Canopy1/Cnpy1 is required for proper V2R processing and transport; its functional loss impairs basal/V2R vomeronasal sensory neurons function and circuit organization.
The Vomeronasal Organ (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-2e, 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 300 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. Only previously studied in zebrafish, Cnpy1 was found to affect Fgfr1 signaling and thus is 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. Before symptoms appear, the basal VSNs of KO mice display reduced V2R protein immunoreactivity in the soma and a complete absence of the protein in 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.  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. Overall design: Whole postnatal vomeronasal organs were dissected from P21 (Cnpy1 KO) C57BL/6J-Cnpy1em1cyagen mice, cells were isolated then analyzed using scRNAseq.