An ancient neurotrophin receptor code; a single Runx/Cbfβ complex determines somatosensory neuron fate specification in zebrafish

In terrestrial vertebrates such as birds and mammals, neurotrophin receptor expression is considered fundamental for the specification of distinct somatosensory neuron types where TrkA, TrkB and TrkC specify nociceptors, mechanoceptors and proprioceptors/mechanoceptors, respectively. In turn, Runx transcription factors promote neuronal fate specification by regulating neurotrophin receptor and sensory receptor expression where Runx1 mediates TrkA+ nociceptor diversification while Runx3 promotes a TrkC+ proprioceptive/mechanoceptive fate. Here, we report in zebrafish larvae that orthologs of the neurotrophin receptors in contrast to terrestrial vertebrates mark overlapping and distinct subsets of nociceptors suggesting that TrkA, TrkB and TrkC do not intrinsically promote nociceptor, mechanoceptor and proprioceptor/mechanoceptor neuronal fates, respectively. While we find that zebrafish Runx3 regulates nociceptors in contrast to terrestrial vertebrates, it shares a conserved regulatory mechanism found in terrestrial vertebrate proprioceptors/mechanoceptors in which it promotes TrkC expression and suppresses TrkB expression. We find that Cbfβ, which enhances Runx protein stability and affinity for DNA, serves as an obligate cofactor for Runx in neuronal fate determination. High levels of Runx can compensate for the loss of Cbfβ, indicating that in this context Cbfβ serves solely as a signal amplifier of Runx activity. Our data suggests an alteration/expansion of the neurotrophin receptor code of sensory neurons between larval teleost fish and terrestrial vertebrates, while the essential roles of Runx/Cbfβ in sensory neuron cell fate determination while also expanded are conserved.

在鸟类与哺乳类等陆生脊椎动物中，神经营养因子受体（neurotrophin receptor）的表达对于不同体感神经元（somatosensory neuron）类型的特化至关重要，其中TrkA、TrkB与TrkC分别特化伤害感受器（nociceptor）、机械感受器（mechanoceptor）以及本体感受器/机械感受器（proprioceptor/mechanoceptor）。反过来，Runx转录因子（Runx transcription factor）通过调控神经营养因子受体与感觉受体的表达来促进神经元命运特化：Runx1介导TrkA阳性伤害感受器神经元的多样化，而Runx3则促成TrkC阳性本体/机械感受器命运的形成。本研究以斑马鱼幼体为研究对象，发现与陆生脊椎动物不同，神经营养因子受体的同源基因（ortholog）会标记出重叠且独特的伤害感受器亚群，这表明TrkA、TrkB与TrkC并非分别内在地促成伤害感受器、机械感受器以及本体/机械感受器的神经元命运。尽管我们发现斑马鱼的Runx3调控伤害感受器神经元的过程与陆生脊椎动物存在差异，但二者共享一套保守的调控机制：Runx3可促进TrkC的表达并抑制TrkB的表达，这一机制在陆生脊椎动物的本体/机械感受器中已被证实。我们还发现，Cbfβ蛋白（Cbfβ）作为可增强Runx蛋白稳定性及其与DNA结合亲和力的因子，是Runx在神经元命运决定过程中的必需辅因子（obligate cofactor）。高表达量的Runx可代偿Cbfβ的缺失，这表明在该场景下Cbfβ仅作为Runx活性的信号放大器发挥作用。我们的研究数据表明，硬骨鱼幼体与陆生脊椎动物之间，体感神经元的神经营养因子受体编码模式发生了改变或扩展；而Runx/Cbfβ复合体在感觉神经元细胞命运决定中的核心作用虽得到扩展，但仍保持保守。