The sorting of signaling receptors into and out of cilia relies on the BBSome, a complex of Bardet-Biedl syndrome (BBS) proteins, and on the intraflagellar transport (IFT) machinery. GTP loading onto the Arf-like GTPase ARL6/BBS3 drives assembly of a membrane-apposed BBSome coat that promotes cargo entry into cilia, yet how and where ARL6 is activated remains elusive. Here, we show that the Rab-like GTPase IFT27/RABL4, a known component of IFT complex B, promotes the exit of BBSome and associated cargoes from cilia. Unbiased proteomics and biochemical reconstitution assays show that, upon disengagement from the rest of IFT-B, IFT27 directly interacts with the nucleotide-free form of ARL6. Furthermore, IFT27 prevents aggregation of nucleotide-free ARL6 in solution. Thus, we propose that IFT27 separates from IFT-B inside cilia to promote ARL6 activation, BBSome coat assembly, and subsequent ciliary exit, mirroring the process by which BBSome mediates cargo entry into cilia.

Centrioles are microtubule-based, barrel-shaped structures that initiate the assembly of centrosomes and cilia. How centriole length is precisely set remains elusive. The microcephaly protein CPAP (also known as MCPH6) promotes procentriole growth, whereas the oral-facial-digital (OFD) syndrome protein OFD1 represses centriole elongation. Here we uncover a new subtype of OFD with severe microcephaly and cerebral malformations and identify distinct mutations in two affected families in the evolutionarily conserved C2CD3 gene. Concordant with the clinical overlap, C2CD3 colocalizes with OFD1 at the distal end of centrioles, and C2CD3 physically associates with OFD1. However, whereas OFD1 deletion leads to centriole hyperelongation, loss of C2CD3 results in short centrioles without subdistal and distal appendages. Because C2CD3 overexpression triggers centriole hyperelongation and OFD1 antagonizes this activity, we propose that C2CD3 directly promotes centriole elongation and that OFD1 acts as a negative regulator of C2CD3. Our results identify regulation of centriole length as an emerging pathogenic mechanism in ciliopathies.

OBJECTIVES: To examine whether social determinants of health (SDH) factors are associated with time to diagnosis, treatment selection, and time to recurrent surgical intervention in idiopathic subglottic stenosis (iSGS) patients. METHODS: Adult patients with diagnosed iSGS were recruited prospectively (2015-2017) via clinical providers as part of the North American Airway Collaborative (NoAAC) and via an online iSGS support community on Facebook. Patient-specific SDH factors included highest educational attainment (self-reported), median household income (matched from home zip code via U.S. Census data), and number of close friends (self-reported) as a measure of social support. Main outcomes of interest were time to disease diagnosis (years from symptom onset), treatment selection (endoscopic dilation [ED] vs cricotracheal resection [CTR] vs endoscopic resection with adjuvant medical therapy [ERMT]), and time to recurrent surgical intervention (number of days from initial surgical procedure) as a surrogate for disease recurrence. RESULTS: The total 810 participants were 98.5% female, 97.2% Caucasian, and had a median age of 50 years (IQR, 43-58). The cohort had a median household income of $62 307 (IQR, $50 345-$79 773), a median of 7 close friends (IQR, 4-10), and 64.7% of patients completed college or graduate school. Education, income, and number of friends were not associated with time to diagnosis via multivariable linear regression modeling. Univariable multinominal logistic regression demonstrated an association between education and income for selecting ED versus ERMT, but no associations were noted for CTR. No associations were noted for time to recurrent surgical procedure via Kaplan Meier modeling and Cox proportional hazards regression. CONCLUSIONS: Patient education, income, and social support were not associated with time to diagnosis or time to disease recurrence. This suggests additional patient, procedure, or disease-specific factors contribute to the observed variations in iSGS surgical outcomes.