Actin filaments in sensory hairs of inner ear receptor cells.

Abstract

Receptor cells in the ear are excited through the bending of sensory hairs which project in a bundle from their surface. The individual stereocilia of a bundle contain filaments about 5 nm in diameter. The identity of these filaments has been investigated in the crista ampullaris of the frog and guinea pig by a technique of decoration with subfragment-1 of myosin (S-l). After demembranation with Triton X-100 and incubation with S-l, "arrowhead" formation was observed along the filaments of the stereocilia and their rootlets and also along filaments in the cuticular plate inside the receptor cell. The distance between attached S-1 was 35 nm and arrowheads pointed in towards the cell soma. It is concluded that the filaments of stereocilia are composed of actin. KEY WORDS actininner ear 9 hair cell sensory hairs crista ampullaris sensory transduction Auditory and vestibular receptor cells are equipped with sensory hairs, the bending of which causes excitation of the sensory cells (2). A signal is generated in response to this stimulus and is in turn coded into an impulse message in the auditory nerve. The bending of the hairs is brought about by displacement of auxiliary sensory structures to which the tips of the sensory hairs are attached. The physical displacement of the sensory hairs is thus the first step in the cellular excitation process, and it is of obvious importance to obtain information about the mechanical properties of the sensory hairs and about the arrangement and nature of the responsible structures.The sensory hairs are tubular projections of the cell membrane which arise from the cell surface, 50-100 in number. One of these, the kinocilium (29), resembles an ordinary flagellum and is present in the vestibular system and the hearing organs of lower vertebrates; all the others are called stereocilia and are the only ones present in the mammalian hearing organ. The present paper deals with the stereocilia. Inside each stereocilium is a core of fine filaments which run down its length and collect to a narrow bundle at the slender base of the cilium, continuing as a rootlet into a cuticular plate which occupies the cell apex. The filament core has recently been found to exhibit considerable stiffness, as determined by micromanipulation of sensory hairs where the membrane has been removed by Triton X-100 (10).There are certain structural similarities between stereocilia and microvilli of the intestine as described by Mooseker and Tilney (20). In particular, we were interested in testing the possibility that the core of the stereocilium is composed of actin filaments as is the case in gut microvilli. For this purpose, we used the technique of decoration of actin with the S-1 fragment of myosin (14). MATERIALS AND METHODS PreparationThe crista ampullaris of frogs of the species Rana

Microtubule-associated proteins (MAPs) and the organization of actin filaments in vitro.

“…Discrete bundles of actin filaments have been found in the intestinal brush border microvilli (4,25), the stereocilia of the inner ear (11,40), acrosomal processes of invertebrate sperm (39), filopodia of sea urchin coelomocytes (9), and in cytoplasmic processes of cultured mammalian cells (21). Actin filament bundles in intestinal microvilli and in coelomocytes form in conjunction with specific actin-binding proteins (4,5,24,26).…”

A 13-A map of the actin-scruin filament from the limulus acrosomal process [published erratum appears in J Cell Biol 1993 Dec;123(6 Pt 1):1621]

“…The utility of actin lies in its abili~ to bind different actin binding proteins and generate different functional assemblies. Examples include the contractile apparatus of muscle (Huxley, 1969), microvillar bundles of intestinal epithelial cells (Maroux et al, 1988;Burgess, 1987), stereocilia of the inner ear (Flock and Cheung, 1977;DeRosier et al, 1980), the acrosomal process of Limulus sperm (Tilney, 1975), bundles within the filopodia of Dictyostelium Dharmawardhane et al, 1991;Owen et al, 1992), and the erythrocyte cytoskeleton (Branton et al, 1981). Recent findings suggest that actin structures can also provide indirect control of cellular processes.…”

Molecular mechanisms of sound amplification in the mammalian cochlea

“…Custom software was used to estimate the cell length changes produced by water entry. Sugars were added around the cell through a puffer pipette at different concentrations (3,10,20, and 30 mM) in standard saline solution, adjusted to 325 mosmol⅐kg Ϫ1 in all cases with sucrose. To compensate, 10 mM NaCl was removed from both bathing and perfusing solution in experiments where 50 mM glucose or fructose was tested.…”