Canopy1/Cnpy1 is required for proper V2R processing and transport; its loss impairs the function and circuit organization of basal/V2R vomeronasal sensory neurons

The vomeronasal organ (VNO) is a specialized chemosensory structure in the nasal cavity that detects pheromones and mediates social and reproductive behaviors. The VNO of rodents is populated by different types of vomeronasal sensory neurons (VSNs). Apical VSNs, located near the lumen, express the transcription factor (TF) Meis2, V1R family receptors, and the G protein subunit Gao; the VSNs distributed closer to the basal lamina express the TF Tfap2e/AP-2{varepsilon}, V2R receptors, and the G protein subunit Gai2. In addition, sparse cells in the VNO express the Formyl Peptide Receptors (FPRs). Single-cell mRNA sequencing (scRNA Seq) identified over 980 differentially expressed genes between these cell types, with many linked to the endoplasmic reticulum (ER). Among these ER proteins, Canopy1 (Cnpy1), was found to be among the most enriched genes in V2R+ VSNs. Previously studied only in zebrafish, Cnpy1 was found to affect Fgfr1 signaling and is thus also known as "FGF signaling regulator-1". In a previous study, we discovered that AP-2e upregulates Cnpy1 expression. Although Cnpy1 knockout mice are viable and have normal VNO development at birth, they experience a progressive degeneration and loss of V2R-expressing VSNs. Prior to symptoms, the basal VSNs of KO mice display reduced V2R protein immunoreactivity in the soma and a complete absence of the protein at the lumen of the VNO, rendering the neurons non-functional. Cnpy1 KOs exhibit altered guidance cues and adhesion molecule expression, along with disrupted connectivity to the accessory olfactory bulb (AOB). Our study shows that distinct neuronal types depend on unique ER protein repertoires to maintain proper proteostasis. The loss of Cnpy1 highlights the importance of cell-type-specific ER factors in the differentiation and function of specific neurons, revealing mechanisms that drive neuronal diversity and vulnerability to ER gene disruption.