ATP-dependent intercellular Ca2+ signaling in the developing cochlea: facts, fantasies and perspectives

A pore locus in the E1 domain differentially regulates Cx26 and Cx30 hemichannel function

Connexins (Cxs) function as gap junction (GJ) channels and hemichannels that mediate intercellular and transmembrane signaling, respectively. Here, we investigated the proximal segment of the first extracellular loop, E1, of two closely related Cxs, Cx26 and Cx30, that share widespread expression in the cochlea. Computational studies of Cx26 proposed that this segment of E1 contains a parahelix and functions in gating. The sequence of the parahelix is identical between Cx26 and Cx30 except for an Ala/Glu difference at position 49. We show through cysteine-scanning and mutational analyses that position 49 is pore-lining and interacts with the adjacent Asp50 residue to impact hemichannel functionality. When both positions 49 and 50 are charged, as occurs naturally in Cx30, the hemichannel function is dampened. Co-expression of Cx30 with Cx26(D50N), the most common mutation associated with keratitis-ichthyosis-deafness syndrome, results in robust hemichannel currents indicating that position 49-50 interactions are relevant in heteromerically assembled hemichannels. Cysteine substitution at position 49 in either Cx26 or Cx30 results in tonic inhibition of hemichannels, both through disulfide formation and high-affinity metal coordination, suggestive of a flexible region of the pore that can narrow substantially. These effects are absent in GJ channels, which exhibit wild-type functionality. Examination of postnatal cochlear explants suggests that Cx30 expression is associated with reduced propagation of Ca2+ waves. Overall, these data identify a pore locus in E1 of Cx26 and Cx30 that impacts hemichannel functionality and provide new considerations for understanding the roles of these connexins in cochlear function.

Damage-evoked signals in cochlear neurons and supporting cells

In addition to hearing loss, damage to the cochlea can lead to gain of function pathologies such as hyperacusis. It has been proposed that painful hyperacusis, noxacusis, may be carried to the central nervous system by type II cochlear afferents, sparse, unmyelinated neurons that share morphological and neurochemical traits with nociceptive C-fibers of the somatic nervous system. Also like in skin, damage elicits spreading calcium waves within cochlear epithelia. These are mediated by extracellular ATP combined with IP3-driven release from intracellular calcium stores. Type II afferents are excited by ATP released from damaged epithelia. Thus, the genesis and propagation of epithelial calcium waves is central to cochlear pathology, and presumably hyperacusis. Damage-evoked signals in type II afferents and epithelial cells have been recorded in cochlear explants or semi-intact otic capsules. These efforts have included intracellular electrical recording, use of fluorescent calcium indicators, and visualization of an activity-dependent, intrinsic fluorescent signal. Of relevance to hyperacusis, prior noise-induced hearing loss leads to the generation of prolonged and repetitive activity in type II neurons and surrounding epithelia.

Purinergic Signalling in the Cochlea

The mammalian cochlea is the sensory organ of hearing with a delicate, highly organised structure that supports unique operating mechanisms. ATP release from the secretory tissues of the cochlear lateral wall (stria vascularis) triggers numerous physiological responses by activating P2 receptors in sensory, supporting and neural tissues. Two families of P2 receptors, ATP-gated ion channels (P2X receptors) and G protein-coupled P2Y receptors, activate intracellular signalling pathways that regulate cochlear development, homeostasis, sensory transduction, auditory neurotransmission and response to stress. Of particular interest is a purinergic hearing adaptation, which reflects the critical role of the P2X₂ receptor in adaptive cochlear response to elevated sound levels. Other P2 receptors are involved in the maturation of neural processes and frequency selectivity refinement in the developing cochlea. Extracellular ATP signalling is regulated by a family of surface-located enzymes collectively known as "ectonucleotidases" that hydrolyse ATP to adenosine. Adenosine is a constitutive cell metabolite with an established role in tissue protection and regeneration. The differential activation of A₁ and A_2A adenosine receptors defines the cochlear response to injury caused by oxidative stress, inflammation, and activation of apoptotic pathways. A₁ receptor agonism, A_2A receptor antagonism, and increasing adenosine levels in cochlear fluids all represent promising therapeutic tools for cochlear rescue from injury and prevention of hearing loss.

