Detection of mRNA encoding guanylate cyclase A/atrial natriuretic peptide receptor in the rat cochlea by competitive polymerase chain reaction and in situ hybridization

Atrial Natriuretic Peptide Promotes Neurite Outgrowth and Survival of Cochlear Spiral Ganglion Neurons in vitro Through NPR-A/cGMP/PKG Signaling

Sensorineural hearing loss (SNHL) is a dominant public health issue affecting millions of people around the globe, which is correlated with the irreversible deterioration of the hair cells and spiral ganglion neurons (SGNs) within the cochlea. Strategies using bioactive molecules that regulate neurite regeneration and neuronal survival to reestablish connections between auditory epithelium or implanted electrodes and SGN neurites would become attractive therapeutic candidates for SNHL. As an intracellular second messenger, cyclic guanosine-3’,5’-monophosphate (cGMP) can be synthesized through activation of particulate guanylate cyclase-coupled natriuretic peptide receptors (NPRs) by natriuretic peptides, which in turn modulates multiple aspects of neuronal functions including neuronal development and neuronal survival. As a cardiac-derived hormone, atrial natriuretic peptide (ANP), and its specific receptors (NPR-A and NPR-C) are broadly expressed in the nervous system where they might be involved in the maintenance of diverse neural functions. Despite former literatures and our reports indicating the existence of ANP and its receptors within the inner ear, particularly in the spiral ganglion, their potential regulatory mechanisms underlying functional properties of auditory neurons are still incompletely understood. Our recently published investigation revealed that ANP could promote the neurite outgrowth of SGNs by activating NPR-A/cGMP/PKG cascade in a dose-dependent manner. In the present research, the influence of ANP and its receptor-mediated downstream signaling pathways on neurite outgrowth, neurite attraction, and neuronal survival of SGNs in vitro was evaluated by employing cultures of organotypic explant and dissociated neuron from postnatal rats. Our data indicated that ANP could support and attract neurite outgrowth of SGNs and possess a high capacity to improve neuronal survival of SGNs against glutamate-induced excitotoxicity by triggering the NPR-A/cGMP/PKG pathway. The neuroregenerative and neuroprotective effects of ANP/NPRA/cGMP/PKG-dependent signaling on SGNs would represent an attractive therapeutic candidate for hearing impairment.

Atrial Natriuretic Peptide Improves Neurite Outgrowth from Spiral Ganglion Neurons In Vitro through a cGMP-Dependent Manner

The spiral ganglion neurons (SGNs) are the primary afferent neurons in the spiral ganglion (SG), while their degeneration or loss would cause sensorineural hearing loss. As a cardiac-derived hormone, atrial natriuretic peptide (ANP) plays a critical role in cardiovascular homeostasis through binding to its functional receptors (NPR-A and NPR-C). ANP and its receptors are widely expressed in the mammalian nervous system where they could be implicated in the regulation of multiple neural functions. Although previous studies have provided direct evidence for the presence of ANP and its functional receptors in the inner ear, their presence within the cochlear SG and their regulatory roles during auditory neurotransmission and development remain largely unknown. Based on our previous findings, we investigated the expression patterns of ANP and its receptors in the cochlear SG and dissociated SGNs and determined the influence of ANP on neurite outgrowth in vitro by using organotypic SG explants and dissociated SGN cultures from postnatal rats. We have demonstrated that ANP and its receptors are expressed in neurons within the cochlear SG of postnatal rat, while ANP may promote neurite outgrowth of SGNs via the NPR-A/cGMP/PKG pathway in a dose-dependent manner. These results indicate that ANP would play a role in normal neuritogenesis of SGN during cochlear development and represents a potential therapeutic candidate to enhance regeneration and regrowth of SGN neurites.

