Disruption of SorCS2 reveals differences in the regulation of stereociliary bundle formation between hair cell types in the inner ear

An Ala/Glu difference in E1 of Cx26 and Cx30 contributes to their differential anionic permeabilities

Two closely related connexins, Cx26 and Cx30, share widespread expression in the cochlear cellular networks. Gap junction channels formed by these connexins have been shown to have different permeability profiles, with Cx30 showing a strongly reduced preference for anionic tracers. The pore-forming segment of the first extracellular loop, E1, identified by computational studies of the Cx26 crystal structure to form a parahelix and a narrowed region of the pore, differs at a single residue at position 49. Cx26 contains an Ala and Cx30, a charged Glu at this position, and cysteine scanning in hemichannels identified this position to be pore-lining. To assess whether the Ala/Glu difference affects permeability, we modeled and quantified Lucifer Yellow transfer between HeLa cell pairs expressing WT Cx26 and Cx30 and variants that reciprocally substituted Glu and Ala at position 49. Cx26(A49E) and Cx30(E49A) substitutions essentially reversed the Lucifer Yellow permeability profile when accounting for junctional conductance. Moreover, by using a calcein efflux assay in single cells, we observed a similar reduced anionic preference in undocked Cx30 hemichannels and a reversal with reciprocal Ala/Glu substitutions. Thus, our data indicate that Cx26 and Cx30 gap junction channels and undocked hemichannels retain similar permeability characteristics and that a single residue difference in their E1 domains can largely account for their differential permeabilities to anionic tracers. The higher anionic permeability of Cx26 compared with Cx30 suggests that these connexins may serve distinct signaling functions in the cochlea, perhaps reflected in the vastly higher prevalence of Cx26 mutations in human deafness.

Contemporary perspectives on heterotopic ossification

Heterotopic ossification (HO) is the formation of ectopic bone that is primarily genetically driven (fibrodysplasia ossificans progressiva [FOP]) or acquired in the setting of trauma (tHO). HO has undergone intense investigation, especially over the last 50 years, as awareness has increased around improving clinical technologies and incidence, such as with ongoing wartime conflicts. Current treatments for tHO and FOP remain prophylactic and include NSAIDs and glucocorticoids, respectively, whereas other proposed therapeutic modalities exhibit prohibitive risk profiles. Contemporary studies have elucidated mechanisms behind tHO and FOP and have described new distinct niches independent of inflammation that regulate ectopic bone formation. These investigations have propagated a paradigm shift in the approach to treatment and management of a historically difficult surgical problem, with ongoing clinical trials and promising new targets.

Characterization of the microRNA transcriptomes and proteomics of cochlear tissue-derived small extracellular vesicles from mice of different ages after birth

The cochlea is an important sensory organ for both balance and sound perception, and the formation of the cochlea is a complex developmental process. The development of the mouse cochlea begins on embryonic day (E)9 and continues until postnatal day (P)21 when the hearing system is considered mature. Small extracellular vesicles (sEVs), with a diameter ranging from 30 to 200 nm, have been considered a significant medium for information communication in both physiological and pathological processes. However, there are no studies exploring the role of sEVs in the development of the cochlea. Here, we isolated tissue-derived sEVs from the cochleae of FVB mice at P3, P7, P14, and P21 by ultracentrifugation. These sEVs were first characterized by transmission electron microscopy, nanoparticle tracking analysis, and western blotting. Next, we used small RNA-seq and mass spectrometry to characterize the microRNA transcriptomes and proteomes of cochlear sEVs from mice at different ages. Many microRNAs and proteins were discovered to be related to inner ear development, anatomical structure development, and auditory nervous system development. These results all suggest that sEVs exist in the cochlea and are likely to be essential for the normal development of the auditory system. Our findings provide many sEV microRNA and protein targets for future studies of the roles of cochlear sEVs.

Essential Role of Sptan1 in Cochlear Hair Cell Morphology and Function Via Focal Adhesion Signaling

Hearing loss is the most common human sensory deficit. Hearing relies on stereocilia, inserted into the cuticular plate of hair cells (HCs), where they play an important role in the perception of sound and its transmission. Although numerous genes have been associated with hearing loss, the function of many hair cell genes has yet to be elucidated. Herein, we focused on nonerythroid spectrin αII (SPTAN1), abundant in the cuticular plate, surrounding the rootlets of stereocilia and along the plasma membrane. Interestingly, mice with HC-specific Sptan1 knockout exhibited rapid deafness, abnormal formation of stereocilia and cuticular plates, and loss of HCs from middle and apical turns of the cochlea during early postnatal stages. Additionally, Sptan1 deficiency led to the decreased spreading of House Ear Institute-Organ of Corti 1 cells, and induced abnormal formation of focal adhesions and integrin signaling in mouse HCs. Altogether, our findings highlight SPTAN1 as a critical molecule for HC stereocilia morphology and auditory function via regulation of focal adhesion signaling.

