In this report, we have examined the requirement for the retinoblastoma (Rb) gene family in neuronal determination with a focus on the developing neocortex. To determine whether pRb is required for neuronal determination in vivo, we crossed the Rb−/− mice with transgenic mice expressing β-galactosidase from the early, panneuronal Tα1 α-tubulin promoter (Tα1:nlacZ). In E12.5 Rb−/− embryos, the Tα1:nlacZ transgene was robustly expressed throughout the developing nervous system. However, by E14.5, there were perturbations in Tα1:nlacZ expression throughout the nervous system, including deficits in the forebrain and retina. To more precisely define the temporal requirement for pRb in neuronal determination, we functionally ablated the pRb family in wild-type cortical progenitor cells that undergo the transition to postmitotic neurons in vitro by expression of a mutant adenovirus E1A protein. These studies revealed that induction of Tα1:nlacZ did not require proteins of the pRb family. However, in their absence, determined, Tα1:nlacZ-positive cortical neurons underwent apoptosis, presumably as a consequence of “mixed signals” deriving from their inability to undergo terminal mitosis. In contrast, when the pRb family was ablated in postmitotic cortical neurons, there was no effect on neuronal survival, nor did it cause the postmitotic neurons to reenter the cell cycle. Together, these studies define a critical temporal window of requirement for the pRb family; these proteins are not required for induction of neuronal gene expression or for the maintenance of postmitotic neurons, but are essential for determined neurons to exit the cell cycle and survive.

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Neuroblastoma (NB) is the most frequent solid extracranial tumor in children. Its clinical prognosis correlates with the expression of members of the Trk neurotrophin receptor family, which includes TrkA and TrkB. TrkA expression is associated with favorable prognosis, whereas TrkB expression is associated with poor prognosis. Here we show that TrkA expression induces the apoptosis of NB cells and does so by modulating the levels or activities of a number of proteins involved in regulating cell survival and apoptosis, including p53, Bcl-2, and caspase-3. TrkA increased the expression of p53 target proteins and failed to induce apoptosis in cells where p53 was inactivated by mutation or via expression of dominant inhibitory p53 or E1B55K, indicating that TrkA mediates apoptosis, at least in part, through p53. Treatment with a caspase inhibitor or overexpression of Bcl-X L also prevented TrkA from inducing apoptosis. In contrast, elevated expression of TrkA in non-transformed sympathetic neurons resulted in the suppression of p53 levels and enhanced survival. These results identify apoptosis as a novel biological response of TrkA in NB cells and imply that TrkA is a good prognosis marker for NB due in part to its ability to mediate apoptosis when expressed at sufficient levels. Neuroblastoma (NB)1 is the most common solid extracranial solid tumor of childhood and likely arises from sympathoadrenal precursor cells. The median age at diagnosis is ϳ18 months, and spontaneous tumor regression is frequently observed in patients diagnosed at 1 year of age or younger. In contrast, children older than 1 year diagnosed with NB often experience aggressive tumors that are disseminated, resistant to chemotherapy, and metastasized to bone, and often fatal. An important correlative characteristic of NB is the expression of two of the neurotrophin receptors, the TrkA/nerve growth factor (NGF) receptor and the TrkB/brain-derived neurotrophic factor receptor (1-4). The expression of TrkB, a poor prognosis marker, mediates survival, proliferation, and chemotherapeutic drug resistance (5-10), whereas expression of TrkA, a favorable prognosis marker, induces cell growth arrest and differentiation of cultured NB cells (11)(12)(13). This is consistent with TrkA being expressed in tumors that spontaneously regress. Most NB cell lines, which are derived from malignant tumors, lack or have very low levels of TrkA protein expression. Cell lines with low TrkA expression respond to NGF by differentiating into neuronal-like cells (11). Similarly, expression of TrkA by transfection converts NGF non-responsive NB cells into NGF-responsive cells, both in culture and in vivo, with the typical responses being the cessation of cell growth and differentiation into neural and Schwann cells (12). Thus, TrkA converts malignant NB cells into quiescent differentiated cells.An alternative or additional explanation for why TrkA is a good prognosis marker for NB that might contribute to spontaneous tumor regression is that it may induce apoptosis. Paradoxic...