ABSTRACT
Interactions between FGF10 and the IIIb isoform of FGFR-2 appear to be crucial for the induction and growth of several organs, particularly those that involve budding morphogenesis. We determined their expression patterns in the inner ear and analyzed the inner ear phenotype of mice specifically deleted for the IIIb isoform of FGFR-2. FGF10 and FGFR-2(IIIb) mRNAs showed distinct, largely nonoverlapping expression patterns in the undifferentiated otic epithelium. Subsequently, FGF10 mRNA became confined to the presumptive cochlear and vestibular sensory epithelia and to the neuronal precursors and neurons. FGFR-2(IIIb) mRNA was expressed in the nonsensory epithelium of the otocyst that gives rise to structures such as the endolymphatic and semicircular ducts. These data suggest that in contrast to mesenchymal-epithelial-based FGF10 signaling demonstrated for other organs, the inner ear seems to depend on paracrine signals that operate within the epithelium. Expression of FGF10 mRNA partly overlapped with FGF3 mRNA in the sensory regions, suggesting that they may form parallel signaling pathways within the otic epithelium. In addition, hindbrain-derived FGF3 might regulate otocyst morphogenesis through FGFR-2(IIIb). Targeted deletion of FGFR-2(IIIb) resulted in severe dysgenesis of the cochleovestibular membraneous labyrinth, caused by a failure in morphogenesis at the otocyst stage. In addition to the nonsensory epithelium, sensory patches and the cochleovestibular ganglion remained at a rudimentary stage. Our findings provide genetic evidence that signaling by FGFR-2(IIIb) is critical for the morphological development of the inner ear.
Subject(s)
Ear, Inner/embryology , Fibroblast Growth Factors/genetics , Protein Isoforms/genetics , Receptor Protein-Tyrosine Kinases/genetics , Receptors, Fibroblast Growth Factor/genetics , Signal Transduction/physiology , Animals , Cochlea/cytology , Cochlea/embryology , Cochlea/metabolism , Ear, Inner/cytology , Ear, Inner/metabolism , Embryo, Mammalian , Fibroblast Growth Factor 10 , Fibroblast Growth Factor 3 , Mice , Mice, Knockout , Mice, Transgenic , Mutation/physiology , Phenotype , Protein Isoforms/deficiency , Proto-Oncogene Proteins/genetics , RNA, Messenger/metabolism , Receptor Protein-Tyrosine Kinases/deficiency , Receptor, Fibroblast Growth Factor, Type 2 , Receptors, Fibroblast Growth Factor/deficiencyABSTRACT
We have studied the mechanisms of auditory hair cell death after insults in vitro and in vivo. We show DNA fragmentation of hair cell nuclei after ototoxic drug and intense noise trauma. By using phospho-specific c-Jun-N-terminal kinase (JNK) and c-Jun antibodies in immunohistochemistry, we show that the JNK pathway, associated with stress, injury, and apoptosis, is activated in hair cells after trauma. CEP-1347, a derivative of the indolocarbazole K252a, is a small molecule that has been shown to attenuate neurodegeneration by blocking the activation of JNK (). Subcutaneously delivered CEP-1347 attenuated noise-induced hearing loss. The protective effect was demonstrated by functional tests, which showed less hearing threshold shift in CEP-1347-treated than in nontreated guinea pigs, and by morphometric methods showing less hair cell death in CEP-1347-treated cochleas. In organotypic cochlear cultures, CEP-1347 prevented neomycin-induced hair cell death. In addition to hair cells, CEP-1347 promoted survival of dissociated cochlear neurons. These results suggest that therapeutic intervention in the JNK signaling cascade, possibly by using CEP-1347, may offer opportunities to treat inner ear injuries.
Subject(s)
Carbazoles/pharmacology , Enzyme Inhibitors/pharmacology , Hair Cells, Auditory/cytology , Hearing Loss, Noise-Induced/drug therapy , Indoles/pharmacology , Neurons, Afferent/cytology , Proto-Oncogene Proteins c-jun/antagonists & inhibitors , Aminoglycosides/toxicity , Animals , Cell Death/drug effects , Cell Survival/drug effects , Cells, Cultured , Enzyme Activation/drug effects , Hair Cells, Auditory/drug effects , Hair Cells, Auditory/physiology , Hearing Loss, Noise-Induced/chemically induced , Hearing Loss, Noise-Induced/pathology , Neomycin/toxicity , Neurons, Afferent/drug effects , Neurons, Afferent/physiology , Noise/adverse effects , Proto-Oncogene Proteins c-jun/metabolism , Rats , Rats, Wistar , Signal Transduction/drug effects , Signal Transduction/physiologyABSTRACT
We studied the expression of neurotrophins and their Trk receptors in the chicken cochlea. Based on in situ hybridization, brain-derived neurotrophic factor (BDNF) is the major neurotrophin there, in contrast to the mammalian cochlea, where neurotrophin-3 (NT-3) predominates. NT-3 mRNA labeling was weak and found only during a short time period in the early cochleas. During embryogenesis, BDNF mRNA was first seen in early differentiating hair cells. Afferent cochlear neurons expressed trkB mRNA from the early stages of gangliogenesis onward. In accordance, in vitro, BDNF promoted survival of dissociated neurons and stimulated neuritogenesis from ganglionic explants. High levels of BDNF mRNA in hair cells and trkB mRNA in cochlear neurons persisted in the mature cochlea. In addition, mRNA for the truncated TrkB receptor was expressed in nonneuronal cells, specifically in supporting cells, located adjacent to the site of BDNF synthesis and nerve endings. Following acoustic trauma, regenerated hair cells acquired BDNF mRNA expression at early stages of differentiation. Truncated trkB mRNA was lost from supporting cells that regenerated into hair cells. High levels of BDNF mRNA persisted in surviving hair cells and trkB mRNA in cochlear neurons after noise exposure. These results suggest that in the avian cochlea, peripheral target-derived BDNF contributes to the onset and maintenance of hearing function by supporting neuronal survival and regulating the (re)innervation process. Truncated TrkB receptors may regulate the BDNF concentration available to neurites, and they might have an important role during reinnervation.
