Balanced levels of Espin are critical for stereociliary growth and length maintenance

Abstract

Hearing and balance depend on microvilli-like actin-based projections of sensory hair cells called stereocilia. Their sensitivity to mechanical displacements on the nanometer scale requires a highly organized hair bundle in which the physical dimension of each stereocilium is tightly controlled. The length and diameter of each stereocilium are established during hair bundle maturation and maintained by life-long continuing dynamic regulation. Here, we studied the role of the actin-bundling protein Espin in stereociliary growth by examining the hair cell stereocilia of Espin-deficient jerker mice (Espn(je)), and the effects of transiently overexpressing Espin in the neuroepithelial cells of the organ of Corti cultures. Using fluorescence scanning confocal and electron microscopy, we found that a lack of Espin results in inhibition of stereociliary growth followed by progressive degeneration of the hair bundle. In contrast, overexpression of Espin induced lengthening of stereocilia and microvilli that mirrored the elongation of the actin filament bundle at their core. Interestingly, Espin deficiency also appeared to influence the localization of Myosin XVa, an unconventional myosin that is normally present at the stereocilia tip at levels proportional to stereocilia length. These results indicate that Espin is important for the growth and maintenance of the actin-based protrusions of inner ear neuroepithelial cells.

Espins and the actin cytoskeleton of hair cell stereocilia and sensory cell microvilli

“…Transfection with espin constructs elicits pronounced microvillar PAB elongation both in epithelial cell lines in culture and in epithelial cells in tissue explants [13,34]. A significant lengthening effect can even be observed for the stereociliary PABs of transfected hair cells [34], even though they already express a large amount of espin endogenously. Conversely, as mentioned above, a major consequence of the espin deficiency noted in jerker mice is a marked shortening of stereocilia.…”

“…Occasional defects, such as protrusion and folding of the cuticular plate, were also noted in the cochlear hair cells of newborn jerker mice. The stereocilia of newborn jerker mice may also be significantly shorter and thinner at birth [34]. Degeneration involving disintegration of the cuticular plate, stereociliary disarray and fusion and hair cell expulsion was noted for the vestibular hair cells of jerker mice, but these changes happened later than in the cochlea, and hair cells in the cristae ampullares were largely spared [35].…”

“…It is possible that the stereociliary shrinkage observed in jerker mice in early postnatal life can also be traced to an ~10% difference in rates of actin polymerization and depolymerization in their PAB. The fact that stereociliary bundles form in jerker mice [32][33][34] suggests a greater requirement for espins in the postnatal elongation and steady-state length maintenance than in stereocilium formation per se. The correlation between espin protein level and stereociliary length also extends to hair cells in normal animals.…”

Progressive hearing loss and gradual deterioration of sensory hair bundles in the ears of mice lacking the actin-binding protein Eps8L2

“…Instead it may provide a balance between the conflicting needs for structural stability and the repair or renewal of structures at the stereociliary tip, not least those of the tip link anchorage. Eps8 could have a similar function in the longer stereocilia, possibly by acting as a "gated-capper" protein (15), in which it protects the actin barbed end from other capping proteins but allows those promoting elongation, such as espin (8,38).…”

Espin cytoskeletal proteins in the sensory cells of rodent taste buds

“…Different transcriptional start sites distinguish four major espin isoform size classes, which range from ~110 kDa tõ 25 kDa in apparent molecular mass and are designated 1-4, in order of decreasing size; splice variants are further specified alphabetically (Sekerková et al, 2004). Beyond bund ling actin filaments in a Ca 2+ -resistant fashion (Bartles et al, 1998;Chen et al, 1999), the espins appear to function in part to regulate the length of microvilli and stereocilia by increasing the steady-state length of the dynamic parallel-actin-bundle scaffold at their core Rzadzinska et al, 2005). Recently, we presented immunocytochemical evidence that espins are present at high levels in the microvilli of several other classes of sensory cell besides hair cells -including vomeronasal sensory neurons, Merkel cells, and solitary chemoreceptor cells -and suggested that espins might confer special properties on the microvillar specializations of sensory cells (Sekerková et al, 2004).…”