Receptors, Ion Channels, and Signaling Mechanisms Underlying Microglial Dynamics

“Increases in neuronal activity evoked by glutamate (or analogues like NMDA, AMPA, or kainate), or by blocking GABAergic inhibition, led to microglial process extension and enhanced baseline motility, whereas block of AMPA receptors had the opposite effect (5, 37, 50, 75).”

“Microglia are brain-resident macrophages, but originate from primitive macrophages in the yolk sac during embryogenesis (2).”

“ATP signaling is diversified by enzymatic hydrolysis; the microglia-specific membrane-bound ectonucleotidase NTPDase1 (CD39), or the pyrophosphatase NPP1 (expressed on microglia but also on oligodendrocyte lineage cells (36)), converts ATP and ADP into AMP, which is then degraded into adenosine by ecto-5 -nucleotidase…”

“Fractalkine receptor deficiency leads to a transient reduction of microglial number during early postnatal development (12, 13), microglia-mediated neurotoxicity (65– 67), and impaired phagocytosis of apoptotic newborn neurons, thus causing reduced neurogenesis and deficits in synaptic pruning associated with weaker s…”

“3), where they converge in less than 30 min to form a spherical shield preventing further spread of the affected area (4, 5, 19, 21).”

“This is essential for regulating the number of neurons in the adult brain; progenitor cells produce neurons in excess, only a fraction of which mature and become integrated into neuronal circuits (14, 15).”

“In a more extreme model of visual deprivation (binocular eye enucleation), microglia in the visual cortex had shorter processes that made less frequent contacts with synapses (8), unlike the findings of Tremblay et al.”

“microglial processes, and light re-exposure increased their velocity back to control levels (7).”

“Subsequently, as activation of microglia occurs, P2Y12 receptors are down-regulated, until barely detectable 24 h after the initial damage (29, 34).”

“This is surprising as CX3CR1 receptor deficiency is known to transform microglia from a quiescent surveillance state into an activated state (65), with a larger cell soma and greatly reduced number of processes that are much shorter and less motile.”

“Microglia survey the brain to detect infection or cell damage or death (3).”

“In line with astrocyte-microglia communication via ATP, blockade of gap junction hemichannels, which may mediate astrocytic ATP release (53, 56, 57), led to microglia adopting shorter process lengths, as well as a reduced baseline process velocity (Fig.”

“This allows them to interact physiologically with cells in the brain parenchyma, as well as respond promptly to brain damage or injury (4, 5) and clear cellular debris or accumulating metabolic products by phagocytosis (6).”

“This is essential for regulating the number of neurons in the adult brain; progenitor cells produce neurons in excess, only a fraction of which mature and become integrated into neuronal circuits (14, 15).”

“These release more ATP across the astrocytic network, promoting communication between astrocytes and to neurons (52).”

“Volume-sensitive Cl channels (and channels with similar pharmacology, like pannexins (46)) also mediate ATP release from cells (47) and, if ATP-induced ATP release from astrocytes promotes microglial chemotaxis by amplifying local ATP signals (4, 10), Cl channel blockers may inhibit chemotaxis by reducing ATP release.”

“A major impact of hemichannels on microglial morphology could not, however, be confirmed in a study performed on zebrafish with pharmacological blockade and genetic down-regulation of pannexin-1 hemichannels (9).”

“Other substances that induce microglial chemotaxis in vivo are NO (in the white matter (24)) and fibrinogen (25), which can leak from damaged blood vessels.”

“Chemoattraction of microglial processes by elevated concentrations of ATP ( 1 mM) leaking from damaged cells was first shown in cultured microglia (22) and confirmed in situ and in vivo (4, 5, 19, 23).”

“Increases in neuronal activity evoked by glutamate (or analogues like NMDA, AMPA, or kainate), or by blocking GABAergic inhibition, led to microglial process extension and enhanced baseline motility, whereas block of AMPA receptors had the opposite effect (5, 37, 50, 75).”

“In line with astrocyte-microglia communication via ATP, blockade of gap junction hemichannels, which may mediate astrocytic ATP release (53, 56, 57), led to microglia adopting shorter process lengths, as well as a reduced baseline process velocity (Fig.”

