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PD Dr. Markus Damme

Markus Damme
Markus Damme
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Lysosomal Diseases and Characterization of New Lysosomal (Mem­brane) Proteins
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Research
The research focus of the group is to analyze the function and dysfunction of lysosomes and lysosomal proteins. We combine multidisciplinary approaches including cell biology, mouse genetics, biochemistry and in collaborative effort structural biology and state-of-the-art high-throughput techniques like mass spectrometry and RNA–sequencing.
 
Deciphering the role of lysosomes and lysosomal proteins in neurodegenerative diseases
Lysosomes are membrane-bound organelles in eukaryotic cells in which (mostly soluble) acidic hydrolases mediate the catabolic degradation of macromolecules like proteins, lipids or oligosaccharides to lower molecular weight metabolites like amino acids, monosaccharides or fatty acids. These metabolites become eventually exported from the lysosomal lumen to the cytosol by specific membranous exporter proteins, where they can be reused for a new round of synthesis. The lysosomal membrane is furthermore protected by highly glycosylated integral membrane proteins from self-digestion by the formation of a glycocalyx.
Figur 1 Figure 1. Tmem106b knockout mice show dramatically enlarged, LAMP1-positive organelles in the axon initial segment of motor neurons Lüningschrör et al. Cell Reports; Vol.30, Issue 10, Pages 3506-3519.e6
 
Characterization of new lysosomal membrane proteins of unknown function
Lysosomes are membrane-bound organelles in eukaryotic cells in which (mostly soluble) acidic hydrolases mediate the catabolic degradation of macromolecules like proteins, lipids or oligosaccharides to lower molecular weight metabolites like amino acids, monosaccharides or fatty acids. These metabolites become eventually exported from the lysosomal lumen to the cytosol by specific membranous exporter proteins, where they can be reused for a new round of synthesis. The lysosomal membrane is furthermore protected by highly glycosylated integral membrane proteins from self-digestion by the formation of a glycocalyx.
Although a couple of lysosomal exporters like Sialin, Cystinosin or the Cobalamin exporter where identified and thoroughly characterized in the last years (mostly due to diseases characterized by a deficiency of such exporter proteins and resulting lysosomal storage of the cargo in the lysosomal lumen), a great majority of such proteins was described so far only by means of biochemistry, without identifying the corresponding genes.
Several recent proteomics studies have addressed the identification of new lysosomal membrane proteins including such putative polytopic exporter proteins and integral membrane proteins with unlikely transporter function with only one or two transmembrane domains. Goal of our research is to elucidate the function of such lysosomal membrane proteins of so far unknown function, particularly those whose dysfunction leads to devastating diseases like lysosomal storage diseases or neurodegeneration. We are currently working on several new lysosomal membrane proteins including the putative transporters MFSD1 and its accessory subunit GLMP. Mouse models with loss-of-function are analysed in order to identify putative substrates or to reveal impaired pathological pathways. We are seeking for interaction partners to elucidate the function of the non-transporter proteins.
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Figure 2. The complex of the polytopic lysosomal transporter MFSD1 and its accessory subunit GLMP. MFSD1 is mislocalized in mouse embryonic fibroblasts from Glmp knockout mice. Massa-Lopez et al. eLife 2019;8:e50025

 
PLD3 – a novel lysosomal 5'-specific exonuclease
Mutations in PLD3, a member of the phospholipase D family, have been identified in Alzheimer's disease patients and hereditary ataxia. We found PLD3 to be a resident lysosomal protein that undergoes proteolytic processing after synthesis as a type II transmembrane protein and transport to multivesicular bodies (Gonzalez et al. Cell Rep. 2018; 22(4):1040-1053). In collaborative efforts, we contributed to the characterization of PLD3 as a single stranded DNA-specific 5' exonuclease that is critical in the catabolism of lysosomal nucleic acids (Gavin et al., Nat Immunol. 2018; (9):942-953). This function is essential for preventing continuous activation of the endosomal Toll-like receptor 9 (TLR9) thereby constituting a critical component of the innate immune-system. We are using mouse models and biochemical methods to further understand the function of PLD3 in lysosomes. We developed essential tools like enzymatic assays and aim to get a comprehensive understanding of the lysosomal nucleic acid degradation mediated by PLD3.
Figur 3 Figure 3. PLD3 mediates the final steps in the degradation of nucleic acids in lysosomes.
 
Arylsulfatase G (ARSG) – a lysosomal sulfatase causative for the Usher syndrome type 4
Lysosomal sulfatases mediate the catabolic degradation of sulfated glycosaminoglycans including heparan sulfate. The function of ARSG – a so far orphan lysosomal sulfatase remained enigmatic. We have identified ARSG as the long sought-after 3-O-specific sulfatase previously. In later studies, mutations in ARSG were found to be causative for Usher type 4 syndrome (USH4) (Khateb et al. Genet Med. 2018 Sep;20(9):1004-1012) – a disease leading to late-onset hearing-loss and retina degeneration. We are currently analysing patient-mutations leading to USH4 and try to understand the molecular details underlying this rare and orphan disease.
Figur 4Figure 4: ARSG cleaves off the sulfate group from heparan sulfate in the 3-O position of glucosamine.
 
Letzte Änderung/Last change: August 20, 2021