Large scale gene expression profiles of regenerating inner ear sensory epithelia

The Effects of Viral Infections on the Molecular and Signaling Pathways Involved in the Development of the PAOs

Cytomegalovirus infection contributes to 10-30% of congenital hearing loss in children. Vertebrate peripheral auditory organs include the outer, middle, and inner ear. Their development is regulated by multiple signaling pathways. However, most ear diseases due to viral infections are due to congenital infections and reactivation and affect healthy adults to a lesser extent. This may be due to the fact that viral infections affect signaling pathways that are important for the development of peripheral hearing organs. Therefore, an in-depth understanding of the relationship between viral infections and the signaling pathways involved in the development of peripheral hearing organs is important for the prevention and treatment of ear diseases. In this review, we summarize the effects of viruses on signaling pathways and signaling molecules in the development of peripheral auditory organs.

Hair Cell Regeneration: From Animals to Humans

Cochlear hair cells convert sound into electrical signals that are relayed via the spiral ganglion neurons to the central auditory pathway. Hair cells are vulnerable to damage caused by excessive noise, aging, and ototoxic agents. Non-mammals can regenerate lost hair cells by mitotic regeneration and direct transdifferentiation of surrounding supporting cells. However, in mature mammals, damaged hair cells are not replaced, resulting in permanent hearing loss. Recent studies have uncovered mechanisms by which sensory organs in non-mammals and the neonatal mammalian cochlea regenerate hair cells, and outlined possible mechanisms why this ability declines rapidly with age in mammals. Here, we review similarities and differences between avian, zebrafish, and mammalian hair cell regeneration. Moreover, we discuss advances and limitations of hair cell regeneration in the mature cochlea and their potential applications to human hearing loss.

SoxC transcription factors shape the epigenetic landscape to establish competence for sensory differentiation in the mammalian organ of Corti

The auditory organ of Corti is comprised of only two major cell types-the mechanosensory hair cells and their associated supporting cells-both specified from a single pool of prosensory progenitors in the cochlear duct. Here, we show that competence to respond to Atoh1, a transcriptional master regulator necessary and sufficient for induction of mechanosensory hair cells, is established in the prosensory progenitors between E12.0 and 13.5. The transition to the competent state is rapid and is associated with extensive remodeling of the epigenetic landscape controlled by the SoxC group of transcription factors. Conditional loss of Sox4 and Sox11-the two homologous family members transiently expressed in the inner ear at the time of competence establishment-blocks the ability of prosensory progenitors to differentiate as hair cells. Mechanistically, we show that Sox4 binds to and establishes accessibility of early sensory lineage-specific regulatory elements, including ones associated with Atoh1 and its direct downstream targets. Consistent with these observations, overexpression of Sox4 or Sox11 prior to developmental establishment of competence precociously induces hair cell differentiation in the cochlear progenitors. Further, reintroducing Sox4 or Sox11 expression restores the ability of postnatal supporting cells to differentiate as hair cells in vitro and in vivo. Our findings demonstrate the pivotal role of SoxC family members as agents of epigenetic and transcriptional changes necessary for establishing competence for sensory receptor differentiation in the inner ear.

On the value of diverse organisms in auditory research: From fish to flies to humans

Historically, diverse organisms have contributed to our understanding of auditory function. In recent years, the laboratory mouse has become the prevailing non-human model in auditory research, particularly for biomedical studies. There are many questions in auditory research for which the mouse is the most appropriate (or the only) model system available. But mice cannot provide answers for all auditory problems of basic and applied importance, nor can any single model system provide a synthetic understanding of the diverse solutions that have evolved to facilitate effective detection and use of acoustic information. In this review, spurred by trends in funding and publishing and inspired by parallel observations in other domains of neuroscience, we highlight a few examples of the profound impact and lasting benefits of comparative and basic organismal research in the auditory system. We begin with the serendipitous discovery of hair cell regeneration in non-mammalian vertebrates, a finding that has fueled an ongoing search for pathways to hearing restoration in humans. We then turn to the problem of sound source localization - a fundamental task that most auditory systems have been compelled to solve despite large variation in the magnitudes and kinds of spatial acoustic cues available, begetting varied direction-detecting mechanisms. Finally, we consider the power of work in highly specialized organisms to reveal exceptional solutions to sensory problems - and the diverse returns of deep neuroethological inquiry - via the example of echolocating bats. Throughout, we consider how discoveries made possible by comparative and curiosity-driven organismal research have driven fundamental scientific, biomedical, and technological advances in the auditory field.

Hearing Dysfunction After Treatment With Teprotumumab for Thyroid Eye Disease

PURPOSE

To characterize the frequency, severity, and resolution of hearing dysfunction in patients treated with teprotumumab for thyroid eye disease (TED).

DESIGN

Prospective observational case series.

METHODS

Ophthalmic examination and adverse event assessment, including otologic symptoms, were performed at baseline, after infusions 2, 4, and 8, and at 6-month follow-up in consecutive patients who received at least 4 teprotumumab infusions. Laboratory test results were collected at baseline and during treatment. Audiometry, patulous eustachian tube (PET) testing, and otolaryngology evaluation were obtained for patients with new or worsening otologic symptoms, with a subset obtaining baseline and posttreatment testing.

RESULTS

Twenty-seven patients were analyzed (24 females, 3 males, average 56.3 years old). Twenty-two patients (81.5%) developed new subjective otologic symptoms, after a mean of 3.8 infusions (SD 1.8). At 39.2-week average follow-up after the last infusion, most patients with tinnitus (100%), ear plugging/fullness (90.9%), and autophony (83.3%) experienced symptom resolution, whereas only 45.5% (5 of 11) of patients with subjective hearing loss/decreased word comprehension experienced resolution. Six patients underwent baseline and posttreatment audiometry, 5 of whom developed teprotumumab-related sensorineural hearing loss (SNHL) and 1 patient also developed PET. Three of the 5 patients with teprotumumab-related SNHL had persistent subjective hearing loss at last follow-up. A prior history of hearing loss was discovered as a risk factor for teprotumumab-related SNHL (P = .008).

CONCLUSIONS

Hearing loss is a concerning adverse event of teprotumumab, and its mechanism and reversibility should be further studied. Until risk factors for hearing loss are better understood, we recommend baseline audiometry with PET testing and repeat testing if new otologic symptoms develop. Screening, monitoring, and prevention guidelines are needed.

Key Signaling Pathways Regulate the Development and Survival of Auditory Hair Cells

The loss of auditory sensory hair cells (HCs) is the most common cause of sensorineural hearing loss (SNHL). As the main sound transmission structure in the cochlea, it is necessary to maintain the normal shape and survival of HCs. In this review, we described and summarized the signaling pathways that regulate the development and survival of auditory HCs in SNHL. The role of the mitogen-activated protein kinase (MAPK), phosphoinositide-3 kinase/protein kinase B (PI3K/Akt), Notch/Wnt/Atoh1, calcium channels, and oxidative stress/reactive oxygen species (ROS) signaling pathways are the most relevant. The molecular interactions of these signaling pathways play an important role in the survival of HCs, which may provide a theoretical basis and possible therapeutic interventions for the treatment of hearing loss.

