The molecular mechanism and significance of endocytic processes involved in directional axon elongation are not well understood. The Unc-51 family of serine/threonine kinases was shown to be important for axon growth and was also linked to endocytosis, providing an entry point to study this problem. We found that mouse Unc-51-like kinase 1/2 (Ulk1/2) proteins are localized to vesicular structures in growth cones of mouse spinal sensory neurons. RNAi-mediated knockdown of Ulk1 and/or Ulk2 resulted in impaired endocytosis of nerve growth factor (NGF), excessive axon arborization, and severely stunted axon elongation. The evidence also indicates that Ulk1/2 mediates a non-clathrin-coated endocytosis in sensory growth cones. Interestingly, NGF can induce the interaction of Ulk1 with TrkA receptor complexes through promoting K63-polyubiquitination of Ulk1 and binding of Ulk1 to the scaffolding protein p62. These results and additional studies suggest that Ulk1/2 proteins regulate filopodia extension and neurite branching during sensory axon outgrowth, probably through regulating TrkA receptor trafficking and signaling.axon growth ͉ dorsal root ganglion neurons ͉ endocytosis ͉ p62

SUMMARYThe stratification and differentiation of the epidermis are known to involve the precise control of multiple signaling pathways. By contrast, little is known about the development of the mouse esophagus and forestomach, which are composed of a stratified squamous epithelium. Based on prior work in the skin, we hypothesized that bone morphogenetic protein (BMP) signaling is a central player. To test this hypothesis, we first used a BMP reporter mouse line harboring a BRE-lacZ allele, along with in situ hybridization to localize transcripts for BMP signaling components, including various antagonists. We then exploited a Shh-Cre allele that drives recombination in the embryonic foregut epithelium to generate gain-or loss-of-function models for the Bmpr1a (Alk3) receptor. In gain-of-function (Shh-Cre;Rosa26 CAG-loxpstoploxp-caBmprIa) embryos, high levels of ectopic BMP signaling stall the transition from simple columnar to multilayered undifferentiated epithelium in the esophagus and forestomach. In loss-of-function experiments, conditional deletion of the BMP receptor in Shh-Cre;Bmpr1a flox/flox embryos allows the formation of a multilayered squamous epithelium but this fails to differentiate, as shown by the absence of expression of the suprabasal markers loricrin and involucrin. Together, these findings suggest multiple roles for BMP signaling in the developing esophagus and forestomach.

Mammalian somatosensory topographic maps contain specialized neuronal structures that precisely recapitulate the spatial pattern of peripheral sensory organs. In the mouse, whiskers are orderly mapped onto several brainstem nuclei as a set of modular structures termed barrelettes. Using a dual-color iontophoretic labeling strategy, we found that the precise topography of barrelettes is not a result of ordered positions of sensory neurons within the ganglion. We next explored another possibility that formation of the whisker map is influenced by periphery-derived mechanisms. During the period of peripheral sensory innervation, several TGF-β ligands are exclusively expressed in whisker follicles in a dynamic spatiotemporal pattern. Disrupting TGF-β signaling, specifically in sensory neurons by conditional deletion of Smad4 at the late embryonic stage, results in the formation of abnormal barrelettes in the principalis and interpolaris brainstem nuclei and a complete absence of barrelettes in the caudalis nucleus. We further show that this phenotype is not derived from defective peripheral innervation or central axon outgrowth but is attributable to the misprojection and deficient segregation of trigeminal axonal collaterals into proper barrelettes. Furthermore, Smad4-deficient neurons develop simpler terminal arbors and form fewer synapses. Together, our findings substantiate the involvement of whisker-derived TGF-β/Smad4 signaling in the formation of the whisker somatotopic maps.O ne prominent characteristic of the rodent whisker-somatosensory system is its precisely organized topographic sensory maps (1-3). Each whisker is innervated by peripheral axons of a subset of trigeminal sensory neurons whose cell bodies reside in the trigeminal ganglia (TG) and central axons project to the brainstem (4). Sensory afferents carrying information from individual whiskers segregate and converge to form modular structures termed barrelettes, whose spatial organization exactly mirrors that of the whiskers in the periphery (5). The barrelette map in the brainstem emerges during development and serves as a template for the subsequent generation of homologous upstream structures in the thalamus and cortex, termed barreloids and barrels, respectively (5, 6). Interestingly, induction of an extra whisker by exogenous expression of Shh during early development leads to the formation of an extra barrelette with a topographic position corresponding to that of the ectopic whisker (7). Together with other studies (8-10), these data suggest that the formation of the whisker map is under the strong instructive influence of the periphery. The whisker-derived signals regulating barrelette formation remain mostly unknown, however.Previous work showed that BMP4 signaling induces differential expression of genes in trigeminal sensory neurons innervating different areas of the face along the dorsoventral axis (11,12). At later developmental stages, multiple TGF-β superfamily ligands are expressed in whisker follicles during the period of sensory...