Stem cells are present in adult tissues and are essential for homeostasis and repair. In the brain of most mammals, including humans, adult neural stem cells (NSCs) persist in specific locations, like the dentate gyrus in the hippocampus. New neurons are generated in the dentate gyrus throughout life, which integrate into the existing hippocampal circuitry to modulate mood and memory. One key feature of adult NSCs is that, at any given time, most of them are in an inactive state - also called quiescence - until receiving the right activation stimuli. They also have a limited ability to self-renew, which means that their activation is tightly coupled with exhaustion. In fact, stem cell numbers rapidly decline with age and the subsequent loss of newborn neurons has been linked to symptoms characteristic of neural disorders such as dementia and depression.
Our research is directed towards elucidating the mechanisms driving the transition of NSCs between active and quiescent states, which ultimately will allow us to devise strategies to prevent their exhaustion during ageing. For this, we use and develop tools to manipulate adult neural stem cells in vivo and in vitro.
IDENTIFYING SIGNALS THAT CONTROL NSC QUIESCENCE
Adult neurogenesis can be modulated by a great variety of local signals and systemic stimuli, such as diet or stress. However, the molecular mechanisms regulating adult NSCs are largely unknown. We use novel in vivo and in vitro tools to study the response of NSCs to different stimuli. Our current focus is on signaling cues directly reaching adult NSCs, including systemic signals and those coming from the extracellular matrix. We give particular importance to testing the effects of these stimuli on the maintenance of the stem cell pool, therefore allowing us to determine their long-term consequences on adult neurogenesis and behaviour.
Adult NSCs are exposed to multiple, often opposing, signals at the same time. We are interested in the internal signal integration mechanisms present in adult NSCs that allow them to mount an appropriate response to their complex signaling environment. For this, we are very lucky to have recently teamed up with the SMICH community (further details available soon here).
One of the most important drivers of the activation of adult NSCs is the transcription factor Ascl1. We know Ascl1 protein levels are regulated by the E3-ubiquitin ligase Huwe1 and that this regulation is behind the changes in adult NSC behaviour observed during ageing. One of our most active projects aims at identifying new regulators of the quiescence state by examining how the Huwe1-Ascl1 axis is controlled. This project has been boosted by a SFB collaborative grant, with which we are also investigating what is the role of targeted protein degradation in the quiescence to activation transition. Please check the Targeted Degradation website.
EFFECTS OF SYSTEMIC STIMULI ON ADULT NEUROGENESIS
We use lineage tracing and label retention experiments to characterize the response of adult neural stem cells to “western/cafeteria" diets or dietary restrictions. Western or cafeteria diets with a high content of fat and sugar are key players behind the emergence of a type 2 diabetes epidemic worldwide and they have been shown to decrease adult neurogenesis. On the other hand, dietary restrictions such as calorie restriction or intermittent fasting are seen as “rejuvenating" diets with the potential to prevent metabolic diseases and delay aging. However, the long-term consequences of these diets on adult NSCs and therefore the lasting repercussion on adult neurogenesis have not been studied yet. Our goal is to characterize the relationship between diet and the regulation of adult neural stem cells to broaden our understanding of the impact of diet on adult neurogenesis, memory and behaviour.
We are also interested in finding the molecular effectors underpinning the modulation of adult neurogenesis by diet. Our current focus is to decipher how insulin and IGF, which are systemically changed upon dietary interventions, affect adult NSC behaviour.
Thanks to an SFB collaborative grant, we are investigating how age affects the response of adult NSCs to systemic cues and whether they are regulated by circadian rhythms. More details will be soon available at the Neuromodulation Consortium website.
GENERATING A MODEL OF HUMAN NSC QUIESCENCE
We still know very little about adult NSCs in humans, and there is currently a heated debate as to whether hippocampal neurogenesis is present only in young individuals or persists throughout adulthood. We are currently generating a quiescence model using hiPSCs in order to start exploring the regulation of quiescence in human neural stem cells. In collaboration with the Edenhofer lab in Innsbruck, we will compare the behavior of reprogrammed NSCs (from iPSCs) to that of directly converted NSCs (from adult fibroblasts), which will allow us to elucidate the effects of ageing on NSC quiescence. More details will be soon available at the Neuromodulation Consortium website.
UNTANGLING QUIESCENCE FROM STEMNESS
While in the developing brain proliferation is associated with the retention of stem cell characteristics, this is quite the opposite in the adult brain. Adult NSCs actually lose their self-renewing capacity upon successive divisions. We hypothesize that quiescence-related signals not only keep NSCs out of the cell cycle, but also allow them to retain their stemness. We are exploring how quiescence-inducing signals affect the self-renewing capacity of NSCs. It has been reported that NSCs gradually switch mode of division, from symmetric self-renewing to asymmetric and eventually to symmetric differentiating. To determine whether quiescence can also influence the mode of division of NSCs, we are currently devising strategies to quantify asymmetric vs symmetric divisions in NSCs using live imaging techniques.
Urbán, N., Blomfield, IM., Guillemot, F. (2019). Quiescence of Adult Mammalian Neural Stem Cells: A Highly Regulated Rest. Neuron. 104(5):834-848
Blomfield, IM., Rocamonde, B., Masdeu, MDM., Mulugeta, E., Vaga, S., van den Berg, DL., Huillard, E., Guillemot, F., Urbán, N. (2019). Id4 promotes the elimination of the pro-activation factor Ascl1 to maintain quiescence of adult hippocampal stem cells. Elife. 8
Urbán, N., van den Berg, DL., Forget, A., Andersen, J., Demmers, JA., Hunt, C., Ayrault, O., Guillemot, F. (2016). Return to quiescence of mouse neural stem cells by degradation of a proactivation protein. Science. 353(6296):292-5
Urbán, N., Guillemot, F. (2014). Neurogenesis in the embryonic and adult brain: same regulators, different roles. Front Cell Neurosci. 8:396
Andersen, J., Urbán, N., Achimastou, A., Ito, A., Simic, M., Ullom, K., Martynoga, B., Lebel, M., Göritz, C., Frisén, J., Nakafuku, M., Guillemot, F. (2014). A transcriptional mechanism integrating inputs from extracellular signals to activate hippocampal stem cells. Neuron. 83(5):1085-97
Martynoga, B., Mateo, JL., Zhou, B., Andersen, J., Achimastou, A., Urbán, N., van den Berg, D., Georgopoulou, D., Hadjur, S., Wittbrodt, J., Ettwiller, L., Piper, M., Gronostajski, RM., Guillemot, F. (2013). Epigenomic enhancer annotation reveals a key role for NFIX in neural stem cell quiescence. Genes Dev. 27(16):1769-86
Webb, AE., Pollina, EA., Vierbuchen, T., Urbán, N., Ucar, D., Leeman, DS., Martynoga, B., Sewak, M., Rando, TA., Guillemot, F., Wernig, M., Brunet, A. (2013). FOXO3 shares common targets with ASCL1 genome-wide and inhibits ASCL1-dependent neurogenesis. Cell Rep. 4(3):477-91