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Mesenchymal stem cells (MSC) are increasingly being studied as a source of cell therapy for neurodegenerative diseases, and several groups have reported their beneficial effects on Alzheimer's disease (AD). In this study using AD model mice (APdE9), we found that transplantation of MSC via the tail vein improved spatial memory in the Morris water maze test. Using electron paramagnetic resonance imaging to evaluate the in vivo redox state of the brain, we found that MSC transplantation suppressed oxidative stress in AD model mice. To elucidate how MSC treatment ameliorates oxidative stress, we focused on amyloid-␤ (A␤) pathology and microglial function. MSC transplantation reduced A␤ deposition in the cortex and hippocampus. Transplantation of MSC also decreased Iba1-positive area in the cortex and reduced activated ameboid shaped microglia. On the other hand, MSC transplantation accelerated accumulation of microglia around A␤ deposits and prompted microglial A␤ uptake and clearance as shown by higher frequency of A␤-containing microglia. MSC transplantation also increased CD14-positive microglia in vivo, which play a critical role in A␤ uptake. To confirm the effects of MSC on microglia, we co-cultured the mouse microglial cell line MG6 with MSC. Co-culture with MSC enhanced A␤ uptake by MG6 cells accompanied by upregulation of CD14 expression. Additionally, co-culture of MG6 cells with MSC induced microglial phenotype switching from M1 to M2 and suppressed production of proinflammatory cytokines. These data indicate that MSC treatment has the potential to ameliorate oxidative stress through modification of microglial functions, thereby improving A␤ pathology in AD model mice.

To explore a novel therapy against Parkinson's disease through enhancement of α7 nicotinic acetylcholine receptor (nAChR), we evaluated the neuroprotective effects of 3-[(2,4-dimethoxy)benzylidene]-anabaseine dihydrochloride (DMXBA; GTS-21), a functionally selective α7 nAChR agonist, in a rat 6-hydroxydopamine (6-OHDA)-induced hemiparkinsonian model. Microinjection of 6-OHDA into the nigrostriatal pathway of rats destroys dopaminergic neurons selectively. DMXBA dose dependently inhibited methamphetamine-stimulated rotational behavior and dopaminergic neuronal loss induced by 6-OHDA. The protective effects were abolished by methyllycaconitine citrate salt hydrate, an α7 nAChR antagonist. Immunohistochemical study confirmed abundant α7 nAChR expression in the cytoplasm of dopaminergic neurons. These results indicate that DMXBA prevented 6-OHDA-induced dopaminergic neuronal loss through stimulating α7 nAChR in dopaminergic neurons. Injection of 6-OHDA elevated immunoreactivities to glial markers such as ionized calcium binding adaptor molecule 1, CD68, and glial fibrillary acidic protein in the substantia nigra pars compacta of rats. In contrast, these immunoreactivities were markedly inhibited by comicroinjection of DMXBA. Microglia also expressed α7 nAChR in both resting and activated states. Hence, we hypothesize that DMXBA simultaneously affects microglia and dopaminergic neurons and that both actions lead to dopaminergic neuroprotection. The findings that DMXBA attenuates 6-OHDA-induced dopaminergic neurodegeneration and glial activation in a rat model of Parkinson's disease raisethe possibility that DMXBA could be a novel therapeutic compound to prevent Parkinson's disease development.

Parkinson's disease (PD) is characterized by relatively selective degeneration of dopaminergic neurons in the substantia nigra and loss of dopamine in the striatum. More than 50 epidemiological studies confirmed the low incidence of PD in smokers. Examining the distribution of subtypes of nicotinic acetylcholine receptors (nAChRs) in dopaminergic neurons of nigrostriatal system and its change in PD patients is quite important to elucidate possible neuroprotective cascade triggered by nicotine. Evidences of nAChR-mediated protection against neurotoxicity induced by rotenone, 6- hydroxydopamine (6-OHDA), and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are briefly reviewed. In rotenone- and 6-OHDA-induced PD models, nAChR-mediated neuroprotection was blocked not only by α4β2 but also by α7 nAChR antagonists. The survival signal transduction, α7 nAChR-Src family-PI3K-Akt/PKB cascade and subsequent upregulation of Bcl-2, would lead to neuroprotection. These findings suggest that nAChR-mediated neuroprotection is achieved through subtypes of nAChRs and common signal cascades. An early diagnosis and protective therapy with specific nAChR modulations could be effective in delaying the progression of PD.