Blockade of Glial Connexin 43 Hemichannels Reduces Food Intake

The circadian rhythms regulated by Cx43-signaling in the pathogenesis of Neuromyelitis Optica

Introduction

Neuromyelitis Optica (NMO) is an inflammatory demyelinating disease of the central nervous system (CNS). NMO manifests as selective and severe attacks on axons and myelin of the optic nerve and spinal cord, resulting in necrotic cavities. The circadian rhythms are well demonstrated to profoundly impact cellular function, behavior, and disease. This study is aimed to explore the role and molecular basis of circadian rhythms in NMO.

Methods

We used an Aquaporin 4(AQP4) IgG-induced NMO cell model in isolated astrocytes. The expression of Cx43 and Bmal1 were detected by real-time PCR and Western Blot. TAT-Gap19 and DQP-1105 were used to inhibit Cx43 and glutamate receptor respectively. The knockdown of Bmal1 were performed with the shRNA containing adenovirus. The levels of glutamate, anterior visual pathway (AVP), and vasoactive intestinal peptide (VIP) were quantified by ELISA kits.

Results

We found that Bmal1 and Clock, two essential components of the circadian clock, were significantly decreased in NMO astrocytes, which were reversed by Cx43 activation (linoleic acid) or glutamate. Moreover, the expression levels of Bmal1 and Clock were also decreased by Cx43 blockade (TAT-Gap19) or glutamate receptor inhibition (DQP-1105). Furthermore, adenovirus-mediated Bmal1 knockdown by shRNA (Ad-sh-Bmal1) dramatically decreased the levels of glutamate, AVP, and VIP from neurons, and significantly down-regulated the protein level of Cx43 in NMO astrocytes with Cx43 activation (linoleic acid) or glutamate treatment. However, Bmal1 knockdown did not alter these levels in normal astrocytes with Cx43 blockade (TAT-Gap19) or glutamate receptor inhibition (DQP-1105).

Discussion

Collectively, these results suggest that Cx43-glutamate signaling would be a critical upstream regulator that contributes to the NMO-induced rhythmic damage in SCN astrocytes.

Constitutively active microglial populations limit anorexia induced by the food contaminant deoxynivalenol

Microglia are involved in neuroinflammatory processes during diverse pathophysiological conditions. To date, the possible contribution of these cells to deoxynivalenol (DON)-induced brain inflammation and anorexia has not yet been evaluated. DON, one of the most abundant trichothecenes found in cereals, has been implicated in mycotoxicosis in both humans and farm animals. DON-induced toxicity is characterized by reduced food intake, weight gain, and immunological effects. We previously showed that exposure to DON induces an inflammatory response within the hypothalamus and dorsal vagal complex (DVC) which contributes to DON-induced anorexia. Here, in response to anorectic DON doses, we reported microglial activation within two circumventricular organs (CVOs), the area postrema (AP) and median eminence (ME) located in the DVC and the hypothalamus, respectively. Interestingly, this microglial activation was observed while DON-induced anorexia was ongoing (i.e., 3 and 6 h after DON administration). Next, we took advantage of pharmacological microglia deletion using PLX3397, a colony-stimulating factor 1 receptor (CSF1R)-inhibitor. Surprisingly, microglia-depleted mice exhibited an increased sensitivity to DON since non-anorectic DON doses reduced food intake in PLX3397-treated mice. Moreover, low DON doses induced c-Fos expression within feeding behavior-associated structures in PLX3397-treated mice but not in control mice. In parallel, we have highlighted heterogeneity in the phenotype of microglial cells present in and around the AP and ME of control animals. In these areas, microglial subpopulations expressed IBA1, TMEM119, CD11b and CD68 to varying degrees. In addition, a CD68 positive subpopulation showed, under resting conditions, a noticeable phagocytotic/endocytotic activity. We observed that DON strongly reduced CD68 in the hypothalamus and DVC. Finally, inactivation of constitutively active microglia by intraperitoneal administration of minocycline resulted in anorexia with a DON dose ineffective in control mice. Taken together, these results strongly suggest that various populations of microglial cells residing in and around the CVOs are maintained in a functionally active state even under physiological conditions. We propose that these microglial cell populations are attempting to protect the brain parenchyma from hazardous molecules coming from the blood. This study could contribute to a better understanding of how microglia respond to environmental contaminants.

