Neural stem cells (NSCs) generate new neurons throughout life in distinct areas of the mammalian brain. This process, called adult neurogenesis, is associated with physiologic brain function but has been also implicated in a number of diseases, such as epilepsy and major depression. Understanding the mechanisms underlying adult neurogenesis represents a prerequisite for future therapeutic targeting of adult NSCs for endogenous brain repair. Our group uses imaging-, genome editing-, and transgenesis-based approaches as well as cellular models of human diseases using pluripotent embryonic cells to study the molecular and cellular framework of NSC biology in the developing and adult brain. Aim of our research is to understand how physiologic and disease-associated alterations of the neurogenic niche are translated into stem cell-associated plastic changes of the adult brain on a cellular and behavioral level.
Recent key publications:
Pilz GA, Bottes S, Betizeau M, Jörg DJ, Carta S, Simons BD, Helmchen F, Jessberger S (2018) Live imaging of neurogenesis in the adult hippocampus. Science 359:658-662
Moore DL, Pilz GA, Araúzo-Bravo MJ, Barral Y, Jessberger S (2015) A mechanism for the segregation of age in mammalian neural stem cells. Science 18;349
Knobloch M, Braun SMG, Zurkirchen L, von Schoultz C, Zamboni N, Kovacs WJ, Araùzo-Bravo MJ, Karalay O, Suter U, Machado R, Roccio M, Lutolf MP, Semenkovich CF, Jessberger S (2013) Metabolic control of adult neural stem cell activity by Fasn-dependent lipogenesis. Nature 493(7431):226-30
The cellular principles of stem cell activity in the adult brain
The cellular principles of NSC division and neural cell birth remain largely unknown due to a lack of longitudinal observations of individual NSCs within their endogenous niche. We previously established together with Fritjof Helmchen’s group a chronic in vivo imaging approach using 2-photon microscopy (Pilz, Carta et al., 2016 J Neurosc). We used this method to follow individual NSCs and their progeny in the mouse hippocampus for > 2 months and provided evidence for limited self-renewal of Ascl1-labeled NSCs and direct neurogenic cell divisions of radial glia-like NSCs (Pilz, Bottes et al., 2018 Science). Thus, our data characterized the cellular principles of NSC division in the adult hippocampus allowing for life-long neurogenesis in the mammalian brain. Currently, we use this approach to study the functional integration of new neurons, the clonal relationship of cell death of individual progeny, and the contribution of new vs. old granule cells for dentate gyrus-dependent computation. The new chronic in vivo imaging tools will provide fundamental new insights into the cellular and molecular mechanisms regulating somatic stem cell activity in the adult brain.
Fig. 1: In vivo imaging of NSCs and their progeny. Shown are selected time points of two radial glia-like NSCs (R cells; depicted with open and closed arrowhead) over the course of 2 months resulting in two neuronal clones. Lineage tree deduced from tracking one R cell (open arrowhead) and its progeny. Each circle in the lineage tree represents an imaging time point. Y axis shows the duration of the imaging. Post hoc immunhistochemical analyses of the clone shown confirm neuronal progeny with newborn cells positive for Prox1 (green) and negative for Sox2 (white). Note that the horizontal view of the DG corresponds to the view obtained during in vivo imaging. For details refer to Pilz et al., 2018 Science.
Asymmetric segregation of age and fate in stem cells
A strong interest of our group is to identify mechanisms of how asymmetry is achieved during somatic stem cell division. This is of relevance for fate decisions but also in the context of segregation of age and cellular components during cell division (e.g., Moore and Jessberger, 2017 TiCB). We currently analyze the segregation of cellular components during NSC division in mouse and human tissues using novel genome-engineering approaches. Further, e have recently identified a novel diffusion barrier in the membrane of the endoplasmic reticulum (ER) of mammalian NSCs that may be critically involved in segregating cellular components during cell division (Moore et al., 2015 Science). Ongoing experiments aim to identify the molecular constituents of the mammalian ER diffusion barrier and to test the influence of manipulating the barrier on cellular behavior. In addition, we aim to define the molecular and behavioral consequences of previous cell divisions of somatic stem cells. To this end, we currently develop novel tools to identify the cell division history of somatic stem cells in live tissue.
Fig. 2: ER diffusion barrier in Neural Stem Cells. NSCs Fluorescence loss in photobleaching (FLIP) experiments are performed during anaphase in NSCs by creating a small region of interest (ROI) that is bleached repeatedly during timelapse imaging. If no diffusion barrier is present between the NSCs, all GFP molecules in the cell will have access to bleaching in the ROI, leading to bleaching of GFP molecules in both daughter cells and a corresponding quantification of reduced fluorescence in both compartments over time. If there is a barrier present, only the GFP molecules on the side that is being bleached will have access to the bleaching ROI, and thus only the GFP in the daughter cell with the ROI will be bleached, leading to the quantification showing a retention of fluorescence in the unbleached daughter cell with a loss in the bleached daughter cell over time. Lower panels show FLIP experiments in rat NSCs overexpressing LumER-GFP (green; targeted to ER lumen) and histone H2B-mCherry (red; DNA), or MemER-GFP (green; targeted to ER membrane) with H2B-mCherry. Note the compartmentalized loss of fluorescence with MemER-GFP bleaching that occurs selectively on the bleached side of the dividing NSC. White outline indicates bleached ROI. Modified from Moore et al., 2015 Science.
Molecular control of neural stem cell activity
Our previous work has identified several pathways/genes that are critically involved in certain steps, from the dividing NSC to the integrating newborn neuron, during the developmental course of adult neurogenesis (e.g., Karalay et al., 2011 PNAS; Bracko et al., 2012 J Neurosc; Vadodaria et al., 2013 J Neurosc; Braun et al., 2015 Cell Reports; Rolando et al., 2016 Cell Stem Cell). Further, we have participated in efforts to characterize the functional role of adult neurogenesis on a single cell but also behavioral level (e.g., Clelland et al., 2009 Science; Brunner et al., 2015 eLife).
Fig. 3: Identifying novel mechanisms regulating adult neurogenesis. Using unbiased gene expression analyses combined with metabolomics we identified a requirement for a specialized lipid metabolism in proliferating NSCs. Shown are hippocampal NSCs expressing Spot14 (green) that is expressed in nestin (blue) and Sox2 (red)-labeled NSCs. Spot14 is a novel regulator of the proliferative activity of adult NSCs (Knobloch et al., 2013 Nature).
Metabolic control of stem cells
Besides a number of morphogenic signaling pathways and transcriptional codes that govern the distinct developmental steps from the dividing NSC to a functional neuron, a critical role of cellular metabolism has been recently identified (e.g, Knobloch and Jessberger, 2017 Current Opinion in Neurobiology). Specifically, we showed that NSCs depend on high levels of fatty acid synthase (FASN)-dependent de novo lipogenesis for proper proliferation (e.g., Knobloch et al., 2013 Nature). We have now identified a novel metabolic shift regulating quiescence vs. proliferation of NSCs (Knobloch et al., 2017 Cell Reports). Strikingly, we found that superimposing a metabolic state is sufficient to regulate the behavior of NSCs, indicating an - at least partially - instructive role for metabolism on somatic stem cell function. We currently test experimentally altered cell metabolism to enhance stem cell activity in mouse in vivo and human in vitro systems using wildtype and genome-edited human pluripotent stem cells. Further, we have participated in experiments understanding specialized lipid metabolism in other somatic cells (Wong et al., 2017 Nature) and mitochondrial metabolism in NSCs (Beckervordersandforth et al., 2017 Neuron).