Calcium signaling in neurogenesis: regulation of proliferation, differentiation and migration of neural stem cells

Abstract

The calcium ion (Ca2+) is a highly versatile and ubiquitous signaling messenger in all cell types. Signal transduction occurs through changes in the cytosolic Ca2+ concentration after the opening of Ca2+ channels in the plasma membrane (PM) and endoplasmic reticulum (ER). The difference in Ca2+ concentration between the extracellular space and the cytosol is large, around 10,000 fold, creating a steep gradient that causes Ca2+ to rapidly flow into the cell. Signaling via Ca2+ is fundamental for triggering numerous vital processes in the cell, ranging from fertilization to cell death. Calcium signaling is also critical for regulating neurogenesis in various ways, some of which have been explored in this work.

Proliferation of neural progenitors is dependent on spontaneous Ca2+ activity that occurs in small-scale networks. Ca2+ activity is correlated with electrical activity both in vitro and in vivo and depends on connexin 43 gap junction and PM channels. Differentiation of neural progenitors is also regulated by Ca2+ signaling. We have found that T α1h voltage-dependent Ca2+ channels promote spontaneous Ca2+ activity and direct the differentiation of human neuroepithelial stem cells towards neurons, depending on caspase-3 enzymatic activity. These results were confirmed with T α1h knockout mice that showed a decreased number of neurons in the dorsal cortex. Neuronal migration also depends on Ca2+ signaling. We demonstrated that glial derived neurotrophic factor (GDNF) stimulates a Ca2+ response through the activation of the receptor tyrosine kinase (RET). The subsequent downstream signaling cascade includes phospholipase Cγ, which binds to RET Tyr1015. Mutating RET at Tyr1015 inhibits neuronal progenitor migration towards the cortical plate. We also showed that neurogenesis was altered by the addition of non-cytotoxic concentrations of polychlorinated biphenyls that disrupt spontaneous Ca2+ activity. Polychlorinated biphenyls are common food contaminants. In addition, methyl mercury, another food contaminant, disrupts neuronal differentiation in the opposite direction. Altogether, these data demonstrate the huge impact of Ca2+ signaling on the development of the embryonic brain.

To conclude, we have analyzed Ca2+ signaling during three critical steps of neurogenesis: proliferation, differentiation, and migration. All of these processes are known to be dependent on Ca2+. A deeper understanding of how Ca2+ regulates such different physiological processes is crucial for the field of regenerative medicine, in which control of the expansion and differentiation of neural stem cells can increase the production of neuronal cells in vitro for use in cell replacement therapies.