Defects in sensory organ morphogenesis and generation of cochlear hair cells in Gata3-deficient mouse embryos

Depolarization shift in the resting membrane potential of inferior colliculus neurons explains their hyperactivity induced by an acoustic trauma

Introduction

Neuronal hyperactivity has been associated with many brain diseases. In the auditory system, hyperactivity has been linked to hyperacusis and tinnitus. Previous research demonstrated the development of hyperactivity in inferior colliculus (IC) neurons after sound overexposure, but the underlying mechanism of this hyperactivity remains unclear. The main goal of this study was to determine the mechanism of this hyperactivity.

Methods

Experiments were performed on CBA/CaJ mice in a restrained, unanesthetized condition using intracellular recordings with sharp microelectrodes. Recordings were obtained from control (unexposed) and unilaterally sound overexposed groups of mice.

Results

Our data suggest that sound exposure-induced hyperactivity was due to a depolarizing shift of the resting membrane potential (RMP) in the hyperactive neurons. The half width of action potentials in these neurons was also decreased after sound exposure. Surprisingly, we also found an RMP gradient in which neurons have more hyperpolarized RMPs with increasing depth in the IC. This gradient was altered in the overexposed animals.

Gata3 is required in late proneurosensory development for proper sensory cell formation and organization

It has previously been shown that the zinc-finger transcription factor Gata3 has dynamic expression within the inner ear throughout embryonic development and is essential for cochlear neurosensory development. However, the temporal window for which Gata3 is required for proper formation of the cochlear neurosensory epithelia remains unclear. To investigate the role of Gata3 in cochlear neurosensory development in the late prosensory stages, we used the Sox2-creERT2 mouse line to target and conditionally delete Gata3 at E11.5, a timepoint before cells have fully committed to a neurosensory fate. While the inner ears of Sox2-creERT2: Gata3 f/f mice appear normal with no gross structural defects, the sensory cells in the organ of Corti are partially lost and disorganized in an increasing severity from base to apex. Additionally, spiral ganglion neurons display aberrant peripheral projections, including increased distances between radial bundles and disorganization upon reaching the organ of Corti. Furthermore, heterozygous Sox2-creERT2: Gata3 f/+ mice show a reduced aberrant phenotype in comparison to the homozygous mutant, supporting the hypothesis that Gata3 is not only required for proper formation at the later proneurosensory stage, but also that a specific expression level of Gata3 is required. Therefore, this study provides evidence that Gata3 plays a time-sensitive and dose-dependent role in the development of sensory and neuronal cells in late proneurosensory stages.

Gata3 is Required in Late Proneurosensory Development for Proper Sensory Cell Formation and Organization

It has been previously shown that zinc-finger transcription factor Gata3 has dynamic expression within the inner ear throughout embryonic development and is essential for cochlear neurosensory development. However, the temporal window to which Gata3 is required for the formation of the cochlear neurosensory epithelia remains unclear. To investigate the role of Gata3 on cochlear neurosensory development in the late prosensory stages, we used the Sox2-cre ERT2 mouse line to target and conditionally delete Gata3 at E11.5 before the cells have fully committed to a neurosensory fate. While the inner ears of Sox2-cre ERT2 : Gata3 f/f mice appear morphologically normal, the sensory cells in the organ of Corti are partially lost and disorganized in a basal to apical gradient with the apex demonstrating the more severe phenotype. Additionally, spiral ganglion neurons display aberrant peripheral projections, such as increased distances between radial bundles and disorganization upon reaching the organ of Corti. Furthermore, heterozygous Sox2-cre ERT2 : Gata3 f/+ mice show a reduced phenotype in comparison to the homozygous mutant, supporting the concept that Gata3 is not only required for proper formation at the later proneurosensory stage, but also that a specific level of Gata3 is required. Therefore, our studies confirm that Gata3 plays a time-sensitive and dose-dependent role in the development of sensory cells in the late proneurosensory stages.

Gata3 is required for the functional maturation of inner hair cells and their innervation in the mouse cochlea

KEY POINTS

The physiological maturation of auditory hair cells and their innervation requires precise temporal and spatial control of cell differentiation. The transcription factor gata3 is essential for the earliest stages of auditory system development and for survival and synaptogenesis in auditory sensory afferent neurons. We show that during postnatal development in the mouse inner ear gata3 is required for the biophysical maturation, growth and innervation of inner hair cells; in contrast, it is required only for the survival of outer hair cells. Loss of gata3 in inner hair cells causes progressive hearing loss and accounts for at least some of the deafness associated with the human hypoparathyroidism, deafness and renal anomaly (HDR) syndrome. The results show that gata3 is critical for later stages of mammalian auditory system development where it plays distinct, complementary roles in the coordinated maturation of sensory hair cells and their innervation.

