Loss of inhibitory interneurons in the dorsal spinal cord and elevated itch in Bhlhb5 mutant mice

Dynorphin is expressed primarily by GABAergic neurons that contain galanin in the rat dorsal horn

Background

The opioid peptide dynorphin is expressed by certain neurons in the superficial dorsal horn of the spinal cord, but little is known about the types of cell that contain dynorphin. In this study, we have used an antibody against the dynorphin precursor preprodynorphin (PPD), to reveal the cell bodies and axons of dynorphin-expressing neurons in the rat spinal cord. The main aims were to estimate the proportion of neurons in each of laminae I-III that express dynorphin and to determine whether they are excitatory or inhibitory neurons.

Results

PPD-immunoreactive cells were concentrated in lamina I and the outer part of lamina II (IIo), where they constituted 17% and 8%, respectively, of all neurons. Around half of those in lamina I and 80% of those in lamina II were GABA-immunoreactive. We have previously identified four non-overlapping neurochemical populations of inhibitory interneurons in this region, defined by the presence of neuropeptide Y, galanin, parvalbumin and neuronal nitric oxide synthase. PPD co-localised extensively with galanin in both cell bodies and axons, but rarely or not at all with the other three markers. PPD was present in around 4% of GABAergic boutons (identified by the presence of the vesicular GABA transporter) in laminae I-II.

Conclusions

These results show that most dynorphin-expressing cells in the superficial dorsal horn are inhibitory interneurons, and that they largely correspond to the population that is defined by the presence of galanin. We estimate that dynorphin is present in ~32% of inhibitory interneurons in lamina I and 11% of those in lamina II. Since the proportion of GABAergic boutons that contain PPD in these laminae was considerably lower than this, our findings suggest that these neurons may generate relatively small axonal arborisations.

Regulation of spinal interneuron development by the Olig-related protein Bhlhb5 and Notch signaling

The neural circuits that control motor activities depend on the spatially and temporally ordered generation of distinct classes of spinal interneurons. Despite the importance of these interneurons, the mechanisms underlying their genesis are poorly understood. Here, we demonstrate that the Olig-related transcription factor Bhlhb5 (recently renamed Bhlhe22) plays two central roles in this process. Our findings suggest that Bhlhb5 repressor activity acts downstream of retinoid signaling and homeodomain proteins to promote the formation of dI6, V1 and V2 interneuron progenitors and their differentiated progeny. In addition, Bhlhb5 is required to organize the spatially restricted expression of the Notch ligands and Fringe proteins that both elicit the formation of the interneuron populations that arise adjacent to Bhlhb5(+) cells and influence the global pattern of neuronal differentiation. Through these actions, Bhlhb5 helps transform the spatial information established by morphogen signaling into local cell-cell interactions associated with Notch signaling that control the progression of neurogenesis and extend neuronal diversity within the developing spinal cord.

Transmitters and pathways mediating inhibition of spinal itch-signaling neurons by scratching and other counterstimuli

Scratching relieves itch, but the underlying neural mechanisms are poorly understood. We presently investigated a role for the inhibitory neurotransmitters GABA and glycine in scratch-evoked inhibition of spinal itch-signaling neurons in a mouse model of chronic dry skin itch. Superficial dorsal horn neurons ipsilateral to hindpaw dry skin treatment exhibited a high level of spontaneous firing that was significantly attenuated by cutaneous scratching, pinch and noxious heat. Scratch-evoked inhibition was nearly abolished by spinal delivery of the glycine antagonist, strychnine, and was markedly attenuated by respective GABA_A and GABA_B antagonists bicuculline and saclofen. Scratch-evoked inhibition was also significantly attenuated (but not abolished) by interruption of the upper cervical spinal cord, indicating the involvement of both segmental and suprasegmental circuits that engage glycine- and GABA-mediated inhibition of spinal itch-signaling neurons by noxious counterstimuli.

