ATP activates P2X receptors to mediate gap junctional coupling in the cochlea

Engineering a human P2X2 receptor with altered ligand selectivity in yeast

P2X receptors are a family of ligand gated ion channels found in a range of eukaryotic species including humans but are not naturally present in the yeast Saccharomyces cerevisiae. We demonstrate the first recombinant expression and functional gating of the P2X2 receptor in baker's yeast. We leverage the yeast host for facile genetic screens of mutant P2X2 by performing site saturation mutagenesis at residues of interest, including SNPs implicated in deafness and at residues involved in native binding. Deep mutational analysis and rounds of genetic engineering yield mutant P2X2 F303Y A304W, which has altered ligand selectivity toward the ATP analog AMP-PNP. The F303Y A304W variant shows over 100-fold increased intracellular calcium amplitudes with AMP-PNP compared to the WT receptor and has a much lower desensitization rate. Since AMP-PNP does not naturally activate P2X receptors, the F303Y A304W P2X2 may be a starting point for downstream applications in chemogenetic cellular control. Interestingly, the A304W mutation selectively destabilizes the desensitized state, which may provide a mechanistic basis for receptor opening with suboptimal agonists. The yeast system represents an inexpensive, scalable platform for ion channel characterization and engineering by circumventing the more expensive and time-consuming methodologies involving mammalian hosts.

Alternative Pharmacological Strategies for the Treatment of Alzheimer's Disease: Focus on Neuromodulator Function

Alzheimer's disease (AD) is a neurodegenerative disorder, comprising 70% of dementia diagnoses worldwide and affecting 1 in 9 people over the age of 65. However, the majority of its treatments, which predominantly target the cholinergic system, remain insufficient at reversing pathology and act simply to slow the inevitable progression of the disease. The most recent neurotransmitter-targeting drug for AD was approved in 2003, strongly suggesting that targeting neurotransmitter systems alone is unlikely to be sufficient, and that research into alternate treatment avenues is urgently required. Neuromodulators are substances released by neurons which influence neurotransmitter release and signal transmission across synapses. Neuromodulators including neuropeptides, hormones, neurotrophins, ATP and metal ions display altered function in AD, which underlies aberrant neuronal activity and pathology. However, research into how the manipulation of neuromodulators may be useful in the treatment of AD is relatively understudied. Combining neuromodulator targeting with more novel methods of drug delivery, such as the use of multi-targeted directed ligands, combinatorial drugs and encapsulated nanoparticle delivery systems, may help to overcome limitations of conventional treatments. These include difficulty crossing the blood-brain-barrier and the exertion of effects on a single target only. This review aims to highlight the ways in which neuromodulator functions are altered in AD and investigate how future therapies targeting such substances, which act upstream to classical neurotransmitter systems, may be of potential therapeutic benefit in the sustained search for more effective treatments.

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.

Efferent neurons control hearing sensitivity and protect hearing from noise through the regulation of gap junctions between cochlear supporting cells

It is critical for hearing that the descending cochlear efferent system provides a negative feedback to hair cells to regulate hearing sensitivity and protect hearing from noise. The medial olivocochlear (MOC) efferent nerves project to outer hair cells (OHCs) to regulate OHC electromotility, which is an active cochlear amplifier and can increase hearing sensitivity. Here, we report that the MOC efferent nerves also could innervate supporting cells (SCs) in the vicinity of OHCs to regulate hearing sensitivity. MOC nerve fibers are cholinergic, and acetylcholine (ACh) is a primary neurotransmitter. Immunofluorescent staining showed that MOC nerve endings, presynaptic vesicular acetylcholine transporters (VAChTs), and postsynaptic ACh receptors were visible at SCs and in the SC area. Application of ACh in SCs could evoke a typical inward current and reduce gap junctions (GJs) between them, which consequently enhanced the direct effect of ACh on OHCs to shift but not eliminate OHC electromotility. This indirect, GJ-mediated inhibition had a long-lasting influence. In vivo experiments further demonstrated that deficiency of this GJ-mediated efferent pathway decreased the regulation of active cochlear amplification and compromised the protection against noise. In particular, distortion product otoacoustic emission (DPOAE) showed a delayed reduction after noise exposure. Our findings reveal a new pathway for the MOC efferent system via innervating SCs to control active cochlear amplification and hearing sensitivity. These data also suggest that this SC GJ-mediated efferent pathway may play a critical role in long-term efferent inhibition and is required for protection of hearing from noise trauma.NEW & NOTEWORTHY The cochlear efferent system provides a negative feedback to control hair cell activity and hearing sensitivity and plays a critical role in noise protection. We reveal a new efferent control pathway in which medial olivocochlear efferent fibers have innervations with cochlear supporting cells to control their gap junctions, therefore regulating outer hair cell electromotility and hearing sensitivity. This supporting cell gap junction-mediated efferent control pathway is required for the protection of hearing from noise.