Cell-type identity of the avian cochlea

In contrast to mammals, birds recover naturally from acquired hearing loss, which makes them an ideal model for inner ear regeneration research. Here, we present a validated single-cell RNA sequencing resource of the avian cochlea. We describe specific markers for three distinct types of sensory hair cells, including a previously unknown subgroup, which we call superior tall hair cells. We identify markers for the supporting cells associated with tall hair cells, which represent the facultative stem cells of the avian inner ear. Likewise, we present markers for supporting cells that are located below the short cochlear hair cells. We further infer spatial expression gradients of hair cell genes along the tonotopic axis of the cochlea. This resource advances neurobiology, comparative biology, and regenerative medicine by providing a basis for comparative studies with non-regenerating mammalian cochleae and for longitudinal studies of the regenerating avian cochlea.

Signaling pathways (Notch, Wnt, Bmp and Fgf) have additive effects on hair cell regeneration in the chick basilar papilla after streptomycin injury in vitro: Additive effects of signaling pathways on hair cell regeneration

Hair cells can be regenerated after damage by transdifferentiation in which a supporting cell directly differentiates into a hair cell without mitosis. However, such regeneration is at the cost of exhausting the support cells in the mammalian mature cochlea. Thus, more effective methods should be found to promote mitotic regeneration but partially preserve support cells after damage. To address the issue, we first injured hair cells in the chick basilar papillae (BP) by treatment with streptomycin in vitro. We then compared the mitotic regeneration on the neural side in the middle part of BP after treatment with a pharmacological inhibitor or agonist of the Notch (DAPT), Wnt (LiCl), Bmp (Noggin) or Fgf (SU5402) signaling pathway, with that after treatment with combinations of two or three inhibitors or agonist of these pathways. Our results indicate that treatments with a single inhibitor or agonist of the Notch, Wnt, Bmp or Fgf signaling pathway could significantly increase mitotic regeneration as well as direct transdifferentiation. The results also show that hair cells (Myosin 7a+), support cells (Sox2+) and mitotically regenerated hair cells (Myosin 7a+/Sox2+/BrdU+) increased significantly on the neural side in the middle part of BP after two or three combinations of the inhibition of Notch, Bmp or Fgf signaling pathway or the activation of Wnt signaling pathway, besides the reported coregulatory effects of Notch and Wnt signaling. The study of the effects of systematic combinations of pathway modulators provided more insight into hair cell regeneration from mitosis.

Perspectives on Human Hearing Loss, Cochlear Regeneration, and the Potential for Hearing Restoration Therapies

Most adults who acquire hearing loss find it to be a disability that is poorly corrected by current prosthetics. This gap drives current research in cochlear mechanosensory hair cell regeneration and in hearing restoration. Birds and fish can spontaneously regenerate lost hair cells through a process that has become better defined in the last few years. Findings from these studies have informed new research on hair cell regeneration in the mammalian cochlea. Hair cell regeneration is one part of the greater problem of hearing restoration, as hearing loss can stem from a myriad of causes. This review discusses these issues and recent findings, and places them in the greater social context of need and community.

Transcriptomic analysis of mouse cochleae suffering from gentamicin damage reveals the signalling pathways involved in hair cell regeneration

There is a strong capacity for hair cell regeneration after damage in the inner ear of non-mammals. However, mammalian hair cells are substantially unable to regenerate. To obtain insights into the mechanism of this difference, we analyzed the transcriptomic changes in the mouse cochleae suffered from gentamicin damage and compared them with those in the chick cochleae suffered from the same damage. The results indicated that 2,230 genes had significantly differential expression between the gentamicin- and saline-treated mouse cochleae. Some of the differentially expressed genes were grouped into 265 signaling pathways, including the Notch, Wnt (Wingless and INT-1), Bmp (bone morphogenetic protein), FGF (fibroblast growth factor) and Shh (sonic hedgehog) pathways. Using pharmacological inhibitors or agonists of these pathways, the effects of these pathways on hair cell regeneration were further studied. The results indicated that Bmp alone and its coregulation with the Notch or Wnt signaling pathways increased the numbers of generated cells from transdifferentiation or proliferation in the mouse cochlea after damage, in addition to the reported coregulation of Notch and Wnt. Thus, this work indicates a new signaling pathway (Bmp) and its synergetic coregulation in mammalian hair cell regeneration, providing potential therapeutic targets to increase mammalian hair cell regeneration.

Stem Cells and the Bird Cochlea-Where Is Everybody?

In sharp contrast to the adult mammalian cochlea, which lacks regenerative ability, the mature avian cochlea, or basilar papilla (BP) is capable of complete recovery from hearing loss after damage. Avian sensory hair cell regeneration relies on rousing quiescent supporting cells to proliferate or transdifferentiate after hair cell death. Unlike mammalian cochlear supporting cells, which have clearly defined subtypes, avian BP supporting cells are deceptively indistinguishable and molecular markers have yet to be identified. Despite the importance of supporting cells as the putative stem cells in avian regeneration, it is unknown whether all supporting cells possess equal capability to give rise to a hair cell or if a specialized subpopulation exists. In this perspective, we reinvigorate the concept of a stem cell in the BP, and form comparisons to other regenerating tissues that show cell-cycle reentry after damage. Special emphasis is given to the structure of the BP and how anatomy informs both the potential, intrinsic heterogeneity of the supporting cell layer as well as the choice between mitotic and nonmitotic regenerative strategies.

Complementary and distinct roles of autophagy, apoptosis and senescence during early inner ear development

The development of the inner ear complex cytoarchitecture and functional geometry requires the exquisite coordination of a variety of cellular processes in a temporal manner. At early stages of inner ear development several rounds of cell proliferation in the otocyst promote the growth of the structure. The apoptotic program is initiated in exceeding cells to adjust cell type numbers. Apoptotic cells are cleared by phagocytic cells that recognize the phosphatidylserine residues exposed in the cell membrane thanks to the energy supplied by autophagy. Specific molecular programs determine hair and supporting cell fate, these populations are responsible for the functions of the adult sensory organ: detection of sound, position and acceleration. The neurons that transmit auditory and balance information to the brain are also born at the otocyst by neurogenesis facilitated by autophagy. Cellular senescence participates in tissue repair, cancer and aging, situations in which cells enter a permanent cell cycle arrest and acquire a highly secretory phenotype that modulates their microenvironment. More recently, senescence has also been proposed to take place during vertebrate development in a limited number of transitory structures and organs; among the later, the endolymphatic duct in the inner ear. Here, we review these cellular processes during the early development of the inner ear, focusing on how the most recently described cellular senescence participates and cooperates with proliferation, apoptosis and autophagy to achieve otic morphogenesis and differentiation.

