Differential Susceptibility to Axonopathy in Necrotic and Non-Necrotic Perinatal White Matter Injury

Abstract

Background and Purpose White matter injury (WMI) is the leading cause of brain injury in preterm survivors and results in myelination failure. Although axonal degeneration occurs in necrotic lesions, the role of axonopathy in myelination failure remains controversial for diffuse non-necrotic WMI, which is currently the major form of WMI. We determined the burden of axonopathy in diffuse lesions. Methods We analyzed WMI in a preterm fetal sheep model of global cerebral ischemia that replicates the relative burden of necrotic and non-necrotic human WMI. WMI was analyzed at 1-or 2-weeks after ischemia and identified by ex vivo high-field (11.7 Tesla) MRI of fixed brain tissue. Axonal integrity was analyzed by immunohistochemical detection of axon injury markers and by transmission electron microscopy (EM) to quantify axon loss and degeneration in MRI-defined lesions. Results Axonal degeneration, defined by staining for neurofilament protein and β-amyloid precursor protein, was restricted to discrete necrotic foci with robust microglial activation. Unexpectedly, axonal degeneration was not visualized in the major form of WMI, which comprised large non-necrotic lesions with diffuse reactive astrogliosis. In these major lesions, quantitative EM studies confirmed no significant differences in the density of intact and degenerating axons or in the distribution of axon diameters relative to controls. Conclusions The mechanism of myelination failure differs significantly in perinatal WMI dependent upon the burden of necrosis. Axonopathy is associated with focal necrotic injury but not with primary diffuse non-necrotic lesions, which supports that intact axons in the primary lesions are potential targets for myelination.

The Instrumented Fetal Sheep as a Model of Cerebral White Matter Injury in the Premature Infant

“…During the chronic phase of WMI from this same fetal sheep preparation [46] and human [54], microscopic foci of necrosis were observed that were rich in microglia, but lacked astrocytes or axons. However, no significant axonal degeneration, axonal loss or shift in the distribution of axon calibers was observed by quantitative electron microscopy studies in preterm fetal sheep [53]. Hence, the contribution of microscopic necrosis to axonal loss with secondary myelination failure appears to be low, but there is a need for further human neuropathological studies with sensitive markers of axonal injury that are applied to cases of diffuse WMI that lack necrosis.…”

“…This shortcoming, for example, limits the relevance of rodent hypoxia-ischemia models for the study of myelination disturbances associated with chronic human WMI. Necrotic injury to cerebral gray matter contributes substantially to neuro-axonal degeneration as a cause of dysmyelination, which is not a prominent feature of WMI in either fetal sheep [53] or contemporary human cases of WMI [54]. The basis for this difference in susceptibility to gray matter injury in rodents relative to sheep and humans is likely to be multifactorial.…”

“…During the acute phase of WMI, after global hypoxia-ischemia, fetal sheep that lacked necrotic WMI did not show evidence of axonal degeneration [41]. We used quantitative electron microscopy studies in our preterm fetal sheep model of global cerebral ischemia to define the extent of axonal degeneration in MRI-defined diffuse WMI [53]. During the chronic phase of WMI from this same fetal sheep preparation [46] and human [54], microscopic foci of necrosis were observed that were rich in microglia, but lacked astrocytes or axons.…”

Astrocytes and microglia in acute cerebral injury underlying cerebral palsy associated with preterm birth

“…Rather, it is closely correlated with the relative vulnerability of late oligodendrocyte progenitors in rabbit, sheep, and human (11,12). Furthermore, in vitro, central premyelinating axons are reported to be much more vulnerable to ischemic injury than adjacent myelinated axons or smaller axons that had not reached the stage of radial expansion (13), likely contributing to the high risk of axonopathy in necrotic periventricular white matter lesions (14).…”

What brakes the preterm brain? An arresting story

“…Disruption of early contact between the cortex and thalamus may also disrupt connectivity and lead to remodeling of cortical circuitry due to target deprivation-induced ("Wallerian") neurodegeneration (72). Nevertheless, although historical data demonstrate axonal injury within and around periventricular white matter necrosis (9), there does not appear to be significant axonal damage in nonnecrotic fetal ovine WMI (73). Similarly, in a postmortem cohort of preterm infants, axonal degeneration was only seen in necrotic foci, in association with microglial activation, but not in areas…”