Failure of remyelination is largely responsible for sustained neurologic symptoms in multiple sclerosis (MS). MS lesions contain hyaluronan deposits that inhibit oligodendrocyte precursor cell (OPC) maturation. However, the mechanism behind this inhibition is unclear. We report here that Toll-like receptor 2 (TLR2) is expressed by oligodendrocytes and is up-regulated in MS lesions. Pathogenderived TLR2 agonists, but not agonists for other TLRs, inhibit OPC maturation in vitro. Hyaluronan-mediated inhibition of OPC maturation requires TLR2 and MyD88, a TLR2 adaptor molecule. Ablated expression of TLR2 also enhances remyelination in a lysolecithin animal model. Hyaluronidases expressed by OPCs degrade hyaluronan to hyaluronan oligomers, a requirement for hyaluronan/TLR2 signaling. MS lesions contain both TLR2 + oligodendrocytes and lowmolecular-weight hyaluronan, consistent with their importance to remyelination in MS. We thus have defined a mechanism controlling remyelination failure in MS where hyaluronan is degraded by hyaluronidases into hyaluronan oligomers that block OPC maturation and remyelination through TLR2-MyD88 signaling.hyaluronidase | MyD88 | innate immunity I n multiple sclerosis (MS), destruction of CNS myelin accounts for a majority of neurologic symptoms. MS patients typically exhibit a relapsing/remitting course in which relapses result from inflammation and demyelination and remissions result from resolution of inflammation and remyelination. Secondary progressive MS, which typically begins in patients after a decade of relapsing/ remitting MS, exhibits irreversible neurologic disability.Most chronic MS lesions show little if any remyelination. The failure of remyelination in MS theoretically could be the consequence of a deficiency in the number of oligodendrocyte progenitor cells (OPCs), absence of a promyelination signal, or the presence of inhibitory influences on OPCs. It therefore is of interest that the histopathology of the MS lesion has revealed the presence of OPCs and premyelinating oligodendrocytes in chronic MS lesions. The premyelinating oligodendrocytes extend processes that contact but fail to myelinate axons (1-4), thus suggesting failure of remyelination is caused by the loss of promyelination signals or the presence of inhibitory signals.Recently, the glycosaminoglycan hyaluronan was identified within MS lesions and found to inhibit OPC maturation and remyelination in an MS animal model (5). The mechanism underlying this inhibition is unknown. Because hyaluronan can function as an endogenous mammalian ligand for Toll-like receptors (TLRs) 2 and 4 in innate immune cell activation (6-9), we reasoned that TLR stimulation also may be required for hyaluronan-mediated blocking of OPC maturation.Although TLRs and the Drosophila ortholog Toll have welldescribed functions in innate immune cells (6-12), TLRs also have potent functions outside the immune system. Toll and TLR have diverse roles in axonal pathfinding, dorsoventral patterning, and cell-fate determination (13). In particul...

Summary:Iron is important for brain oxygen transport, electron transfer, neurotransmitter synthesis, and myelin production. Though iron deposition has been observed in the brain with normal aging, increased iron has also been shown in many chronic neurological disorders including Alzheimer's disease, Parkinson's disease, and multiple sclerosis. In vitro studies have demonstrated that excessive iron can lead to free radical production, which can promote neurotoxicity. However, the link between observed iron deposition and pathological processes underlying various diseases of the brain is not well understood. It is not known whether excessive in vivo iron directly contributes to tissue damage or is solely an epiphenomenon. In this article, we focus on the imaging of brain iron and the underlying physiology and metabolism relating to iron deposition. We conclude with a discussion of the potential implications of iron-related toxicity to neurotherapeutic development.

