Freeze-Fracture and Cytochemical Evidence for Structural and Functional Alteration in the Axolemma and Myelin Sheath of Adult Guinea Pig Optic Nerve Fibers After Stretch Injury

Abstract

Recent work in animal models of human diffuse axonal injury has generated the hypothesis that, rather than there being physical disruption of the axolemma at the time of injury, a pertubation of the membrane occurs, which leads, over time, to a dysfunction of the physiology of the axolemmal. This dysfunction is posited to lead to a disruption of ionic homeostasis within the injured axon, leading to secondary axotomy some hours after the initial insult. We decided to test the hypothesis that membrane pump/ion channel activity or function is compromised and this would be reflected in structural changes within the axolemma and myelin sheath. We used freeze fracture and cytochemical techniques to provide evidence for change in membrane structure and the activity of membrane pumps after nondisruptive axonal injury in the adult guinea pig optic nerve. Within 10 min of injury, structural changes occurred in the distribution and number of intramembranous particles (IMPs) in the internodal axolemma. By 4 h, there was novel labeling for Ca-ATPase membrane pump activity at the same site. There was loss of IMPs from the nodal axolemma extending over several hours after injury. There was loss of both membrane pump Ca-ATPase and p-nitro-phenylphosphatase (p-NPPase) activity of the node. There was loss of ecto-Ca-ATPase activity but increased labeling for p-NPPase activity at sites of dissociation of compacted myelin. Quantitative freeze-fracture demonstrated statistically significant changes in membrane structure. We provide support for the hypothesis that structural and functional changes occur in the axolemma and myelin sheath at nondisruptive axonal injury.

Therapy Development for Diffuse Axonal Injury

“…39,78,79 Owing, in part, to the isolated nature of the white matter injury, these models have proven very useful in evaluating ultrastructural changes in axons after trauma as well as identifying pathological chemical cascades that may represent important therapeutic targets. 34,39,47,[78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93] These models, however, are not widely used or practical for high-throughput therapy evaluation. Nonetheless, they represent a potential stepwise bridge between in vitro and in vivo therapy development.…”

Multiple functions of the paranodal junction of myelinated nerve fibers

“…Disruption of axoplasmic transport follows and ultimately, axonal interruption. The final pathological changes are thought to be mediated by increased influx of calcium through the stretch-induced nodal defects (Maxwell et al, 1999). In the PNS as well, slow, artificial stretch results in nodal elongation, leading to conduction block (Ikeda et al, 2000).…”

Traumatically Induced Axotomy Adjacent to the Soma Does Not Result in Acute Neuronal Death

“…Once thought to occur immediately as a result of tearing evoked by TBI, it is now known that tearing rarely occurs (Maxwell et al, 1997). Rather, TAI typically involves a more progressive response involving a transient, traumatically induced disruption of the axonal membrane, allowing for unregulated calcium entry (Maxwell et al, 1995(Maxwell et al, , 1999Pettus et al, 1994;Pettus and Povlishock, 1996;Wolf et al, 2001). This calcium influx initiates calpain activation (Buki et al, 1999;Shields et al, 2000) and mitochondrial injury/swelling (Okonkwo and Povlishock, 1999) with cytochrome c release and caspase activation (Buki et al, 2000), leading to further axonal injury and detachment over time.…”