Acute inhibition of signalling phenotype of spinal GABAergic neurons by tumour necrosis factor-alpha

Spinal application of TNFα induces both allodynia and hyperalgesia, and at least part of the pronociceptive effects of TNFα have been suggested as due to the impaired function of spinal inhibitory neurons (disinhibition). The present study explores the effects of TNFα on the signalling phenotype of spinal GABAergic neurons identified in transgenic mice expressing green fluorescent protein at the glutamic acid decarboxylase 67 (GAD67) promoter. Acute application of TNFα directly inhibits the excitability of a subset of GAD67(+) spinal neurons. TNFα-induced inhibition was dependent on the activation of p38 mitogen-activated protein kinase (MAPK) within these GAD67(+) neurons. TNFα receptor 1 (TNFR1) but not receptor 2 (TNFR2) was identified on spinal GAD67(+) neurons, suggesting that TNFα signals through TNFR1. Voltage-clamp recordings of GAD67(+) neurons indicated that the inhibitory effect of TNFα was through suppression of the hyperpolarization-activated cation current (I(h)). This study defines a novel mechanism of spinal disinhibition mediated by a TNFα-TNFR1-p38 pathway within GABAergic inhibitory interneurons.

A quantitative study of neuronal nitric oxide synthase expression in laminae I-III of the rat spinal dorsal horn

Nitric oxide produced by neuronal nitric oxide synthase (nNOS) in the spinal cord is required for development of hyperalgesia in inflammatory and neuropathic pain states. nNOS is expressed by some dorsal horn neurons, and an early study that used a histochemical method to identify these cells suggested that they were mainly inhibitory interneurons. We have carried out a quantitative analysis of nNOS-immunoreactivity in laminae I-III of the rat dorsal horn, to determine the proportion of inhibitory and excitatory neurons and axonal boutons that express the protein. nNOS was present in ∼5% of neurons in laminae I and III, and 18% of those in lamina II. Although most cells with strong nNOS immunostaining were GABA-immunoreactive, two-thirds of the nNOS-positive cells in lamina II and half of those in lamina III were not GABAergic, and some of these expressed protein kinase Cγ (PKCγ). We estimate that nNOS is present in 17-19% of the inhibitory interneurons in laminae I-II, and 6% of those in lamina III. However, our results suggest that nNOS is also expressed at a relatively low level by a significant proportion (∼17%) of excitatory interneurons in lamina II. nNOS was seldom seen in boutons that contained vesicular glutamate transporter 2, which is expressed by excitatory interneurons, but was co-localised with the vesicular GABA transporter (VGAT, a marker for GABAergic and glycinergic axons). nNOS was detected in 13% of VGAT boutons in lamina I and in 7-8% of those in laminae II-III. However, it was only found in 2-4% of the VGAT boutons that were presynaptic to PKCγ-expressing interneurons in this region. These results indicate that nNOS is more widely expressed than previously thought, being present in both inhibitory and excitatory neurons. They provide further evidence that axons of neurochemically defined populations of inhibitory interneuron are selective in their post-synaptic targets.

The distinct roles of two GPCRs, MrgprC11 and PAR2, in itch and hyperalgesia

Itch has been defined as an unpleasant skin sensation that triggers the urge to scratch. Primary sensory dorsal root ganglia neurons detect itch stimuli through peripheral axons in the skin, playing an important role in generating itch. Itch is broadly categorized as histaminergic (sensitive to antihistamines) or nonhistaminergic. The peptide Ser-Leu-Ile-Gly-Arg-Leu (SLIGRL) is an itch-inducing agent widely used to study histamine-independent itch. Here, we show that Mrgprs (Mas-related G protein-coupled receptors), particularly MrgprC11, rather than PAR2 (protease-activated receptor 2) as previously thought, mediate this type of itch. A shorter peptide, SLIGR, which specifically activates PAR2 but not MrgprC11, induced thermal pain hypersensitivity in mice but not a scratch response. Therefore, although both Mrgpr and PAR2 are SLIGRL-responsive G protein-coupled receptors present in dorsal root ganglia, each plays a specific role in mediating itch and pain.

