Importance of being Nernst: Synaptic activity and functional relevance in stem cell-derived neurons

Abstract

Functional synaptogenesis and network emergence are signature endpoints of neurogenesis. These behaviors provide higher-order confirmation that biochemical and cellular processes necessary for neurotransmitter release, post-synaptic detection and network propagation of neuronal activity have been properly expressed and coordinated among cells. The development of synaptic neurotransmission can therefore be considered a defining property of neurons. Although dissociated primary neuron cultures readily form functioning synapses and network behaviors in vitro, continuously cultured neurogenic cell lines have historically failed to meet these criteria. Therefore, in vitro-derived neuron models that develop synaptic transmission are critically needed for a wide array of studies, including molecular neuroscience, developmental neurogenesis, disease research and neurotoxicology. Over the last decade, neurons derived from various stem cell lines have shown varying ability to develop into functionally mature neurons. In this review, we will discuss the neurogenic potential of various stem cells populations, addressing strengths and weaknesses of each, with particular attention to the emergence of functional behaviors. We will propose methods to functionally characterize new stem cell-derived neuron (SCN) platforms to improve their reliability as physiological relevant models. Finally, we will review how synaptically active SCNs can be applied to accelerate research in a variety of areas. Ultimately, emphasizing the critical importance of synaptic activity and network responses as a marker of neuronal maturation is anticipated to result in in vitro findings that better translate to efficacious clinical treatments.

Keywords: Synapses, Neurotransmission, In vitro techniques, Induced pluripotent stem cells, Neuronal networks, Neurogenesis, Neural stem cells, Embryonic stem cells, Induced neurons

Core tip: During stem cell neuronal differentiation, functional synaptogenesis and the emergence of coordinated, networked activity are critical behaviors in confirming that cells have developed into a relevant neuronal population. As the number of stem cell-derived neuron (SCN) models continues to proliferate, the use of specific functional readouts to evaluate SCN maturity will become increasingly important compared to morphological or proteomic characterization of neuronal maturation. The review provides diverse options for reliably assaying the development of synaptic neurotransmission in derived neurons and describes the strengths, weaknesses and potential applications of several stem cell-based neuron models.