Objective Generation and differentiation of new oligodendrocytes in demyelinated white matter is the best described repair process in the adult human brain. However, remyelinating capacity falters with age in patients with multiple sclerosis. (MS). Since demyelination of cerebral cortex is extensive in brains from MS patients, we investigated the capacity of cortical lesions to remyelinate and directly compared the extent of remyelination in lesions that involve cerebral cortex and adjacent subcortical white matter. Methods Postmortem brain tissue from 22 patients with MS (age 27 to 77 years) and 6 subjects without brain disease were analyzed. Regions of cerebral cortex with reduced myelin were examined for remyelination, oligodendrocyte progenitor cells, reactive astrocytes, and molecules that inhibit remyelination. Results “New” oligodendrocytes that were actively forming myelin sheaths were identified in 30/42 remyelinated subpial cortical lesions, including lesions from three patients in their 70's. Oligodendrocyte progenitor cells were not decreased in demyelinated or remyelinated cortices when compared to adjacent normal-appearing cortex or controls. In demyelinated lesions involving cortex and adjacent white matter, the cortex showed greater remyelination, more actively remyelinating oligodendrocytes and fewer reactive astrocytes. Astrocytes in the white-matter, but not in cortical portions of these lesions, significantly up-regulate CD44, hyaluronan, and versican, molecules that form complexes that inhibit oligodendrocyte maturation and remyelination. Interpretation Endogenous remyelination of the cerebral cortex occurs in individuals with MS regardless of disease duration or chronological age of the patient. Cortical remyelination should be considered as a primary outcome measure in future clinical trials testing remyelination therapies.

Magnetic resonance imaging (MRI) has emerged as a powerful noninvasive tool to assist in the diagnosis and monitoring of multiple sclerosis (MS). In addition, investigators have used MRI metrics as supportive outcome measures to explore drug efficacy in clinical trials. Conventional MRI surrogates provide information at the macroscopic level but lack sensitivity and specificity in identifying the full extent of underlying MS pathology. They also show relatively weak relationships to clinical status such as predictive strength for clinical change. Advanced MRI techniques involving quantitative measures of diffuse damage in normal appearing (NA) white matter (WM) and gray matter (GM) may help in resolving this apparent clinical MRI paradox. T2 hypointensity has been described in the GM of patients with MS and has been linked to physical disability, cognitive dysfunction, and brain atrophy. While this T2 hypointensity is thought to represent iron deposition, this awaits pathologic confirmation. Advanced MRI measures of iron deposition such as R2, R2*, R2' relaxometry, 3T imaging and other new approaches are beginning to be applied to studies of MS and should yield interesting information. Both T1 and T2 relaxometry have a role in detecting damage in NA brain tissue that escapes detection by conventional MRI lesion measures. For example, T2 mapping may allow an assessment of myelin content in NAWM. In this review, we will focus on MRI advances in the last 10 years pertaining to T1 and T2 measures of diffuse GM and WM damage.

During mammalian development, myelin-forming oligodendrocytes are generated and axons ensheathed according to a tightly regulated sequence of events. Excess premyelinating oligodendrocytes are eliminated by apoptosis and the timing of the onset of myelination in any specific CNS region is highly reproducible. Although the developing CNS recovers more effectively than the adult CNS from similar insults, it is unknown whether early loss of oligodendrocyte lineage cells leads to long-term functional deficits. To directly assess whether the loss of oligodendrocytes during early postnatal spinal cord development impacted oligodendrogenesis, myelination, and remyelination, transgenic mouse lines were generated in which a modified caspase-9 molecule allowed spatial and temporal control of the apoptotic pathway specifically in mature, myelin basic protein expressing oligodendrocytes (MBP-iCP9). Activating apoptosis in MBP ϩ cells of the developing spinal cord during the first postnatal week inhibited myelination. This inhibition was transient, and the levels of myelination largely returned to normal after 2 weeks. Despite robust developmental plasticity, MBP-iCP9-induced oligodendrocyte apoptosis compromised the rate and extent of adult remyelination. Remyelination failure correlated with a truncated proliferative response of oligodendrocyte progenitor cells, suggesting that depleting the oligodendrocyte pool during critical developmental periods compromises the regenerative response to subsequent demyelinating lesions.

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