Kölliker’s organ-supporting cells and cochlear auditory development

The Kölliker’s organ is a transient cellular cluster structure in the development of the mammalian cochlea. It gradually degenerates from embryonic columnar cells to cuboidal cells in the internal sulcus at postnatal day 12 (P12)–P14, with the cochlea maturing when the degeneration of supporting cells in the Kölliker’s organ is complete, which is distinct from humans because it disappears at birth already. The supporting cells in the Kölliker’s organ play a key role during this critical period of auditory development. Spontaneous release of ATP induces an increase in intracellular Ca²⁺ levels in inner hair cells in a paracrine form via intercellular gap junction protein hemichannels. The Ca²⁺ further induces the release of the neurotransmitter glutamate from the synaptic vesicles of the inner hair cells, which subsequently excite afferent nerve fibers. In this way, the supporting cells in the Kölliker’s organ transmit temporal and spatial information relevant to cochlear development to the hair cells, promoting fine-tuned connections at the synapses in the auditory pathway, thus facilitating cochlear maturation and auditory acquisition. The Kölliker’s organ plays a crucial role in such a scenario. In this article, we review the morphological changes, biological functions, degeneration, possible trans-differentiation of cochlear hair cells, and potential molecular mechanisms of supporting cells in the Kölliker’s organ during the auditory development in mammals, as well as future research perspectives.

Allosteric probe initiated triple signal recycles for sensitive analysis of adenosine triphosphate (ATP)

Adenosine triphosphate (ATP) is closely associated with a variety of pathophysiological process, providing energy for cell activities. However, it remains a huge challenge to develop a simple and sensitive ATP detection method. Herein, we describe an ultrasensitive approach for ATP detection by using an elegantly designed allosteric probe to bind with targets and to induce DNAzyme assisted triple signal recycles. To establish the approach, the allosteric probe is designed with three functional parts to bind with target ATP, to induce signal recycles and to assist chain extension. After recognition of ATP by allosteric probe, the triple signal recycles are initiated, including (i) released ATP-allosteric probe complex unfolds H1 probe, constituting the first signal recycle; (ii) DNAzyme based cleavage induces the second signal recycle; and (iii) chain extension induced release of ATP is the third signal recycle. Through a series of experiments, the approach exhibits a favorable ATP detection performance and shows a high anti-interference ability, indicating a potential application value in early diagnosis of diseases.

Pseudo-Temporal Analysis of Single-Cell RNA Sequencing Reveals Trans-Differentiation Potential of Greater Epithelial Ridge Cells Into Hair Cells During Postnatal Development of Cochlea in Rats

The hair cells of the cochlea play a decisive role in the process of hearing damage and recovery, yet knowledge of their regeneration process is still limited. Greater epithelial ridge (GER) cells, a type of cell present during cochlear development that has the characteristics of a precursor sensory cell, disappear at the time of maturation of hearing development. Its development and evolution remain mysterious for many years. Here, we performed single-cell RNA sequencing to profile the gene expression landscapes of rats’ cochlear basal membrane from P1, P7, and P14 and identified eight major subtypes of GER cells. Furthermore, single-cell trajectory analysis for GER cells and hair cells indicated that among the different subtypes of GER, four subtypes had transient cell proliferation after birth and could transdifferentiate into inner and outer hair cells, and two of them mainly transdifferentiated into inner hair cells. The other two subtypes eventually transdifferentiate into outer hair cells. Our study lays the groundwork for elucidating the mechanisms of the key regulatory genes and signaling pathways in the trans-differentiation of GER cell subtypes into hair cells and provides potential clues to understand hair cell regeneration.

