Spastin Binds to Lipid Droplets and Affects Lipid Metabolism

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.

Spastin tethers lipid droplets to peroxisomes and directs fatty acid trafficking through ESCRT-III

“…Depletion of Spastin in Drosophila and C. elegans leads to aberrant FA metabolism in LDs (Papadopoulos et al, 2015). In addition, HSP patient-derived olfactory neurosphere-derived cells with mutations in Spastin showed impaired peroxisome movement and distribution (Wali et al, 2016).…”

“…M1 Spastin is an isoform generated from the first translation initiation codon of Spastin (Claudiani et al, 2005). It contains an integral membrane hairpin motif in the Nterminal region that localizes it to LDs and other membrane compartments (Connell et al, 2009;Papadopoulos et al, 2015;Park et al, 2010;Reid et al, 2005). The shorter M87 Spastin isoform lacking this region resides primarily in the cytoplasm.…”

Spastin, atlastin, and ER relocalization are involved in axon but not dendrite regeneration

“…In mammals, the long M1 spastin isoform has an N-terminal region that contains a hydrophobic domain, and this isoform is an integral membrane protein (Park et al ., 2010) believed to localize to the endoplasmic reticulum (ER), endosomes (Sanderson et al ., 2006), and lipid droplets (Papadopoulos et al ., 2015). In Drosophila , there are also spastin isoforms with an N-terminal hydrophobic region predicted to be a transmembrane domain (Trotta et al ., 2004), and this region directs overexpressed spastin to punctate membranes (Roll-Mecak and Vale, 2005).…”

“…However, so far there is no evidence to support this role in mature neurons. Atlastin and spastin also regulate lipid droplet formation (Klemm et al ., 2013; Papadopoulos et al ., 2015) and BMP signaling (Tsang et al ., 2009; Fassier et al ., 2010; Zhao and Hedera, 2013; Summerville et al ., 2016), and so these are other potential common functions that could be relevant to HSP. Beyond this group of interacting proteins, other HSP genes encode proteins broadly involved in ER morphogenesis, endosomal trafficking, microtubule-based transport, and mitochondrial function (Noreau et al ., 2014), but most are not as well characterized as spastin and atlastin.…”

Patient-Derived Stem Cell Models in SPAST HSP: Disease Modelling and Drug Discovery

“…Spastin regulates multiple cellular functions, including microtubule dynamics [10], shaping endoplasmic reticulum [11], and regulating lipid droplet metabolism [12]. Our review focuses on the microtubule dynamics regulation function of spastin.…”