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.

Use of the guinea pig in studies on the development and prevention of acquired sensorineural hearing loss, with an emphasis on noise

Guinea pigs have been used in diverse studies to better understand acquired hearing loss induced by noise and ototoxic drugs. The guinea pig has its best hearing at slightly higher frequencies relative to humans, but its hearing is more similar to humans than the rat or mouse. Like other rodents, it is more vulnerable to noise injury than the human or nonhuman primate models. There is a wealth of information on auditory function and vulnerability of the inner ear to diverse insults in the guinea pig. With respect to the assessment of potential otoprotective agents, guinea pigs are also docile animals that are relatively easy to dose via systemic injections or gavage. Of interest, the cochlea and the round window are easily accessible, notably for direct cochlear therapy, as in the chinchilla, making the guinea pig a most relevant and suitable model for hearing. This article reviews the use of the guinea pig in basic auditory research, provides detailed discussion of its use in studies on noise injury and other injuries leading to acquired sensorineural hearing loss, and lists some therapeutics assessed in these laboratory animal models to prevent acquired sensorineural hearing loss.

Purinergic Signaling and Cochlear Injury-Targeting the Immune System?

Hearing impairment is the most common sensory deficit, affecting more than 400 million people worldwide. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy largely due to the insufficient knowledge of the pathomechanism. Purinergic signaling plays a substantial role in cochlear (patho)physiology. P2 (ionotropic P2X and the metabotropic P2Y) as well as adenosine receptors expressed on cochlear sensory and non-sensory cells are involved mostly in protective mechanisms of the cochlea. They are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics. Cochlear blood flow is also regulated by purines. Here, we propose to comprehend this field with the purine-immune interactions in the cochlea. The role of harmful immune mechanisms in sensorineural hearing losses has been emerging in the horizon of cochlear pathologies. In addition to decreasing hearing sensitivity and increasing cochlear blood supply, influencing the immune system can be the additional avenue for pharmacological targeting of purinergic signaling in the cochlea. Elucidating this complexity of purinergic effects on cochlear functions is necessary and it can result in development of new therapeutic approaches in hearing disabilities, especially in the noise-induced ones.

Spontaneous and Acetylcholine Evoked Calcium Transients in the Developing Mouse Utricle

Spontaneous calcium transients are present during early postnatal development in the mouse retina and cochlea, and play an important role in maturation of the sensory organs and neural circuits in the central nervous system (CNS). It is not known whether similar calcium transients occur during postnatal development in the vestibular sensory organs. Here we demonstrate spontaneous intracellular calcium transients in sensory hair cells (HCs) and supporting cells (SCs) in the murine utricular macula during the first two postnatal weeks. Calcium transients were monitored using a genetically encoded calcium indicator, GCaMP5G (G5), at 100 ms-frame⁻¹ in excised utricle sensory epithelia, including HCs, SCs, and neurons. The reporter line expressed G5 and tdTomato (tdT) in a Gad2-Cre dependent manner within a subset of utricular HCs, SCs and neurons. Kinetics of the G5 reporter limited temporal resolution to calcium events lasting longer than 200 ms. Spontaneous calcium transients lasting 1-2 s were observed in the expressing population of HCs at birth and slower spontaneous transients lasting 10-30 s appeared in SCs by P3. Beginning at P5, calcium transients could be modulated by application of the efferent neurotransmitter acetylcholine (ACh). In mature mice, calcium transients in the utricular macula occurred spontaneously, had a duration 1-2 s, and could be modulated by the exogenous application of acetylcholine (ACh) or muscarine. Long-lasting calcium transients evoked by ACh in mature mice were blocked by atropine, consistent with previous reports describing the role of muscarinic receptors expressed in calyx bearing afferents in efferent control of vestibular sensation. Large spontaneous and ACh evoked transients were reversibly blocked by the inositol trisphosphate receptor (IP₃R) antagonist aminoethoxydiphenyl borate (2-APB). Results demonstrate long-lasting calcium transients are present in the utricular macula during the first postnatal week, and that responses to ACh mature over this same time period.