Failure Of Hearing Acquisition in Mice With Reduced Expression of Connexin 26 Correlates With the Abnormal Phasing of Apoptosis Relative to Autophagy and Defective ATP-Dependent Ca2+ Signaling in Kölliker’s Organ

Mutations in the GJB2 gene that encodes connexin 26 (Cx26) are the predominant cause of prelingual hereditary deafness, and the most frequently encountered variants cause complete loss of protein function. To investigate how Cx26 deficiency induces deafness, we examined the levels of apoptosis and autophagy in Gjb2^loxP/loxP; ROSA26^CreER mice injected with tamoxifen on the day of birth. After weaning, these mice exhibited severe hearing impairment and reduced Cx26 expression in the cochlear duct. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positive cells were observed in apical, middle, and basal turns of Kölliker’s organ at postnatal (P) day 1 (P1), associated with increased expression levels of cleaved caspase 3, but decreased levels of autophagy-related proteins LC3-II, P62, and Beclin1. In Kölliker’s organ cells with decreased Cx26 expression, we also found significantly reduced levels of intracellular ATP and hampered Ca²⁺ responses evoked by extracellular ATP application. These results offer novel insight into the mechanisms that prevent hearing acquisition in mouse models of non-syndromic hearing impairment due to Cx26 loss of function.

The Cl--channel TMEM16A is involved in the generation of cochlear Ca2+ waves and promotes the refinement of auditory brainstem networks in mice

Before hearing onset (postnatal day 12 in mice), inner hair cells (IHCs) spontaneously fire action potentials, thereby driving pre-sensory activity in the ascending auditory pathway. The rate of IHC action potential bursts is modulated by inner supporting cells (ISCs) of Kölliker’s organ through the activity of the Ca²⁺-activated Cl^--channel TMEM16A (ANO1). Here, we show that conditional deletion of Ano1 (Tmem16a) in mice disrupts Ca²⁺ waves within Kölliker’s organ, reduces the burst-firing activity and the frequency selectivity of auditory brainstem neurons in the medial nucleus of the trapezoid body (MNTB), and also impairs the functional refinement of MNTB projections to the lateral superior olive. These results reveal the importance of the activity of Kölliker’s organ for the refinement of central auditory connectivity. In addition, our study suggests the involvement of TMEM16A in the propagation of Ca²⁺ waves, which may also apply to other tissues expressing TMEM16A.

Identification of a Novel Stop Loss Mutation in P2RX2 Gene in an Iranian Family with Autosomal Nonsyndromic Hearing Loss

Background:

Hearing loss, a congenital genetic disorder in human, is difficult to diagnose. WES is a powerful approach for ethiological disgnosis of such disorders.

Methods:

One Iranian family with two patients were attented in the study. Sequencing of known NSHL genes was carried out to recognize the genetic causes of HL.

Results:

Molecular analyses identified a novel stop loss mutation, c.1048T>G (p.Term350Glu), whitin the P2RX2 gene, causing a termination-site modification.This event would lead to continued translation into the 3' UTR of the gene, which in turn may result in a longer protein product. The mutation was segregating with the disease phenotype and predicted to be pathogenic by bioinformatic tools.

Conclusion:

This study is the first Iranian case report of a diagnosis of ADNSHL caused by P2RX2 mutation. The recognition of other causative mutations in P2RX2 gene more supports the probable function of this gene in causing ADNSHL.

Organ-on-chip model shows that ATP release through connexin hemichannels drives spontaneous Ca2+ signaling in non-sensory cells of the greater epithelial ridge in the developing cochlea

Prior work supports the hypothesis that ATP release through connexin hemichannels drives spontaneous Ca2+ signaling in non-sensory cells of the greater epithelial ridge (GER) in the developing cochlea; however, direct proof is lacking. To address this issue, we plated cochlear organotypic cultures (COCs) and whole cell-based biosensors with nM ATP sensitivity (ATP-WCBs) at the bottom and top of an ad hoc designed transparent microfluidic chamber, respectively. By performing dual multiphoton Ca2+ imaging, we monitored the propagation of intercellular Ca2+ waves in the GER of COCs and ATP-dependent Ca2+ responses in overlying ATP-WCBs. Ca2+ signals in both COCs and ATP-WCBs were inhibited by supplementing the extracellular medium with ATP diphosphohydrolase (apyrase). Spontaneous Ca2+ signals were strongly depressed in the presence of Gjb6-/- COCs, in which connexin 30 (Cx30) is absent and connexin 26 (Cx26) is strongly downregulated. In contrast, spontaneous Ca2+ signals were not affected by replacement of Panx1-/- with Panx1+/+ COCs in the microfluidic chamber. Similar results were obtained by estimating ATP release from COCs using a classical luciferin-luciferase bioluminescence assay. Therefore, connexin hemichannels and not pannexin 1 channels mediate the release of ATP that is responsible for Ca2+ wave propagation in the developing mouse cochlea. The technological advances presented here have the potential to shed light on a plethora of unrelated open issues that involve paracrine signaling in physiology and pathology and cannot be addressed with standard methods.

