Cbl is the product of the protooncogene c-cbl and is involved in T cell antigen receptor (TCR)-mediated signaling. To understand the role of Cbl for immune system development and function, we generated a Cbl-deficient mouse strain. In Cbl-deficient mice, positive selection of the thymocytes expressing major histocompatibility complex class IIrestricted transgenic TCR was significantly enhanced. Two factors may have contributed to the altered thymic selection. First, Cbl deficiency markedly up-regulated the activity of ZAP-70 and mitogen-activated protein kinases. The mitogenactivated protein kinase pathway was shown previously to be involved in thymic positive selection. Second, Cbl-deficient thymocytes expressed CD3 and CD4 molecules at higher levels, which consequently may increase the avidity of TCR͞ major histocompatibility complex͞coreceptor interaction. Thus, Cbl plays a novel role in modulating TCR-mediated multiple signaling pathways and fine-tunes the signaling threshold for thymic selection.

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The apoptosis-linked gene product, ALG-2, is a member of the family of intracellular Ca 2؉ -binding proteins and a part of the apoptotic machinery controlled by T-cell receptor (TCR), Fas, and glucocorticoid signals. To explore the physiologic function of ALG-2 in T-cell development and function, we generated mice harboring a null mutation in the alg-2 gene. The alg-2 null mutant mice were viable and fertile and showed neither gross developmental abnormality nor immune dysfunction. Analyses of apoptotic responses of ALG-2-deficient T cells demonstrated that ALG-2 deficiency failed to block apoptosis induced by TCR, Fas, or dexamethasone signals. These findings indicate that ALG-2 is physiologically dispensable for apoptotic responses induced by the above signaling pathways and suggest that other functionally redundant proteins might exist in mammalian cells.Apoptosis is a unique biological process of multicellular organisms and one of the driving forces underlying tissue/ organ remodeling during animal development. This process also plays an important role during immune system development and function (1). Under physiological conditions, apoptotic response of lymphocytes is elicited by specialized molecular networks comprising of multiple signaling pathways and appears to be pivotal for shaping lymphocyte repertoires and preventing autoimmune diseases (8). Although large numbers of molecules involved in such networks have been identified recently (28), regulation and coordination between these molecules during immune system development and function remain elusive.ALG-2 is a 22-kDa Ca 2ϩ -binding protein belonging to the penta-EF (PEF) hand protein family that contains the Ca 2ϩ -binding helix-loop-helix structure (14,15,25). The PEF hand protein family includes peflin, sorcin, and grancalcin, as well as the large and small subunits of calpains (2,10,15,24). ALG-2 is ubiquitously expressed in mouse tissues, with its highest level of expression detected in thymus and liver. Recent data from T-cell lines indicate that ALG-2 protein plays a critical role for T-cell receptor (TCR)-, Fas-, and glucocorticoid-induced apoptosis, because depletion of ALG-2 in these cells blocks apoptosis whereas its overexpression significantly promotes apoptosis induced by these signals (5, 25). Although it is unclear how ALG-2 affects apoptosis of lymphocytes, evidence indicates that ALG-2 becomes rapidly associated with ALG-2-interacting protein-1 (AIP-1), a proapoptotic protein in a Ca 2ϩ -dependent manner, suggesting that the regulation is likely controlled by a secondary messenger, Ca 2ϩ (17,26). Detailed biochemical analysis also places ALG-2 downstream of the ICE/Ced-3 signaling cascade activated by TCR, Fas, and dexamethasone stimulation (11).To study the function of ALG-2 under physiological conditions, we generated ALG-2-deficient mice by using the gene targeting approach. Our data indicate that the general development and survival of mutant mice, as well as their immune system development and differentiation, appear to be nor...

Abstract. Spectrin is a major structural protein associated with the cytoplasmic surface of plasma membranes of many types of cells. To study the functions of spectrin, we transfected Caco-2 intestinal epithelial cells with a plasmid conferring neomycin resistance and encoding either actin-binding or ankyrin-binding domains of beta~-spectrin fused with beta-galactosidase. These polypeptides, in principle, could interfere with the interaction of spectrin with actin or ankyrin, as well as block normal assembly of alpha-and betaspectrin subunits. Cells expressing the fusion proteins represented only a small fraction of neomycin-resistant cells, but they could be detected based on expression of beta-galactosidase. Cells expressing spectrin domains exhibited a progressive decrease in amounts of endogenous betac-spectrin, although alpha-spectrin was still present. Betac-spectrin-deficient cells lost epithelial cell morphology, became multinucleated, and eventually disappeared after 10-14 d in culture. Spectrin-associated membrane proteins, ankyrin and adducin, as well as the Na÷,K÷-ATPase, which binds to ankyrin, exhibited altered distributions in cells transfected with betac-spectrin domains. E-cadherin and F-actin, in contrast to ankyrin, adducin, and the Na÷,K÷-ATPase, were expressed, and they exhibited unaltered distribution in betao-spectrin-deficient cells. Cells transfected with the same plasmid encoding betagalactosidase alone survived in culture as the major population of neomycin-resistant cells, and they exhibited no change in morphology or in the distribution of spectrin-associated membrane proteins. These results establish that betac-spectrin is essential for the normal morphology of epithelial cells, as well as for their maintenance in monolayer culture. S PV.CTRIN is an elongated actin-binding protein that is the principal component of a system of structural proteins associated with the cytoplasmic surface of plasma membranes of most metazoan cells (reviewed by Bennett and Gilligan, 1993). Spectrin is comprised of two subunits, termed alpha and beta, that are aligned side-to-side to form heterodimers, and the dimers are linked head-to-head to form tetramers. Beta subunits contain most of the recognition sites of spectrin for other proteins including ankyrin, protein 4.1 actin, as well as the site for ankyrin-independent association of spectrin with membranes. Beta~-spectrin is the most common type of beta subunit, and it is expressed in most vertebrate tissues (Hu et al., 1992). The structure and function of spectrin has been best characterized in mammalian erythrocytes from both in vivo and in vitro studies (Palek and Lambert, 1990;Delaunay and Dhermy, 1993;Gallagher and Forget, 1993;Bennett and Gilligan, 1993). Erythrocyte spectrin forms a membrane-associated polygonal network by the associations of spectrin with actin illaments and with integral membrane proteins through linkages with ankyrin and protein 4.1. Defects and deficiencies in components of the spectrin-actin network result in abnormally fr...

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