Reticulons 1 and 3 are essential for axonal growth and synaptic maintenance associated with intellectual development

Reticulon (RTN) proteins are a family of proteins biochemically identified for shaping tubular endoplasmic reticulum, a subcellular structure important for vesicular transport and cell-to-cell communication. In our recent study of mice with knockout of both reticulon 1 (Rtn1) and Rtn3, we discovered that Rtn1-/-;Rtn3-/- (brief as R1R3dKO) mice exhibited neonatal lethality, despite the fact that mice deficient in either RTN1 or RTN3 alone exhibit no discernible phenotypes. This has been the first case to find early lethality in animals with deletion of partial members of RTN proteins. The complete penetrance for neonatal lethality can be attributed to multiple defects including the impaired neuromuscular junction found in the diaphragm. We also observed significantly impaired axonal growth in a regional-specific manner, detected by immunohistochemical staining with antibodies to neurofilament light chain and neurofilament medium chain. Ultrastructural examination by electron microscopy revealed a significant reduction in synaptic active zone length in the hippocampus. Mechanistic exploration by unbiased proteomic assays revealed reduction of proteins such as FMR1, Staufen2, Cyfip1, Cullin-4B and PDE2a, which are known components in the fragile X mental retardation pathway. Together, our results reveal that RTN1 and RTN3 are required to orchestrate neurofilament organization and intact synaptic structure of the central nervous system.