Computational analysis of mRNA expression profiling in the inner ear reveals candidate transcription factors associated with proliferation, differentiation, and deafness

BACKGROUND

Hearing loss is a major cause of disability worldwide, impairing communication, health, and quality of life. Emerging methods of gene therapy aim to address this morbidity, which can be employed to fix a genetic problem causing hair cell dysfunction and to promote the proliferation of supporting cells in the cochlea and their transdifferentiation into hair cells. In order to extend the applicability of gene therapy, the scientific community is focusing on discovery of additional deafness genes, identifying new genetic variants associated with hearing loss, and revealing new factors that can be manipulated in a coordinated manner to improve hair cell regeneration. Here, we addressed these challenges via genome-wide measurement and computational analysis of transcriptional profiles of mouse cochlea and vestibule sensory epithelium at embryonic day (E)16.5 and postnatal day (P)0. These time points correspond to developmental stages before and during the acquisition of mechanosensitivity, a major turning point in the ability to hear.

RESULTS

We hypothesized that tissue-specific transcription factors are primarily involved in differentiation, while those associated with development are more concerned with proliferation. Therefore, we searched for enrichment of transcription factor binding motifs in genes differentially expressed between the tissues and between developmental ages of mouse sensory epithelium. By comparison with transcription factors known to alter their expression during avian hair cell regeneration, we identified 37 candidates likely to be important for regeneration. Furthermore, according to our estimates, only half of the deafness genes in human have been discovered. To help remedy the situation, we developed a machine learning classifier that utilizes the expression patterns of genes to predict how likely they are to be undiscovered deafness genes.

CONCLUSIONS

We used a novel approach to highlight novel additional factors that can serve as points of intervention for enhancing hair cell regeneration. Given the similarities between mouse and human deafness, our predictions may be of value in prioritizing future research on novel human deafness genes.

Mechanism Underlying the Effects of Estrogen Deficiency on Otoconia

Otoconia-related vertigo and balance deficits, particularly benign paroxysmal positional vertigo (BPPV), are common. Our recent studies in humans show that, while BPPV prevalence greatly increases with age in both genders, peri-menopausal women are especially susceptible. In the present study, we show that bilateral ovariectomized (OVX) mice have significant balance behavioral deficits, and that estrogen deficiency compromises otoconia maintenance and anchoring by reducing the expression of otoconial component and anchoring proteins. There is ectopic debris formation in the ampulla under estrogen deficiency due to aberrant matrix protein expression. Furthermore, phytoestrogen is effective in rescuing the otoconia abnormalities. By comparing the expression levels of known estrogen receptor (Esr) subtypes, and by examining the otoconia phenotypes of null mice for selected receptors, we postulate that Esr2 may be critical in mediating the effects of estrogen in otoconia maintenance.

New role of LRP5, associated with nonsyndromic autosomal-recessive hereditary hearing loss

Human hearing loss is a common neurosensory disorder about which many basic research and clinically relevant questions are unresolved. At least 50% of hearing loss are due to a genetic etiology. Although hundreds of genes have been reported, there are still hundreds of related deafness genes to be found. Clinical, genetic, and functional investigations were performed to identify the causative mutation in a distinctive Chinese family with postlingual nonsyndromic sensorineural hearing loss. Whole-exome sequencing (WES) identified lipoprotein receptor-related protein 5 (LRP5), a member of the low-density lipoprotein receptor family, as the causative gene in this family. In the zebrafish model, lrp5 downregulation using morpholinos led to significant abnormalities in zebrafish inner ear and lateral line neuromasts and contributed, to some extent, to disabilities in hearing and balance. Rescue experiments showed that LRP5 mutation is associated with hearing loss. Knocking down lrp5 in zebrafish results in reduced expression of several genes linked to Wnt signaling pathway and decreased cell proliferation when compared with those in wild-type zebrafish. In conclusion, the LRP5 mutation influences cell proliferation through the Wnt signaling pathway, thereby reducing the number of supporting cells and hair cells and leading to nonsyndromic hearing loss in this Chinese family.

Early growth response protein 1 regulates promoter activity of α-plasma membrane calcium ATPase 2, a major calcium pump in the brain and auditory system

BACKGROUND

Along with sodium/calcium (Ca2+) exchangers, plasma membrane Ca2+ ATPases (ATP2Bs) are main regulators of intracellular Ca2+ levels. There are four ATP2B paralogs encoded by four different genes. Atp2b2 encodes the protein pump with the fastest activation, ATP2B2. In mice, the Atp2b2 transcript has several alternate transcriptional start site variants: α, β, µ and δ. These variants are expressed in developmental and tissue specific manners. The α and β Atp2b2 transcripts are equally expressed in the brain. αAtp2b2 is the only transcript found in the outer hair cells of young mice (Silverstein RS, Tempel BL. in Neuroscience 141:245-257, 2006). Mutations in the coding region of the mouse Atp2b2 gene indicate a narrow window for tolerated dysfunction of the ATP2B2 protein, specifically in the auditory system. This highlights the necessity of tight regulation of this gene for normal cell physiology.

RESULTS

Although ATP2Bs are important regulators of Ca2+ in many cell types, little is known about their transcriptional regulation. This study identifies the proximal promoter of the αAtp2b2 transcript. Further investigations indicate that ATOH1 and EGR1 modulate promoter activity. Additionally, we report that EGR1 increases endogenous expression of Atp2b2 transcript in two cell lines. Electrophoretic mobility shift assays (EMSA) indicate that EGR1 binds to a specific site in the CpG island of the αAtp2b2 promoter.

CONCLUSION

This study furthers our understanding of Atp2b2 regulation by: (I) elucidating transcriptional regulatory mechanisms for Atp2b2, and (II) identifying transcription factors that modulate expression of Atp2b2 in the brain and peripheral auditory system and (III) allows for future studies modulating gene expression of Atp2b2.

In Vivo Interplay between p27Kip1, GATA3, ATOH1, and POU4F3 Converts Non-sensory Cells to Hair Cells in Adult Mice

Hearing loss is widespread and persistent because mature mammalian auditory hair cells (HCs) are nonregenerative. In mice, the ability to regenerate HCs from surrounding supporting cells (SCs) declines abruptly after postnatal maturation. We find that combining p27^Kip1 deletion with ectopic ATOH1 expression surmounts this age-related decline, leading to conversion of SCs to HCs in mature mouse cochleae and after noise damage. p27^Kip1 deletion, independent of canonical effects on Rb-family proteins, upregulated GATA3, a co-factor for ATOH1 that is lost from SCs with age. Co-activation of GATA3 or POU4F3 and ATOH1 promoted conversion of SCs to HCs in adult mice. Activation of POU4F3 alone also converted mature SCs to HCs in vivo. These data illuminate a genetic pathway that initiates auditory HC regeneration and suggest p27^Kip1, GATA3, and POU4F3 as additional therapeutic targets for ATOH1-mediated HC regeneration.