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

Expression patterns of atrial natriuretic peptide and its receptors within the cochlear spiral ganglion of the postnatal rat

The spiral ganglion, which is primarily composed of spiral ganglion neurons and satellite glial cells, transmits auditory information from sensory hair cells to the central nervous system. Atrial natriuretic peptide (ANP), acting through specific receptors, is a regulatory peptide required for a variety of cardiac, neuronal and glial functions. Although previous studies have provided direct evidence for the presence of ANP and its functional receptors (NPR-A and NPR-C) in the inner ear, their presence within the cochlear spiral ganglion and their regulatory roles during auditory neurotransmission and development is not known. Here we investigated the expression patterns and levels of ANP and its receptors within the cochlear spiral ganglion of the postnatal rat using immunofluorescence and immunoelectron microscopy techniques, reverse transcription-polymerase chain reaction and Western blot analysis. We have demonstrated that ANP and its receptors colocalize in both subtypes of spiral ganglion neurons and in perineuronal satellite glial cells. Furthermore, we have analyzed differential expression levels associated with both mRNA and protein of ANP and its receptors within the rat spiral ganglion during postnatal development. Collectively, our research provides direct evidence for the presence and synthesis of ANP and its receptors in both neuronal and non-neuronal cells within the cochlear spiral ganglion, suggesting possible roles for ANP in modulating neuronal and glial functions, as well as neuron-satellite glial cell communication, within the spiral ganglion during auditory neurotransmission and development.

Expression and localization of atrial natriuretic peptide and its receptors in rat spiral ganglion neurons

Spiral ganglion neurons (SGNs) are the primary auditory neurons in the inner ear, conveying auditory information between sensory hair cells and the central nervous system. Atrial natriuretic peptide (ANP), acting through specific receptors, is a regulatory peptide required for a variety of cardiac and neuronal functions. While the localization of ANP and its receptors (NPR-A and NPR-C) in the inner ear has been widely studied, there is only limited information regarding their localization in cochlear SGNs and their regulatory roles during primary auditory neurotransmission. Here we have investigated the presence of ANP and its receptors in the cochlear spiral ganglion of the postnatal rat using immunohistochemistry, reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis. ANP and its receptors are expressed in the cochlear SGNs at both the mRNA and protein level, and co-localize in the cochlear SGNs as shown by immunofluorescence. Our research provides a direct evidence for the presence and synthesis of ANP as well as its receptors in the cochlear SGNs, suggesting a possible role for ANP in modulating the neuronal functions of SGNs via its receptors.

C-type natriuretic peptide-like immunoreactivity in the rat inner ear

C-type natriuretic peptide (CNP) is a member of the atrial natriuretic peptide family (ANP family). The family also includes ANP and brain natriuretic peptide (BNP). These peptides regulate the homeostasis of body fluid and blood pressure as a neuropeptide in the central nervous system as well as a cardiac hormone in the periphery. We have recently reported the expression of CNP mRNA in the inner ear. To assess the possible physiological role of CNP in the inner ear, we investigated the localization of CNP peptide in the rat inner ear by immunohistochemistry at the light and electron microscopic level. CNP-like immunoreactivity was widely distributed in the secretory and the neuronal portion of the inner ear, i.e. the spiral ligament, the dark cell region of the utriculus, the epithelium of the endolymphatic sac, the spiral ganglion cells and the vestibular ganglion cells. The results suggest that CNP may play a role in the homeostasis of the perilymph and endolymph and may also influence nerve activities in the inner ear.

Expression pattern of aquaporin water channels in the inner ear of the rat. The molecular basis for a water regulation system in the endolymphatic sac

