Compensatory engulfment and Müller glia reactivity in the absence of microglia

Abstract

Microglia are known for important phagocytic functions in the vertebrate retina. Reports also suggest that Müller glia have phagocytic capacity, though the relative levels and contexts in which this occurs remain to be thoroughly examined. Here, we investigate Müller glial engulfment of dying cells in the developing zebrafish retina in the presence and absence of microglia, using a genetic mutant in which microglia do not develop. We show that in normal conditions clearance of dying cells is dominated by microglia; however, Müller glia do have a limited clearance role. In retinas lacking intact microglial populations, we found a striking increase in the engulfment load assumed by the Müller glia, which displayed prominent cellular compartments containing apoptotic cells, several of which localized with the early phagosome/endosome marker Rab5. Consistent with increased engulfment, lysosomal staining was also increased in Müller glia in the absence of microglia. Increased engulfment load led to evidence of Müller glia reactivity including upregulation of gfap but did not trigger cell cycle re‐entry by differentiated Müller glia. Our work provides important insight into the phagocytic capacity of Müller glia and the ability for compensatory functions and downstream effects. Therefore, effects of microglial deficiency or depletion on other glial cell types should be well‐considered in experimental manipulations, in neurodegenerative disease, and in therapeutic approaches that target microglia. Our findings further justify future work to understand differential mechanisms and contexts of phagocytosis by glial cells in the central nervous system, and the significance of these mechanisms in health and disease.

What microglia depletion approaches tell us about the role of microglia on synaptic function and behavior

“…Other pressing questions pertain the limitations of microglia depletion techniques. These include establishing similarities and differences between genetic and pharmacological depletion strategies, understanding the effects of microglia depletion on other cells regulating plasticity such as astrocytes and understanding potential compensatory mechanisms that might take place in absence of microglia (Thiel et al, 2022).…”

Intercellular contact and cargo transfer between Müller glia and to microglia precede apoptotic cell clearance in the developing retina

“…In some cases, these reports seem to contrast with evidence that microglia dominate clearance of dying cells during retinal development (Blume et al, 2020; Francisco-Morcillo et al, 2014; Thiel et al, 2022b) or following induced retinal damage (Mitchell et al, 2018; White et al, 2017). Our recent work demonstrated the ability of Müller glia to substantially increase phagocytosis in the absence of microglia in an otherwise normally developing retina (Thiel et al, 2022b). However, to our knowledge to date, studies reporting Müller glia phagocytosis in vivo involve primarily fixed tissue or static samples and the full process of Müller glial phagocytosis in vivo, in real-time, has not been documented.…”

“…The phagocytic function of microglia is indeed well-appreciated, yet also in the central nervous system exist neuroglial cells that have become known to engage in phagocytic activity (Jung and Chung, 2018). Within the retina in particular are the Müller glia, which have been reported to engage in phagocytosis in a variety of species and contexts (Bailey et al, 2010; Lew et al, 2022; Nomura-Komoike et al, 2020; Sakami et al, 2019; Thiel et al, 2022b). Despite this body of work, the extent of such phagocytic activity is not yet fully defined and has not been directly observed in vivo in real-time.…”

“…Further, Müller glia-microglia interactions appear to influence such regenerative potential (Fischer et al, 2014; Fogerty et al, 2022; Todd et al, 2020; White et al, 2017). Phagocytosis or engulfment of dying neurons by retinal Müller glia has been described in several studies through static visualization of dying cell markers within Müller cells (Bailey et al, 2010; Egensperger et al, 1996; Krylov et al, 2023; Lew et al, 2022; Morris et al, 2005; Nomura-Komoike et al, 2020; Sakami et al, 2019; Thiel et al, 2022b). In some cases, these reports seem to contrast with evidence that microglia dominate clearance of dying cells during retinal development (Blume et al, 2020; Francisco-Morcillo et al, 2014; Thiel et al, 2022b) or following induced retinal damage (Mitchell et al, 2018; White et al, 2017).…”

“…Phagocytosis or engulfment of dying neurons by retinal Müller glia has been described in several studies through static visualization of dying cell markers within Müller cells (Bailey et al, 2010; Egensperger et al, 1996; Krylov et al, 2023; Lew et al, 2022; Morris et al, 2005; Nomura-Komoike et al, 2020; Sakami et al, 2019; Thiel et al, 2022b). In some cases, these reports seem to contrast with evidence that microglia dominate clearance of dying cells during retinal development (Blume et al, 2020; Francisco-Morcillo et al, 2014; Thiel et al, 2022b) or following induced retinal damage (Mitchell et al, 2018; White et al, 2017). Our recent work demonstrated the ability of Müller glia to substantially increase phagocytosis in the absence of microglia in an otherwise normally developing retina (Thiel et al, 2022b).…”