inner ear

Stability in an Unstable World

By Timothy S. Balmer, Ph.D., and Laurence O Trussell, Ph.D.

Balmer & Trussell traced the direct and indirect pathways that carry vestibular information to the cerebellum for controlling balance and posture. Shown here is a primary afferent axon (green) expressing the light-gated ion channel, Channelrhodopsin. Postsynaptic cells, in this case a unipolar brush cell (magenta), were recorded from during stimulation of the input axons by light flashes. This technique was used to discover how direct and indirect vestibular pathways are processed in the cerebellum.

Balmer & Trussell traced the direct and indirect pathways that carry vestibular information to the cerebellum for controlling balance and posture. Shown here is a primary afferent axon (green) expressing the light-gated ion channel, Channelrhodopsin. Postsynaptic cells, in this case a unipolar brush cell (magenta), were recorded from during stimulation of the input axons by light flashes. This technique was used to discover how direct and indirect vestibular pathways are processed in the cerebellum.

Mice are helping scientists to understand how the world around us remains looking stable even as we move.

While out jogging, you have no trouble keeping your eyes fixed on objects in the distance even though your head and eyes are moving with every step. Humans owe this stability of the visual world partly to a region of the brain called the vestibular cerebellum. From its position underneath the rest of the brain, the vestibular cerebellum detects head motion and then triggers compensatory movements to stabilize the head, body and eyes.

The vestibular cerebellum receives sensory input from the body via direct and indirect routes. The direct input comes from five structures within the inner ear, each of which detects movement of the head in one particular direction. The indirect input travels to the cerebellum via the brainstem, which connects the brain with the spinal cord. The indirect input contains information on head movements in multiple directions combined with input from other senses such as vision.

Balmer & Trussell traced the direct and indirect pathways that carry vestibular information to the cerebellum for controlling balance and posture. Direct projections from the vestibular inner ear (green) and indirect projections from the brainstem (magenta) were shown to target different populations of neurons in the cerebellum.

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By studying the mouse brain, Balmer and Trussell have now mapped the direct and indirect circuits that carry sensory information to the vestibular cerebellum. Both types of input activate cells within the vestibular cerebellum called unipolar brush cells (UBCs). There are two types of UBCs: ON and OFF. Direct sensory input from the inner ear activates only ON UBCs. These cells respond to the arrival of sensory input by increasing their activity. Indirect input from the brainstem activates both ON UBCs and OFF UBCs. The latter respond to the input by decreasing their activity.

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The vestibular cerebellum thus processes direct and indirect inputs via segregated pathways containing different types of UBCs. The next step in understanding how the cerebellum maintains a stable visual world is to identify the circuitry beyond the UBCs. Understanding these circuits will ultimately provide insights into balance disorders, such as vertigo.

A 2017 Emerging Research Grants (ERG) scientist who received the Les Paul Foundation Award for Tinnitus Research, Timothy Balmer, Ph.D., is a postdoctoral fellow at the Oregon Hearing Research Center at Oregon Health & Science University (OHSU). Laurence Trussell, Ph.D., a 1991 ERG recipient, is a professor of otolaryngology–head and neck surgery at OHSU.

This research summary was repurposed with permission from eLife with permission.

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Improved TMC1 Gene Therapy Restores Hearing and Balance in Mice

By Christopher Geissler, Ph.D.

Half of all inner ear disorders, which have a negative impact on hearing and/or balance, are caused by genetic mutations. A study published in January 2019 in Nature Communications demonstrates the effectiveness of a gene therapy targeting one specific gene mutation, TMC1 (transmembrane channel-like 1). The research was conducted by Carl A. Nist-Lund in the Harvard Medical School lab of Gwenaëlle S. Géléoc, Ph.D., and Jeffrey R. Holt, Ph.D., with contributions from colleagues including 2017 Emerging Research Grants (ERG) recipient Jennifer Resnik, Ph.D., and her ERG co-principal investigator Daniel B. Polley, Ph.D., both also of Harvard Medical School.

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So far, 35 TMC1 mutations have been identified in humans, including several that are responsible for moderate to severe hearing loss, representing between 3 to 8 percent of cases of genetic hearing loss. This TMC1 gene therapy has had an encouraging level of success in mice and may prove capable of addressing similar genetic mutations in humans in the future.

Previous studies targeting this gene were only moderately successful in restoring function in inner hair cells, with little or no success in outer hair cells. Both types of hair cell are necessary for hearing.

