Identification of the GlialCAM interactome: the G protein-coupled receptors GPRC5B and GPR37L1 modulate megalencephalic leukoencephalopathy proteins.

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.

Structural basis for the dominant or recessive character of GLIALCAM mutations found in leukodystrophies.

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a type of leukodystrophy characterized by white matter edema, and it is caused mainly by recessive mutations in MLC1 and GLIALCAM genes. These variants are called MLC1 and MLC2A with both types of patients sharing the same clinical phenotype. In addition, dominant mutations in GLIALCAM have also been identified in a subtype of MLC patients with a remitting phenotype. This variant has been named MLC2B. GLIALCAM encodes for an adhesion protein containing two immunoglobulin (Ig) domains and it is needed for MLC1 targeting to astrocyte-astrocyte junctions. Most mutations identified in GLIALCAM abolish GlialCAM targeting to junctions. However, it is unclear why some mutations behave as recessive or dominant. Here, we used a combination of biochemistry methods with a new developed anti-GlialCAM nanobody, double-mutants and cysteine cross-links experiments, together with computer docking, to create a structural model of GlialCAM homo-interactions. Using this model, we suggest that dominant mutations affect different GlialCAM-GlialCAM interacting surfaces in the first Ig domain, which can occur between GlialCAM molecules present in the same cell (cis) or present in neighbouring cells (trans). Our results provide a framework that can be used to understand the molecular basis of pathogenesis of all identified GLIALCAM mutations.

GPR37 Receptors and Megalencephalic Leukoencephalopathy with Subcortical Cysts.

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare type of vacuolating leukodystrophy (white matter disorder), which is mainly caused by defects in MLC1 or glial cell adhesion molecule (GlialCAM) proteins. In addition, autoantibodies to GlialCAM are involved in the pathology of multiple sclerosis. MLC1 and GLIALCAM genes encode for membrane proteins of unknown function, which has been linked to the regulation of different ion channels and transporters, such as the chloride channel VRAC (volume regulated anion channel), ClC-2 (chloride channel 2), and connexin 43 or the Na+/K+-ATPase pump. However, the mechanisms by which MLC proteins regulate these ion channels and transporters, as well as the exact function of MLC proteins remain obscure. It has been suggested that MLC proteins might regulate signalling pathways, but the mechanisms involved are, at present, unknown. With the aim of answering these questions, we have recently described the brain GlialCAM interactome. Within the identified proteins, we could validate the interaction with several G protein-coupled receptors (GPCRs), including the orphan GPRC5B and the proposed prosaposin receptors GPR37L1 and GPR37. In this review, we summarize new aspects of the pathophysiology of MLC disease and key aspects of the interaction between GPR37 receptors and MLC proteins.

GlialCAM/MLC1 modulates LRRC8/VRAC currents in an indirect manner: Implications for megalencephalic leukoencephalopathy.

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare type of leukodystrophy caused by mutations in either MLC1 or GLIALCAM genes. Previous work indicated that chloride currents mediated by the volume-regulated anion channel (VRAC) and ClC-2 channels were affected in astrocytes deficient in either Mlc1 or Glialcam. ClC-2 forms a ternary complex with GlialCAM and MLC1. LRRC8 proteins have been identified recently as the molecular components of VRAC, but the relationship between MLC and LRRC8 proteins is unknown. Here, we first demonstrate that LRRC8 and MLC1 are functionally linked, as MLC1 cannot potentiate VRAC currents when LRRC8A, the main subunit of VRAC, is knocked down. We determine that LRRC8A and MLC1 do not co-localize or interact and, in Xenopus oocytes, MLC1 does not potentiate LRRC8-mediated VRAC currents, indicating that VRAC modulation in astrocytes by MLC1 may be indirect. Investigating the mechanism of modulation, we find that a lack of MLC1 does not influence either mRNA or total and plasma membrane protein levels of LRRC8A; and neither does it affect LRRC8A subcellular localization. In agreement with recent results that indicated that overexpression of MLC1 decreases the phosphorylation of extracellular signal-regulated kinases (ERK), we find that astrocytes lacking MLC1 show an increase in ERK phosphorylation. In astrocytes with reduced or increased levels of MLC1 we observe changes in the phosphorylation state of the VRAC subunit LRRC8C. Our results thus reinforce previous suggestions that indicated that GlialCAM/MLC1 might modify signal transduction pathways that influence the activity of different proteins, such as VRAC.

Split-Tobacco Etch Virus (Split-TEV) Method in G Protein-Coupled Receptor Interacting Proteins.

Split-TEV assay enables the identification of protein-protein interaction in mammalian cells. This method is based on the split of tobacco etch virus (TEV) protease in two fragments, where each fragment is fused to the candidate proteins predicted to interact. If there is indeed an interaction between both proteins, TEV protease reconstitutes its proteolytic activity and this activity is used to induce the expression of some reporter genes. However, some studies have detected unspecific interaction between membrane proteins due to its higher tendency to aggregate. Here we describe a variation of the Split-TEV method developed with the aim to increase the specificity in the study of G protein-coupled receptor (GPCR) interacting proteins. This approach for monitoring interactions between GPCRs is an easy and robust assay and offers good perspectives in drug discovery.

Megalencephalic leukoencephalopathy with subcortical cysts: A personal biochemical retrospective.

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare type of leukodystrophy characterized by dysfunction of the role of glial cells in controlling brain fluid and ion homeostasis. Patients affected by MLC present macrocephaly, cysts and white matter vacuolation, which lead to motor and cognitive impairments. To date, there is no treatment for MLC, only supportive care. MLC is caused by mutations in the MLC1 and GLIALCAM genes. MLC1 is a membrane protein with low identity to the Kv1.1 potassium channel and GlialCAM belongs to an adhesion molecule family. Both proteins form a complex with an as-yet-unknown function that is expressed mainly in the astrocytes surrounding the blood-brain barrier and in Bergmann glia. GlialCAM also acts as an auxiliary subunit of the chloride channel ClC-2, thus regulating its localization at cell-cell junctions and modifying its functional properties by affecting the common gate of ClC-2. Recent studies in Mlc1-, GlialCAM- and Clcn2-knockout mice or Mlc1-knockout zebrafish have provided fresh insight into the pathophysiology of MLC and further details about the molecular interactions between these three proteins. Additional studies have shown that GlialCAM/MLC1 also regulates other ion channels (TRPV4, VRAC) or transporters (Na+/K+-ATPase) in a not-understood manner. Furthermore, it has been shown that GlialCAM/MLC1 may influence signal transduction mechanisms, thereby affecting other proteins not related with transport such as the EGF receptor. Here, we offer a personal biochemical retrospective of the work that has been performed to gain knowledge of the pathophysiology of MLC, and we discuss future strategies that may be used to identify therapeutic solutions for MLC patients.