Role of the lysosomal membrane protein LIMP-2 as a transport receptor for Glucocerebrosidase in disease

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Acid hydrolases are essential for the degradation of macromolecules within lysosomes. The physiological role of this process is highlighted by a large number of disorders caused by dysfunctional lysosomal hydrolases. In general are these enzymes transported to lysosomes by the mannose-6-phosphat pathway, which is the common trafficking pathway with enzyme and tissue specific exceptions. One of such an exception is glucocerebrosidase (GCase), in which mutations cause Gaucher Disease (GD). We have demonstrated that GCase utilizes the lysosomal membrane protein (LIMP-2) for lysosomal transport and that lysosomal function of GCase is intimately connected to LIMP-2. Null-mutations in the gene encoding LIMP-2 cause Action Myoclonus-Renal Failure syndrome, an autosomal-recessive lysosomal storage disorder (LSD) with the combination of kidney failure and progressive myoclonus epilepsy. Some patients also develop a demyelinating peripheral neuropathy and hearing loss. LIMP-2-deficient mice have a very similar phenotype, making this mouse model a valuable tool to analyze consequences of AMRF causing null-mutations.By solving the structure of the LIMP-2 ectodomain, we found that LIMP-2 is likely bifunctional. It consists of a partial β-barrel core, important for lipid binding and transport and a three helix bundle that mediates interaction with GCase. LIMP-2/GCase interaction is necessary for lysosomal transport of GCase from the Endoplasmic Reticulum to lysosomes. LIMP-2 deficiency, caused by AMRF null-mutations is affecting both, lysosomal lipid transport and lysosomal trafficking of GCase. As already mentioned cause mutations in GBA1, the gene encoding GCase, GD, the most prevalent LSD, characterized by hepatosplenomegaly, bone abnormalities and in rare cases central nervous system manifestations, such as ataxia or epilepsy. More recent have been variants in SCARB2 and GBA1 identified as risk factors for Parkinson`s disease (PD) and Dementia with Lewy Bodies (DLB). In particular, mutations in GCase are a major genetic contributor to PD and DLB and decreased GCase activity has also been found in sporadic PD brains. To dissect the physiological role of LIMP-2 as a lysosomal trafficking receptor for GCase and as a lipid/cholesterol transporter, we have generated a knockin (KI) mouse model, expressing a LIMP-2 mutant, carrying a point mutation in the GCase binding site. Our preliminary results show that KI mice are expressing GCase binding deficient LIMP-2, resulting in significantly reduced lysosomal GCase levels in liver, which are in a comparable range of LIMP-2 deficient mice.Our preliminary results strongly suggest that the LIMP-2 KI mice provide a unique tool to analyze the physiological consequences of a significant depletion of lysosomal GCase activity without disrupting LIMP-2 expression. The experiments proposed here will establish whether the pathology mediated by LIMP-2 deficiency may at least be in part dependent of GCase.