Stroke incidence increases with age and this has been attributed to vascular factors. We show here that CNS white matter (WM) is intrinsically more vulnerable to ischemic injury in older animals and that the mechanisms of WM injury change as a function of age. The mouse optic nerve was used to study WM function. WM function in older animals (12 months) was not protected from ischemic injury by removal of extracellular Ca 2ϩ or by blockade of reverse Na ϩ /Ca 2ϩ exchange, as is the case with young adults. Ischemic WM injury in older mice is predominately mediated by glutamate release and activation of AMPA/kainate-type glutamate receptors. Glutamate release, attributable to reverse glutamate transport, occurs earlier and is more robust in older mice that show greater expression of the glutamate transporter. The observation that WM vulnerability to ischemic injury is age dependent has possible implications for the pathogenesis of other age-related CNS conditions.

Ischemic preconditioning (IPC) is a robust neuroprotective phenomenon whereby brief ischemic exposure confers tolerance to a subsequent ischemic challenge. IPC has not been studied selectively in CNS white matter (WM), although stroke frequently involves WM. We determined whether IPC is present in WM and, if so, its mechanism. We delivered a brief in vivo preconditioning ischemic insult (unilateral common carotid artery ligation) to 12-to 14-week-old mice and determined WM ischemic vulnerability [oxygen-glucose deprivation (OGD)] 72 h later, using acutely isolated optic nerves (CNS WM tracts) from the preconditioned (ipsilateral) and control (contralateral) hemispheres. Functional and structural recovery was assessed by quantitative measurement of compound action potentials (CAPs) and immunofluorescent microscopy. Preconditioned mouse optic nerves (MONs) showed better functional recovery after OGD than the non-preconditioned MONs (31 Ϯ 3 vs 17 Ϯ 3% normalized CAP area, p Ͻ 0.01). Preconditioned MONs also showed improved axon integrity and reduced oligodendrocyte injury compared with non-preconditioned MONs. Toll-like receptor-4 (TLR4) and type 1 interferon receptor (IFNAR1), key receptors in innate immune response, are implicated in gray matter preconditioning. Strikingly, IPC-mediated WM protection was abolished in both TLR4 Ϫ/Ϫ and IFNAR1 Ϫ/Ϫ mice. In addition, IPC-mediated protection in WM was also abolished in IFNAR1 fl/fl LysM cre , but not in IFNAR1 fl/fl control, mice. These findings demonstrated for the first time that IPC was robust in WM, the phenomenon being intrinsic to WM itself. Furthermore, WM IPC was dependent on innate immune cell signaling pathways. Finally, these data demonstrated that microglial-specific expression of IFNAR1 plays an indispensable role in WM IPC.

A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. Aglycemia was produced by switching to 0 glucose ACSF. Supra-maximal compound action potentials (CAPs) were elicited using suction electrodes and axon function was quantified as the area under the CAP. Amino acid release was measured using HPLC. Extracellular [lactate] was measured using an enzyme electrode.Results: About 50% of MON axons were injured after 60 min of aglycemia (90%

Matrix metalloproteinases (MMPs) and their specific inhibitors the TIMPs play significant roles in angiogenesis. We investigated how the expression of specific MMPs and TIMPs by human microvascular endothelial cells (hmECs) was modulated by culture of the cells in 3-dimensional (3D) type I collagen gels versus 2-dimensional (2D) collagen-coated surfaces. By reverse-transcription polymerase chain reaction (RT-PCR), levels of mRNA for MMPs-1, -2, and -13, MT1-MMP, and TIMPs-1 and -2 were similar in 2D versus 3D cultures. By Western blot assay, TIMP-1 and proMMP-1 were present and were expressed similarly in media from 2D versus 3D cultures, whereas active MMPs-1, -9, and -13 were not detected. Active MMP-13 was present in cell lysates (CL) and was increased in lysates from 3D cultures relative to 2D cultures. Relative to 2D cultures, CL and media from 3D cultures exhibited a decrease in expression of TIMP-2 and an increased conversion of proMMP-2 and proMT1-MMP to active or processed forms. The MMP inhibitor GM6001 interfered with the migration of hmECs in 3D cultures, but not in 2D cultures. Addition of active MMP-1 or blocking antibodies to TIMP-1 did not affect the migration of hmECs in 3D collagen. Migration in 3D collagen was decreased by TIMP-2 (an inhibitor of MT1-MMP), but not by TIMP-1 (a poor inhibitor of MT1-MMP, but an efficient inhibitor of MMP-2). Collectively, our data indicate that MT1-MMP contributes significantly to the movement of hmECs through 3D collagen, in contrast to secretory-type MMPs-1, -2, -9, and -13, which are not critical for this movement.

The mammalian central nervous system (CNS) is generally believed to be completely dependent on the presence of oxygen (O 2 ) to maintain energy levels necessary for excitability. However, previous studies on CNS white matter (WM) have shown that a large subset of CNS-myelinated axons of mice aged 4 to 6 weeks remains excitable in the absence of O 2 . We investigated whether this surprising WM tolerance to anoxia varied with age. Acutely isolated mouse optic nerve (MON), a purely myelinated WM tract, was studied electrophysiologically. Excitability in the MONs from 1-month-, 4-month-, and 8-month-old mice was assessed quantitatively as the area under the supramaximal compound action potential (CAP). Anoxia-resistant WM function declined with age. After 60 minutes of anoxia, B23% of the CAP remained in 1-month-old mice, 8% in 4-month-old mice, and B0 in the 8-month-old group. Our results indicated that although some CNS axons function anaerobically in young adult animals, they lose this ability in later adulthood. This finding may help explain the clinical impression that favorable outcome after stroke and other brain injuries declines with age.