Caveolae (CAV) constitute a novel subcellular transport vesicle that has received special attention based on its proven and postulated participation in transcytosis, potocytosis, and in cell signaling events. One of the principal components of CAV are caveolin protein isoforms. Here, we have undertaken the immunochemical identification of CAV and the known caveolin isoforms (1alpha, 1beta, 2 and 3) in cultured rat C6 glioma cells. Immunoblot analysis revealed that particulate fractions from rat C6 glioma cells express caveolin-1 and caveolin-2. The relative detergent-insolubility of these caveolin isoforms was also determined by Western blot analysis. Indirect immunofluorescence analysis with caveolin-1 and -2 antibodies revealed staining patterns typical of CAV's known subcellular distribution and localization. For both caveolin isoforms immunocytochemical staining was characterized by intensely fluorescent puncta throughout the cytoplasm and diffuse micropatches at the level of the plasmalemma. Perinuclear staining was also detected, consistent and suggestive of caveolin's localization in the trans Golgi region. The caveolin-1 and -2 immunoreactivity seen in Western blots and immunocytochemically is related to structurally relevant CAV as supported by the isolation of caveolin-enriched membrane complexes using two different methods. Light-density, Triton X-100-insoluble caveolin-1- and caveolin-2-enriched fractions were obtained after fractionation of rat C6 glioma cells and their separation over 5-40% discontinuous sucrose-density gradients. Similar fractions were obtained using a detergent-free, sodium carbonate-based fractionation method. These results further support the localization of CAV and caveolins in glial cells. In addition, they demonstrate that cultured C6 glioma cells can be useful as a model system to study the role of CAV and caveolins in subcellular transport and signal transduction events in glial cells and the brain.

Caveolae, a specialized form of lipid rafts, are cholesterol- and sphingolipid-rich membrane microdomains implicated in potocytosis, endocytosis, transcytosis, and as platforms for signal transduction. One of the major constituents of caveolae are three highly homologous caveolin isoforms (caveolin-1, caveolin-2, and caveolin-3). The present study expands the analysis of caveolin isoform expression in C6 glioma cells. Three complementary approaches were used to assess their differential expression during the dibutyryl-cyclic AMP-induced differentiation of C6 cells into an astrocyte-like phenotype. Immunoblotting, conventional RT-PCR, and real-time RT-PCR analysis established the expression of the caveolin-3 isoform in C6 cells, in addition to caveolin-1 and caveolin-2. Similar to the other isoforms, caveolin-3 was associated with light-density, detergent-insoluble caveolae membrane fractions obtained using sucrose-density gradient centrifugation. The three caveolin isoforms display different temporal patterns of mRNA/protein expression during the differentiation of C6 cells. Western blot and real-time RT-PCR analysis demonstrate that caveolin-1 and caveolin-2 are up-regulated during the late stages of the differentiation of C6 cells. Meanwhile, caveolin-3 is gradually down-regulated during the differentiation process. Indirect immunofluorescence analysis via laser-scanning confocal microscopy reveals that the three caveolin isoforms display similar subcellular distribution patterns. In addition, co-localization of caveolin-1/caveolin-2 and caveolin-1/caveolin-3 was detected in both C6 glioma phenotypes. The findings reveal a differential temporal pattern of caveolin gene expression during phenotypic differentiation of C6 glioma cells, with potential implications to developmental and degenerative events in the brain.

