Changes in ultrastructural characteristics of endolymphatic sac ribosome-rich cells of the rat during development

SOX9 and SOX10 control fluid homeostasis in the inner ear for hearing through independent and cooperative mechanisms

The in vivo mechanisms underlying dominant syndromes caused by mutations in SRY-Box Transcription Factor 9 (SOX9) and SOX10 (SOXE) transcription factors, when they either are expressed alone or are coexpressed, are ill-defined. We created a mouse model for the campomelic dysplasia SOX9Y440X mutation, which truncates the transactivation domain but leaves DNA binding and dimerization intact. Here, we find that SOX9Y440X causes deafness via distinct mechanisms in the endolymphatic sac (ES)/duct and cochlea. By contrast, conditional heterozygous Sox9-null mice are normal. During the ES development of Sox9Y440X/+ heterozygotes, Sox10 and genes important for ionic homeostasis are down-regulated, and there is developmental persistence of progenitors, resulting in fewer mature cells. Sox10 heterozygous null mutants also display persistence of ES/duct progenitors. By contrast, SOX10 retains its expression in the early Sox9Y440X/+ mutant cochlea. Later, in the postnatal stria vascularis, dominant interference by SOX9Y440X is implicated in impairing the normal cooperation of SOX9 and SOX10 in repressing the expression of the water channel Aquaporin 3, thereby contributing to endolymphatic hydrops. Our study shows that for a functioning endolymphatic system in the inner ear, SOX9 regulates Sox10, and depending on the cell type and target gene, it works either independently of or cooperatively with SOX10. SOX9Y440X can interfere with the activity of both SOXE factors, exerting effects that can be classified as haploinsufficient/hypomorphic or dominant negative depending on the cell/gene context. This model of disruption of transcription factor partnerships may be applicable to congenital deafness, which affects ∼0.3% of newborns, and other syndromic disorders.

Prox1 expression in the endolymphatic sac revealed by whole-mount fluorescent imaging of Prox1-GFP transgenic mice

This study describes a technical breakthrough in endolymphatic sac research, made possible by the use of the recently generated Prox1-GFP transgenic mouse model. Whole-mount imaging techniques through the decalcified temporal bone and three-dimensional observations of Prox1-GFP mouse tissue revealed the positive labeling of the endolymphatic sac in adult stage, and allowed, for the first time, the GFP-based identification of endolymphatic sac epithelial cells. Prox1 expression was observed in all parts of the endolymphatic sac epithelia. In intermediate portion of the endolymphatic sac, mitochondria-rich cells did not express Prox1, although ribosome-rich cells showed strong GFP labeling. The anatomical relationship between the endolymphatic sac and the surrounding vasculature was directly observed. In the endolymphatic sac, expression of Prox1 may suggest progenitor cell-like pluripotency or developmental similarity to systemic lymphatic vessels in other organs. This whole-mount imaging technique of the endolymphatic sac can be combined with other conventional histological, sectioning, and labeling techniques and will be very useful for future endolymphatic sac research.

Experimental hyperactivity of the endolymphatic sac

Injury to the endolymphatic sac may play an important role in the pathogenesis of Ménière's disease, an inner ear disorder characterized by hearing loss, tinnitus and attacks of vertigo. Isoimmunization of 16 inbred Lewis rats with a crude endolymphatic sac extract and complete Freund's adjuvant induced hyperactivity of the endolymphatic sac. One group of rats was immunized by a single dose whereas a second group was immunized twice. Control animals were injected with Freund's adjuvant in saline only. Serum was collected from all rats by the end of the study and harvested autoantibodies were tested by immunohistochemistry. The endolymphatic sacs were investigated by transmission electron microscopy. Endolymphatic sac stimulation was observed in all immunized rats. Based on detailed ultrastructural observations, the degree of reactivity seemed proportional to the number of injections and the extent of immunization. Moreover, the ribosome-rich cells seemed hyperactive with an extravagant content of intracellular components: numerous rough endoplasmic reticulum and free ribosomes, morphological signs of extensive endo- and exocytosis, vesicles of material with a density similar to the homogeneous substance of which many were observed to fuse with primary lysozymes. Basolateral foldings were numerous and in the subepithelial capillaries formation of multiple and apposing fenestrations were observed. No endolymphatic sac stimulation was observed in the control animals. Specific ribosome-rich cell alterations identical to those present in the endolymphatic sac of Ménière's disease were observed 21 days after the first immunization. The observations suggest that either an autoantigen or a trophic factor, capable of inducing a hyperactivity of the ribosome-rich cells and an imbalance of the homogeneous substance metabolism, exists in the endolymphatic sac of the rat.

