Hearing sensitivity in mammals is enhanced by more than 40 dB (that is, 100-fold) by mechanical amplification thought to be generated by one class of cochlear sensory cells, the outer hair cells. In addition to the mechano-electrical transduction required for auditory sensation, mammalian outer hair cells also perform electromechanical transduction, whereby transmembrane voltage drives cellular length changes at audio frequencies in vitro. This electromotility is thought to arise through voltage-gated conformational changes in a membrane protein, and prestin has been proposed as this molecular motor. Here we show that targeted deletion of prestin in mice results in loss of outer hair cell electromotility in vitro and a 40-60 dB loss of cochlear sensitivity in vivo, without disruption of mechano-electrical transduction in outer hair cells. In heterozygotes, electromotility is halved and there is a twofold (about 6 dB) increase in cochlear thresholds. These results suggest that prestin is indeed the motor protein, that there is a simple and direct coupling between electromotility and cochlear amplification, and that there is no need to invoke additional active processes to explain cochlear sensitivity in the mammalian ear.

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It is a central tenet of cochlear neurobiology that mammalian ears rely on a local, mechanical amplification process for their high sensitivity and sharp frequency selectivity. While it is generally agreed that outer hair cells provide the amplification, two mechanisms have been proposed: stereociliary motility and somatic motility. The latter is driven by the motor protein prestin. Electrophysiological phenotyping of a prestin knockout mouse intimated that somatic motility is the amplifier. However, outer hair cells of knockout mice have significantly altered mechanical properties, making this mouse model unsatisfactory. Here, we study a mouse model without alteration to outer hair cell and organ of Corti mechanics or to mechanoelectric transduction, but with diminished prestin function. These animals have knockout-like behavior, demonstrating that prestin-based electromotility is required for cochlear amplification.

Polycystin-1 (PC1) has an essential function in renal tubular morphogenesis and disruption of its function causes cystogenesis in human autosomal dominant polycystic kidney disease. We have previously shown that recombinant human PC1 is cis -autoproteolytically cleaved at the G protein-coupled receptor proteolytic site domain. To investigate the role of cleavage in vivo , we generated by gene targeting a Pkd1 knockin mouse ( Pkd1 V/V ) that expresses noncleavable PC1. The Pkd1 V/V mice show a hypomorphic phenotype, characterized by a delayed onset and distal nephron segment involvement of cystogenesis at postnatal maturation stage. We show that PC1 is ubiquitously and incompletely cleaved in wild-type mice, so that uncleaved and cleaved PC1 molecules coexist. Our study establishes a critical but restricted role of cleavage for PC1 function and suggests a differential function of the two types of PC1 molecules in vivo .

The striated ciliary rootlet is a prominent cytoskeleton originating from basal bodies of ciliated cells. Although a familiar structure in cell biology, its function has remained unresolved. In this study, we carried out targeted disruption in mice of the gene for rootletin, a component of the rootlet. In the mutant, ciliated cells are devoid of rootlets. Phototransduction and ciliary beating in sensory and motile cilia initially exhibit no apparent functional deficits. However, photoreceptors degenerate over time, and mutant lungs appear prone to pathological changes consistent with insufficient mucociliary clearance. Further analyses revealed a striking fragility at the ciliary base in photoreceptors lacking rootlets. In vitro assays suggest that the rootlet is among the least dynamic of all cytoskeletons and interacts with actin filaments. Thus, a primary function of the rootlet is to provide structural support for the cilium. Inasmuch as photoreceptors elaborate an exceptionally enlarged sensory cilium, they are especially dependent on the rootlet for structural integrity and long-term survival.The ciliary rootlet originates from the proximal ends of basal bodies, the centriole-related structure that anchors the cilia, and extends proximally toward the cell nuclei (5, 25). Prominent rootlets are associated with sensory cilia of photoreceptor cells and motile cilia, such as those lining the respiratory tract, oviduct, and brain ventricles. The retinal photoreceptor elaborates a single enlarged distal cilium known as the outer segment. The outer segment is packed with photosensitive membranous disks and specializes in phototransduction. It is among the largest of all mammalian cilia, spanning approximately 25 m in length and more than 1 m in diameter. The outer segment is linked to the cell body, the inner segment, through a thin bridge called the connecting cilium (3). In a photoreceptor, the rootlet appears as a very thick striated filament that traverses the entire cell body all the way to the synaptic terminal (17,19,26). In epithelial cells bearing motile cilia, rootlets appear as robust subapical filamentous networks.