syk family kinases in 1998. 3BP2 displays the typical modular organization of an adapter protein with an amino-terminal PH domain, a central proline rich region and a carboxyl-terminal SH2 domain. Although its physiological function remains unknown, studies have implicated a role for 3BP2 in immunoreceptor signaling through its interaction with a number of signaling molecules including Src and Syk families of protein tyrosine kinases, the membrane adaptor LAT, Vav exchange factors, PLC-gamma, and 14-3-3 proteins. Recently, the 3bp2/sh3bp2 locus was shown to be mutated in a rare human disease involved in cranial-facial development called cherubism, suggesting a role for 3BP2 in regulating osteoclast and hematopoietic cell function.

SH3BP2 is rarely mutated in exon 9 in giant cell lesions outside cherubism

Giant cell tumor of bone and giant cell reparative granuloma are benign lesions with prominent giant (multinucleated) cells, and an understanding of the molecular biology and genetics of these lesions will likely aid in more effective treatment. Cherubism is a benign lesion of the maxilla and mandible histologically similar to giant cell tumor of bone and giant cell reparative granuloma. Germline mutations in exon 9 of the gene encoding Src homology 3 binding protein 2 (SH3BP2) occur in most patients with cherubism. We therefore hypothesized SH3BP2 and its putative downstream effector nuclear factor of activated T cells c1 isoform (NFATc1) are highly expressed in sporadic nonsyndromic giant cell lesions and associated with somatic SH3BP2 mutations. We analyzed giant cell lesions for SH3BP2 and NFATc1 expression by RNA blot and/or immunohistochemistry and for exon 9 SH3BP2 mutations. We found the SH3BP2 transcripts and protein were abundantly expressed in giant cell tumors of bone, as well as NFATc1 protein. Sequencing of exon 9 of SH3BP2 was normal in all sporadic nonsyndromic giant cell lesions. Although many multinucleated giant cell lesions of bone share histologic features, the primary genetic defect in cherubism and these other giant cell lesions appears different.

Increased myeloid cell responses to M-CSF and RANKL cause bone loss and inflammation in SH3BP2 "cherubism" mice

While studies of the adaptor SH3BP2 have implicated a role in receptor-mediated signaling in mast cells and lymphocytes, they have failed to identify its function or explain why SH3BP2 missense mutations cause bone loss and inflammation in patients with cherubism. We demonstrate that Sh3bp2 "cherubism" mice exhibit trabecular bone loss, TNF-alpha-dependent systemic inflammation, and cortical bone erosion. The mutant phenotype is lymphocyte independent and can be transferred to mice carrying wild-type Sh3bp2 alleles through mutant fetal liver cells. Mutant myeloid cells show increased responses to M-CSF and RANKL stimulation, and, through mechanisms of increased ERK 1/2 and SYK phosphorylation/activation, they form macrophages that express high levels of TNF-alpha and osteoclasts that are unusually large. M-CSF and RANKL stimulation of myeloid cells that overexpress wild-type SH3BP2 results in similar large osteoclasts. This indicates that the mutant phenotype reflects gain of SH3BP2 function and suggests that SH3BP2 is a critical regulator of myeloid cell responses to M-CSF and RANKL stimulation.

Abl-SH3 binding protein 2, 3BP2, interacts with CIN85 and HIP-55

The adapter 3BP2 is involved in leukocyte signaling downstream Src/Syk-kinases coupled immunoreceptors. Here, we show that 3BP2 directly interacts with the endocytic scaffold protein CIN85 and the actin-binding protein HIP-55. 3BP2 co-localized with CIN85 and HIP-55 in T cell rafts and at the T cell/APC synapse, an active zone of receptors and proteins recycling. A binding region of CIN85 SH3 domains on 3BP2 was mapped to a PVPTPR motif in the first proline-rich region of 3BP2, whereas the C-terminal SH3 domain of HIP-55 bound a more distal proline-rich domain of 3BP2. Together, our data suggest an unexpected role of 3BP2 in endocytic and cytoskeletal regulation through its interaction with CIN85 and HIP-55.

The 3BP2 adapter protein is required for optimal B-cell activation and thymus-independent type 2 humoral response

3BP2 is a pleckstrin homology domain- and Src homology 2 (SH2) domain-containing adapter protein that is mutated in the rare human bone disorder cherubism and which has also been implicated in immunoreceptor signaling. However, a function for this protein has yet to be established. Here we show that mice lacking 3BP2 exhibited a perturbation in the peritoneal B1 and splenic marginal-zone B-cell compartments and diminished thymus-independent type 2 antigen response. 3BP2(-/-) B cells demonstrated a proliferation defect in response to antigen receptor cross-linking and a heightened sensitivity to B-cell receptor-induced death via a caspase-3-dependent apoptotic pathway. We show that 3BP2 binds via its SH2 domain to the CD19 signaling complex and is required for optimum Syk phosphorylation and calcium flux.

