[Show abstract][Hide abstract] ABSTRACT: Prestin is the motor protein of cochlear outer hair cells (OHCs) with the unique capability of performing direct, rapid, and reciprocal electromechanical conversion. Prestin consists of 744 amino acids with a molecular mass of approximately 81.4 kDa. The predicted membrane topology and molecular mass of a single prestin molecule appear inadequate to account for the size of intramembrane particles (IMPs) expressed in the OHC membrane. Although recent biochemical evidence suggests that prestin forms homo-oligomers, most likely as a tetramer, the oligomeric structure of prestin in OHCs remains unclear. We obtained the charge density of prestin in the gerbil OHCs by measuring their nonlinear capacitance (NLC). The average charge density (22,608 microm(-2) measured was four times the average IMP density (5686 microm(-2) reported in the freeze-fracture study. This suggests that each IMP contains four prestin molecules, based on the general notion that each prestin transfers a single elementary charge. We subsequently compared the voltage dependency and the values of slope factor of NLC and somatic motility simultaneously measured from the same OHCs to determine whether NLC and motility are fully coupled and how prestin subunits function within the tetramer. We showed that the voltage dependency and slope factors of NLC and motility were not statistically different, suggesting that NLC and motility are fully coupled. The fact that the slope factor is the same between NLC and motility suggests that each prestin monomer in the tetramer is in parallel, each interacting independently with cytoplasmic or other partners to facilitate the mechanical response.
[Show abstract][Hide abstract] ABSTRACT: Fms-like tyrosine kinase 3 ligand (Flt3L) reverses the features of allergic airway inflammation and increases a Th2-suppressive regulatory lung CD11c(high)CD11b(low) dendritic cell (DC) subset in a mouse model. We examined the migratory pattern and Ag uptake efficiency of lung DC subsets in the therapeutic effect of Flt3L. Lung CD11c(high)CD11b(low) and CD11c(low)CD11b(high) DCs from PBS-treated, OVA-sensitized, and Flt3L-treated/OVA-sensitized BALB/c mice were sorted using MACS and FACS for phenotype analysis. Lymphatic chemokine expression in thoracic lymph nodes was determined by immunohistochemistry. Migration of two lung DC subsets to lymphatic chemokines was examined in vitro using a Transwell chemotaxis assay. Labeled Ag was intranasally delivered into mouse lung to track the migration and Ag uptake of lung DCs. The in vitro cytokine secretion of mediastinal lymph node cells was determined using ELISA. CD11c(low)CD11b(high) DCs have higher expression of CCR5, CCR6, and CCR7, but lower expression of CCR2 than CD11c(high)CD11b(low) DCs. CD11c(low)CD11b(high) DCs in Flt3L-treated/OVA-sensitized mice demonstrated a less mature phenotype, inefficiency in Ag uptake, and impaired migration in vitro to lymphatic chemokine than those in OVA-sensitized mice. Administration of Flt3L decreased the expression of CCR5 and CCR7 in CD11c(low)CD11b(high) DCs in OVA-sensitized mice. Fewer Ag-carrying cells were detected in the lungs and lymph nodes in Flt3L-treated/OVA-sensitized mice than OVA-sensitized mice with a greater decrease in CD11c(low)CD11b(high) DCs. Mediastinal lymph node cells from Flt3L-treated mice secreted higher levels of Th1 cytokines and IL-10 than OVA-sensitized mice in vitro. In conclusion, Flt3L-generated lung immunogenic CD11c(low)CD11b(high) DCs have a less mature phenotype, impaired Ag uptake, and impaired migration to draining lymph nodes.
Full-text · Article · Nov 2009 · The Journal of Immunology
[Show abstract][Hide abstract] ABSTRACT: The Smads are a group of related intracellular proteins critical for transmitting the signals to the nucleus from the transforming growth factor-beta superfamily at the cell surface. Knockout of the Smad5 is embryonic lethal. However, the Smad5 knockout of single allele (+/-) could survive. We used Smad5 heterozygous knockout (+/-) to determine the role of Smad5 in the development of inner ear morphology and function. In situ hybridization showed that Smad5 was expressed predominantly in hair cells, spiral ganglion, and supporting cells. Measurements of hearing thresholds using auditory brainstem response showed that Smad5 defect resulted in progressive hearing loss between 4 and 24 weeks after birth. Morphological examination revealed apoptosis in the inner ear, with significant loss of outer hair cells in adult Smad5 mutant mice. Our results indicated that deficiency in the Smad5-mediated signaling resulted in apoptosis of hair cells, suggesting Smad5 is a gene that may be related with presbycusis.
