Fodrin degradation by calcium-activated neutral proteinase (CANP) in retinal ganglion cell neurons and optic glia: preferential localization of CANP activities in neurons

Abstract

The activity of calcium-activated neutral proteinases (CANPs) toward endogenous substrates was measured in axons of retinal ganglion cell (RGC) neurons and separately in adjacent optic glia under in vitro conditions that preserved the ultrastructure and anatomic relationships between these cellular elements. RGC neurons and optic glia both expressed CANP activity. In contrast to RGC axons, which contained at least two CANP activities with calcium requirements in the millimolar (CANP A) and micromolar (CANP B) range (Nixon et al., 1985), CANP activity in optic glia was detectable only at millimolar calcium concentrations. When maximally activated, CANP(s) in optic glia exhibited a broad specificity for endogenous proteins but degraded larger proteins at a faster rate. The cytoskeletal protein fodrin (brain spectrin) was among the most susceptible endogenous substrates in RGC axons or glia. The similar properties of fodrin in neurons and glia, including its susceptibility to a purified millimolar calcium-sensitive brain CANP (mCANP), provided the basis for using this protein as a substrate to compare the relative activity of neuronal and glial CANPs in situ. Fodrin degradation mediated by CANPs proceeded at least 6 X more rapidly in intact RGC axons than in optic glia. Comparable differences in the relative degradation rates of the total neuronal and glial protein pools were also observed. These results indicate that the potential activity of CANPs is substantially greater in RGC neurons than in glia. The enormous potential activity and preferential localization of multiple CANP activities in RGC neurons support previously hypothesized roles for CANPs in the processing of axonally transported proteins and in the regulation of neuronal cytoskeletal dynamics and geometry.(ABSTRACT TRUNCATED AT 250 WORDS)