Glutamate acts as a neurotransmitter for gastrin releasing peptide-sensitive and insensitive itch-related synaptic transmission in mammalian spinal cord

Itch sensation is one of the major sensory experiences of human and animals. Recent studies have proposed that gastrin releasing peptide (GRP) is a key neurotransmitter for itch in spinal cord. However, no direct evidence is available to indicate that GRP actually mediate responses between primary afferent fibers and dorsal horn neurons. Here we performed integrative neurobiological experiments to test this question. We found that a small population of rat dorsal horn neurons responded to GRP application with increases in calcium signaling. Whole-cell patch-clamp recordings revealed that a part of superficial dorsal horn neurons responded to GRP application with the increase of action potential firing in adult rats and mice, and these dorsal horn neurons received exclusively primary afferent C-fiber inputs. On the other hands, few A_δ inputs receiving cells were found to be GRP positive. Finally, we found that evoked sensory responses between primary afferent C fibers and GRP positive superficial dorsal horn neurons are mediated by glutamate but not GRP. CNQX, a blocker of AMPA and kainate (KA) receptors, completely inhibited evoked EPSCs, including in those Fos-GFP positive dorsal horn cells activated by itching. Our findings provide the direct evidence that glutamate is the principal excitatory transmitter between C fibers and GRP positive dorsal horn neurons. Our results will help to understand the neuronal mechanism of itch and aid future treatment for patients with pruritic disease.

Facial injections of pruritogens or algogens elicit distinct behavior responses in rats and excite overlapping populations of primary sensory and trigeminal subnucleus caudalis neurons

In the present study, we investigated whether intradermal cheek injection of pruritogens or algogens differentially elicits hindlimb scratches or forelimb wipes in Sprague-Dawley rats, as recently reported in mice. We also investigated responses of primary sensory trigeminal ganglion (TG) and dorsal root ganglion (DRG) cells, as well as second-order neurons in trigeminal subnucleus caudalis (Vc), to pruritic and algesic stimuli. 5-HT was the most effective chemical to elicit dose-dependent bouts of hindlimb scratches directed to the cheek, with significantly less forelimb wiping, consistent with itch. Chloroquine also elicited significant scratching but not wiping. Allyl isothiocyanate (AITC; mustard oil) elicited dose-dependent wiping with no significant scratching. Capsaicin elicited equivalent numbers of scratch bouts and wipes, suggesting a mixed itch and pain sensation. By calcium imaging, ∼ 6% of cultured TG and DRG cells responded to 5-HT. The majority of 5-HT-sensitive cells also responded to chloroquine, AITC, and/or capsaicin, and one-third responded to histamine. Using a chemical search strategy, we identified single units in Vc that responded to intradermal cheek injection of 5-HT. Most were wide dynamic range (WDR) or nociceptive specific (NS), and a few were mechanically insensitive. The large majority additionally responded to AITC and/or capsaicin and thus were not pruritogen selective. These results suggest that primary and second-order neurons responsive to pruritogens and algogens may utilize a population coding mechanism to distinguish between itch and pain, sensations that are behaviorally manifested by distinct hindlimb scratching and forelimb wiping responses.

Pirt, a TRPV1 modulator, is required for histamine-dependent and -independent itch

Itch, or pruritus, is an important clinical problem whose molecular basis has yet to be understood. Recent work has begun to identify genes that contribute to detecting itch at the molecular level. Here we show that Pirt, known to play a vital part in sensing pain through modulation of the transient receptor potential vanilloid 1 (TRPV1) channel, is also necessary for proper itch sensation. Pirt^−/− mice exhibit deficits in cellular and behavioral responses to various itch-inducing compounds, or pruritogens. Pirt contributes to both histaminergic and nonhistaminergic itch and, crucially, is involved in forms of itch that are both TRPV1-dependent and -independent. Our findings demonstrate that the function of Pirt extends beyond nociception via TRPV1 regulation to its role as a critical component in several itch signaling pathways.

