Mechanisms underlying microglial colonization of developing neural retina in zebrafish

Abstract

Microglia are brain-resident macrophages that function as the first line of defense in brain. Embryonic microglial precursors originate in peripheral mesoderm and migrate into brain during development. However, the mechanism by which they colonize the brain is incompletely understood. The retina is one of the first brain regions to accommodate microglia. In zebrafish, embryonic microglial precursors use intraocular hyaloid blood vessels as a pathway to migrate into the optic cup via the choroid fissure. Once retinal progenitor cells exit from the cell cycle, microglial precursors associated with hyaloid blood vessels start to infiltrate the retina preferentially through neurogenic regions, suggesting that colonization of retinal tissue depends upon the neurogenic state. Upstream of blood vessels and retinal neurogenesis, IL34 also promotes microglial precursor colonization of the retina. Altogether, CSF receptor signaling, blood vessels, and neuronal differentiation, function as guidance cues, and create an essential path for microglial migration into developing retina.

Single-cell transcriptome analysis reveals heterogeneity and a dynamic regenerative response of quiescent radial glia in adult zebrafish brain

“…Therefore, we examined microglial morphology by measuring the cell sphericity index, which is defined as the ratio of the surface area of a sphere of the same volume as the given object to the surface area of the object (Fernandez-Arjona et al, 2017). We scanned microglia in adult telencephalon using the transgenic line Tg[mpeg1.1:EGFP] , which visualizes microglia (Ranawat and Masai, 2021), and calculated their sphericity indices. Indeed, microglia show a range of sphericity from 0.2 to 0.7 along the axis of cell shape complexity from high to low (Fig.…”

“…Tg[GFAP:GFP] mi2001 (Bernardos and Raymond, 2006) and Tg[GFAP:dTomato] nns17Tg (Satou et al, 2012) were used to visualize qRG. Tg[mpeg1.1:EGFP] oki053 (Ranawat and Masai, 2021) was used to visualize microglia. Tg[mpeg1:NTR-EYFP] w202 (Petrie et al, 2014) was used for depletion of microglia.…”

“…Fish were maintained in water containing 2.5 mM MTZ from 2 days prior to injury until harvesting of brains. To evaluate levels of microglia elimination, the transgenic line Tg[mpeg1.1:NTR-eYFP] was combined with another transgenic line Tg[mpeg1.1:EGFP] (Ranawat and Masai, 2021), which labels microglia or microglial precursor cells more efficiently. This double transgenic line was used for sphericity analysis (Fig.…”

Strip1 regulates retinal ganglion cell survival by suppressing Jun-mediated apoptosis to promote retinal neural circuit formation

“…Embryos produced by intercrosses of wild-type or strip1 rw147 heterozygous fish were injected with antisense morpholino oligonucleotides at 1-cell stage. MO-strip1, MO-strn3, and MO-ath5 (Ranawat and Masai, 2021) were injected at a concentration of 250 μM, whereas MO-jun (Han et al, 2016) was injected at a concentration of 125 μM. For each morpholino experiment, the same concentration of the standard control morpholino (STD-MO) was used as a negative control.…”

Microglial colonisation of the developing brain is facilitated by clonal expansion of highly proliferative progenitors and follows an allometric scaling

“…Microglia, the brain’s parenchymal macrophages, primarily derive from yolk sac macrophages which are generated at embryonic day 7.5 (E7.5) in mice and appear first within the brain at around E9.5 (Alliot et al, 1999; Ginhoux et al, 2010). The number of infiltrating yolk sac macrophages is dramatically reduced following the establishment of the blood brain barrier at E14.5 (Ranawat and Masai, 2021; Stremmel et al, 2018). The entry of ‘foreign’ microglial precursors is crucial for normal brain development processes: microglial cells participate in the wiring of neuronal networks through pruning of dopaminergic axons and the establishment of the correct positioning of interneurons in their respective laminae (Squarzoni et al, 2014); they support myelination (Wlodarczyk et al, 2017) and clear myelin debris and apoptotic cells (Cunningham et al, 2013); and they promote the maturation and differentiation of neurons and oligodendrocytes (Decoeur et al, 2022; Marsters et al, 2020).…”