Protein kinase Cα and integrin-linked kinase mediate the negative axon guidance effects of Sonic hedgehog.

In addition to its role as a morphogen, Sonic hedgehog (Shh) has also been shown to function as a guidance factor that directly acts on the growth cones of various types of axons. However, the noncanonical signaling pathways that mediate the guidance effects of Shh protein remain poorly understood. We demonstrate that a novel signaling pathway consisting of protein kinase Cα (PKCα) and integrin-linked kinase (ILK) mediates the negative guidance effects of high concentration of Shh on retinal ganglion cell (RGC) axons.

The COPI vesicle complex binds and moves with survival motor neuron within axons.

Spinal muscular atrophy (SMA), an inherited disease of motor neuron dysfunction, results from insufficient levels of the survival motor neuron (SMN) protein. Movement of the SMN protein as granules within cultured axons suggests that the pathogenesis of SMA may involve defects in neuronal transport, yet the nature of axon transport vesicles remains enigmatic. Here we show that SMN directly binds to the α-subunit of the coat protein I (COPI) vesicle coat protein.

Myosin IIB isoform plays an essential role in the formation of two distinct types of macropinosomes.

The function and mechanism of macropinocytosis in cells outside of the immune system remain poorly understood. We used a neuroblastoma cell line, Neuro-2a, to study macropinocytosis in neuronal cells. We found that phorbol 12-myristate 13-acetate (PMA) and insulin-like growth factor 1 (IGF-1) induced two distinct types of macropinocytosis in the Neuro-2a cells.

Negative guidance factor-induced macropinocytosis in the growth cone plays a critical role in repulsive axon turning.

Macropinocytosis is a type of poorly characterized fluid-phase endocytosis that results in formation of relatively large vesicles. We report that Sonic hedgehog (Shh) protein induces macropinocytosis in the axons through activation of a noncanonical signaling pathway, including Rho GTPase and nonmuscle myosin II. Macropinocytosis induced by Shh is independent of clathrin-mediated endocytosis but dependent on dynamin, myosin II, and Rho GTPase activities.

Intraretinal projection of retinal ganglion cell axons as a model system for studying axon navigation.

The initial step of retinal ganglion cell (RGC) axon pathfinding involves directed growth of RGC axons toward the center of the retina, the optic disc, a process termed "intraretinal guidance". Due to the accessibility of the system, and with various embryological, molecular and genetic approaches, significant progress has been made in recent years toward understanding the mechanisms involved in the precise guidance of the RGC axons. As axons are extending from RGCs located throughout the retina, a multitude of factors expressed along with the differentiation wave are important for the guidance of the RGC axons.

Notch signaling plays a key role in cardiac cell differentiation.

Results from lineage tracing studies indicate that precursor cells in the ventricles give rise to both cardiac muscle and conduction cells. Cardiac conduction cells are specialized cells responsible for orchestrating the rhythmic contractions of the heart. Here, we show that Notch signaling plays an important role in the differentiation of cardiac muscle and conduction cell lineages in the ventricles.

Sema3D and Sema7A have distinct expression patterns in chick embryonic development.

By RT-PCR, we isolated a partial cDNA clone for the chick Semaphorin7A (Sema7A) gene. We further analyzed its expression patterns and compared them with those of the Sema3D gene, in chick embryonic development. Sema3D and Sema7A appeared to be expressed in distinct cell populations.

Sema3D, Sema3F, and Sema5A are expressed in overlapping and distinct patterns in chick embryonic heart.

An increasing number of axon guidance cues have been shown recently to play important roles in the development of non-neural tissues. Semaphorins comprise one of the largest conserved families of axon guidance factors. We analyzed the expression patterns of Sema3D, Sema3F, and Sema5A genes in the chick embryonic heart by in situ hybridization.

Sonic hedgehog has a dual effect on the growth of retinal ganglion axons depending on its concentration.

The stereotypical projection of retinal ganglion cell (RGC) axons to the optic disc has served as a good model system for studying axon guidance. By both in vitro and in vivo experiments, we show that a secreted molecule, Sonic hedgehog (Shh), may play a critical role in the process. It is expressed in a dynamic pattern in the ganglion cell layer with a relatively higher expression in the center of the retina.

Disruption of gradient expression of Zic3 resulted in abnormal intra-retinal axon projection.

The targeting of retinal ganglion axons toward the optic disc is the first step in axon pathfinding in the visual system. The molecular mechanisms involved in guiding the retinal axons to project towards the optic disc are not well understood. We report that a gene encoding a zinc-finger transcription factor, Zic3, is expressed in a periphery-high and center-low gradient in the retina at the stages of active axon extension inside the retina.

Irx4-mediated regulation of Slit1 expression contributes to the definition of early axonal paths inside the retina.

Although multiple axon guidance cues have been discovered in recent years, little is known about the mechanism by which the spatiotemporal expression patterns of the axon guidance cues are regulated in vertebrates. We report that a homeobox gene Irx4 is expressed in a pattern similar to that of Slit1 in the chicken retina. Overexpression of Irx4 led to specific downregulation of Slit1 expression, whereas inhibition of Irx4 activity by a dominant negative mutant led to induction of Slit1 expression, indicating that Irx4 is a crucial regulator of Slit1 expression in the retina.