The length of motor axons entails significant engineering for their formation and maintenance. Failures of this are seen in the Hereditary Spastic Paraplegias (HSPs), with lower limb weakness, and degeneration of longer upper motor axons. To date over 60 causative Spastic Paraplegia Genes (SPGs) are known. Several encode endoplasmic reticulum membrane proteins, with intramembrane hairpins that curve and model ER membrane, suggesting the axonal ER network as an important component to maintain function and integrity of long axons.
We have explored this model in Drosophila by testing for abnormalities of the axonal ER network in Drosophila that lack SPG-encoded intramembrane hairpin proteins. We have focused on two of these protein families, reticulons and REEPs, which are together required for formation of most tubular ER in yeast. We have developed axonal ER markers in Drosophila, and mutants that lack one or more members of these protein families. We see widespread if not ubiquitous occurrence of tubular ER in axons, and a range of loss-of-function phenotypes, and partial redundancy of gene function. Loss of either reticulon, or one REEP gene, leads to partial loss of ER from distal but not proximal axons, recapitulating the susceptibility of longer distal axons in the human diseases. Loss of multiple genes can lead to phenotypes consistent with loss of ER membrane curvature, including some loss of continuity of the ER network in axons.
We therefore propose a role for SPG genes in establishing or maintaining the axonal ER network, and a role for ER continuity in axons, which could account for the preferential susceptibility of longer axons to HSP.