“Astrocytes can amplify a local rise in extracellular [ATP] through this ATP-induced ATP release (53, 54), facilitating microglial chemotaxis after focal brain damage (4) and enhancing excitatory neurotransmission following microglia activation by LPS (10).”

“Fractalkine receptor deficiency leads to a transient reduction of microglial number during early postnatal development (12, 13), microglia-mediated neurotoxicity (65– 67), and impaired phagocytosis of apoptotic newborn neurons, thus causing reduced neurogenesis and deficits in synaptic pruning associated with weaker s…”

“Further work is needed to reveal which other mechanisms exist to evoke a targeted response of microglial processes under pathological conditions when activated microglia have substantially down-regulated their P2Y12 receptor expression (29, 45).”

“PI3K/Akt and integrins play a crucial role in actin filament remodeling (41, 42), which presumably underlies their role in promoting targeted microglial process motility on P2Y12 receptor activation.”

“Because ATP functions in many cell types as an auto- or paracrine signaling molecule acting in the direct vicinity of its release site (74), a similar scenario may hold for microglia (during either baseline surveillance or chemotaxis).”

“This may reflect co-release of ATP with principal neurotransmitters from neurons (63) or secondary neuronal ATP release following NMDA receptor activation (37, 75), or may involve communication to astrocytes, which release ATP in response to neuronal activity (52).”

“Increases in neuronal activity evoked by glutamate (or analogues like NMDA, AMPA, or kainate), or by blocking GABAergic inhibition, led to microglial process extension and enhanced baseline motility, whereas block of AMPA receptors had the opposite effect (5, 37, 50, 75).”

“This contradicts data from cultured microglia, showing expression of glutamate-sensing AMPA and kainate receptors, as well as glutamate transporters (61, 62).”

“Indeed, in microglia in situ, only extracellular nucleotides such as ATP, ADP, and UDP evoke inward membrane currents through ionotropic P2X (presumably P2X4 and P2X7) receptors and outward K currents via G protein-coupled (P2Y) purinergic receptors (23, 31–33, 35, 50), whereas other neurotransmitters (glutamate, GABA…”

“Consistent with this signaling contributing to chemotaxis, blockade of PLC or PI3K or chelation of intracellular Ca(2) with 1,2-bis(o-aminophenoxy)ethane-N,N,N ,N -tetraacetic acid, acetoxymethyl ester (BAPTA-AM) suppresses ATP/ADP-induced chemotaxis in vitro (30, 38).”

“Increases in neuronal activity evoked by glutamate (or analogues like NMDA, AMPA, or kainate), or by blocking GABAergic inhibition, led to microglial process extension and enhanced baseline motility, whereas block of AMPA receptors had the opposite effect (5, 37, 50, 75).”

“Astrocyte Ca(2) waves can be sensed by some microglia via purinergic receptors that generate an outward membrane current (55).”

“In line with astrocyte-microglia communication via ATP, blockade of gap junction hemichannels, which may mediate astrocytic ATP release (53, 56, 57), led to microglia adopting shorter process lengths, as well as a reduced baseline process velocity (Fig.”

“First, both of these modes depend on cytoskeletal changes arising from highly motile bundles of actin filaments (17).”

“Volume-sensitive Cl channels (and channels with similar pharmacology, like pannexins (46)) also mediate ATP release from cells (47) and, if ATP-induced ATP release from astrocytes promotes microglial chemotaxis by amplifying local ATP signals (4, 10), Cl channel blockers may inhibit chemotaxis by reducing ATP release.”

“Although process velocity was reduced by 30% for baseline surveillance and in response to tissue damage, microglial morphology and tissue density were unaltered (71).”

“5 m/min (4, 5, 9, 23, 48), whereas thicker processes closer to the soma are less motile.”

“Noradrenaline also abolished ATP-induced process outgrowth, suggesting a regulation of purinergic signaling by adrenergic signaling in microglia (58).”