The team decided to look at improving the mechanism that encodes TCM1 in affected mice, using a synthetic delivery vehicle they hoped would be more effective than the conventional one used in previous studies. In mice with this TCM1 mutation, hair cells begin to die when the mouse reaches 4 weeks of age. The treated mice in this study showed improved rates of survival in both inner and outer hair cells.

Most importantly, the improvement in hearing in the mice that received this intervention occurred primarily in the lower frequencies. Human speech is at the low to mid frequency range of the auditory spectrum, so if future human trials are able to replicate the success of this study, speech perception may improve.

The study additionally provided evidence of improved responses in the brain of the treated mice. This indicates that treatment of the cochlea by injection had knock-on effects in the auditory cortex, the part of the brain that plays an important role in hearing.

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Finally, the team recorded improved balance function in the mice that received the gene therapy. While only very young mice experienced better hearing, even older mice showed improvement in balance. The team writes that this improvement in balance function in mature mice may contribute to eventually developing a way to treat balance disorders in humans.

Jennifer Resnik, Ph.D., is a postdoctoral fellow in the Polley Lab, part of the Eaton Peabody Laboratories, Massachusetts Eye and Ear/Harvard Medical School. Her 2017 Emerging Research Grant was generously funded by Hyperacusis Research Ltd. Christopher Geissler, Ph.D., is HHF’s director of program and research support.

Empower groundbreaking research toward better treatments and cures for hearing loss and tinnitus. If you are able, please make a contribution today.

 
 
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Uncovering a Signaling Molecule That Modulates Avian Hair Cell Regeneration

By Rebecca M. Lewis, Au.D., Ph.D., and Jennifer Stone, Ph.D.

Mammals including humans cannot regenerate hair cells, but other species such as birds and fish readily regenerate hair cells after damage to restore auditory function. The gene ATOH1 produces a protein that pushes supporting cells—cells that neighbor hair cells—to either directly convert into a hair cell or to divide and form a new hair cell. However, ATOH1 expression (when the gene is turned on) does not guarantee that hair cells develop in birds or mammals, which suggests that there are factors that prevent supporting cells from changing into hair cells. Identifying these factors in birds may help us better understand the lack of hair cell regeneration in mammals.

This schematic depicts our current ideas for how BMP4 regulates ATOH1 expression and therefore hair cell regeneration in the avian hearing organ. It shows (from left) typical hair cells, hair cell damage, and hair cell regeneration. Typical hair cells secrete BMP4. When hair cells die, BMP4 signaling is reduced, which allows ATOH1 to be expressed in supporting cells and pushes supporting cells to turn into hair cells. The newly regenerated hair cells secrete BMP4, suppressing ATOH1 in supporting cells and restoring the normal condition.

This schematic depicts our current ideas for how BMP4 regulates ATOH1 expression and therefore hair cell regeneration in the avian hearing organ. It shows (from left) typical hair cells, hair cell damage, and hair cell regeneration. Typical hair cells secrete BMP4. When hair cells die, BMP4 signaling is reduced, which allows ATOH1 to be expressed in supporting cells and pushes supporting cells to turn into hair cells. The newly regenerated hair cells secrete BMP4, suppressing ATOH1 in supporting cells and restoring the normal condition.

We examined the avian auditory system to characterize a potential inhibitor to ATOH1 during hair cell regeneration: bone morphogenetic protein 4 (BMP4). Bone morphogenetic proteins are secreted signaling molecules that regulate cellular processes in many regions of the body, including the nervous system. We found that BMP4 localizes to hair cells of the mature avian hearing organ and disappears when hair cells die or sustain damage. From this, we hypothesized that BMP4 may prevent ATOH1 expression in supporting cells and loss of BMP4 when hair cells die may enable ATOH1 to be expressed in supporting cells, driving them to convert into hair cells.

When we exposed avian auditory organs to BMP4 after selectively killing hair cells, this prevented ATOH1 expression and hair cell regeneration. When we antagonized BMP4 using an inhibitor, we found a generally opposite result: an increase in the number of regenerated hair cells.

We conclude that BMP4 is a potent inhibitor of ATOH1 and therefore suppresses hair cell regeneration. We recommend that BMP4 be explored further in studies of mammalian hair cell regeneration.

Published in Hearing Research on May 2, 2018, this study detailing BMP4’s negative effect on ATOH1 expands our knowledge of signaling molecules that suppress hair cell regeneration in birds and may also modulate hair cell regeneration in humans.

