September 2, 2003
Chicken embryo research tunes into inner ear
WEST LAFAYETTE, Ind. Purdue University biologists have learned how to control the development of stem cells in the inner ears of embryonic chickens, a discovery which could potentially improve the ability to treat human diseases that cause deafness and vertigo.
By introducing new genes into the cell nuclei, researchers instructed the embryonic cells to develop into different adult cells than they would have ordinarily. Instead of forming the tiny hairs that the inner ear uses to detect sound waves, the stem cells matured into tissue with different kind of hairs the sort used to keep balance. This ability to guide the choice of cell types could expand researchers' knowledge of the inner ear and its disorders.
"We've essentially switched the fate of these cells," said Donna Fekete (pronounced FEH-ka-tee), associate professor of biology in Purdue's School of Science. "We now know at least one gene that determines what these embryonic ear cells will eventually become. As a result, we can control the outcome ourselves using gene transduction. Because so many people suffer from deafness later in life, we hope this research will yield treatments for them down the line."
The research appears in the current (9/1) issue of Developmental Biology.
Fekete's group stumbled onto these results after setting out to determine the function of a family of genes found in many embryonic cells. These genes, called "Wnt" genes, influence the development of organs from the brain to muscles, but they also seemed connected in some unknown capacity to the ear. Some evidence that pointed in this direction came from Fekete's collaborators in England, who work in the lab of Julian Lewis.
"We knew the Wnt genes were present in the ears of embryonic chicks," Fekete said. "We thought that altering the genes would perturb ear development in some way, and from a pure research perspective we wanted to know what that perturbation was. So, just to see what would happen, we used a modified retrovirus to deliver a souped-up version of a gene to make more cells experience the Wnt signal."
Retroviruses are the Trojan horses of the gene therapy world the infectious genetic material within their shells can be replaced with genes of the researcher's choosing, which the retrovirus then delivers to the nucleus of the target cell. The technique, when used on the chick cells, caused them to develop into otherwise healthy tissue that ordinarily appeared in different places in the inner ear.
"The inner ear uses two kinds of tiny hairs to sense sound and bodily motion," Fekete said. "These hairs are microscopic, and they are very different than the hairs you have on your head. The two kinds of inner ear hairs are different in one obvious respect both types grow in tufts, but those tufts used for balance also have single, long cilia that stretch out from among the hairs. After we turned the Wnt genes on, we saw these cilia growing in places usually reserved for the non-ciliated auditory hairs."
Many researchers overseas are trying to make stem cells develop into different types of adult cells in order to cure diseases, and Fekete said she believes this is the kind of information they will eventually need to help humans.
"More than half of the U.S. population over the age of 60 has some sort of hearing loss," she said. "These cases are often caused by degeneration of inner ear cells damaged over the long term. Many young people also lose their hearing from sudden acoustic trauma. If we are to replace the damaged cells, we will presumably need to know how to grow the right cell type."
Another problem this research could address is the set of disorders that cause vertigo, which includes Meniere's disease. This disorder, which strikes approximately one person out of 2,000 annually, causes bouts of severe disequilibrium and tinnitus and lasts for life.
"An added benefit of this discovery is that it not only switches the type of surface cells responsible for hairs, it switches the type of supporting cells as well. In other words, we can make entire sections of the inner ear grow one way or the other, which might permit doctors more options."
It will, however, be many years before such therapies might be ready for human testing.
"There's still a great deal of work to be done here," Fekete said. "We still are not sure what happens when you completely deactivate the Wnt signal, for example, and that's where our research is headed next. In any case, a cure for deafness based on this discovery won't be appearing in your drugstore anytime soon."
Fekete did say, however, that the research was yet another example of the potential of stem cell research.
"Even if we cannot do research on human stem cells, those taken from animals can still contribute to our understanding of how living things develop," she said. "It's work that needs to continue."
This research was supported in part by the National Institutes of Health.
Writer: Chad Boutin, (765) 494-2081, firstname.lastname@example.org
Source: Donna Fekete, (765) 496-3058, email@example.com
Purdue News Service: (765) 494-2096; firstname.lastname@example.org
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PHOTO CAPTION:Figure 1 shows part of the cochlea in an embryonic chicken's inner ear, where patches of vestibular hairs, used to detect balance, grew in place of those that detect sound waves. The arrow indicates one such patch. Figure 2 is a close-up that shows both types of inner-ear hairs, which grow in tufts in different locations. The inset shows the type that detects bodily motion, with the hairs themselves stained red and the telltale cilia that extend from motion-detecting tufts stained green.
Forced activation of Wnt signaling alters morphogenesis and sensory organ identity
Julian H. Lewis, and Donna M. Fekete
Components of the Wnt signaling pathway are expressed in the developing inner ear. To explore their role in ear patterning, we used retroviral gene transfer to force the expression of an activated form of beta-catenin that should constitutively activate targets of the canonical Wnt signaling pathway. At embryonic day 9 (E9) and beyond, morphological defects were apparent in the otic capsule and the membranous labyrinth, including ectopic and fused sensory patches. Most notably, the basilar papilla, an auditory organ, contained infected sensory patches with a vestibular phenotype. Vestibular identity was based on: (1) stereociliary bundle morphology; (2) spacing of hair cells and supporting cells; (3) the presence of otoliths; (4) immunolabeling indicative of vestibular supporting cells; and (5) expression of Msx1, a marker of certain vestibular sensory organs. Retrovirus-mediated misexpression of Wnt3a also gave rise to ectopic vestibular patches in the cochlear duct. In situ hybridization revealed that genes for three Frizzled receptors, c-Fz1, c-Fz7, and c-Fz10, are expressed in and adjacent to sensory primordia, while Wnt4 is expressed in adjacent, nonsensory regions of the cochlear duct. We hypothesize that Wnt/beta-catenin signaling specifies otic epithelium as macular and helps to define and maintain sensory/nonsensory boundaries in the cochlear duct.