Efficient introduction of an isogenic homozygous mutation to induced pluripotent stem cells from a hereditary hearing loss family using CRISPR/Cas9 and single-stranded donor oligonucleotides

BACKGROUND

Heterozygous purinergic receptor p2x gene ( P2RX2) c.178G>T (p.V60L) mutations can lead to progressive hearing loss (HL) and increased susceptibility to noise. However, the underlying mechanisms remain unclear. A combination of human induced pluripotent stem cell (hiPSC) technology with clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein (Cas)9-mediated gene editing may provide a promising tool to study gene function and treat hereditary deafness in humans.

METHODS

hiPSC technology and CRISPR/Cas9-mediated gene editing were used to generate heterozygous and homozygous P2RX2 c.178G>T (p.V60L) cell models.

RESULTS

We generated non-integrative hiPSCs from urine samples derived from three members of a large Chinese family carrying heterozygous P2RX2 c.178G>T mutations (designated P2RX2+/-) as a model to study P2RX2-mediated hereditary HL. Furthermore, we used CRISPR/Cas9 and single-stranded donor oligonucleotides to genetically establish homozygous P2RX2 c.178G>T hiPSCs (designated P2RX2-/-) from heterozygous patient-specific hiPSCs as a control to further study the pathological gene function.

CONCLUSIONS

Heterozygous and homozygous P2RX2-mutated hiPSC lines are good models to investigate the pathological mechanisms of P2RX2 mutations in HL pathogenesis. Our findings confirmed our hypothesis that it is feasible and convenient to introduce precise point mutations into genomic loci of interest to generate gene-mutated hiPSC models.

Purinergic signaling in the organ of Corti: Potential therapeutic targets of sensorineural hearing losses

Purinergic signaling is deeply involved in the development, functions and protective mechanisms of the cochlea. Release of ATP and activation of purinergic receptors on sensory and supporting/epithelial cells play a substantial role in cochlear (patho)physiology. Both the ionotropic P2X and the metabotropic P2Y receptors are widely distributed on the inner and outer hair cells as well as on the different supporting cells in the organ of Corti and on other epithelial cells in the scala media. Among others, they are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics acting on outer hair cells and supporting cells. Cochlear blood flow is also regulated by purines. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy. Decreasing hearing sensitivity and increasing cochlear blood supply by pharmacological targeting of purinergic signaling in the cochlea are potential new therapeutic approaches in these hearing disabilities, especially in the noise-induced ones.

Lysosomal exocytosis of ATP is coupled to P2Y2 receptor in marginal cells in the stria vascular in neonatal rats

