Transforming growth factor beta(TGFbeta) is a 25 kD protein which belongs to a family of ligands that regulate cell growth and differentiation. Although a number of TGFbeta binding species have been identified, the most commonly observed receptors are referred to as the type 1, type 2, and type 3 (betaglycan) TGFbeta receptor. At present, two general receptor models have been presented to account for the various cellular responses to TGFbeta. The first model proposes that all TGFbeta signaling results from the generation of a heteromeric type 1/type 2 TGFbeta receptor complex. A fundamental tenet to this model is that the type 1 TGFbeta receptor. The second model proposes that distinct TGFbeta receptor combinations mediate various aspects of TGFbeta signaling. For instance, effects of TGFbeta on extracellular matrix production and growth inhibition might result from the activation of differing receptor complexes. There is a large amount of evidence in support of both these models and it is likely that aspects of each will turn out to be t rue. In the present application two distinct approaches are proposed to address these general questions related to TGFbeta signaling. The first will use chimeric receptors consisting of the extracellular ligand binding domain of the GM-CSF alpha or beta receptor fused to the transmembrane and cytoplasmic domains of the type 1 or type 2 TGFbeta receptors. Since high affinity GM-CSF binding requires dimerization of the alpha and beta ligand binding domains, defined type 1 or type 2 TGFbeta cytoplasmic domain homodimers or heterodimers can be generated with essentially 100% specificity. Moreover, the absence of endogenous GM-CSF receptors and ability to isolate stable cell liens with equal receptor numbers and affinity provides the means to determine whether specific cellular responses are dependent upon activation of type 1/type 1 homodimers type 2/type 2 homodimers, type 1/type 2 heterodimers, and/or higher order complexes. The second approach will utilize a genetic screen to identify the signaling pathway(s) stimulated following TGFbeta receptor activation.
The specific aims to this application are: (1) determination of the TGFbeta receptor profile mediating endogenous biological and transcriptional responses; (2) determination of the effects of specific chimeric receptor complexes on downstream events dependent upon TGFbeta signaling; and (3) identification of the TGFbeta signaling pathway(s) using genetic analysis of yeast. The first two aims provide a comprehensive analysis of the receptor isoforms mediating distinct TGFbeta responses. In the third aim functional TGFbeta signaling will be reconstituted in the genetically amenable organism Saccharomyces cerevisiae. These studies have the potential to answer one of the more pressing questions in understanding TGFbeta action; that being what are the pathways activated following TGFbeta receptor activation?
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