MYORG

MYORG (NET37) is an ER-luminal α-galactosidase of the GH31 glycoside hydrolase family that functions in astrocyte biology and brain calcium homeostasis, with loss-of-function causing primary familial brain calcification. Crystal structures with substrate and inhibitor define its active site, and disease-associated mutations map to catalytically critical residues; mutation of the conserved catalytic residue abolishes enzymatic function and the protein's ability to support cellular differentiation (PMID:36129849, PMID:19706595). MYORG is expressed specifically in S100β-positive astrocytes, and knockout in mice or knockdown in zebrafish produces brain calcification, with human autopsy revealing shortened astrocyte foot processes and reduced AQP4 at the blood–brain barrier as downstream consequences of MYORG loss (PMID:29910000, PMID:35870928, PMID:39180105). In myoblasts, MYORG associates with pro-IGF-II and is required for IGF-II secretion and Akt activation during differentiation, indicating a role in glycoprotein processing within the ER lumen (PMID:19706595).

1. 2009 High

   Establishing that MYORG (NET37) is an ER/nuclear-envelope transmembrane protein with a lumen-facing glycosidase domain whose catalytic activity is required for myoblast differentiation, answering the basic question of where the protein acts and whether its enzymatic fold is functionally relevant.

   Evidence Protease protection topology mapping, RNAi knockdown in C2C12 myoblasts with catalytic-dead mutant rescue, co-immunoprecipitation with pro-IGF-II, IGF-II secretion and Akt phosphorylation assays

   PMID:19706595

   Open questions at the time

   • The endogenous substrate cleaved by MYORG in the ER lumen was not identified
   • Whether the IGF-II interaction is direct or bridged by other ER-resident factors was not resolved
   • Relevance of the myoblast differentiation function to in vivo physiology was unknown
2. 2018 High

   Demonstrating that MYORG is astrocyte-specific in brain and that its loss of function causes brain calcification in mice, establishing MYORG as a causal gene for primary familial brain calcification and shifting attention from muscle to the nervous system.

   Evidence In situ hybridization and immunostaining for cell-type localization; Myorg knockout mouse model with CT and histological analysis of brain calcification

   PMID:29910000

   Open questions at the time

   • The molecular mechanism linking ER-luminal glycosidase activity to calcium deposition was not determined
   • Whether calcification arises cell-autonomously in astrocytes or involves non-cell-autonomous effects on neighboring cells was unclear
   • The endogenous glycoprotein substrates in astrocytes were not identified
3. 2022 High

   Defining MYORG's precise enzymatic specificity as an α-galactosidase (not α-glucosidase) and solving its crystal structure with substrate and inhibitor, which revealed that disease mutations cluster at the active site and that pharmacological chaperones can stabilize pathogenic variants.

   Evidence In vitro enzymatic assays with defined substrates; X-ray crystallography of MYORG–substrate and MYORG–inhibitor complexes; active-site mutagenesis; thermal shift assay with pharmacological chaperone

   PMID:36129849

   Open questions at the time

   • The physiological galactose-containing glycan substrate in the ER remains unidentified
   • Whether chaperone-mediated stabilization of mutant MYORG restores function in vivo has not been tested
   • The structural basis for MYORG's interaction with pro-IGF-II or other client proteins is unknown
4. 2022 High

   Confirming evolutionary conservation of the MYORG loss-of-function brain calcification phenotype by showing that morpholino knockdown in zebrafish produces brain calcification that is rescued by cDNA replenishment.

   Evidence Morpholino knockdown at splice and translation sites in zebrafish; calcein staining; mRNA rescue

   PMID:35870928

   Open questions at the time

   • The specific cell types affected in zebrafish brain were not determined
   • Whether zebrafish calcification involves the same astrocyte-BBB mechanism as in mammals is unknown
5. 2024 Medium

   Linking MYORG loss to astrocyte foot process retraction and reduced AQP4 at brain capillaries in a human autopsy, implicating blood–brain barrier disruption as a downstream pathophysiological consequence.

   Evidence Neuropathological autopsy with AQP4 immunohistochemistry and astrocyte morphometry in a MYORG-PFBC patient

   PMID:39180105

   Open questions at the time

   • Based on a single autopsy case; independent replication in additional patients or animal models is needed
   • Whether AQP4 loss and foot process changes are a direct consequence of MYORG enzymatic deficiency or secondary to calcification is unresolved
   • The molecular link between ER-luminal α-galactosidase activity and astrocyte endfeet maintenance is unknown

• The identity of MYORG's physiological glycan or glycoprotein substrate in astrocytes remains the central open question; without it, the causal chain from α-galactosidase deficiency to calcium deposition cannot be mechanistically completed.
• No endogenous ER-luminal substrate has been identified in astrocytes
• The cell-autonomous versus non-cell-autonomous basis of calcification has not been resolved
• Whether MYORG's role in IGF-II processing is relevant to astrocyte function or brain calcification is untested