Adenosine triphosphate (ATP) is stored as lysosomal vesicles in marginal cells of the stria vascular in neonatal rats, but the mechanisms of ATP release are unclear. Primary cultures of marginal cells from 1-day-old Sprague-Dawley rats were established. P2Y₂ receptor and inositol 1,4,5-trisphosphate (IP3) receptor were immunolabelled in marginal cells of the stria vascular. We found that 30 μM ATP and 30 μM uridine triphosphate (UTP) evoked comparable significant increases in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the absence of extracellular Ca²⁺, whereas the response was suppressed by 100 μM suramin, 10 μM 1-(6-(17β-3-methoxyester-1,3,5(10)-trien-17-yl)amino)-hexyl)-1H-pyrrole-2,5-dione(U-73122), 100 μM 2-aminoethoxydiphenyl borate (2-APB) and 5 μM thapsigargin (TG), thus indicating that ATP coupled with the P2Y₂R-PLC-IP3 pathway to evoke Ca²⁺ release from the endoplasmic reticulum (ER). Incubation with 200 μM Gly-Phe-β-naphthylamide (GPN) selectively disrupted lysosomes and caused significant increases in [Ca²⁺]_I; this effect was partly inhibited by P2Y₂R-PLC-IP3 pathway antagonists. After pre-treatment with 5 μM TG, [Ca²⁺]_i was significantly lower than that after treatment with P2Y₂R-PLC-IP3 pathway antagonists under the same conditions, thus indicating that lysosomal Ca²⁺ triggers Ca²⁺ release from ER Ca²⁺ stores. Baseline [Ca²⁺]_i declined after treatment with the Ca²⁺ chelator 50 μM bis-(aminophenolxy) ethane-N,N,N',N'-tetra-acetic acid acetoxyme-thyl ester (BAPTA-AM) and 4 IU/ml apyrase. 30 μM ATP decrease of the number of quinacrine-positive vesicles via lysosome exocytosis, whereas the number of lysosomes did not change. However, lysosome exocytosis was significantly suppressed by pre-treatment with 5 μM vacuolin-1. Release of ATP and β-hexosaminidase both increased after treatment with 200 μM GPN and 5 μM TG, but decreased after incubation with 50 μM BAPTA-AM, 4 IU/ml apyrase and 5 μM vacuolin-1. We suggest that ATP triggers Ca²⁺ release from the ER, thereby contributing to secretion of lysosomal ATP via lysosomal exocytosis. Lysosomal stored Ca²⁺ triggers Ca²⁺ release from the ER directly though the IP3 receptors, and lysosomal ATP evokes Ca²⁺ signals indirectly via the P2Y₂R-PLC-IP3 pathway.

Knockout of Pannexin-1 Induces Hearing Loss

Mutations of gap junction connexin genes induce a high incidence of nonsyndromic hearing loss. Pannexin genes also encode gap junctional proteins in vertebrates. Recent studies demonstrated that Pannexin-1 (Panx1) deficiency in mice and mutation in humans are also associated with hearing loss. So far, several Panx1 knockout (KO) mouse lines were established. In general, these Panx1 KO mouse lines demonstrate consistent phenotypes in most aspects, including hearing loss. However, a recent study reported that a Panx1 KO mouse line, which was created by Genentech Inc., had no hearing loss as measured by the auditory brainstem response (ABR) threshold at low-frequency range (<24 kHz). Here, we used multiple auditory function tests and re-examined hearing function in the Genentech Panx1 (Gen-Panx1) KO mouse. We found that ABR thresholds in the Gen-Panx1 KO mouse were significantly increased, in particular, in the high-frequency region. Moreover, consistent with the increase in ABR threshold, distortion product otoacoustic emission (DPOAE) and cochlear microphonics (CM), which reflect active cochlear amplification and auditory receptor current, respectively, were significantly reduced. These data demonstrated that the Gen-Panx1 KO mouse has hearing loss and further confirmed that Panx1 deficiency can cause deafness.

P2X2 Dominant Deafness Mutations Have No Negative Effect on Wild-Type Isoform: Implications for Functional Rescue and in Deafness Mechanism

The P2X2 receptor is an ATP-gated ion channel, assembled by three subunits. Recently, it has been found that heterozygous mutations of P2X2 V60L and G353R can cause autosomal dominant nonsyndromic hearing loss. However, the underlying mechanism remains unclear. The fact that heterozygous mutations cause deafness suggests that the mutations may have dominant-negative effect (DNE) on wild-type (WT) P2X2 isoforms and/or other partners leading to hearing loss. In this study, the effect of these dominant deafness P2X2 mutations on WT P2X2 was investigated. We found that sole transfection of both V60L and G353R deafness mutants could efficiently target to the plasma membrane, like WT P2X2, but exhibit a significantly reduced response to ATP stimulation. Both mutants reduced the channel conductance, but G353R mutation also altered the voltage dependency. Co-expression with WT P2X2 could restore the response to ATP. As the ratio of WT P2X2 vs. mutants increased, the response to ATP was also increased. Computer modeling confirmed that both V60L and G353R dominant-deafness mutant subunits do not have any negative effect on WT P2X2 subunit, when assembled as a heterotrimer. Improper docking or defective gating is the more likely mechanism for impaired channel function by these P2X2 deafness mutations. These results suggest that P2X2 dominant deafness mutations do not have negative effects on WT P2X2 isoforms, and that adding additional WT P2X2 could rescue the lost channel function caused by the deafness mutations. These P2X2 dominant deafness mutations may have negative-effects on other partners leading to hearing loss.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

