Induction of Somatosensory Neural Lineage from Human and Mouse Fibroblasts

Somatic sensory neurons of the dorsal root ganglion and trigeminal nerve mediate the sensation of chemical, thermal, painful, pruritogenic and mechanical stimuli based on cell type-specific expression of various receptor proteins and ion channels. Treatment of human pain, itch or inherited disorders affecting somatic sensory neurons would be aided by a means to produce these neurons in vitro from genetically diverse populations of humans.   However, at present the only method to generate human sensory neurons in vitro is to differentiate induced pluripotent stem cells into a restricted sub-class of nociceptors that do not recapitulate the endogenous neuronal diversity of in vivo sensory neurons.  Here we report that transient ectopic expression of two transcription factors rapidly and efficiently converts both human and mouse fibroblasts to mature neurons, without transiting through a proliferative intermediate.  Using immunostaining and RT-PCR we show that this conversion is largely selective for the somatosensory lineage, with the majority of neurons(~90%) expressing one of the three major somatic sensory sub-lineages markers, TrkA, TrkB or TrkC.  Remarkably, induced somatic sensory lineage neurons (iSLN) also exhibit pseudounipolar morphologies and subset-specific soma sizes that are characteristic of their in vivo counterparts, indicating that specific neuronal subtype morphology may be specified through direct reprogramming.  Additionally, iSLNs display functional diversity that is appropriate for somatic sensory neurons.  Through calcium imaging we show that subsets of iSLNs respond to distinct combinations of painful and pruritogenic compounds.  These findings demonstrate that discrete neural lineages can be induced and patterned through transient ectopic expression of only two transcription factors, showing that neither cell division nor exogenous factors are required for lineage specification. These results also provide rapid and efficient methods for generating human sensory neurons, delivering new technology for therapy, investigating fundamentals of pain, somatosensory biology, and therapeutic screening.