Cochlear hair cell regeneration after noise-induced hearing loss: Does regeneration follow development?

Noise-induced hearing loss (NIHL) affects a large number of military personnel and civilians. Regenerating inner-ear cochlear hair cells (HCs) is a promising strategy to restore hearing after NIHL. In this review, we first summarize recent transcriptome profile analysis of zebrafish lateral lines and chick utricles where spontaneous HC regeneration occurs after HC damage. We then discuss recent studies in other mammalian regenerative systems such as pancreas, heart and central nervous system. Both spontaneous and forced HC regeneration occurs in mammalian cochleae in vivo involving proliferation and direct lineage conversion. However, both processes are inefficient and incomplete, and decline with age. For direct lineage conversion in vivo in cochleae and in other systems, further improvement requires multiple factors, including transcription, epigenetic and trophic factors, with appropriate stoichiometry in appropriate architectural niche. Increasing evidence from other systems indicates that the molecular paths of direct lineage conversion may be different from those of normal developmental lineages. We therefore hypothesize that HC regeneration does not have to follow HC development and that epigenetic memory of supporting cells influences the HC regeneration, which may be a key to successful cochlear HC regeneration. Finally, we discuss recent efforts in viral gene therapy and drug discovery for HC regeneration. We hope that combination therapy targeting multiple factors and epigenetic signaling pathways will provide promising avenues for HC regeneration in humans with NIHL and other types of hearing loss.

The Transcriptomics to Proteomics of Hair Cell Regeneration: Looking for a Hair Cell in a Haystack

Mature mammals exhibit very limited capacity for regeneration of auditory hair cells, while all non-mammalian vertebrates examined can regenerate them. In an effort to find therapeutic targets for deafness and balance disorders, scientists have examined gene expression patterns in auditory tissues under different developmental and experimental conditions. Microarray technology has allowed the large-scale study of gene expression profiles (transcriptomics) at whole-genome levels, but since mRNA expression does not necessarily correlate with protein expression, other methods, such as microRNA analysis and proteomics, are needed to better understand the process of hair cell regeneration. These technologies and some of the results of them are discussed in this review. Although there is a considerable amount of variability found between studies owing to different species, tissues and treatments, there is some concordance between cellular pathways important for hair cell regeneration. Since gene expression and proteomics data is now commonly submitted to centralized online databases, meta-analyses of these data may provide a better picture of pathways that are common to the process of hair cell regeneration and lead to potential therapeutics. Indeed, some of the proteins found to be regulated in the inner ear of animal models (e.g., IGF-1) have now gone through human clinical trials.

Retinoic Acid Signaling Mediates Hair Cell Regeneration by Repressing p27kip and sox2 in Supporting Cells

During development, otic sensory progenitors give rise to hair cells and supporting cells. In mammalian adults, differentiated and quiescent sensory cells are unable to generate new hair cells when these are lost due to various insults, leading to irreversible hearing loss. Retinoic acid (RA) has strong regenerative capacity in several organs, but its role in hair cell regeneration is unknown. Here, we use genetic and pharmacological inhibition to show that the RA pathway is required for hair cell regeneration in zebrafish. When regeneration is induced by laser ablation in the inner ear or by neomycin treatment in the lateral line, we observe rapid activation of several components of the RA pathway, with dynamics that position RA signaling upstream of other signaling pathways. We demonstrate that blockade of the RA pathway impairs cell proliferation of supporting cells in the inner ear and lateral line. Moreover, in neuromast, RA pathway regulates the transcription of p27(kip) and sox2 in supporting cells but not fgf3. Finally, genetic cell-lineage tracing using Kaede photoconversion demonstrates that de novo hair cells derive from FGF-active supporting cells. Our findings reveal that RA has a pivotal role in zebrafish hair cell regeneration by inducing supporting cell proliferation, and shed light on the underlying transcriptional mechanisms involved. This signaling pathway might be a promising approach for hearing recovery.

SIGNIFICANCE STATEMENT

Hair cells are the specialized mechanosensory cells of the inner ear that capture auditory and balance sensory input. Hair cells die after acoustic trauma, ototoxic drugs or aging diseases, leading to progressive hearing loss. Mammals, in contrast to zebrafish, lack the ability to regenerate hair cells. Here, we find that retinoic acid (RA) pathway is required for hair cell regeneration in vivo in the zebrafish inner ear and lateral line. RA pathway is activated very early upon hair cell loss, promotes cell proliferation of progenitor cells, and regulates two key genes, p27(kip) and sox2. Our results position RA as an essential signal for hair cell regeneration with relevance in future regenerative strategies in mammals.

Gene expression profiling of the inner ear

The identification of transcriptional differences has served as an important starting point in understanding the molecular mechanisms behind biological processes and systems. The developmental biology of the inner ear, the biology of hearing and of course the pathology of deafness are all processes that warrant a molecular description if we are to improve human health. To this end, technological innovation has meant that larger scale analysis of gene transcription has been possible for a number of years now, extending our molecular analysis of genes to beyond those that are currently in vogue for a given system. In this review, some of the contributions gene profiling has made to understanding developmental, pathological and physiological processes in the inner ear are highlighted.

Sensory hair cell development and regeneration: similarities and differences

Sensory hair cells are mechanoreceptors of the auditory and vestibular systems and are crucial for hearing and balance. In adult mammals, auditory hair cells are unable to regenerate, and damage to these cells results in permanent hearing loss. By contrast, hair cells in the chick cochlea and the zebrafish lateral line are able to regenerate, prompting studies into the signaling pathways, morphogen gradients and transcription factors that regulate hair cell development and regeneration in various species. Here, we review these findings and discuss how various signaling pathways and factors function to modulate sensory hair cell development and regeneration. By comparing and contrasting development and regeneration, we also highlight the utility and limitations of using defined developmental cues to drive mammalian hair cell regeneration.

Sensory hair cell regeneration in the zebrafish lateral line

BACKGROUND

Damage or destruction of sensory hair cells in the inner ear leads to hearing or balance deficits that can be debilitating, especially in older adults. Unfortunately, the damage is permanent, as regeneration of the inner ear sensory epithelia does not occur in mammals.

