Downregulation of BM88 after optic nerve injury.

Purpose: BM88 is a cell-cycle exit and neuronal differentiation protein that has been used as a marker of surviving retinal ganglion cells (RGCs) after optic nerve injury. Thy1.1 has also been used as a marker for RGC loss, but after optic nerve crush (ONC) a decrease in Thy1.1 expression precedes the loss of RGCs. The purpose of this study was to determine if BM88 expression was correlated with RGC loss after ONC and optic nerve transection (ONT) injuries.
Methods: Rats were injected with Fluorogold (FG) into the superior colliculus to label RGCs and received ONC or ONT 7 days later. Eyes were collected 2 to 28 days after injury. Retinas were labeled with BM88 and intensity of the BM88 cell labeling was measured.
Results: In control retinas, 98.9% of RGCs were immunoreactive (-IR) for BM88. There was a significant downregulation of BM88 by 52% to 80% of RGCs 7 days after ONC or ONT. The staining intensity of the remaining labeled cells was reduced to 41% to 51% of the control after 28 days of optic nerve injury. However, early in the injury there was a significant increase in the staining intensity of BM88.
Conclusions: Nearly all BM88-IR RGCs colocalized with FG-labeled RGCs in control retinas. However, both the number of BM88-IR RGCs and their intensity decreased gradually between 4 and 28 days, preceding the loss of FG-labeled cells. These findings indicate that BM88 is not a good marker of surviving RGCs but may indicate abnormal RGC functioning, which precedes cell death.