Mast Cell and Astrocyte Hemichannels and Their Role in Alzheimer's Disease, ALS, and Harmful Stress Conditions

Abstract

Considered relevant during allergy responses, numerous observations have also identified mast cells (MCs) as critical effectors during the progression and modulation of several neuroinflammatory conditions, including Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS). MC granules contain a plethora of constituents, including growth factors, cytokines, chemokines, and mitogen factors. The release of these bioactive substances from MCs occurs through distinct pathways that are initiated by the activation of specific plasma membrane receptors/channels. Here, we focus on hemichannels (HCs) formed by connexins (Cxs) and pannexins (Panxs) proteins, and we described their contribution to MC degranulation in AD, ALS, and harmful stress conditions. Cx/Panx HCs are also expressed by astrocytes and are likely involved in the release of critical toxic amounts of soluble factors-such as glutamate, adenosine triphosphate (ATP), complement component 3 derivate C3a, tumor necrosis factor (TNFα), apoliprotein E (ApoE), and certain miRNAs-known to play a role in the pathogenesis of AD, ALS, and other neurodegenerative disorders. We propose that blocking HCs on MCs and glial cells offers a promising novel strategy for ameliorating the progression of neurodegenerative diseases by reducing the release of cytokines and other pro-inflammatory compounds.

Keywords: connexin; degranulation; gap junction channels; glial cells; hemichannels; inflammation; mast cells; neurodegeneration; pannexin; pro-inflammatory compounds.

Figure 1

General mechanisms of degranulation from activated MCs. (A) Mast cells (MCs) are resident immune cell characterized by presenting numerous secretory granules in their cytoplasm in which they store pre-formed inflammatory mediators, such as biogenic amines (e.g., histamine, serotonin), growth factors (e.g., nerve growth factor (NGF), vascular endothelial growth factor (VEGF)), specific proteases (e.g., tryptase, chymase, carboxypeptidase A3), serglycin proteoglycan (such as heparin and chondroitin sulfate), cytokines (e.g., IL-4 and tumor necrosis factor (TNFα)), and adenosine triphosphate (ATP). These mediators are differentially packed in at least three different types (Types I, II, and III) of heterogenous secretory granules along the cell. Upon activation, these pre-formed mediators are rapidly released to the extracellular medium in a process called degranulation or regulated exocytosis, leading to immediate inflammatory reaction. Depending on the stimulus, MCs activation can also trigger the de novo synthesis of inflammatory mediators in a process called constitutive secretion, releasing molecules such as neuropeptides (e.g., substance P), growth factors (e.g., NGF, VEGF, platelet derived growth factor (PDGF)), cytokines (e.g., IL-6, IL-1β, TNFα, SCF), and chemokines (e.g., Monocyte chemoattractant protein-1 (MCP-1)), modulating late phase inflammation responses. Both docking and fusion to the plasma membrane are dependent on increased levels of the Ca²⁺ signal. The intracellular events triggered after MCs activation includes increasing membrane permeability to Ca²⁺, ATP release, and purinergic receptor recruitment, which are key components for MCs degranulation. (B) Different agents can induce MCs degranulation, including physical changes (e.g., temperature, vibrations, pressure, stress, pH) and ligand–receptor interaction (e.g., antigen–Immunoglobulin E (IgE)–FcεRI, ATP–P2Y/X receptors, and Aβ peptide–CD47). For the latter case, after intracellular signals are generated, connexins (Cxs) (potentially mediated by Cx43 and/or Cx32) and Panx1 HCs are eventually activated. HCs opening leads to massive ATP release, activating adjacent purinergic receptors in an autocrine way. Purinergic P2X receptor activation allows Ca²⁺ influx and the direct recruitment of more Panx1 HCs opening, leading to a vicious loop resulting in degranulation. (C) Different types of degranulation have been reported for MCs, including full exocytosis, kiss-and-run exocytosis, piecemeal degranulation, multigranular, and sequential compound exocytosis. However, compound exocytosis and piecemeal degranulation are the prevailing forms of degranulation in human, mouse, and rat MCs. Although we have proposed HCs and purinergic receptors activation as key components for MCs degranulation, contribution on constitutive secretion have not been evaluated. For more information about inflammatory mediators, we recommend [10,20]; while for detailed reviews of regulated exocytosis mechanism, we recommend revising [13,14,21,22,23]. Finally, the evidence of hemichannels (HCs) contribution on MCs degranulation is indicated in [24,25].