Deletion of Tmtc4 activates the unfolded protein response and causes postnatal hearing loss

Hearing loss is a significant public health concern, affecting over 250 million people worldwide. Both genetic and environmental etiologies are linked to hearing loss, but in many cases the underlying cellular pathophysiology is not well understood, highlighting the importance of further discovery. We found that inactivation of the gene Tmtc4 (transmembrane and tetratricopeptide repeat 4), which was broadly expressed in the mouse cochlea, caused acquired hearing loss in mice. Our data showed Tmtc4 enriched in the endoplasmic reticulum, and that it functioned by regulating Ca2+ dynamics and the unfolded protein response (UPR). Given this genetic linkage of the UPR to hearing loss, we demonstrated a direct link between the more common noise-induced hearing loss (NIHL) and the UPR. These experiments suggested a novel approach to treatment. We demonstrated that the small-molecule UPR and stress response modulator ISRIB (integrated stress response inhibitor), which activates eIF2B, prevented NIHL in a mouse model. Moreover, in an inverse genetic complementation approach, we demonstrated that mice with homozygous inactivation of both Tmtc4 and Chop had less hearing loss than knockout of Tmtc4 alone. This study implicated a novel mechanism for hearing impairment, highlighting a potential treatment approach for a broad range of human hearing loss disorders.

Connexin hemichannels and cochlear function

Connexins play vital roles in hearing, including promoting cochlear development and sustaining auditory function in the mature cochlea. Mutations in connexins expressed in the cochlear epithelium, Cx26 and Cx30, cause sensorineural deafness and in the case of Cx26, is one of the most common causes of non-syndromic, hereditary deafness. Connexins function as gap junction channels and as hemichannels, which mediate intercellular and transmembrane signaling, respectively. Both channel configurations can play important, but very different roles in the cochlea. The potential roles connexin hemichannels can play are discussed both in normal cochlear function and in promoting pathogenesis that can lead to hearing loss.

Connexin-Mediated Signaling in Nonsensory Cells Is Crucial for the Development of Sensory Inner Hair Cells in the Mouse Cochlea

Mutations in the genes encoding for gap junction proteins connexin 26 (Cx26) and connexin 30 (Cx30) have been linked to syndromic and nonsyndromic hearing loss in mice and humans. The release of ATP from connexin hemichannels in cochlear nonsensory cells has been proposed to be the main trigger for action potential activity in immature sensory inner hair cells (IHCs), which is crucial for the refinement of the developing auditory circuitry. Using connexin knock-out mice, we show that IHCs fire spontaneous action potentials even in the absence of ATP-dependent intercellular Ca²⁺ signaling in the nonsensory cells. However, this signaling from nonsensory cells was able to increase the intrinsic IHC firing frequency. We also found that connexin expression is key to IHC functional maturation. In Cx26 conditional knock-out mice (Cx26^Sox10-Cre), the maturation of IHCs, which normally occurs at approximately postnatal day 12, was partially prevented. Although Cx30 has been shown not to be required for hearing in young adult mice, IHCs from Cx30 knock-out mice exhibited a comprehensive brake in their development, such that their basolateral membrane currents and synaptic machinery retain a prehearing phenotype. We propose that IHC functional differentiation into mature sensory receptors is initiated in the prehearing cochlea provided that the expression of either connexin reaches a threshold level. As such, connexins regulate one of the most crucial functional refinements in the mammalian cochlea, the disruption of which contributes to the deafness phenotype observed in mice and DFNB1 patients.

