Developmental changes in cell–extracellular matrix interactions limit proliferation in the mammalian inner ear

Abstract

Hair cell losses can produce severe hearing and balance deficits in mammals and nonmammals alike, but nonmammals recover after epithelial supporting cells divide and give rise to replacement hair cells. Here, we describe cellular changes that appear to underlie the permanence of hair cell deficits in mammalian vestibular organs. In sensory epithelia isolated from the utricles of embryonic day 18 (E18) mice, supporting cells readily spread and proliferated, but spreading and proliferation were infrequent in supporting cells from postnatal day 6 (P6) mice. Cellular spreading and proliferation were dependent on alpha6 integrin, which disappeared from lateral cell membranes by P6 and colocalized with beta4 integrin near the basement membrane at both ages. In the many well-spread, proliferating E18 supporting cells, beta4 was localized at cell borders, but it was localized to hemidesmosome-like structures in the columnar, nondividing supporting cells that were prevalent in P6 cultures. We treated cultures with phorbol myristate acetate (PMA) to activate protein kinase C (PKC) in an initial test of the possibility that maturational changes in supporting cell cytoskeletons or their anchorage might restrict the proliferation of these progenitor cells in the developing mammalian inner ear. That treatment triggered the disassembly of the hemidesmosome-like beta4 structures and resulted in significantly increased cellular spreading and S-phase entry in the P6 epithelia. The results suggest that maturational changes in cytoskeletal organization and anchorage restrict proliferation of mammalian supporting cells whose counterparts are the progenitors of replacement hair cells in nonmammals, thereby leaving mammals vulnerable to persistent sensory deficits caused by hair cell loss.

Over Half the Hair Cells in the Mouse Utricle First Appear After Birth, with Significant Numbers Originating from Early Postnatal Mitotic Production in Peripheral and Striolar Growth Zones

“…Similar signs of plasticity have been observed in utricular sensory epithelia isolated and cultured from neonatal mice, where exogenous growth factors promote rapid supporting cell spreading and proliferation (Montcouquiol and Corwin 2001a, b;Davies et al 2007;Gu et al 2007;Lu and Corwin 2008). Balance organs from non-mammals produce new hair cells and supporting cells in vivo.…”

Shape Change Controls Supporting Cell Proliferation in Lesioned Mammalian Balance Epithelium

“…Maturational changes in the capacity for supporting cells to change shape and proliferate also appear to correlate with changes in integrin expression and hemidesmosome development, which both affect cytoskeletal anchorage (Davies et al, 2007). If cytoskeletal anchorage and rigidity are responsible for limiting proliferation in the mature ears of mammals, then similar cytoskeletal and anchorage changes may not occur during the maturation of nonmammalian vertebrates, in which supporting cells retain the capacity to change shape, proliferate, and replace hair cells after damage.…”

Inner ear hair cells produced in vitro by a mesenchymal-to-epithelial transition

“…In vitro pharmacological treatments that help to restore proliferation in mature mammalian vestibular epithelia have recently been identified (13)(14)(15), but the achievement of effective regeneration in mammalian ears is likely to depend in part on discovering how hair cell differentiation is controlled.…”

“…The primary cultures [designated passage (P)0] contained many solitary cells but also small epithelial islands composed of preexisting hair cells and supporting cells, which were visibly linked by epithelial junctions. The supporting cells that grew in isolation as individual cells and those that were near the edges of the epithelial islands were the first to flatten and spread as thin polygonal shaped cells (13)(14)(15), followed in turn by the remaining supporting cells within those islands. In contrast to the supporting cells, the hair cells remained essentially cylindrical and were readily identifiable by their circular apical surfaces and hair bundles.…”