Induction of connexin43 and gap junctional communication in PC12 cells overexpressing the carboxy terminal region of amyloid precursor protein

Connexin channel and its role in diabetic retinopathy

Diabetic retinopathy is the leading cause of blindness in the working age population. Unfortunately, there is no cure for this devastating ocular complication. The early stage of diabetic retinopathy is characterized by the loss of various cell types in the retina, namely endothelial cells and pericytes. As the disease progresses, vascular leakage, a clinical hallmark of diabetic retinopathy, becomes evident and may eventually lead to diabetic macular edema, the most common cause of vision loss in diabetic retinopathy. Substantial evidence indicates that the disruption of connexin-mediated cellular communication plays a critical role in the pathogenesis of diabetic retinopathy. Yet, it is unclear how altered communication via connexin channel mediated cell-to-cell and cell-to-extracellular microenvironment is linked to the development of diabetic retinopathy. Recent observations suggest the possibility that connexin hemichannels may play a role in the pathogenesis of diabetic retinopathy by allowing communication between cells and the microenvironment. Interestingly, recent studies suggest that connexin channels may be involved in regulating retinal vascular permeability. These cellular events are coordinated at least in part via connexin-mediated intercellular communication and the maintenance of retinal vascular homeostasis. This review highlights the effect of high glucose and diabetic condition on connexin channels and their impact on the development of diabetic retinopathy.

Protective effects of carbenoxolone are associated with attenuation of oxidative stress in ischemic brain injury

Accumulating evidence has suggested that the gap junction plays an important role in the determination of cerebral ischemia, but the underlying mechanisms remain to be elucidated. In this study, we assessed the effect of a gap-junction blocker, carbenoxolone (CBX), on ischemia/reperfusion-induced brain injury and the possible mechanisms. By using the transient cerebral ischemia model induced by occlusion of the middle cerebral artery for 30 min followed by reperfusion for 24 h, we found that pre-administration of CBX (25 mg/kg, intracerebroventricular injection, 30 min before cerebral ischemic surgery) diminished the infarction size in rats. And this was associated with a decrease of reactive oxygen species generation and inhibition of the activation of astrocytes and microglia. In PC12 cells, H2O2 treatment induced more coupling and apoptosis, while CBX partly inhibited the opening of gap junctions and improved the cell viability. These results suggest that cerebral ischemia enhances the opening of gap junctions. Blocking the gap junction with CBX may attenuate the brain injury after cerebral ischemia/reperfusion by partially contributing to amelioration of the oxidative stress and apoptosis.

Glial connexin expression and function in the context of Alzheimer's disease

A hallmark of neurodegenerative diseases is the reactive gliosis characterized by a phenotypic change in astrocytes and microglia. This glial response is associated with modifications in the expression and function of connexins (Cxs), the proteins forming gap junction channels and hemichannels. Increased Cx expression is detected in most reactive astrocytes located at amyloid plaques, the histopathological lesions typically present in the brain of Alzheimer's patients and animal models of the disease. The activity of Cx channels analyzed in vivo as well as in vitro after treatment with the amyloid β peptide is also modified and, in particular, hemichannel activation may contribute to neuronal damage. In this review, we summarize and discuss recent data that suggest glial Cx channels participate in the neurodegenerative process of Alzheimer's disease. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Astrocytic gap junctional communication is reduced in amyloid-β-treated cultured astrocytes, but not in Alzheimer's disease transgenic mice

Alzheimer's disease is characterized by accumulation of amyloid deposits in brain, progressive cognitive deficits and reduced glucose utilization. Many consequences of the disease are attributed to neuronal dysfunction, but roles of astrocytes in its pathogenesis are not well understood. Astrocytes are extensively coupled via gap junctions, and abnormal trafficking of metabolites and signalling molecules within astrocytic syncytia could alter functional interactions among cells comprising the neurovascular unit. To evaluate the influence of amyloid-beta on astrocyte gap junctional communication, cultured astrocytes were treated with monomerized amyloid-β(1-40) (1 μmol/l) for intervals ranging from 2 h to 5 days, and the areas labelled by test compounds were determined by impaling a single astrocyte with a micropipette and diffusion of material into coupled cells. Amyloid-β-treated astrocytes had rapid, sustained 50-70% reductions in the area labelled by Lucifer Yellow, anionic Alexa Fluor® dyes and energy-related compounds, 6-NBDG (a fluorescent glucose analogue), NADH and NADPH. Amyloid-β treatment also caused a transient increase in oxidative stress. In striking contrast with these results, spreading of Lucifer Yellow within astrocytic networks in brain slices from three regions of 8.5-14-month-old control and transgenic Alzheimer's model mice was variable, labelling 10-2000 cells; there were no statistically significant differences in the number of dye-labelled cells among the groups or with age. Thus amyloid-induced dysfunction of gap junctional communication in cultured astrocytes does not reflect the maintenance of dye transfer through astrocytic syncytial networks in transgenic mice; the pathophysiology of Alzheimer's disease is not appropriately represented by the cell culture system.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Connexins and gap junctions of astrocytes and oligodendrocytes in the CNS

