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Adult neurogenesis in the hippocampal subgranular zone (SGZ) is involved in learning and memory throughout life but declines with aging. Mice lacking the CD44 transmembrane receptor for the glycosaminoglycan hyaluronan (HA) demonstrate a number of neurological disturbances including hippocampal memory deficits, implicating CD44 in the processes underlying hippocampal memory encoding, storage, or retrieval. Here, we found that HA and CD44 play important roles in regulating adult neurogenesis, and we provide evidence that HA contributes to age-related reductions in neural stem cell (NSC) expansion and differentiation in the hippocampus. CD44-expressing NSCs isolated from the mouse SGZ are self-renewing and capable of differentiating into neurons, astrocytes, and oligodendrocytes. Mice lacking CD44 demonstrate increases in NSC proliferation in the SGZ. This increased proliferation is also observed in NSCs grown in vitro, suggesting that CD44 functions to regulate NSC proliferation in a cell-autonomous manner. HA is synthesized by NSCs and increases in the SGZ with aging. Treating wild type but not CD44-null NSCs with HA inhibits NSC proliferation. HA digestion in wild type NSC cultures or in the SGZ induces increased NSC proliferation, and CD44-null as well as HA-disrupted wild type NSCs demonstrate delayed neuronal differentiation. HA therefore signals through CD44 to regulate NSC quiescence and differentiation, and HA accumulation in the SGZ may contribute to reductions in neurogenesis that are linked to age-related decline in spatial memory.

The Olig2 basic-helix-loop-helix transcription factor promotes oligodendrocyte specification in early neural progenitor cells (NPCs), including radial glial cells, in part by recruiting SWI/SNF chromatin remodeling complexes to the enhancers of genes involved in oligodendrocyte differentiation. How Olig2 expression is regulated during oligodendrogliogenesis is not clear. Here, we find that the Brg1 subunit of SWI/SNF complexes interacts with a proximal Olig2 promoter and represses Olig2 transcription in the mouse cortex at E14, when oligodendrocyte progenitors (OPCs) are not yet found in this location. Brg1 does not interact with the Olig2 promoter in the E14 ganglionic eminence, where NPCs differentiate into Olig2-positive OPCs. Consistent with these findings, Brg1-null NPCs demonstrate precocious expression of Olig2 in the cortex. However, these cells fail to differentiate into OPCs. We further find that Brg1 is necessary for neuroepithelial-to-radial glial cell transition, but not neuronal differentiation despite a reduction in expression of the pro-neural transcription factor Pax6. Collectively, these and earlier findings support a model whereby Brg1 promotes neurogenic radial glial progenitor cell specification but is dispensable for neuronal differentiation. Concurrently, Brg1 represses Olig2 expression and the specification of OPCs, but is required for OPC differentiation and oligodendrocyte maturation.

CD44 is a transmembrane receptor for the glycosaminoglycan hyaluronan, a component of the extracellular matrix. CD44 is expressed by neural stem/progenitor cells, astrocytes, and some neurons but its function in the central nervous system is unknown. To determine the role of CD44 in brain function, we behaviorally analyzed CD44-null (KO) and wild-type (WT) mice. KO mice showed increased activity levels in the light-dark test and a trend towards increased activity in the open field. In addition, KO mice showed impaired hippocampus-dependent spatial memory retention in the probe trial following the first hidden-platform training day in the Morris water maze: WT mice showed spatial memory retention and spent more time in the target quadrant than any other quadrant, while KO mice did not. Although there were no genotype differences in swim speeds during the water maze training sessions with the visible or hidden platform, sensorimotor impairments were seen in other behavioral tests. In the inclined screen and balance beam tests, KO mice moved less than WT mice. In the wire hang test, KO mice also fell off of the wire faster than WT mice. In contrast, there was no genotype difference when emotional learning and memory were assessed in the passive avoidance test. These data support an important role for CD44 in locomotor and sensorimotor functions, and in spatial memory retention.

