The mammalian nervous system comprises many distinct neuronal subtypes, each with its own phenotype and differential sensitivity to degenerative disease. Although specific neuronal types can be isolated from rodent embryos or engineered from stem cells for translational studies, transcription factor-mediated reprogramming might provide a more direct route to their generation. Here we report that the forced expression of select transcription factors is sufficient to convert mouse and human fibroblasts into induced motor neurons (iMNs). iMNs displayed a morphology, gene expression signature, electrophysiology, synaptic functionality, in vivo engraftment capacity, and sensitivity to degenerative stimuli similar to those of embryo-derived motor neurons. We show that the converting fibroblasts do not transit through a proliferative neural progenitor state, and thus form bona fide motor neurons via a route distinct from embryonic development. Our findings demonstrate that fibroblasts can be converted directly into a specific differentiated and functional neural subtype, the spinal motor neuron.

Introduction

The mammalian central nervous system (CNS) is assembled from a diverse collection of neurons, each with its own unique properties. These discrete characteristics underlie the proper integration and function of each neuron within the circuitry of the brain and spinal cord. However, their individual qualities also render particular neurons either resistant or sensitive to distinct degenerative stimuli. Thus, for each neurodegenerative disease, a stereotyped set of neuronal subtypes is destroyed, causing the hallmark presentation of that condition. Therefore, if we are to comprehend the mechanisms that underlie the development, function, and degeneration of the CNS, we must first deeply understand the properties of individual neuronal subtypes.

Wichterle et al., 2002 Wichterle H.

Lieberam I.

Porter J.A.

Jessell T.M. Directed differentiation of embryonic stem cells into motor neurons. Peljto and Wichterle, 2011 Peljto M.

Wichterle H. Programming embryonic stem cells to neuronal subtypes. Physiological and biochemical studies of individual neuronal types have been greatly facilitated by the ability to isolate distinct classes of neurons and interrogate them in vitro. Most studies have focused on neurons isolated from the developing rodent CNS. However, it is not routinely possible to isolate analogous populations of human neurons or to isolate and fully study differentiated central neurons. Pluripotent stem cells, such as embryonic stem cells (ESCs), may provide an inexhaustible reservoir of diverse neural subtypes, offering an attractive approach for in vitro studies (). Although stem cells have shown great promise, to date, only a handful of neural subtypes have been produced in this way. Furthermore, in many cases the neuronal populations produced from stem cells have not been shown to possess refined, subtype-specific properties, and may only superficially resemble their counterparts from the CNS ().

To determine whether transcription factors can bestow a precise neural subtype identity, we sought factors that could reprogram fibroblasts into spinal motor neurons. Motor neurons control the contraction of muscle fibers actuating movement. Damage to motor neurons caused by either injury or disease can result in paralysis or death; consequently, there is significant interest in understanding how motor neurons regenerate after nerve injury and why they are selective targets of degeneration in diseases such as spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). We therefore attempted induction of motor neurons both because of their significant translational utility and because the developmental origins and functional properties of this neural subtype are among the most well understood.