White matter injury following ischemic stroke is a major cause of functional disability. Injury to both myelinated axons and oligodendrocytes, the myelin producing cells in the central nervous system, occurs in experimental models of ischemic stroke. Age-related changes in white matter vulnerability to ischemia have been extensively studied and suggest that both the perinatal and the aged periods are times of increased white matter vulnerability. However, sensitivity of white matter following stroke in the juvenile brain has not been evaluated. Interestingly, the late pediatric period is an important developmental stage, as it is the time of maximal myelination. The current study demonstrates that neurons in late pediatric/juvenile striatum are vulnerable to ischemic damage, with neuronal injury being comparable in juvenile and adult mice following ischemia. By contrast, actively myelinating striatal oligodendrocytes in the juvenile brain are resistant to ischemia, whereas adult oligodendrocytes are quite sensitive. As a result, myelin sheaths are remarkably intact and axons survive well in the injured striatum of juvenile mice. In addition to relative resistance of juvenile white matter, other glial responses were very different in juvenile and adult mice following cerebral ischemia, including differences in astrogliosis, fibrosis, NG2-cell reactivity, and vascular integrity. Together, these responses lead to long-term preservation of brain parenchyma in juvenile mice, compared to severe tissue loss and scarring in adult mice. Overall, the current study suggests that equivalent ischemic insults may result in less functional deficit in children compared to adults and an environment more conducive to long-term recovery.

The incidence of stroke in children is 2.4 per 100,000 person-years and results in long-term motor and cognitive disability. In ischemic stroke, white matter (WM) is frequently injured, but is relatively understudied compared to grey matter injury. Previous research suggests that the cellular response to WM ischemic injury is different at different ages. Little is known about whether WM repair mechanisms differ in children and adults. We utilized a model of focal ischemic WM injury to determine the oligodendrocyte (OL) response to focal WM ischemic injury in juvenile and adult mice. Methods: Juvenile (21–25 days of age) versus adult (2–3 months of age) mice underwent stereotaxic injection of the potent vasoconstrictor N5-(1-iminoethyhl)-L-ornithine (L-NIO) into the lateral corpus callosum (CC). Animals were sacrificed on postoperative day 3 (acute) or 21 (chronic). Cell birth-dating was performed acutely after WM stroke with 5-ethynyl-2-deoxyuridine (EdU) injected intraperitoneally. Immunohistochemistry was performed, as well as stereology, to measure injury volume. The acute oligodendrocyte progenitor cell (OPC) proliferation and the chronic OL cell fate were determined with immunohistochemistry. Compound action potentials were measured in the CC at acute and chronic time points. Results: Acutely WM injury volume was smaller in juveniles. There was significantly greater OPC proliferation in juvenile animals (acute) compared to adults, but newly born OLs did not survive and mature into myelinating cells at chronic time points. In addition, juveniles did not have improved histological or functional recovery when compared to adults. Protecting newly born OPCs is a potential therapeutic target in children with ischemic stroke.

Shp2 is a nonreceptor protein tyrosine phosphatase that has been shown to influence neurogenesis, oligodendrogenesis, and oligodendrocyte differentiation. Furthermore, Shp2 is a known regulator of the Akt/mammalian target of rapamycin and ERK signaling pathways in multiple cellular contexts, including oligodendrocytes. Its role during later postnatal CNS development or in response to demyelination injury has not been examined. Based on the current studies, we hypothesize that Shp2 is a negative regulator of CNS myelination. Using transgenic mouse technology, we show that Shp2 is involved in oligodendrocyte differentiation and early myelination, but is not necessary for myelin maintenance. We also show that Shp2 regulates the timely differentiation of oligodendrocytes following lysolecithin-induced demyelination, although apparently normal remyelination occurs at a delayed time point. These data suggest that Shp2 is a relevant therapeutic target in demyelinating diseases such as multiple sclerosis. In the present study, we show that the protein phosphatase Shp2 is an important mediator of oligodendrocyte differentiation and myelination, both during developmental myelination as well as during myelin regeneration. We provide important insight into the signaling mechanisms regulating myelination and propose that Shp2 acts as a transient brake to the developmental myelination process. Furthermore, we show that Shp2 regulates oligodendrocyte differentiation following demyelination and therefore has important therapeutic implications in diseases such as multiple sclerosis.

Pediatric arterial ischemic stroke affects hundreds of children in the United States each year, with a majority of surviving children suffering long-term neurologic deficit. Initial studies suggest that recovery from stroke is greater among juveniles compared to neonates or adults. We have developed a middle cerebral artery occlusion (MCAO) mouse model of pediatric stroke in order to understand the cellular responses unique to the juvenile developmental time period. One strikingly understudied aspect of stroke, especially in juvenile subjects, is its impact on glia, in particular myelinating glia. Therefore, the glial responses following experimentally-induced stroke in juvenile mice were investigated. Methods: P20-25 mice were subjected to 45 min MCAO with an intraluminal filament. Animals were analyzed at 24 hr, 3, 7 and 30 days of recovery. Results: Due to the high metabolic activity of myelinating oligodendrocytes during pediatric development, we hypothesized that oligodendrocytes would be particularly sensitive to ischemia. Surprisingly, mature oligodendrocytes and gross myelin production were unaffected during the acute (24 hr and 3 d post MCAO) and subacute (7 d) recovery phases. However, during the chronic (30 d) phase, some myelin debris and signs of mild axon pathology were detected in the lateral striatum, where gliosis was most severe. GFAP immunoreactivity steadily increased in the lesioned striatum from 24 hr to 30 d following MCAO. Similarly, Iba1-positive microglia and NG2-positive cells proliferated in the lesioned area 24 hr after stroke, and remained increased after 30 d. Adult mice subject to identical ischemic insult resulted in decreased numbers of oligodendrocytes, severe axon pathology, and significantly greater tissue loss. Conclusions: Together, these results suggest that myelinating oligodendrocytes in the pediatric brain are resistant to the initial ischemic insult. Furthermore, retaining healthy oligodendrocytes after stroke could serve an important role in maintaining axon integrity and limiting long-term tissue loss after stroke. Understanding the unique cellular responses in the juvenile brain could yield valuable insight into enhancing cellular resistance and promoting recovery after ischemia.