Abstract
Axonal degeneration is a common feature of multiple neurodegenerative diseases, yet the mechanisms underlying its various manifestations are incompletely understood. We previously demonstrated that axonal degeneration is associated with externalization of phosphatidylserine (PS), which precedes morphological evidence of degeneration, is redox-sensitive, and is delayed in Wallerian degeneration slow (WldS) mutant animals. Phosphatidylethanolamine (PE) is the other major membrane phospholipid in the inner leaflet of the cell membrane, and given that PS signals apoptosis, phagocytosis, and degeneration, we hypothesized that PS and PE membrane dynamics play distinct roles in axonal degeneration. To test this hypothesis, axonal degeneration was induced with calcium ionophores in postnatal rat retinal ganglion cells, and PS- and PE-specific fluorescent probes used to measure their externalization over time. In untreated cells, cell-surface PS was prominent in the cell body alone. Elevation of intracellular calcium with calcium ionophores resulted in significantly increased levels of PS externalization in the cell body, axon, and axon growth cone. Unlike PS, cell-surface PE was diffusely distributed in untreated cells, with comparable levels across the soma, axons, and axon terminals. After exposure to calcium ionophores, PE externalization significantly increased in the cell body and axon. Elevated intracellular calcium also resulted in the formation of axonal blebs which exclusively contained externalized PS, but not PE. Together, these results indicated distinct patterns of externalized PS and PE in normal and degenerating neurons, suggesting a differential role for these phospholipids in transducing neuronal injury.
Highlights
Cell death is characterized by biochemical changes in all cell compartments, including events at the plasma membrane
In healthy neurons there is an asymmetric distribution of phospholipids in the plasma membrane bilayer, with phosphatidylserine (PS) and phosphatidylethanolamine (PE) mostly concentrated in the inner leaflet, and much less in the outer leaflet [1]
We previously found that axonal injury to retinal ganglion cells (RGCs) results in a wave of propagation of PS externalization that travels proximally and distally along the axon from the site of injury [5]
Summary
Cell death is characterized by biochemical changes in all cell compartments, including events at the plasma membrane. The asymmetric distribution of PS and PE is maintained by lipid transporters that move phospholipids between the membrane leaflets [2]. In the early phase of cell death, altered regulation of these lipid transporters results in the loss of membrane asymmetry and externalization of phospholipids such as PS and PE into the outer leaflet of the membrane (Fig. 1A) [3]. Probes that bind PS, e.g., annexins A5 [4] and B12 [5], have enabled studies of PS externalization, including live imaging of neurons undergoing apoptosis [5, 6]. Simultaneous imaging of PS and PE externalization has the potential to identify differences in their regulation, purpose, and distribution in
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