ABSTRACT

The zinc finger transcription factor gata3 regulates inner ear development from the formation of the embryonic otic placode. Throughout development, gata3 is expressed dynamically in all the major cochlear cell types. Its role in afferent formation is well established but its possible involvement in hair cell maturation remains unknown. Here, we find that in heterozygous gata3 null mice (gata3+/- ) outer hair cells (OHCs) differentiate normally but their numbers are significantly lower. In contrast, inner hair cells (IHCs) survive normally but they fail to acquire adult basolateral membrane currents, retain pre-hearing current and efferent innervation profiles and have fewer ribbon synapses. Targeted deletion of gata3 driven by otoferlin-cre recombinase (gata3fl/fl otof-cre+/- ) in IHCs does not affect OHCs or the number of IHC afferent synapses but it leads to a failure in IHC maturation comparable to that observed in gata3+/- mice. Auditory brainstem responses in gata3fl/fl otof-cre+/- mice reveal progressive hearing loss that becomes profound by 6-7 months, whilst distortion product otoacoustic emissions are no different to control animals up to this age. Our results, alongside existing data, indicate that gata3 has specific, complementary functions in different cell types during inner ear development and that its continued expression in the sensory epithelium orchestrates critical aspects of physiological development and neural connectivity. Furthermore, our work indicates that hearing loss in human hypoparathyroidism, deafness and renal anomaly (HDR) syndrome arises from functional deficits in IHCs as well as loss of function from OHCs and both afferent and efferent neurons.

GATA transcription factors in development and disease

The GATA family of transcription factors is of crucial importance during embryonic development, playing complex and widespread roles in cell fate decisions and tissue morphogenesis. GATA proteins are essential for the development of tissues derived from all three germ layers, including the skin, brain, gonads, liver, hematopoietic, cardiovascular and urogenital systems. The crucial activity of GATA factors is underscored by the fact that inactivating mutations in most GATA members lead to embryonic lethality in mouse models and are often associated with developmental diseases in humans. In this Primer, we discuss the unique and redundant functions of GATA proteins in tissue morphogenesis, with an emphasis on their regulation of lineage specification and early organogenesis.

HDR syndrome with a novel mutation in GATA3 mimicking a congenital X-linked stapes gusher: a case report

Background

Hypoparathyroidism, sensorineural hearing loss, and renal disease (HDR) syndrome, also known as Barakat syndrome, is a rare genetic disorder with high phenotypic heterogeneity caused by haploinsufficiency of the GATA3 gene on chromosome 10p14-p15. For these reasons, the diagnosis of HDR syndrome is challenging and requires a high index of suspicion as well as genetic analysis.

Case presentation

A 14-month-old boy, with sensorineural hearing loss in both ears, showed typical radiological features of X-linked stapes gusher on preoperative temporal bone computed tomography (CT) for cochlear implantations. Then after his discharge from hospital, he suffered a hypocalcemic seizure and we discovered a renal cyst during investigation of hypocalcemia. He was finally diagnosed with HDR syndrome by clinical findings, which were confirmed by molecular genetic testing. Direct sequencing of the GATA3 gene showed a heterozygous 2-bp deletion (c.1201_1202delAT), which is predicted to cause a frameshift of the reading frame (p.Met401Valfs*106).

Conclusions

To our knowledge, this is the first case of HDR syndrome with a novel de novo variant mimicking a congenital X-linked stapes gusher syndrome. Novel mutations and the diversity of clinical manifestations expand the genotypic and phenotypic spectrum of HDR syndrome.

Diagnosis of HDR syndrome is still challenging, but clinicians should consider it in their differential diagnosis for children with a wide range of clinical manifestations including hypocalcemia induced seizures and deafness. We hope that this case will contribute to further understanding and studies of HDR-associated GATA3 mutations.