SIGNIFICANCE STATEMENT The correct development and function of the mammalian cochlea relies not only on the sensory hair cells, but also on the surrounding nonsensory cells. Although the nonsensory cells have been largely implicated in the general homeostasis in the mature cochlea, their involvement in the initial functional differentiation of the sensory inner hair cells is less clear. Using mutant mouse models for the most common form of congenital deafness in humans, which are knock-outs for the gap-junction channels connexin 26 and connexin 30 genes, we show that defects in nonsensory cells prevented the functional maturation of inner hair cells. In connexin knock-outs, inner hair cells remained stuck at a prehearing stage of development and, as such, are unable to process sound information.

Ca2+ signaling, apoptosis and autophagy in the developing cochlea: Milestones to hearing acquisition

In mammals, the sense of hearing arises through a complex sequence of morphogenetic events that drive the sculpting of the auditory sensory epithelium into its terminally functional three-dimensional shape. While the majority of the underlying mechanisms remain unknown, it has become increasingly clear that Ca²⁺ signaling is at center stage and plays numerous fundamental roles both in the sensory hair cells and in the matrix of non-sensory, epithelial and supporting cells, which embed them and are tightly interconnected by a dense network of gap junctions formed by connexin 26 (Cx26) and connexin 30 (Cx30) protein subunits. In this review, we discuss the intricate interplay between Ca²⁺ signaling, connexin expression and function, apoptosis and autophagy in the crucial steps that lead to hearing acquisition.

Critical role of ATP-induced ATP release for Ca2+ signaling in nonsensory cell networks of the developing cochlea

Spatially and temporally coordinated variations of the cytosolic free calcium concentration ([Ca²⁺]_c) play a crucial role in a variety of tissues. In the developing sensory epithelium of the mammalian cochlea, elevation of extracellular adenosine trisphosphate concentration ([ATP]_e) triggers [Ca²⁺]_c oscillations and propagation of intercellular inositol 1,4,5-trisphosphate (IP₃)-dependent Ca²⁺ waves. What remains uncertain is the relative contribution of gap junction channels and connexin hemichannels to these fundamental mechanisms, defects in which impair hearing acquisition. Another related open question is whether [Ca²⁺]_c oscillations require oscillations of the cytosolic IP₃ concentration ([IP₃]_c) in this system. To address these issues, we performed Ca²⁺ imaging experiments in the lesser epithelial ridge of the mouse cochlea around postnatal day 5 and constructed a computational model in quantitative adherence to experimental data. Our results indicate that [Ca²⁺]_c oscillations are governed by Hopf-type bifurcations within the experimental range of [ATP]_e and do not require [IP₃]_c oscillations. The model replicates accurately the spatial extent and propagation speed of intercellular Ca²⁺ waves and predicts that ATP-induced ATP release is the primary mechanism underlying intercellular propagation of Ca²⁺ signals. The model also uncovers a discontinuous transition from propagating regimes (intercellular Ca²⁺ wave speed > 11 μm⋅s^-1) to propagation failure (speed = 0), which occurs upon lowering the maximal ATP release rate below a minimal threshold value. The approach presented here overcomes major limitations due to lack of specific connexin channel inhibitors and can be extended to other coupled cellular systems.

Differential effects of pannexins on noise-induced hearing loss

Hearing loss, including noise-induced hearing loss, is highly prevalent and severely hinders an individual's quality of life, yet many of the mechanisms that cause hearing loss are unknown. The pannexin (Panx) channel proteins, Panx1 and Panx3, are regionally expressed in many cell types along the auditory pathway, and mice lacking Panx1 in specific cells of the inner ear exhibit hearing loss, suggesting a vital role for Panxs in hearing. We proposed that Panx1 and/or Panx3 null mice would exhibit severe hearing loss and increased susceptibility to noise-induced hearing loss. Using the auditory brainstem response, we surprisingly found that Panx1^-/- and Panx3^-/- mice did not harbor hearing or cochlear nerve deficits. Furthermore, while Panx1^-/- mice displayed no protection against loud noise-induced hearing loss, Panx3^-/- mice exhibited enhanced 16- and 24-kHz hearing recovery 7 days after a loud noise exposure (NE; 12 kHz tone, 115 dB sound pressure level, 1 h). Interestingly, Cx26, Cx30, Cx43, and Panx2 were up-regulated in Panx3^-/- mice compared with wild-type and/or Panx1^-/- mice, and assessment of the auditory tract revealed morphological changes in the middle ear bones of Panx3^-/- mice. It is unclear if these changes alone are sufficient to provide protection against loud noise-induced hearing loss. Contrary to what we expected, these data suggest that Panx1 and Panx3 are not essential for baseline hearing in mice tested, but the therapeutic targeting of Panx3 may prove protective against mid-high-frequency hearing loss caused by loud NE.

