Sonic hedgehog (Shh) signaling is required for specification of six different cell types in the ventral neural tube. The core of the Hedgehog signaling pathway is evolutionarily conserved, but we have identified more than six genes that are required for Hedgeghog signaling in the mouse but not in Drosophila.
At least five of these novel Shh pathway genes affect the primary cilium. Primary cilia are microtubule-based organelles that extend from the surface of most non-dividing vertebrate cells (Fig. 1).
Figure 1. Cilia in the mouse node. In the wild-type node, long microtubule-based cilia are present, in addition to the much shorter actin-based microvilli. A specific dynein is required to transport material from the tip of the cilium back to the base; mutants that lack that dynein (Dnchc2lln) have bulges along the length of the cilium.
For example, a dynein heavy chain gene is required for retrograde transport. We identified four different mutations in the gene that encodes the heavy chain of the dynein motor required for retrograde transport in the cilium. These mutants have two phenotypes: abnormally shaped cilia and loss of ventral cell types in the neural tube (Fig.2)
Figure 2. Ventral neural cell types, marked by expression of Shh and Nkx2.2, fail to be specified in mutants that lack the heavy chain dynein retrograde IFT motor of cilia (Dnchc2lln).
Our studies have led to the conclusion that cilia are required for the ability of mammalian cells to respond to Hedgehog ligands, and that organized trafficking within the cilium is essential for Hedgehog signal transduction.
The lab is currently pursuing studies to define the events that occur within cilia and to test whether other developmental signaling pathways depend on cilia. We also continue to identify and characterize other new mutations that affect neural patterning.