3BP2 deficiency impairs the response of B cells, but not T cells, to antigen receptor ligation

The adapter protein 3BP2 is expressed in lymphocytes; binds to Syk/ZAP-70, Vav, and phospholipase C-gamma (PLC-gamma); and is thought to be important for interleukin-2 gene transcription in T cells. To define the role of 3BP2 in lymphocyte development and function, we generated 3BP2-deficient mice. T-cell development, proliferation, cytokine secretion, and signaling in response to T-cell receptor (TCR) ligation were all normal in 3BP2(-/-) mice. 3BP2(-/-) mice had increased accumulation of pre-B cells in the bone marrow and a block in the progression of transitional B cells in the spleen from the T1 to the T2 stage, but normal numbers of mature B cells. B-cell proliferation, cell cycle progression, PLC-gamma2 phosphorylation, calcium mobilization, NF-ATp dephosphorylation, and Erk and Jnk activation in response to B-cell receptor (BCR) ligation were all impaired. These results suggest that 3BP2 is important for BCR, but not for TCR signaling.

The GTPase RhoA increases utrophin expression and stability, as well as its localization at the plasma membrane

The Rho family of small GTPases are signalling molecules involved in cytoskeleton remodelling and gene transcription. Their activities are important for many cellular processes, including myogenesis. In particular, RhoA positively regulates skeletal-muscle differentiation. We report in the present study that the active form of RhoA increases the expression of utrophin, the autosomal homologue of dystrophin in the mouse C2C12 and rat L8 myoblastic cell lines. Even though this RhoA-dependent utrophin increase is higher in proliferating myoblasts, it is maintained during myogenic differentiation. This occurs via two mechanisms: (i) transcriptional activation of the utrophin promoter A and (ii) post-translational stabilization of utrophin. In addition, RhoA increases plasma-membrane localization of utrophin. Thus RhoA activation up-regulates utrophin levels and enhances its localization at the plasma membrane.

Impaired FcepsilonRI-dependent gene expression and defective eicosanoid and cytokine production as a consequence of Fyn deficiency in mast cells

Fyn kinase is a key contributor in coupling FcepsilonRI to mast cell degranulation. A limited macroarray analysis of FcepsilonRI-induced gene expression suggested potential defects in lipid metabolism, eicosanoid and glutathione metabolism, and cytokine production. Biochemical analysis of these responses revealed that Fyn-deficient mast cells failed to secrete the inflammatory eicosanoid products leukotrienes B4 and C4, the cytokines IL-6 and TNF, and chemokines CCL2 (MCP-1) and CCL4 (MIP-1beta). FcepsilonRI-induced generation of arachidonic acid and normal induction of cytokine mRNA were defective. Defects in JNK and p38 MAPK activation were observed, whereas ERK1/2 and cytosolic phospholipase A2 (S505) phosphorylation was normal. Pharmacological studies revealed that JNK activity was associated with generation of arachidonic acid. FcepsilonRI-mediated activation of IkappaB kinase beta and IkappaBalpha phosphorylation and degradation was defective resulting in a marked decrease of the nuclear NF-kappaB DNA binding activity that drives IL-6 and TNF production in mast cells. However, not all cytokine were affected, as IL-13 production and secretion was enhanced. These studies reveal a major positive role for Fyn kinase in multiple mast cell inflammatory responses and demonstrate a selective negative regulatory role for certain cytokines.

Constrictor-induced translocation of NFAT3 in human and rat pulmonary artery smooth muscle

The transcription factor nuclear factor of activated T cells (NFAT) resides in the cytoplasm in resting cells and upon stimulation is dephosphorylated, translocates to the nucleus, and becomes transcriptionally active. NFAT is commonly activated by stimulation of receptors coupled to Ca2+mobilization; however, little is known about the regulation of NFAT in pulmonary vascular smooth muscle. The aim of this study was to investigate regulation of NFAT in human and rat intralobar pulmonary artery by two constrictors: phenylephrine (PE) and 20-hydroxyeicosatetraenoic acid (20-HETE), a cytochrome P-450 metabolite formed endogenously in lungs. Immunostaining of smooth muscle cells revealed cytoplasmic localization of NFAT in untreated cells, and PE or 20-HETE induced translocation to the nucleus, with maximal effect at 30 min. Cyclosporin A and FK-506 (both 1 μM) inhibited NFAT translocation, indicating involvement of calcineurin. Moreover, the Rho-kinase blocker Y-27632 prevented translocation. Translocation of NFAT was confirmed by Western blots, with NFAT3 the prominent isoform in pulmonary artery. Constrictors caused calcineurin-sensitive translocation of NFAT to nuclei in intact arteries, demonstrating regulation in native tissue. To investigate a role for Ca2+, cells were loaded with fura-2. Whereas PE caused an acute transient rise of [Ca2+]i, 20-HETE caused a prolonged low amplitude rise of [Ca2+]i. The involvement of Rho-kinase in PE- and 20-HETE-induced NFAT3 translocation in pulmonary artery suggests a level of control not previously recognized in smooth muscle. Constrictors of the pulmonary vasculature not only cause acute responses but also activate NFAT, which may alter gene expression in pulmonary health and disease.

NFAT transcription factors--new players in the pathogenesis of inflammatory arthropathies?

Inflammatory arthropathies are characterized by major changes in gene expression, which-ultimately-result from differential activities of intracellular signaling pathways and their associated inducible transcription factors. The nuclear factor of activated T cells' (NFAT) family of transcription factors plays diverse roles in a variety of processes in the immune system and other tissues. Preliminary evidence has recently emerged implicating NFAT family members directly in the pathogenesis of inflammatory arthropathies. Specific anti-NFAT drug therapy may add to the pharmacologic armamentarium against rheumatoid arthritis, other inflammatory arthropathies, and related autoimmune disorders.