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.

Adenosine Triphosphate (ATP) Inhibits Voltage-Sensitive Potassium Currents in Isolated Hensen's Cells and Nifedipine Protects Against Noise-Induced Hearing Loss in Guinea Pigs

Background

There is increasing evidence that adenosine triphosphate (ATP), a well-known neurotransmitter and neuromodulator in the central nervous system, plays an important role as an extracellular chemical messenger in the cochlea.

Material/Methods

Using a whole-cell recording technique, we studied the effects of ATP on isolated Hensen’s cells, which are supporting cells in the cochlea, to determine if they are involved in the transduction of ions with hair cells.

Results

ATP (0.1–10 μM) reduced the potassium current (I_K⁺) in the majority of the recorded Hensen’s cells (21 out of 25 cells). An inward current was also induced by high concentrations of ATP (100 μM to 10 mM), which was reversibly blocked by 100 μM suramin (a purinergic antagonist) and blocked by nifedipine (an L-type calcium channel blocker). After the cochleas were perfused with artificial perilymph solutions containing nifedipine and exposed to noise, the amplitude increase in the compound action potential (CAP) threshold and the reduction in cochlear microphonics was lower than when they were exposed to noise alone.

Conclusions

Our results suggest that ATP can block I_K⁺ channels at a low concentration and induce an inward Ca²⁺ current at high concentrations, which is reversed by purinergic receptors. Nifedipine may have a partially protective effect on noise-induced hearing loss (NIHL).

Expression and function of pannexins in the inner ear and hearing

Pannexin (Panx) is a gene family encoding gap junction proteins in vertebrates. So far, three isoforms (Panx1, 2 and 3) have been identified. All of three Panx isoforms express in the cochlea with distinct expression patterns. Panx1 expresses in the cochlea extensively, including the spiral limbus, the organ of Corti, and the cochlear lateral wall, whereas Panx2 and Panx3 restrict to the basal cells of the stria vascularis in the lateral wall and the cochlear bony structure, respectively. However, there is no pannexin expression in auditory sensory hair cells. Recent studies demonstrated that like connexin gap junction gene, Panx1 deficiency causes hearing loss. Panx1 channels dominate ATP release in the cochlea. Deletion of Panx1 abolishes ATP release in the cochlea and reduces endocochlear potential (EP), auditory receptor current/potential, and active cochlear amplification. Panx1 deficiency in the cochlea also activates caspase-3 cell apoptotic pathway leading to cell degeneration. These new findings suggest that pannexins have a critical role in the cochlea in regard to hearing. However, detailed information about pannexin function in the cochlea and Panx mutation induced hearing loss still remain largely undetermined. Further studies are required.

Molecular Structure and Regulation of P2X Receptors With a Special Emphasis on the Role of P2X2 in the Auditory System

The P2X purinergic receptors are cation-selective channels gated by extracellular adenosine 5'-triphosphate (ATP). These purinergic receptors are found in virtually all mammalian cell types and facilitate a number of important physiological processes. Within the past few years, the characterization of crystal structures of the zebrafish P2X4 receptor in its closed and open states has provided critical insights into the mechanisms of ligand binding and channel activation. Understanding of this gating mechanism has facilitated to design and interpret new modeling and structure-function experiments to better elucidate how different agonists and antagonists can affect the receptor with differing levels of potency. This review summarizes the current knowledge on the structure, activation, allosteric modulators, function, and location of the different P2X receptors. Moreover, an emphasis on the P2X2 receptors has been placed in respect to its role in the auditory system. In particular, the discovery of three missense mutations in P2X2 receptors could become important areas of study in the field of gene therapy to treat progressive and noise-induced hearing loss. J. Cell. Physiol. 231: 1656-1670, 2016. © 2015 Wiley Periodicals, Inc.

