Regualaton and function of the K channel KCa3.1 and lymphocyte activation
The Ca2+-activated K+ channel, KCa3.1, is required for Ca2+ influx and the subsequent activation of B and T cells. Inhibitors of KCa3.1 are in development to treat autoimmune diseases and transplant rejection, underscoring the importance in understanding how these channels are regulated. We recently identified several new signaling molecules that are critical for regulating KCa3.1 channel activity in human CD4+ T cells. We found that: 1) the lipid phophatidylinositol 3 phosphate (PI3P) is required for KCa3.1 channel activity and that the PI3P phosphatase, myotubularin related protein 6 (MTMR6) negatively regulates KCa3.1 by dephosphorylating PI3P; 2) the class 2 PI3K-C2b is activated following TCR activation and generates the PI3P pool required for KCa3.1 activation; 3) Nucleoside Diphosphate Kinase Beta (NDPK-B), a mammalian histidine kinase, functions downstream of PI3P and is required for KCa3.1 channel activation by phosphorylating histidine (H) 358 in the carboxyl terminus of KCa3.1; and 4) the histidine phosphatase protein histidine phosphatase-1 (PHPT-1) directly binds and dephosphorylates H358 on KCa3.1 leading to KCa3.1 channel inhibition. These findings provide one of the best examples whereby a mammalian histidine kinase and histidine phosphatase regulates a biological process in mammals. Moreover, these studies identify for the first time that PI3K-C2b and NDPK-B are required for activation of a subset of human CD4+ T cells and that MTMR6 and PHPT-1 function to inhibit activation of these cells. We are currently working to understand the mechanism(s) whereby PI3K-C2b, NDPK-B, PHPT-1, and MTMR6 are regulated in CD4+ T cells. We have also generated knockout mice to study the role of these molecules in vivo. These studies should uncover novel pathways that regulate T cell activation and may identify new mechanisms whereby aberrant activation of these pathways can contribute to autoimmune diseases.
Inhibiting KCa3.1 is a potential treatment for autoimmune disease
We found that both Ca2+ influx and cytokine production by Th1 and Th2 CD4 T cells were impaired in KCa3.1-/- mice, while T-regulatory and Th17 function were normal. Morevoer, KCa3.1-/- mice were protected from developing severe colitis in two mouse models of inflammatory bowel disease (IBD), which included the adoptive transfer of KCa3.1-/- or wild-type naïve CD4 T cells into rag2-/- recipients and trinitobencene sulfonic acid (TNBS)-induced colitis. Pharmacologic inhibitors of KCa3.1 have already been shown to be safe in humans. Thus, if these preclinical studies continue to show efficacy, it may be possible to rapidly test whether KCa3.1 inhibitors are efficacious in patients with IBD.
Autosomal Dominant Polycystic Kidney Disease
Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the presence of innumerous fluid-filled cysts in the kidneys and is a common cause of renal failure. Net fluid secretion into renal cysts is driven by transepithelial Cl- secretion mediated by apical cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels and is an important factor in kidney enlargement. Our recent studies demonstrated that KCa3.1 also plays a critical role in the regulation of CFTR-mediated Cl- secretion and cyst formation in normal human kidney epithelia (NHK) cells and epithelial cells derived from the cysts of ADPKD kidneys. Moreover, we found that treatment of various mouse models of polycystic kidney disease with the KCa3.1 inhibitor, TRAM34, significantly decreased the formation of cysts in these animals. We currently exploring the use of KCa3.1 inhibitors as a potential new therapy to treat patients with ADPKD.