Figure 2

Mechanisms of storage and release of bioactive substances in astrocytes: a focus on neurodegenerative diseases. Model showing (A) a general overview and (B–D) specific details of the release of signaling molecules from astrocytes into the extracellular space through three classical mechanisms. These mechanisms involve (B) diffusion through plasma membrane channels, (C) exocytosis, and (D) translocation by transporters. For simplicity, we show only a few membrane proteins, along with some bioactive substances and secretion signaling mechanisms that are critical for communicating with neurons and glial cells and are involved in diverse neurodegenerative diseases, particularly in Alzheimer´s disease and amyotrophic lateral sclerosis (ALS)/frontotemporal dementia (FTD). (B) Plasma membrane channels/receptors include HCs (mainly Cx43 and Panx1), purinergic receptors (e.g., P2XR, P2YR), growth factor receptors (e.g., fibroblast growth factor receptor (FGFR)) and ionic channels (e.g., VRAC, TREK-1; not shown). The influx of Ca²⁺ through HCs and ion channels—along with the initiation of intracellular signaling cascades through purinergic and growth factor receptors—promotes the release of Ca²⁺ from intracellular stores (predominantly endoplasmic reticulum) and increases cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i), which is a process that leads to the extracellular release of many bioactive substances by exocytosis. [Ca²⁺]_i promotes the opening of Cx43 HCs and Panx HCs to directly release ATP into the extracellular environment. Cx43 HCs also release other small substances such as glutamate, NAD⁺, and glutathione. Extracellular Aβ also activates HCs. (C) Vesicles, such as SLMVs (synaptic-like microvesicles), DCVs (dense-core vesicles), lysosomes and EVs (extracellular vesicles, mainly microvesicles) contain diverse bioactive substances, including neurotransmitters, hormones and peptides, metabolic substrates, growth factors, and inflammatory factors, among others. Particular membrane fusion molecules (e.g., VAMP2 and TiVAMP) regulate the release of the vesicles. While many membrane proteins and bioactive substances are identified, little is known about the spatial and temporal aspects of vesicle release, including if membrane receptors/channels are localized in specific microdomains to trigger localized intracellular signaling pathways, and if elevated, [Ca²⁺]_i promotes the fusion of specific vesicle types with a membrane that is adjacent either to neuronal synapses or away from the tripartite synapse. Understanding the mechanisms that underlie the differential release of vesicles would potentially open therapeutic avenues for constraining the secretion of critical toxic factors—such as glutamate, ATP, C3a, TNFα, apoliprotein E (ApoE), and certain miRNAs identified to play a relevant role in the pathogenesis of Alzheimer´s disease and ALS/FTD—without affecting the release of beneficial factors. Although less intensively studied, several of these toxic factors have been implicated in other neurodegenerative diseases. Of interest, a recent report shows that mutant huntingtin (mHtt) associates with Rab3a, which is a small GTPase localized on the membranes of DCVs, and it impairs brain-derived neurotrophic factor (BDNF) release from astrocytes. Another emerging topic is the potential role of connexons (mediated by Cx43 and Cx26) in recruiting molecules such as RNA and DNA into microvesicles and exosomes and the subsequent intercellular transfer of these vesicles. For simplicity, mHtt and connexons are not shown in the model. (D) The critical bioactive substances glutamate and lactate can also be released by transmembrane transporters such as the cystiene/glutamate antiporter (xCT), and monocarboxylate transporters (MCT1/4), respectively, independent of [Ca²⁺]_i changes. The Na⁺-dependent glutamate excitatory amino acid transporters (EAAT1/2) are relevant for the uptake of extracellular glutamate and make a well-established contribution to excitotoxicity in ALS. For additional information on the release of bioactive substances and on the role of HCs in astrocytes, see [47,144,145,146,147]; on the Aβ-mediated opening of HCs, see [68]; on mHtt and impaired BDNF release from astrocytes, see [148]; and on connexons in the intercellular transfer of vesicles, see [149].