Functional mutation analysis provides evidence for a role of REEP1 in lipid droplet biology.
Hereditary axonopathies are frequently caused by mutations in proteins that reside in the endoplasmic reticulum (ER). Which of the many ER functions are pathologically relevant, however, remains to be determined. REEP1 is an ER protein mutated in hereditary spastic paraplegia (HSP) and hereditary motor neuropathy (HMN). We found that HSP-associated missense variants at the N-terminus of REEP1 abolish ER targeting, whereas two more central variants are either rare benign SNPs or confer pathogenicity via a different mechanism. The mis-targeted variants accumulate at lipid droplets (LDs). N-terminal tagging, deletion of the N-terminus, and expression of a minor REEP1 isoform had the same effect. We also confirmed an increase in LD size upon cooverexpression of atlastins and REEP1. Neither wild-type REEP1, LD-targeted HSP variants, nor a non-LD-targeted HMN variant reproduced this effect when expressed alone. We conclude that the N-terminus of REEP1 is necessary for proper targeting to and/or retention in the ER. The protein's potential to also associate with LDs corroborates a synergistic effect with atlastins on LD size. Interestingly, LD size is also altered upon knockdown of seipin, mutations of which also cause HSP and HMN. Regulation of LDs may thus be an ER function critical for long-term axonal maintenance.
Department of Clinical Chemistry and Laboratory Medicine, Jena University Hospital, Jena, Germany.
Julia
J
Falk
Magdalena
M
Rohde
Mohamed M
MM
Bekhite
Sophie
S
Neugebauer
Peter
P
Hemmerich
Michael
M
Kiehntopf
Thomas
T
Deufel
Christian A
CA
Hübner
Christian
C
Beetz
D000818:Animals; D045744:Cell Line, Tumor; D004252:DNA Mutational Analysis; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D014644:Genetic Variation; D006367:HeLa Cells; D006801:Humans; D066292:Lipid Droplets / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000235:genetics* / Q000378:metabolism*; D051379:Mice; D009134:Muscular Atrophy, Spinal / Q000235:genetics; D009154:Mutation; D015419:Spastic Paraplegia, Hereditary / Q000235:genetics
D026901:Membrane Transport Proteins
REEP1; hereditary motor neuropathy; hereditary spastic paraplegia; lipid droplets