NF-κB and AP-1 are required for the lipopolysaccharide-induced expression of MCP-1, CXCL1, and Cx43 in cultured rat dorsal spinal cord astrocytes

TLR4 and Cx43 signaling in dorsal spinal cord has been shown to be involved in the development of neuropathic pain. However, it is not clear whether TLR4 signaling is associated with the expression of MCP-1, CXCL1, and Cx43 in LPS (lipopolysaccharide)-treated rat dorsal spinal cord astrocytes under in vitro condition. In the present study, we found that TLR4 antagonist TAK-242 significantly inhibited LPS-induced MCP-1, CXCL1, and Cx43 expression, suggesting the role of TLR4 in response to LPS in cultured dorsal spinal cord astrocytes. Application of TAK-242 significantly blocked LPS-induced NF-κB and AP-1 activity and the expression of MCP-1, CXCL1 and Cx43. Furthermore, NF-κB inhibitor PDTC and AP-1 inhibitor SR11302 significantly blocked LPS-induced MCP-1, CXCL1, and Cx43 expression. DNA-binding activity of NF-κB, its effect on MCP-1 expression was suppressed by PDTC and SR11302. On the other hand, DNA-binding activity of AP-1, its effect on CXCL1 or Cx43 expression was also suppressed by PDTC and SR11302. In addition, PDTC was found to inhibit the nuclear translocation of AP-1 and the expression of c-Jun induced by LPS, which suggested that NF-κBp65 is essential for the AP-1 activity. Similarly, SR11302 significantly blocked LPS-induced the nuclear translocation of NF-κBp65 and the expression of NF-κBp65 induced by LPS. Pretreatment with CBX, Gap26, or Gap19 (Cx43 blockers) significantly inhibited abnormal astrocytic hemichannel opening and chemokines (MCP-1 and CXCL1) release in LPS-stimulated astrocytes. In summary, cell culture experiments revealed that LPS stimulation could evoke TLR4 signaling with the subsequent activation of NF-κB and AP-1, resulting in the expression of MCP-1, CXCL1, and Cx43. TLR4 activation increased Cx43 hemichannel, but not gap-junction activities and induced the release of the MCP-1 and CXCL1 from astrocytes via Cx43 hemichannel. These findings may help us to understand the role of astrocytic signaling in inflammatory response within dorsal spinal cord tissue.

Glial Modulation of Energy Balance: The Dorsal Vagal Complex Is No Exception

The avoidance of being overweight or obese is a daily challenge for a growing number of people. The growing proportion of people suffering from a nutritional imbalance in many parts of the world exemplifies this challenge and emphasizes the need for a better understanding of the mechanisms that regulate nutritional balance. Until recently, research on the central regulation of food intake primarily focused on neuronal signaling, with little attention paid to the role of glial cells. Over the last few decades, our understanding of glial cells has changed dramatically. These cells are increasingly regarded as important neuronal partners, contributing not just to cerebral homeostasis, but also to cerebral signaling. Our understanding of the central regulation of energy balance is part of this (r)evolution. Evidence is accumulating that glial cells play a dynamic role in the modulation of energy balance. In the present review, we summarize recent data indicating that the multifaceted glial compartment of the brainstem dorsal vagal complex (DVC) should be considered in research aimed at identifying feeding-related processes operating at this level.

