A teneurin-3 microphthalmia mutation disrupts trans adhesion for specific alternative splicing isoforms

Microphthalmia (MCOP) is a developmental eye disorder in which one or both eyes are abnormally small. This condition is often accompanied by cognitive impairments. Teneurin-3, a cell adhesion molecule with important functions in axon pathfinding and synaptic organization, has been repeatedly implicated with MCOP, suggesting its potential role in the etiology of this complex developmental disorder. It was previously shown that two small alternatively spliced exons - inserts A and B - instigate large structural reorganizations of the teneurin-3 covalent homodimer. Here, we map the MCOP mutation R2579W to an intramolecular interface specific to splice insert A-containing isoforms. We demonstrate that the MCOP mutation leads to a loss of transcellular adhesion, predominantly affecting these isoforms. Using small-angle X-ray scattering we establish that, despite these functional disruptions, the structural compactness of all mutant isoforms is preserved. Molecular dynamics simulations predict a global stabilization of the A1 mutant molecule compared to its wildtype counterpart, due to changes in local compact dimer interactions around the mutated residue. We experimentally validate these findings with biophysical assays. With cryo-EM analysis, we confirm the overall compactness with reduced conformational flexibility and reveal that the mutant tryptophan binds to a hydrophobic pocket in the structure. With these data, we provide a model for how increased stability in cis disrupts trans oligomerization needed for cellular adhesion. Altogether, we demonstrate that the MCOP-associated mutation in teneurin-3 causally disrupts cellular adhesion in an isoform-specific manner, thereby likely directly contributing to the development of the disease in MCOP patients.