Hmx gene conservation identifies the evolutionary origin of vertebrate cranial ganglia

The evolutionary origin of vertebrates included innovations in sensory processing associated with the acquisition of a predatory lifestyle1. Vertebrates perceive external stimuli through sensory systems serviced by cranial sensory ganglia (CSG) which develop from cranial placodes; however understanding the evolutionary origin of placodes and CSGs is hampered by the gulf between living lineages and difficulty in assigning homology between cell types and structures. Here we use the Hmx gene family to address this question. We show Hmx is a constitutive component of vertebrate CSG development and that Hmx in the tunicate Ciona is able to drive the differentiation program of Bipolar Tail Neurons (BTNs), cells previously thought neural crest homologs2,3. Using Ciona and lamprey transgenesis we demonstrate that a unique, tandemly duplicated enhancer pair regulated Hmx in the stem-vertebrate lineage. Strikingly, we also show robust vertebrate Hmx enhancer function in Ciona, demonstrating that deep conservation of the upstream regulatory network spans the evolutionary origin of vertebrates. These experiments demonstrate regulatory and functional conservation between Ciona and vertebrate Hmx, and confirm BTNs as CSG homologs. Our analysis also identifies derived evolutionary changes, including a genetic basis for secondary simplicity in Ciona and unique regulatory complexity in vertebrates.

Neuroblast sensory quiescence depends of vascular cytoneme contacts and sensory neuronal differentiation requires initiation of blood flow

SummaryIn many organs, stem cell function depends on the communication with their niche partners. Cranial sensory neurons develop in close proximity to blood vessels, however whether vasculature is an integral component of their niches is yet unknown. Here, two separate, novel roles for vasculature in cranial sensory neurogenesis in zebrafish are uncovered. The first involves precise spatiotemporal endothelial-neuroblast cytoneme contacts and Dll4-Notch signalling to restrain neuroblast proliferation. Secondly, we find that blood flow onset triggers a transcriptional response to modify neuroblast metabolic status and is required for sensory neuron differentiation. In contrast, no role of sensory neurogenesis in vascular development is found, suggesting a unidirectional signalling from vasculature to sensory neuroblasts. Altogether, we demonstrate that the cranial vasculature constitutes a hitherto unrecognized niche component of the sensory ganglia that regulates the pace of their growth and differentiation dynamics.Highlights♦ Vasculature is part of the cranial sensory ganglia niche and regulates neurogenesis.♦ Cytoneme contacts between endothelial cells and sensory neuroblasts are required for neuroblast quiescence.♦ Endothelial Dll4 and neuroblast Notch1 signal to regulate the growth of cranial sensory ganglia.♦ Initiation of blood flow triggers a transcriptional metabolic switch and sensory neuronal differentiation.

Cornichon-like Protein Facilitates Secretion of HB-EGF and Regulates Proper Development of Cranial Nerves

During their migration to the periphery, cranial neural crest cells (NCCs) are repulsed by an ErbB4-dependent cue(s) in the mesenchyme adjoining rhombomeres (r) 3 and 5, which are segmented hindbrain neuromeres. ErbB4 has many ligands, but which ligand functions in the above system has not yet been clearly determined. Here we found that a cornichon-like protein/cornichon homolog 2 (CNIL/CNIH2) gene was expressed in the developing chick r3 and r5. In a cell culture system, its product facilitated the secretion of heparin-binding epidermal growth factor-like growth factor (HB-EGF), one of the ligands of ErbB4. When CNIL function was perturbed in chick embryos by forced expression of a truncated form of CNIL, the distribution of NCCs was affected, which resulted in abnormal nerve fiber connections among the cranial sensory ganglia. Also, knockdown of CNIL or HB-EGF with siRNAs yielded a similar phenotype. This phenotype closely resembled that of ErbB4 knockout mouse embryos. Because HB-EGF was uniformly expressed in the embryonic hindbrain, CNIL seems to confine the site of HB-EGF action to r3 and r5 in concert with ErbB4.