Liquid in the outer ear is a problem. Water that infiltrates a swimmer’s ear can cause an infection. But fluid in the inner ear is a necessity. The soundwaves that strike the eardrum only become interpretable words and noises if sensory hair cells in the cochlea, the portion of the inner ear dedicated to hearing, relay the information to the brain. For that to happen, the fluid that fills the cochlea must move the hair cells to the beat of the eardrum.

At the same time, the fluid in the cochlea nourishes and protects the cells it bathes. This nutritive role caught the eye of Maiken Nedergaard, a neuroscientist at the University of Rochester Medical Center. The movements of fluid around the head have always fascinated her. “It’s like a plumbing system you turn on and off, and it’s neural activity that determines that,” she said. Conversations with her colleagues led her to wonder how liquid moves into the cochlea. “There has to be some fluid flow into the ear,” she said. “You need to transport glucose and oxygen.” 

Gene delivery to the inner ear has been very, very difficult because the inner ear is embedded in the skull, and the only way you can get into those cells is basically by breaking all that you need to be able to hear. 
- Maiken Nedergaard, University of Rochester Medical Center

While bony walls separate the cochlea from the brain’s cavity in mammals, there are understudied tubes called the cochlear aqueducts that bridge both sides. By investigating these cochlear aqueducts in mice, Nedergaard and her colleagues found that these bony channels connect the fluid of the inner ear to the cerebrospinal fluid (CSF) that fills the spaces around the brain and spinal cord. Nedergaard’s team went on to successfully administer gene therapy to mutant cochlear hair cells via an injection to the base of the skull, which could indicate a new, less destructive method for treating genetic hearing loss in humans, according to their results published in Science Translational Medicine (1). 

“Gene delivery to the inner ear has been very, very difficult because the inner ear is embedded in the skull, and the only way you can get into those cells is basically by breaking all that you need to be able to hear,” Nedergaard said.

Before they could even think about using the CSF to deliver viruses programmed with beneficial genes to the cochlea, Nedergaard’s team did some basic anatomical work. First, they injected tracer molecules into the brain-containing compartment of living mouse skulls and used an MRI to watch the molecules spread through the cochlear aqueducts into the inner ears. Then, they dissected out the animals’ cochlear aqueducts, thinly sliced them, and stained them for various proteins to see what type of tissue the cells of the aqueduct most resemble. The researchers found evidence that the cochlear aqueduct has similar properties to the lymphatic system, which protects the body from infection. 

Equipped with new knowledge about the basic structure and function of mouse cochlear aqueducts, Nedergaard and her colleagues felt confident that they could use the tubes as delivery routes for genetic hearing loss therapeutics. The researchers raised mice afflicted with a genetic disorder that rendered their cochlear hair cells incapable of releasing signaling molecules to relay information about sound to the brain. Then the researchers rescued hearing in the mice by injecting a virus equipped with a working copy of the mutated mouse gene into the base of their skulls. Micrographic imaging confirmed that the mutant hair cells regained the ability to release signaling molecules after exposure to the virus.

That the cochlear aqueducts are open to fluid flow in adult mice raises questions about prevailing wisdom among clinicians. “The cochlear aqueducts seem to close off at some point postnatally,” said Lawrence Lustig, an otolaryngologist at the Columbia University Irving Medical Center who studies hearing loss but was not involved in this study. 

Nedergaard, however, thinks that the theory is likely out of date. She pointed out that doctors have used the eardrums to measure intracranial pressure, which is only possible if the fluid around the brain is continuous with the fluid in the inner ears (2). Lustig acknowledged that some medical literature has indicated that changes in CSF pressure can affect hearing, but he still thinks those cases represent exceptions rather than rules (3). “It’s not consistent, and it’s not in everybody,” he said.

Yet Lustig is impressed with what Nedergaard’s team accomplished in mice. “As a tool to rapidly screen different kinds of genes in mice models of deafness, this may be a really cool way to go,” said Lustig. However, he added, “I don’t think this would be a viable way to do this in humans.” Without more evidence that human cochlear aqueducts consistently remain open throughout life, Lustig thinks that only a small subset of patients could benefit from this delivery route for gene therapy.

Nedergaard is more optimistic. She pointed out that a separate set of researchers had delivered gene therapy to hair cells in the ears of nonhuman primates via an injection to their CSF (4). She’s confident that it’s only a matter of time before enough evidence emerges for clinicians to rethink their understanding of the human cochlear aqueduct. 

Her only concern about delivering gene therapy to the inner ear via the CSF is whether the viruses and genes used for the treatment are specific enough. In the current study, her team found little evidence that the viral therapy had infected other cells in the mouse bodies and no evidence of inflammation. Still, Nedergaard said, “There’s a lot of work to be done, basically looking at better promoters [for the genes being delivered], better [casings] for the viruses so they primarily target the inner ear hair cells.” Although treating genetic hearing loss via the CSF promises to be less invasive, she highlighted the importance of proceeding with caution to minimize risks. 

References

1. Mathiesen, B. et al. Delivery of gene therapy through a cerebrospinal fluid conduit to rescue hearing in adult mice. Science Translational Medicine  15, eabq3916 (2023).
2. Kostick, N. et al. The “Brain Stethoscope”: A non-invasive method for detecting elevated intracranial pressure. Cureus  13, e13865 (2021).
3. Girardi, F. et al. Sudden sensorineural hearing loss after spinal surgery under general anesthesia. Journal of Spinal Disorders  14, 180-183 (2001).
4. Ranum, P. et al. Cochlear transduction via cerebrospinal fluid delivery of AAV in non-human primates. Molecular Therapy  31, 609-612 (2023).