Abstract
BackgroundEarly inner ear development requires the strict regulation of cell proliferation, survival, migration and differentiation, coordinated by the concerted action of extrinsic and intrinsic factors. Deregulation of these processes is associated with embryonic malformations and deafness. We have shown that insulin-like growth factor I (IGF-I) plays a key role in embryonic and postnatal otic development by triggering the activation of intracellular lipid and protein kinases. RAF kinases are serine/threonine kinases that regulate the highly conserved RAS-RAF-MEK-ERK signaling cascade involved in transducing the signals from extracellular growth factors to the nucleus. However, the regulation of RAF kinase activity by growth factors during development is complex and still not fully understood.Methodology/Principal FindingsBy using a combination of qRT-PCR, Western blotting, immunohistochemistry and in situ hybridization, we show that C-RAF and B-RAF are expressed during the early development of the chicken inner ear in specific spatiotemporal patterns. Moreover, later in development B-RAF expression is associated to hair cells in the sensory patches. Experiments in ex vivo cultures of otic vesicle explants demonstrate that the influence of IGF-I on proliferation but not survival depends on RAF kinase activating the MEK-ERK phosphorylation cascade. With the specific RAF inhibitor Sorafenib, we show that blocking RAF activity in organotypic cultures increases apoptosis and diminishes the rate of cell proliferation in the otic epithelia, as well as severely impairing neurogenesis of the acoustic-vestibular ganglion (AVG) and neuron maturation.Conclusions/SignificanceWe conclude that RAF kinase activity is essential to establish the balance between cell proliferation and death in neuroepithelial otic precursors, and for otic neuron differentiation and axonal growth at the AVG.
Highlights
The vertebrate inner ear is responsible for the detection of sound and balance, and it contains two main functional parts, the auditory system dedicated to hearing and the vestibular system that controls balance
insulin-like growth factor I (IGF-I) can activate the RAF-MEK-ERK cascade in explanted otic vesicles and by blocking RAF kinases with Sorafenib, we show that RAF activity is essential for cell proliferation
The expression of transcripts encoding these RAF isoforms was comparable from HH18 to HH24, the RNA transcripts for both these RAF kinases were strongly downregulated at stage HH27
Summary
The vertebrate inner ear is responsible for the detection of sound and balance, and it contains two main functional parts, the auditory system dedicated to hearing and the vestibular system that controls balance. This complex sensory organ derives from an ectodermic region adjacent to the hindbrain, the otic placode. Inner ear development requires the strict regulation of cell proliferation, survival, migration and differentiation, coordinated by the concerted action of extrinsic and intrinsic factors. Deregulation of these processes is associated with embryonic malformations and deafness. The regulation of RAF kinase activity by growth factors during development is complex and still not fully understood
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