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Molecular Dissection of Lymphocyte Signal Transduction Pathways

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Molecular Dissection of Lymphocyte Signal Transduction Pathways 003 I-3998/93/3301-OOS9$03.00/0 PEDIATRIC RESEARCH Vol. 33 (Suppl), No. 1, 1993 Copyright O 1992 International Pediatric Research Foundation, Inc. Printed in U.S.A. Molecular Dissection of Lymphocyte Signal Transduction Pathways ROGER M. PERLMUTTER Department of Immunology, Howard Hughes Medical Institute, University of Washington School ofMedicine, Seattle, Washington 981 95 ABSTRACT. The earliest biochemical changes in lympho- ble for coupling these receptors to the cell interior. We were cyte activation include the accumulation of phosphotyro- particularly intrigued by the idea that lymphocyte-specific pro- sine-containing proteins. This accumulation, catalyzed by tein tyrosine kinases might couple these receptors to other bio- specific protein tyrosine kinases, is required for subsequent chemical pathways. Our theory was not derived from detailed cell activation. Experimental manipulation of these protein knowledge about the importance of tyrosine phosphorylation. It tyrosine kinases results in improved or decreased transit simply seemed that in many respects the signal transduction through the activation sequence. Analysis of transgenic cascade initiated by lymphocyte cell surface receptors resembled animals bearing altered kinase genes permits assembly of the kind of signal transduction processes occurring with stimu- a model for signal transduction from the T-cell antigen lation of growth factor receptors known to possess intrinsic receptor. (Pediatr Res 33 (Suppl): S9-S15, 1993) protein tyrosine kinase activity (5). With this hypothesis in mind, we studied a variety of lymphoid cell lines, searching for those with elevated tyrosine kinase activ- ity. One of the cell lines-a mouse Moloney murine leukemia Shortly after crosslinking of the T-cell antigen receptor by virus-transformed cell line called LSTRA-was particularly in- cognate ligand, a series of biochemical changes results. The formative. earliest observable events are changes in the accumulation of The LSTRA cell line was found to contain high levels of a phosphorylated substrates, particularly a set of proteins specifi- specialized protein tyrosine kinase of 56 kD apparent molecular cally phosphorylated on tyrosine residues. This accumulation of mass. We were able to use molecular biologic techniques to substrates begins within seconds after ligand occupancy of the identify the gene that encodes this molecule (6) and learned that T-cell antigen receptor. Thereafter, changes in the turnover of it is a typical nonreceptor protein tyrosine kinase of the type membrane phospholipids (Fig. 1) and the accumulation of high exemplified by p6OSrc. concentrations of intracellular calcium occur transiently. All The src-family of protein tyrosine kinases all share a similar these alterations precede changes in gene expression, especially structure. These molecules are myristoylated at their amino the activation of a set of genes important for proliferation. termini, which presumably assists their interaction with the inner Included among these are genes encoding the transcription fac- leaflet of the plasma membrane. Across the entire structure, three tors, genes encoding lymphokine receptors, and the lymphokine distinct regions can be identified (Fig. 2). First, a unique amino genes themselves (1). These events together permit lymphocytes terminal domain structure, about 70 amino acids in length, to proceed along a course that ultimately will lead to DNA appears to control direct interaction with receptor structures. replication and proliferation. Then a set of src homology motifs appears, including sequences Two fundamental problems exist in understanding this se- that are involved in the binding of phosphotyrosine itself, and, quence of events. First, we have no idea of the causal relation- finally, there is a carboxyterminal kinase domain, the business ships among these various biochemical alterations. Is tyrosine end of the molecule responsible for phosphorylating substrates phosphorylation a cause of these subsequent events? Second, we (7). Included in the kinase domain is a critical site of regulation: have no idea how the receptor molecules on the surfaces of a tyrosine near the carboxy terminus that is ordinarily phos- lymphoid cells effect these biochemical changes. This article phorylated in vivo (8). If the phosphate is stripped from this site, focuses on that problem. the kinase becomes much more active. Study of this activity can Arrayed on the T lymphocyte surface is a set of molecules that be useful in investigating the function of these molecules. transmit signals to the cell interior. These structurally related There are now eight well-described members