Conference Agenda

Beyond the canonical neurogenic niches, there are dormant neuronal precursors in several regions of the adult mammalian brain. Dormant precursors maintain latent post-mitotic immaturity from birth to adulthood, followed by staggered awakening events in a process which is still largely unresolved. Due to the slow rate of awakening, some precursors remain immature until old age. Therefore, we questioned whether aging affects the precursor awakening and maturation. To this end, we studied the maturation process in transgenic mice (DCX-CreER^T2/fl-EGFP) in which immature precursors were labelled permanently in vivo at different ages.

We found that dormant precursors are capable to awaken at earlier age, becoming adult-matured (AM) neurons, as well as to awaken at later age, becoming late-AM neurons. Thus, protracted immaturity does not prevent late awakening and maturation. However, late-AM neurons diverged morphologically and functionally from AM. Moreover, AM were functionally most similar to neonatal-matured (NM) neurons. Conversely, late AM neurons were endowed with high intrinsic excitability and high input resistance, and received a smaller amount of spontaneous synaptic input. Thus, even if late awakening occurs, aging affects the maturation process.

Several groups have identified subsets of neural progenitors residing in the meninges during development and in adulthood in different mammalian species. Interestingly, these immature neural cells are able to migrate from the meninges to the neural parenchyma and, without proliferation, differentiate into functional cortical neurons or oligodendrocytes. Immature neural cells residing in the meninges promptly react to brain disease. Injury-induced expansion and migration of meningeal neural progenitors have been observed following experimental demyelination, traumatic spinal cord and brain injury, amygdala lesion, stroke, and progressive ataxia. Whether a neural stem/progenitor cell population is present in human brain is still not clear. We analyzed different fetal and adult human brain samples and found cells in meninges expressing the immature stem progenitor cell markers nestin, sox2 and dcx. Moreover, to test their neural stem/progenitor cell properties, we in vitro expanded neural stem/progenitor cells from adult human brain samples and verified their neural multipotent differentiation potential and their in vitro cell assembly properties generating brain organoids. Adult human brain cells present in meninges in vitro differentiated into neurons, oligodendrocytes and astrocytes and they were able to differentiate into neurons following in vivo transplantation into the adult hippocampus of rodent mice.

In the mammalian brain, different populations of “young”, undifferentiated neurons can be found at various locations: within “canonical” neurogenic sites, wherein they are produced by stem cell-driven neurogenesis, and in cortical and subcortical regions, wherein they can be dormant elements generated during embryogenesis but retaining immaturity through adulthood. We have recently demonstrated that cortical immature neurons show remarkable interspecies variation, being restricted to paleocortex in rodents while expanding in the whole neocortex of large-brained mammals, with high cell densities in some species (e.g., cat, chimpanzee). We have introduced a method for comparing different species in a “comparable” way, by counting the doublecortin-positive cells in the cortical layer II and in subcortical regions (amygdala, claustrum), and by checking their possible co-expression with Ki67 antigen, in mammals widely differing for their brain size, gyrencephaly and socio-ecological niche. In the emerging complexity of brain structural plasticity, non-newly generated “immature” neurons seem to represent an evolutionary choice for large-brained mammals endowed with higher cognitive capabilities that display lower rates of stem cell-driven adult neurogenesis. Important aspects are at stake: the possible role(s) that these neuronal populations may play in prevention/repair of neurological disorders, and the correct translation of results obtained from laboratory rodents.

In the adult brain two stem cell niches exist, where astrocytes sporadically undergo neurogenic activation fueling a population of intermediate progenitors (TAPs) that transiently expand before differentiating into neurons. Unexpectedly, our and other labs demonstrated that brain lesions can induce astrocytes neurogenic activation outside these niches, particularly in the striatum. After excitotoxic lesion striatal astrocytes generate neurons through clusters of proliferating TAPs-like cells. For at least six months numerous neurogenic foci populate the striatum, but the spatio-temporal dynamics supporting their proliferative and differentiative activities remain unclear.

Here, we evaluated if striatal neurogenic foci are stable structures, supported by the continuous activity of a small and anatomically restricted astrocyte population, or transient structures originated by the sporadic activation of a widespread population. To this end, we employed lineage-tracing, BrdU birth-dating analyses and 3D-reconstructions coupled with mathematical modelling and spatial analyses.

Our results indicate that striatal astrocytes sparsely activate in a random pattern generating new spatially independent clusters of TAPs cells that locally expand and mature into proliferating and post-mitotic neuroblasts. These neurogenic foci live transiently for about 10 days during which they progressively mature, until all cells become post-mitotic. A homeostatic turnover, where the exhaustion of old neurogenic foci is continuously counterbalanced by the establishment of new ones, stabilizes the number of these structures for at least 5 weeks after neurogenesis onset.

These spatio-temporal turnover dynamics remind those of canonical niches progenitors, though diluted in a larger volume. In conclusion our results indicate that neurogenic potential is widespread among striatal astrocytes and that the parenchyma is largely permissive for the establishment of neurogenic niches. Understanding the mechanisms supporting such progenitor activity in the mature brain parenchyma could be exploited in the design of strategies to stimulate brain repair.