Histological and ultrastructural analysis of white matter damage after naturally-occurring spinal cord injury

Atrophy and degeneration in sciatic nerve of presymptomatic mice carrying the Huntington's disease mutation

Huntington's disease (HD) is a progressive neurological disorder characterised by motor impairments caused by degeneration in the striatum. The mechanism by which the HD mutation leads to the neurodegenerative pathology of HD is still unknown. Recently it was shown that, in HD patients, early pathological changes in white matter precede selective cell death in the striatum. We wondered whether axonal pathology is also an early pathological feature in a transgenic mouse model carrying the HD mutation (R/2 line). R6/2 mice show brain atrophy, a progressive neurological deterioration and skeletal muscle atrophy that resemble those seen in HD patients. However, there is very little neuronal cell loss seen in these animals, even when they show severe symptoms. Here we used sciatic nerve to look for evidence of early neurodegenerative changes in axons of the R6/2 mouse at an ultrastructural level. We observed ultrastructural changes that preferentially affected large myelinated fibres of the sciatic nerve in 10-week-old asymptomatic R6/2 mice. The changes included a significant decrease in the axoplasm diameter of myelinated neurons and an increase in the number of degenerating myelinated fibres compared to age-matched wild type littermates. Myelin thickness and unmyelinated fibre diameter were not affected. The abnormalities described here precede the appearance of overt motor symptoms in the R6/2 mouse and occur in parallel with pathophysiological changes at the neuromuscular junction. We suggest that degenerative changes in axons are likely to contribute to the early pathological phenotype in HD, even in the absence of frank neuronal cell loss.

Calponin is expressed by fibroblasts and meningeal cells but not olfactory ensheathing cells in the adult peripheral olfactory system

Olfactory ensheathing cells (OECs), the principal glial cells of the peripheral olfactory system, have many phenotypic similarities with Schwann cells of the peripheral nervous system. This makes reliably distinguishing these two cells types difficult, especially following transplantation into areas of injury in the central nervous system. In an attempt to identify markers by which these two cells types can be distinguished, a recent proteomic analysis of fetal OECs and adult Schwann cells identified the actin-binding protein calponin as a potential marker expressed by OECs but not Schwann cells. Since many studies designed with the translational goal of autologous transplantation in mind have used adult OECs, this study examined the expression of calponin by adult OECs, both in vivo within the peripheral olfactory system and in vitro. Calponin colocalized with strongly fibronectin positive fibroblasts in the olfactory mucosa (OM) and meningeal cells in the olfactory bulb (OB) but not with S100beta or neuropeptide-Y positive OECs. In tissue culture, calponin was strongly expressed by fibronectin-expressing fibroblasts from OM, sciatic nerve and skin and by meningeal cells from the OB, but not by p75(NTR)- and S100beta-expressing OECs. These data, supported by Western blotting, indicate that calponin can not be used to distinguish adult OECs and Schwann cells.