Deaf Mice Study: Training the Brain for Sound Processing
The Surprising Discovery of Near-Normal Neural Activity
During the first two weeks of life, mice with an inherited form of deafness have near-normal neural activity in the auditory system, according to a new study by Johns Hopkins Medicine scientists. This unexpected finding suggests a molecular mechanism for understanding the effectiveness of cochlear implants, electronic devices that enhance hearing in people with hearing loss. The study demonstrates that even in the absence of a crucial protein called connexin 26, support cells in the cochlea continue to produce spontaneous activity, shaping the development of the auditory system.
The Role of Connexin 26 in Hearing Loss
The connexin 26 protein encoded by the Gjb2 gene plays a critical role in hearing. Mutations in Gjb2 are responsible for more than a quarter of all inherited forms of hearing loss at birth. This protein belongs to a family of proteins known as GAP junctions, which are responsible for exchanging ions, metabolites, and other molecules to maintain balance between cells.
Understanding Cochlear Implants and Hearing Improvement
The researchers found that people with the inherited mutation in connexin 26 respond well to cochlear implants, which mimic sound conduction in the inner ear. Cochlear implants have been shown to improve hearing in individuals with hearing problems. This aligns with the observation that despite the absence of connexin 26, the cochlea is still capable of generating spontaneous activity, preparing the brain for sound processing.
The Enrichment of Connexin 26 in Support Cells
The connexin 26 protein is highly enriched in support cells located in the cochlea, the spiral-shaped structure in the inner ear. These support cells provide structural and nutritional support to the surrounding hair cells and auditory neurons. Without connexin 26, the cochlea fails to develop its normal shape and is unable to amplify sound-induced vibrations necessary for efficient sound detection.
The Training Role of Support Cells
Support cells, also known as glial cells, play a crucial role in training the auditory system and preparing it to process sound. The research shows that during the early stages of development, before the ear canals open, support cells generate bursts of electrical activity. This spontaneous activity stimulates the mechanically sensitive hair cells in the fluid-filled cochlea. It’s as if the cochlea is making its own “sounds” to prepare the auditory neurons and circuits in the brain for the start of hearing.
Deaf Mice Study on Connexin 26
The study by neuroscientist Dwight Bergles, Ph.D., and Calvin Kersbergen, MD/Ph.D. candidate, focused on a mouse model lacking connexin 26 in the supporting cells of the cochlea. By measuring electrical responses in the auditory nerve using external electrodes, they found that mice without connexin 26 were deaf. However, they also discovered that these mice exhibited bursts of electrical activity in auditory neurons similar to those with intact connexin 26, indicating the crucial role of support cells in shaping neural activity.
Hypersensitivity to Sound in Deaf Mice
Mice lacking connexin 26 showed hypersensitivity to sound, similar to a condition called hyperacusis in humans. This hypersensitivity can result in hearing loss and a constant ringing in the ears known as tinnitus. Understanding this phenomenon provides insights into the molecular mechanisms behind hyperacusis and tinnitus and offers potential avenues for future research.
Implications for Early Cochlear Implant Placement
Furthermore, the study suggests that the presence of spontaneous activity in the supporting cells of the cochlea may explain why people with the inherited mutation who receive cochlear implants early tend to have better outcomes. Early implantation may leverage this spontaneous activity pathway to enhance the brain’s response to sound, leading to improved hearing outcomes.
Expanding the Understanding of Hearing Loss and Treatment
The findings from this study offer a new perspective on the role of connexin 26 and support cells in hearing loss and the development of the auditory system. Further research can explore harnessing the spontaneous activity pathway in support cells to develop treatments for conditions such as tinnitus and other hearing-related issues. Understanding the underlying mechanisms of sound processing can have significant implications for improving the quality of life for individuals with hearing loss.
Conclusion
In conclusion, the research conducted by Johns Hopkins Medicine scientists highlights the significant role of support cells and the connexin 26 protein in training the auditory system and preparing it to process sound. Despite its absence, support cells continue to generate spontaneous activity, shaping neural development. This discovery provides insights into why cochlear implants are effective in individuals with the inherited mutation and opens new possibilities for understanding and treating hearing-related conditions. Further research in this field will continue to unravel the mysteries of the auditory system, offering hope for improved treatments and interventions.
Summary
The recent study by Johns Hopkins Medicine scientists reveals that mice with an inherited form of deafness exhibit near-normal neural activity in the auditory system during the first two weeks of life. This early neural activity, generated by support cells in the cochlea, plays a vital role in training the auditory system to respond to sound. The connexin 26 protein, crucial for normal hearing, is highly enriched in these support cells. The study’s findings suggest a molecular mechanism for the effectiveness of cochlear implants in individuals with the inherited mutation. Furthermore, the research indicates that support cells generate spontaneous activity, even in the absence of connexin 26, shaping brain development to process sound. Understanding the role of support cells and the connexin 26 protein opens new avenues for improving treatments for hearing loss and related conditions, such as hyperacusis and tinnitus.