of the src family. molecules are all encoded by genes that are members of the Ig It is important to remember that these molecules are usually superfamily. They include the T-cell antigen receptor a and P specialized for expression in hematopoietic cells. For example, chains (2), the associated CD3 components of the T-cell receptor the blk kinase identified by Steve Desiderio and his colleagues at signaling complex (3), the coreceptors CD4 and CD8 (4), and Johns Hopkins University (9) is expressed only in B lymphocytes. other auxiliary signaling structures. All these molecules also share It performs some unique signal transduction function in those another aspect of structure: none has any obvious means of cells. The hck-encoded kinase is expressed primarily in granulo- transmitting signals to the cell interior in that they all have cytes (10). The lckgene, which we originally cloned from LSTRA relatively short cytoplasmic extensions that lack known catalytic cells, is expressed only in lymphoid cells, particularly T lympho- function. cytes (6). Thus, the Ick gene is an ideal candidate for a signal Some years ago, my group and others began studying the transduction molecule that might link lymphoid cell surface various signal transduction mechanisms that might be responsi- receptors to the cell interior. But what precise function does this protein tyrosine kinase Correspondence: Roger M. Perlmutter, M.D, Ph.D., Department of Immunol- perform? Lck transcripts accumulate to especially high levels in ogy, University of Washington School of Medicine, Health Sciences Building, Mail Stop SL-05, Seattle, WA 98195. thymocytes. This suggests that the Ick gene performs some func- Supported in part by a grant from the NIH (CA045682) and by the Howard tion critical to T lymphocyte development. Moreover, the lck Hughes Medical Institute. gene is expressed at the earliest time that one can identify T-cell slo PERLMUTTER tyrosine None of this theorizing advances our understanding of what phosphorylotion I the lck gene actually does; however, it suggests that p56lCkmay PIP2 turnover behave as a signal transduction molecule involved in the control of replication or those early activation events in T lymphocytes co '+influx Lymphokine that give rise to proliferation. Recent studies demonstrate that I fi expression p56lCkphysically associates with the CD4/CD8 coreceptor mole- cules on the T-cell surface. These observations argue that p56lCk may be brought into the T-cell receptor complex at the time of antigen recognition and may be directly involved in transmitting signals from that antigen receptor complex. Studies by our group and others have defined the mechanism Hours Post-Stimulation of association of CD4 and CD8 with p56" and have shown that cysteine residues near the N terminus of p56lCkand the C termi- Fig. 1. Early metabolic events after T-cell activation. Shown is a nus of these coreceptors are crucial for this interaction. Interrup- hypothetical time line for activation listing some of the major early tion of any of these four cysteine residues completely destroys biochemical changes that occur after T-cell receptor stimulation. NFAT the association (14). is a transcription factor complex involved in regulating IL-2 expression. Thus, the amino terminal domain of p56lCkis involved in ODC is ornithine decarboxylase. PIP2 is phosphatidylinositol-bis-phos- permitting the molecule to associate with the T-cell antigen phate. IL-2 rcp is interleukin-2-receptor. receptor complex by its interaction with the coreceptors, CD4 and CD8. This observation gives rise to a fairly simple hypothesis: Under ordinary circumstances, p56Ick is in some sort of equilib- rium state interacting with this coreceptor. At the time of antigen recognition, p56lCkbecomes involved in the complex and be- comes activated, presumably by dephosphorylation of a carboxy terminal tyrosine residue. Indeed, several groups have demon- strated that the CD45 phosphotyrosine phosphatase can dephos- Cysteine Mdii Tyrosine 394 phorylate tyrosine 505 and hence activate p56Ick in precisely the (site of CD4lCD8 interaction) (autophosphorylation site) I + I fashion previously explored using mutant lck cDNA introduced Putative hubstrate I into fibroblasts (1 5). Interadon Region Tyrosine 505 (regulatwy phoJphorylation site) To test this model, we hoped to artificially augment or decrease Fig. 2. Structural concept of a typical src-family protein tyrosine the amount of p56Ick activity present in otherwise normal T kinase. Shown is a schematic representation of p56Ick illustrating the lymphocytes, that is, in transgenic mice. Such experiments have positions of the amino-terminal and "src-homology" (SH3, SH2) do- numerous advantages over conventional transfection studies. mains. See text for details. First, the signaling molecules are expressed in a normal cellular
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