LIMP-2: A fascinating lysosomal membrane protein with multiple functions. Analysis of its role in late endocytotic compartments
Prof. Dr. Paul Saftig (2008 – 2017)
PD Dr. Michael Schwake (2008 – 2015)
Dr. Judith Blanz (2012 – 2015)
Kiel University, Institute of Biochemistry
The highly glycosylated lysosomal integral membrane protein type-2 (LIMP-2/ SCARB2) is an abundant protein of the lysosomal membrane fulfilling diverse functions. Mutations within SCARB2, the gene encoding LIMP-2 were identified as disease causing for the Action Myoclonus Renal Failure Syndrome (AMRF), a progressive myoclonic epilepsy associated with renal failure (Berkovic et al., 2008,Am J Hum Genet). LIMP-2 has been also identified as the receptor for the uptake of the enterovirus 71 and coxsackievirus A16 which are most frequently associated with hand, foot and mouth disease (Yamayoshi et al., 2009, Nat Medicine).
In 2007, we demonstrated that LIMP-2 is the long sought after receptor for mannose-6-phophate independent trafficking of the acid hydrolase β-glucocerebrosidase (GC) to the lysosome and that activity of GC is tightly linked to expression of LIMP-2 (Reczek and Schwake et al., 2007, Cell). GC is required for the intralysosomal breakdown of the sphingolipid glucosylceramide (GluCer). Loss of GC activity leads to intralysosomal accumulation of GluCer and Gaucher disease (GD), the most common lysosomal storage disorder. GD is genetically linked to Parkinson`s disease (PD) and mutations in GBA1, the gene encoding GC represent to date the highest genetic risk factor for developing PD. Recently, in collaboration our group has solved the structure of the LIMP-2 ectodomain highlighting a helical bundle as the GC interaction site (Neculai et al., 2014, Nature). In addition, the LIMP-2 structure revealed the presence of a lipid translocating tunnel suggesting an important role of LIMP-2 in lipid metabolism independent of its function as a GC transporter. Since trafficking of GC requires LIMP-2 and genetic variations within the LIMP-2 locus have been linked to synucleinopthies (Hopfner et al., 2013, Movement Disorder and Bras et al., 2014, Human Molecular Genetics) we examined the role of LIMP-2 in α-syn aggregation by using LIMP-2 knockout mice (Rothaug et al., 2014, PNAS).
In LIMP-2-deficient brains, lysosomal GC activity was strongly reduced which was accompanied by pronounced accumulation of lipids including the GC substrate GluCer. GluCer has been suggested to stabilize soluble oligomeric α-syn species thereby promoting neurotoxic aggregation of α-syn which leads to a blockage of ER-Golgi transport of GC forming a pathogenic loop boosting disease progression (Mazzulli et al., 2011, Cell). In LIMP-2-deficient brains and in primary cultured neurons, we observed aggregation of α-syn, selective shrinkage of dopaminergic (DA) neurons, apoptotic cell death and glial activation. Heterologous expression of lysosomal targeted LIMP-2 increased lysosomal GC activity and concomitantly enhanced α-syn clearance in various cellular systems. In addition, midbrain sections of sporadic PD patients displayed increased levels of LIMP-2 in surviving DA neurons possibly as a compensatory mechanism to overcome impaired targeting of GC to lysosomes (Rothaug et al., 2014, PNAS). Hence, we suggest that targeting of GC to lysosomes can be modulated via its interaction with LIMP-2 which represents a novel therapeutic target for the treatment of synucleinopathies.
Our research focuses i) on the impact of LIMP-2 on GC transport to determine the stoichiometry and the structure of the LIMP-2/GC complex, ii) to unravel the role of LIMP-2 in α-syn metabolism and iii) to elucidate unknown functions of this diverse protein by searching for novel interaction partners of LIMP-2 using biochemical and genetic approaches.