Reconstruction of single cortical projection neurons reveals primary spine loss in multiple sclerosis.

Grey matter pathology has emerged as an important contributor to long-term disability in multiple sclerosis. To better understand where and how neuronal damage in the grey matter is initiated, we used high resolution confocal microscopy of Golgi-Cox impregnated tissue sections and reconstructed single cortical projection neurons in autopsies from eight patients with long-standing relapsing-remitting or secondary progressive multiple sclerosis and eight control patients without neurological disease. Analysis of several hundred individual neurons located in the insular, frontotemporal and occipital lobe revealed a widespread and pronounced loss of dendritic spines in multiple sclerosis cortex that occurs independent of cortical demyelination and axon loss. The presence of a primary synaptic pathology in the normal-appearing cortex of multiple sclerosis patients challenges current disease concepts and has important implications for our understanding of disease progression.

Oligodendroglia in cortical multiple sclerosis lesions decrease with disease progression, but regenerate after repeated experimental demyelination.

Cerebral cortex shows a high endogenous propensity for remyelination. Yet, widespread subpial cortical demyelination (SCD) is a common feature in progressive multiple sclerosis (MS) and can already be found in early MS. In the present study, we compared oligodendroglial loss in SCD in early and chronic MS. Furthermore, we addressed in an experimental model whether repeated episodes of inflammatory SCD could alter oligodendroglial repopulation and subsequently lead to persistently demyelinated cortical lesions. NogoA(+) mature oligodendrocytes and Olig2(+) oligodendrocyte precursor cells were examined in SCD in patients with early and chronic MS, normal-appearing MS cortex, and control cortex as well as in the rat model of repeated targeted cortical experimental autoimmune encephalomyelitis (EAE). NogoA(+) and Olig2(+) cells were significantly reduced in SCD in patients with chronic, but not early MS. Repeated induction of SCD in rats resulted only in a transient loss of NogoA(+), but not Olig2(+) cells during the demyelination phase. This phase was followed by complete oligodendroglial repopulation and remyelination, even after four episodes of demyelination. Our data indicate efficient oligodendroglial repopulation in subpial cortical lesions in rats after repeated SCD that was similar to early, but not chronic MS cases. Accordingly, four cycles of experimental de- and remyelination were not sufficient to induce sustained remyelination failure as found in chronic cortical MS lesions. This suggests that alternative mechanisms contribute to oligodendrocyte depletion in chronic cortical demyelination in MS.

Late motor decline after accomplished remyelination: impact for progressive multiple sclerosis.

OBJECTIVE: To investigate the impact of single or repeated episodes of reversible demyelination on long-term locomotor performance and neuroaxonal integrity, and to analyze the myelin proteome after remyelination and during aging. METHODS: Long-term locomotor performance of previously cuprizone-treated animals was monitored using the motor skill sequence (MOSS). Quantitative analysis of myelin proteome and histopathological analysis of neuronal/axonal integrity was performed after successful remyelination. Histopathological findings observed in experimental chronic remyelinated lesions were verified in chronic remyelinated lesions from multiple sclerosis (MS) patients. RESULTS: Following cessation of cuprizone treatment, animals showed an initial recovery of locomotor performance. However, long after remyelination was completed (approximately 6 months after the last demyelinating episode), locomotor performance again declined in remyelinated animals as compared to age-matched controls. This functional decline was accompanied by brain atrophy and callosal axonal loss. Furthermore, the number of acutely damaged amyloid precursor protein-positive (APP+) axons was still significantly elevated in long-term remyelinated animals as compared to age-matched controls. Confocal analysis revealed that a substantial proportion of these APP+ spheroids were ensheathed by myelin, a finding that was confirmed in the chronic remyelinated lesions of MS patients. Moreover, quantitative analysis of myelin proteome revealed that remyelinated myelin displays alterations in composition that are in some aspects similar to the myelin of older animals. INTERPRETATION: We propose that even after completed remyelination, axonal degeneration continues to progress at a low level, accumulating over time, and that once a threshold is passed axonal degeneration can become functionally apparent in the long-term. The presented model thus mimics some of the aspects of axonal degeneration in chronic progressive MS.

Toll-like receptor activation reveals developmental reorganization and unmasks responder subsets of microglia.

The sentinel and immune functions of microglia require rapid and appropriate reactions to infection and damage. Their Toll-like receptors (TLRs) sense both as threats. However, whether activated microglia mount uniform responses or whether subsets conduct selective tasks is unknown. We demonstrate that murine microglia reorganize their responses to TLR activations postnatally and that this process comes with a maturation of TLR4-organized functions. Although induction of MHCI for antigen presentation remains as a pan-populational feature, synthesis of TNFα becomes restricted to a subset, even within adult central nervous system regions. Response heterogeneity is evident ex vivo, in situ, and in vivo, but is not limited to TNFα production or to TLR-triggered functions. Also, clearance activities for myelin under physiological and pathophysiological conditions, IFNγ-enforced upregulation of MHCII, or challenged inductions of other proinflammatory factors reveal dissimilar microglial contributions. Notably, response heterogeneity is also confirmed in human brain tissue. Our findings suggest that microglia divide by constitutive and inducible capacities. Privileged production of inflammatory mediators assigns a master control to subsets. Sequestration of clearance of endogenous material versus antigen presentation in exclusive compartments can separate potentially interfering functions. Finally, subsets rather than a uniform population of microglia may assemble the reactive phenotypes in responses during infection, injury, and rebuilding, warranting consideration in experimental manipulation and therapeutic strategies.

Phagocyte-mediated synapse removal in cortical neuroinflammation is promoted by local calcium accumulation.

Cortical pathology contributes to chronic cognitive impairment of patients suffering from the neuroinflammatory disease multiple sclerosis (MS). How such gray matter inflammation affects neuronal structure and function is not well understood. In the present study, we use functional and structural in vivo imaging in a mouse model of cortical MS to demonstrate that bouts of cortical inflammation disrupt cortical circuit activity coincident with a widespread, but transient, loss of dendritic spines. Spines destined for removal show local calcium accumulations and are subsequently removed by invading macrophages or activated microglia. Targeting phagocyte activation with a new antagonist of the colony-stimulating factor 1 receptor prevents cortical synapse loss. Overall, our study identifies synapse loss as a key pathological feature of inflammatory gray matter lesions that is amenable to immunomodulatory therapy.

Propagation of spreading depression inversely correlates with cortical myelin content.

OBJECTIVE: Cortical myelin can be severely affected in patients with demyelinating disorders of the central nervous system. However, the functional implication of cortical demyelination remains elusive. In this study, we investigated whether cortical myelin influences cortical spreading depression (CSD). METHODS: CSD measurements were performed in rodent models of toxic and autoimmune induced cortical demyelination, in neuregulin-1 type I transgenic mice displaying cortical hypermyelination, and in glial fibrillary acidic protein-transgenic mice exhibiting pronounced astrogliosis. RESULTS: Cortical demyelination, but not astrogliosis or inflammation per se, was associated with accelerated CSD. In contrast, hypermyelinated neuregulin-1 type I transgenic mice displayed a decelerated CSD propagation. INTERPRETATION: Cortical myelin may be crucially involved in the stabilization and buffering of extracellular ion content that is decisive for CSD propagation velocity and cortical excitability, respectively. Our data thus indicate that cortical involvement in human demyelinating diseases may lead to relevant alterations of cortical function.