Heregulin enhances regenerative proliferation in postnatal rat utricular sensory epithelium after ototoxic damage

Hair cell loss due to acoustic and ototoxic damage often leads to hearing a nd balance impairments. Although a spontaneous event in chicks and lower ve rtebrates, hair cell replacement occurs at a much lower frequency in mammal s presumably due to a very low rate of supporting cell proliferation follow ing injury. We report here that heregulin, a member of the neuregulin famil y, dramatically enhances proliferation of supporting cells in postnatal rat utricular epithelial sheet cultures after gentamicin treatment, as reveale d by bromo-deoxyuridine (BrdU) immunocytochemistry. A dose-dependent study shows that the maximal effects of heregulin are achieved at 3 nM. The mitog enic effects of heregulin are confirmed in utricular whole mount cultures. Autoradiography of the utricular whole mount cultures shows that heregulin also enhances the number of tritiated thymidine-labeled cells within the ha ir cell layer. TaqMan quantitative RT-PCR analysis and immunocytochemistry reveal that heregulin and its binding receptors (ErbB-2, ErbB-3 and ErbB-4) are expressed in the inner ear sensory epithelium. Of several ligands acti vating various ErbB receptors, including heregulin, neuregulin-3, beta-cell ulin, heparin binding-epidermal growth factor (HB-EGF), transforming growth factor-alpha (TGF-alpha) and EGF, heregulin shows the most potent mitogeni c effects on supporting cells. Because neuregulin-3 that signals only throu gh ErbB-4 does not show an effect, these data suggest that activation of th e ErbB-2-ErbB-3 heterodimeric complexes, rather than ErbB-4, is critical fo r the proliferative response in the utricular sensory epithelium. In additi on, gentamicin treatment induces an upregulation of heregulin mRNA. Conside red together, heregulin may play an important role in hair cell regeneratio n following ototoxic damage.