Source: PLOS Biology
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During the first two weeks of life, mice with an inherited form of deafness have near-normal neural activity in the auditory system, according to a new study by Johns Hopkins Medicine scientists. Their previous studies indicate that this early auditory activity, before the start of hearing, provides a kind of training to prepare the brain to process sound when hearing begins.
The findings are published June 27 in PLOS Biology.
Mutations in Gjb2 cause more than a quarter of all inherited forms of hearing loss at birth in people, according to some estimates. The connexin 26 protein encoded by the gene belongs to a family of proteins known as GAP junctions, because these proteins span the small space between cells and form a kind of tube that connects two cells to exchange ions, metabolites, and other molecules that communicate. or maintain. a balance
This unexpected finding, the researchers say, suggests a molecular mechanism for the observation that people with this inherited mutation respond well to cochlear implants, electronic devices that are designed to mimic sound conduction in the inner ear and can improve hearing. hearing in people with hearing problems. loss. According to the National Institutes of Health, between December 2019 and March 2021, about 118,100 cochlear implants were implanted in adults and 65,000 in children.
The connexin 26 protein in the cochlea, the spiral-shaped structure in the inner ear, is highly enriched in support cells which, as the name implies, provide structural and nutritional support to the surrounding hair cells and auditory neurons.
Previous studies have shown that without connexin 26, the cochlea fails to develop its normal shape and is unable to amplify sound-induced vibrations necessary for efficient sound detection. Despite this disruption to cochlear structure, this research shows that the cochlea is still capable of producing the “spontaneous” activity needed to shape brain development.
“Support cells are extremely important to tissues and organs,” says neuroscientist Dwight Bergles, Ph.D., the Diana Sylvestre and Charles Homcy Professor at the Johns Hopkins University School of Medicine. “The new study shows how critical they are in training the auditory system and preparing it to process sound.”
For the study, Bergles and Calvin Kersbergen, MD/Ph.D. candidate in the Johns Hopkins Medical Scientist Training Program, created a mouse model that lacked connexin 26 specifically in the supporting cells in the cochlea.
By using external electrodes to measure electrical responses in the auditory nerve in response to tones or clicks, they found that mice lacking connexin 26 only in the supporting cells of the cochlea were, in fact, deaf, demonstrating the crucial role of these intercellular channels in hearing.
However, Bergles and Kersbergen wondered if this change in support cells and the shape of the cochlea would also disrupt spontaneous activity in younger mice, less than 2 weeks old, before their ear canals open.
The researchers found that mice without connexin 26 still exhibit bursts of electrical activity in auditory neurons at nearly the same levels as young mice with intact connexin 26. Subsequent investigations revealed that spontaneous activity in the support cells was able to activate sensory hair cells in the inner ear. , leading to normal neural activity in the sound processing areas of the brain.
“Even in the absence of connexin 26, we still found robust spontaneous activity in the cochlea of these young mice,” says Bergles.
Bergles says there is now evidence that the role of support cells in this early period is to “train” the auditory system to respond to sound at certain frequencies. Since the ear canal is not yet open, the supporting cells spontaneously generate their own activity to stimulate the mechanically sensitive hair cells in the fluid-filled cochlea.
“It’s as if the cochlea is making its own ‘sounds’ at this stage of development,” says Bergles. “This practice may help auditory neurons and circuits in the brain mature before the ear canal opens.”
“It’s like a baseball player in a batting cage, learning the basics of his swing and preparing to deal with the unpredictability of a real pitcher,” Bergles says.
Finally, the researchers found that spontaneous activity in the support cells of the deaf mice stops once the ear canal is opened. At the same time, because the mice can’t process sound, their auditory neurons actually increase their sensitivity to sound.
This hypersensitivity to sound is similar to the phenomenon of hyperacusis, in which normal sound levels can be painful. In humans, this hypersensitivity-induced hearing loss can also cause a constant ringing in the ears, called tinnitus.
Bergles says the research also suggests a molecular mechanism for why people with this inherited mutation who receive cochlear implants early tend to have better outcomes than those who receive them later.
“Spontaneous activity in the supporting cells of the cochlea may provide molecular evidence for empirical data showing better outcomes among people who have cochlear implants placed earlier in life,” says Bergles.
The research team plans to study whether they can harness the spontaneous activity pathway in support cells to treat tinnitus and other hearing conditions.
Scientists Travis Babola and Patrick Kanold also contributed to this research.
Funding was provided by the National Institutes of Health (F30DC018711, F32DC019842, U19NS107464, R01DC009607, R01DC008860, P30NS050274).
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