Bile acid-permeation enhancement for inner ear cochlear drug – pharmacological uptake: bio-nanotechnologies in chemotherapy-induced hearing loss

Ototoxicity is the damage to inner ear sensory epithelia due to exposure to certain medications and chemicals. This occurs when toxins enter the tightly controlled inner ear environment inducing hair cell death, resulting in hearing loss. Recent studies have explored hydrogel-based bio-nanotechnologies and new drug delivery formulations to prevent drug-induced hearing loss, with much attention given to administration of antioxidant drugs. Bile acids have been recognized as promising excipients due to their biocompatibility and unique physiochemical properties. As yet bile acids have not been explored in improving drug delivery to the inner ear despite improving drug stability and delivery in other systems and demonstrating positive biological effects in their own right.

Conserved and Divergent Principles of Planar Polarity Revealed by Hair Cell Development and Function

Planar polarity describes the organization and orientation of polarized cells or cellular structures within the plane of an epithelium. The sensory receptor hair cells of the vertebrate inner ear have been recognized as a preeminent vertebrate model system for studying planar polarity and its development. This is principally because planar polarity in the inner ear is structurally and molecularly apparent and therefore easy to visualize. Inner ear planar polarity is also functionally significant because hair cells are mechanosensors stimulated by sound or motion and planar polarity underlies the mechanosensory mechanism, thereby facilitating the auditory and vestibular functions of the ear. Structurally, hair cell planar polarity is evident in the organization of a polarized bundle of actin-based protrusions from the apical surface called stereocilia that is necessary for mechanosensation and when stereociliary bundle is disrupted auditory and vestibular behavioral deficits emerge. Hair cells are distributed between six sensory epithelia within the inner ear that have evolved unique patterns of planar polarity that facilitate auditory or vestibular function. Thus, specialized adaptations of planar polarity have occurred that distinguish auditory and vestibular hair cells and will be described throughout this review. There are also three levels of planar polarity organization that can be visualized within the vertebrate inner ear. These are the intrinsic polarity of individual hair cells, the planar cell polarity or coordinated orientation of cells within the epithelia, and planar bipolarity; an organization unique to a subset of vestibular hair cells in which the stereociliary bundles are oriented in opposite directions but remain aligned along a common polarity axis. The inner ear with its complement of auditory and vestibular sensory epithelia allows these levels, and the inter-relationships between them, to be studied using a single model organism. The purpose of this review is to introduce the functional significance of planar polarity in the auditory and vestibular systems and our contemporary understanding of the developmental mechanisms associated with organizing planar polarity at these three cellular levels.

A level set-based approach for modeling cellular rearrangements in tissue morphogenesis

Among morphological phenomena, cellular patterns in developing sensory epithelia have gained attention in recent years. Although physical models for cellular rearrangements are well-established thanks to a large bulk of experimental work, their computational implementation lacks solid mathematical background and involves experimentally unreachable parameters. Here we introduce a level set-based computational framework as a tool to rigorously investigate evolving cellular patterns, and study its mathematical and computational properties. We illustrate that a significant feature of the method is its ability to correctly handle complex topology changes, including frequent cell intercalations. Combining this accurate numerical scheme with an established mathematical model, we show that the new framework features minimum possible number of parameters and is capable of reproducing a wide range of tissue morphological phenomena, such as cell sorting, engulfment or internalization. In particular, thanks to precise mathematical treatment of cellular intercalations, this method is the first to successfully simulate experimentally observed development of cellular mosaic patterns in sensory epithelia.

ATP and ACh Evoked Calcium Transients in the Neonatal Mouse Cochlear and Vestibular Sensory Epithelia

