Lipofuscin is an autofluorescent (AF) pigment formed by lipids and misfolded proteins, which accumulates in postmitotic cells with advanced age. Here, we immunophenotyped microglia in the brain of old C57BL/6 mice (>18 months old) and demonstrate that in comparison to young mice, one-third of old microglia are AF, characterized by profound changes in lipid and iron content, phagocytic activity, and oxidative stress. Pharmacological depletion of microglia in old mice eliminated the AF microglia following repopulation and reversed microglial dysfunction. Age-related neurological deficits and neurodegeneration after traumatic brain injury (TBI) were attenuated in old mice lacking AF microglia. Furthermore, increased phagocytic activity, lysosomal burden, and lipid accumulation in microglia persisted for up to 1 year after TBI, were modified by APOE4 genotype, and chronically driven by phagocyte-mediated oxidative stress. Thus, AF may reflect a pathological state in aging microglia associated with increased phagocytosis of neurons and myelin and inflammatory neurodegeneration that can be further accelerated by TBI.

Traumatic brain injury (TBI) is a progressive and complex pathological condition that results in multiple adverse consequences, including impaired learning and memory. Transplantation of mesenchymal stem cells (MSCs) has produced limited benefits in experimental TBI models. Fibroblast growth factor 21 (FGF21) is a novel metabolic regulator that has neuroprotective effects, promotes remyelination, enhances angiogenesis, and elongates astrocytic processes. In this study, MSCs were genetically engineered to overexpress FGF21 in order to improve their efficacy in TBI. MSCs overexpressing FGF21 (MSC-FGF21) were transplanted to mouse brain by intracerebroventricular injection 24 h after TBI was induced by controlled cortical impact (CCI). Hippocampus-dependent spatial learning and memory, assessed by the Morris water maze test, was markedly decreased 3-4 weeks after TBI, a deficit that was robustly recovered by treatment with MSC-FGF21, but not MSC-mCherry control. Hippocampus-independent learning and memory, assessed by the novel object recognition test, was also impaired; these effects were blocked by treatment with both MSC-FGF21 and MSC-mCherry control. FGF21 protein levels in the ipsilateral hippocampus were drastically reduced 4 weeks post-TBI, a loss that was restored by treatment with MSC-FGF21, but not MSC-mCherry. MSC-FGF21 treatment also partially restored TBIinduced deficits in neurogenesis and maturation of immature hippocampal neurons, whereas MSC-mCherry was less effective. Finally, MSC-FGF21 treatment also normalized TBI-induced impairments in dendritic arborization of hippocampal neurons. Taken together, the results indicate that MSC-FGF21 treatment significantly improved TBI-induced spatial memory deficits, impaired hippocampal neurogenesis, and abnormal dendritic morphology. Future clinical investigations using MSC-FGF21 to improve post-TBI outcomes are warranted.

Mesenchymal stem cells (MSCs) are emerging as a potential therapeutic intervention for brain injury due to their neuroprotective effects and safe profile. However, the homing ability of MSCs to injury sites still needs to be improved. Fibroblast Growth Factor 21 (FGF21) was recently reported to enhance cells migration in different cells type. In this study, we investigated whether MSCs that overexpressing FGF21 (MSC-FGF21) could exhibit enhanced homing efficacy in brain injury. We used novel Molday IONEverGreen™ (MIEG) as cell labeling probe that enables a non-invasive, high-sensitive and real-time MRI tracking. Using a mouse model of traumatic brain injury (TBI), MIEG labeled MSCs were transplanted into the contralateral lateral ventricle followed by real-time MRI tracking. FGF21 retained MSC abilities of proliferation and morphology. MSC-FGF21 showed significantly greater migration in transwell assay compared to control MSC. MIEG labeling showed no effects on MSCs’ viability, proliferation and differentiation. Magnetic resonance imaging (MRI) revealed that FGF21 significantly enhances the homing of MSC toward injury site. Histological analysis further confirmed the MRI findings. Taken together, these results show that FGF21 overexpression and MIEG labeling of MSC enhances their homing abilities and enables non-invasive real time tracking of the transplanted cells, provides a promising approach for MSC based therapy and tracking in TBI.