Mammalian aquaporins constitute a family of so far 10 related water channel proteins which mediate osmotically driven water fluxes across the plasma membrane. Because regulation of the ionic composition and osmolality of inner ear fluids is of great functional significance, we investigated the expression patterns of aquaporins in five defined areas of the rat inner ear by RT-PCR. The tissues used were stria vascularis, endolymphatic sac, Reissner's membrane, vestibulum and organ of Corti. Aquaporin 1 transcripts were detected in all tissues and are probably constitutive. Aquaporin 5 was only expressed in the organ of Corti and in Reissner's membrane. We show that aquaporin 2, so far considered to be specific to the principal cells of the renal collecting duct, is expressed in the endolymphatic sac. Aquaporin 2 expression was not detected in any other inner ear region. The postnatal appearance of aquaporin 2 transcripts in the endolymphatic sac resembled that in the kidney, i.e. it increased postnatally until day 4. The full-length DNA for aquaporin 2 was cloned from cDNA of the endolymphatic sac. It had an irrelevant Ile54Thr mutation because it could be functionally expressed in Xenopus oocytes. Also exclusively in the endolymphatic sac of the inner ear, we detected transcripts for aquaporin isoforms 3 and 4 which are known to be expressed in the renal principal cells. In the kidney, aquaporin 2 regulation involves vasopressin-stimulated, cAMP-dependent phosphorylation of Ser256 of aquaporin 2 which is stored in cytosolic vesicles. These storage vesicles also contain a serpentine calcium/polycation-sensing receptor. Vesicle shuffling to the plasma membrane involves proteins such as vesicle-associated membrane protein VAMP2, syntaxin-4 and the small GTPase Rab3a. Using RT-PCR we were able to demonstrate the expression of all of these components. By analogy the data suggest that in the endolymphatic sac of the inner ear a system for cellular water permeability is in place which may share many similarities with that characterized in the principal cells of the renal collecting duct. These findings may have a number of interesting pharmacological implications which need to be addressed in future studies.

Expression pattern of adenylyl cyclase isoforms in the inner ear of the rat by RT-PCR and immunochemical localization of calcineurin in the organ of Corti

Most studies concerning adenylyl cyclases in the inner ear were carried out before the advent of molecular biology. In a PCR approach using cDNAs of six inner ear tissues (stria vascularis, endolymphatic sac, organ of Corti, vestibulum, cochlear and vestibular nerve) we found tissue specific expression of adenylyl cyclase isoforms. Adenylyl cyclases types 2 and 4 are predominant in the fluid controlling tissues, i.e. in the stria vascularis and endolymphatic sac. In the organ of Corti and vestibulum the Ca2+-modulated isoforms types 1, 6 and 9 were expressed. The regulation of adenylyl cyclase 9, which is the major isoform expressed in the organ of Corti, proceeds via the Ca2+-activated protein phosphatase 2B (calcineurin, PPP3). PCR with specific primers for calcineurin demonstrated its abundant expression in the organ of Corti. Using a monoclonal antibody we localized calcineurin immunochemically to the cochlear nerve, the nerve fibers and the inner hair cells. In the cochlear and vestibular nerves a characteristic neuronal expression pattern of adenylyl cyclase isoforms was observed, i.e. adenylyl cyclases types 2, 3 and 8. The functional consequences of the adenylyl cyclase expression pattern in the inner ear are discussed in conjunction with its unique sensory performance.

Expression of mRNAs encoding hormone receptors in the endolymphatic sac of the rat

The endolymphatic sac (ES) is believed to absorb the endolymphatic fluid produced by the stria vascularis and vestibular dark cells. Recent studies have implied that the function of the ES may be controlled by circulating hormones, suggesting that hormone receptors should exist there. In the present study, the expression of genes encoding receptors for aldosterone, atrial natriuretic peptide (ANP) and vasopressin in the ES was examined by reverse transcription-polymerase chain reaction (RT-PCR). Next, the cellular localization of the expression of these genes was investigated by in situ hybridization. RT-PCR indicated that aldosterone. ANP-A and vasopressin V1a receptor genes were expressed in the ES. In contrast, neither ANP-B nor vasopressin V2 receptor gene expression was detected. In situ hybridization experiments demonstrated aldosterone receptor gene expression in epithelial cells of the intermediate potion of the ES, while expression of ANP-A or V1a receptor genes was not detected. The present results suggested that aldosterone may play a specific role in the function of the ES. However, we could not conclude that ANP and vasopressin play physiological roles in the ES because receptors for these hormones were detected only by highly sensitive PCR.

