Accounting for specificity in receptor tyrosine kinase signaling.

the pathways may be functionally redundant (Figure 1A). Many signal transduction mechanisms transmit informaIf this view is correct, RTK pathways may function merely tion from the extracellular space or an intracellular comto activate transcription factors generically, whose acpartment to the nucleus where they influence gene extivities would determine the consequences of signaling pression. This flow can be viewed metaphorically as the and whose identities would be specified by cell type reverse of gene expression where information moves (Figure 1B). As the authors themselves pointed out, it outward from the genome. However, the idea that signal is important to extend these studies beyond laboratorytransduction processes convey detailed amounts of inadapted tissue culture cells (Fambrough et al., 1999). formation has been recently debated with respect to Engineered animals offer an appropriate test since in receptor tyrosine kinases (RTKs). Upon binding to their principle ligands and receptors are expressed at physioligands, RTK molecules dimerize and undergo autophoslogical levels and in the correct temporal pattern and phorylation on specific tyrosine residues. In turn, these tissue context. Knockin studies in mice of the PDGFR phosphotyrosines and their adjacent sequences specifidescribed above have yielded some evidence for discally recruit downstream signaling molecules via their tinct functions for SH2 docking sites in vivo. Mice conSH2 domains, causing their subsequent activation. The taining a mutation in the PI-3K docking site were fully activation of a single type of RTK thus leads to the viable (in contrast to the null which dies during developactivation of multiple intracellular signal transduction ment due to cardiovascular, renal, and hematological pathways. The naive expectation would be that each defects), but exhibited a defect in interstitial fluid hopathway would have a distinct function. However, a meostasis (Heuchel et al., 1999). A double mutant in the number of studies have called this presumption into PI-3K and PLC docking sites again resulted in viable question. For example, a study of mutants of the platemice, but these showed a defect in vascular cell fitness let-derived growth factor receptor RTK led to the surin chimeric mice (Tallquist et al., 2000). Studies in mice prising conclusion that the downstream pathways are of other RTKs have also supported the view that the in fact redundant in terms of what genes they activate docking sites are not totally redundant. For example, (Fambrough et al., 1999). I will first focus on these mutation of PLC docking site of the essential FGFR1 thought-provoking cell culture experiments before deRTK yields viable mice with a specific homeotic transforscribing studies in whole animals that indicate that a mation (Partanen et al., 1998). The c-Kit RTK is required significant amount of specific (i.e., nonredundant) inforfor normal hematopoesis, melanogenesis, and gametomation can be transmitted during RTK signaling. genesis. Strikingly, mutation of the PI-3K docking site To address whether distinct SH2 domain-containing of this receptor results in a specific defect in gametogensignaling molecules send qualitatively different signals, esis (Blume-Jensen et al., 2000; Kissel et al., 2000). Two Fambrough et al. (1999) created mutations in SH2 bindrecent papers published in Molecular Cell inform further ing sites in the platelet-derived growth factor receptor the issue of RTK specificity using engineered mice. (PDGFR ). NIH 3T3 cells were transfected with conIn experiments by Klinghoffer and colleagues, the instructs encoding hybrid receptors with an intracellular tracellular domains of the and isoforms of the PDGF domain from the PDGFR and the extracellular domain receptor (encoded by distinct genes) were exchanged of the M-CSF receptor. Remarkably, simultaneous muto address the issue of RTK effector redundancy (Klingtation of five SH2 binding sites, which drastically rehoffer et al., 2001). The experiment asked whether reduces the biological responsiveness of PDGFR funcplacement of the PDGFR intracellular domain with that tion in cultured cells, had only modest quantitative of would result in a functional receptor as assayed in effects on immediate-early gene inductions in response a knockin mouse. The reciprocal experiment was also to the heterologous ligand M-CSF (which avoided actiperformed. While the two isoforms are known to bind vation of endogenous PDGF receptors) as measured many of the same effectors such as Src, PI-3K, SHP-2,