Functional characterization of the putative lysosomal transporter protein Major facilitator superfamily domain containing 1 (Mfsd1) and its role in sinusoidal obstruction syndrome

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Lysosomes are membrane bound organelles which mediate the degradation of most classes of macromolecules from intracellular or extracellular sources by a concerted action of acid hydrolases. Upon enzymatic degradation of these macromolecules to low molecular weight monomers like amino acids or monosaccharides, the latter become exported from the lysosomal lumen to the cytosol for metabolic reuse by specific transmembrane proteins, so called exporter proteins. Even though lysosomal transporters for a few metabolites are known, the exporter proteins for the great majority including most amino acids and monosaccharides are currently unidentified. Some lysosomal amino acid exporters were shown to play a pivotal role in mechanistic Target of Rapamycin complex (mTORC1)-mediated nutrient signalling at the lysosomal membrane. The ultimate goal of this application is to decipher the function and the substrate(s) of a lysosomal membrane protein from the solute carrier superfamily, called Mfsd1. The protein is assumed to be a transporter for small solutes and was shown in our preliminary experiments to localize to lysosomes, but its substrate(s) is (are) unknown so far. We envisage deciphering its physiological role and transporting function by characterizing the protein in detail, generating loss and gain of function cell lines and analyse an established Mfsd1-knockout mouse model in detail. A possible contribution of Mfsd1 in mTORC1-mediated nutrient-sensing at the lysosomal membrane should also be evaluated. Our preliminary analyses of Mfsd1-knockout mice revealed an early liver phenotype characterised by sinusoidal obstruction and liver sinusoidal endothelial cell death, reflecting the situation in a human hepatic disease called sinusoidal obstruction syndrome. The role of Mfsd1 in the pathogenesis of sinusoidal obstruction syndrome should therefore be investigated. Furthermore the interaction of Mfsd1 with another lysosomal membrane protein called Glmp, will be examined since our preliminary data point to a direct interaction of both proteins and knockout mice of both proteins have a strikingly similar phenotype. In summary, our findings will significantly gain deeper insight into the understanding of transporter functions at the lysosomal membrane. We will identify the metabolites which are transported by Mfsd1 and will determine the physiological function of the protein in knockout mice and particularly in sinusoidal obstruction syndrome.