Aquaporin-4 Cell-Surface Expression and Turnover Are Regulated by Dystroglycan, Dynamin, and the Extracellular Matrix in Astrocytes

Abstract

The water-permeable channel aquaporin-4 (AQP4) is highly expressed in perivascular astrocytes of the mammalian brain and represents the major conduit for water across the blood-brain barrier. Within these cells, AQP4 is found in great quantities at perivascular endfoot sites but is detected in lesser amounts at the membrane domains within the brain parenchyma. We had previously established that this polarization was regulated by the interaction between dystroglycan (DG), an extracellular matrix receptor that is co-expressed with AQP4, and the laminin that is contained within the perivascular basal lamina. In the present study, we have attempted to describe the mechanisms that underlie this regulation, using primary astrocyte cultures. Via biotinylation, we found that the cell-surface expression of AQP4 is DG-dependent and is potentiated by laminin. We also determined that this laminin-dependent increase occurs not through an upregulation of total AQP4 levels, but rather from a redirection of AQP4 from an intracellular, EEA-1-associated pool to the cell surface. We then demonstrated an association between DG and dynamin and showed that dynamin functioned in conjunction with clathrin to regulate surface AQP4 amounts. Furthermore, we observed that DG preferentially binds to the inactive forms of dynamin, suggesting that this interaction was inhibitory for AQP4 endocytosis. Finally, we showed that laminin selectively upregulates the cell-surface expression of the M23 isoform of AQP4. Our data therefore indicate that the dual interation of DG with laminin and dynamin is involved in the regulation of AQP4 internalization, leading to its asymmetric enrichment at perivascular astrocyte endfeet.

Neurochemical regulation of the expression and function of glial fibrillary acidic protein in astrocytes

“…Since these proteins are involved in the package, secretion, and exocytosis proteins, their expressions in astrocytes make it possible for GFAP to participate in the membrane protein recycling. This proposal is supported by the finding that a dual interaction of dystrophin with laminin and dynamin is involved in the regulation of AQP4 internalization, leading to asymmetric enrichment of AQP4 at perivascular astrocyte endfeet (Tham, Joshi, & Moukhles, ).…”

Indirect Role of AQP4b and AQP4d Isoforms in Dynamics of Astrocyte Volume and Orthogonal Arrays of Particles

“…The hypoosmotically-triggered redistribution (increased colocalization) of intracellular AQP4 isoforms to early endosomes coincided with an increase in the average area of early endosomes. The pool of early endosomes was predicted to be important for the redistribution of AQP4 between the intracellular space and the plasma membrane in primary rat astrocytes and in vivo, which could potentially influence the water transport across cell membranes [41][42][43]. This is not a unique characteristic of AQP4, because early endosomes were considered to be important for the redistribution of another aquaporin-AQP2 [44,45].…”

Agrin plays a major role in the coalescence of the aquaporin‐4 clusters induced by gamma‐1‐containing laminin

“…Indeed, AQP4 coimmunoprecipitates with syntrophin, and mutations in the dystrophin gene, or the ablation of syntrophin expression dramatically reduces AQP4 expression at the astrocyte endfoot (Amiry‐Moghaddam et al, ; Connors & Kofuji, ; Vajda et al, ). In addition, decreasing DG expression via siRNA treatment similarly reduces the cell‐surface expression of AQP4 and abrogates the AQP4 clustering in the presence of laminin (Noel, Tham, & Moukhles, ; Tham, Joshi, & Moukhles, ).…”