The Neuronal Primary Cilium is a Key Regulator of Homeostatic Plasticity

The capacity of neurons to maintain stable activity levels through homeostatic plasticity is essential for proper brain function. Primary cilia, which are non-motile, antenna-like organelles projecting from the surface of most vertebrate cells, serve as key hubs for signal transduction, playing crucial roles in tissue development and cellular homeostasis. In this study, we identify a previously unrecognized role for primary cilia in mediating neuronal homeostatic plasticity using human induced pluripotent stem cell-derived neurons. We show that neuronal cilia exhibit dynamic, bidirectional changes in volume in response to alterations in network activity: elongating during chronic activity suppression and shortening after increased activity. To assess the functional relevance of this ciliary plasticity, we modelled ciliary dysfunction in neurons carrying homozygous loss-of-function mutations in genes associated with neuronal ciliopathies, including NPHP1 and CEP290. Mutations affecting ciliary function either increased ciliary length or led to ciliary loss, and these mutant neurons exhibited severe impairments in homeostatic regulation across multiple domains -morphological, functional, and transcriptional. Specifically, NPHP1 and CEP290 deficient neurons failed to adapt synaptic strength, intrinsic excitability, and ciliary morphology in response to prolonged activity suppression. They also displayed dysregulated baseline network activity, and exhibited blunted gene expression changes. Together, these findings establish the primary cilium as a critical regulator of homeostatic plasticity in human neurons and provide a new framework through which to examine neurodevelopmental and neuropsychiatric disorders linked to ciliary dysfunction.