MRNA expression of members of the IGF system in the organ of Corti, the modiolus and the stria vascularis of newborn rats

IGF-1 Mediated Neuroprotective Effects of Olfactory-Derived Mesenchymal Stem Cells on Auditory Hair Cells

IMPORTANCE

Mesenchymal stem cells (MSCs) have the capability of providing ongoing paracrine support to degenerating tissues. Since MSCs can be extracted from a broad range of tissues, their specific surface marker profiles and growth factor secretions can be different. We hypothesized that MSCs derived from different sources might also have different neuroprotective potential.

OBJECTIVE

In this study, we extracted MSCs from rodent olfactory mucosa and compared their neuroprotective effects on auditory hair cell survival with MSCs extracted from rodent adipose tissue.

METHODS

Organ of Corti explants were dissected from 41 cochlea and incubated with olfactory mesenchymal stem cells (OMSCs) and adipose mesenchymal stem cells (AMSCs). After 72 hours, Corti explants were fixed, stained, and hair cells counted. Growth factor concentrations were determined in the supernatant and cell lysate using Enzyme-Linked Immunosorbent Assay (ELISA).

RESULTS

Co-culturing of organ of Corti explants with OMSCs resulted in a significant increase in inner and outer hair cell stereocilia survival, compared to control. Comparisons between both stem cell lines, showed that co-culturing with OMSCs resulted in superior inner and outer hair cell stereocilia survival rates over co-culturing with AMSCs. Assessment of growth factor secretions revealed that the OMSCs secrete significant amounts of insulin-like growth factor 1 (IGF-1). Co-culturing OMSCs with organ of Corti explants resulted in a 10-fold increase in IGF-1 level compared to control, and their secretion was 2 to 3 times higher compared to the AMSCs.

CONCLUSIONS

This study has shown that OMSCs may mitigate auditory hair cell stereocilia degeneration. Their neuroprotective effects may, at least partially, be ascribed to their enhanced IGF-1 secretory abilities compared to AMSCs.

IGF-1 Controls Metabolic Homeostasis and Survival in HEI-OC1 Auditory Cells through AKT and mTOR Signaling

Insulin-like growth factor 1 (IGF-1) is a trophic factor for the nervous system where it exerts pleiotropic effects, including the regulation of metabolic homeostasis. IGF-1 deficiency induces morphological alterations in the cochlea, apoptosis and hearing loss. While multiple studies have addressed the role of IGF-1 in hearing protection, its potential function in the modulation of otic metabolism remains unclear. Here, we report that "House Ear Institute-organ of Corti 1" (HEI-OC1) auditory cells express IGF-system genes that are regulated during their differentiation. Upon binding to its high-affinity receptor IGF1R, IGF-1 activates AKT and mTOR signaling to stimulate anabolism and, concomitantly, to reduce autophagic catabolism in HEI-OC1 progenitor cells. Notably, IGF-1 stimulation during HEI-OC1 differentiation to mature otic cells sustained both constructive metabolism and autophagic flux, possibly to favor cell remodeling. IGF1R engagement and downstream AKT signaling promoted HEI-OC1 cell survival by maintaining redox balance, even when cells were challenged with the ototoxic agent cisplatin. Our findings establish that IGF-1 not only serves an important function in otic metabolic homeostasis but also activates antioxidant defense mechanisms to promote hair cell survival during the stress response to insults.

Prevention of Noise-Induced Hearing Loss In Vivo: Continuous Application of Insulin-like Growth Factor 1 and Its Effect on Inner Ear Synapses, Auditory Function and Perilymph Proteins

As noise-induced hearing loss (NIHL) is a leading cause of occupational diseases, there is an urgent need for the development of preventive and therapeutic interventions. To avoid user-compliance-based problems occurring with conventional protection devices, the pharmacological prevention is currently in the focus of hearing research. Noise exposure leads to an increase in reactive oxygen species (ROS) in the cochlea. This way antioxidant agents are a promising option for pharmacological interventions. Previous animal studies reported preventive as well as therapeutic effects of Insulin-like growth factor 1 (IGF-1) in the context of NIHL. Unfortunately, in patients the time point of the noise trauma cannot always be predicted, and additive effects may occur. Therefore, continuous prevention seems to be beneficial. The present study aimed to investigate the preventive potential of continuous administration of low concentrations of IGF-1 to the inner ear in an animal model of NIHL. Guinea pigs were unilaterally implanted with an osmotic minipump. One week after surgery they received noise trauma, inducing a temporary threshold shift. Continuous IGF-1 delivery lasted for seven more days. It did not lead to significantly improved hearing thresholds compared to control animals. Quite the contrary, there is a hint for a higher noise susceptibility. Nevertheless, changes in the perilymph proteome indicate a reduced damage and better repair mechanisms through the IGF-1 treatment. Thus, future studies should investigate delivery methods enabling continuous prevention but reducing the risk of an overdosage.

Insulin-like Growth Factor 1 Signaling in Mammalian Hearing

Insulin-like growth factor 1 (IGF-1) is a peptide hormone belonging to the insulin family of proteins. Almost all of the biological effects of IGF-1 are mediated through binding to its high-affinity tyrosine kinase receptor (IGF1R), a transmembrane receptor belonging to the insulin receptor family. Factors, receptors and IGF-binding proteins form the IGF system, which has multiple roles in mammalian development, adult tissue homeostasis, and aging. Consequently, mutations in genes of the IGF system, including downstream intracellular targets, underlie multiple common pathologies and are associated with multiple rare human diseases. Here we review the contribution of the IGF system to our understanding of the molecular and genetic basis of human hearing loss by describing, (i) the expression patterns of the IGF system in the mammalian inner ear; (ii) downstream signaling of IGF-1 in the hearing organ; (iii) mouse mutations in the IGF system, including upstream regulators and downstream targets of IGF-1 that inform cochlear pathophysiology; and (iv) human mutations in these genes causing hearing loss.

