Arl13b regulates Shh signaling from both inside and outside the cilium.
The regulatory GTPase Arl13b localizes to primary cilia, where it regulates Sonic hedgehog (Shh) signaling. Missense mutations in ARL13B can cause the ciliopathy Joubert syndrome, while the mouse null allele is embryonic lethal. We used mouse embryonic fibroblasts as a system to determine the effects of Arl13b mutations on Shh signaling. We tested a total of seven different mutants, three JS-causing variants, two point mutants predicted to alter guanine nucleotide handling, one that disrupts cilia localization, and one that prevents palmitoylation and thus membrane binding, in assays of transcriptional and non-transcriptional Shh signaling. We found that mutations disrupting Arl13b's palmitoylation site, cilia localization signal, or GTPase handling altered the Shh response in distinct assays of transcriptional or non-transcriptional signaling. In contrast, JS-causing mutations in Arl13b did not affect Shh signaling in these same assays, suggesting these mutations result in more subtle defects, likely affecting only a subset of signaling outputs. Finally, we show that restricting Arl13b from cilia interferes with its ability to regulate Shh-stimulated chemotaxis, despite previous evidence that cilia themselves are not required for this non-transcriptional Shh response. This points to a more complex relationship between the ciliary and non-ciliary roles of this regulatory GTPase than previously envisioned.
*Department of Human Genetics, Emory University, Atlanta, GA, USA Neuroscience Graduate Program, Emory University, Atlanta, GA, USA.
Laura E
LE
Mariani
Laboratory for Experimental Oncology and Radiobiology, Center for Experimental and Molecular Medicine, Academic Medical Center and Cancer Center Amsterdam, Amsterdam, The Netherlands.
Maarten F
MF
Bijlsma
Department of Biochemistry, Emory University, Atlanta, GA, USA.
Anna A
AA
Ivanova
*Department of Human Genetics, Emory University, Atlanta, GA, USA Genetics and Molecular Biology Graduate Program, Emory University, Atlanta, GA, USA.
Sarah K
SK
Suciu
Department of Biochemistry, Emory University, Atlanta, GA, USA.
Richard A
RA
Kahn
*Department of Human Genetics, Emory University, Atlanta, GA, USA tcaspar@emory.edu.
Tamara
T
Caspary
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