ATP-Evoked Intracellular Ca²⁺ Signaling of Different Supporting Cells in the Hearing Mouse Hemicochlea

Hearing and its protection is regulated by ATP-evoked Ca(2+) signaling in the supporting cells of the organ of Corti, however, the unique anatomy of the cochlea hampers observing these mechanisms. For the first time, we have performed functional ratiometric Ca(2+) imaging (fura-2) in three different supporting cell types in the hemicochlea preparation of hearing mice to measure purinergic receptor-mediated Ca(2+) signaling in pillar, Deiters' and Hensen's cells. Their resting [Ca(2+)]i was determined and compared in the same type of preparation. ATP evoked reversible, repeatable and dose-dependent Ca(2+) transients in all three cell types, showing desensitization. Inhibiting the Ca(2+) signaling of the ionotropic P2X (omission of extracellular Ca(2+)) and metabotropic P2Y purinergic receptors (depletion of intracellular Ca(2+) stores) revealed the involvement of both receptor types. Detection of P2X2,3,4,6,7 and P2Y1,2,6,12,14 receptor mRNAs by RT-PCR supported this finding and antagonism by PPADS suggested different functional purinergic receptor population in pillar versus Deiters' and Hensen's cells. The sum of the extra- and intracellular Ca(2+)-dependent components of the response was about equal with the control ATP response (linear additivity) in pillar cells, and showed supralinearity in Deiters' and Hensen's cells. Calcium-induced calcium release might explain this synergistic interaction. The more pronounced Ca(2+) leak from the endoplasmic reticulum in Deiters' and Hensen's cells, unmasked by cyclopiazonic acid, may also suggests the higher activity of the internal stores in Ca(2+) signaling in these cells. Differences in Ca(2+) homeostasis and ATP-induced Ca(2+) signaling might reflect the distinct roles these cells play in cochlear function and pathophysiology.

Design and Construction of a Cost-Effective Spinning Disk System for Live Imaging of Inner Ear Tissue

Confocal imaging of fluorescent probes offers a powerful, non-invasive tool which enables data collection from vast population of cells at high spatial and temporal resolution. Spinning disk confocal microscopy parallelizes the imaging process permitting the study of dynamic events in populations of living cells on the millisecond time scale. Several spinning disk microscopy solutions are commercially available, however these are often poorly configurable and relatively expensive. This chapter describes a procedure to assemble a cost-effective homemade spinning disk system for fluorescence microscopy, which is highly flexible and easily configurable. We finally illustrate a reliable protocol to obtain high-quality Ca(2+) and voltage imaging data from cochlear preparations.

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Developmental expression of inositol 1, 4, 5-trisphosphate receptor in the post-natal rat cochlea

Inositol 1, 4, 5-trisphosphate receptor (IP3R) has been established to be essential for hearing. However, the expression of IP3R in the cochlea in the period of auditory development remains unknown. We investigated the expression of IP3R in the developing rat cochlea using immunohistochemistry and real-time reverse transcription polymerase chain reaction (RT-PCR). We observed its presence in the developing rat cochlea, and changes in IP3R protein expressions from the early post-natal period to adult. At birth (post-natal day 0, P0), IP3R expression was only found in Hensen's cell. IP3R immunoreactivity first appeared in the sensory hair cells in the organ of Corti at P2. This localization was confirmed by means of double-labeling experiments with Myosin VIIA, a marker for cochlear hair cells. Colocalization of IP3R and Myosin VIIA from P2 to the second post-natal week suggested early expression of IP3R in developing inner and outer hair cells. Claudius' cells near the spiral ligament were labelled for IP3R from P8 onwards. Transient IP3R expression was observed in the stria vascularis in early post-natal rat from P4 to P8. Spiral ganglion neurons also exhibited weaker IP3R fluorescence signals during post-natal development. The results of RT-PCR demonstrated that all three IP3R isoforms (IP3R1, IP3R2, and IP3R3) were present in rat cochlea during four different developmental stages of cochlea, from P0 to P28. Present immunohistochemical evidence for both change and maintenance of expression of IP3R during post-natal development of the rat cochlea indicated the possible involvement of IP3R-mediated calcium signaling in cochlear development.

