Our research focuses one side on the molecular and cellular mechanisms underlying physiological adult neurogenesis while addressing on the other the possibility of inducing via cellular reprogramming de novo genesis of neurons in brain areas that are naturally devoid of neurogenesis.
Two regions in the mammalian brain are capable of life-long production of new neurons and glia: the subgranular zone (located right under the granule cell layer of the dentate gyrus of the hippocampus) and the subventricular zone bordering the lateral ventricle. Both regions harbor neural stem cells capable of self-renewal as well of producing progeny that differentiates into distinct types of neurons and glia. The latter comprise oligodendroglia which is responsible for the formation of the myelin sheaths enwrapping nerve fibers that degenerate in diseases like multiple sclerosis. We investigate the mode of cell division of neural stem cells and the mechanisms that regulate the decision whether a undifferentiated cell gives rise to neuron or glia (Costa et al., 2011; Ortega et al., 2011). A key question which we study intensely concerns the issue whether one and the same neural stem cell generates both neuron and oligodendroglia. By means of time-lapse videomicroscopy we were able to show that adult subventricular zone neural stem cells in vitro produce either neuronal or oligodendroglial progeny, but never both (Ortega et al., 2013). Moreover, we found that the generation of new oligodendroglia can be selectively enhanced upon stimulation of the Wnt signaling pathway. These findings were fully corroborated in the adult subventricular zone in vivo. This data provide important evidence for the possibility of selectively enhancing oligodendrogliogenesis, thereby opening eventually new avenues for the treatment of diseases like multiple sclerosis.
The video shows the lineage progression of a single neural stem cell isolated from the adult subventricular Zone.
Another theme of our research concerns the question how newly generated neurons integrate into a pre-existing neural network. Surprisingly, many adult-generated neurons die soon after their birth (for review see (Bergami and Berninger, 2012)). To better understand the process of incorporation of adult-generated neurons into the neuronal circuitry we developed a method which allows for following the establishment of synaptic contacts onto newly generated neurons in vivo over time (Deshpande et al., 2013). Neurons generated in the subgranular zone migrate over short distance into the granular layer where they differentiate in granule neurons. With the help of a modified rabies virus we can examine how the innervation of newly generated granule neurons by their presynaptic partners evolves over time. Using this approach we found that 1-2 weeks after their birth new neurons receive synaptic contacts from neurons of their immediate vicinity, i.e., local interneurons. Only 1-2 weeks later synaptic inputs from more distant brain areas arrive, such as cholinergic input from the medial septum. Finally, only at the end of their maturation process (3-5 weeks) new granule neurons become innervated by pyramidal neurons in the entorhinal cortex and thus form part of the classical trisynaptic hippocampal circuit. The changes in the innervation pattern suggest that during the course of their maturation newly generated neurons exert distinct influences on hippocampal information processing.
Within the adult nervous system neurogenesis is a highly restricted phenomenon. Other brain areas such as the cerebral cortex are devoid of ongoing neurogenesis. However, the cerebral cortex is afflicted in many neurodegenerative diseases such as Alzheimer’s Disease and stroke, causing massive neurological deficits. We follow an approach to induce de novo neurogenesis via cellular reprogramming. To this end we introduce transcription factors which are known to play pivotal roles in the generation of neurons during embryonic development into non-neuronal cells using virus-based vectors. Then we examine whether such genetically modified cells adopt neuronal properties. Indeed we could previously show that astroglia isolated from the brain of young mice can be reprogrammed in vitro into excitatory and inhibitory neurons by forced expression of neurogenin-2 (Neurog2) and achaete-scute complex homologue-1 (Ascl1), respectively (Heinrich et al., 2010; Heinrich et al., 2011). In a recent study we succeeded to translate these findings to cells of human origin. Using the two transcription factors Ascl1 and Sox2 we were able to convert brain pericytes, i.e., a cell type tightly associated with the brain microvasculature, into functional induced neurons (Karow et al., 2012). This finding supports our hypothesis that brain resident cells can be recruited towards the de novo genesis of neurons (for review see (Karow and Berninger, 2013)). Based on these studies our current research aims now at converting non-neuronal cells into functional nerve cells directly in the cerebral cortex in situ.