Connexin26 gap junction mediates miRNA intercellular genetic communication in the cochlea and is required for inner ear development

Organ development requires well-established intercellular communication to coordinate cell proliferations and differentiations. MicroRNAs (miRNAs) are small, non-coding RNAs that can broadly regulate gene expression and play a critical role in the organ development. In this study, we found that miRNAs could pass through gap junctions between native cochlear supporting cells to play a role in the cochlear development. Connexin26 (Cx26) and Cx30 are predominant isoforms and co-express in the cochlea. Cx26 deficiency but not Cx30 deficiency can cause cochlear developmental disorders. We found that associated with Cx26 deletion induced the cochlear developmental disorders, deletion of Cx26 but not Cx30 disrupted miRNA intercellular transfer in the cochlea, although inner ear gap junctions still retained permeability after deletion of Cx26. Moreover, we found that deletion of Cx26 but not Cx30 reduced miR-96 expression in the cochlea during postnatal development. The reduction is associated with the cochlear tunnel developmental disorder in Cx26 knockout (KO) mice. These data reveal that Cx26-mediated intercellular communication is required for cochlear development and that deficiency of Cx26 can impair miRNA-mediated intercellular genetic communication in the cochlea, which may lead to cochlear developmental disorders and eventually congenital deafness as previously reported.

Expression of Pannexin3 in human odontoblast-like cells and its hemichannel function in mediating ATP release

OBJECTIVE

The aim of this study is to investigate the expression of pannexin3 (Panx3) in human odontoblast-like cells (hOBs) and its hemichannel function in mediating ATP release.

METHODS

RT-PCR and immunofluorescence analysis were used to detect the expression of pannexins (Panxs) in human dental pulp tissue and cultured cells. To determine the role of Panx3 in ATP release, hOBs were infected with Panx3-overexpression lentivirus, Panx3-shRNA lentivirus or control lentivirus and then stimulated with cold buffer. Intracellular ATP was monitored using quinacrine, and then semi-quantitatively analyzed. In the meantime, the ATP release was quantitatively analyzed using the bioluminescence method when the cells were exposed to cold stimulus.

RESULTS

Panx3 mRNA and protein were found in dental pulp tissue and cultured cells. Upon cold stimulus, intracellular ATP was released into the extracellular space. Overexpression of Panx3 accelerated ATP release, whereas inhibition of Panx3 suppressed this process.

CONCLUSION

Panx3 hemichannel is expressed in human odontoblast-like cells and mediates ATP release into the extracellular space.

Cellular signaling protective against noise-induced hearing loss – A role for novel intrinsic cochlear signaling involving corticotropin-releasing factor?

Hearing loss afflicts approximately 15% of the world's population, and crosses all socioeconomic boundaries. While great strides have been made in understanding the genetic components of syndromic and non-syndromic hearing loss, understanding of the mechanisms underlying noise-induced hearing loss (NIHL) have come much more slowly. NIHL is not simply a mechanism by which older individuals loose their hearing. Significantly, the incidence of NIHL is increasing, and is now involving ever younger populations. This may predict future increased occurrences of hearing loss. Current research has shown that even short-term exposures to loud sounds generating what was previously considered temporary hearing loss, actually produces an almost immediate and permanent loss of specific populations of auditory nerve fibers. Additionally, recurrent exposures to intense sound may hasten age-related hearing loss. While NIHL is a significant medical concern, to date, few compounds have delivered significant protection, arguing that new targets need to be identified. In this commentary, we will explore cellular signaling processes taking place in the cochlea believed to be involved in protection against hearing loss, and highlight new data suggestive of novel signaling not previously recognized as occurring in the cochlea, that is perhaps protective of hearing. This includes a recently described local hypothalamic-pituitary-adrenal axis (HPA)-like signaling system fully contained in the cochlea. This system may represent a local cellular stress-response system based on stress hormone release similar to the systemic HPA axis. Its discovery may hold hope for new drug therapies that can be delivered directly to the cochlea, circumventing systemic side effects.