Preview · Article · Feb 2009 · Developmental Neurobiology
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Glucose transporter 5 (Glut5) is a high-affinity fructose transporter. It was proposed to be a motor protein or part of the motor complex required for cochlear amplification in outer hair cells (OHCs). Here we show that, in contrast to previous reports, Glut5 is undetectable, and possibly absent, in OHCs harvested from wildtype mice. Further, Glut5-deficient mice display normal OHC morphology and motor function (i.e., nonlinear capacitance and electromotility) and normal cochlear sensitivity and frequency selectivity. We conclude that Glut5 is not required for OHC motility or cochlear amplification.
[Show abstract][Hide abstract] ABSTRACT: The cochlear outer hair cell (OHC), which plays a crucial role in mammalian hearing through its unique voltage-dependent motility, has been established as a primary target of the ototoxicity of aminoglycoside antibiotics. These polycationic drugs are also known to block a wide variety of ion channels, purinergic ionotropic channels, and nicotinic ACh receptors in hair cells in vitro. The OHC motor protein, prestin, is a voltage-sensitive transmembrane protein containing several negatively charged residues on both intra- and extracellular surface. The acidic sites may be susceptible to polycationic-charged aminoglycoside binding, which may result in disruption of motility. We attempted to examine whether aminoglycosides such as streptomycin and gentamicin could affect OHC motility and its electrical signature, the nonlinear capacitance (NLC) in adult gerbil OHCs. Somatic motility and NLC were measured under the whole-cell voltage-clamp mode. Streptomycin and gentamicin were applied extracellularly or intracellularly. Results show that streptomycin and gentamicin did not change either the magnitude of motility or the NLC. Theses results suggest that, although streptomycin and gentamicin can block mechanotransduction channels as well as ACh receptors in hair cells, they have no direct affect on OHC somatic motility.
No preview · Article · Jan 2008 · Hearing Research
[Show abstract][Hide abstract] ABSTRACT: The remarkable sensitivity and frequency selectivity of the mammalian cochlea is attributed to a unique amplification process that resides in outer hair cells (OHCs). Although the mammalian-specific somatic motility is considered a substrate of cochlear amplification, it has also been proposed that somatic motility in mammals simply acts as an operating-point adjustment for the ubiquitous stereocilia-based amplifier. To address this issue, we created a mouse model in which a mutation (C1) was introduced into the OHC motor protein prestin, based on previous results in transfected cells. In C1/C1 knockin mice, localization of C1-prestin, as well as the length and number of OHCs, were all normal. In OHCs isolated from C1/C1 mice, nonlinear capacitance and somatic motility were both shifted toward hyperpolarization, so that, compared with WT controls, the amplitude of cycle-by-cycle (alternating, or AC) somatic motility remained the same, but the unidirectional (DC) component reversed polarity near the OHC's presumed in vivo resting membrane potential. No physiological defects in cochlear sensitivity or frequency selectivity were detected in C1/C1 or C1/+ mice. Hence, our results do not support the idea that OHC somatic motility adjusts the operating point of a stereocilia-based amplifier. However, they are consistent with the notion that the AC component of OHC somatic motility plays a dominant role in mammalian cochlear amplification.
Full-text · Article · Aug 2007 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Prestin, a member of the solute carrier (SLC) family SLC26A, is the molecular motor that drives the somatic electromotility of mammalian outer hair cells (OHCs). Its closest reported homologue, zebrafish prestin (zprestin), shares approximately 70% strong amino acid sequence similarity with mammalian prestin, predicting an almost identical protein structure. Immunohistochemical analysis now shows that zprestin is expressed in hair cells of the zebrafish ear. Similar to mammalian prestin, heterologously expressed zprestin is found to generate voltage-dependent charge movements, giving rise to a non-linear capacitance (NLC) of the cell membrane. Compared with mammalian prestin, charge movements mediated by zprestin display a weaker voltage dependence and slower kinetics; they occur at more positive membrane voltages, and are not associated with electromotile responses. Given this functional dissociation of NLC and electromotility and the structural similarity with mammalian prestin, we anticipate that zprestin provides a valuable tool for tracing the molecular and evolutionary bases of prestin motor function.
Full-text · Article · May 2007 · The Journal of Physiology