Astroglial Hemichannels and Pannexons: The Hidden Link between Maternal Inflammation and Neurological Disorders

Maternal inflammation during pregnancy causes later-in-life alterations of the offspring’s brain structure and function. These abnormalities increase the risk of developing several psychiatric and neurological disorders, including schizophrenia, intellectual disability, bipolar disorder, autism spectrum disorder, microcephaly, and cerebral palsy. Here, we discuss how astrocytes might contribute to postnatal brain dysfunction following maternal inflammation, focusing on the signaling mediated by two families of plasma membrane channels: hemi-channels and pannexons. [Ca²⁺]_i imbalance linked to the opening of astrocytic hemichannels and pannexons could disturb essential functions that sustain astrocytic survival and astrocyte-to-neuron support, including energy and redox homeostasis, uptake of K⁺ and glutamate, and the delivery of neurotrophic factors and energy-rich metabolites. Both phenomena could make neurons more susceptible to the harmful effect of prenatal inflammation and the experience of a second immune challenge during adulthood. On the other hand, maternal inflammation could cause excitotoxicity by producing the release of high amounts of gliotransmitters via astrocytic hemichannels/pannexons, eliciting further neuronal damage. Understanding how hemichannels and pannexons participate in maternal inflammation-induced brain abnormalities could be critical for developing pharmacological therapies against neurological disorders observed in the offspring.

Over-activated hemichannels: A possible therapeutic target for human diseases

In our body, all the cells are constantly sharing chemical and electrical information with other cells. This intercellular communication allows them to respond in a concerted way to changes in the extracellular milieu. Connexins are transmembrane proteins that have the particularity of forming two types of channels; hemichannels and gap junction channels. Under normal conditions, hemichannels allow the controlled release of signaling molecules to the extracellular milieu. However, under certain pathological conditions, over-activated hemichannels can induce and/or exacerbate symptoms. In the last decade, great efforts have been put into developing new tools that can modulate these over-activated hemichannels. Small molecules, antibodies and mimetic peptides have shown a potential for the treatment of human diseases. In this review, we summarize recent findings in the field of hemichannel modulation via specific tools, and how these tools could improve patient outcome in certain pathological conditions.

Tanycytic networks mediate energy balance by feeding lactate to glucose-insensitive POMC neurons

Hypothalamic glucose sensing enables an organism to match energy expenditure and food intake to circulating levels of glucose, the main energy source of the brain. Here, we established that tanycytes of the hypothalamic arcuate nucleus, specialized glia that line the wall of the third ventricle, convert brain glucose supplies into lactate that they transmit through monocarboxylate transporters to arcuate proopiomelanocortin neurons, which integrate this signal to drive their activity and to adapt the metabolic response to meet physiological demands. Furthermore, this transmission required the formation of extensive Connexin-43 gap-junction-mediated metabolic networks by arcuate tanycytes. Selectively suppressing either tanycytic monocarboxylate transporters or gap junctions resulted in altered feeding behavior and energy metabolism. Tanycytic intercellular communication and lactate production are thus integral to the mechanism by which hypothalamic neurons that regulate energy and glucose homeostasis efficiently perceive alterations in systemic glucose levels as a function of the physiological state of the organism.

Purinergic signaling in tanycytes and its contribution to nutritional sensing

Tanycytes are hypothalamic radial glial-like cells with an important role in the regulation of neuroendocrine axes and energy homeostasis. These cells have been implicated in glucose, amino acids, and fatty acid sensing in the hypothalamus of rodents, where they are strategically positioned. While their cell bodies contact the cerebrospinal fluid, their extensive processes contact neurons of the arcuate and ventromedial nuclei, protagonists in the regulation of food intake. A growing body of evidence has shown that purinergic signaling plays a relevant role in this homeostatic role of tanycytes, likely regulating the release of gliotransmitters that will modify the activity of satiety-controlling hypothalamic neurons. Connexin hemichannels have proven to be particularly relevant in these mechanisms since they are responsible for the release of ATP from tanycytes in response to nutritional signals. On the other hand, either ionotropic or metabotropic ATP receptors are involved in the generation of intracellular Ca²⁺ waves in response to hypothalamic nutrients, which can spread between glial cells and towards neighboring neurons. This review will summarize recent evidence that supports a nutrient sensor role for tanycytes, highlighting the participation of purinergic signaling in this process.