Rebecca M. Lewis, Au.D., Ph.D., is a clinical audiologist and auditory neuroscientist at Massachusetts Eye and Ear/Harvard Medical School in Boston. HRP researcher Jennifer Stone, Ph.D., is the director of research in the department of otolaryngology–head and neck surgery at the Virginia Merrill Bloedel Hearing Research Center at the University of Washington.

Empower the Hearing Restoration Project's life-changing research. If you are able, please make a contribution today.

 
 
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The Countdown to Operation Regrow

By Gina Russo

Hearing Health Foundation (HHF) is counting down the days until the start of Operation Regrow, a two-week movement when you can help us to further progress toward better treatments and cures for hearing loss.

Beginning Tuesday, June 5, at 8:00 AM EDT, you can support the team of scientists conducting life-changing research to restore lost hearing, and more importantly, your generosity will have double the impact! All contributions received by 11:59 PM EDT on Tuesday, June 19 will be matched by an anonymous donor.

Transverse section through the embryonic day 20 chicken utricle (inner ear organ) at 20X magnification. Photo by Amanda Janesick, Ph.D., of the lab of Stefan Heller, Ph.D., a Hearing Restoration Project consortium member

Transverse section through the embryonic day 20 chicken utricle (inner ear organ) at 20X magnification. Photo by Amanda Janesick, Ph.D., of the lab of Stefan Heller, Ph.D., a Hearing Restoration Project consortium member

With just five days remaining until launch, you can share the five most important facts about Operation Regrow with friends and family:

  1. The Hearing Restoration Project (HRP) is the HHF-funded scientific consortium dedicated to finding biological cures for hearing loss.

  2. Damage to the sensory cells in the human inner ear causes irreversible hearing loss.

  3. The HRP members know that the key to hearing loss cures is the human ability to regrow cells in the inner ear. This phenomenon is already possible in certain species. The HRP has observed cell regrowth in chickens, fish, and young mice.

  4. The HRP is comprised of 15 senior scientists who work collaboratively by openly sharing data and ideas, and this collaboration helps to speed up the research process.

  5. HHF maintains stellar charity ratings from Better Business Bureau Wise Giving Alliance, Guidestar, Charity Navigator, and CharityWatch for using 100% of donations to support critical research, ensuring that all Operation Regrow contributions will directly help the HRP.

If you are able to make a gift to Operation Regrow, please visit www.hhf.org/regrow between June 5 and June 19. Gifts may also be made by phone during business hours, 9:00 AM to 5:30 PM EDT, at 212-257-6140. We’ll be sure to keep you updated on our progress. Thank you for supporting HRP and hearing health!

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New Data-Driven Analysis Procedure for Diagnostic Hearing Test

By Carol Stoll

Stimulus frequency otoacoustic emissions (SFOAEs) are sounds generated by the inner ear in response to a pure-tone stimulus. Hearing tests that measure SFOAEs are noninvasive and effective for those who are unable to participate, such as infants and young children. They also give valuable insight into cochlear function and can be used to diagnose specific types and causes of hearing loss. Though interpreting SFOAEs is simpler than other types of emissions, it is difficult to extract the SFOAEs from the same-frequency stimulus and from background noise caused by patient movement and microphone slippage in the ear canal.

2014 Emerging Research Grants (ERG) recipient Srikanta Mishra, Ph.D., and colleagues have addressed SFOAE analysis issues by developing an efficient data-driven analysis procedure. Their new method considers and rejects irrelevant background noise such as breathing, yawning, and subtle movements of the subject and/or microphone cable. The researchers used their new analysis procedure to characterize the standard features of SFOAEs in typical-hearing young adults and published their results in Hearing Research.

Mishra and team chose 50 typical-hearing young adults to participate in their study. Instead of using a discrete-tone procedure that measures SFOAEs one frequency at a time, they used a more efficient method: a single sweep-tone stimulus that seamlessly changes frequencies from 500 to 4,000 Hz, and vice versa, over 16 and 24 seconds. The sweep tones were interspersed with suppressor tones that reduce the response to the previous tone. The tester manually paused and restarted the sweep recording when they detected background noises from the subject’s movements.

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The SFOAEs generated were analyzed using a mathematical model called a least square fit (LSF) and a series of algorithms based on statistical analysis of the data. This model objectively minimized the potential error from extraneous noises. Conventional SFOAE features such as level, noise floor, and signal-to-noise ratio (SNR) were described for the typical-hearing subjects.

