Ionic modulators of stem cell state

Abstract: Tissue generation during development and maintenance throughout life relies on the proliferation and sequential specification of a group of cells. Stem cells are defined by the properties of self-renewal (division to produce daughter cells equipotent to the mother) and differentiation (division where at least one daughter is of a more restricted potential). In adult systems, two additional states - quiescence, a dormant state of infrequent proliferation, and activation, a state of increased proliferation, are described. Regulation of these states is a key determinant of health and fitness on a tissue and organism level, as it ensures proper development and regeneration. The aim of this thesis is to investigate how another key system at the cellular level – regulation of ion availability – can modulate cell states of embryonic and adult stem cells. In paper I the effect of lithium chloride (LiCl) on juvenile mouse neural stem progenitor cells (NSPCs) from the subgranular zone (SGZ) of the hippocampus was investigated. Under maintenance conditions, treatment with LiCl increased NSPC proliferation, reducing the fraction of cells in G0/G1. Pre-treatment of NSPCs with LiCl prior to ionizing radiation (IR) exposure reduced DNA damage response activation, and attenuated the IR-induced G1 block, restoring proliferation, although cell death was not reduced. In paper II the effect of ZD7288, a specific blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, on mouse embryonic stem cell (ESCs) was examined. The blocker attenuated proliferation by extending G1 and S phases. This did not compromise pluripotency, but facilitated spontaneous serum-induced differentiation while reducing the efficiency of directed differentiation towards the neuronal lineage. In paper III expression of HCN family channels and effects of their inhibition in adult NSPCs were described. Hcn2 and Hcn3 are expressed throughout the NSPC hierarchy, but only functional in S and G2/M phases. HCN inhibition or knockdown attenuated proliferation due to a reversible G0/G1 accumulation which was accompanied by alterations in activation marker expression, metabolism, and the molecular clock network. A small molecular agonist of Rev-erb-α, a clock component, recapitulated the proliferative effects. HCN inhibition-induced G0/G1 block was shown to have a protective effect during IR exposure of juvenile mice, reducing apoptosis and maintaining proliferation. In conclusion, lithium, which is proposed to inhibit a number of enzymes by replacing magnesium as a cofactor, and HCN currents, which are involved in regulation of the electrochemical state of the cell, were shown to modulate stem cell state. This suggests that further investigation of these and other ionic modulators is warranted, both for therapy development and in the interests of basic science.