Programming embryonic stem cells to neuronal subtypes

Richness of neural circuits and specificity of neuronal connectivity depend on the diversification of nerve cells into functionally and molecularly distinct subtypes. Although efficient methods for directed differentiation of embryonic stem cells (ESCs) into multiple principal neuronal classes have been established, only a few studies systematically examined the subtype diversity of in vitro derived nerve cells. Here we review evidence based on molecular and in vivo transplantation studies that ESC-derived spinal motor neurons and cortical layer V pyramidal neurons acquire subtype specific functional properties. We discuss similarities and differences in the role of cell-intrinsic transcriptional programs, extrinsic signals and cell–cell interactions during subtype diversification of the two classes of nerve cells. We conclude that the high degree of fidelity with which differentiating ESCs recapitulate normal embryonic development provides a unique opportunity to explore developmental processes underlying specification of mammalian neuronal diversity in a simplified and experimentally accessible system.

Introduction

One of the tantalizing features of the central nervous system (CNS) is the precision with which nerve cells establish stereotypical neural circuits. Ongoing characterization of the developing CNS is revealing an unprecedented degree of neuronal diversity that corresponds with and likely underlies the richness and specificity of neuronal connectivity and function. Subtype diversification of spinal motor neurons, retinal amacrine cells, cortical layer V pyramidal neurons or olfactory sensory neurons underlies functionally relevant differences in axonal projections, dendritic arborization, and molecular and electrophysiological properties of these neurons [1, 2, 3, 4, 5]. How such neuronal diversity is established during mammalian development remains poorly understood in part due to technical challenges inherent to the analysis of complex and heterogeneous CNS tissue under poorly accessible experimental conditions. Recapitulation of normal neural development with pluripotent embryonic stem cells (ESCs) in vitro might therefore become a powerful and indispensable tool in the field of developmental neurobiology. It provides an unparalleled experimental access to the developing mammalian CNS previously enjoyed only by researchers studying oviparous vertebrates and lower organisms.

Major advances have been reported in directed differentiation of pluripotent cells into distinct classes of nerve cells (reviewed in [6, 7, 8, 9]). These studies firmly established that differentiating pluripotent cells respond correctly to developmental signals that pattern the developing CNS along the rostro-caudal or dorso-ventral axes and give rise to nerve cells expressing congruent region specific markers and neurotransmitter phenotypes. By contrast, less attention has been paid to whether in vitro generated neurons also acquire refined subtype specific properties. This is largely due to the lack of understanding of mechanisms underlying subtype diversification of mammalian nerve cells in vivo. Modeling the acquisition of subtype specific nerve cell identities in a simplified in vitro system might therefore provide a powerful tool to dissect the roles of cell-intrinsic genetic programs, extrinsic diffusible signals, and cell–cell interactions in the fine-grained diversification of developing nerve cells.