RESULTS

Zebrafish and other non-mammalian vertebrates have the remarkable ability to regenerate sensory hair cells and understanding the molecular and cellular basis for this regenerative ability will hopefully aid us in designing therapies to induce regeneration in mammals. Zebrafish not only possess hair cells in the ear but also in the sensory lateral line system. Hair cells in both organs are functionally analogous to hair cells in the inner ear of mammals. The lateral line is a mechanosensory system found in most aquatic vertebrates that detects water motion and aids in predator avoidance, prey capture, schooling, and mating. Although hair cell regeneration occurs in both the ear and lateral line, most research to date has focused on the lateral line due to its relatively simple structure and accessibility.

CONCLUSIONS

Here we review the recent discoveries made during the characterization of hair cell regeneration in zebrafish.

The transcriptome of utricle hair cell regeneration in the avian inner ear

Sensory hair cell loss is the major cause of hearing and balance disorders. Mammals are incapable of sustained hair cell regeneration, but lower vertebrates can regenerate these mechano-electrical transducers. We present the first comprehensive transcriptome (by mRNA-Seq) of hair cell regeneration in the chick utricle. We provide pathway and pattern annotations and correlate these with the phenotypic events that occur during regeneration. These patterns are surprisingly synchronous and highly punctuated. We show how these patterns are a new resource for identifying components of the hair cell transcriptome and identify 494 new putative hair-cell-specific genes and validate three of these (of three tested) by immunohistochemical staining. We describe many surprising new components and dynamic expression patterns, particularly within NOTCH signaling. For example, we show that HES7 is specifically expressed during utricle hair cell regeneration and closely parallels the expression of HES5. Likewise, the expression of ATOH1 is closely correlated with HEYL and the HLH inhibitory transcription factors ID1, ID2, and ID4. We investigate the correlation between fibroblast growth factor signaling and supporting cell proliferation and show that FGF20 inhibits supporting cell proliferation. We also present an analysis of 212 differentially expressed transcription factor genes in the regenerative time course that fall into nine distinct gene expression patterns, many of which correlate with phenotypic events during regeneration and represent attractive candidates for future analysis and manipulation of the regenerative program in sensory epithelia and other vertebrate neuroepithelia.

Responses to cell loss become restricted as the supporting cells in mammalian vestibular organs grow thick junctional actin bands that develop high stability

Sensory hair cell (HC) loss is a major cause of permanent hearing and balance impairments for humans and other mammals. Yet, fish, amphibians, reptiles, and birds readily replace HCs and recover from such sensory deficits. It is unknown what prevents replacement in mammals, but cell replacement capacity declines contemporaneously with massive postnatal thickening of F-actin bands at the junctions between vestibular supporting cells (SCs). In non-mammals, SCs can give rise to regenerated HCs, and the bands remain thin even in adults. Here we investigated the stability of the F-actin bands between SCs in ears from chickens and mice and Madin-Darby canine kidney cells. Pharmacological experiments and fluorescence recovery after photobleaching (FRAP) of SC junctions in utricles from mice that express a γ-actin-GFP fusion protein showed that the thickening F-actin bands develop increased resistance to depolymerization and exceptional stability that parallels a sharp decline in the cell replacement capacity of the maturing mammalian ear. The FRAP recovery rate and the mobile fraction of γ-actin-GFP both decreased as the bands thickened with age and became highly stabilized. In utricles from neonatal mice, time-lapse recordings in the vicinity of dying HCs showed that numerous SCs change shape and organize multicellular actin purse strings that reseal the epithelium. In contrast, adult SCs appeared resistant to deformation, with resealing responses limited to just a few neighboring SCs that did not form purse strings. The exceptional stability of the uniquely thick F-actin bands at the junctions of mature SCs may play an important role in restricting dynamic repair responses in mammalian vestibular epithelia.

Gene-expression analysis of hair cell regeneration in the zebrafish lateral line

Deafness caused by the terminal loss of inner ear hair cells is one of the most common sensory diseases. However, nonmammalian animals (e.g., birds, amphibians, and fish) regenerate damaged hair cells. To understand better the reasons underpinning such disparities in regeneration among vertebrates, we set out to define at high resolution the changes in gene expression associated with the regeneration of hair cells in the zebrafish lateral line. We performed RNA-Seq analyses on regenerating support cells purified by FACS. The resulting expression data were subjected to pathway enrichment analyses, and the differentially expressed genes were validated in vivo via whole-mount in situ hybridizations. We discovered that cell cycle regulators are expressed hours before the activation of Wnt/β-catenin signaling following hair cell death. We propose that Wnt/β-catenin signaling is not involved in regulating the onset of proliferation but governs proliferation at later stages of regeneration. In addition, and in marked contrast to mammals, our data clearly indicate that the Notch pathway is significantly down-regulated shortly after injury, thus uncovering a key difference between the zebrafish and mammalian responses to hair cell injury. Taken together, our findings lay the foundation for identifying differences in signaling pathway regulation that could be exploited as potential therapeutic targets to promote either sensory epithelium or hair cell regeneration in mammals.

Sensational placodes: neurogenesis in the otic and olfactory systems

For both the intricate morphogenetic layout of the sensory cells in the ear and the elegantly radial arrangement of the sensory neurons in the nose, numerous signaling molecules and genetic determinants are required in concert to generate these specialized neuronal populations that help connect us to our environment. In this review, we outline many of the proteins and pathways that play essential roles in the differentiation of otic and olfactory neurons and their integration into their non-neuronal support structures. In both cases, well-known signaling pathways together with region-specific factors transform thickened ectodermal placodes into complex sense organs containing numerous, diverse neuronal subtypes. Olfactory and otic placodes, in combination with migratory neural crest stem cells, generate highly specialized subtypes of neuronal cells that sense sound, position and movement in space, odors and pheromones throughout our lives.

A brief history of hair cell regeneration research and speculations on the future

Millions of people worldwide suffer from hearing and balance disorders caused by loss of the sensory hair cells that convert sound vibrations and head movements into electrical signals that are conveyed to the brain. In mammals, the great majority of hair cells are produced during embryogenesis. Hair cells that are lost after birth are virtually irreplaceable, leading to permanent disability. Other vertebrates, such as fish and amphibians, produce hair cells throughout life. However, hair cell replacement after damage to the mature inner ear was either not investigated or assumed to be impossible until studies in the late 1980s proved this to be false. Adult birds were shown to regenerate lost hair cells in the auditory sensory epithelium after noise- and ototoxic drug-induced damage. Since then, the field of hair cell regeneration has continued to investigate the capacity of the auditory and vestibular epithelia in vertebrates (fishes, birds, reptiles, and mammals) to regenerate hair cells and to recover function, the molecular mechanisms governing these regenerative capabilities, and the prospect of designing biologically-based treatments for hearing loss and balance disorders. Here, we review the major findings of the field during the past 25 years and speculate how future inner ear repair may one day be achieved.

