👤 Francisco Ciruela

Megalencephalic leukoencephalopathy with subcortical cyst (MLC) is a rare leukodystrophy primarily caused by mutations in two genes: MLC1, encoding a membrane protein of unknown function, and GlialCAM, a cell adhesion molecule. Although MLC1 has been implicated in downregulating signaling pathways, its molecular mechanisms remain elusive. Recently, the orphan G-protein-coupled receptor GPRC5B was identified as a novel interactor of both GlialCAM and MLC1, with dominant heterozygous mutations found in MLC patients, suggesting that GlialCAM and MLC1 may regulate cell signaling via GPRC5B. Here, we show that GPRC5B exhibits constitutive activity, which is inhibited by MLC1, likely through interference with GPRC5B oligomerization. Conversely, GlialCAM enhances β-arrestin 2 recruitment, leading to its own mislocalization from cell-cell junctions. MLC-associated GPRC5B mutants show enhanced maturation and increased stability at the plasma membrane, retain normal constitutive activity and responsiveness to MLC1 and GlialCAM but display increased affinity for GlialCAM and localize to cell-cell junctions in its presence. Notably, coexpression of GlialCAM with these mutants does not induce GlialCAM mislocalization. We propose a model in which finely tuned interactions among GPRC5B, GlialCAM, and MLC1 regulate receptor signaling. These findings provide the first biochemical evidence of GlialCAM and MLC1 modulating GPRC5B activity, suggesting a biochemical explanation for the gain-of-function phenotype observed in GPRC5B MLC mutants. Importantly, our work supports the potential of targeting GPRC5B as a therapeutic strategy in MLC. Show less

Megalencephalic Leukoencephalopathy with subcortical Cysts (MLC) is a type of vacuolating leukodystrophy, which is mainly caused by mutations in MLC1 or GLIALCAM. The two MLC-causing genes encode for membrane proteins of yet unknown function that have been linked to the regulation of different chloride channels such as the ClC-2 and VRAC. To gain insight into the role of MLC proteins, we have determined the brain GlialCAM interacting proteome. The proteome includes different transporters and ion channels known to be involved in the regulation of brain homeostasis, proteins related to adhesion or signaling as several G protein-coupled receptors (GPCRs), including the orphan GPRC5B and the proposed prosaposin receptor GPR37L1. Focusing on these two GPCRs, we could validate that they interact directly with MLC proteins. The inactivation of Gpr37l1 in mice upregulated MLC proteins without altering their localization. Conversely, a reduction of GPRC5B levels in primary astrocytes downregulated MLC proteins, leading to an impaired activation of ClC-2 and VRAC. The interaction between the GPCRs and MLC1 was dynamically regulated upon changes in the osmolarity or potassium concentration. We propose that GlialCAM and MLC1 associate with different integral membrane proteins modulating their functions and acting as a recruitment site for various signaling components as the GPCRs identified here. We hypothesized that the GlialCAM/MLC1 complex is working as an adhesion molecule coupled to a tetraspanin-like molecule performing regulatory effects through direct binding or influencing signal transduction events. Show less