Single neonatal dexamethasone administration has long-lasting outcome on depressive-like behaviour, Bdnf, Nt-3, p75ngfr and sorting receptors (SorCS1-3) stress reactive expression

Elevated glucocorticoid level in the early postnatal period is associated with glucocorticoid therapy prescribed at preterm delivery most often has severe long-lasting neurodevelopmental and behavioural effects. Detailed molecular mechanisms of such programming action of antenatal glucocorticoids on behaviour are still poorly understood. To address this question we studied neurotrophins: Bdnf, Nt-3, Ngf and their receptors: p75ngfr, Sorcs3 expression changes after subcutaneous dexamethasone (DEX) 0.2 mg/kg injection to P2 rat pups. Neurotrophins expression level was studied in the hippocampus (HPC). Disturbances in these brain regions have been implicated in the emergence of multiple psychopathologies. p75ngfr and Sorcs3 expression was studied in the brainstem—region where monoamine neurons are located. Immunohistochemically P75NTR protein level changes after DEX were investigated in the brainstem Locus Coereleus norepinephrine neurons (NE). In the first hours after DEX administration elevation of neurotrophins expression in HPC and decline of receptor’s expression in the NE brainstem neurons were observed. Another critical time point during maturation is adolescence. Impact of elevated glucocorticoid level in the neonatal period and unpredictable stress (CMUS) at the end of adolescence on depressive-like behaviour was studied. Single neonatal DEX injection leads to decrease in depressive-like behaviour, observed in FST, independently from chronic stress. Neonatal DEX administration decreased Ntf3 and SorCS1 expression in the brainstem. Also Bdnf mRNA level in the brainstem of these animals didn’t decrease after FST. CMUS at the end of adolescence changed p75ngfr and SorCS3 expression in the brainstem in the animals that received single neonatal DEX administration.

Retinoic acid degradation shapes zonal development of vestibular organs and sensitivity to transient linear accelerations

Each vestibular sensory epithelium in the inner ear is divided morphologically and physiologically into two zones, called the striola and extrastriola in otolith organ maculae, and the central and peripheral zones in semicircular canal cristae. We found that formation of striolar/central zones during embryogenesis requires Cytochrome P450 26b1 (Cyp26b1)-mediated degradation of retinoic acid (RA). In Cyp26b1 conditional knockout mice, formation of striolar/central zones is compromised, such that they resemble extrastriolar/peripheral zones in multiple features. Mutants have deficient vestibular evoked potential (VsEP) responses to jerk stimuli, head tremor and deficits in balance beam tests that are consistent with abnormal vestibular input, but normal vestibulo-ocular reflexes and apparently normal motor performance during swimming. Thus, degradation of RA during embryogenesis is required for formation of highly specialized regions of the vestibular sensory epithelia with specific functions in detecting head motions.

Development and Patterning of the Cochlea: From Convergent Extension to Planar Polarity

Within the mammalian cochlea, sensory hair cells and supporting cells are aligned in curvilinear rows that extend along the length of the tonotopic axis. In addition, all of the cells within the epithelium are uniformly polarized across the orthogonal neural-abneural axis. Finally, each hair cell is intrinsically polarized as revealed by the presence of an asymmetrically shaped and apically localized stereociliary bundle. It has been known for some time that many of the developmental processes that regulate these patterning events are mediated, to some extent, by the core planar cell polarity (PCP) pathway. This article will review more recent work demonstrating how components of the PCP pathway interact with cytoskeletal motor proteins to regulate cochlear outgrowth. Finally, a signaling pathway originally identified for its role in asymmetric cell divisions has recently been shown to mediate several aspects of intrinsic hair cell polarity, including kinocilia migration, bundle shape, and elongation.

New insights into regulation and function of planar polarity in the inner ear

Acquisition of cell polarity generates signaling and cytoskeletal asymmetry and thus underpins polarized cell behaviors during tissue morphogenesis. In epithelial tissues, both apical-basal polarity and planar polarity, which refers to cell polarization along an axis orthogonal to the apical-basal axis, are essential for epithelial morphogenesis and function. A prime example of epithelial planar polarity can be found in the auditory sensory epithelium (or organ of Corti, OC). Sensory hair cells, the sound receptors, acquire a planar polarized apical cytoskeleton which is uniformely oriented along an axis orthogonal to the longitudinal axis of the cochlear duct. Both cell-intrinsic and tissue-level planar polarity are necessary for proper perception of sound. Here we review recent insights into the novel roles and mechanisms of planar polarity signaling gained from genetic analysis in mice, focusing mainly on the OC but also with some discussions on the vestibular sensory epithelia.

