[Show abstract][Hide abstract]ABSTRACT: We previously reported that the mice deficient for SPA-1, a Rap1 GTPase-activating protein, developed hematopoietic stem cell disorders. Here, we demonstrate that SPA-1(-/-) mice show an age-dependent increase in B220(high) B1a cells producing anti-dsDNA antibody and lupus-like nephritis. SPA-1(-/-) peritoneal B1 cells revealed the altered Vkappa gene repertoire, including skewed Vkappa4 usage and the significant Igkappa/Iglambda isotype inclusion indicative of extensive receptor editing. Rap1GTP induced OcaB gene activation via p38MAPK-dependent Creb phosphorylation, and consistently, SPA-1(-/-) immature BM B cells showing high Rap1GTP exhibited the augmented expression of OcaB and Vkappa4 genes. SPA-1(-/-) BM cells could transfer the autoimmunity in association with the generation of peritoneal B220(high) B1a cells in Rag-2(-/-) recipients. Finally, a portion of SPA-1(-/-) mice developed B1 cell leukemia with hemolytic autoantibody. Present results suggest that the regulated Rap1 signal in the immature B cells plays a role in modifying the B cell receptor repertoire and in maintaining the self-tolerance.
[Show abstract][Hide abstract]ABSTRACT: Rap1 is a member of the Ras family of GTPases and, depending on the cellular context, has an important role in the regulation of proliferation or cell adhesion. In lymphohematopoietic tissues, SPA-1 is a principal Rap1 GTPase-activating protein. Mice that are deficient for the SPA-1 gene develop age-dependent progression of T-cell immunodeficiency followed by a spectrum of late onset myeloproliferative disorders, mimicking human chronic myeloid leukemia. Recent studies reveal that deregulated Rap1 activation in SPA-1-deficient mice causes enhanced expansion of the bone marrow hematopoietic progenitors, but induces progressive unresponsiveness or anergy in T cells. Rap1 and its regulator, SPA-1, could, therefore, provide unique molecular targets for the control of human hematologic malignancy.
[Show abstract][Hide abstract]ABSTRACT: SPA-1 is a principal Rap1 GTPase-activating protein in the hematopoietic progenitors and peripheral T cells, and SPA-1-deficient mice develop a spectrum of myeloproliferative stem cell disorders of late onset. In the present study, we show that SPA-1-deficient mice develop age-dependent T cell unresponsiveness preceding the myeloid disorders, whereas the T cell numbers remained unchanged. Progression of the T cell dysfunction was attributed to the age-dependent increase in CD44high T cell population that was unresponsive to T cell receptor stimulation. Younger SPA-1-deficient mice exhibited selectively impaired recall T cell responses against a T-dependent antigen with normal primary antibody response. These results suggested that the unresponsiveness of CD44high T cells was antigen-driven in vivo. T cells from younger SPA-1-/- mice showed much greater and more persisted Rap1 activation by anti-CD3 stimulation than control T cells. Furthermore, freshly isolated T cells from SPA-1-/- mice exhibited progressive accumulation of Rap1GTP as mice aged. T cells from aged SPA-1-/- mice with high amounts of Rap1GTP showed normal or even enhanced Ras activation with little extracellular signal-regulated kinase activation in response to anti-CD3 stimulation, indicating that excess Rap1GTP induced the uncoupling of Ras-mediated extracellular signal-regulated kinase activation. These results suggested that antigenic activation of naïve T cells in SPA-1-/- mice was followed by anergic rather than memory state due to the defective down-regulation of Rap1 activation, resulting in the age-dependent progression of overall T cell immunodeficiency.
Full-text Article · Oct 2003 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract]ABSTRACT: SPA-1 (signal-induced proliferation-associated gene-1) is a principal Rap1 GTPase-activating protein in hematopoietic progenitors. SPA-1-deficient mice developed a spectrum of myeloid disorders that resembled human chronic myelogenous leukemia (CML) in chronic phase, CML in blast crisis, and myelodysplastic syndrome as well as anemia. Preleukemic SPA-1-deficient mice revealed selective expansion of marrow pluripotential hematopoietic progenitors, which showed abnormal Rap1GTP accumulation. Overexpression of an active form of Rap1 promoted the proliferation of normal hematopoietic progenitors, while SPA-1 overexpression markedly suppressed it. Furthermore, restoring SPA-1 gene in a SPA-1-deficient leukemic blast cell line resulted in the dissolution of Rap1GTP accumulation and concomitant loss of the leukemogenicity in vivo. These results unveiled a role of Rap1 in myeloproliferative stem cell disorders and a tumor suppressor function of SPA-1.