Spastin Binds to Lipid Droplets and Affects Lipid Metabolism

PLoS Genet. 2015 Apr 13;11(4):e1005149. doi: 10.1371/journal.pgen.1005149. eCollection 2015 Apr.

Abstract

Mutations in SPAST, encoding spastin, are the most common cause of autosomal dominant hereditary spastic paraplegia (HSP). HSP is characterized by weakness and spasticity of the lower limbs, owing to progressive retrograde degeneration of the long corticospinal axons. Spastin is a conserved microtubule (MT)-severing protein, involved in processes requiring rearrangement of the cytoskeleton in concert to membrane remodeling, such as neurite branching, axonal growth, midbody abscission, and endosome tubulation. Two isoforms of spastin are synthesized from alternative initiation codons (M1 and M87). We now show that spastin-M1 can sort from the endoplasmic reticulum (ER) to pre- and mature lipid droplets (LDs). A hydrophobic motif comprised of amino acids 57 through 86 of spastin was sufficient to direct a reporter protein to LDs, while mutation of arginine 65 to glycine abolished LD targeting. Increased levels of spastin-M1 expression reduced the number but increased the size of LDs. Expression of a mutant unable to bind and sever MTs caused clustering of LDs. Consistent with these findings, ubiquitous overexpression of Dspastin in Drosophila led to bigger and less numerous LDs in the fat bodies and increased triacylglycerol levels. In contrast, Dspastin overexpression increased LD number when expressed specifically in skeletal muscles or nerves. Downregulation of Dspastin and expression of a dominant-negative variant decreased LD number in Drosophila nerves, skeletal muscle and fat bodies, and reduced triacylglycerol levels in the larvae. Moreover, we found reduced amount of fat stores in intestinal cells of worms in which the spas-1 homologue was either depleted by RNA interference or deleted. Taken together, our data uncovers an evolutionarily conserved role of spastin as a positive regulator of LD metabolism and open up the possibility that dysfunction of LDs in axons may contribute to the pathogenesis of HSP.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenosine Triphosphatases / chemistry
  • Adenosine Triphosphatases / genetics
  • Adenosine Triphosphatases / metabolism*
  • Amino Acid Motifs
  • Animals
  • Drosophila / genetics
  • Drosophila / metabolism
  • Drosophila Proteins / chemistry
  • Drosophila Proteins / genetics
  • Drosophila Proteins / metabolism*
  • Endoplasmic Reticulum / metabolism
  • Intestinal Mucosa / metabolism
  • Lipid Droplets / metabolism*
  • Lipid Metabolism*
  • Muscle, Skeletal / metabolism
  • Neurons / metabolism
  • Protein Transport
  • Triglycerides / metabolism

Substances

  • Drosophila Proteins
  • Triglycerides
  • Adenosine Triphosphatases
  • spas protein, Drosophila

Grant support

This work was supported by grants from the Spastic Paraplegia Foundation, the Fritz Thyssen Foundation, and the Tom Wahlig Foundation to EIR. EIR, ATr and SH received funding by the Center of Molecular Medicine Cologne (CMMC). GO received funding by the Fondazione Cariparo (Young Investigator Grant on Pediatric Research 14/03) and Italian Ministry of Health (RC2014). EIR and GO received support from the E-Rare Research Programme within the framework of the ERA-NET E-Rare 2 (Federal Ministry of Education and Research to EIR, Ministry of Health to GO). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.