Calcium‐Mediated Degeneration of the Axonal Cytoskeleton in the Ola Mouse

Abstract

The C57BL/Ola (Ola) mouse is a mutant substrain in which transected axons undergo very slow Wallerian degeneration. Because axonal degradation during Wallerian degeneration is calcium dependent, we tested whether Ola axons are susceptible to calcium-mediated axonal degeneration by comparing neurofilament degradation between Ola and C57BL/6 mice in sciatic nerve explants. Using immunoblot analysis of neurofilament degradation and electron microscopy we found that as in normal axons, axonal degeneration in the Ola is calcium dependent. However, when compared with normal animals, higher levels of calcium were required for complete degradation of neurofilaments in Ola nerve, suggesting a relative insensitivity to calcium-mediated degeneration in the Ola. We conclude that calcium-activated proteases are present and active in Ola axons but that higher levels of calcium are required to accomplish complete axonal degradation. These results suggest a possible mechanism for prolonged survival of transected Ola axons and provide potential insight into the pathophysiology of axonal degeneration in injury and disease.

Wallerian Degeneration in the Optic Nerve Stretch-Injury Model of Traumatic Brain Injury: A Stereological Analysis

“…39 For example, channel-mediated influx of extracellular Ca 2+ is essential for the initiation of acute axonal degeneration (AAD), for Ca 2+ activated serinethreonine protease calpain breakdown of tubulin of axonal microtubules, spectrin of the subaxolemma cytoskeleton and neurofilaments, 53, 54, 40 disruption of mitochondrial structure and function leading to opening of the mitochondrial membrane transition pore 55,56 and activation of calpain in the distal axon after axotomy. 57,40 The mode of loss of axons from the proximal part of the left optic tract differed from that reported previously in the proximal segment of the injured optic nerve. 14 It is suggested that a reason is that the optic tract contains axons from parts of two retinae while the optic nerve contains axons from a single retina.…”

Axon regeneration in the absence of growth cones: Acceleration by cyclic AMP

“…The retraction involves creation of densely packed NF domains (McHale et al,1995) initiated by proteolysis of the NF sidearms (Hall and Lee,1995), and is an active process, since it is greatly slowed in the isolated spinal cord (Zhang et al,2005). NF proteins are degraded by calpains (Schlaepfer and Hasler,1979; Pant et al,1982; Zimmerman and Schlaepfer,1982; Glass et al,1994). The mechanism for this effect is unknown but reduction of axon terminal retraction has been seen in porcine photoreceptors exposed in vitro to either 8‐(4‐chlorophenylthio)‐cAMP or forskolin (Khodair et al,2005).…”

Axon degeneration: Molecular mechanisms of a self-destruction pathway

“…Culturing neurons in a reduced calcium environment by switching to low Ca 2+ media (below 200uM) or chelating external Ca 2+ with EGTA robustly delays axon degeneration for 4 d after axotomy, whereas adding Ca 2+ ionophores is sufficient to revert the protective phenotype and induce degeneration in uninjured neurites (Schlaepfer and Bunge, 1973; George et al, 1995). Similar to AAD, the Ca 2+ -dependent protease calpain is also activated in the distal axons from rising Ca 2+ levels after axotomy (Glass et al, 2002); however, chemically inhibiting calpain activity only delays axon degeneration for 12–24 h in vitro (Glass et al, 1994; Wang et al, 2000). The significantly weaker axonal protection from calpain inhibition alone compared with Ca 2+ chelation (Finn et al, 2000; Zhai et al, 2003) suggests that additional Ca 2+ -dependent proteases or pathways mediate the cytoskeletal breakdown in Wallerian degeneration.…”

“…Events similar to Wallerian axon degeneration also occur in genetic mutants such as pmn that are impaired in axonal transport (Martin et al, 2002), where there is an absence of direct trauma and where the source of Ca 2+ influx into the axon is not apparent (Coleman, 2005). Moreover, exogenous addition of Ca 2+ is sufficient to abolish WldS-mediated axonal protection (Glass et al, 1994), indicating that the calcium-dependent activities in the axon are downstream of molecular components that suppress the activation of Wallerian degeneration. Thus, although Ca 2+ signaling is clearly critical for axonal degradation, more upstream cellular processes likely trigger the increased intra-axonal Ca 2+ levels by promoting channel-mediated influx or modulating intracellular Ca 2+ buffering and release, or both, while subsequent Ca 2+ -dependent proteolytic events mediate the physical destruction of the axon.…”