Deubiquitinating enzyme CYLD negatively regulates the ubiquitin-dependent kinase Tak1 and prevents abnormal T cell responses.

The deubiquitinating enzyme CYLD has recently been implicated in the regulation of signal transduction, but its physiological function and mechanism of action are still elusive. In this study, we show that CYLD plays a pivotal role in regulating T cell activation and homeostasis. T cells derived from Cyld knockout mice display a hyperresponsive phenotype and mediate the spontaneous development of intestinal inflammation. Interestingly, CYLD targets a ubiquitin-dependent kinase, transforming growth factor-beta-activated kinase 1 (Tak1), and inhibits its ubiquitination and autoactivation. Cyld-deficient T cells exhibit constitutively active Tak1 and its downstream kinases c-Jun N-terminal kinase and IkappaB kinase beta. These results emphasize a critical role for CYLD in preventing spontaneous activation of the Tak1 axis of T cell signaling and, thereby, maintaining normal T cell function.

Defective feedback regulation of NF-kappaB underlies Sjogren's syndrome in mice with mutated kappaB enhancers of the IkappaBalpha promoter.

Feedback regulation of transcription factor NF-kappaB by its inhibitor IkappaBalpha plays an essential role in control of NF-kappaB activity. To understand the biological significance of IkappaBalpha-mediated feedback regulation of NF-kappaB, we generated mice harboring mutated kappaB enhancers in the promoter of the IkappaBalpha gene (IkappaBalpha(M/M)) to inhibit NF-kappaB-regulated IkappaBalpha expression. Here, we report that these mutant mice are defective in NF-kappaB-induced expression of IkappaBalpha. This defective feedback regulation of NF-kappaB by IkappaBalpha not only altered activity of NF-kappaB, but also the expression of NF-kappaB-regulated genes. As a result, IkappaBalpha(M/M), the homozygous knock-in mice with mutated kappaB enhancers in the IkappaBalpha promoter, acquire shorten life span, hypersensitivity to septic shock, abnormal T-cell development and activation, and Sjögren's Syndrome. These findings therefore demonstrate that the IkappaBalpha-mediated feedback regulation of NF-kappaB has an essential role in controlling T-cell development and functions, provide mechanistic insight into the development of Sjögren's Syndrome, and suggest the potential of NF-kappaB signaling as a therapeutic target for Sjögren's Syndrome and other autoimmune diseases.

Regulation of early wave of germ cell apoptosis and spermatogenesis by deubiquitinating enzyme CYLD.

Spermatogenesis involves an early wave of germ cell apoptosis, which is required for maintaining the balance between germ cells and supporting Sertoli cells. However, the signaling mechanism regulating this apoptotic event is poorly defined. Here we show that genetic deficiency of Cyld, a recently identified deubiquitinating enzyme, attenuates the early wave of germ cell apoptosis and causes impaired spermatogenesis in mice. Interestingly, the loss of CYLD in testicular cells leads to activation of the transcription factor NF-kappaB and aberrant expression of antiapoptotic genes. We further show that CYLD negatively regulates a ubiquitin-dependent NF-kappaB activator, RIP1. CYLD binds to RIP1 and inhibits its ubiquitination and signaling function. These findings establish CYLD as a pivotal deubiquitinating enzyme (DUB) that regulates germ cell apoptosis and spermatogenesis and suggest an essential role for CYLD in controlling the RIP1/NF-kappaB signaling axis in testis.

CARMA1 regulation of regulatory T cell development involves modulation of interleukin-2 receptor signaling.

T cell receptor-stimulated NF-kappaB activation requires CARMA1 and is negatively regulated by the deubiquitinase CYLD. Recent studies suggest that CARMA1 regulates regulatory T cell (Treg) development, although the role of NF-kappaB in this event is incompletely understood. We show that CYLD deficiency causes constitutive NF-kappaB activation in thymocytes, which is associated with enhanced frequency of Treg cells. The NF-kappaB activation in CYLD-deficient thymocytes is independent of CARMA1, because the NF-kappaB activation was also detected in CYLD/CARMA1 double knock-out thymocytes. Interestingly, although loss of CYLD causes NF-kappaB activation in the CARMA1-deficient thymocytes, the CYLD deficiency fails to rescue the defect of CARMA1 knock-out mice in Treg development. Furthermore, inhibition of canonical NF-kappaB by an IkappaBalpha transgene only partially inhibits Treg development. We demonstrate that CARMA1 regulates IL-2 receptor signaling and controls the IL-2-stimulated maturation of Treg precursors to mature Tregs. These results suggest that the role of CARMA1 in Treg regulation involves both NF-kappaB activation and IL-2 receptor signaling.