In vivo monitoring of stem cells after grafting is essential for a better understanding of their migrational dynamics and differentiation processes and of their regeneration potential. Migration of endogenous or grafted stem cells and neurons has been described in vertebrate brain, both under normal conditions from the subventricular zone along the rostral migratory stream and under pathophysiological conditions, such as degeneration or focal cerebral ischemia. Those studies, however, relied on invasive analysis of brain sections in combination with appropriate staining techniques. Here, we demonstrate the observation of cell migration under in vivo conditions, allowing the monitoring of the cell dynamics within individual animals, and for a prolonged time. Embryonic stem (ES) cells, constitutively expressing the GFP, were labeled by a lipofection procedure with a MRI contrast agent and implanted into rat brains. Focal cerebral ischemia had been induced 2 weeks before implantation of ES cells into the healthy, contralateral hemisphere. MRI at 78-m isotropic spatial resolution permitted the observation of the implanted cells with high contrast against the host tissue, and was confirmed by GFP registration. During 3 weeks, cells migrated along the corpus callosum to the ventricular walls, and massively populated the borderzone of the damaged brain tissue on the hemisphere opposite to the implantation sites. Our results indicate that ES cells have high migrational dynamics, targeted to the cerebral lesion area. The imaging approach is ideally suited for the noninvasive observation of cell migration, engraftment, and morphological differentiation at high spatial and temporal resolution.embryonic stem cells ͉ cerebral ischemia ͉ cell labeling S everal studies have been able to demonstrate the migrational capacity of endogenous stem or progenitor cells in rat and mouse brains during normal (1, 2) and pathophysiological conditions (3, 4). The therapeutical potential of stem cell grafting has recently been studied in various pathological conditions of the brain showing extensive cell migration after implantation. However, all investigations so far have required the invasive analysis of brain sections postmortem in various groups of animals for different survival periods. A recent investigation described the detection of labeled cells, injected into rats, but reported no specific cell migration in the in vivo MRI data (5). All other studies have investigated the potential of MRI to detect pretreated cells (5-7) by registering the MRI data ex vivo, thus permitting observation of only one time point. In the present investigation we demonstrate sufficient spatial and temporal resolution of experimental MRI at high fields to allow longitudinal studies on individual animals after stem cell implantation into the brain. We have investigated the spatial dynamics of implanted embryonic stem (ES) cells and demonstrated their high migrational potential from the implantation site in the normal brain hemisphere toward the ischemic le...

Impairment of insulin signaling in the brain has been linked to neurodegenerative diseases. To test the hypothesis that neuronal insulin resistance contributes to defects in neuronal function, we have performed a detailed analysis of brain͞neuron-specific insulin receptor knockout (NIRKO) mice. We find that NIRKO mice exhibit a complete loss of insulin-mediated activation of phosphatidylinositol 3-kinase and inhibition of neuronal apoptosis. In intact animals, this loss results in markedly reduced phosphorylation of Akt and GSK3␤, leading to substantially increased phosphorylation of the microtubule-associated protein Tau, a hallmark of neurodegenerative diseases. Nevertheless, these animals exhibit no alteration in neuronal proliferation͞survival, memory, or basal brain glucose metabolism. Thus, lack of insulin signaling in the brain may lead to changes in Akt and GSK3␤ activity and Tau hyperphosphorylation but must interact with other mechanisms for development of Alzheimer's disease.