The Meckel-Gruber Syndrome protein TMEM67 (meckelin) regulates basal body planar polarization and non-canonical Wnt signalling via Wnt5a and ROR2

Objective

Ciliopathies are a group of developmental disorders that manifest with multi-organ anomalies. Mutations in TMEM67 have been reported in human ciliopathies that include Meckel-Gruber and Joubert syndromes. In this study we describe multi-organ developmental abnormalities in the Tmem67^tm1Dgen/H1 knockout mouse that closely resemble those of Wnt5a and Ror2 knockout mice.

Methods

We used anatomical assessment, immunofluorescence confocal microscopy and biochemical methods to determine mutant phenotypes at the organismal, cellular and molecular levels.

Results

Tmem67^-/- mutant phenotypes include pulmonary hypoplasia, ventricular septal defects, shortening of the body longitudinal axis, limb abnormalities, and cochlear hair cell stereociliary bundle orientation and basal body/kinocilium positioning defects. The basal body/kinocilium complex was often uncoupled from the hair bundle, suggesting aberrant basal body migration. TMEM67 (meckelin) is essential for phosphorylation of the non-canonical Wnt receptor ROR2 (receptor tyrosine kinase-like orphan receptor 2) upon Wnt5a stimulation. ROR2 interacts with the intracellular C-terminal domain of TMEM67 and co-localizes with TMEM67 at the ciliary transition zone. The N-terminal domain of TMEM67 preferentially binds to Wnt5a in an in vitro binding assay. Tmem67 mutant embryonic lungs in ex vivo culture failed to respond to Wnt5a stimulation of epithelial morphogenesis. However, stimulating the non-canonical Wnt pathway downstream of the receptor by activating RhoA resulted in an elicited response and the rescue of lung hypoplasia phenotypes.

Conclusion

Our data suggest that TMEM67 is a novel receptor that has a major role in non-canonical Wnt signalling by Wnt5a and ROR2. We propose that this signalling ensures correct basal body positioning.