Signal Transduction In B Lymphocytes: Identification Of Key Signaling Molecules

National Institute Of Allergy And Infectious Diseases

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Abstract

CD38 is a cell surface receptor capable of generating calcium-mobilizing second messengers. It has been implicated in host defense and cancer biology, but signaling mechanisms downstream of CD38 remain unclear. Mutations in LRRK2 (leucine-rich repeat kinase 2) are the most common genetic cause of Parkinson disease; it is also a risk factor for Crohn disease, leprosy, and certain types of cancers. The pathogenesis of these diseases involves inflammation and macroautophagy/autophagy, processes both CD38 and LRRK2 are implicated in. We have mechanistically and functionally linked CD38 and LRRK2 as upstream activators of TFEB (transcription factor EB), a host defense transcription factor and the master transcriptional regulator of the autophagy/lysosome machinery. In B-lymphocytes and macrophages, we show that CD38 and LRRK2 exist in a complex on the plasma membrane. Ligation of CD38 with the monoclonal antibody clone 90 results in internalization of the CD38-LRRK2 complex and its targeting to the endolysosomal system. This generates an NAADP-dependent calcium signal, which requires LRRK2 kinase activity, and results in the downstream activation of TFEB. lrrk2 KO macrophages accordingly have TFEB activation defects following CD38 or LPS stimulation and fail to switch to glycolytic metabolism after LPS treatment. In overexpression models, the pathogenic LRRK2G2019S mutant promotes hyperactivation of TFEB even in the absence of CD38, both by stabilizing TFEB and promoting its nuclear translocation via aberrant calcium signaling. In sum, we have identified a physiological CD38-LRRK2-TFEB signaling axis in immune cells. The common pathogenic mutant, LRRK2G2019S, appears to hijack this pathway. The cytosolic pattern recognition receptor NLRP3 senses host-derived danger signals and certain microbe-derived products in both humans and rodents. NLRP3 activation assembles an inflammasome complex, which contains the adapter protein ASC and caspase-1, whose activation triggers the maturation and release of the proinflammatory cytokines IL-1 and IL-18. Serine 5 (S5) phosphorylation of NLRP3 prevents its oligomerization and activation, while dephosphorylation of this residue by the phosphatase PP2A allows NLRP3 activation. However, the protein kinase that mediates NLRP3 S5 phosphorylation is unknown. We have shown that AKT associates with NLRP3 and phosphorylates it on S5, limiting NLRP3 oligomerization. This phosphorylation event also stabilizes NLRP3 by reducing its ubiquitination on lysine 496, which inhibits its proteasome mediated degradation by the E3 ligase Trim31. Pharmacologic manipulation of AKT kinase activity reciprocally modulates NLRP3 inflammasome mediated IL-1beta production. Inhibition of AKT reduced IL-1beta production following the intraperitoneal injection of LPS into mice. We propose that AKT, Trim31 and PP2A together modulate NLRP3 protein levels and tendency to oligomerize, thereby setting a tightly regulated threshold for NLRP3 activation. Ligation of the B cell integrin LFA-1 with ICAM-1 promotes B cell activation and immune synapse formation when membrane-bound antigen is limiting by promoting B cell adhesion. In this study we showed using super-resolution imaging of primary B cells that LFA-1: ICAM-1 interaction also promotes the formation of an actomyosin network that dominates the B cell immune synapse. This network is created by the formin mDia1 and organized into concentric, contractile arcs by bipolar filaments of myosin 2A. Quantitative time-lapse imaging showed that this network and the B cell receptor: antigen clusters present within it flow inward at the same rate. The imaging revealed individual B cell receptor microclusters being swept inward by individual actomyosin arcs. Under conditions where integrin co-stimulation promotes synapse formation, inhibiting myosin contractility impaired synapse formation, as evidenced by reduced antigen centralization, diminished B cell receptor-dependent signaling, and defects in signaling protein distribution at the synapse. Together, these results argue that a contractile actomyosin arc network created downstream of integrin ligation plays an important role in the mechanism by which LFA-1 co-stimulation promotes B cell activation and immune synapse formation (This study is an ongoing collaboration with John Hammers laboratory).