Transected axons are common in the lesions of multiple sclerosis, and axonal transection may be the pathologic correlate of the irreversible neurologic impairment in this disease.

Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system that causes motor, sensory, and cognitive deficits. The present study characterized demyelinated lesions in the cerebral cortex of MS patients. One hundred twelve cortical lesions were identified in 110 tissue blocks from 50 MS patients. Three patterns of cortical demyelination were identified: Type I lesions were contiguous with subcortical white matter lesions; Type II lesions were small, confined to the cortex, and often perivascular; Type III lesions extended from the pial surface to cortical layer 3 or 4. Inflammation and neuronal pathology were studied in tissue from 8 and 7 patients, respectively. Compared to white matter lesions, cortical lesions contained 13 times fewer CD3-positive lymphocytes (195 vs 2,596/mm3 of tissue) and 6 times fewer CD68-positive microglia/macrophages (11,948 vs 67,956/mm3 of tissue). Transected neurites (both axons and dendrites) occurred at a density of 4,119/mm3 in active cortical lesions, 1,107/mm3 in chronic active cortical lesions, 25/mm3 in chronic inactive cortical lesions, 8/mm3 in myelinated MS cortex, and 1/mm3 in control cortex. In active and chronic active cortical lesions, activated microglia closely apposed and ensheathed apical dendrites, neurites, and neuronal perikarya. In addition, apoptotic neurons were increased significantly in demyelinated cortex compared to myelinated cortex. These data support the hypothesis that demyelination, axonal transection, dendritic transection, and apoptotic loss of neurons in the cerebral cortex contribute to neurological dysfunction in MS patients.

Objective: Degeneration of chronically demyelinated axons is a major cause of irreversible neurological disability in multiple sclerosis (MS) patients. Development of neuroprotective therapies will require elucidation of the molecular mechanisms by which neurons and axons degenerate. Methods: We report ultrastructural changes that support Ca2؉-mediated destruction of chronically demyelinated axons in MS patients. We compared expression levels of 33,000 characterized genes in postmortem motor cortex from six control and six MS brains matched for age, sex, and postmortem interval. As reduced energy production is a major contributor to Ca2؉-mediated axonal degeneration, we focused on changes in oxidative phosphorylation and inhibitory neurotransmission. Results: Compared with controls, 488 transcripts were decreased and 67 were increased (p < 0.05, 1.5-fold) in the MS cortex. Twenty-six nuclear-encoded mitochondrial genes and the functional activities of mitochondrial respiratory chain complexes I and III were decreased in the MS motor cortex. Reduced mitochondrial gene expression was specific for neurons. In addition, pre-synaptic and postsynaptic components of GABAergic neurotransmission and the density of inhibitory interneuron processes also were decreased in the MS cortex. Interpretation: Our data supports a mechanism whereby reduced ATP production in demyelinated segments of upper motor neuron axons impacts ion homeostasis, induces Ca2؉-mediated axonal degeneration, and contributes to progressive neurological disability in MS patients. Neurol 2006;59:478 -489 Rapid communication between neurons requires energy and the insulation of axons by discontinuous segments of myelin. Voltage-gated Na ϩ channels produce nerve impulses and are concentrated at the nodes of Ranvier, 1 the short unmyelinated axon segment between individual myelin internodes. The nerve impulse rapidly jumps from node to node by a process called saltatory conduction. Multiple sclerosis (MS) is an inflammatory, demyelinating disease of the central nervous system (CNS) that destroys myelin, oligodendrocytes, axons, and neurons. Ann2 Pathologically, demyelination predominates during early stages of MS. Neurological disability associated with demyelinating lesions is initially reversible because of a variety of adaptive changes in the MS brain. As part of these changes, Na ϩ channels are distributed diffusely along the surface of demyelinated axons, 3 resulting in slow but effective nerve communication. This also increases the energy demands of neuronal communication and renders the demyelinated axon more susceptible to hypoxic/ischemic damage (for review, see Stys 4 ). After an initial stage (commonly 10 -15 years) of relapses and remissions (RRMS), most MS patients enter a course of irreversible and continuous neurological decline, termed secondary progressive multiple sclerosis (SPMS).2 During SPMS, new inflammatory brain lesions substantially decrease with age, 5 but neurological decline continues due in part to degeneration of chronically...

Multiple sclerosis (MS) is characterized by multifocal loss of myelin, oligodendrocytes, and axons. Potential MS therapies include enhancement of remyelination by transplantation or manipulation of endogenous oligodendrocyte progenitor cells. Characteristics of endogenous oligodendrocyte progenitors in normal human brain and in MS lesions have not been studied extensively. This report describes the distribution of cells in sections from normal adult human brain and MS lesions by using antibodies directed against NG2, an integral membrane chondroitin sulfate proteoglycan expressed by oligodendrocyte progenitor cells. Stellate-shaped NG2-positive cells were detected in the white and gray matter of normal adult human brain and appeared as abundant as, but distinct from, astrocytes, oligodendrocytes, and microglia. Stellate-shaped or elongated NG2-positive cells also were detected in chronic MS lesions. A subpopulation of the elongated NG2-positive cells expressed the putative apoptotic signaling molecule p75(NTR). TUNEL-positive cells in three active, nine chronic active, and four chronic inactive lesions, however, were p75(NTR)-negative. These studies identify cells with phenotypic markers of endogenous oligodendrocyte progenitors in the mature human CNS and suggest that functional subpopulations of NG2-positive cells exist in MS lesions. Endogenous oligodendrocyte progenitor cells may represent a viable target for future therapies intended to enhance remyelination in MS patients.