Rugarli, Elena - C 8

Lipid droplets (LDs) are complex and dynamic organelles whose function is to assemble, store, and supply neutral lipids, mainly sterol esters and triacylglycerols (TAGs). Initially recognized in specialized cells, such as adipocytes, it is now clear that any cell has the ability to form LDs. Dysfunctions of LDs have been implicated in several pathologic conditions, such as obesity, atherosclerosis, and lipodystrophies. LDs are occasionally found in ultrastructural studies of neurons, however very little is known about their role in these cells. Notably, LDs appear to be increased in the brain of Alzheimer patients. Moreover, α-synuclein, the major constituent of Lewy bodies in Parkinson’s disease, was shown to accumulate on the surface of LDs in cells loaded with lipids. Recently, a link is emerging between LDs and axonopathies of the central nervous system, such as hereditary spastic paraplegia (HSP).

HSP is clinically defined by the association of weakness and spasticity of the lower limbs (pure HSP). The disease is caused by progressive retrograde degeneration of the longest axons of the central nervous system, those composing the corticospinal tract. HSP is one of the most heterogeneous neurological conditions from a genetic point of view. More than 80 genes have currently been identified to be responsible for this disease. A unifying view of HSP pathogenesis posits that alterations of the endoplasmic reticulum morphogenesis underlie this condition. Notably, several proteins implicated in HSP, such as Atlastin, REEP1, Seipin, Spartin, and DDHD2, have been linked to some aspect of LD function. 

Our group has a long-standing interest in SPAST, encoding spastin, which is the most commonly mutated gene in patients affected by autosomal dominant hereditary spastic paraplegia (HSP). Spastin is a conserved microtubule (MT)-severing protein, involved in several 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 (spastin-M1 and spastin-M87) are synthesized from alternative initiation codons located in the first exon of the gene.

Recently, we found that in mammalian cells spastin-M1 can sort from the endoplasmic reticulum to pre- and mature lipid droplets (LDs) (Figure 1).

We further identified the N-terminal hydrophobic domain in cooperation with a basic motif in spastin-M1 to be necessary and sufficient for recruitment to LDs (Figure 2).

Increased levels of spastin-M1 expression lead to reduced number of bigger LDs, while expression of a spastin mutant unable to bind and sever MTs causes clustering of LDs. The role of spastin in LD formation and lipid metabolism is evolutionary conserved.

Downregulation of Dspastin and expression of a dominant-negative variant decrease LD number in Drosophila nerves and skeletal muscle, while Dspastin overexpression leads to an opposite phenotype. Ubiquitous downregulation or upregulation of Dspastin levels affects triacylglycerol levels in a consistent fashion.

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 a role of spastin in LD metabolism and strongly suggest that dysfunction of LDs in axons may contribute to the pathogenesis of HSP.