Recently, our studies have focused on genes expressed at the earliest stages of inner ear development. Our aim is to identify and characterize genes that are involved in determining the axes of the semicircular canals, in otic crest delamination and in early innervation of the inner ear. Many elegant studies of auditory development have been done in animal models. However, the need for large amounts of well-characterized embryonic material for molecular studies makes the development of otocyst cell lines with different genetic repertoires attractive. We have therefore derived immortalized otocyst cells from two of the most widely used animal models of ear development: avians and mice. Avian cell isolates were produced from quail otocysts (embryonic stage 19) that were transformed with temperature-sensitive variants of the Rous sarcoma virus (RSV). Among the individual transformed cells are those that produce neuron-like derivatives in response to treatment with 10(-9) M retinoic acid. Mammalian cell isolates were derived from otocysts, of 9 day (post coitus) embryos of the H2kbtsA58 transgenic mouse (Immortomouse), which carries a temperature-sensitive variant of the Simian Virus 40 Tumor antigen. The vast majority of cells of the Immortomouse are capable of being immortalized at 33 degrees C, the permissive temperature for transgene expression, in the presence of gamma-interferon. Several putative clones et these cells differentiated into neuron-like cells after temperature shift and withdrawal of gamma-interferon; another isolate of cells assumed a neuron-like morphology on exposure to brain-derived neurotrophic factor even at the permissive temperature. We describe also a cell isolate that expresses the Pax-2 protein product and two putative cell lines that express the protein product of the chicken equivalent of the Drosophila segmentation gene engrailed. These genes and their protein products are expressed in specific subpopulation of otocyst cells at early stages. Both mouse and quail immortalized cell lines will be used to study inner ear development at the molecular level.
Early regionalized gene expression patterns within the otocyst appear to correlate with and contribute to development of mature otic structures. In the chick, the transcription factor Pax2 becomes restricted to the dorsal and entire medial side of the otocyst by stage 16/17. The dorsal region of the otocyst forms the endolymphatic duct and sac (ED/ES), and the cochlear duct is derived from the ventromedial region. In the mouse, however, Pax2 expression is reported only in the ventromedial and not the dorsal otocyst. In Pax2 null mice, the cochlea is missing or truncated, but vestibular structures differentiate normally. Here we demonstrate that in the chick, the emerging ED/ES express high levels of Pax2 even when the position of the emerging ED is altered with respect to its environment, either by 180 degrees otocyst rotations about the anterior/posterior axis or transplantation of the otocyst into the hindbrain cavity. However, the Pax2 expression pattern is plastic in the rest of the otic epithelium after 180 degrees rotation of the otocyst. Pax2 is upregulated on the medial side (formerly lateral), and downregulated on the lateral side (formerly medial and expressing Pax2) indicating that Pax2 expression is influenced by the environment. Although Pax2 is upregulated in the epithelium after 180 degrees rotations in the region that should form the cochlear duct, cochlear ducts are truncated or absent, and the ED/ES emerge in a new ventrolateral position. Ablation of the hindbrain at the placode or early otic pit stage alters the timing of regionalized Pax2 expression in the otocyst. The resulting otocysts and ears are generally smaller, vestibular structures are abnormal, ED/ES are missing but cochlear ducts are of normal length. The hindbrain and dorsal periotic mesenchyme provide unique trophic and patterning information to the dorsal otocyst. Our results demonstrate that the ED is the earliest structure patterned in the inner ear and that the hindbrain is important for its specification. We also show that, although normal Pax2 expression is required for cochlear duct development, it is downstream of ventral otocyst patterning events.
Differential display of mRNA is a technique that enables the researcher to compare genes expressed in two or more different tissues or in the same tissue or cell under different conditions. The method is based on polymerase chain reaction amplification and comparison of specific subsets of mRNA. We have used this method to clone partial complementary DNAs (cDNAs; amplicons) for genes expressed in the otocyst in order to identify genes that may be involved in development of the inner ear. A full length cDNA was isolated from an embryonic quail head library with an amplicon (KH121) obtained from the otocyst. This avian cDNA encoded a novel, 172-amino acid acidic protein and detected a major transcript of ca 0.8 kb in RNA from chick embryos and several neonatal chick tissues. The full length avian cDNA had high sequence identity to several human cDNAs (expressed sequence tags) from human fetal tissues, including cochlea, brain, liver/spleen and lung, and from placenta. The human homologue of the avian gene encoded a protein that was 183 amino acids long and had 75.6% amino acid sequence identity to the avian protein. These results identified both the avian and human homologues of an evolutionarily conserved gene encoding a small acidic protein of unknown function; however, expression of this gene was not restricted to otocysts.
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