Demyelination occurs following numerous insults to the CNS and is the hallmark of multiple sclerosis (MS), causing conduction deficits that compromise motor, sensory and cognitive functions. Some recovery of function is associated with the recruitment of oligodendrocyte progenitor cells (OPCs) to demyelinating lesions, generating oligodendrocytes (OLs) that remyelinate spared axons.1 However, OPCs often accumulate in chronically demyelinated lesions and fail to give rise to myelinating OLs.2–7 Strategies that promote OPC maturation within demyelinating lesions therefore have the potential to promote remyelination and functional recovery in affected individuals. Multiple signals within the microenvironments of demyelinating lesions contribute to the failure of OPC maturation and remyelination.8, 9 We previously found that high molecular weight (HMW) forms of the glycosaminoglycan hyaluronan (HA) are among these signals. HA is synthesized by transmembrane synthases and is composed of multiple disaccharide units of glucuronic acid and N-acetylglucosamine. HA molecules range in size from ≤2.5×105 Da to ≥4×106 Da. Different molecular weight forms of HA have distinct functions in the nervous system including the regulation of cell motility, growth, and differentiation.10, 11 We found that HA accumulates coincident with astrogliosis in demyelinating MS lesions,12 traumatic spinal cord injuries,13 perinatal white matter injuries,14 lesions associated with vascular cognitive impairment15 and during normal aging.16 HA blocks OPC maturation and remyelination in lysolecithin-induced demyelinating lesions, suggesting that HMW HA accumulation contributes to remyelination failure.12 We reasoned that one strategy to promote remyelination within demyelinating lesions is to degrade accumulated HMW HA using hyaluronidases. During inflammatory responses outside the central nervous system (CNS), activated fibroblasts or other cells secrete hyaluronidases that degrade HA, generating HA digestion products that act as immune regulators.17 Reactive oxygen species at sites of inflammation further promote this degradation.18 Although hyaluronidases are expressed in the CNS,19, 20 it is unclear if HA is similarly degraded at extracellular sites of CNS inflammation where astrocytes are the principle source of HA.21 Paradoxically, we and others have found that the digestion of HMW HA by some hyaluronidases prevents OPC maturation. A recent study found that treatment of OPCs with HA degraded by a combination of hyaluronidases and β-glucuronidase blocked OPC maturation in vitro through a mechanism involving toll-like receptor-2 (TLR2).20 This study also demonstrated that lower MW forms of HA accumulate in MS lesions, that OPCs in vitro express multiple hyaluronidases, and that a broad spectrum hyaluronidase inhibitor can promote OPC maturation in vitro. The focus of the present study was to determine if hyaluronidases are expressed in human and rodent demyelinating lesions; if specific hyaluronidases alone can block OPC maturation; and if blocking hyaluronidase activity can promote remyelination in vivo. Here, we report that an extracellular hyaluronidase, called PH20, is elevated in rodent and human demyelinating lesions. We demonstrate that PH20 expression is elevated in OPCs and astrocytes in chronic demyelinated MS lesions and in acute lesions in mice with experimental autoimmune encephalomyelitis (EAE). Elevated expression of PH20, but not other hyaluronidases, is sufficient to block OPC maturation. Digestion products of PH20, but not those generated by another hyaluronidase, potently block remyelination in vivo. Furthermore, inhibiting HA synthesis does not influence OPC maturation but blocking hyaluronidase activity promotes OPC maturation and remyelination. Collectively, these data suggest that PH20 is a potential therapeutic target to promote remyelination.

The CD44 transmembrane glycoprotein family has been implicated in cell-cell adhesion and cell signaling in response to components of the extracellular matrix but its role in the nervous system is not understood. CD44 proteins are elevated in Schwann cells and oligodendrocytes following nervous system insults, in inflammatory demyelinating lesions, and in tumors. Here, we tested the hypothesis that elevated CD44 expression influences Schwann cell and oligodendrocyte functions by generating transgenic mice that express CD44 under the control of the 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase) promoter. These mice failed to develop peripheral nerve or CNS tumors. However, they did develop severe tremors that were associated with CNS dysmyelination and progressive demyelination. Loss of CNS myelin was not due to alterations in early oligodendrocyte precursor differentiation, proliferation, or survival. Myelination in the PNS appeared normal. In no instance was there any evidence of an inflammatory response that could account for the loss of CNS myelin. These findings suggest that CNPase-CD44 mice are a novel model for noninflammatory progressive demyelinating disease and support a potential role for CD44 proteins expressed by glial cells in promoting demyelination.

The extravasation of lymphocytes across central nervous system (CNS) vascular endothelium is a key step in inflammatory demyelinating diseases including multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE). The glycosaminoglycan hyaluronan (HA) and its receptor, CD44, have been implicated in this process but their precise roles are unclear. We find that CD44(-/-) mice have a delayed onset of EAE compared with wild type animals. Using an in vitro lymphocyte rolling assay, we find that fewer slow rolling (

International audience; Epigenetic alterations in cell-type-specific gene expression control the transition of neural stem cells (NSCs) from predominantly neurogenic to predominantly gliogenic phases of differentiation, but how this switch occurs is unclear. Here, we show that brahma-related gene 1 (Brg1), an ATP-dependent chromatin remodeling factor, is required for the repression of neuronal commitment and the maintenance of NSCs in a state that permits them to respond to gliogenic signals. Loss of Brg1 in NSCs in conditional brg1 mutant mice results in precocious neuronal differentiation, such that cells in the ventricular zone differentiate into post-mitotic neurons before the onset of gliogenesis. As a result, there is a dramatic failure of astrocyte and oligodendrocyte differentiation in these animals. The ablation of brg1 in gliogenic progenitors in vitro also prevents growth-factor-induced astrocyte differentiation. Furthermore, proteins implicated in the maintenance of stem cells, including Sox1, Pax6 and Musashi-1, are dramatically reduced in the ventricular zones of brg1 mutant mice. We conclude that Brg1 is required to repress neuronal differentiation in NSCs as a means of permitting glial cell differentiation in response to gliogenic signals, suggesting that Brg1 regulates the switch from neurogenesis to gliogenesis.