Rapid clearance of cellular debris by microglia limits secondary neuronal cell death after brain injury in vivo

Abstract

Moderate or severe traumatic brain injury (TBI) causes widespread neuronal cell death. Microglia, the resident macrophages of the brain, react to injury by migrating to the lesion site, where they phagocytose cellular debris. Microglial phagocytosis can have both beneficial (e.g. debris clearance) and detrimental (e.g. respiratory burst, phagoptosis) consequences. Hence, whether the overall effect of microglial phagocytosis after brain injury in vivo is neuroprotective or neurotoxic is not known. Here, we establish a system with which to carry out dynamic real-time analyses of the mechanisms regulating cell death after brain injury in vivo. We show that mechanical injury to the larval zebrafish brain induces distinct phases of primary and secondary cell death. Excitotoxicity contributes to secondary cell death in zebrafish, reflecting findings from mammals. Microglia arrive at the lesion site within minutes of injury, where they rapidly engulf dead cells. Importantly, the rate of secondary cell death is increased when the rapid removal of cellular debris by microglia is reduced pharmacologically or genetically. In summary, our results provide evidence that microglial debris clearance is neuroprotective after brain injury in vivo.

Microglia in the developing retina couple phagocytosis with the progression of apoptosis via P2RY12 signaling

“…For example, dysregulated microglial phagocytosis may contribute to pathology in a variety of neurodegenerative diseases . In contrast, microglial phagocytosis limits secondary cell death in contexts of acute CNS injury and is important in response to widespread retinal lesion in zebrafish . Therefore, a better understanding of molecular mechanisms and pathways functioning in microglial phagocytosis in healthy vs damaged or diseased tissue are likely to yield important targets to modulate and balance microglial behavior in vivo.…”

Microglial Corpse Clearance: Lessons From Macrophages

“…In summary, phagocytosis is a powerful double-edged sword that must be kept under a tight rein, as is exemplified by two recent papers in zebrafish and Drosophila. Phagocytosis of apoptotic cells is beneficial during brain trauma, as it prevents secondary damage spread in zebrafish larvae (128). In control larvae, the initial necrotic and apoptotic cells resulting from traumatic brain injury in the optic tectum are cleared by microglia within the first 24 h. When phagocytosis is pharmacologically and genetically disturbed by targeting PS and the zebrafish ortholog of PS receptor BAI1, a larger wave of secondary cell death spread over the brain (128).…”

“…Phagocytosis of apoptotic cells is beneficial during brain trauma, as it prevents secondary damage spread in zebrafish larvae (128). In control larvae, the initial necrotic and apoptotic cells resulting from traumatic brain injury in the optic tectum are cleared by microglia within the first 24 h. When phagocytosis is pharmacologically and genetically disturbed by targeting PS and the zebrafish ortholog of PS receptor BAI1, a larger wave of secondary cell death spread over the brain (128). In contrast, overexpression of phagocytosis receptors Six-Microns-Under (SIMU) and Draper (Drpr), homologs of Stabilin2 and MEGF10 (Multiple EGF Like Domains 10, a complement receptor), respectively, in adult Drosophila leads to phagocytosis of live neurons, motor dysfunction and a shortened lifespan (129).…”

Live-imaging of endothelial Erk activity reveals dynamic and sequential signalling events during regenerative angiogenesis

“…Also, other types of fluorescence-based kinase activity reporters such as separation of phases-based activity reporter of kinases (SPARK), that had been shown to work in zebrafish could be applied to visualise EC Erk activity in zebrafish (Zhang et al, 2018). (Baek et al, 2019), Tg(fli1a:EGFP) y1 (Lawson and Weinstein, 2002), Tg(ubb:Mmu.Elk1-KTR-mClover) vi1 (Mayr et al, 2018), Tg(actb2:GCaMP6f) zf3076 (Herzog et al, 2019), Tg(kdrl:mCherry-CAAX) y171 (Fujita et al, 2011), Tg(mpeg1:mCherry) gl23 (Ellett et al, 2011), and Tg(kdrl:EGFP) s843 (Beis et al, 2005).…”

“…To determine whether Ca 2+ signalling is rapidly activated in ablated ISV ECs in our model, we measured the dynamic expression of a ubiquitously expressed GCamp, a GFP-based Ca 2+ probe, using the Tg(actb2:GCaMP6f);Tg(kdrl:mCherry-CAAX) transgenic line (Herzog et al, 2019). ISVs in non-ablated 4 dpf larvae did not show We next tested whether Ca 2+ signalling is required for maintaining Erk activity in ablated ISV ECs 3 hpa.…”