Screen of FDA-approved drug library reveals compounds that protect hair cells from aminoglycosides and cisplatin

Loss of mechanosensory hair cells in the inner ear accounts for many hearing loss and balance disorders. Several beneficial pharmaceutical drugs cause hair cell death as a side effect. These include aminoglycoside antibiotics, such as neomycin, kanamycin and gentamicin, and several cancer chemotherapy drugs, such as cisplatin. Discovering new compounds that protect mammalian hair cells from toxic insults is experimentally difficult because of the inaccessibility of the inner ear. We used the zebrafish lateral line sensory system as an in vivo screening platform to survey a library of FDA-approved pharmaceuticals for compounds that protect hair cells from neomycin, gentamicin, kanamycin and cisplatin. Ten compounds were identified that provide protection from at least two of the four toxins. The resulting compounds fall into several drug classes, including serotonin and dopamine-modulating drugs, adrenergic receptor ligands, and estrogen receptor modulators. The protective compounds show different effects against the different toxins, supporting the idea that each toxin causes hair cell death by distinct, but partially overlapping, mechanisms. Furthermore, some compounds from the same drug classes had different protective properties, suggesting that they might not prevent hair cell death by their known target mechanisms. Some protective compounds blocked gentamicin uptake into hair cells, suggesting that they may block mechanotransduction or other routes of entry. The protective compounds identified in our screen will provide a starting point for studies in mammals as well as further research discovering the cellular signaling pathways that trigger hair cell death.

Transcriptomic analysis of the developing and adult mouse cochlear sensory epithelia

The adult mammalian cochlea lacks regenerative ability and the irreversible degeneration of cochlear sensory hair cells leads to permanent hearing loss. Previous data show that early postnatal cochlea harbors stem/progenitor-like cells and shows a limited regenerative/repair capacity. These properties are progressively lost later during the postnatal development. Little is known about the genes and pathways that are potentially involved in this difference of the regenerative/repair potentialities between early postnatal and adult mammalian cochlear sensory epithelia (CSE). The goal of our study is to investigate the transcriptomic profiles of these two stages. We used Mouse Genome 430 2.0 microarray to perform an extensive analysis of the genes expressed in mouse postnatal day-3 (P3) and adult CSE. Statistical analysis of microarray data was performed using SAM (Significance Analysis of Microarrays) software. We identified 5644 statistically significant differentially expressed transcripts with a fold change (FC) >2 and a False Discovery Rate (FDR) ≤0.05. The P3 CSE signature included 3,102 transcripts, among which were known genes in the cochlea, but also new transcripts such as, Hmga2 (high mobility group AT-hook 2) and Nrarp (Notch-regulated ankyrin repeat protein). The adult CSE overexpressed 2,542 transcripts including new transcripts, such as Prl (Prolactin) and Ar (Androgen receptor), that previously were not known to be expressed in the adult cochlea. Our comparative study revealed important genes and pathways differentially expressed between the developing and adult CSE. The identification of new candidate genes would be useful as potential markers of the maintenance or the loss of stem cells and regenerative/repair ability during mammalian cochlear development.

Perspectives of pharmacological treatment in otosclerosis

To review our current knowledge of the pathologic bone metabolism in otosclerosis and to discuss the possibilities of non-surgical, pharmacological intervention. Otosclerosis has been suspected to be associated with defective measles virus infection, local inflammation and consecutive bone deterioration in the human otic capsule. In the early stages of otosclerosis, different pharmacological agents may delay the progression or prevent further deterioration of the disease and consecutive hearing loss. Although effective anti-osteoporotic drugs have become available, the use of sodium fluoride and bisphosphonates in otosclerosis has not yet been successful. Bioflavonoids may relieve tinnitus due to otosclerosis, but there is no data available on long-term application and effects on sensorineural hearing loss. In the initial inflammatory phase, corticosteroids or non-steroidal anti-inflammatory drugs may be effective; however, extended systemic application may lead to serious side effects. Vitamin D administration may have effects on the pathological bone loss, as well as on inflammation. No information has been reported on the use of immunosuppressive drugs. Anti-cytokine targeted biological therapy, however, may be feasible. Indeed, one study on the local administration of infliximab has been reported. Potential targets of future therapy may include osteoprotegerin, RANK ligand, cathepsins and also the Wnt-β-catenin pathway. Finally, anti-measles vaccination may delay the progression of the disease and potentially decrease the number of new cases. In conclusion, stapes surgery remains to be widely accepted treatment of conductive hearing loss due to otosclerosis. Due to lack of solid evidence, the place of pharmacological treatment targeting inflammation and bone metabolism needs to be determined by future studies.

Probing the Xenopus laevis inner ear transcriptome for biological function

Background

The senses of hearing and balance depend upon mechanoreception, a process that originates in the inner ear and shares features across species. Amphibians have been widely used for physiological studies of mechanotransduction by sensory hair cells. In contrast, much less is known of the genetic basis of auditory and vestibular function in this class of animals. Among amphibians, the genus Xenopus is a well-characterized genetic and developmental model that offers unique opportunities for inner ear research because of the amphibian capacity for tissue and organ regeneration. For these reasons, we implemented a functional genomics approach as a means to undertake a large-scale analysis of the Xenopus laevis inner ear transcriptome through microarray analysis.

Results

Microarray analysis uncovered genes within the X. laevis inner ear transcriptome associated with inner ear function and impairment in other organisms, thereby supporting the inclusion of Xenopus in cross-species genetic studies of the inner ear. The use of gene categories (inner ear tissue; deafness; ion channels; ion transporters; transcription factors) facilitated the assignment of functional significance to probe set identifiers. We enhanced the biological relevance of our microarray data by using a variety of curation approaches to increase the annotation of the Affymetrix GeneChip® Xenopus laevis Genome array. In addition, annotation analysis revealed the prevalence of inner ear transcripts represented by probe set identifiers that lack functional characterization.

Conclusions

We identified an abundance of targets for genetic analysis of auditory and vestibular function. The orthologues to human genes with known inner ear function and the highly expressed transcripts that lack annotation are particularly interesting candidates for future analyses. We used informatics approaches to impart biologically relevant information to the Xenopus inner ear transcriptome, thereby addressing the impediment imposed by insufficient gene annotation. These findings heighten the relevance of Xenopus as a model organism for genetic investigations of inner ear organogenesis, morphogenesis, and regeneration.

Concise review: Inner ear stem cells--an oxymoron, but why?

Hearing loss, caused by irreversible loss of cochlear sensory hair cells, affects millions of patients worldwide. In this concise review, we examine the conundrum of inner ear stem cells, which obviously are present in the inner ear sensory epithelia of nonmammalian vertebrates, giving these ears the ability to functionally recover even from repetitive ototoxic insults. Despite the inability of the mammalian inner ear to regenerate lost hair cells, there is evidence for cells with regenerative capacity because stem cells can be isolated from vestibular sensory epithelia and from the neonatal cochlea. Challenges and recent progress toward identification of the intrinsic and extrinsic signaling pathways that could be used to re-establish stemness in the mammalian organ of Corti are discussed.