TRPA1 is required for histamine-independent, Mas-related G protein-coupled receptor-mediated itch

Itch, the unpleasant sensation that evokes a desire to scratch, accompanies numerous skin and nervous system disorders. In many cases, pathological itch is insensitive to antihistamine treatment. Recent studies have identified members of the Mas-related G protein-coupled receptor (Mrgpr) family that are activated by mast cell mediators and promote histamine-independent itch. MrgprA3 and MrgprC11 act as receptors for the pruritogens chloroquine and BAM8-22, respectively. However, the signaling pathways and transduction channels activated downstream of these pruritogens are largely unknown. We found that TRPA1 is the downstream target of both MrgprA3 and MrgprC11 in cultured sensory neurons and heterologous cells. TRPA1 is required for Mrgpr-mediated signaling, as sensory neurons from TRPA1-deficient mice exhibited markedly diminished responses to chloroquine and BAM8-22. Similarly, TRPA1-deficient mice displayed little to no scratching in response to these pruritogens. Our findings indicate that TRPA1 is an essential component of the signaling pathways that promote histamine-independent itch.

The molecular basis of making spiral ganglion neurons and connecting them to hair cells of the organ of Corti

The bipolar spiral ganglion neurons apparently delaminate from the growing cochlear duct and migrate to Rosenthal's canal. They project radial fibers to innervate the organ of Corti (type I neurons to inner hair cells, type II neurons to outer hair cells) and also project tonotopically to the cochlear nuclei. The early differentiation of these neurons requires transcription factors to regulate migration, pathfinding and survival. Neurog1 null mice lack formation of neurons. Neurod1 null mice show massive neuronal death combined with aberrant central and peripheral projections. Prox1 protein is necessary for proper type II neuron process navigation, which is also affected by the neurotrophins Bdnf and Ntf3. Neurotrophin null mutants show specific patterns of neuronal loss along the cochlea but remaining neurons compensate by expanding their target area. All neurotrophin mutants have reduced radial fiber growth proportional to the degree of loss of neurotrophin alleles. This suggests a simple dose response effect of neurotrophin concentration. Keeping overall concentration constant, but misexpressing one neurotrophin under regulatory control of another one results in exuberant fiber growth not only of vestibular fibers to the cochlea but also of spiral ganglion neurons to outer hair cells suggesting different effectiveness of neurotrophins for spiral ganglion neurite growth. Finally, we report here for the first time that losing all neurons in double null mutants affects extension of the cochlear duct and leads to formation of extra rows of outer hair cells in the apex, possibly by disrupting the interaction of the spiral ganglion with the elongating cochlea.

Itch: Cells, Molecules, and Circuits

The itch field has made great advances in recent years, building upon earlier work to form a clearer picture of the biology behind this important sensory modality. Models for how itch is encoded have emerged that fit with physiological, molecular, and behavioral data. The molecular mechanisms of itch, both peripherally and centrally, are being revealed with the aid of newer animal models. Future work must address shortcomings in our current understanding of itch including limitations of current experimental methods. Here we review what is known about the cells, molecules, and circuits involved in itch and highlight key questions that remain to be answered.

Chemosensory properties of the trigeminal system

The capacity of cutaneous, including trigeminal endings, to detect chemicals is known as chemesthesis or cutaneous chemosensation. This sensory function involves the activation of nociceptor and thermoreceptor endings and has a protective or defensive function, as many of these substances are irritants or poisonous. However, humans have also developed a liking for the distinct sharpness or pungency of many foods, beverages, and spices following activation of the same sensory afferents. Our understanding of the cellular and molecular mechanisms of chemosensation in the trigeminal system has experienced enormous progress in the past decade, following the cloning and functional characterization of several ion channels activated by physical and chemical stimuli. This brief review attempts to summarize our current knowledge in this field, including a functional description of various sensory channels, especially TRP channels, involved in trigeminal chemosensitivy. Finally, some of these new findings are discussed in the context of the pathophysiology of trigeminal chemosensation, including pain, pruritus, migraine, cough, airway inflammation, and ophthalmic diseases.

