Stem cell mediated regeneration of cerebral lesions
The following aspects are in the center of our attention:
- Using noninvasive imaging modalities, we investigate the temporo-spatial dynamics of the stem cell graft and explore the conditions for stem cell differentiation and integration into the brain.
- In this context we investigate the role of the inflammatory response on stem cell behavior after stroke to identify players of protective and destructive phases.
- To assess therapeutic success of the stem cell implantation, we investigate structural and functional deficit and improvement, using various cutting-edge MRI modalities permitting to unravel conditions for recovery of originally damaged tissue areas and of plastic reorganization. These imaging based studies are complemented by sensorimotor behaviour tests to assess therapy based progress.
Methodologically, our investigations rely heavily on longitudinal in vivo MRI , and optical imaging experiments, complemented by invasive histological and immunohistochemical staining techniques. Physiological and electrophysiological monitoring are an integral part of our protocols. A major focus is on molecular biological techniques: transgenic cells are generated to produce their own cell fate imaging reporters for in vivo imaging, both MRI and optical imaging.
(1) In vivo cell fate imaging of stem cell grafts in brain disease models
In cerebral disease models, the transplantation of stem cells has enormous potential to improve the health of the animal, resulting in reduced neurobehavioral deficits and attenuation of damage. However, the mechanism underlying the active role of the stem cell graft is currently not well understood. This is largely due to the invasiveness of analytic investigations, permitting only the analysis of one time point. For a better understanding of the regenerative process of stem cell grafts, the dynamics of the cells need to be characterized in real-time. We have developed various non-invasive strategies allowing us to monitor the whole time profile of the graft dynamics during a chronic phase following stem cell implantation. We have been the first to demonstrate stem cell migration from the graft towards the ischemic hemisphere. Using imaging reporter technology, we have now unraveled the time profile of differentiation of human neural stem cells after intra-cortical implantation in vivo in a longitudinal study. Identification of the time windows of early neuronal differentiation and of late synaptogenesis permits us to now discriminate between paracrine effects by the stem cells and their later functional integration into the tissue. By using optogenetics and DREADD concepts, we are presently investigating the role of the (late appearing) integration into cortex for the functional improvement.
(2) Structural and functional correlates of behavioral improvement after stem cell treatment
In the past, spontaneous improvement has been shown to occur after various brain lesions. However, the underlying plastic reorganization of this process has not been elucidated in many cases. Using structural and functional connectivity approaches with high field MRI, we have discriminated the intact from the lesioned fiber connection between various brain nuclei, with a particular focus on the thalamo-cortical connections (intra- and inter-hemispheric). Functional deficit and improvement is monitored with stimulus-based fMRI which allow us to design a reliable protocol of discriminating spontaneous from stem cell-treated functional improvement. In the next step, the spontaneously existing functional network is determined using resting state fMRI. Thus, structural and functional reorganizations following lesion and therapy are followed. With this combination of high-end technologies, we are presently investigating the structural correlate of functional plasticity. Using functional connectivity network analysis, we study the influence of the focal lesion on the whole hierarchical functional network, and aim to unravel the mechanisms modulating the functional network by the stem cell graft.
(3) Role of inflammatory cells for stem cell-mediated CNS therapy
Recent evidence has shown that monocytes and microglia play a janus-faced beneficial or detrimental role during cerebral inflammation. Furthermore, these cells share various cytokines and growth factors with stem cells, indicating a mutual influence of these cell types. The time profile of activation of the inflammatory cells after stroke is not clear and the population of infiltrating monocytes has not yet been quantified. We follow the hypothesis that a large contribution of the observed therapy success of stem cells is due to mutual influence of stem and inflammatory cells. Recently, we have developed imaging-based approaches to monitor the monocyte infiltration into the cerebral lesion over time in a quantitative manner. Moreover, using virally injected cell-specific imaging reporters, we intend to study the activation of microglia over time, as well as their shift between the beneficial and detrimental phase. By having a time-resolved approach, we can modulate their spontaneously occurring polarization for neuroprotection and maximal functional recovery. By injecting microRNA-124, we have recently shifted the microglia and monocyte polarization towards the beneficial phase, paralleled by a faster functional improvement after stroke.