Identification of modulators of hair cell regeneration in the zebrafish lateral line

The external location of the zebrafish lateral line makes it a powerful model for studying mechanosensory hair cell regeneration. We have developed a chemical screen to identify FDA-approved drugs and biologically active compounds that modulate hair cell regeneration in zebrafish. Of the 1680 compounds evaluated, we identified two enhancers and six inhibitors of regeneration. The two enhancers, dexamethasone and prednisolone, are synthetic glucocorticoids that potentiated hair cell numbers during regeneration and also induced hair cell addition in the absence of damage. BrdU analysis confirmed that the extra hair cells arose from mitotic activity. We found that dexamethasone and prednisolone, like other glucocorticoids, suppress zebrafish caudal fin regeneration, indicating that hair cell regeneration occurs by a distinctly different process. Further analyses of the regeneration inhibitors revealed that two of the six, flubendazole and topotecan, significantly suppress hair cell regeneration by preventing proliferation of hair cell precursors. Flubendazole halted support cell division in M-phase, possibly by interfering with normal microtubule activity. Topotecan, a topoisomerase inhibitor, killed both hair cells and proliferating hair cell precursors. A third inhibitor, fulvestrant, moderately delayed hair cell regeneration by reducing support cell proliferation. Our observation that hair cells do not regenerate when support cell proliferation is impeded confirms previous observations that cell division is the primary route for hair cell regeneration after neomycin treatment in zebrafish.

Stem cell therapy for the inner ear: recent advances and future directions

In vertebrates, perception of sound, motion, and balance is mediated through mechanosensory hair cells located within the inner ear. In mammals, hair cells are only generated during a short period of embryonic development. As a result, loss of hair cells as a consequence of injury, disease, or genetic mutation, leads to permanent sensory deficits. At present, cochlear implantation is the only option for profound hearing loss. However, outcomes are still variable and even the best implant cannot provide the acuity of a biological ear. The recent emergence of stem cell technology has the potential to open new approaches for hair cell regeneration. The goal of this review is to summarize the current state of inner ear stem cell research from a viewpoint of its clinical application for inner ear disorders to illustrate how complementary studies have the potential to promote and refine stem cell therapies for inner ear diseases. The review initially discusses our current understanding of the genetic pathways that regulate hair cell formation from inner ear progenitors during normal development. Subsequent sections discuss the possible use of endogenous inner ear stem cells to induce repair as well as the initial studies aimed at transplanting stem cells into the ear.

High throughput gene expression analysis of the inner ear

The mouse auditory and vestibular epithelia consist of a complex array of many different cell types. Over the last decade microarrays were used to characterize gene expression in the inner ear. Studies were performed on wild type mice to identify deafness genes, transcriptional networks activated during development, or identify miRNA with a functional role in the ear. Other studies focused on the molecular response of the inner ear to stimuli ranging from ototoxic medications to hypergravity and caloric restriction. Finally, microarrays were used to identify transcriptional networks activated downstream of deafness genes. As template-free high throughput gene expression profiling methods such as RNA-seq are increasingly popular, we offer a critical review of the data generated over the last decade relating to microarrays for gene expression profiling of the inner ear. Moreover, as most of the published data is available through the gene expression omnibus (GEO), we demonstrate the feasibility of integrating data from independent experiments to reach novel insights.

Serial analysis of gene expression in the chicken otocyst

The inner ear arises from multipotent placodal precursors that are gradually committed to the otic fate and further differentiate into all inner ear cell types, with the exception of a few immigrating neural crest-derived cells. The otocyst plays a pivotal role during inner ear development: otic progenitor cells sub-compartmentalize into non-sensory and prosensory domains, giving rise to individual vestibular and auditory organs and their associated ganglia. The genes and pathways underlying this progressive subdivision and differentiation process are not entirely known. The goal of this study was to identify a comprehensive set of genes expressed in the chicken otocyst using the serial analysis of gene expression (SAGE) method. Our analysis revealed several hundred transcriptional regulators, potential signaling proteins, and receptors. We identified a substantial collection of genes that were previously known in the context of inner ear development, but we also found many new candidate genes, such as SOX4, SOX5, SOX7, SOX8, SOX11, and SOX18, which previously were not known to be expressed in the developing inner ear. Despite its limitation of not being all-inclusive, the generated otocyst SAGE library is a practical bioinformatics tool to study otocyst gene expression and to identify candidate genes for developmental studies.

An RNA interference-based screen of transcription factor genes identifies pathways necessary for sensory regeneration in the avian inner ear

Sensory hair cells of the inner ear are the mechanoelectric transducers of sound and head motion. In mammals, damage to sensory hair cells leads to hearing or balance deficits. Nonmammalian vertebrates such as birds can regenerate hair cells after injury. In a previous study, we characterized transcription factor gene expression during chicken hair cell regeneration. In those studies, a laser microbeam or ototoxic antibiotics were used to damage the sensory epithelia (SE). The current study focused on 27 genes that were upregulated in regenerating SEs compared to untreated SEs in the previous study. Those genes were knocked down by siRNA to determine their requirement for supporting cell proliferation and to measure resulting changes in the larger network of gene expression. We identified 11 genes necessary for proliferation and also identified novel interactive relationships between many of them. Defined components of the WNT, PAX, and AP1 pathways were shown to be required for supporting cell proliferation. These pathways intersect on WNT4, which is also necessary for proliferation. Among the required genes, the CCAAT enhancer binding protein, CEBPG, acts downstream of Jun Kinase and JUND in the AP1 pathway. The WNT coreceptor LRP5 acts downstream of CEBPG, as does the transcription factor BTAF1. Both of these genes are also necessary for supporting cell proliferation. This is the first large-scale screen of its type and suggests an important intersection between the AP1 pathway, the PAX pathway, and WNT signaling in the regulation of supporting cell proliferation during inner ear hair cell regeneration.

Bisphenol A induces otolith malformations during vertebrate embryogenesis

Background

The plastic monomer and plasticizer bisphenol A (BPA), used for manufacturing polycarbonate plastic and epoxy resins, is produced at over 2.5 million metric tons per year. Concerns have been raised that BPA acts as an endocrine disruptor on both developmental and reproductive processes and a large body of evidence suggests that BPA interferes with estrogen and thyroid hormone signaling. Here, we investigated BPA effects during embryonic development using the zebrafish and Xenopus models.