Electronic supplementary material

The online version of this article (10.1186/s12881-017-0484-6) contains supplementary material, which is available to authorized users.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

Transactivation of human osteoprotegerin promoter by GATA-3

Osteoprotegerin (OPG) is a key regulator of bone remodeling. Mutations in OPG are involved in a variety of human diseases. We have shown that cochlear spiral ganglion cells secrete OPG at high levels and lack of OPG causes sensorineural hearing loss in addition to the previously described conductive hearing loss. In order to study the regulation of OPG expression, we conducted a database search on regulatory elements in the promoter region of the OPG gene, and identified two potential GATA-3 binding sites. Using luciferase assays and site directed mutagenesis, we demonstrate that these two elements are GATA-3 responsive and support GATA-3 transactivation in human HEK and HeLa cells. The expression of wild type GATA-3 activated OPG mRNA and protein expression, while the expression of a dominant negative mutant of GATA-3 or a GATA-3 shRNA construct reduced OPG mRNA and protein levels. GATA-3 deficient cells generated by expressing a GATA-3 shRNA construct were sensitive to apoptosis induced by etoposide and TNF-α. This apoptotic effect could be partly prevented by the co-treatment with exogenous OPG. Our results suggest new approaches to rescue diseases due to GATA-3 deficiency – such as in hypoparathyroidism, sensorineural deafness, and renal (HDR) syndrome – by OPG therapy.

Audiometric characteristics of a dutch family with a new mutation in GATA3 causing HDR syndrome

We present the case of a Dutch family with a new mutation (c523_528dup) in GATA3 causing HDR syndrome. HDR syndrome is characterised by hypoparathyroidism, deafness and renal defects. In this study, we describe the audiometric characteristics of 5 patients from this family. Their hearing impairment was congenital, bilateral and symmetric. Audiograms showed mild-to-moderate hearing impairment with a flat audiogram configuration. Higher frequencies tended to be affected more strongly. Cross-sectional analyses showed no progression, and a mean audiogram was established. Psychophysical measurements in 3 HDR patients - including speech reception in noise, loudness scaling, gap detection and difference limen for frequency - were obtained to assess hearing function in greater detail. Overall, the results of the psychophysical measurements indicated characteristics of outer hair cell loss. CT scanning showed no anomalies in 3 of the HDR patients. Although 2 patients displayed vestibular symptoms, no anomalies in the vestibular system were found by vestibulo-ocular examination. Our results are in agreement with the theory that outer hair cell malfunctioning can play a major role in HDR syndrome.

Meta-analysis of genome-wide association studies identifies six new Loci for serum calcium concentrations

Calcium is vital to the normal functioning of multiple organ systems and its serum concentration is tightly regulated. Apart from CASR, the genes associated with serum calcium are largely unknown. We conducted a genome-wide association meta-analysis of 39,400 individuals from 17 population-based cohorts and investigated the 14 most strongly associated loci in ≤ 21,679 additional individuals. Seven loci (six new regions) in association with serum calcium were identified and replicated. Rs1570669 near CYP24A1 (P = 9.1E-12), rs10491003 upstream of GATA3 (P = 4.8E-09) and rs7481584 in CARS (P = 1.2E-10) implicate regions involved in Mendelian calcemic disorders: Rs1550532 in DGKD (P = 8.2E-11), also associated with bone density, and rs7336933 near DGKH/KIAA0564 (P = 9.1E-10) are near genes that encode distinct isoforms of diacylglycerol kinase. Rs780094 is in GCKR. We characterized the expression of these genes in gut, kidney, and bone, and demonstrate modulation of gene expression in bone in response to dietary calcium in mice. Our results shed new light on the genetics of calcium homeostasis.

Ahsa1 and Hsp90 activity confers more severe craniofacial phenotypes in a zebrafish model of hypoparathyroidism, sensorineural deafness and renal dysplasia (HDR)

The severity of most human birth defects is highly variable. Our ability to diagnose, treat and prevent defects relies on our understanding of this variability. Mutation of the transcription factor GATA3 in humans causes the highly variable hypoparathyroidism, sensorineural deafness and renal dysplasia (HDR) syndrome. Although named for a triad of defects, individuals with HDR can also exhibit craniofacial defects. Through a forward genetic screen for craniofacial mutants, we isolated a zebrafish mutant in which the first cysteine of the second zinc finger of Gata3 is mutated. Because mutation of the homologous cysteine causes HDR in humans, these zebrafish mutants could be a quick and effective animal model for understanding the role of gata3 in the HDR disease spectrum. We demonstrate that, unexpectedly, the chaperone proteins Ahsa1 and Hsp90 promote severe craniofacial phenotypes in our zebrafish model of HDR syndrome. The strengths of the zebrafish system, including rapid development, genetic tractability and live imaging, make this an important model for variability.