This review article summarizes early and recent literature on the structure, distribution and composition of gap junctions between astrocytes and oligodendrocytes, and the differential expression of glial connexins in adult and developing mammalian CNS. In addition to an overview of the topic, discussion is focused on the organization of homologous gap junctional interactions between astrocytes and between oligodendrocytes as well as on heterologous junctional coupling between astrocytes and oligodendrocytes. The homotypic and heterotypic nature of these gap junctions is related to the connexins known to be produced by glial cells in the intact brain and spinal cord. Emphasis is placed on the ultrastructural level of analysis required to attribute gap junction and connexin deployment to particular cell types and subcellular locations. Our aim is to provide a firm basis for consideration of anticipated rapid advances in understanding of structural relationships of gap junctions and connexins within the glial gap junctional syncytium. Conclusions to date suggest that the glial syncytium is more complex than previously appreciated and that glial pathways of junctional communication may not only be determined by the presence of gap junctions, but also by the connexin composition and conductance regulation of junctional channels.

Connexin32-null mice develop demyelinating peripheral neuropathy

Mutations in the gene encoding the gap junction protein connexin32 (Cx32) cause X-linked Charcot-Marie-Tooth disease (CMTX), a common form of inherited demyelinating peripheral neuropathy. To learn more about the pathogenesis of CMTX, we examined the PNS and CNS of cx32-null mice (cx32-/Y males and cx32-/-females) by light and electron microscopy. These mice develop a progressive demyelinating peripheral neuropathy beginning by 3 months of age, and at all ages, motor fibers are more affected than sensory fibers. Like other genes of the X chromosome, the cx32 gene appears to be randomly inactivated, since only some myelinating Schwann cells express Cx32 in heterozygous cx32 +/- females. Heterozygous cx32 +/- females have fewer demyelinated and remyelinated axons than age-matched homozygous cx32-/- females and cx32-/Y males. Although oligodendrocytes also express Cx32, no abnormalities in CNS myelin were found. These findings indicate that a null cx32 allele in myelinating Schwann cells is sufficient to cause an inherited demyelinating neuropathy, so that Cx32 has an essential role in myelinating Schwann cells both in mice and in humans.

Identification of connexin43 as a functional target for Wnt signalling

Wnt mediated signal transduction is considered to regulate activity of target genes. In Xenopus embryos, ectopic Wnt1 and Wnt8 expression induces gap-junctional communication. During murine brain formation, Wnt1 and the gap-junctional protein connexin43 (Cx43) are co-expressed at the mid/hindbrain border, while interference with Wnt1 or Cx43 expression during embryogenesis leads to severe brain defects in the mid/hindbrain region. In PC12 cells, Wnt1 expression leads to an apparent increase in cell-cell adhesion. We investigated the effects of Wnt1 overexpression on gap-junctional communication in PC12 cells. Wnt1 expressing clones displayed an increased electrical and chemical coupling. This coincides with an increased expression of Cx43 mRNA and protein, while other connexins, Cx26, Cx32, Cx37, Cx40 and Cx45, were not up-regulated. Also, induction of Wnt1 expression in a mammary epithelial cell line leads to an increase in gap-junctional communication and Cx43 protein expression. In transient transactivation assays in P19 EC cells we found that Wnt1 and Li+, an ion that mimics Wnt signalling, increased transcription from the rat Cx43 promoter, potentially via TCF/LEF binding elements, in a pathway separate from cAMP-induced Cx43 transactivation. The results demonstrate that Cx43 acts as a functional target of Wnt1 signalling, and Cx43 expression can be regulated by Wnt1 at the transcriptional level. Our data suggest that Wnt1-induced cell fate determination is likely to involve regulation of gap-junctional communication.

Altered trafficking of mutant connexin32

We examined the cellular localization of nine different connexin32 (Cx32) mutants associated with X-linked Charcot-Marie-Tooth disease (CMTX) in communication-incompetent mammalian cells. Cx32 mRNA was made, but little or no protein was detected in one class of mutants. In another class of mutants, Cx32 protein was detectable in the cytoplasm and at the cell surface, where it appeared as plaques and punctate staining. Cx32 immunoreactivity in a third class of mutants was restricted to the cytoplasm, where it often colocalized with the Golgi apparatus. Our studies suggest that CMTX mutations have a predominant effect on the trafficking of Cx32 protein, resulting in a potentially toxic cytoplasmic accumulation of Cx32 in these cells. These results and evidence of cytoplasmic accumulation of other mutated myelin proteins suggest that diseases affecting myelinating cells may share a common pathophysiology.