The DABBEC phenotyping system: towards a mechanistic understanding of CP/CPPS

There is an urgent need to elucidate the mechanistic basis of chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS), as the current methods of symptom-based diagnosis and treatment have failed. Here, we propose a phenotyping system that bridges the gap between the symptom-based diagnosis and treatment of the present and the mechanistic approach of the future. Our phenotyping system uses the Chronic Prostatitis Collaborative Research Network (CPCRN)-recommended algorithm in combination with the NIH Chronic Prostatitis Symptom Index (NIH-CPSI) as a basis for diagnosis, while incorporating novel domains for quantitative assessment and stratification of CP/CPPS patients. We believe this novel system will serve to help advance our understanding of the roles of the patient's genome and proteome in the etiology of CP/CPPS. We predict that, as we begin to understand the mechanistic basis of CP/CPPS pathology and progression, we will develop specific treatments that will aim to cure the disease, rather than merely quell the symptoms.

VGLUT2-dependent glutamate release from nociceptors is required to sense pain and suppress itch

Itch can be suppressed by painful stimuli, but the underlying neural basis is unknown. We generated conditional null mice in which vesicular glutamate transporter type 2 (VGLUT2)-dependent synaptic glutamate release from mainly Nav1.8-expressing nociceptors was abolished. These mice showed deficits in pain behaviors, including mechanical pain, heat pain, capsaicin-evoked pain, inflammatory pain, and neuropathic pain. The pain deficits were accompanied by greatly enhanced itching, as suggested by (1) sensitization of both histamine-dependent and histamine-independent itch pathways and (2) development of spontaneous scratching and skin lesions. Strikingly, intradermal capsaicin injection promotes itch responses in these mutant mice, as opposed to pain responses in control littermates. Consequently, coinjection of capsaicin was no longer able to mask itch evoked by pruritogenic compounds. Our studies suggest that synaptic glutamate release from a group of peripheral nociceptors is required to sense pain and suppress itch. Elimination of VGLUT2 in these nociceptors creates a mouse model of chronic neurogenic itch.

An itch to be scratched

The description of itch (formally known as pruritus) as an "unpleasant sensation that elicits the desire or reflex to scratch" (Ikoma et al., 2006) is immediately familiar. Research in the field of pruritoception has added to our understanding of this area of sensory neurobiology as it pertains to both normal and pathological conditions. In particular, much progress has been made on the mechanisms and circuits of itch, which we review here.

VGLUT2-dependent sensory neurons in the TRPV1 population regulate pain and itch

The natural response to itch sensation is to scratch, which relieves the itch through an unknown mechanism. Interaction between pain and itch has been frequently demonstrated, and the selectivity hypothesis of itch, based on data from electrophysiological and behavioral experiments, postulates the existence of primary pain afferents capable of repressing itch. Here, we demonstrate that deletion of vesicular glutamate transporter (VGLUT) 2 in a subpopulation of neurons partly overlapping with the vanilloid receptor (TRPV1) primary afferents resulted in a dramatic increase in itch behavior accompanied by a reduced responsiveness to thermal pain. The increased itch behavior was reduced by administration of antihistaminergic drugs and by genetic deletion of the gastrin-releasing peptide receptor, demonstrating a dependence on VGLUT2 to maintain normal levels of both histaminergic and nonhistaminergic itch. This study establishes that VGLUT2 is a major player in TRPV1 thermal nociception and also serves to regulate a normal itch response.

