Molecular pathways of proprioceptive dorsal root ganglion (DRG) sensory neuron specification

Dorsal root ganglia (DRG) sensory neurons can be classified into distinct neuronal subpopulations based on the expression of specific receptor tyrosine kinases known as neurotrophic factor receptors. Nociceptive and thermoceptive DRG neurons express the nerve growth factor (NGF) receptor tyrosine kinase TrkA, mechanoreceptive DRG neurons express TrkB, the receptor for brain derived neurotrophic factor (BDNF) and neurotrophin 4/5 (NT4/5), whereas proprioceptive DRG neurons express the neurotrophin 3 (NT3) receptor TrkC. Moreover, a subpopulation of DRG neurons expresses the receptor tyrosine kinase Ret mediating signaling in response to glial cell line derived neurotrophic factor (GDNF) family proteins.
Neurotrophic factor receptor expression is known to play important roles not only in mediating survival of DRG sensory neuron subpopulations, but also in controlling the acquisition of phenotypic traits. The choice to express a particular neurotrophic factor receptor is associated with the establishment of unique functional properties and patterns of connectivity during embryonic development. By eliminating the pro-apoptotic Bcl2 (B-cell leukemia/ lymphoma 2) family member gene Bax (Bcl2-associated X protein) concomitant with the gene encoding NT3 we could show that muscle derived NT3 is responsible for inducing expression of the ETS transcription factor Er81 in TrkC positive proprioceptive DRG neurons. In absence of NT3 signaling, Er81 expression is severely reduced in proprioceptive sensory neurons, and therefore proprioceptive afferents fail to project into the ventral horn of the spinal cord and to establish monosynaptic connections with spinal α-motor neurons. Hence, peripheral NT3 signaling is required for establishment of sensory-motor connectivity during development via induction of Er81 expression in proprioceptive DRG neurons.
In contrast to the downstream molecular signaling events elicited by neurotrophic factors, very little is known about the molecular mechanisms underlying the selectivity of neurotrophic factor receptor choice in early differentiating DRG sensory neurons. In a differential screen aimed at the identification of novel marker genes expressed by differentiating DRG neurons we identified the runt related transcription factor Runx3. At
early developmental stages, Runx3 expression is tightly associated with a sensory neuron subpopulation that expresses TrkC alone, but excluded from TrkA, TrkB and Ret expressing DRG sensory neurons. Using genetic manipulations in the mouse, we found that Runx3 is specifically required within TrkC expressing DRG neurons to promote a neuronal phenotype associated with a proprioceptive identity by suppressing TrkB expression within presumptive proprioceptive DRG neurons, thus contributing to the consolidation of a proprioceptive fate through the coordinate extinction of an alternative neuronal fate. In contrast, Runx3 is not sufficient to promote conversion of TrkA expressing cutaneous DRG neurons into proprioceptive sensory neurons. Together, these findings demonstrate that the status of Runx3 expression is of key importance for the proper differentiation of proprioceptive afferents by controlling neurotrophic factor receptor choice in DRG neurons.