Results

We report that BPA exposure leads to severe malformations of the otic vesicle. In zebrafish and in Xenopus embryos, exposure to BPA during the first developmental day resulted in dose-dependent defects in otolith formation. Defects included aggregation, multiplication and occasionally failure to form otoliths. As no effects on otolith development were seen with exposure to micromolar concentrations of thyroid hormone, 17-ß-estradiol or of the estrogen receptor antagonist ICI 182,780 we conclude that the effects of BPA are independent of estrogen receptors or thyroid-hormone receptors. Na⁺/K⁺ ATPases are crucial for otolith formation in zebrafish. Pharmacological inhibition of the major Na⁺/K⁺ ATPase with ouabain can rescue the BPA-induced otolith phenotype.

Conclusions

The data suggest that the spectrum of BPA action is wider than previously expected and argue for a systematic survey of the developmental effects of this endocrine disruptor.

Gene expression analysis of forskolin treated basilar papillae identifies microRNA181a as a mediator of proliferation

Background

Auditory hair cells spontaneously regenerate following injury in birds but not mammals. A better understanding of the molecular events underlying hair cell regeneration in birds may allow for identification and eventually manipulation of relevant pathways in mammals to stimulate regeneration and restore hearing in deaf patients.

Methodology/Principal Findings

Gene expression was profiled in forskolin treated (i.e., proliferating) and quiescent control auditory epithelia of post-hatch chicks using an Affymetrix whole-genome chicken array after 24 (n = 6), 48 (n = 6), and 72 (n = 12) hours in culture. In the forskolin-treated epithelia there was significant (p<0.05; >two-fold change) upregulation of many genes thought to be relevant to cell cycle control and inner ear development. Gene set enrichment analysis was performed on the data and identified myriad microRNAs that are likely to be upregulated in the regenerating tissue, including microRNA181a (miR181a), which is known to mediate proliferation in other systems. Functional experiments showed that miR181a overexpression is sufficient to stimulate proliferation within the basilar papilla, as assayed by BrdU incorporation. Further, some of the newly produced cells express the early hair cell marker myosin VI, suggesting that miR181a transfection can result in the production of new hair cells.

Conclusions/Significance

These studies have identified a single microRNA, miR181a, that can cause proliferation in the chicken auditory epithelium with production of new hair cells.

Comparative gene expression analysis of avian embryonic facial structures reveals new candidates for human craniofacial disorders

Mammals and birds have common embryological facial structures, and appear to employ the same molecular genetic developmental toolkit. We utilized natural variation found in bird beaks to investigate what genes drive vertebrate facial morphogenesis. We employed cross-species microarrays to describe the molecular genetic signatures, developmental signaling pathways and the spectrum of transcription factor (TF) gene expression changes that differ between cranial neural crest cells in the developing beaks of ducks, quails and chickens. Surprisingly, we observed that the neural crest cells established a species-specific TF gene expression profile that predates morphological differences between the species. A total of 232 genes were differentially expressed between the three species. Twenty-two of these genes, including Fgfr2, Jagged2, Msx2, Satb2 and Tgfb3, have been previously implicated in a variety of mammalian craniofacial defects. Seventy-two of the differentially expressed genes overlap with un-cloned loci for human craniofacial disorders, suggesting that our data will provide a valuable candidate gene resource for human craniofacial genetics. The most dramatic changes between species were in the Wnt signaling pathway, including a 20-fold up-regulation of Dkk2, Fzd1 and Wnt1 in the duck compared with the other two species. We functionally validated these changes by demonstrating that spatial domains of Wnt activity differ in avian beaks, and that Wnt signals regulate Bmp pathway activity and promote regional growth in facial prominences. This study is the first of its kind, extending on previous work in Darwin's finches and provides the first large-scale insights into cross-species facial morphogenesis.

Two short children born small for gestational age with insulin-like growth factor 1 receptor haploinsufficiency illustrate the heterogeneity of its phenotype

CONTEXT

Small for gestational age (SGA)-born children comprise a heterogeneous group in which only few genetic causes have been identified.

OBJECTIVE

To determine copy number variations in 18 growth-related genes in 100 SGA children with persistent short stature.

METHODS

Copy number variations in 18 growth-related genes (SHOX, GH1, GHR, IGF1, IGF1R, IGF2, IGFBP1-6, NSD1, GRB10, STAT5B, ALS, SOCS2, and SOCS3) were determined by an "in house" multiplex ligation-dependent probe amplification kit. The deletions were further characterized by single-nucleotide polymorphism array analysis.

RESULTS

Two heterozygous de novo insulin-like growth factor 1 receptor (IGF1R) deletions were found: a deletion of the complete IGF1R gene (15q26.3, exons 1-21), including distally flanking sequences, and a deletion comprising exons 3-21, extending further into the telomeric region. In one case, serum IGF-I was low (-2.78 sd score), probably because of a coexisting growth hormone (GH) deficiency. Both children increased their height during GH treatment (1 mg/m(2) per day). Functional studies in skin fibroblast cultures demonstrated similar levels of IGF1R autophosphorylation and a reduced activation of protein kinase B/Akt upon a challenge with IGF-I in comparison with controls.

CONCLUSIONS

IGF1R haploinsufficiency was present in 2 of 100 short SGA children. GH therapy resulted in moderate catch-up growth in our patients. A review of the literature shows that small birth size, short stature, small head size, relatively high IGF-I levels, developmental delay, and micrognathia are the main predictors for an IGF1R deletion.

Molecular signaling networks that choreograph epimorphic fin regeneration in zebrafish - a mini-review

This short review provides a current synopsis of caudal fin regeneration in zebrafish with an emphasis on the molecular signaling networks that dictate epimorphic regeneration. At the outset, the fundamentals of caudal fin architecture and the stages of epimorphic regeneration are described. This is followed by a detailed look at the main networks implicated in fin regeneration, namely the Wnt, fibroblast growth factor, activin-betaA, retinoic acid and hedgehog signaling pathways. Throughout this mini-review, these molecular networks are examined through the lens of wound healing, blastema formation or regenerative outgrowth, three of the main stages of epimorphic regeneration. Next, the emerging role of noncoding RNAs as regulators of regeneration and mechanisms of regenerative termination are discussed. Finally, the implications for future research and the broader field of regenerative medicine are examined.

Cellular targets of estrogen signaling in regeneration of inner ear sensory epithelia

Estrogen signaling in auditory and vestibular sensory epithelia is a newly emerging focus propelled by the role of estrogen signaling in many other proliferative systems. Understanding the pathways with which estrogen interacts can provide a means to identify how estrogen may modulate proliferative signaling in inner ear sensory epithelia. Reviewed herein are two signaling families, EGF and TGFbeta. Both pathways are involved in regulating proliferation of supporting cells in mature vestibular sensory epithelia and have well characterized interactions with estrogen signaling in other systems. It is becoming increasingly clear that elucidating the complexity of signaling in regeneration will be necessary for development of therapeutics that can initiate regeneration and prevent progression to a pathogenic state.