The multiple pathways for itch and their interactions with pain

Multiple neural pathways and molecular mechanisms responsible for producing the sensation of itch have recently been identified, including histamine-independent pathways. Physiological, molecular, behavioral and brain imaging studies are converging on a description of these pathways and their close association with pain processing. Some conflicting results have arisen and the precise relationship between itch and pain remains controversial. A better understanding of the generation of itch and of the intrinsic mechanisms that inhibit itch after scratching should facilitate the search for new methods to alleviate clinical pruritus (itch). In this review we describe the current understanding of the production and inhibition of itch. A model of itch processing within the CNS is proposed.

Labeled lines meet and talk: population coding of somatic sensations

The somatic sensory system responds to stimuli of distinct modalities, including touch, pain, itch, and temperature sensitivity. In the past century, great progress has been made in understanding the coding of these sensory modalities. From this work, two major features have emerged. First, there are specific neuronal circuits or labeled lines transmitting specific sensory information from the skin to the brain. Second, the generation of specific sensations often involves crosstalk among distinct labeled lines. These features suggest that population coding is the mechanism underlying somatic sensation.

Loss of LMO4 in the retina leads to reduction of GABAergic amacrine cells and functional deficits

BACKGROUND

LMO4 is a transcription cofactor expressed during retinal development and in amacrine neurons at birth. A previous study in zebrafish reported that morpholino RNA ablation of one of two related genes, LMO4b, increases the size of eyes in embryos. However, the significance of LMO4 in mammalian eye development and function remained unknown since LMO4 null mice die prior to birth.

METHODOLOGY/PRINCIPAL FINDINGS

We observed the presence of a smaller eye and/or coloboma in ∼40% LMO4 null mouse embryos. To investigate the postnatal role of LMO4 in retinal development and function, LMO4 was conditionally ablated in retinal progenitor cells using the Pax6 alpha-enhancer Cre/LMO4flox mice. We found that these mice have fewer Bhlhb5-positive GABAergic amacrine and OFF-cone bipolar cells. The deficit appears to affect the postnatal wave of Bhlhb5+ neurons, suggesting a temporal requirement for LMO4 in retinal neuron development. In contrast, cholinergic and dopaminergic amacrine, rod bipolar and photoreceptor cell numbers were not affected. The selective reduction in these interneurons was accompanied by a functional deficit revealed by electroretinography, with reduced amplitude of b-waves, indicating deficits in the inner nuclear layer of the retina.

CONCLUSIONS/SIGNIFICANCE

Inhibitory GABAergic interneurons play a critical function in controlling retinal image processing, and are important for neural networks in the central nervous system. Our finding of an essential postnatal function of LMO4 in the differentiation of Bhlhb5-expressing inhibitory interneurons in the retina may be a general mechanism whereby LMO4 controls the production of inhibitory interneurons in the nervous system.

Facilitation of the inhibitory transmission by gastrin-releasing peptide in the anterior cingulate cortex

Gastrin-releasing peptide (GRP) has been proposed as a peptidergic molecule for behavioral fear and itching. Immunohistochemistry and in situ hybridization studies have shown that GRP and GRP receptor are widely distributed in forebrain areas. Less information is available for the functional action for GRP in the prefrontal cortex including the anterior cingulate cortex (ACC). Here we used whole-cell patch-clamp recording technique to study the modulation of synaptic transmission by GRP in the ACC. We found that GRP increased the frequency of sIPSCs recorded while had no significant effect on sEPSCs in ACC pyramidal neurons. The facilitatory effect of GRP on sIPSCs was blocked by the GRP receptor antagonist, RC3095. In the presence of TTX, however, GRP had no effect on the mIPSCs. Therefore, activation of GRP receptor may facilitate the excitation of the interneurons and enhanced spontaneous GABAergic, but not glutamatergic neurotransmission. Similar results on GRP modulation of GABAergic transmission were observed in the insular cortex and amygdala, suggesting a general possible effect of GRP on cortical inhibitory transmission. Our results suggest that GRP receptor is an important regulator of inhibitory circuits in forebrain areas.

