As Andrea McQuate looked through fluorescence image after fluorescence image of zebrafish hair cells, each seemed to tell the same story: highly metabolically active hair cells, which in humans reside in the inner ear and allow us to hear and balance, looked like they only had one mitochondrion to power the whole cell.
“It was just like, really? I don't know if that's true,” said McQuate, a postdoctoral researcher and neuroscientist in David Raible’s laboratory at the University of Washington. To see what hair cell mitochondria really looked like, she needed to visualize them under higher resolution.
Hair cells get their name from the hair-like cilia poking out of their tops. When air pressure in the cochlea moves the cilia, it sets off a signaling cascade in the cell that synapses with downstream neurons, leading to the perception of sound. Mitochondria are especially important for proper hair cell function. Mitochondrial dysfunction due to intense noise exposure or age-related hearing loss contributes to the death of hair cells. Scientists have also found more than 30 mutations in mitochondrial genes that cause deafness.
“Most fascinating is that these mutations will be in every cell of the body,” said McQuate, “but what you get is hearing loss because the hair cells are just so sensitive to these changes.”
Despite mitochondria’s clear importance in hearing, scientists still know very little about what hair cell mitochondria look like and how exactly they contribute to hearing loss. In a study published in eLife, McQuate and her colleagues used serial block-face scanning electron microscopy to characterize hair cell mitochondria, providing a deeper understanding of the potential mechanisms involved in mitochondrial deafness (1).
In most animals, hair cells lie deep within the ear canal, making the cells difficult to study without damaging them, but in zebrafish, hair cells run along the length of the body where they help the fish sense changes in water flow. With a structure and function similar to human inner ear hair cells, zebrafish hair cells are a useful model for investigating hair cell mitochondria.
To image these hair cells, McQuate fixed and stained individual zebrafish in a block of resin. Similar to traditional electron microscopy (EM), serial block-face scanning electron microscopy takes snapshots of samples at high enough resolution to see subcellular structures, but it also allows scientists to visualize those structures in three dimensions. After taking an image of the sample, the microscope uses a diamond knife to slice 50 nanometers off the surface of the sample. Then it images the new surface and repeats the process until the sample is gone and replaced with a stack of 600 to 700 top to bottom images.
“I still get so excited watching the [process],” McQuate said. “The technician who was helping me with this is very jaded in terms of like, ‘Yeah, whatever, we see this.’ I was like, ‘Can we watch the knife?’”
McQuate manually traced the outlines of the mitochondria in each of the hundreds of images the serial block-face scanning electron microscope generated. To her astonishment, she saw that there were two distinct populations of mitochondria within one individual hair cell.
At the top of the hair cells close to the sensory cilia, oodles of tiny mitochondria crowded together.
“The sheer volume of mitochondria that these cells have is pretty wild,” said McQuate. “They're about on par with a cardiac muscle cell… Your heart is beating all the time. It needs mitochondria. You just don't think about hearing as also being that energetically demanding as well, but according to the hair cells, it is.”
As she followed the mitochondrial outlines at the base of the cell, she realized that one massive mitochondrion dominated the space.
“It looks like spaghetti, and it's all interconnected,” she said. “When I was doing these reconstructions, that was just absolutely incredible to actually see it in EM and go through three dimensions and be like, ‘Oh, that's connected, that's connected, that's connected,’ just reconstructing these monster mitochondria.”
McQuate plans to investigate the function of these two different mitochondrial populations. She speculates that the smaller mitochondria at the top of the cell may be important for buffering the influx of calcium that enters the cell when the cilia deflect in response to sound. Taking up too much calcium can damage the mitochondria, and it’s easier for cells to remove smaller, damaged mitochondria than larger ones. She also hypothesizes that because hair cells synapse onto neurons at the base of the cell, the large, highly networked mitochondrion may support the high metabolic demands of synaptic transmission.
“We have this textbook image of the mitochondria as this little bean,” said McQuate, “but it's actually differentiated in such a way to be essential for a particular cell’s function. That concept just blows my mind.”
- McQuate et al. Activity regulates a cell type-specific mitochondrial phenotype in zebrafish lateral line hair cells. eLife 12, e80468 (2023). https://elifesciences.org/articles/80468