The meningeal stem cell niche in health and disease

Our group have demonstrated for the first time that a new niche for stem/precursor cells with neural differentiation potential resides in brain meninges (arachnoid and pia mater) of postnatal rats. Meningeal stem/progenitor cells express the neural stem progenitor marker nestin and can be extracted and expanded in vitro as neurospheres. Moreover, they can be induced to differentiate into neurons both in vitro and in vivo (Bifari et al., 2009). Thanks to their superficial location, meninges might represent a new easy accessible tissue hosting neural stem/progenitor cell in the Central Nervous system (CNS). This represents an important aspect that may open new perspective for the possible collection of Neural Stem Cells (NSCs) for regenerative medicine and autologous transplantation. Moreover, every parenchymal vessels inside the CNS are surrounded by a perivascular space (Virchow–Robin space) formed by the extroflexions of meninges filled with cerebrospinal fluid suggesting that meningeal stem/progenitor cells might be widely distributed also in CNS parenchyma. Thus, we hypothesized that meningeal stem/progenitor cells may contribute to CNS homeostasis in health and disease. Verifying this hypothesis could offer new insights for the generation of novel pharmacological approaches to treat neurodegenerative diseases. Based on the great potential and the relevance of our previous finding, during my PhD period, I addressed the following main questions: How is the distribution of the meningeal stem/progenitor cell niche in adult brain and spinal cord? Is the meningeal stem/progenitor cell niche modified by pathological conditions? The experimental plan of these two years of PhD has been focused on the study of the meningeal stem/progenitor cells and the meningeal stem cell niche in healthy and disease animal models (rat and mice). To analyze the meningeal niche at the cellular and molecular levels, we used the combinations of different technical approaches such as immunofluorescence confocal microcopy, real time PCR, western blot and in vitro cell culture. To describe the molecular and cellular features of the meningeal stem/progenitor cells and the organization of the meningeal stem cell niche in adult animals, we analyzed the expression and 4 distribution of markers of stem/progenitor cells (nestin/dcx/cxcr4), proliferation (ki67), self renewal (oct4, BrdU) and extracellular matrix components (laminin, fibronectin, condroitin sulphate, collagen 1a). We found that stem/progenitor cells with self-renewal and proliferative properties are present in adult brain and spinal cord meninges. Moreover, we have shown that the presence of immature nestin/positive cells population is a conserved feature across species including human. The complex dynamic equilibrium present in healthy adult CNS also involves the participation of functional NSC niches. In CNS, various pathogenic events acting by different mechanisms may cause neural cell loss and chronic inflammation. Several agents and mediators sustaining these mechanisms also act on niche homeostasis and it is therefore expected that these conditions may have a deep impact on NSC biology and NSC niche properties. To investigate the influence of CNS disease conditions on the meningeal stem cell niche, we have analyzed meninges of severe combined immunodeficient (SCID) mice and spinal cord injured (SCI) rats. Meningeal stem cell niche in SCID mice was deeply changed. The number of the stem/progenitor cells was statistically significantly decreased associated with a dramatically increase in the cellular and extracellular matrix components related to fibrosis (i.e. fibroblasts, fibronectin and collagene). Furthermore, stem/progenitor cells of meninges have shown a lower proliferation rate in vitro. These data indicate that the lack of the adaptive immune system decreases the stemness properties of the meningeal stem cell niche. In SCI mice model we found that meningeal stem/progenitor cell niche is activated. Following the contusion the meningeal niche increase in thickens, stem/progenitor cells largely increase their proliferation and number. Moreover, we found that SCI induced a global increase in the stemness related gene expression profile. This observation suggests that SCI induces in spinal cord meninges an amplification of the stemness properties of the niche. In conclusion the main results of this work are: I) A stem/precursor cell population, is present in adult meninges and is conserved across species; II) The meningeal niche, including the immature nestin positive cell population, of adult mice brain result perturbed in immunodeficient animal model; 5 III) Meningeal niche is activated by contusive spinal cord injury: meningeal stem/precursor cells proliferate and increase in number. All together our data suggest a novel role for meninges as a potential niche harboring endogenous stem/precursor cells that can be functionally modulated in disease conditions. Depending on specific disease-related stimuli, the meningeal stem cell niche can react both by increasing or decreasing its stem cell properties. This differential response to specific conditions, suggests a potential role and contribution of the meningeal stem/progenitor cells in the physiopathological events occurring in CNS diseases. Further evaluation of the molecular mechanisms involved in the meningeal stem/progenitor cells contribution to the physiopathology of different diseases, will open new prospective for the research on pharmacological treatments and regenerative medicine applied to CNS disease.