Investigations of Regeneration in Mammalian Hair Cell Epithelia

In the ears of fish, amphibians, and birds, divisions of supporting cells produce thousands of hair cells throughout life. Traumatic loss of hair cells in their ears quickly evokes cell divisions in neighboring supporting cells. Many of the newly produced cells become replacement hair cells, so hearing and balance deficits are only temporary in those species. Yet, millions of people have permanent hearing impairments and balance deficits that result from losses of hair cells that are not effectively replaced. Low numbers of supporting cell divisions can occur after damage however, to balance epithelia from the ears of adult humans and other mammals, and a few cells appear to become hair cells. We have focused on experimental treatments with drugs and growth factors that greatly increase supporting cell divisions that occur in hair cell epithelia from mammals and experimental measurements of the intracellular signals that underlie those pharmacological responses. The findings show that large numbers of supporting cells (more than 40% of the total) divide within 72 hours of exposure to recombinant human glial growth factor 2 (rhGGF2) in cultured sheets of pure hair cell epithelium from utricles of newborn rats. The number of cells that respond to such short-term exposures to rhGGF2 and other mitogens we have tested, however, declines to less than 2% over the first 2 weeks of the mammalian ear's postnatal maturation. In longer-term cultures from adult rats, rhGGF2 increases the divisions that occur by 20-fold compared with controls, but the numbers of responding cells are much smaller than in neonates. The other experiments show that activation of a PI-3K signal cascade, elevation of intracellular calcium, and elevation of cyclic adenosine monophosphate (cAMP) can each trigger intracellular signaling that leads to divisions of mammalian supporting cells. In fact, raising intracellular levels of cAMP via a 15-minute pretreatment with forskolin will reliably double the number of supporting cells that respond to rhGGF2. Together these results indicate that the decline in cell divisions, which appears to leave mammalian ears uniquely vulnerable to permanent sensory deficits resulting from hair cell loss, actually progresses throughout the first 2 weeks of postnatal life and not just during embryogenesis. The developmental changes underlying that limit to mammalian hair cell replacement therefore should be accessible to molecular identification.