Galectin-3 Promotes Müller Glia Clearance Phagocytosis via MERTK and Reduces Harmful Müller Glia Activation in Inherited and Induced Retinal Degeneration

Abstract

Clearance phagocytosis is a documented function of Müller glia in the retina. However, the molecular mechanisms of Müller glia phagocytosis remain largely undefined. Here, we show that extracellular galectin-3 and protein S promote clearance phagocytosis by immortalized human MIO-M1 Müller cells in an additive, saturable manner. Galectin-3 promotes phagocytosis by primary Müller glia from wild-type (WT) mice but not from mice that lack the engulfment receptor MERTK and therefore develop postnatal photoreceptor degeneration. Probing a possible functional link between Müller galectin-3 and MERTK, we discovered that mertk−/− Müller glia in situ show excess galectin-3 at postnatal day 20 (P20), an age prior to detectable photoreceptor degeneration. Moreover, double knockout (DKO) mice lacking both galectin-3 and MERTK show increased activation of Müller cells (but not of microglia) at P20 and more pronounced photoreceptor loss at P35 compared to mice lacking MERTK alone. Exploring the well-established sodium iodate injury model, we also found more severe activation specifically of Müller glia, and worse retinal damage in mice lacking galectin-3 compared to WT mice. Indeed, galectin-3 deficiency significantly increased sensitivity to injury, yielding Müller activation and retinal damage at a sodium iodate concentration that had no effect on the WT retina. Altogether, our results from both inherited and acutely induced models of retinal degeneration agree that eliminating galectin-3 exacerbates Müller cell activation and retinal degeneration. These data identify an important protective role for the MERTK ligand galectin-3 in the retina in restraining Müller glia activation.

Heterogeneity in quiescent Müller glia in the uninjured zebrafish retina drive differential responses following photoreceptor ablation

“…Thus, we assessed if the differences in capacity for activation of Müller glia are influenced by signals local to the site of injury. Specifically, we investigated phagocytosis of dying cells and their debris, which can influence behaviour Müller glia following injury (Sakami et al, 2019; Lew et al, 2022) including Müller glia proliferation (Bailey et al, 2010; Nomura-Komoike et al, 2020). For this, we quantified the number of mCherry containing presumed phagocytic Müller glia and compared this to the Müller glia re-entering the cell cycle (PCNA-positive) after widespread Lws2, smaller Lws2 and rod ablation, as these resulted in a substantial Müller glia proliferative response (Figure 4).…”

Clearance phagocytosis by the retinal pigment epithelial during photoreceptor outer segment renewal: Molecular mechanisms and relation to retinal inflammation

“…Our recent studies revealed, however, pro‐inflammatory signaling in RCS rat retina by postnatal Day 14, an age just prior to eye opening in RCS rats and thus prior to onset of robust diurnal outer segment renewal 47 . Microglia and Müller glia activation including elevated Iba‐1, CD68, and GFAP are pronounced by postnatal Day 20, an age prior to any evidence for photoreceptor apoptosis in RCS rat retina 47,176 . These results imply that loss of MERTK elicits retinal inflammation independently of the loss of RPE clearance function.…”

“…Moreover, immune activation of microglia triggers their mobilization: dormant microglia reside in the inner retina, but in disease or tissue stress, microglia translocate to the site of injury 174 . Inactive and activated Müller glia and microglia phagocytose tissue and cell debris in the PS‐dependent manner similar to the RPE, although the underlying molecular mechanisms are still only poorly understood 175–177 …”

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

“…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).…”