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Neuronal cell fate decisions are directed in Drosophila by NUMB, a signaling adapter protein with two protein-protein interaction domains: a phosphotyrosinebinding domain and a proline-rich region (PRR) that functions as an SH3-binding domain. Here we show that there are at least four human NUMB isoforms and that these serve two distinct developmental functions in the neuronal lineage: differentiation (but not proliferation) is promoted by human NUMB protein isoforms with a type I (short) PRR. In contrast, proliferation (but not differentiation) is directed by isoforms that have a type II (long) PRR. The two types of PRR may promote distinct intracellular signaling pathways downstream of the NOTCH receptor during mammalian neurogenesis.During Drosophila peripheral nervous system development, a sensory organ precursor cell divides twice to produce the four cells that form a functional sensory organ (neuron, sheath, hair, and socket). NUMB functions in this lineage to direct specific binary cell fate choices: the IIb vs. IIa fate at the first division and at the second division, neuron vs. sheath as well as hair vs. socket (1). Absence of NUMB results in production of two IIa cells that then divide to give four sockets while ectopic expression of NUMB generates two IIb cells and, subsequently, four neurons and no hairs (2). The fate choices directed by Drosophila NUMB occur in a stereotypical lineage within which NUMB functions solely to control binary differentiation decisions and not the proliferation dynamics of the cells. NUMB has been hypothesized to function in directing cell fate choice by directly interacting with NOTCH and inhibiting NOTCH function (3).The recent cloning of a mammalian NUMB homologue suggested an evolutionarily conserved function for mammalian NUMB (4, 5). We showed that ectopic expression of this mammalian NUMB protein (mNUMB) promotes neuronal commitment in both Drosophila and cultured mammalian cells (4). Recently, it has been observed that a human NUMB homologue (hNUMB) is transported to the nucleus and associates with the modulator of mitogenesis, MDM2 (6, 7). Our previous studies showed no mitogenic component to murine NUMB function (4). Here we report the existence of four hNUMB isoforms. Based on the structure of the proline-rich region (PRR), the NUMB isoforms regulate either cell fate or cellular proliferation, but not both, during mammalian neurogenesis.

We have isolated a human cDNA for the signaling adapter molecule FRS-2/suc1-associated neurotrophic factor target and shown that it is tyrosine-phosphorylated in response to nerve growth factor (NGF) stimulation. Importantly, we demonstrate that the phosphotyrosine binding domain of FRS-2 directly binds the Trk receptors at the same phosphotyrosine residue that binds the signaling adapter Shc, suggesting a model in which competitive binding between FRS-2 and Shc regulates differentiation versus proliferation. Consistent with this model, FRS-2 binds Grb-2, Crk, the SH2 domain containing tyrosine phosphatase SH-PTP-2, the cyclindependent kinase substrate p13 suc1 , and the Src homology 3 (SH3) domain of Src, providing a functional link between TrkA, cell cycle, and multiple NGF signaling effectors. Importantly, overexpression of FRS-2 in cells expressing an NGF nonresponsive TrkA receptor mutant reconstitutes the ability of NGF to stop cell cycle progression and to stimulate neuronal differentiation.