Dynamics of oligodendrocyte generation and myelination in the human brain.

The myelination of axons by oligodendrocytes has been suggested to be modulated by experience, which could mediate neural plasticity by optimizing the performance of the circuitry. We have assessed the dynamics of oligodendrocyte generation and myelination in the human brain. The number of oligodendrocytes in the corpus callosum is established in childhood and remains stable after that. Analysis of the integration of nuclear bomb test-derived (14)C revealed that myelin is exchanged at a high rate, whereas the oligodendrocyte population in white matter is remarkably stable in humans, with an annual exchange of 1/300 oligodendrocytes. We conclude that oligodendrocyte turnover contributes minimally to myelin modulation in human white matter and that this instead may be carried out by mature oligodendrocytes, which may facilitate rapid neural plasticity.

Publisher Correction: Dynamics of oligodendrocyte generation in multiple sclerosis.

In this Letter, the vertical error bars were missing from Fig. 3b and 3c. This figure has been corrected online.

Dynamics of oligodendrocyte generation in multiple sclerosis.

Oligodendrocytes wrap nerve fibres in the central nervous system with layers of specialized cell membrane to form myelin sheaths1. Myelin is destroyed by the immune system in multiple sclerosis, but myelin is thought to regenerate and neurological function can be recovered. In animal models of demyelinating disease, myelin is regenerated by newly generated oligodendrocytes, and remaining mature oligodendrocytes do not seem to contribute to this process2-4. Given the major differences in the dynamics of oligodendrocyte generation and adaptive myelination between rodents and humans5-9, it is not clear how well experimental animal models reflect the situation in multiple sclerosis. Here, by measuring the integration of 14C derived from nuclear testing in genomic DNA10, we assess the dynamics of oligodendrocyte generation in patients with multiple sclerosis. The generation of new oligodendrocytes was increased several-fold in normal-appearing white matter in a subset of individuals with very aggressive multiple sclerosis, but not in most subjects with the disease, demonstrating an inherent potential to substantially increase oligodendrocyte generation that fails in most patients. Oligodendrocytes in shadow plaques-thinly myelinated lesions that are thought to represent remyelinated areas-were old in patients with multiple sclerosis. The absence of new oligodendrocytes in shadow plaques suggests that remyelination of lesions occurs transiently or not at all, or that myelin is regenerated by pre-existing, and not new, oligodendrocytes in multiple sclerosis. We report unexpected oligodendrocyte generation dynamics in multiple sclerosis, and this should guide the use of current, and the development of new, therapies.

Efficient reprogramming of adult neural stem cells to monocytes by ectopic expression of a single gene.

Neural stem cells have a broad differentiation repertoire during embryonic development and can be reprogrammed to pluripotency comparatively easily. We report that adult neural stem cells can be reprogrammed at very high efficiency to monocytes, a differentiated fate of an unrelated somatic lineage, by ectopic expression of the Ets transcription factor PU.1. The reprogrammed cells display a marker profile and functional characteristics of monocytes and integrate into tissues after transplantation. The failure to reprogram lineage-committed neural cells to monocytes with PU.1 suggests that neural stem cells are uniquely amenable to reprogramming.

The age of olfactory bulb neurons in humans.

Continuous turnover of neurons in the olfactory bulb is implicated in several key aspects of olfaction. There is a dramatic decline postnatally in the number of migratory neuroblasts en route to the olfactory bulb in humans, and it has been unclear to what extent the small number of neuroblasts at later stages contributes new neurons to the olfactory bulb. We have assessed the age of olfactory bulb neurons in humans by measuring the levels of nuclear bomb test-derived (14)C in genomic DNA. We report that (14)C concentrations correspond to the atmospheric levels at the time of birth of the individuals, establishing that there is very limited, if any, postnatal neurogenesis in the human olfactory bulb. This identifies a fundamental difference in the plasticity of the human brain compared to other mammals.

Forebrain ependymal cells are Notch-dependent and generate neuroblasts and astrocytes after stroke.

Neurons are continuously generated from stem cells in discrete regions in the adult mammalian brain. We found that ependymal cells lining the lateral ventricles were quiescent and did not contribute to adult neurogenesis under normal conditions in mice but instead gave rise to neuroblasts and astrocytes in response to stroke. Ependymal cell quiescence was actively maintained by canonical Notch signaling. Inhibition of this pathway in uninjured animals allowed ependymal cells to enter the cell cycle and produce olfactory bulb neurons, whereas forced Notch signaling was sufficient to block the ependymal cell response to stroke. Ependymal cells were depleted by stroke and failed to self-renew sufficiently to maintain their own population. Thus, although ependymal cells act as primary cells in the neural lineage to produce neurons and glial cells after stroke, they do not fulfill defining criteria for stem cells under these conditions and instead serve as a reservoir that is recruited by injury.