Aberrant Cx26 hemichannels and keratitis-ichthyosis-deafness syndrome: insights into syndromic hearing loss

Mutation of the GJB2 gene, which encodes the connexin 26 (Cx26) gap junction (GJ) protein, is the most common cause of hereditary, sensorineural hearing loss. Cx26 is not expressed in hair cells, but is widely expressed throughout the non-sensory epithelial cells of the cochlea. Most GJB2 mutations produce non-syndromic deafness, but a subset produces syndromic deafness in which profound hearing loss is accompanied by a diverse array of infectious and neoplastic cutaneous disorders that can be fatal. Although GJ channels, which are assembled by the docking of two, so-called hemichannels (HCs), have been the main focus of deafness-associated disease models, it is now evident that the HCs themselves can function in the absence of docking and contribute to signaling across the cell membrane as a novel class of ion channel. A notable feature of syndromic deafness mutants is that the HCs exhibit aberrant behaviors providing a plausible basis for disease that is associated with excessive or altered contributions of Cx26 HCs that, in turn, lead to compromised cell integrity. Here we discuss some of the aberrant Cx26 HC properties that have been described for mutants associated with keratitis-ichthyosis-deafness (KID) syndrome, a particularly severe Cx26-associated syndrome, which shed light on genotype-phenotype relationships and causes underlying cochlear dysfunction.

Functional analysis and regulation of purified connexin hemichannels

Gap-junction channels (GJCs) are aqueous channels that communicate adjacent cells. They are formed by head-to-head association of two hemichannels (HCs), one from each of the adjacent cells. Functional HCs are connexin hexamers composed of one or more connexin isoforms. Deafness is the most frequent sensineural disorder, and mutations of Cx26 are the most common cause of genetic deafness. Cx43 is the most ubiquitous connexin, expressed in many organs, tissues, and cell types, including heart, brain, and kidney. Alterations in its expression and function play important roles in the pathophysiology of very frequent medical problems such as those related to cardiac and brain ischemia. There is extensive information on the relationship between phosphorylation and Cx43 targeting, location, and function from experiments in cells and organs in normal and pathological conditions. However, the molecular mechanisms of Cx43 regulation by phosphorylation are hard to tackle in complex systems. Here, we present the use of purified HCs as a model for functional and structural studies. Cx26 and Cx43 are the only isoforms that have been purified, reconstituted, and subjected to functional and structural analysis. Purified Cx26 and Cx43 HCs have properties compatible with those demonstrated in cells, and present methodologies for the functional analysis of purified HCs reconstituted in liposomes. We show that phosphorylation of serine 368 by PKC produces a partial closure of the Cx43 HCs, changing solute selectivity. We also present evidence that the effect of phosphorylation is highly cooperative, requiring modification of several connexin subunits, and that phosphorylation of serine 368 elicits conformational changes in the purified HCs. The use of purified HCs is starting to provide critical data to understand the regulation of HCs at the molecular level.

Spontaneous Network Activity and Synaptic Development

Throughout development, the nervous system produces patterned spontaneous activity. Research over the past two decades has revealed a core group of mechanisms that mediate spontaneous activity in diverse circuits. Many circuits engage several of these mechanisms sequentially to accommodate developmental changes in connectivity. In addition to shared mechanisms, activity propagates through developing circuits and neuronal pathways (i.e., linked circuits in different brain areas) in stereotypic patterns. Increasing evidence suggests that spontaneous network activity shapes synaptic development in vivo Variations in activity-dependent plasticity may explain how similar mechanisms and patterns of activity can be employed to establish diverse circuits. Here, I will review common mechanisms and patterns of spontaneous activity in emerging neural networks and discuss recent insights into their contribution to synaptic development.

