Characterization of NG2+ macrophage in glaucoma : an investigation focusing on its origin and potential roles in ischemic retina

Abstract

Macrophage manipulation as immunomodulatory intervention in glaucoma, a leading cause to blindness characterized by retinal ganglion cell (RGC) loss, has been proposed as a promising strategy to protect RGCs from dying. Three types of macrophage in damaged retinas, microglia-derived macrophage, monocyte-derived macrophage and perivascular macrophage, together consist a highly heterogeneous population with both beneficial and detrimental functions partly resulted from their distinct origins.

NG2 (Nerve/glial antigen 2) positive macrophage, a subtype of macrophage, has been considered to fulfill specific functions with neuroprotective and neurogenic potentials. Whether NG2+ macrophage can be a possible target for treatment in glaucoma is unknown. Basic information of this subtype of macrophage in ischemic retinas of a mouse glaucomatous model called acute ocular hypertension (AOH) model was obtained in this study to answer three questions.

First, do NG2+ macrophage exist in mouse retina, if so, what do they like? In normal mouse retinas, NG2+ macrophage did not exist. After AOH, NG2 could be induced on macrophage. And they are unevenly distributed in the whole retina while reside in the ganglion cell layer, inner plexiform layer or inner nuclear layer. Moreover, NG2+ macrophage all contained two nuclei with typical amoeboid-like morphologies of macrophage in activation state except the rounded type.

Secondly, are they associated with the surrounding cells, if so, how? Close association of RGCs and Müller cells with NG2+ macrophage was found due to their co-localization. To know how they functioned, potential roles regarding its proliferating and phagocytic abilities were revealed by its co-localization with proliferating and phagocytic markers. However, unlike NG2+ macrophage in ischemic brain, NG2+ macrophage in ischemic retina did not express neurotrophic factors while some of them were found to express interlukin-10, an anti-inflammatory cytokine. Possible neurogenic potential was also observed by its co-localization with Nestin, a neural stem cell marker.

Thirdly, where do they come from? To know their origins, bone marrow chimeras combined with specific markers were used to distinguish microglia-derived macrophage, monocyte-derived macrophage and perivascular macrophage. And it was found that majority of NG2+ macrophage was originated from resident microglia, rather than monocyte-derived macrophage or perivascular macrophage.

Taken together, the present study showed the basic profile of a subtype of macrophage in a mouse model of glaucoma, more accurately, NG2+ macrophage in AOH mouse retinas. NG2 expression was induced mostly in resident microglia in AOH retina as a proliferating population with phagocytic, anti-inflammatory functions, and possible neurogenic potentials. Therefore, NG2+ macrophage may be a potential candidate target for treatment of glaucoma.