Research topic: Lysosomal sorting

Main topics are:

1. Analysis of GGA structure (ubiquitin binding domain) and function for the lysosomal enzyme sorting and autophagy in neurons
2. Analysis of the cargo recognition motifs in LERP

The classical item for lysosomes reduced them to ”degradative and waste recycling organelles”, characterised by the rare specific lysosomal storage diseases. In recent years, however, lysosomes have been recognized as active participants in cellular metabolic processes, in signalling nutrition status and growth and in apoptosis of cells. Their dysfunction has been detected in many neurological diseases causing disturbed autophagy. The resulting aggregates are blocking the metabolism and the providing of monomers for reuse.
In mammalian cells two mannose-6-phosphate-receptors (MPR) are sorting the lysosomal acid hydrolases to lysosomes by interaction with specific sorting adaptors. These include three different Golgi-adaptor proteins (GGAs). The recognition motifs for interaction have been established, however, the specific meaning of the three single GGAs for sorting requires further analyses. We found that in Drosophila a similar system with only one lysosomal enzyme receptor protein (LERP) and only one GGA exists with high GGA levels in pigment cells of the retina and the mushroom bodies which are involved in the chemosensory memory and learning in insects. Ubiquitious and eye-specific GGA-knockdown by RNAi in fly as well as modulation of the expression levels resulted in impaired lysosomal sorting and disturbed LERP trafficking. To establish these findings a knockout is done using the CRISPR/CAS system in fly for GGA and LERP.

Model for lysosomal enzyme trafficking and effect of GGA mutation/knockdown on the trafficking mechanism.
A. Normal trafficking of vesicles to the lysosome Lysosomal biogenesis in wt cells. LERP sorts lysosomal cargo/cathepsins in cooperation with the adaptors GGA and AP1 for functional equipment of lysosomes. Lysosomes get material from the endocytotic route (green) for breakdown of polymer material (broken membrane line). Lysosomes fuse with autophagosomes (blue) to yield functional autophagolysosomes mediating successive degradation of the autophagic cargo (yellow-green-blueish color).
B: Lysosomal biogenesis in GGA-KOs. Depletion of GGA (X) leaves LERP sorting of lysosomal cargo to AP1 which is insufficient for functional biogenesis, with undegraded endocytosed membranes (continuous membrane line) and missorting of lysosomal hydrolase cargo to the extracellular space (light yellow colour in lysosomes and constitutive secretion). The ratio of transport via GGA or AP1 might also vary in tissues or metabolism. Thus autophagosomes find dysfunctional lysosome partners, the resulting autophagolysosomes present a diminished content of hydrolases (faint yellow colour contribution) resulting in a delay in degradation and subsequent accumulation of the autophagic cargo. The question how and whether the ubiquitin binding function of GGA contributes to autophagy is open, indicated by (?).