title: Hemichannels in the neurovascular unit and white matter under normal and inflamed conditions.
abstract: In the normal brain, cellular types that compose the neurovascular unit, including neurons, astrocytes and endothelial cells express pannexins and connexins, which are protein subunits of two families that form plasma membrane channels. Most available evidence in mammals indicated that endogenously expressed pannexins only form hemichannels, and connexins form both gap junction channels and hemichannels. While gap junction channels connect the cytoplasm of contacting cells and coordinate electrical and metabolic activities, hemichannels communicate intra- and extracellular compartments and serve as diffusional pathways for ions and small molecules. Here, evidence supporting the functional role of hemichannels in the neurovascular unit and white matter under physiological and pathological conditions are reviewed. A sub-threshold acute pathological threatening condition (e.g., stroke and brain infection) leads to glial cell activation, which maintains an active defense and restores the normal function of the neurovascular unit. However, if the stimulus is deleterious, microglia and the endothelium become overactivated, both releasing bioactive molecules (e.g., glutamate, cytokines, prostaglandins and ATP) that increase the activity of astroglial hemichannels, reducing the astrocyte neuroprotective functions, and further reducing neuronal cell viability. Moreover, ATP is known to contribute to myelin degeneration of axons. Consequently, hemichannels might play a relevant role in the excitotoxic response of oligodendrocytes observed in ischemia and encephalomyelitis. Regulated changes in hemichannel permeability in healthy brain cells can have positive consequences in terms of paracrine/autocrine signaling, whereas persistent changes in cells affected by neurological disorders can be detrimental. Therefore, blocking hemichannels expressed by glial cells and/or neurons of the inflamed central nervous system might prevent neurovascular unit dysfunction and neurodegeneration.
authors.affiliation: Departamento de Fisiología, Pontificia Universidad Católica de Chile, Santiago, Chile. jaorella@uc.cl
authors.forename: Juan A
authors.initials: JA
authors.lastname: Orellana
authors.forename: Xavier F
authors.initials: XF
authors.lastname: Figueroa
authors.forename: Helmuth A
authors.initials: HA
authors.lastname: Sánchez
authors.forename: Susana
authors.initials: S
authors.lastname: Contreras-Duarte
authors.forename: Victoria
authors.initials: V
authors.lastname: Velarde
authors.forename: Juan C
authors.initials: JC
authors.lastname: Sáez
mesh_terms: D017630:Connexins / Q000502:physiology*; D003711:Demyelinating Diseases / Q000378:metabolism / Q000503:physiopathology*; D006801:Humans; D007249:Inflammation / Q000378:metabolism / Q000503:physiopathology*; D007473:Ion Channels / Q000502:physiology*; D058990:Molecular Targeted Therapy; D009417:Nerve Tissue / Q000502:physiology / Q000503:physiopathology*; D009419:Nerve Tissue Proteins / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology
chemical_list: D017630:Connexins; D007473:Ion Channels; D009419:Nerve Tissue Proteins; C492764:PANX1 protein, human