Expression of membrane-bound and cytosolic guanylyl cyclases in the rat inner ear

Membrane-bound guanylyl cyclases (GCs) are peptide hormone receptors whereas the cytosolic isoforms are receptors for nitric oxide. In the inner ear, the membrane-bound GCs may be involved in the regulation of fluid homeostasis and the cytosolic forms possibly play a role in signal processing and regulation of local blood flow. In this comprehensive study, we examined, qualitatively and quantitatively, the transcription pattern of all known GC isoforms in the inner ear from rat by RT-PCR. The tissues used were endolymphatic sac, stria vascularis, organ of Corti, organ of Corti outer hair cells, cochlear nerve, Reissner's membrane, vestibular dark cells, and vestibular sensory cells. We show that multiple particulate (GC-A, GC-B, GC-D, GC-E, GC-F and GC-G) and several subunits of the heterodimeric cytosolic GCs (alpha1, alpha2, beta1 and beta2) are expressed, albeit at highly different levels. GC-C was not found. GC-A and the soluble subunits alpha1 and beta1 were transcribed ubiquitously. GC-B was present in all tissues except stria vascularis, which contained GC-A and traces of GC-E and GC-G. GC-B was by far the predominant membrane-bound isoform in the organ of Corti (86%), Reissner's membrane (75%) and the vestibulum (80%). Surprisingly, GC-E, a retinal isoform, was detected in significant amounts in the cochlear nerve (8%) and in the organ of Corti (4%). Although the cytosolic GC is a heterodimer composed of an alpha and a beta subunit, the mRNA transcription of these subunits was not stoichiometric. Particularly in the vestibulum, the transcription of the beta1 subunits was at least four-fold higher than of the alpha1 subunit. The data are compatible with earlier suggestions that membrane receptor GCs may be involved in the control of inner ear electrolyte and fluid composition whereas NO-stimulated GC isoforms mainly participate in the regulation of blood flow and supporting cell physiology.

RT-PCR analysis of mRNA expression of natriuretic peptide family and their receptors in rat inner ear

To assess the possible physiological role of the atrial natriuretic peptide (ANP) family, we investigated the expression of mRNA of ANP, brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), and their receptors in rat inner ear using the reverse transcription-polymerase chain reaction method. ANP and CNP message bands were detected in the inner ear, but the BNP message band was not. Amplification products of the expected sizes of ANP-A, ANP-B and ANP-C receptors were detected in the inner ear. These results suggest that natriuretic peptide family may influence the function of the inner ear through the ANP-A, ANP-B, and ANP-C receptors.

Expression of mRNA encoding vasopressin V1a, vasopressin V2, and ANP-B receptors in the rat cochlea

The expression of mRNAs encoding vasopressin V1a, V2, and ANP-B receptors in the rat cochlea was examined by PCR and in situ hybridization. After reverse-transcription of rat cochlear RNA, cDNA was amplified by PCR using pairs of primers specific to these receptors. After subcloning of the PCR products, clones with sequences identical to those cloned previously from the rat liver (V1a receptor), kidney (V2 receptor) and brain (ANP-B receptor) were obtained. The localization of expression of those receptors in the developing and adult rat cochlea was examined by in situ hybridization using 35S-labeled cRNA probes. The V1a and V2 receptors were expressed throughout the whole of the neonatal rat cochlea, while no expression was detected in the adult cochlea. The ANP-B receptor was expressed throughout the whole of the neonatal cochlea. In the adult cochlea, expression was observed in the spiral ganglion and the spiral ligament. These results suggest that vasopressin may play a role in the development of the cochlea, and that natriuretic peptide may play a role in the function of the spiral ganglion and the spiral ligament.

Protein kinase C mediates P2U purinergic receptor inhibition of K+ channel in apical membrane of strial marginal cells