“Indeed, in microglia in situ, only extracellular nucleotides such as ATP, ADP, and UDP evoke inward membrane currents through ionotropic P2X (presumably P2X4 and P2X7) receptors and outward K currents via G protein-coupled (P2Y) purinergic receptors (23, 31–33, 35, 50), whereas other neurotransmitters (glutamate, GABA…”

“Another indication that extracellular nucleotides regulate microglial process ramification came from mice lacking the ectonucleotidase CD39 (51), the hippocampal microglia of which had a less ramified morphology, with shorter processes covering a smaller cortical area than for wild-type mice, whereas their tissue dens…”

“In vitro, P2Y12 receptor activation also increases expression and activation of integrin M 1 (CD11b/CD29), a cell adhesion molecule that mediates interactions with the extracellular matrix (39, 40).”

“5 m/min (4, 5, 9, 23, 48), whereas thicker processes closer to the soma are less motile.”

“Other substances that induce microglial chemotaxis in vivo are NO (in the white matter (24)) and fibrinogen (25), which can leak from damaged blood vessels.”

“Volume-sensitive Cl channels (and channels with similar pharmacology, like pannexins (46)) also mediate ATP release from cells (47) and, if ATP-induced ATP release from astrocytes promotes microglial chemotaxis by amplifying local ATP signals (4, 10), Cl channel blockers may inhibit chemotaxis by reducing ATP release.”

“In addition to P2Y12, P2X4 receptors might contribute to ATP-evoked chemotaxis of microglia in vitro (30), but expression of P2X4 in situ only occurs after microglial activation, not in quiescent, ramified microglia (35).”

“Consistent with this signaling contributing to chemotaxis, blockade of PLC or PI3K or chelation of intracellular Ca(2) with 1,2-bis(o-aminophenoxy)ethane-N,N,N ,N -tetraacetic acid, acetoxymethyl ester (BAPTA-AM) suppresses ATP/ADP-induced chemotaxis in vitro (30, 38).”

“Furthermore, the small Rho-GTPase Rac, a key regulator of actin dynamics (20), is highly enriched in the motile endings of microglial processes, and Rac down-regulation prevents directed process movement in response to neuronal activity (9).”

“In cultured microglia, although chemotactic migration was not induced by adenosine alone, it was reduced with adenosine A1 receptors blocked or knocked out, or with the ectonucleotidase CD39 deleted (43), which seems at variance with a role only for P2Y12 receptors in regulating chemotaxis (34).”

“Furthermore, contradicting the idea that process motility correlates with neuronal activity, highfrequency stimulation inducing long-term potentiation in hippocampal slices did not change microglial baseline surveillance (59).”

“In another study, selective P2Y12 receptor activation by 2-methylthioADP led to microglial process outgrowth and migration only on co-stimulation with adenosine, which, in contrast to the aforementioned study, was attributed to adenosine acting via A3 adenosine receptors (44).”

“High amounts of filamentous actin are present in microglial cells (18), and agents blocking actin polymerization such as cytochalasin B or latrunculin B inhibit both microglial baseline surveillance and chemotaxis in situ (19).”

“Indeed, in microglia in situ, only extracellular nucleotides such as ATP, ADP, and UDP evoke inward membrane currents through ionotropic P2X (presumably P2X4 and P2X7) receptors and outward K currents via G protein-coupled (P2Y) purinergic receptors (23, 31–33, 35, 50), whereas other neurotransmitters (glutamate, GABA…”

“In vitro, P2Y12 receptor activation also increases expression and activation of integrin M 1 (CD11b/CD29), a cell adhesion molecule that mediates interactions with the extracellular matrix (39, 40).”

“Overall surveillance of the surrounding neuronal environment is accomplished via a large number of signaling mechanisms [99,102].”

“These cells communicate with surrounding environments including neurons, astrocytes and blood vessels to maintain the development and homeostasis of the CNS.(163,164) When microglia recognize the insults of the CNS, they respond to the injury or invasion by a morphological change, resulting in cell enlargement and mig…”

“In response to various chemotactic signals, microglia are attracted to and interact with neurons and synapses (Badimon et al., 2020; Cserép et al., 2020; Madry and Attwell, 2015).”