Overall, the results of this study demonstrate the effectiveness of the automated noise rejection procedure of sweep-tone–evoked SFOAEs in adults. The features of SFOAEs characterized in this study from a large group of typical-hearing young adults should be useful for developing tests for cochlear function that can be useful in the clinic and laboratory.

Srikanta Mishra, Ph.D, was a 2014 Emerging Research Grants scientist and a General Grand Chapter Royal Arch Masons International award recipient. For more, see Sweep-tone evoked stimulus frequency otoacoustic emissions in humans: Development of a noise-rejection algorithm and normative features” in Hearing Research.

We need your help supporting innovative hearing and balance science through our Emerging Research Grants program. Please make a contribution today.

 
 


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Ménière's Disease Grantee Featured in Reader's Digest

Credit: Agnieszka Marcinska, Shutterstock

Credit: Agnieszka Marcinska, Shutterstock

Ian Swinburne, Ph.D., a 2018 Ménière's Disease Grant (MDG) recipient, shared his expertise regarding vertigo with Reader's Digest in an article called "What Causes Vertigo? 15 Things Neurologists Wish You Knew" published in March 2018. 

"The spinning, dizzying loss of balance which earmarks vertigo can come without warning," the article opens. Various professionals provide information about its duration, how it feels, and different types.

HHF-funded Dr. Swinburne notes specifically that the inner ear and balance disorder Ménière's disease can cause vertigo. He explains that "[b]outs of vertigo likely arise in patients with Ménière's disease, because the inner ear's tissue tears from too much fluid pressure—causing the ear's internal environment to become abnormal.'" He is currently pursuing a research project to understand the inner ear stabilizes fluid composition, which he believes will help to identify ways to restore or elevate this function to mitigate or cure Ménière's disease.

View the full article from Reader's Digest, here.

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Cellular Changes and Ménière’s Disease Symptoms

By Carol Stoll

Ménière’s disease is characterized by fluctuating hearing loss, vertigo, tinnitus, and ear fullness, but the causes of these symptoms are not well understood. Past research has suggested that a damaged blood labyrinthine barrier (BLB) in the inner ear may be involved in the pathophysiology of inner ear disorders. Hearing Health Foundation (HHF)’s 2016 Emerging Research Grants (ERG) recipient Gail Ishiyama, M.D., was the first to test this proposition by using electron microscopy to analyze the BLB in both typical and Ménière’s disease patients. Ishiyama’s research was fully funded by HHF and was recently published in Nature publishing group, Scientific Reports.

The BLB in a Meniere’s disease capillary. a) Capillary located in the stroma of the macula utricle from a Meniere’s subject (55-year-old-male). The lumen (lu) of the capillary is narrow, vascular endothelial cells (vec) are swollen and the cytoplasm is vacuolated (pink asterisks). b. Diagram showing the alterations in the swollen vec, microvacuoles are also abundant (v). Abbreviations, rbc: red blood cells, tj: tight junctions, m: mitochondria, n: cell nucleus, pp: pericyte process; pvbm: perivascular basement membrane. Bar is 2 microns.

The BLB in a Meniere’s disease capillary. a) Capillary located in the stroma of the macula utricle from a Meniere’s subject (55-year-old-male). The lumen (lu) of the capillary is narrow, vascular endothelial cells (vec) are swollen and the cytoplasm is vacuolated (pink asterisks). b. Diagram showing the alterations in the swollen vec, microvacuoles are also abundant (v). Abbreviations, rbc: red blood cells, tj: tight junctions, m: mitochondria, n: cell nucleus, pp: pericyte process; pvbm: perivascular basement membrane. Bar is 2 microns.

The BLB is composed of a network of vascular endothelial cells (VECs) that line all capillaries in the inner ear organs to separate the vasculature (blood vessels) from the inner ear fluids. A critical function of the BLB is to maintain proper composition and levels of inner ear fluid via selective permeability. However, the inner ear fluid space in patients with Ménière’s has been shown to be ballooned out due to excess fluid. Additionally, the group had identified permeability changes in magnetic resonance imaging studies of Meniere’s patients, which may be an indication of BLB malfunction.

Ishiyama’s research team used transmission electron microscopy (TEM) to investigate the fine cellular structure of the BLB in the utricle, a balance-regulating organ of the inner ear. Two utricles were taken by autopsy from individuals with no vestibular or auditory disease. Five utricles were surgically extracted from patients with severe stage IV Ménière’s disease with profound hearing loss and intractable recurrent vertigo spells, who were undergoing surgery as curative treatment.