Strial marginal cells (SMC) electrogenically secrete K+ via slowly activating K+ (I[sK]) channels, consisting of I(sK) regulatory and KvLQT1 channel subunits, and the associated short circuit current (I[sc]) is inhibited by agonists of the apical P2U receptor [Liu et al., Audit. Neurosci. 2 (1995) 331-340]. Measurements of relative K+ flux (JK) with a self-referencing K+-selective probe demonstrated a decrease in JK after apical perfusion of 100 microM ATP. On-cell macro patch recordings from the apical membrane of gerbil SMC showed a decrease of the I(sK) channel current (I[IsK]) by 88 +/- 8% during pipette perfusion of 100 microM ATP. The magnitude of the decrease of L(sc) by ATP was diminished in the presence of inhibitors of phospholipase C (PLC) and protein kinase C (PKC), U-73122 and GF109203X. Activation of PKC by phorbol 12-myristate 13-acetate (20 nM) decreased I(IsK) (gerbil: by 62 +/- 10%; rat: by 72 +/- 6%) in perforated-patch whole-cell recordings while the inactive analog, 4alphaPMA, had no effect. By contrast, elevation of cytosolic [Ca2+] by A23187 increased the whole-cell I(IsK). The expression of the isk gene transcript was confirmed and the serine responsible for the species-specific response to PKC was found to be present in the gerbil I(sK) sequence. These data provide evidence consistent with a direct effect of the PKC branch of the PLC pathway on the I(sK) channel of SMC in response to activation of the apical P2U receptor and predict that the secretion of endolymph in the human cochlea may be controlled by PKC in the same way as in our animal model.

Localization of soluble guanylate cyclase activity in the guinea pig cochlea suggests involvement in regulation of blood flow and supporting cell physiology

Although the nitric oxide/cGMP pathway has many important roles in biology, studies of this system in the mammalian cochlea have focused on the first enzyme in the pathway, nitric oxide synthase (NOS). However, characterization of the NO receptor, soluble guanylate cyclase (sGC), is crucial to determine the cells targeted by NO and to develop rational hypotheses of the function of this pathway in auditory processing. In this study we characterized guinea pig cochlear sGC by determining its enzymatic activity and cellular localization. In cytosolic fractions of auditory nerve, lateral wall tissues, and cochlear neuroepithelium, addition of NO donors resulted in three- to 15-fold increases in cGMP formation. NO-stimulated sGC activity was not detected in particulate fractions. We also localized cochlear sGC activity through immunocytochemical detection of NO-stimulated cGMP. sGC activity was detected in Hensen's and Deiters' cells of the organ of Corti, as well as in vascular pericytes surrounding small capillaries in the lateral wall tissues and sensory neuroepithelium. sGC activity was not observed in sensory cells. Using NADPH-diaphorase histochemistry, NOS was localized to pillar cells and nerve fibers underlying hair cells. These results indicate that the NO/cGMP pathway may influence diverse elements of the auditory system, including cochlear blood flow and supporting cell physiology.

Transcripts encoding three types of guanylyl-cyclase-coupled trans-membrane receptors in inner ear tissues of guinea pigs

The distribution of membrane-bound guanylyl cyclase (GC) transcription in inner ear tissues of the guinea pig was addressed by a reverse transcription-PCR approach using consensus primers flanking a region of about 630 bp in the intracellular domains in the target sequences. Restriction mapping of such amplificates obtained from cochlear and vestibular specimens permitted us to demonstrate GC-A, GC-B, and GC-C expression by differentiating overall PCR signals. This assay indicated that GC-A was expressed in the cochlea and vestibular organ. PCR products resulting from transcripts of the GC-B gene were obtained at considerably lower abundance than amplificates typical of the GC-A gene. The consensus primer approach with subsequent restriction mapping provided the opportunity to examine at the same time expression of GC-C in the inner ear and revealed the occurrence of GC-C transcripts in both inner ear compartments under investigation. The distribution pattern found by analysing the intracellular domains of membrane-bound guanylyl cyclases was confirmed by demonstrating transcription of the corresponding extracellular receptor domains. In addition, single-strand conformation polymorphism analysis of cDNA amplificates comprising the catalytic domain of guanylyl cyclases also indicated the presence of GC-C expression in the inner ear tissues examined. The GC-C transcripts detected in inner ear tissues appeared to correlate with functional receptor expression, since the production of cyclic GMP catalyzed by cochlear and vestibular specimens was stimulated by 1 microM of heat-stable enterotoxin to 18 and 80% above basal levels, respectively. Thus, GC-C may be involved in the fluid regulation by typical ligands (e.g., the peptide hormone guanylin or the toxins causing travellers' diarrhea), not only in the intestine but also in the organs responsible for hearing and gravitational orientation.