The role of bHLH genes in ear development and evolution: revisiting a 10-year-old hypothesis

In mouse ear development, two bHLH genes, Atoh1 and Neurog1, are essential for hair cell and sensory neuron differentiation. Evolution converted the original simple atonal-dependent neurosensory cell formation program of diploblasts into the derived developmental program of vertebrates that generates two neurosensory cell types, the sensory neuron and the sensory hair cell. This transformation was achieved through gene multiplication in ancestral triploblasts resulting in the expansion of the atonal bHLH gene family. Novel genes of the Neurogenin and NeuroD families are upregulated prior to the expression of Atoh1. Recent data suggest that NeuroD and Neurogenin were lost or their function in neuronal specification reduced in flies, thus changing our perception of the evolution of these genes. This sequence of expression changes was accompanied by modification of the E-box binding sites of these genes to regulate different downstream genes and to form inhibitory loops among each other, thus fine-tuning expression transitions.

Spontaneous itch in the absence of hyperalgesia in a mouse hindpaw dry skin model

We presently investigated if chronic dry skin treatment of the mouse hindpaw results in itch-related behavior, with or without accompanying hyperalgesia and allodynia. Following a 10-day period of treatment of one hindpaw with a mixture of acetone-diethylether-water (AEW), mice exhibited a significant increase in spontaneous biting behavior directed to the treated hindpaw compared to control animals in which one hindpaw was similarly treated with water only (W). Biting in the AEW group was significantly attenuated by the μ-opioid antagonist naltrexone but was unaffected by the μ-agonist morphine. There were no significant differences in hindpaw heat withdrawal latency, mechanical withdrawal threshold, or cold-plate latency between W and AEW treatment groups. These results indicate that chronic AEW treatment induces spontaneous itch but does not alter pain sensitivity, supporting the utility of this model for studies of chronic dry skin itch.

Neurod1 suppresses hair cell differentiation in ear ganglia and regulates hair cell subtype development in the cochlea

BACKGROUND

At least five bHLH genes regulate cell fate determination and differentiation of sensory neurons, hair cells and supporting cells in the mammalian inner ear. Cross-regulation of Atoh1 and Neurog1 results in hair cell changes in Neurog1 null mice although the nature and mechanism of the cross-regulation has not yet been determined. Neurod1, regulated by both Neurog1 and Atoh1, could be the mediator of this cross-regulation.

METHODOLOGY/PRINCIPAL FINDINGS

We used Tg(Pax2-Cre) to conditionally delete Neurod1 in the inner ear. Our data demonstrate for the first time that the absence of Neurod1 results in formation of hair cells within the inner ear sensory ganglia. Three cell types, neural crest derived Schwann cells and mesenchyme derived fibroblasts (neither expresses Neurod1) and inner ear derived neurons (which express Neurod1) constitute inner ear ganglia. The most parsimonious explanation is that Neurod1 suppresses the alternative fate of sensory neurons to develop as hair cells. In the absence of Neurod1, Atoh1 is expressed and differentiates cells within the ganglion into hair cells. We followed up on this effect in ganglia by demonstrating that Neurod1 also regulates differentiation of subtypes of hair cells in the organ of Corti. We show that in Neurod1 conditional null mice there is a premature expression of several genes in the apex of the developing cochlea and outer hair cells are transformed into inner hair cells.

CONCLUSIONS/SIGNIFICANCE

Our data suggest that the long noted cross-regulation of Atoh1 expression by Neurog1 might actually be mediated in large part by Neurod1. We suggest that Neurod1 is regulated by both Neurog1 and Atoh1 and provides a negative feedback for either gene. Through this and other feedback, Neurod1 suppresses alternate fates of neurons to differentiate as hair cells and regulates hair cell subtypes.