Capillary blood flow around microglial somata determines dynamics of microglial processes in ischemic conditions

Abstract

Microglia are the resident immune cells in the brain. Under normal conditions resting ramified microglia constantly extend and retract fine processes while performing immunological surveillance. In ischemia, microglia become activated as demonstrated by morphological changes during deramification leading to transformation from ramified to amoeboid form. In vivo two-photon microscopy of EGFP-expressing microglia in mouse neocortex was used to examine microglial dynamics during the early periods of focal and global ischemia. A penumbra-like “area-at-risk” surrounded by a square-shaped area of severely hypoperfused tissue was created by laser-induced photothrombosis. The dynamics of microglial processes in the area-at-risk were strongly correlated with capillary blood flow (BF) measured within 10 μm of microglial somata. Changes in BF around distal microglial processes (>30 μm from somata) had no effect on microglial dynamics. A severe reduction of capillary BF near somata by 84±6% resulted in initiation of microglial deramification, suggesting activation. A moderate decrease in BF near somata by 22±5% or increase by 87±10%, reflecting a redistribution of capillary BF, had no effect on microglial morphology. Complete BF loss during cardiac arrest (CA) or transient bilateral common carotid artery occlusion (BCCAO) entirely stalled all microglial processes without structural changes. Reperfusion after BCCAO induced recovery of microglial dynamics to pre-occlusion values. These findings suggest that during ischemia, the severe drop in BF around microglial somata coincides with morphological activation. However, this activation requires some residual BF because complete perfusion loss (as during BCCAO and CA) did not support microglial deramification.

A close look at brain dynamics: Cells and vessels seen by in vivo two-photon microscopy

“…In a model of spreading depression in organotypic slice cultures that causes loss of synaptic activity, microglia showed displacement by Lé vy flightlike movements in response to diminished synaptic activity (Grinberg et al, 2011). In contrast, in models such as traumatic brain injury (induced by laser ablation, Davalos et al, 2005), the photothrombotic stroke model and global or focal ischemia (Davalos et al, 2005;Fumagalli et al, 2013;Masuda et al, 2011), microglia did not show any body movement in response to damage.…”

“…Analysis of cell dynamics over time can provide information on microglial activity in an inflamed territory. In the last few years, microglial dynamics has been widely studied by in vivo 2-PM using a transgenic mouse model expressing GFP under the control of the fractalkine receptor (cx3cr1), which is constitutively expressed by resident microglia (Davalos et al, 2005;Fumagalli et al, 2013;Liang et al, 2009;Masuda et al, 2011;Nimmerjahn et al, 2005;Ohsawa and Kohsaka, 2011). These studies indicated the high dynamic nature of microglia within the CNS.…”

Diabetes Impairs Cortical Plasticity and Functional Recovery Following Ischemic Stroke

“…al., 2010), microglial activity (Masuda et al, 2011) and angiogenesis (Zhang and Chopp, 2009). Deficits in any one aspect of this coordinate response could explain the impaired cortical plasticity and recovery in our model of diabetes.…”

Microglial calcium signaling in the adult, aged and diseased brain

“…Motility of microglial processes was reversibly reduced in vivo also under experimentally induced ischemic conditions [36]. Another in vivo study has shown that in the ischemic brain the duration of the contacts between microglia and neuronal structures is prolonged [19].…”