Epithelial cell stretching and luminal acidification lead to a retarded development of stria vascularis and deafness in mice lacking pendrin

Loss-of-function mutations of SLC26A4/pendrin are among the most prevalent causes of deafness. Deafness and vestibular dysfunction in the corresponding mouse model, Slc26a4^−/−, are associated with an enlargement and acidification of the membranous labyrinth. Here we relate the onset of expression of the HCO₃⁻ transporter pendrin to the luminal pH and to enlargement-associated epithelial cell stretching. We determined expression with immunocytochemistry, cell stretching by digital morphometry and pH with double-barreled ion-selective electrodes. Pendrin was first expressed in the endolymphatic sac at embryonic day (E) 11.5, in the cochlear hook-region at E13.5, in the utricle and saccule at E14.5, in ampullae at E16.5, and in the upper turn of the cochlea at E17.5. Epithelial cell stretching in Slc26a4^−/− mice began at E14.5. pH changes occurred first in the cochlea at E15.5 and in the endolymphatic sac at E17.5. At postnatal day 2, stria vascularis, outer sulcus and Reissner's membrane epithelial cells, and utricular and saccular transitional cells were stretched, whereas sensory cells in the cochlea, utricle and saccule did not differ between Slc26a4^+/− and Slc26a4^−/− mice. Structural development of stria vascularis, including vascularization, was retarded in Slc26a4^−/− mice. In conclusion, the data demonstrate that the enlargement and stretching of non-sensory epithelial cells precedes luminal acidification in the cochlea and the endolymphatic sac. Stretching and luminal acidification may alter cell-to-cell communication and lead to the observed retarded development of stria vascularis, which may be an important step on the path to deafness in Slc26a4^−/− mice, and possibly in humans, lacking functional pendrin expression.

The effect of changes in perilymphatic K+ on the vestibular evoked potential in the guinea pig

To investigate the effect on the functioning of the vestibular system of a rupture of Reissner's membrane, artificial endolymph was injected in scala media of ten guinea pigs and vestibular evoked potentials (VsEPs), evoked by vertical acceleration pulses, were measured. Directly after injection of a sufficient volume to cause rupture, all ears showed a complete disappearance of VsEP, followed by partial recovery. To investigate the effect of perilymphatic potassium concentration on the vestibular sensory and neural structures, different concentrations of KCl were injected directly into the vestibule. The KCl injections resulted in a dose-dependent decrease of VsEP, followed by a dose-dependent slow recovery. This animal model clearly shows a disturbing effect of a higher than normal K(+) concentration in perilymph on the vestibular and neural structures in the inner ear. Potassium intoxication is the most probable explanation for the observed effects. It is one of the explanations for Menière attacks.

Morphology of the endolymphatic sac in the guinea pig after an acute endolymphatic hydrops

The role of the endolymphatic sac (ES) in endolymph volume homeostasis is speculative. The present study investigates changes of the ES's epithelia and luminal filling after induction of an acute endolymphatic hydrops. After microinjection of 1.1 mul artificial endolymph into scala media of the cochlea, guinea pigs were terminated immediately (n = 6) or after different time intervals ; 1/2 h (n = 3), 1 h (n = 4) and 2 h (n = 4). Inner ear specimens were processed for light and/or transmission electron microscopy. The non-injected contralateral ear served as a histological control. Correct injection was confirmed by detection of microspheres in the endolymphatic compartment after the same microinjection procedure. In all specimens, ribosome rich cells and intraluminal macrophages appeared to be actively involved in degradation of homogeneous substance (HS) by secreting lytic enzymes and digestion, respectively. Amazingly, in our study no ES differences were found between injected and non-injected ears and no distinct changes were observed in guinea pigs terminated after different time intervals. The ES's luminal HS was always present and often to a large extent. This is in contrast with [Hear. Res. 138, 81] dramatic changes were observed. Endolymph volume homeostasis is a complex mechanism, in which the role of HS remains obscure.

Co-expression of pendrin, vacuolar H+-ATPase alpha4-subunit and carbonic anhydrase II in epithelial cells of the murine endolymphatic sac

The endolymph in the endolymphatic sac (ES) is acidic (pH 6.6-7). Maintaining this acidic lumen is believed to be important for the normal function of the ES. The acid-base regulation mechanisms of the ES are unknown. Here we investigated the expression patterns of acid-base regulators, including vacuolar (v)H+-ATPase (proton pump), carbonic anhydrase (CA) II, and pendrin in the murine ES epithelium by immunohistochemistry (IHC) and compared their expression patterns by double immunostaining. We found that pendrin and vH+-ATPase were co-localized in the apical membrane of a specific type of ES epithelial cell. Pendrin- and vH+-ATPase-positive cells also expressed cytoplasmic CA II. Co-expression of pendrin, vH+-ATPase, and CA II in the same subgroup of ES cells suggests that this specific type of ES cell is responsible for the acid-base balance processes in the ES and pendrin, vH+-ATPase, and CA II are involved in these processes.

Genetic disorders of transporters/channels in the inner ear and their relation to the kidney

Inner ear physiology is reviewed with emphasis on features common to renal physiology. Genetic disorders in transporters/channels for chloride (ClC-K), bicarbonate (Cl(-)/HCO(3)(-) exchanger), protons (H(+)-ATPase), sodium (ENaC, NKKC1, NBC3, NHE3), potassium (KCNQ1/KCNE1, Kcc4), and water (AQP4) in the inner ear and their relation to the kidney are discussed. Based on data from human disorders (with or without mouse counterparts) and mouse models (without human counterparts) this article focuses on the involvement of these transporters/channels in hearing loss.