Beyond the Flow: Epithelial Sodium Channel and Mechanical Forces Regulate Adult Neural Stem and Glial Cells
Ludwig-Maximilian University Munich, Helmholtz Center Munich (Germany)
In the adult mammalian brain, there are specialized regions, called neurogenic niches, containing resident adult neural stem cells (aNSCs) that generate newborn neurons. In the hippocampus, the new granule cell neurons are critical for learning and memory and for mood control. In the walls of the lateral ventricles, in the subependymal zone (SEZ), aNSCs give rise to neuroblasts that migrate to the olfactory bulb, where they differentiate into olfactory interneurons. aNSCs in SEZ are in contact with the lateral ventricles; however, the role of electrochemical and mechanical forces imposed by the circulating cerebrospinal fluid (CSF) on their biology has not been explored.
We identified the Epithelial Sodium Channel (ENaC) to be enriched in aNSC and to be necessary for the shear stress sensing. For the first time in aNSCs, we directly measured changes in intracellular sodium concentration and in calcium dynamics with respect to fluid flow. Our results show that the increased fluid flow increases proliferation of aNSCs and their progeny and blocking or knocking-out of ENaC abrogates this effect. Moreover, our calcium imaging experiments demonstrate that increased fluid flow increases calcium oscillations in aNSCs but only if they are at the surface of the lateral wall, but not deeper in the niche depth and not in the absence of ENaC.
To further examine ENaC function in vivo, we used transgenic mice, in which floxed ENaC alpha subunit can be knocked-out in GLAST-expressing aNSCs by Tamoxifen (Tam) administration. Following the GFP+ recombined cell population at different time points after Tam revealed reduced proliferation of the aNSCs, the transit-amplifying progenitors and proliferating neuroblasts in SEZ. This proliferation deficit occurs without increasing cell death and was confirmed in vivo by knock-down in fast proliferating cells or in vitro by time-lapse imaging of primary SEZ cells, which had ENaC blocked or knocked-down. Mechanistically, the flow-induced increase in SEZ proliferation requires the calcium release activated channel (CRAC) that acts upstream of Erk kinase activation. In summary, our results suggest for the first time that cells in adult neurogenesis are responsive to mechanical forces and require ENaC as an electrochemical sensor to instruct them to proliferate.
Finally, we have been exploring the possible roles of ENaC in the central nervous system but outside of the neurogenic niches. Our preliminary results suggest that ENaC is critical for function of the Müller glia, the major microglia of the retina, as well as for proliferating astrocytes in the white matter. These findings provide additional functional understanding of neural sodium homeostasis controlled by ENaC, an ion channel traditionally associated with sodium recycling in kidneys or lungs.
This seminar is partially supported by the Campus of Biscay of the UPV/EHU.