FIG4 regulates lysosome membrane homeostasis independent of phosphatase function

Abstract

FIG4 is a phosphoinositide phosphatase that is mutated in several diseases including Charcot-Marie-Tooth Disease 4J (CMT4J) and Yunis-Varon syndrome (YVS). To investigate the mechanism of disease pathogenesis, we generated Drosophila models of FIG4-related diseases. Fig4 null mutant animals are viable but exhibit marked enlargement of the lysosomal compartment in muscle cells and neurons, accompanied by an age-related decline in flight ability. Transgenic animals expressing Drosophila Fig4 missense mutations corresponding to human pathogenic mutations can partially rescue lysosomal expansion phenotypes, consistent with these mutations causing decreased FIG4 function. Interestingly, Fig4 mutations predicted to inactivate FIG4 phosphatase activity rescue lysosome expansion phenotypes, and mutations in the phosphoinositide (3) phosphate kinase Fab1 that performs the reverse enzymatic reaction also causes a lysosome expansion phenotype. Since FIG4 and FAB1 are present together in the same biochemical complex, these data are consistent with a model in which FIG4 serves a phosphatase-independent biosynthetic function that is essential for lysosomal membrane homeostasis. Lysosomal phenotypes are suppressed by genetic inhibition of Rab7 or the HOPS complex, demonstrating that FIG4 functions after endosome-to-lysosome fusion. Furthermore, disruption of the retromer complex, implicated in recycling from the lysosome to Golgi, does not lead to similar phenotypes as Fig4, suggesting that the lysosomal defects are not due to compromised retromer-mediated recycling of endolysosomal membranes. These data show that FIG4 plays a critical noncatalytic function in maintaining lysosomal membrane homeostasis, and that this function is disrupted by mutations that cause CMT4J and YVS.

PIKfyve activity regulates reformation of terminal storage lysosomes from endolysosomes

“…These observations are in line with previous studies, although more extensive PIKfyve inhibition was also reported to induce enlargement of early endosomes . As observed before, knockdown of the phosphatase FIG4 resulted in the same phenotype as knockdown of the PIKfyve kinase, which may be explained by the scaffolding function of FIG4 that regulates PIKfyve activity . While the protein content of the limiting membrane of these enlarged compartments was already described, little is known about their lipid and lumenal content.…”

“…5 As observed before, knockdown of the phosphatase FIG4 resulted in the same phenotype as knockdown of the PIKfyve kinase, 2,3 which may be explained by the scaffolding function of FIG4 that regulates PIKfyve activity. 11,12 While the protein content of the limiting membrane of these enlarged compartments was already described, little is known about their lipid and lumenal content. Late endosomes are filled with intralumenal vesicles (ILVs) that contain lysobisphosphatidic acid (LBPA) and cholesterol.…”

TFEB/Mitf links impaired nuclear import to autophagolysosomal dysfunction in C9-ALS

“…In Drosophila, G4C2 repeats cause loss of autophagosomes and disrupt lysosomal structure and function. This accumulation of autolysosomes and lysosome-related organelles called multilamellar bodies (MLBs) has been 5 observed in lysosomal storage disorders and has been reported in spinal cord tissue from sporadic ALS patients (Bharadwaj, Cunningham, Zhang, & Lloyd, 2016;Parkinson-Lawrence et al, 2010;Sasaki, 2011). Regulation of protein and lipid homeostasis by the lysosome may be particularly important in neurons since they are post-mitotic and have high energy demands (Fraldi, Klein, Medina, & Settembre, 2016).…”

Yunis-Varón syndrome caused by biallelic VAC14 mutations

“…3 PtdIns(3,5)P 2 is enriched at the cytosolic surface of endolysosomal membranes where it is synthesized by a ternary complex consisting of FIG4, the lipid kinase PIKfyve and the adapter protein VAC14; all three proteins are essential for PtdIns(3,5)P 2 synthesis. 4,5 Cells from YVS patients and orthologous (Fig4 -/-) mice have altered endolysosomal trafficking, as evidenced by the presence of multiple, enlarged vacuoles expressing endolysosomal markers including LAMP2. 3,6,7 Pathogenic variants of the PtdIns(3,5)P 2 synthetic complex cause several disorders.…”