Mutations in the ER-shaping protein reticulon 2 cause the axon-degenerative disorder hereditary spastic paraplegia type 12

Abstract

Hereditary spastic paraplegias (HSPs) are a group of genetically heterogeneous neurodegenerative conditions. They are characterized by progressive spastic paralysis of the legs as a result of selective, lengthdependent degeneration of the axons of the corticospinal tract. Mutations in 3 genes encoding proteins that work together to shape the ER into sheets and tubules -receptor accessory protein 1 (REEP1), atlastin-1 (ATL1), and spastin (SPAST) -have been found to underlie many cases of HSP in Northern Europe and North America. Applying Sanger and exome sequencing, we have now identified 3 mutations in reticulon 2 (RTN2), which encodes a member of the reticulon family of prototypic ER-shaping proteins, in families with spastic paraplegia 12 (SPG12). These autosomal dominant mutations included a complete deletion of RTN2 and a frameshift mutation predicted to produce a highly truncated protein. Wild-type reticulon 2, but not the truncated protein potentially encoded by the frameshift allele, localized to the ER. RTN2 interacted with spastin, and this interaction required a hydrophobic region in spastin that is involved in ER localization and that is predicted to form a curvature-inducing/sensing hairpin loop domain. Our results directly implicate a reticulon protein in axonopathy, show that this protein participates in a network of interactions among HSP proteins involved in ER shaping, and further support the hypothesis that abnormal ER morphogenesis is a pathogenic mechanism in HSP. IntroductionThe ER is a continuous membrane system comprising the nuclear envelope and a dynamic network of proximal sheets and peripheral tubules. Proteins of 2 classes -the reticulons and the REEP/DP1/yop1p family (referred to herein as the REEPs) -are fundamental to the generation of both sheets and tubules. These proteins share a characteristic sequence feature; in the case of the reticulons this feature is termed the reticulon homology domain (RHD) and consists of 2 long hydrophobic stretches separated by a hydrophilic sequence. Each hydrophobic stretch is thought to sit in the ER membrane as a hairpin loop. Such loop domains have been suggested to generate membrane curvature by occupying more space in the outer leaflet of the membrane than the inner in a process termed "hydrophobic wedging" (1-3). These proteins are thus critical for producing

Protrudin binds atlastins and endoplasmic reticulum-shaping proteins and regulates network formation

“…Protrudin facilitated the interaction of KIF5 not only with Rab11 but also with ER proteins VAP-A/B and reticulon 3 (25). This is of particular interest because KIF5A is mutated in another HSP, SPG10 (26), and reticulon 2 is mutated in SPG12 (27). Thus, protrudin potentially links a large number of autosomal dominant HSPs: SPG3A, SPG4, SPG10, SPG12, and SPG31.…”

Ascorbate peroxidase proximity labeling coupled with biochemical fractionation identifies promoters of endoplasmic reticulum–mitochondrial contacts

“…Differential up-and down-regulation of RTNs have been linked to neurodegenerative diseases (e.g. Alzheimer's disease, amyotrophic lateral sclerosis, multiple sclerosis, as well as hereditary spastic paraplegia) (62)(63)(64). RTN1A has been implicated in the onset of several neurodegenerative diseases; however, what role it plays is unknown (62,65).…”

Inhibition of TFG function causes hereditary axon degeneration by impairing endoplasmic reticulum structure

“…REEP1 mutations also cause an uncomplicated form of HSP (22). Together with reticulons, which have also been implicated in HSP (23), members of the REEP family are thought to generate and/or maintain the highly curved shape of ER membranes (7). Interestingly, spastin has been shown to interact biochemically with members of the atlastin, REEP, and reticulon families (11,18,24), suggesting that these proteins function together.…”