Defective autophagy in spastizin mutated patients with hereditary spastic paraparesis type 15

Abstract

Hereditary spastic paraparesis type 15 is a recessive complicated form of the disease clinically characterized by slowly progressive spastic paraparesis and mental deterioration with onset between the first and second decade of life. Thinning of corpus callosum is the neuroradiological distinctive sign frequently associated with white matter abnormalities. The causative gene, ZFYVE26, encodes a large protein of 2539 amino acid residues, termed spastizin, containing three recognizable domains: a zinc finger, a leucine zipper and a FYVE domain. Spastizin protein has a diffuse cytoplasmic distribution and co-localizes partially with early endosomes, the endoplasmic reticulum, microtubules and vesicles involved in protein trafficking. In addition, spastizin localizes to the mid-body during the final step of mitosis and contributes to successful cytokinesis. Spastizin interacts with Beclin 1, a protein required for cytokinesis and autophagy, which is the major lysosome-mediated degradation process in the cell. In view of the Beclin 1-spastizin interaction, we investigated the possible role of spastizin in autophagy. We carried out this analysis by using lymphoblast and fibroblast cells derived from four different spastizin mutated patients (p.I508N, p.L243P, p.R1209fsX, p.S1312X) and from control subjects. Of note, the truncating p.R1209fsX and p.S1312X mutations lead to loss of spastizin protein. The results obtained indicate that spastizin interacts with the autophagy related Beclin 1-UVRAG-Rubicon multiprotein complex and is required for autophagosome maturation. In cells lacking spastizin or with mutated forms of the protein, spastizin interaction with Beclin 1 is lost although the formation of the Beclin 1-UVRAG-Rubicon complex can still be observed. However, in these cells we demonstrate an impairment of autophagosome maturation and an accumulation of immature autophagosomes. Autophagy defects with autophagosome accumulation can be observed also in neuronal cells upon spastizin silencing. These results indicate that autophagy is a central process in the pathogenesis of complicated forms of hereditary spastic paraparesis with thin corpus callosum.

Autophagy and human diseases

“…They contain seven-bladed β-propellers formed by seven WD40 repeats and bind to phosphatidylinositol 3-phosphate and the lysosomal/vacuolar lipid phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P 2 ) [17,20]. [44,45] Atg18/WIPIs also interact with Atg2 [20][21][22]. The crystal structure of Hsv2, a yeast Atg18 paralogue, shows two phosphoinositide-binding sites at blades five and six, and an Atg2-binding region at blade 2 [23][24][25].…”

“…Recently, Vantaggiato et al reported that ZFYVE26/ SPG15, the causative gene of another recessive complicated form of HSP (HSP type 15), is also involved in the autophagy process [44]. ZFYVE26/SPG15 encodes a zinc-finger protein with a FYVE domain and a leucine zipper, termed spastizin [45].…”

“…Spastizin interacts with the Beclin 1-UVRAG-Rubicon complex and mediates autophagosome maturation. Both spastizin-mutated fibroblast cells derived from HSP patients and spastizin knockdown cells showed impaired autophagic flux and accumulation of autophagosomes due to reduced autophagosome-lysosome fusion [44]. However, as this complex also plays an important role in the endocytic pathway [46][47][48][49], and as spastizin is not present on the autophagic membranes [44], whether spastizin specifically regulates autophagosome-lysosome fusion needs to be clarified.…”

Autophagy and neurodegeneration

“…The exact function of this protein is unknown; however, it bears homology to TECPR1, which has been implicated in sequestration of bacteria into autophagosomes, suggesting a role as an autophagy adaptor (77). Type 15 HSP is caused by mutations in ZFYVE26 (79) encoding spastizin (also known as FYVE-CENT), which interacts with the core autophagy protein, beclin 1. Disease-associated mutations in spastizin compromise this interaction, leading to impaired autophagy (79).…”

“…Type 15 HSP is caused by mutations in ZFYVE26 (79) encoding spastizin (also known as FYVE-CENT), which interacts with the core autophagy protein, beclin 1. Disease-associated mutations in spastizin compromise this interaction, leading to impaired autophagy (79).…”