Distinct Modes of Balancing Glomerular Cell Proteostasis in Mucolipidosis Type II and III Prevent Proteinuria
Authors: Sachs Wiebke , Sachs Marlies , Krüger Elke , Zielinski Stephanie , Kretz Oliver , Huber Tobias B. , Baranowsky Anke , Westermann Lena Marie , Voltolini Velho Renata , Ludwig Nataniel Floriano , Yorgan Timur Alexander , Di Lorenzo Giorgia , Kollmann Katrin , Braulke Thomas , Schwartz Ida Vanessa , Schinke Thorsten , Danyukova Tatyana , Pohl Sandra , Meyer-Schwesinger Catherine 
Published: July 2020

Significance Statement
Patients with the severe lysosomal storage disorder mucolipidosis II (MLII) have mild microalbuminuria, among other symptoms, but patients with the milder MLIII do not have proteinuria. Both conditions result from mutations in the same gene. Mouse models of each disorder reveal that distinct mechanisms compensate for the disruption in protein synthesis balance in glomeruli. Both MLII and MLIII downregulate the protein complex mTORC1 (mammalian target of rapamycin complex 1) signaling to dampen protein synthesis, but MLII also increases the integrated stress response and MLIII activates the proteasome system.

Background
The mechanisms balancing proteostasis in glomerular cells are unknown. Mucolipidosis (ML) II and III are rare lysosomal storage disorders associated with mutations of the Golgi-resident GlcNAc-1-phosphotransferase, which generates mannose 6-phosphate residues on lysosomal enzymes. Without this modification, lysosomal enzymes are missorted to the extracellular space, which results in lysosomal dysfunction of many cell types. Patients with MLII present with severe skeletal abnormalities, multisystemic symptoms, and early death; the clinical course in MLIII is less progressive. Despite dysfunction of a major degradative pathway, renal and glomerular involvement is rarely reported, suggesting organ-specific compensatory mechanisms.

Methods
MLII mice were generated and compared with an established MLIII model to investigate the balance of protein synthesis and degradation, which reflects glomerular integrity. Proteinuria was assessed in patients. High-resolution confocal microscopy and functional assays identified proteins to deduce compensatory modes of balancing proteostasis.

Results
Patients with MLII but not MLIII exhibited microalbuminuria. MLII mice showed lysosomal enzyme missorting and several skeletal alterations, indicating that they are a useful model. In glomeruli, both MLII and MLIII mice exhibited reduced levels of lysosomal enzymes and enlarged lysosomes with abnormal storage material. Nevertheless, neither model had detectable morphologic or functional glomerular alterations. The models rebalance proteostasis in two ways: MLII mice downregulate protein translation and increase the integrated stress response, whereas MLIII mice upregulate the proteasome system in their glomeruli. Both MLII and MLIII downregulate the protein complex mTORC1 (mammalian target of rapamycin complex 1) signaling, which decreases protein synthesis.

Conclusions
Severe lysosomal dysfunction leads to microalbuminuria in some patients with mucolipidosis. Mouse models indicate distinct compensatory pathways that balance proteostasis in MLII and MLIII.