Microscopic examination revealed significant structural differences of the BLB within the utricle between individuals with and without Ménière’s disease. In the normal utricle samples, the VECs of the BLB contained numerous mitochondria and very few fluid-containing organelles called vesicles and vacuoles. The cells were connected by tight junctions to form a smooth, continuous lining, and were surrounded by a uniform membrane.

However, samples with confirmed Ménière’s disease showed varying degrees of structural changes within the VECs; while the VECs remained connected by tight junctions, an increased number of vesicles and vacuoles was found, which may cause swelling and degeneration of other organelles. In the most severe case, there was complete VEC necrosis, or cell death, and a severe thickening of the basal membrane surrounding the VECs.

The documentation of the cellular changes in the utricle of Ménière’s patients was the first of its kind and has important implications for future treatments. Ishiyama’s study concluded that the alteration and degeneration of the BLB likely contributes to fluid changes in the inner ear organs that regulate hearing and balance, thus causing the Ménière’s symptoms. Further scientific understanding of the specific cellular and molecular components affected by Ménière’s can lead to the development of new drug therapies that target the BLB to decrease vascular damage in the inner ear.

Gail Ishiyama, M.D., is a 2016 Emerging Research Grants recipient. Her grant was generously funded by The Estate of Howard F. Schum.

WE NEED YOUR HELP IN FUNDING THE EXCITING WORK OF HEARING AND BALANCE SCIENTISTS. DONATE TODAY TO HEARING HEALTH FOUNDATION AND SUPPORT GROUNDBREAKING RESEARCH: HHF.ORG/DONATE.

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Gaining Better Clarity of Neural Networks

By Pranav Parikh

The ear, just like any other organ in the human body, uses nerves to function properly. One of the most vital nerves that the ear uses is the cochlear nerve, which connects the inner ear to the brain, or more specifically to the tonotopically-based regions of the cochlear nuclear complex located in the brainstem. This nerve shares the same shape and design of most nerves in the body, with dendrites absorbing information from various sources, sending the signal down the axon of the nerve through action potentials, and terminating the signal in a synapse so the message can be spread. In order to allow for this process to occur expediently, the nerve encounters a process known as myelination (providing a myelin sheath to propagate a signal faster). This is done through a glial cell known as an oligodendrocyte. Oligodendrocytes form a layer of lipid (fat) and protein around the axon to provide insulation, thereby allowing for signals to be sent to the brain more efficiently.

The immunoreactivity of Olig2 was detected during postnatal day (PND) 0 to 7, which became weaker after PND 10. Before PND 7, the majority of Olig2-expressing cells were found within the modiolus at the basal cochlear turn, while a few cells were located peripherally to the DIC-PCTZ and in close proximity to the spiral lamina at the basal cochlea turn. After PND 7, Olig2-expressing cells were fully overlapped with the DIC-PCTZ within modiolus at the spiral lamina in the basal cochlea.

The immunoreactivity of Olig2 was detected during postnatal day (PND) 0 to 7, which became weaker after PND 10. Before PND 7, the majority of Olig2-expressing cells were found within the modiolus at the basal cochlear turn, while a few cells were located peripherally to the DIC-PCTZ and in close proximity to the spiral lamina at the basal cochlea turn. After PND 7, Olig2-expressing cells were fully overlapped with the DIC-PCTZ within modiolus at the spiral lamina in the basal cochlea.

A team of scientists led by Dr. Zhengqing Hu, funded by Hearing Health Foundation through its Emerging Research Grants program (2010 & 2011) was able to analyze oligodendrocyte protein expression in the cochlear nerve of postnatal mice. Through the use of Differential Interference Contrast (DIC) microscopy, they were able to investigate the cochlear nerve at staggered postnatal days, meaning the period following birth.

Their findings indicate oligodendrocytes are found to migrate along with the transition zone between the central and peripheral nervous systems. As the fetus develops after birth, and myelination occurs in the nerves connecting to the brain, the oligodendrocyte protein marker Oligo2 was observed. This could mean loss of hearing function could be connected to unmyelinated axons. There are many other neurodegenerative autoimmune diseases, such as multiple sclerosis, caused by demyelination, and hearing loss could potentially be added to that list. Dr. Hu’s work improves clarity of the neural network connecting the inner ear and the brain.

Zhengqing Hu, M.D., Ph.D. , is a 2010 and 2011 Emerging Research Grants recipient. Hu's research was published by Otolaryngology-Head and Neck Surgery on July 11, 2017.

We need your help supporting innovative hearing and balance science through our Emerging Research Grants program. Please make a contribution today.

 
 
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