Identification of the cytoskeletal regulatory protein alpha-adducin as a target of T cell receptor signaling.

Quiescent T lymphocytes reorganize the actin cytoskeleton subsequent to interaction with antigen presenting cells bearing the appropriate peptide antigen. Although both biochemical and genetic evidence indicate that T cell receptor-dependent cytoskeletal reorganization is critical to T cell activation, the mechanisms that mediate this process remain poorly defined. In this study, the cytoskeletal regulatory protein alpha-adducin was identified as a novel target of TCR signaling in primary T lymphocytes through the biochemical purification of an unknown 120 kDa protein (p120) defined by a fortuitously cross-reactive phospho-sensitive antiserum. The epitope identified by this antiserum defines a previously unrecognized site of phosphorylation localized to amino acids 590-620. Both TCR cross-linking and exposure to phorbol ester resulted in the phosphorylation-dependent elimination of this epitope. However, while phorbol ester induced rapid phosphorylation of both the phospho-sensitive epitope and the functionally defined major protein kinase C site present near the carboxy-terminus (serine 724) of alpha-adducin, only the phospho-sensitive epitope was modified upon activation through the TCR. Moreover, inhibition of actin polymerization by cytochalasin D blocked this modification. Of particular importance, alpha-adducin was not expressed in T cell lines, was completely down-regulated in primary T cells within 24h of activation and was reduced in quiescent memory T cells. These results suggest a model in which reorganization of the unique cytoskeletal network that defines a primary quiescent T lymphocyte is mediated in part through the TCR-dependent modification and subsequent down-regulation of alpha-adducin, thereby resulting in a cytoskeletal architecture compatible with T cell effector and memory functions.

NFAT5/TonEBP mutant mice define osmotic stress as a critical feature of the lymphoid microenvironment.

Osmotic stress responses are critical not only to the survival of unicellular organisms but also to the normal function of the mammalian kidney. However, the extent to which cells outside the kidney rely on osmotic stress responses in vivo remains unknown. Nuclear factor of activated T cells 5 (NFAT5)/tonicity enhancer binding protein (TonEBP), the only known osmosensitive mammalian transcription factor, is expressed most abundantly in the thymus and is induced upon lymphocyte activation. Here we report that NFAT5/TonEBP is not only essential for normal cell proliferation under hyperosmotic conditions but also necessary for optimal adaptive immunity. Targeted deletion of exons 6 and 7 of the Nfat5 gene, which encode a critical region of the DNA-binding domain, gave rise to a complete loss of function in the homozygous state and a partial loss of function in the heterozygous state. Complete loss of function resulted in late gestational lethality. Furthermore, hypertonicity-induced NFAT5/TonEBP transcriptional activity and hsp70.1 promoter function were completely eliminated, and cell proliferation under hyperosmotic culture conditions was markedly impaired. Partial loss of NFAT5/TonEBP function resulted in lymphoid hypocellularity and impaired antigen-specific antibody responses in viable heterozygous animals. In addition, lymphocyte proliferation ex vivo was reduced under hypertonic, but not isotonic, culture conditions. Direct measurement of tissue osmolality further revealed lymphoid tissues to be hyperosmolar. These results indicate that lymphocyte-mediated immunity is contingent on adaptation to physiologic osmotic stress, thus providing insight into the lymphoid microenvironment and the importance of the NFAT5/TonEBP osmotic stress response pathway in vivo.