Research

Hair cells are specialized sensory cells important for hearing and balance in mammals. Hair cell death and dysfunction are major causes of human hearing loss. Hair cells are sensitive to a number of insults including loud noises, aging, certain therapeutic medications such as aminoglycoside antibiotics and chemotherapeutics, and environmental toxins such as heavy metals. My research uses zebrafish to uncover the cellular processes involved in hair cell development and death. By conducting a forward genetic screen, I discovered that genes regulating cilia function play important roles in hair cells (Stawicki et al. 2016). Cilia are microtubule-based projections on cells that are involved in multiple cell signaling processes. Early in their development, all hair cells contain a single primary cilium called the kinocilium. One of the main goals of my research moving forward is to gain further insight into the roles cilia genes play in hair cell function and dysfunction and to gain new insight into cilia biology. I am also interested in further understanding how other toxins lead to hair cell death and have recently started investigating heavy metal-induced hair cell death.

The Zebrafish Lateral Line

Hair cells of the zebrafish lateral line are morphologically and functionally similar to the sensory hair cells of the mammalian inner ear. They are also sensitive to the same therapeutic medications that kill mammalian hair cells. The surface location of zebrafish lateral line hair cells makes them easy to manipulate and visualize in vivo. You can see a 5-day old transgenic zebrafish larva below expressing green fluorescent protein (GFP) specifically in hair cells. These hair cells cluster together in structures called neuromasts. This collection of neuromasts along the length of the fish’s body makes up the lateral line system.

Lateral line hair cells have very long kinocilia making them a great system to study the localization and trafficking of cilia-localized genes. You can see this in the magnified view of a single neuromast below.

Cilia Genes and Hair Cell Death

I have found that mutations in a number of cilia-related genes lead to resistance to aminoglycoside-induced hair cell death. These genes fall into two major classes, transition zone, and intraflagellar transport (IFT) genes. Transition zone proteins are located at the base of the cilia and are believed to regulate which proteins exit and enter the cilia. Mutations in the transition zone genes cc2d2a and mks1 lead to moderate protection against aminoglycoside-induced hair cell death, whereas a mutation in cep290 leads to milder protection. Hair cells in these mutants appear otherwise normal.

IFT proteins are important for trafficking other proteins within the cilia. I have found that mutations in the anterograde and retrograde IFT genes lead to strong resistance in aminoglycoside-induced hair cell death. Additionally, these mutants show decreased control hair cell number and a loss of kinocilia. Mutations in most  IFT genes I have tested also show significant reductions in the uptake of aminoglycosides and the vital dye FM1-43 into hair cells suggesting a decrease in hair cell mechanotransduction activity. Below is an image of neuromasts from wild type and dync2h1, a retrograde IFT gene, mutant zebrafish exposed to an aminoglycoside conjugated to a red fluorescent dye to see the difference in uptake.

Mutations in cilia genes are known to cause a category of human diseases known as ciliopathies. Similar to the different hair cell phenotypes I see between transition zone and IFT mutants, mutations in these two different classes of cilia genes cause different diseases and symptom profiles in humans. My lab is currently investigating how these genes differentially affect cell signaling pathways in hopes to better understand how these different phenotypes arise.

Video of me presenting my work on cilia genes in hair cells from the 2016 Allied Genetics Conference is available online.

Heavy Metal-Induced Hair Cell Death

I have recently expanded my research interests to look at hair cell death in response to environmental toxins, such as heavy metals. Our lab has found that the heavy metal cadmium can reliably kill hair cells of the zebrafish lateral line as shown below (Schmid et al., 2020).

 

Similar to aminoglycoside-induced hair cell death mechanotransduction activity seems to be important for cadmium-induced hair cell death. Also while cilia genes with altered mechanotransduction activity are resistant to cadmium-induced hair cell death, similar to aminoglycosides, transition zone mutants with normal hair cell mechanotransduction activity show now resistance to cadmium-induced hair cell death. This suggests heavy metals and aminoglycosides might have different intracellular mechanisms once inside the hair cells resulting in cell death. The lab is currently investigating what the intracellular mechanisms of cadmium-induced hair cell death are.