Pannexin1 channels dominate ATP release in the cochlea ensuring endocochlear potential and auditory receptor potential generation and hearing

Pannexin1 (Panx1) is a gap junction gene in vertebrates whose proteins mainly function as non-junctional channels on the cell surface. Panx1 channels can release ATP under physiological conditions and play critical roles in many physiological and pathological processes. Here, we report that Panx1 deficiency can reduce ATP release and endocochlear potential (EP) generation in the cochlea inducing hearing loss. Panx1 extensively expresses in the cochlea, including the cochlear lateral wall. We found that deletion of Panx1 in the cochlear lateral wall almost abolished ATP release under physiological conditions. Positive EP is a driving force for current through hair cells to produce auditory receptor potential. EP generation requires ATP. In the Panx1 deficient mice, EP and auditory receptor potential as measured by cochlear microphonics (CM) were significantly reduced. However, no apparent hair cell loss was detected. Moreover, defect of connexin hemichannels by deletion of connexin26 (Cx26) and Cx30, which are predominant connexin isoforms in the cochlea, did not reduce ATP release under physiological conditions. These data demonstrate that Panx1 channels dominate ATP release in the cochlea ensuring EP and auditory receptor potential generation and hearing. Panx1 deficiency can reduce ATP release and EP generation causing hearing loss.

Cellular and Deafness Mechanisms Underlying Connexin Mutation-Induced Hearing Loss - A Common Hereditary Deafness

Hearing loss due to mutations in the connexin gene family, which encodes gap junctional proteins, is a common form of hereditary deafness. In particular, connexin 26 (Cx26, GJB2) mutations are responsible for ~50% of non-syndromic hearing loss, which is the highest incidence of genetic disease. In the clinic, Cx26 mutations cause various auditory phenotypes ranging from profound congenital deafness at birth to mild, progressive hearing loss in late childhood. Recent experiments demonstrate that congenital deafness mainly results from cochlear developmental disorders rather than hair cell degeneration and endocochlear potential reduction, while late-onset hearing loss results from reduction of active cochlear amplification, even though cochlear hair cells have no connexin expression. However, there is no apparent, demonstrable relationship between specific changes in connexin (channel) functions and the phenotypes of mutation-induced hearing loss. Moreover, new experiments further demonstrate that the hypothesized K⁺-recycling disruption is not a principal deafness mechanism for connexin deficiency induced hearing loss. Cx30 (GJB6), Cx29 (GJC3), Cx31 (GJB3), and Cx43 (GJA1) mutations can also cause hearing loss with distinct pathological changes in the cochlea. These new studies provide invaluable information about deafness mechanisms underlying connexin mutation-induced hearing loss and also provide important information for developing new protective and therapeutic strategies for this common deafness. However, the detailed cellular mechanisms underlying these pathological changes remain unclear. Also, little is known about specific mutation-induced pathological changes in vivo and little information is available for humans. Such further studies are urgently required.

Investigation of olfactory function in a Panx1 knock out mouse model

Pannexin 1 (Panx1), the most extensively investigated member of a channel-forming protein family, is able to form pores conducting molecules up to 1.5 kDa, like ATP, upon activation. In the olfactory epithelium (OE), ATP modulates olfactory responsiveness and plays a role in proliferation and differentiation of olfactory sensory neurons (OSNs). This process continuously takes place in the OE, as neurons are replaced throughout the whole lifespan. The recent discovery of Panx1 expression in the OE raises the question whether Panx1 mediates ATP release responsible for modulating chemosensory function. In this study, we analyzed pannexin expression in the OE and a possible role of Panx1 in olfactory function using a Panx1^−/− mouse line with a global ablation of Panx1. This mouse model has been previously used to investigate Panx1 functions in the retina and adult hippocampus. Here, qPCR, in-situ hybridization, and immunohistochemistry (IHC) demonstrated that Panx1 is expressed in axon bundles deriving from sensory neurons of the OE. The localization, distribution, and expression of major olfactory signal transduction proteins were not significantly altered in Panx1^−/− mice. Further, functional analysis of Panx1^−/− animals does not reveal any major impairment in odor perception, indicated by electroolfactogram (EOG) measurements and behavioral testing. However, ATP release evoked by potassium gluconate application was reduced in Panx1^−/− mice. This result is consistent with previous reports on ATP release in isolated erythrocytes and spinal or lumbar cord preparations from Panx1^−/− mice, suggesting that Panx1 is one of several alternative pathways to release ATP in the olfactory system.