Glial cells prevail in number and in diversity of cellular phenotypes in the nervous system. They have also gained prominence due to their multiple physiological and pathophysiological roles. Our current knowledge of the asymmetry and heterogeneity of the plasma membrane demands an in depth analysis of the diverse array of membrane microdomains postulated to exist in the context of glial cells. This review focuses and analyzes the studies reported to date on the detection of caveolae membrane rafts and the caveolin family members in glial cell model systems, the conditions leading to changes in their level of expression, and their functional and clinical significance. Outstanding in this work emerge the ubiquitous expression of caveolins, including the typically regarded 'muscle-specific' cav3, in diverse glial cell model systems, their participation in reactive astrogliosis, cancer, and their key relevance to calcium signaling. The knowledge obtained to date demands incorporation of the caveolins and caveolae membrane rafts in our current models on the role of glial cells in heath and neurological disease. The relative abundance of glia parallels their increasingly evident roles in nervous system physiology and pathophysiology. The diversity in functional roles of glia relates to the main different types of glia: the myelinating oligodendrocytes (OL) and Schwann cells (SC), the fibrous (white matter) and protoplasmic (gray matter) astrocytes, perivascular astrocytes, radial glia, and the mesodermally derived microglia, which are key players in nervous system inflammatory responses. Astrocytes are known to participate in nutrient transport, ionic homeostasis, mechanical support, synaptic plasticity, and blood-brain barrier integrity (Hansson and Ronnback 2003). Astrocyte activation, known as reactive astrogliosis, ensues during pathophysiological processes such as injury, trauma, ischemia, stroke, neurodegenerative disorders, aging, and brain tumor formation. Depending on the type of insult, extent and time point astrocytes may exert opposite cytoprotective or cytotoxic actions (Fellin and Carmignoto 2004).Paramount to the understanding of the functional roles of glia in the nervous system is the recognition of plasmalemma lipid heterogeneity, asymmetry, and distinct membrane microdomains. This review focuses on the caveolae (CAV) membrane microdomain, and its constituent or marker proteins the caveolins. The present review addresses the studies performed in glial cell model systems in the following areas: ultrastructural analysis of CAV, detection of caveolin1 (cav1) and 2, the expression of the 'muscleAddress correspondence and reprint requests to Dr Walter I. Silva, Department of Physiology, School of Medicine, University of Puerto Rico, PO Box 365067, San Juan 00936-5067, Puerto Rico. E-mail: wsilva@rcm.upr.edu 1 Membrane rafts are small (10-200 nm), heterogeneous, highly dynamic, and sterol-and sphingolipid-enriched domains that compartmentalize cellular processes. Small rafts can sometimes be stabi...

Cholesterol modulates the plasmalemma's biophysical properties and influences the function and trafficking of membrane proteins. A fundamental phenomenon that remains obscure is how the plasmalemma's lipid composition regulates the activatable pool of membrane receptors. An outstanding model to study this phenomenon is the nicotinic acetylcholine receptor (nAChR), since the nAChR activatable pool has been estimated to be but a small fraction of the receptors present in the plasmalemma. Studies on the effect of cholesterol depletion in the function of the Torpedo californica nAChR, using the lipid-exposed nAChR mutation (αC418W) that produces a congenital myasthenic syndrome (CMS), demonstrated that cholesterol depletion causes a remarkable increase in the αC418W nAChR's macroscopic current whereas not in the wild-type (WT). A variety of approaches were used to define the mechanism responsible for the cholesterol depletion mediated-increase in the αC418W nAChR's macroscopic current. The present study suggests that a substantial fraction of the αC418W nAChRs is located in caveolin-1-positive domains, "trapped" in a non-activatable state, and that membrane cholesterol depletion results in the relocation of these receptors to the activatable pool. Co-fractionation and co-immunoprecipitation of the αC418W nAChR and the membrane raft protein caveolin-1 (cav1) support the notion that interactions at lipid-exposed domains regulate the partition of the receptor into membrane raft microdomains. These results have potential implications as a novel mechanism to fine-tune cholinergic transmission in the nervous system and in the pathogenesis associated to the αC418W nAChR.

Cholesterol modulates the biophysical properties of cell membranes and influences the function and trafficking of membrane proteins. A fundamental phenomenon that remains obscure is the regulation of the functional pool of membrane receptors. We examined this phenomenon in an acetylcholine receptor, lipid‐exposed, mutation that produces congenital myasthenic syndrome. A large fraction of this mutation is located in caveolin‐1‐positive membrane domains, trapped in a nonfunctional state. Lowering membrane cholesterol levels results in a transfer of these mutated receptors to the functional pool. The present data demonstrate that interactions at lipid‐exposed domains with lipid rafts microdomains regulate the functional pool of the acetylcholine receptor, and could provide a novel mechanism to fine‐tune cholinergic transmission in the central nervous system.