GATA3 controls the specification of prosensory domain and neuronal survival in the mouse cochlea

HDR syndrome (also known as Barakat syndrome) is a developmental disorder characterized by hypoparathyroidism, sensorineural deafness and renal disease. Although genetic mapping and subsequent functional studies indicate that GATA3 haplo-insufficiency causes human HDR syndrome, the role of Gata3 in sensorineural deafness and auditory system development is largely unknown. In this study, we show that Gata3 is continuously expressed in the developing mouse inner ear. Conditional knockout of Gata3 in the developing inner ear disrupts the morphogenesis of mouse inner ear, resulting in a disorganized and shortened cochlear duct with significant fewer hair cells and supporting cells. Loss of Gata3 function leads to the failure in the specification of prosensory domain and subsequently, to increased cell death in the cochlear duct. Moreover, though the initial generation of cochleovestibular ganglion (CVG) cells is not affected in Gata3-null mice, spiral ganglion neurons (SGNs) are nearly depleted due to apoptosis. Our results demonstrate the essential role of Gata3 in specifying the prosensory domain in the cochlea and in regulating the survival of SGNs, thus identifying a molecular mechanism underlying human HDR syndrome.

Gata3 is a critical regulator of cochlear wiring

Spiral ganglion neurons (SGNs) play a key role in hearing by rapidly and faithfully transmitting signals from the cochlea to the brain. Identification of the transcriptional networks that ensure the proper specification and wiring of SGNs during development will lay the foundation for efforts to rewire a damaged cochlea. Here, we show that the transcription factor Gata3, which is expressed in SGNs throughout their development, is essential for formation of the intricately patterned connections in the cochlea. We generated conditional knock-out mice in which Gata3 is deleted after SGNs are specified. Cochlear wiring is severely disrupted in these animals, with premature extension of neurites that follow highly abnormal trajectories toward their targets, as shown using in vitro neurite outgrowth assays together with time-lapse imaging of whole embryonic cochleae. Expression profiling of mutant neurons revealed a broad shift in gene expression toward a more differentiated state, concomitant with minor changes in SGN identity. Thus, Gata3 appears to serve as an "intermediate regulator" that guides SGNs through differentiation and preserves the auditory fate. As the first auditory-specific regulator of SGN development, Gata3 provides a useful molecular entry point for efforts to engineer SGNs for the restoration of hearing.

Continued expression of GATA3 is necessary for cochlear neurosensory development

Hair cells of the developing mammalian inner ear are progressively defined through cell fate restriction. This process culminates in the expression of the bHLH transcription factor Atoh1, which is necessary for differentiation of hair cells, but not for their specification. Loss of several genes will disrupt ear morphogenesis or arrest of neurosensory epithelia development. We previously showed in null mutants that the loss of the transcription factor, Gata3, results specifically in the loss of all cochlear neurosensory development. Temporal expression of Gata3 is broad from the otic placode stage through the postnatal ear. It therefore remains unclear at which stage in development Gata3 exerts its effect. To better understand the stage specific effects of Gata3, we investigated the role of Gata3 in cochlear neurosensory specification and differentiation utilizing a LoxP targeted Gata3 line and two Cre lines. Foxg1^Cre∶Gata3^f/f mice show recombination of Gata3 around E8.5 but continue to develop a cochlear duct without differentiated hair cells and spiral ganglion neurons. qRT-PCR data show that Atoh1 was down-regulated but not absent in the duct whereas other hair cell specific genes such as Pou4f3 were completely absent. In addition, while Sox2 levels were lower in the Foxg1^Cre:Gata3^f/f cochlea, Eya1 levels remained normal. We conclude that Eya1 is unable to fully upregulate Atoh1 or Pou4f3, and drive differentiation of hair cells without Gata3. Pax2-Cre∶Gata3^f/f mice show a delayed recombination of Gata3 in the ear relative to Foxg1^Cre:Gata3^f/f. These mice exhibited a cochlear duct containing patches of partially differentiated hair cells and developed only few and incorrectly projecting spiral ganglion neurons. Our conditional deletion studies reveal a major role of Gata3 in the signaling of prosensory genes and in the differentiation of cochlear neurosenory cells. We suggest that Gata3 may act in combination with Eya1, Six1, and Sox2 in cochlear prosensory gene signaling.

Transcription factors that control inner ear development and their potential for transdifferentiation and reprogramming

Transcription factors (TFs) participate during various processes throughout inner ear development such as induction, morphogenesis and determination of cell fate and differentiation. The analysis of mouse mutants has been essential to define the requirement of different members of TF families during these processes. Next to their roles during normal development TFs have also been tested for their capacity to induce differentiation or reprogram cells upon misexpression. Recently the capacity of TFs to transdifferentiate easily accessible cells such as fibroblasts to highly specialized cell types has opened a new pathway for regenerative therapies. In this review the influence of TFs acting during different phases and processes of inner ear development will be summarized. A special focus will be given to TFs with a potential to reprogram or transdifferentiate cells to sensory cell types of the inner ear such as hair cells or neurons and thus may form part of future protocols directed to generate replacement cells in a clinical context.