PINK1 Protects Auditory Hair Cells and Spiral Ganglion Neurons from Cisplatin-induced Ototoxicity via Inducing Autophagy and Inhibiting JNK Signaling Pathway

Phosphatase and tensin homologue (PTEN)-induced putative kinase 1 (PINK1) gene encodes a serine/threonine kinase, which acts as a molecular sensor of mitochondrial health necessary for mitochondrial quality control. The present study was designed to examine whether PINK1 expressed in C57BL/6 murine cochlea and HEI-OC1 cells and, if so, to investigate the possible mechanisms underlying the action of PINK1 in cisplatin-induced death of sensory hair cells (HCs) and spiral ganglion neurons (SGNs) in vitro. The expression pattern of PINK1, formation of parkin particles, and autophagy were determined by immunofluorescent staining. The expressions of PINK1, LC3B, cleaved-caspase 3 and p-JNK were measured by Western blotting. The levels of reactive oxygen species (ROS) were evaluated by DCFH-DA and Mito-Sox Red staining. The mitochondrial membrane potential was detected by Tetramethylrhodamine methyl ester perchlorate (TMRM) and Rhodamine 123. Cell viability and apoptosis were examined by CCK8 assay, TUNEL staining and Annexin V Apoptosis Detection Kit, respectively. We found that PINK1 was widely expressed in the cytoplasm in HCs, SGNs, stria vascularis of C57BL/6 cochlea and HEI-OC1 cells and, notably, the expression level in cochlear HCs and SGNs of postnatal day 4 (P4) mice was higher than that in adult mice. Moreover, treatment with 30 μM cisplatin elicited the formation of ROS, which, in turn, led to PINK1 activation, parkin recruitment, autophagy formation and JNK pathway relevant to apoptosis in HEI-OC1 cells, HCs, and SGNs. Meanwhile, co-treatment with ROS scavenger N-acetyl-L-cysteine (NAC) or H₂O₂ consumer catalase-polyethylene glycol (PEG-catalase) inhibited parkin recruitment, alleviated autophagy formation, and mitigated JNK pathway related apoptosis. In addition, PINK1 silencing resulted in a lower level of autophagy, but, a higher mortality in HEI-OC1 cells treated with cisplatin. Taken together, data from this work reveal that PINK1 possesses the protective effect via induction of autophagy and resistance of apoptosis under cisplatin stimulus in sensory HCs and SGNs, implying that PINK1 might serve as an important regulator of cisplatin-elicited ototoxicity.

Daple coordinates organ-wide and cell-intrinsic polarity to pattern inner-ear hair bundles

Each hair cell of our auditory and vestibular systems transduces stimuli into electrical signals through its mechanosensitive hair bundle. Because the bundle is responsive along only a single axis, its orientation is crucial. Two systems determine hair-bundle polarity: planar cell polarity proteins, which establish axes along which hair cells are oriented, and the proteins Gαi and LGN. Investigating how these two systems are coordinated so that each hair bundle is appropriately aligned, we identified Daple. In mutants lacking Daple, hair bundles are misoriented and misshapen, a phenotype suggestive of both organ-wide and cell-intrinsic defects. Our study indicates how Daple interacts with proteins of the two systems and proposes a model for its role in determining hair-bundle polarity.

The establishment of planar polarization by mammalian cells necessitates the integration of diverse signaling pathways. In the inner ear, at least two systems regulate the planar polarity of sensory hair bundles. The core planar cell polarity (PCP) proteins coordinate the orientations of hair cells across the epithelial plane. The cell-intrinsic patterning of hair bundles is implemented independently by the G protein complex classically known for orienting the mitotic spindle. Although the primary cilium also participates in each of these pathways, its role and the integration of the two systems are poorly understood. We show that Dishevelled-associating protein with a high frequency of leucine residues (Daple) interacts with PCP and cell-intrinsic signals. Regulated by the cell-intrinsic pathway, Daple is required to maintain the polarized distribution of the core PCP protein Dishevelled and to position the primary cilium at the abneural edge of the apical surface. Our results suggest that the primary cilium or an associated structure influences the domain of cell-intrinsic signals that shape the hair bundle. Daple is therefore essential to orient and pattern sensory hair bundles.