Many neuro-psychiatric diseases result in the death of neurons and lead to severe and often terminal failure of brain function. The discovery that new nerve cells are born in various brain regions throughout adulthood not only challenged the conceptual understanding of brain function and structural plasticity but also opened novel approaches to restore brain function in disease states (Braun and Jessberger, 2014 Development; Jessberger and Gage, 2014 TiCB). Our laboratory aims 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 but also behavioral level.
Fig. 1: Neural Stem Cells in vitro and in vivo. NSCs can be grown as free-floating neurospheres, (A), or under adherent culture conditions forming monolayers (B). Mechanisms of NSC proliferation and neural differentiation can be studied in vitro (in C are GFP-labeled, rat neural stem cells expressing the neuronal marker MAP2ab in red). Within the adult brain newborn neurons can be visualized and manipulated using retroviral vectors that selectively integrate into the genome of proliferating cells and their progeny (green cells in D represent 4 weeks-old neurons born in the adult mouse hippocampus).
Molecular mechanisms underlying stem cell activity and neuronal differentiation
We have previously used unbiased gene expression profiling of adult NSCs and their neuronal progeny to identify regulators of adult neurogenesis (Bracko et al., 2012 J Neurosc). Currently, we characterize novel molecular pathways that regulate the activity of adult NSCs or distinct developmental steps in the course of neuronal integration using transgenesis- and virus-based strategies (e.g., Porcheri et al., 2014 J Neurosc; Knobloch et al., 2013 Nature; Vadodaria et al., 2013 J Neurosc; Karalay et al., 2011 PNAS). In this context we developed novel techniques to image the neurogenic process and to manipulate newborn cells in the adult brain (Kleine Borgmann et al., 2013 Development; Braun et al., 2013 Stem Cell Reports). Current efforts in our group focus on characterizing and understanding metabolic switches from quiescent NSCs toward more proliferative and finally differentiating NSCs. Furthermore, we currently establish novel imaging approaches together with Fritjof Helmchen’s group to study the process of adult neurogenesis directly in the behaving animal.
Fig. 2: Identifying novel mechanisms regulating adult neurogenesis. Using unbiased gene expression analyses combined with metabolomics (together with Nicola Zamboni, ETH Zurich) 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).
Mechanisms underlying age-dependent decrease of neurogenesis
Adult neurogenesis has been associated with a number of neuropsychiatric diseases, including affective disorders, epilepsy, and age-dependent cognitive decline. Specifically, cognitive performance in aged rodents correlates with decreased levels of neurogenesis, suggesting that enhancing decreased neurogenesis may be also beneficial for certain forms of cognition (e.g., Jessberger and Gage, 2008 Psychology and Aging). To understand the cellular and molecular basis of the age-dependent decrease of neurogenesis, current projects aim to identify the cellular mechanism of stem cell proliferation/maintenance using state-of-the-art longitudinal light microscopy imaging and gene expression profiling. With this approach we try to understand how aging affects stem cell behavior on a cellular and molecular level. One aspect we currently focus on is based on our recent finding that NSCs establish a diffusion barrier in the endoplasmic reticulum (ER), similar to what has been previously described in budding yeast (Moore et al., 2015 Science). The ER diffusion barrier may participate in the asymmetric segregation of cellular components during cell division and we currently aim to identify the molecular constituents of the barrier and how previous cell division history relates to the establishment of the barrier.
Fig. 3: 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.
In vivo reprogramming of adult neural stem cells
Under normal conditions the vast majority of newborn cells born in the adult hippocampus differentiate toward a neuronal fate. However, we have previously shown that adult NSCs can be reprogrammed in vivo to adopt an oligodendrocytic phenotype (Jessberger et al., 2008 Nature neuroscience). Thus, we have tested the efficacy of reprogrammed oligodendrocytic cells for endogenous brain repair using genetic and spatially restricted models of demyelination in the adult dentate gyrus. The aim of this project was to provide a proof-of-principle that reprogramming of NSCs within the adult brain could be utilized as a cellular source for endogenous repair in demyelinating disease (Braun et al., 2015 Cell Reports).
Fig. 4: Reprogrammed NSCs as a source for endogenous brain repair. We currently test if reprogrammed oligodendrocytes generated within the dentate niche are capable to regenerate lesions in the context of demyelinating disease (Braun et al., 2015 Cell Reports). To this end we developed a focal model of conditional demyelination (using a diphtheria-toxin based approach) in combination with virus-mediated reprogramming. Shown is the degree of demyelination (as measured by myelin basic protein (MBP) expression in red) after inducing the lesion and examples of reprogrammed cells expressing GFP (green).
Stem cell-based disease modeling
Our previous work identified several genes to be implicated in NSC activity and/or neuronal differentiation that have been associated with human cognition and disease. At this time we use transgenesis to model human mutations in murine models to test for a link between altered neurogenesis and hippocampal phenotypes in humans. In addition, we use genome-editing of human embryonic stem cells and induced pluripotent stem cells (iPSC) technology followed by differentiation into NSCs and their neural progeny to model human diseases.
Functional relevance of adult neurogenesis
Previous studies have shown the activation of newborn neurons during processes underlying certain forms of learning and memory (e.g. Jessberger and Kempermann, 2003 EJN; Clelland et al., 2009 Science; Jessberger et al., 2009 Learning & Memory). We currently analyze the functional significance of adult neurogenesis on a variety of behavioral tasks by using novel transgenic strategies combined with novel imaging approaches that will help to further dissect the function of new neurons for adult brain behavior.