Investigating Brain Repair and Development Using Stem Cells

Abstract

The directed differentiation of human pluripotent stem cells into neuronal subtypes has generated immense enthusiasm that they could be used therapeutically or to study development in vitro. Two areas of particular interest are the replacement of midbrain dopaminergic neurons that degenerate in Parkinson’s disease and the ability to study the early development of the human cortex.

The loss of midbrain dopamine neurons in Parkinson’s disease leads to a breakdown in basal ganglia circuitry and motor dysfunction. Previous clinical trials utilizing fetal dopamine tissue have provided proof-of-principle that transplantation of new dopamine neurons can relieve motor symptoms for up to two decades. However, the widespread use of fetal tissue in the clinic presents multiple ethical and logistical hurdles. As a result, the generation of human pluripotent stem cell-derived dopamine neurons has been an area of intense focus in recent years. Current protocols are not amenable to clinical translation and generate dopamine neurons that poorly reinnervate the striatum following transplantation. Here we develop a fully-defined protocol for midbrain dopamine generation and improve transplantation survival, plasticity and function via overexpression of glial cell line-derived neurotrophic factor.

In contrast to dopaminergic differentiation strategies, protocols for deriving neurons of the neocortex are relatively limited, generating heterogenous populations of progenitors, neurons and glia. The human neocortex is arguably the most altered structure during mammalian evolution and underlies the cognitive abilities that define human intelligence. However, the majority of research into cortical development has been carried out in frog, chick or mouse models. Therefore, there is an unmet need to investigate how the relative complexity of the human cortex is developed. In particular, the intrinsic and extrinsic cues that control temporal development and drive the progressive generation of neuronal subtypes that form the six-layered mammalian neocortex remain unknown. Here we investigated the role of FGF-ERK signaling in the development of early-born, deep layer neurons of the neocortex in a reductionist pluripotent stem cell model. We find that FGF-ERK signaling, in part, modulates the timing and temporal progression of neocortical progenitors. Together, these studies advance the use of pluripotent stem cell tools to model neural development and to aid in neuroregeneration.