A novel P2RX2 mutation in an Italian family affected by autosomal dominant nonsyndromic hearing loss

Hereditary hearing loss (HHL) is a common disorder accounting for at least 60% of prelingual deafness. It is characterized by a large genetic heterogeneity, and despite the presence of a major gene, still there is a need to search for new causative mutations/genes. Very recently, a mutation within ATP-gated P2X(2) receptor (ligand-gated ion channel, purinergic receptor 2) gene (P2RX2) at DNFA41 locus has been reported leading to a bilateral and symmetrical sensorineural non-syndromic autosomal dominant HHL in two Chinese families. We performed a linkage analysis in a large Italian family with a dominant pattern of inheritance showing a significant 3.31 LOD score in a 2Mb region overlapping with the DNFA41 locus. Molecular analyses of P2RX2 identified a novel missense mutation (p.Gly353Arg) affecting a residue highly conserved across species. Visual inspection of the protein structure as obtained from comparative modeling suggests that substitution of the small glycine residue with a charged bulky residue such as an arginine that is close to the 'neck' of the region responsible for ion channel gating should have a high energetic cost and should lead to a severely destabilization of the fold. The identification of a second most likely causative mutation in P2RX2 gene further supports the possible role of this gene in causing autosomal dominant HHL.

A rapid and sensitive assay of intercellular coupling by voltage imaging of gap junction networks

Background

A variety of mechanisms that govern connexin channel gating and permeability regulate coupling in gap junction networks. Mutations in connexin genes have been linked to several pathologies, including cardiovascular anomalies, peripheral neuropathy, skin disorders, cataracts and deafness. Gap junction coupling and its patho–physiological alterations are commonly assayed by microinjection experiments with fluorescent tracers, which typically require several minutes to allow dye transfer to a limited number of cells. Comparable or longer time intervals are required by fluorescence recovery after photobleaching experiments. Paired electrophysiological recordings have excellent time resolution but provide extremely limited spatial information regarding network connectivity.

Results

Here, we developed a rapid and sensitive method to assay gap junction communication using a combination of single cell electrophysiology, large–scale optical recordings and a digital phase–sensitive detector to extract signals with a known frequency from Vf2.1.Cl, a novel fluorescent sensor of plasma membrane potential. Tests performed in HeLa cell cultures confirmed that suitably encoded Vf2.1.Cl signals remained confined within the network of cells visibly interconnected by fluorescently tagged gap junction channels. We used this method to visualize instantly intercellular connectivity over the whole field of view (hundreds of cells) in cochlear organotypic cultures from postnatal mice. A simple resistive network model reproduced accurately the spatial dependence of the electrical signals throughout the cellular network. Our data suggest that each pair of cochlear non−sensory cells of the lesser epithelial ridge is coupled by ~1500 gap junction channels, on average. Junctional conductance was reduced by 14% in cochlear cultures harboring the T5M mutation of connexin30, which induces a moderate hearing loss in connexin30^T5M/T5M knock–in mice, and by 91% in cultures from connexin30^−/− mice, which are profoundly deaf.

Conclusions

Our methodology allows greater sensitivity (defined as the minimum magnitude of input signal required to produce a specified output signal having a specified signal−to−noise ratio) and better time resolution compared to classical tracer–based techniques. It permitted us to dynamically visualize intercellular connectivity down to the 10th order in non−sensory cell networks of the developing cochlea. We believe that our approach is of general interest and can be seamlessly extended to a variety of biological systems, as well as to other connexin−related disease conditions.

The role of transcription-independent damage signals in the initiation of epithelial wound healing

Wound healing is an essential biological process that comprises sequential steps aimed at restoring the architecture and function of damaged cells and tissues. This process begins with conserved damage signals, such as Ca(2+), hydrogen peroxide (H2O2) and ATP, that diffuse through epithelial tissues and initiate immediate gene transcription-independent cellular effects, including cell shape changes, the formation of functional actomyosin structures and the recruitment of immune cells. These events integrate the ensuing transcription of specific wound response genes that further advance the wound healing response. The immediate importance of transcription-independent damage signals illustrates that healing a wound begins as soon as damage occurs.