Megalencephalic Leukoencephalopathy: Insights Into Pathophysiology and Perspectives for Therapy

Abstract

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare genetic disorder belonging to the group of vacuolating leukodystrophies. It is characterized by megalencephaly, loss of motor functions, epilepsy, and mild mental decline. In brain biopsies of MLC patients, vacuoles were observed in myelin and in astrocytes surrounding blood vessels. It is mainly caused by recessive mutations in MLC1 and HEPACAM (also called GLIALCAM) genes. These disease variants are called MLC1 and MLC2A with both types of patients sharing the same clinical phenotype. Besides, dominant mutations in HEPACAM were also identified in a subtype of MLC patients (MLC2B) with a remitting phenotype. MLC1 and GlialCAM proteins form a complex mainly expressed in brain astrocytes at the gliovascular interface and in Bergmann glia at the cerebellum. Both proteins regulate several ion channels and transporters involved in the control of ion and water fluxes in glial cells, either directly influencing their location and function, or indirectly regulating associated signal transduction pathways. However, the MLC1/GLIALCAM complex function and the related pathological mechanisms leading to MLC are still unknown. It has been hypothesized that, in MLC, the role of glial cells in brain ion homeostasis is altered in both physiological and inflammatory conditions. There is no therapy for MLC patients, only supportive treatment. As MLC2B patients show an MLC reversible phenotype, we speculated that the phenotype of MLC1 and MLC2A patients could also be mitigated by the re-introduction of the correct gene even at later stages. To prove this hypothesis, we injected in the cerebellar subarachnoid space of Mlc1 knockout mice an adeno-associated virus (AAV) coding for human MLC1 under the control of the glial-fibrillary acidic protein promoter. MLC1 expression in the cerebellum extremely reduced myelin vacuolation at all ages in a dose-dependent manner. This study could be considered as the first preclinical approach for MLC. We also suggest other potential therapeutic strategies in this review.

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

“…They might also regulate the trafficking and biosynthetic processing of these partners ( 66 ). Considering MLC1, although the sequence homology to tetraspanins is very low (~20%), it passes the membrane eight times, but biochemical studies indicate that each four transmembrane domains can be considered as a duplicate ( 6 ). As tetraspanins, MLC1 also oligomerizes with himself and forms a tight complex with GlialCAM, a single-pass transmembrane domain of the Ig family ( 4 ).…”

“…These genes encode for membrane proteins that form a complex located at cell–cell junctions in brain perivascular astrocytic processes or in Bergmann glia at the cerebellum ( 5 ). A reduced number of patients (2%) do not harbor mutations in MLC1 or GLIALCAM , suggesting the existence of other unknown disease genes ( 6 ).…”

MLC1: A New Calcium-regulated Protein Conferring Calcium Dependence to Volume-regulated Anion Channels (VRAC) in Astrocytes.

“…Cycloheximide (CHX) treatment to assess MLC1 degradation kinetics, was carried out by stimulating cells with CHX (100 µg/mL, Sigma-Aldrich, Darmstadt, Germany) for different time lengths (1,3,4 or 5 hours, h). After all stimulations, cells were washed in phosphate buffered saline (PBS), collected by scraping and centrifuged at 2700g at 4°C for 10 min.…”

“…MLC1 is a 377-amino acid membrane protein with eight predicted transmembrane domains and short cytoplasmic amino and carboxylic tails that shows very low similarity with some ion channels (reviewed in [1]). MLC1 is almost exclusively expressed in the brain where it localizes preferentially at astrocyte end-feet contacting blood vessels and meninges, and in the Bergmann glia of the cerebellum [2][3][4].…”

Transmembrane topology and oligomeric nature of an astrocytic membrane protein, MLC1

“…The pathological outcome of MLC disease is impaired motor coordination, cognitive function and epilepsy. The genetic aetiology indicates that approximately 75% of affected patients carry genetic mutations in the MLC1 gene [1][2][3][4][5][6][7][8][9][10]. Previous studies have suggested that MLC1 is crucial for astroglial interactions and water homeostasis, which might cause the pathological outcomes of MLC disease [11][12][13].…”