Synaptic mitochondria regulate hair-cell synapse size and function

Hearing depends upon specialized cells deep within the ear called hair cells. These cells take their name from the bundles of hair-like fibers found on their surface, which move when sound waves enter the ear. This movement activates the hair cells, which send signals to nearby neurons at contact points called synapses. Hair cells must send messages to their synaptic partners rapidly and continuously in order for humans to follow complex streams of sound, such as speech. This requires large amounts of energy, which are produced by compartments inside the hair cells called mitochondria. Wong et al. show that mitochondria, which are often described as the ‘power plants’ of cells, are critical for hair cell synapses to form and work correctly. But rather than studying hair cells in the human ear, Wong et al. took advantage of the fact that another species – the zebrafish – has hair cells on its body surface. These cells detect movements in water rather than sound waves, but they work in much the same way as hair cells in the ear, and are easier to access and study. Wong et al. report that in zebrafish larvae, developing hair cells send spontaneous signals to their contact neurons even before they start receiving any sensory input. But if mitochondria in the hair cells fail to detect these signals, the synapses fail to form correctly. In older zebrafish, mature hair cells send signals to their synaptic partners whenever they detect sensory input. But if mitochondria fail to detect these signals, the synapses stop working and ultimately break down. These findings help explain why damage to mitochondria in the inner ear can lead to hearing loss. Moreover, because mitochondria are present in almost all cells, their disruption causes a wide range of diseases. Many of these involve the brain, which requires large amounts of energy and so is particularly vulnerable to mitochondrial damage. These results may provide insights into such disorders.