We are interested in the fundamental mechanisms governing neural stem cell maintenance, division, elimination, and differentiation. We develop innovative animal models and approaches to visualize stem cells, trace their lineage, and analyze their signaling landscape, Using these tools, we have delineated sequential stages in the progression from stem cells to mature neurons, revealing the basic blueprint for the differentiation cascade within the adult hippocampus (J Comp Neurol, 2004; PNAS, 2006; Cell Stem Cell, 2011). These investigations have also led to a novel model of the life cycle of neural stem cells (Cell Stem Cell, 2011; CSH Persp, 2015; Behav Brain Res, 2019; see also Cell Stem Cell, 2010; Nature, 2012; Cell Stem Cell, 2015; Cell Reports, 2019).
We found that adult neural stem cells may remain quiescent for most of their postnatal life, but, when activated, rapidly divide several times to bud off daughter cells that eventually yield neuronal progeny, while the stem cell itself differentiates into a mature astrocyte, effectively leaving the stem cell pool.Hence, in contrast to the conventional model of recurring stem cell quiescence, a hippocampal stem cell can be described as a “single-use” or a “disposable” unit – used in adulthood only once and then abandoned in its stem cell capacity. This model has now been corroborated by others, using independent approaches.
Our findings also indicate that continuous loss of stem cells via their division-coupled astrocytic differentiation underlies age-dependent diminished production of new neurons and may contribute to age-related cognitive impairment. We are presently exploring whether our model of differentiation-driven stem cell attrition may be relevant to other normal and tumor-initiating stem cell contexts.
Tissue stem cells
We design transgenic reporter lines and their combinations to visualize, track, and isolate stem cells; some of our reporter lines became widely employed genetic tools, used by over 200 groups worldwide. The reporter lines’ approach allowed us to discover stem and progenitor cells in a range of tissues and organs, as diverse as anterior pituitary, skeletal muscle, testis, hair follicles, liver, pancreas, retina, adrenal medulla, and bone marrow. We identified some of those stem cell species in our group and studied other types in close collaboration with other research groups. Most important results pertain to neural stem cells (J Comp Neurol, 2004; PNAS, 2005; Cell Stem Cell, 2011), mesenchymal stem cells (Nature, 2010; Nature, 2012), liver oval cells (Dev Dyn, 2005; Cell, 2014), ovarian stem cells (Nature, 2013), stem cells in the skin (Cell Stem Cell, 2011) and hair follicle (PNAS, 2003; J Comp Neurol, 2007), precursors to Leydig cells (J Cell Biol, 2004), and adult anterior pituitary stem cells (PNAS, 2009). We now use this reporter-based approach to search for new types of tissue-specific stem and progenitor cells and explore the cross-talk between different stem cell compartments.
Nitric oxide and development
Our longstanding research interest has been focused on the diverse spectrum of biological functions mediated by nitric oxide (NO). Our group was the first to discover the essential role that NO plays in organism development, tissue differentiation, and stem cell regulation. This includes the findings that NO potentiates weak calcium signals (Nature, 1993), mediates neuronal differentiation (Nature, 1995), regulates Drosophila imaginal disk development (Cell, 1996), controls brain development (J Neurosci, 2001) and adult neurogenesis (PNAS, 2003), contributes to the regulation of hematopoietic stem cells (Mol Ther, 2004; Mol Med, 2008), and coordinates cell proliferation and cell movements during early developmental stages (Cell Cycle, 2007). Our current focus is on a novel function of NO that was discovered in collaboration with Natalia Peunova – specifically, the role that nNOS/NO plays in the development and function of the motile cilia in the airways (Life Sci Alliance, 2021) and the brain. Our recent results suggest that a range of inborn and acquired human ciliopathies and related disorders may be associated with decreased availability of NO and may benefit from NO-based therapeutic strategies.
The defects in cell division, elimination, and migration are essential contributors to the neurobiological (and ultimately behavioral) manifestations of various neurodevelopmental and neurodegenerative disorders. In recent years, we have developed a suite of methods for 2D, 3D, and 4D visualization and quantification of dividing cells in the whole embryonic, perinatal, and adult mouse brain. This includes methods for multitagging the dividing and differentiating cells (Meth Cell Biol, 2008; Stem Cell Reports, 2018; Cells, 2022a; Cells, 2022b) and novel computational algorithms for automatic cell detection and counting in 3D and spatiotemporal 3D image registration (collaboration with Alex Koulakov, CSHL) (Front Neuroanat, 2017; Sci Reports, 2022). Leveraging these methods, we have generated 3D representations of dividing and migrating progenitor cells in the developing and adult mouse brain, as well as a spatio-temporal 4D pseudotime description of the stem cell division and migration. We now use these approaches to determine the changes in progenitors’ division and migration after various treatments and in neurodevelopmental mutants.
Social conflict circuitry
Establishment of social hierarchy in animal groups through conflict behavior helps to deflect excessive violence and injury, protects group’s valuable resources, and molds the overall societal structure. We use a comprehensive mouse model of conflict behavior (see Kudryavtseva, Nature Prot, 2014) to examine changes in behavior, neurogenesis, and neuronal activity. We found that repeated positive fighting experience heightens aggression, increases proliferation of neuronal progenitors and production of young neurons in the hippocampus, and alters neuronal activity in the amygdala (Front Neurosci, 2015). We also found that if adolescent male mice are exposed to adult males with elevated levels of aggression, they demonstrate high levels of anxiety and heightened aggression when they become adult (PLOS One, 2014).
Our current focus is on neural circuits underlying acquisition, escalation, and reversal of aggressive behavior in a social conflict setting. Several brain regions and circuits involved in aggressive behavior and social dominancy have been identified; however, the global maps of brain networks involved in establishing, maintaining, and reversing social hierarchy are not known. In close collaboration with Alex Koulakov’s group at CSHL and Natalia Kudryavtseva’s group at the Institute of Cytology and Genetic, Novosibirsk, we applied high-throughput global mapping of neuronal activation patterns to determine network activity across the brain regions, infer the circuits involved in aggression, dominancy, and reversal of hierarchical status, and determine the unique signatures that characterize each state.