Hair cells in the mammalian inner ear sensory epithelia are surrounded by supporting cells which are essential for function of cochlear and vestibular systems. In mice, support cells exhibit spontaneous intracellular Ca2+ transients in both auditory and vestibular organs during the first postnatal week before the onset of hearing. We recorded long lasting (>200 ms) Ca2+ transients in cochlear and vestibular support cells in neonatal mice using the genetic calcium indicator GCaMP5. Both cochlear and vestibular support cells exhibited spontaneous intracellular Ca2+ transients (GCaMP5 ΔF/F), in some cases propagating as waves from the apical (endolymph facing) to the basolateral surface with a speed of ∼25 μm per second, consistent with inositol trisphosphate dependent calcium induced calcium release (CICR). Acetylcholine evoked Ca2+ transients were observed in both inner border cells in the cochlea and vestibular support cells, with a larger change in GCaMP5 fluorescence in the vestibular support cells. Adenosine triphosphate evoked robust Ca2+ transients predominantly in the cochlear support cells that included Hensen’s cells, Deiters’ cells, inner hair cells, inner phalangeal cells and inner border cells. A Ca2+ event initiated in one inner border cells propagated in some instances longitudinally to neighboring inner border cells with an intercellular speed of ∼2 μm per second, and decayed after propagating along ∼3 cells. Similar intercellular propagation was not observed in the radial direction from inner border cell to inner sulcus cells, and was not observed between adjacent vestibular support cells.

A level set-based approach for modeling cellular rearrangements in tissue morphogenesis

Among morphological phenomena, cellular patterns in developing sensory epithelia have gained attention in recent years. Although physical models for cellular rearrangements are well-established thanks to a large bulk of experimental work, their computational implementation lacks solid mathematical background and involves experimentally unreachable parameters. Here we introduce a level set-based computational framework as a tool to rigorously investigate evolving cellular patterns. We investigate its mathematical and computational properties, showing that it significantly surpasses existing schemes in its ability to correctly handle complex topology changes, including frequent cell intercalations. Combining this accurate numerical scheme with an established mathematical model, we show that the new framework features minimum possible number of parameters and is capable of reproducing a wide range of tissue morphological phenomena, such as cell sorting, engulfment or internalization. In particular, thanks to precise mathematical treatment of cellular intercalations, this method is the first to successfully simulate experimentally observed development of cellular mosaic patterns in sensory epithelia.

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Cav1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

Differential role of type I and type II vestibular hair cells in two anti-gravity reflexes in the rat

The tail-lift reflex and the air-righting reflex in rats are anti-gravity reflexes that depend on vestibular function. We assessed reflex loss in relationship to the graded lesions caused in the vestibular sensory epithelia by varying doses of an ototoxic compound. Using high-speed video recording, we obtained nose-back of the neck-tail angles from the tail-lift reflex and time to right in the air-righting test. We then correlated these measures with type I (HCI), type II (HCII) and all hair cell (HC) counts in central and peripheral zones of the crista, utricle, and saccule. Correlations varied with the cell type, zone and end-organ considered, and those of tail-lift angles were strikingly greater with HCI counts that HCII counts. A similar HCI vs HCII difference was not recorded for air-righting times. We conclude that these two reflexes depend differently on HCI and HCII function and that the tail-lift angle measures HCI function.

A level set-based approach for modeling cellular rearrangements in tissue morphogenesis

Mathematical models and numerical simulations can provide an essential insight into the mechanisms through which local cell-cell interactions affect tissue-level cell morphology. Among such morphological phenomena, cellular patterns observed in developing sensory epithelia have gained keen attention of researchers in recent years, because they are thought to be of utmost importance for accurate sensory functions. However, most of current computational approaches to cellular rearrangements lack solid mathematical background and involve experimentally unreachable parameters, whereby only weak and ambiguous conclusions can be made based on simulation results. Here we present a simple mathematical model for tissue morphogenesis together with a level set-based numerical scheme for its solution as a tool to rigorously investigate evolving cellular patterns. This combined framework of a model and a numerical method features minimum possible number of physical parameters and guarantees reliability of simulation results, including correct handling of topology changes, such as cell intercalations. In this framework, we adopt the viewpoint of free energy minimization principle, and take cellular rearrangement as a gradient flow of a weighted surface energy associated with cell membrane, where the weights are related to physical parameters of the cells, for example, cell-cell adhesion and cell contractility. We present the applicability of this model to a wide range of tissue morphological phenomena, such as cell sorting, engulfment or internalization. In particular, we stress that this method is the first one to be successful in computationally reproducing the experimentally observed development of cellular mosaic patterns in sensory epithelia. Thanks to its simplicity and reliability, the model is able to capture the essence of biological phenomena, and may give a strong helping hand in deciphering unsolved questions of morphology.