PgmNr M5076: The cilia protein Arl13b regulates axon guidance in the mouse hindbrain.

Authors:
S. K. Suciu 1,3 ; L. E. Mariani 2,3 ; J. Ferent 4 ; J. Guo 5 ; E. S. Anton 5 ; F. Charron 4 ; T. Caspary 3


Institutes
1) Genetics and Molecular Biology Graduate Program; 2) Neuroscience Graduate Program; 3) Emory University, Atlanta, GA 30322; 4) IRCM, Montreal, Quebec, Canada; 5) University of North Carolina, Chapel Hill, NC 27599.


Abstract:

Sonic hedgehog (SHH) signaling is a critical developmental pathway best known to regulate cell fate specification and cell proliferation. This occurs in the cilium through SHH activation of downstream effector smoothened (SMO) and GLI transcription factors. Additionally, SHH regulates axon guidance in the developing spinal cord and optic chiasm through a distinct, transcription-independent pathway. Arl13b encodes a small GTPase enriched in cilia that regulates transcription-dependent SHH signaling at the level of and downstream of SMO. Recessive mutations in ARL13B cause the ciliopathy Joubert Syndrome (JS), a disorder defined by intellectual disability, cerebellar hypoplasia, and physical deformities. JS is diagnosed by MRI-evidence of a hindbrain malformation known as the Molar Tooth Sign, which is caused in part by the white matter tracts called superior cerebellar peduncles (SCPs) failing to cross the midline of the brain. JS patients also exhibit midline crossing defects in the optic chiasm and failure of the corticospinal tract to cross the midline. Collectively, these phenotypes indicate JS patients display axon guidance defects; however, no known mechanism connects cilia genes such as ARL13B to the regulation of axon guidance. Because transcription-independent SHH signaling can regulate axon guidance, and ARL13B mutations lead to axon guidance defects in JS, we hypothesize that ARL13B also regulates transcription-independent SHH signaling to direct axon guidance in the developing brain. To test this hypothesis, we examined SCPs direction and uniformity in mouse brains lacking either SMO or ARL13B in projection neurons by performing diffusion tensor imaging (DTI) MRI. We showed that SCPs lacking SMO display significant midline crossing defects in the hindbrain, indicating axon guidance in these projection neurons is SMO dependent and suggesting SHH as a possible guidance cue. Furthermore, there is a midline crossing defect in SCPs lacking ARL13B, supporting our hypothesis that ARL13B regulates this transcription-independent SHH signaling pathway. Our ongoing work continues to investigate the role of ARL13B and SMO in SCP midline crossing through DTI and fluorescent tract tracing. Through these methods, we aim to define the role of ARL13B in transcription-independent SHH signaling and form a novel connection between ciliary genes and axon guidance.