Corneal epithelial cells upregulate macropinocytosis to engulf metabolically active axonal mitochondria released by injured axons

Abstract

Purpose

To determine the mechanisms used to internalize mitochondria by corneal epithelial cells after in vivo corneal trephine injury and in vitro in corneal epithelial cells.

Methods

Male and female mice were subjected to trephine injury and euthanized immediately, 6, and 24 h after injury. Macropinocytosis was quantified in vivo using 70 kD fluorescent dextran. Mitochondrial content was assessed by immunofluorescence and metabolic activity quantified by Seahorse assay immediately and 6 h after injury. In vitro experiments using human corneal and limbal epithelial (HCLE) cells and isolated mitochondria were performed to assess mitochondrial transfer in the presence of the gap junction inhibitor 18α-glycyrrhetinc acid and the macropincytosis inhibitor ethylisopropylamiloride.

Results

Mitochondria accumulate within apical epithelial cell layers within minutes of trephine injury. Macropinocytosis also increases within minutes of trephine injury. Oxygen Consumption Rates increase in the corneal epithelium 6 h after trephine injury in males and females. Inhibiting gap junctions increases mitochondrial engulfment while inhibiting macropinocytosis prevents engulfment of mitochondria by corneal epithelial cells in vitro.

Conclusions

Molecules released by injured cells and severed axons induce macropinocytosis in corneal epithelial cells within minutes of trephine injury. An increase in oxygen consumption rate in the corneal epithelium after trephine injury indicates that axonal mitochondria can evade lysosomal degradation for at least 6 h. In vitro studies using isolated labeled and unlabeled mitochondria and control and mechanically stressed human corneal epithelial cells confirm the involvement of macropinocytosis in the engulfment of free and vesicle bound mitochondria by corneal epithelial cells.