Project Summary
The primary cilium, present on most cell types of the human body, is required for a broad
array of developmental and physiological functions. These include Hedgehog (Hh) signaling,
which is used for the specification of cell types in the central nervous system, neuronal GPCR
signaling, retinal photoreceptor homeostasis, olfaction, and cochlear hair cell function. However,
the ways by which cilia are assembled and function in signal transduction are only partially
understood. We are studying a kinase, called Cell Cycle-Related Kinase (CCRK), which is
required for both ciliary length regulation, ciliary structure, and proper cilium-dependent Hh
signaling. The mechanism by which CCRK functions is unclear but our published data indicates
it functions to promote the import of Hh signaling components into the cilium. The only established
substrate of CCRK is another kinase, ICK, which functions in ciliary length control, but is largely
dispensable for early development and Hh signaling. We will hypothesize that, although
phosphorylated ICK (in the TDY motif by CCRK) restricts ciliary length, unphosphorylated ICK
functions in an overlapping manner with CCRK to promote ciliogenesis. We will test this
hypothesis by analyzing ciliogenesis and Hh signaling in mouse mutants completely lacking both
kinases. While CCRK appears to control ciliary import of Smo and Gli2, we hypothesize that it
controls the import of other ciliary proteins that assemble the cilium and mediate other types of
neuronal signaling. We will specifically test this hypothesis using fluorescent recovery after
photobleaching of fluorophore-tagged ciliary cargo within the cilia or wild-type and Ccrk null cells.
Finally, we propose that CCRK is continuously required to promote ciliary cargo import in the fully
assembled cilium and that it does so in a dosage-dependent manner. Experiments aimed at
acutely disrupting or increasing CCRK activity after ciliogenesis is complete are proposed to test
this hypothesis. This combination of experiments should significantly clarify open questions
regarding the mechanisms by which cilia are loaded with cargo that execute signaling functions
critical for neural development and establish appropriate ciliary length.