Megalencephalic Leukoencephalopathy with Subcortical Cysts Disease-Linked MLC1 Protein Favors Gap-Junction Intercellular Communication by Regulating Connexin 43 Trafficking in Astrocytes

Abstract

Astrocytes, the most numerous cells of the central nervous system, exert critical functions for brain homeostasis. To this purpose, astrocytes generate a highly interconnected intercellular network allowing rapid exchange of ions and metabolites through gap junctions, adjoined channels composed of hexamers of connexin (Cx) proteins, mainly Cx43. Functional alterations of Cxs and gap junctions have been observed in several neuroinflammatory/neurodegenerative diseases. In the rare leukodystrophy megalencephalic leukoencephalopathy with subcortical cysts (MLC), astrocytes show defective control of ion/fluid exchanges causing brain edema, fluid cysts, and astrocyte/myelin vacuolation. MLC is caused by mutations in MLC1, an astrocyte-specific protein of elusive function, and in GlialCAM, a MLC1 chaperon. Both proteins are highly expressed at perivascular astrocyte end-feet and astrocyte-astrocyte contacts where they interact with zonula occludens-1 (ZO-1) and Cx43 junctional proteins. To investigate the possible role of Cx43 in MLC pathogenesis, we studied Cx43 properties in astrocytoma cells overexpressing wild type (WT) MLC1 or MLC1 carrying pathological mutations. Using biochemical and electrophysiological techniques, we found that WT, but not mutated, MLC1 expression favors intercellular communication by inhibiting extracellular-signal-regulated kinase 1/2 (ERK1/2)-mediated Cx43 phosphorylation and increasing Cx43 gap-junction stability. These data indicate MLC1 regulation of Cx43 in astrocytes and Cx43 involvement in MLC pathogenesis, suggesting potential target pathways for therapeutic interventions.

Varying perivascular astroglial endfoot dimensions along the vascular tree maintain perivascular‐interstitial flux through the cortical mantle

“…The physiological function of MLC1 remains unknow. Previous work done in cell culture in Mlc1null mice suggest that MLC1 may be involved in astrocyte ion and volume regulation through its interaction with different chloride, potassium and cation channels, or with gap junction-forming connexin-43 [30][31][32][33][34][35] . Anatomically, immunogold-electron microscopic images in postmortem human tissue showed that MLC1 localized to the junctions connecting overlapping perivascular astrocytic endfeet 36 .…”

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

“…Furthermore, the Na + /K + -ATPase pump has been identified as a MLC1-interacting protein, whereas the overexpression of MLC1 was observed to reduce its activity ( 11 ). Finally, Cx43 has been identified as a GlialCAM interacting protein ( 12 ) and MLC1 might influence Cx43 stability at gap junctions in astrocytoma cells ( 13 ).…”

Megalencephalic leukoencephalopathy with subcortical cysts 1 (MLC1) promotes glioblastoma cell invasion in the brain microenvironment

“…In ovarian cancer cells Axl signaling leads to reduced Erk1/2 phosphorylation and this correlates with a more epithelial-like cell state due to higher activities of the phosphatase DUSP4 [ 41 ]. MLC1 in astrocytes has also been reported to inhibit Erk1/2 signaling via connexins involved in direct cell-cell communication [ 42 ]. In addition, we have shown that the metastasis suppressor Ndrg1 is hyperphosphorylated in GSCs expressing MLC1 shRNAs, revealing that multiple pathways downstream of Mlc1 likely promote GBM cell polarity and invasion.…”