White matter injury and microglia/macrophage polarization are strongly linked with age-related long-term deficits in neurological function after stroke

Abstract

Most of the successes in experimental models of stroke have not translated well to the clinic. One potential reason for this failure is that stroke mainly afflicts the elderly and the majority of experimental stroke studies rely on data gathered from young adult animals. Therefore, in the present study we established a reliable, reproducible model of stroke with low mortality in aged (18 month) male mice and contrasted their pathophysiological changes with those in young (2 month) animals. To this end, mice were subjected to permanent tandem occlusion of the left distal middle cerebral artery (dMCAO) with ipsilateral common carotid artery occlusion (CCAO). Cerebral blood flow (CBF) was evaluated repeatedly during and after stroke. Reduction of CBF was more dramatic and sustained in aged mice. Aged mice exhibited more severe long-term sensorimotor deficits, as manifested by deterioration of performance in the Rotarod and hanging wire tests up to 35d after stroke. Aged mice also exhibited significantly worse long-term cognitive deficits after stroke, as measured by the Morris water maze test. Consistent with these behavioral observations, brain infarct size and neuronal tissue loss after dMCAO were significantly larger in aged mice at 2d and 14d, respectively. The young versus aged difference in neuronal tissue loss, however, did not persist until 35d after dMCAO. In contrast to the transient difference in neuronal tissue loss, we found significant and long lasting deterioration of white matter in aged animals, as revealed by the loss of myelin basic protein (MBP) staining in the striatum at 35d after dMCAO. We further examined the expression of M1 (CD16/CD32) and M2 (CD206) markers in Iba-1+ microglia by double immunofluorescent staining. In both young and aged mice, the expression of M2 markers peaked around 7d after stroke whereas the expression of M1 markers peaked around 14d after stroke, suggesting a progressive M2-to-M1 phenotype shift in both groups. However, aged mice exhibited significantly reduced M2 polarization compared to young adults. Remarkably, we discovered a strong positive correlation between favorable neurological outcomes after dMCAO and MBP levels or the number of M2 microglia/macrophages. In conclusion, our studies suggest that the distal MCAO stroke model consistently results in ischemic brain injury with long-term behavioral deficits, and is therefore suitable for the evaluation of long-term stroke outcomes. Furthermore, aged mice exhibit deterioration of functional outcomes after stroke and this deterioration is linked to white matter damage and reductions in M2 microglia/macrophage polarization.

A Post-stroke Therapeutic Regimen with Omega-3 Polyunsaturated Fatty Acids that Promotes White Matter Integrity and Beneficial Microglial Responses after Cerebral Ischemia

“…6h–j). A similar correlation between M2 microglia and post-stroke functional recovery was previously noted in aged mice after distal MCAO [53]. In contrast, the M1 microglia correlated poorly with post-stroke sensorimotor deficits (Fig.…”

“…Ischemia/reperfusion rapidly activates microglia, which can exert different impacts on the injury and repair of gray and white matter, depending on their phenotypic polarizing state [12]. Under various pathological conditions such as ischemic stroke, traumatic brain injury or multiple sclerosis, M1 microglia/macrophages are generally considered to exacerbate oligodendrocyte cell death and destroy myelin through excessive proinflammatory responses [14, 27], whereas M2 microglia/macrophages facilitate remyelination and tissue repair [16, 36, 53]. A previous study using microglial cultures has shown that DHA and EPA both promote the M2 polarization of microglia by down-regulating M1 signature genes ( e.g.…”

Aging of cerebral white matter

“…Furthermore, axonal regrowth is also retarded in aged brains (Lamoureux et al, 2010). As a result, the aged brain suffers more severe WM atrophy and axonal degeneration, and worse long-term sensorimotor and cognitive deficits after stroke (Rosenzweig and Carmichael, 2013; Suenaga et al, 2015). …”

The Neural Cell Adhesion Molecule-Derived (NCAM)-Peptide FG Loop (FGL) Mobilizes Endogenous Neural Stem Cells and Promotes Endogenous Regenerative Capacity after Stroke

“…Our results stress the importance of assessing the subtypes of microglia and M1/M2 ratios to assess the shift between the more protective and the more detrimental microglia subpopulations, both in the infarct area and in remote degenerating areas. A shift between M1-and M2-phenotypes due to external stimuli has been described (Suenaga et al 2015;Hamzei Taj et al 2016). Our data suggest that FGL induces such a shift.…”