Familial dysautonomia (FD) is a devastating developmental and progressive peripheral neuropathy caused by a mutation in the gene inhibitor of kappa B kinase complex-associated protein (IKBKAP). To identify the cellular and molecular mechanisms that cause FD, we generated mice in which Ikbkap expression is ablated in the peripheral nervous system and identify the steps in peripheral nervous system development that are Ikbkap-dependent. We show that Ikbkap is not required for trunk neural crest migration or pathfinding, nor for the formation of dorsal root or sympathetic ganglia, or the adrenal medulla. Instead, Ikbkap is essential for the second wave of neurogenesis during which the majority of tropomyosin-related kinase A (TrkA + ) nociceptors and thermoreceptors arise. In its absence, approximately half the normal complement of TrkA + neurons are lost, which we show is partly due to p53-mediated premature differentiation and death of mitotically-active progenitors that express the paired-box gene Pax3 and give rise to the majority of TrkA + neurons. By the end of sensory development, the number of TrkC neurons is significantly increased, which may result from an increase in Runx3 + cells. Furthermore, our data demonstrate that TrkA + (but not TrkC + ) sensory and sympathetic neurons undergo exacerbated Caspase 3-mediated programmed cell death in the absence of Ikbkap and that this death is not due to a reduction in nerve growth factor synthesis. In summary, these data suggest that FD does not result from a failure in trunk neural crest migration, but rather from a critical function for Ikbkap in TrkA progenitors and TrkA + neurons.H ereditary sensory and autonomic neuropathies (HSANs) are a group of five phenotypically diverse but overlapping disorders of the peripheral nervous system (PNS) that result from mutations in 12 distinct genes (1). HSAN type 3, or familial dysautonomia (FD) (also called Riley-Day syndrome), results from an intronic mutation (IVS20 + 6T > C; 99.5% of patients) in a gene called inhibitor of kappa B kinase complex-associated protein or IKBKAP, causing mis-splicing and subsequent tissuespecific reductions in IKAP protein (2, 3). FD is marked by tachycardia, blood pressure lability, autonomic vomiting "crises," decreased pain and temperature sensation, and commonly death during early adulthood (4). The function of IKAP in the nervous system is unclear, nor is it understood why deletions in this broadly expressed gene primarily devastate the PNS. The earliest pathology study, performed on a 2-y-old child with FD, showed that ∼90% of cells in the dorsal root and sympathetic ganglia (SG) were missing (5). To identify IKAP's function in the developing PNS, we first need to establish the steps in which it is essential.The vertebrate PNS derives primarily from the neural crest, a multipotent, heterogeneous cell population that delaminates from the neural tube and migrates throughout the embryo (6). Those neural crest cells that stop laterally to the neural tube give rise to the chain of sensory...

We have developed an in vitro system for studying the interaction of chick dorsal root ganglion neuronal growth cones with a localized source of nerve growth factor (NGF) covalently conjugated to polystyrene beads. Growth cones rapidly turned and migrated under NGF-coated beads in a process that involved the initial formation of persistent contact with a bead, followed by directed flow of cytoplasm toward the point of contact. A role for the local activation of the high-affinity NGF receptor trkA was suggested by a strong inhibition of the turning response by (1) the addition of an antibody against the extracellular portion of trkA, (2) the elevation of the background concentration of NGF to saturate trkA, or (3) the presence of a concentration of the drug K252a that inhibits trkA activation. NGF binding to the pan-neurotrophin receptor p75 is also involved but is not required for turning. These data show a new role for both the trkA and the p75 receptors: the mediation of local events in the guidance of nerve growth cones.

The neural crest is a migratory population of cells that produces many diverse structures within the embryo. Trunk neural crest cells give rise to such structures as the dorsal root ganglia (DRG) and sympathetic ganglia (SG),which form in a metameric pattern along the anterior-posterior axis of the embryo. While static analyses have provided invaluable information concerning the development of these structures, time-lapse imaging of neural crest cells navigating through their normal environment could potentially reveal previously unidentified cellular and molecular interactions integral to DRG and SG development. In this study, we follow fluorescently labeled trunk neural crest cells using a novel sagittal explant and time-lapse confocal microscopy. We show that along their dorsoventral migratory route, trunk neural crest cells are highly motile and interact extensively with neighboring cells and the environment, with many cells migrating in chain-like formations. Surprisingly, the segregated pattern of crest cell streams through the rostral somite is not maintained once these cells arrive alongside the dorsal aorta. Instead, neural crest cells disperse along the ventral outer border of the somite, interacting extensively with each other and their environment via dynamic extension and retraction of filopodia. Discrete sympathetic ganglia arise as a consequence of intermixing and selective reorganization of neural crest cells at the target site. The diverse cell migratory behaviors and active reorganization at the target suggest that cell-cell and cell-environment interactions are coordinated with dynamic molecular processes.

We thank Dr. Ronal d Li ndsay (Regeneron, Tarrytown, NY) for the gift of BDNF and NT-3, and Drs. Hi deaki Tanaka and Gisela Weskamp for the gift of ~75 antibodies. Basic FGF was kindly provi ded by Chi ron Corporation. We also thank Drs.