Kölliker's organ and the development of spontaneous activity in the auditory system: implications for hearing dysfunction

Prior to the “onset of hearing,” developing cochlear inner hair cells (IHCs) and primary auditory neurons undergo experience-independent activity, which is thought to be important in retaining and refining neural connections in the absence of sound. One of the major hypotheses regarding the origin of such activity involves a group of columnar epithelial supporting cells forming Kölliker's organ, which is only present during this critical period of auditory development. There is strong evidence for a purinergic signalling mechanism underlying such activity. ATP released through connexin hemichannels may activate P2 purinergic receptors in both Kölliker's organ and the adjacent IHCs, leading to generation of electrical activity throughout the auditory system. However, recent work has suggested an alternative origin, by demonstrating the ability of IHCs to generate this spontaneous activity without activation by ATP. Regardless, developmental abnormalities of Kölliker's organ may lead to congenital hearing loss, considering that mutations in ion channels (hemichannels, gap junctions, and calcium channels) involved in Kölliker's organ activity share strong links with such types of deafness.

Non-synonymous single nucleotide polymorphisms in the P2X receptor genes: association with diseases, impact on receptor functions and potential use as diagnosis biomarkers

P2X receptors are Ca²⁺-permeable cationic channels in the cell membranes, where they play an important role in mediating a diversity of physiological and pathophysiological functions of extracellular ATP. Mammalian cells express seven P2X receptor genes. Single nucleotide polymorphisms (SNPs) are widespread in the P2RX genes encoding the human P2X receptors, particularly the human P2X7 receptor. This article will provide an overview of the non-synonymous SNPs (NS-SNPs) that have been associated with or implicated in altering the susceptibility to pathologies or disease conditions, and discuss the consequences of the mutations resulting from such NS-SNPs on the receptor functions. Disease-associated NS-SNPs in the P2RX genes have been valuable in understanding the disease etiology and the receptor function, and are promising as biomarkers to be used for the diagnosis and development of stratified therapeutics.

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.

Mutation of the ATP-gated P2X(2) receptor leads to progressive hearing loss and increased susceptibility to noise

Age-related hearing loss and noise-induced hearing loss are major causes of human morbidity. Here we used genetics and functional studies to show that a shared cause of these disorders may be loss of function of the ATP-gated P2X(2) receptor (ligand-gated ion channel, purinergic receptor 2) that is expressed in sensory and supporting cells of the cochlea. Genomic analysis of dominantly inherited, progressive sensorineural hearing loss DFNA41 in a six-generation kindred revealed a rare heterozygous allele, P2RX2 c.178G > T (p.V60L), at chr12:133,196,029, which cosegregated with fully penetrant hearing loss in the index family, and also appeared in a second family with the same phenotype. The mutation was absent from more than 7,000 controls. P2RX2 p.V60L abolishes two hallmark features of P2X(2) receptors: ATP-evoked inward current response and ATP-stimulated macropore permeability, measured as loss of ATP-activated FM1-43 fluorescence labeling. Coexpression of mutant and WT P2X(2) receptor subunits significantly reduced ATP-activated membrane permeability. P2RX2-null mice developed severe progressive hearing loss, and their early exposure to continuous moderate noise led to high-frequency hearing loss as young adults. Similarly, among family members heterozygous for P2RX2 p.V60L, noise exposure exacerbated high-frequency hearing loss in young adulthood. Our results suggest that P2X(2) function is required for life-long normal hearing and for protection from exposure to noise.