Researchers at Washington University School of Medicine have identified a gene that is required for proper development of the inner ear of mice.
Researchers first hypothesized that the fibroblast growth factors sensory (FGF20) had a direct correlation to the loss of hearing in mice. They designed a series of studies to test if mice with the gene inactivated would have a loss of hearing compared to mice with an activated gene.
The study began with the generation of the FGF20 mutant mice. Briefly, exon 1 of Fgf20 was replaced with a βGal-LoxP-neomycin-LoxP cassette. After mating the newly generated mice, they now had mice with the gene inactivated. The next part of their test was the auditory brainstem response test, in which they tested both the mutated mice and the normal mice, to discern the difference. They set up an environment to control, in which each mouse was anesthetized and maintained at 37ºC, all carried out in a single-walled sound-attenuating room. The independent variable was the different volumes of sound; the dependent variable was the auditory profiles of each mouse. These were recorded using platinum sub-dermal needle electrodes placed with the recording electrode behind the right pinna, the reference electrode at the vertex, and the ground electrode in the skin of the back.
After recording the frequencies of their response, the experimenters then switched to different tests. They conducted an RNA extraction, cDNA synthesis, and Quantitive RT-PCR in order to detect the FGF20 gene and to quantify it. βGal staining was also completed, as well as hair cell and supporting cell counting using Image J Software. It was determined that the FGF20 signaling (or its chemical equivalent, FGF9) must occur on or before day 14 of the embryo’s development to produce a normal inner ear. This was ascertained through the establishment of embryonic mouse cochlear cultures with minor modifications – they would modify them at specific times within the incubation period to pinpoint when the mutations must occur for hearing loss.
The main control of this experiment was the procedures completed on the non-modified mice. By making the independent variable the inactivation of the FGF20 sensory gene, the dependent variable, in turn, is the loss of hearing in the mice. The specific controlled variables varied between each different part of their studies.
They found that when they inactivated the FGF20, the mice were viable, fertile and healthy, albeit lacking auditory perception, with no ear twitching in response to loud noise. Specifically, the mice with the inactivated gene had auditory brainstem response thresholds greater than 40 db, which was much above the controlled mice (the ones without the gene inactivated), which had the usual 5-20 kHz range. These results showed that disabling the FGF20 gene causes a loss of outer hair cells, which are a special type of sensory cell in the inner ear responsible for amplifying sound. With the loss of these hairs, the ear no longer amplifies noise, and the mice are left deaf. While about two-thirds of the outer hair cells were missing in mice without FGF20, the number of inner hair cells, the cells responsible for transmitting the amplified signals to the brain, appeared normal.
It is hypothesized that FGF20 signaling will be a required step toward the goal of regenerating outer hair cells in mammals (reversing deafness). It is known that the fibroblast growth factors play important and broad roles in embryonic development, tissue maintenance and wound healing, including the regrowth of zebra-fish fins among other species of vertebrates other than mammals. Ideally, it should next be asked how this relates to humans. The more we understand the mutations of the fibroblast growth factors and their role in the cause of deafness, the closer we are to finding a way to avoid these mutations and possibly cure hearing loss. Are humans affected by this FGF20 signaling gene? Is there any way that it can be modified in order for humans to be able to regrow the inner hair cells, so as to counter hearing loss permanently?
These questions can only be answered as more tests and studies are done. In the future, a cure for hearing loss may be reached, thanks to the bona fide lab rat.
Hayashi T, Ray C. A, Bermingham-McDonogh O. 2008. Fgf20 is required for sensory epithelial specification in the developing cochlea. Journal of Neuroscience. 28: 5991–5999.
Hayashi T, Ray C. A, Younkins C, Bermingham-McDonogh O. 2010. Expression patterns of FGF receptors in the developing mammalian cochlea. Developmental Dynamics. 239: 1019–1026.
Huh S-H, Jones J, Warchol ME, Ornitz DM. 2012. Differentiation of the Lateral Compartment of the Cochlea Requires a Temporally Restricted FGF20 Signal. PLoS Biology. 10: 1-12.
Shim K, Minowada G, Coling D. E, Martin G. R. 2005. Sprouty2, a mouse deafness gene, regulates cell fate decisions in the auditory sensory epithelium by antagonizing FGF signaling. Developmental Cell. 8: 553–564.
Zhang X, Ibrahimi O. A, Olsen S. K, Umemori H, Mohammadi M, Ornitz D. M. 2006. Receptor specificity of the fibroblast growth factor family, The complete mammalian FGF family. Journal of Biological Chemistry. 281: 15694–15700.
Original:
http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001231
Others:
http://onlinelibrary.wiley.com/doi/10.1002/dvdy.21026/full
http://neuro.cjb.net/content/31/49/18104.full.pdf+html?sid=f31b6389-1214-4132-8ee7-a952742e26fd