[Molecular networks of hypoxia and neuronal apoptosis in the cochlea]

BACKGROUND

In organotypic cultures, the modiolus (MOD) region of newborn rats shows a fourfold higher rate of cell death than the organ of Corti (OC). The differing vulnerability of OC and MOD cells is related to differential expression of numerous genes (DEG).

MATERIALS AND METHODS

Organotypic cultures of OC and MOD of 3-5-day-old rats were exposed to a normoxic or a hypoxic (pO₂ 10-20 mmHg; 5 h) atmosphere. Cell death rate and gene expression as detected by c‑DNA microarray analysis were determined 24 h after the culture was created. Genes with modified expression (n = 60) were analyzed for biological processes according to the DAVID Gene Ontology Database (GO). Molecular networks were created using the STRING and ConsensusPathDB databases.

RESULTS

The network of the GO annotations "hypoxia", "inflammation", and "mechanical stimulus" indicates the existence of two gene clusters: a cluster with pro-inflammatory genes (Ccl3, Cxcl2, Cxcr4, Ccl20) and a cluster with hypoxia-associated genes (e.g., c-Jun, Hif1a, and Vegfa). The network of the GO annotations "positive and negative regulation of neuron apoptotic process" suggests that the differential expression of c-Jun, Ngfr, and Casp3 is important for regulation of programmed cell death in neuronal cells of the OC and MOD.

CONCLUSION

While c‑JUN acts as an important modulator of the balance between cell death and survival, the associations of NGFR and CASP3 seem to be significant for the initiation of cell death. The evaluation and application of findings from biostatistical databases is important for understanding the function of individual genes and gene clusters in medically relevant biological processes.

Characterization of Lgr5+ progenitor cell transcriptomes in the apical and basal turns of the mouse cochlea

Lgr5+ supporting cells (SCs) are enriched hair cell (HC) progenitors in the cochlea, and several studies have shown a difference in the proliferation and HC regeneration ability of SCs between the apical and basal turns. However, the detailed differences between the transcriptomes of the apical and basal Lgr5+ SCs have not yet been investigated. We found that when isolated by FACS, Lgr5+ cells from the apex generated significantly more HCs and had significantly higher proliferation and mitotic HC regeneration ability compared to those from the base. Next, we used microarray analysis to determine the transcriptome expression profiles of Lgr5+ progenitors from the apex and the base. We first analyzed the genes that were enriched and differentially expressed in Lgr5+ progenitors from the apex and the base. Then we analyzed the cell cycle genes and the transcription factors that might regulate the proliferation and differentiation of Lgr5+ progenitors. Lastly, to further analyze the role of differentially expressed genes and to gain an overall view of the gene network in cochlear HC regeneration, we created a protein-protein interaction network. Our datasets suggest the possible genes that might regulate the proliferation and HC regeneration ability of Lgr5+ progenitors, and these genes might provide new therapeutic targets for HC regeneration in the future.

The stat3/socs3a pathway is a key regulator of hair cell regeneration in zebrafish. [corrected]

All nonmammalian vertebrates studied can regenerate inner ear mechanosensory receptors (i.e., hair cells) (Corwin and Cotanche, 1988; Lombarte et al., 1993; Baird et al., 1996), but mammals possess only a very limited capacity for regeneration after birth (Roberson and Rubel, 1994). As a result, mammals experience permanent deficiencies in hearing and balance once their inner ear hair cells are lost. The mechanisms of hair cell regeneration are poorly understood. Because the inner ear sensory epithelium is highly conserved in all vertebrates (Fritzsch et al., 2007), we chose to study hair cell regeneration mechanism in adult zebrafish, hoping the results would be transferrable to inducing hair cell regeneration in mammals. We defined the comprehensive network of genes involved in hair cell regeneration in the inner ear of adult zebrafish with the powerful transcriptional profiling technique digital gene expression, which leverages the power of next-generation sequencing ('t Hoen et al., 2008). We also identified a key pathway, stat3/socs3, and demonstrated its role in promoting hair cell regeneration through stem cell activation, cell division, and differentiation. In addition, transient pharmacological inhibition of stat3 signaling accelerated hair cell regeneration without overproducing cells. Taking other published datasets into account (Sano et al., 1999; Schebesta et al., 2006; Dierssen et al., 2008; Riehle et al., 2008; Zhu et al., 2008; Qin et al., 2009), we propose that the stat3/socs3 pathway is a key response in all tissue regeneration and thus an important therapeutic target for a broad application in tissue repair and injury healing.

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.

The role of insulin-like growth factor-I in the physiopathology of hearing

Insulin-like growth factor-I (IGF-I) belongs to the family of polypeptides of insulin, which play a central role in embryonic development and adult nervous system homeostasis by endocrine, autocrine, and paracrine mechanisms. IGF-I is fundamental for the regulation of cochlear development, growth, and differentiation, and its mutations are associated with hearing loss in mice and men. Low levels of IGF-I have been shown to correlate with different human syndromes showing hearing loss and with presbyacusis. Animal models are fundamental to understand the genetic, epigenetic, and environmental factors that contribute to human hearing loss. In the mouse, IGF-I serum levels decrease with aging and there is a concomitant hearing loss and retinal degeneration. In the Igf1(-/-) null mouse, hearing loss is due to neuronal loss, poor innervation of the sensory hair cells, and age-related stria vascularis alterations. In the inner ear, IGF-I actions are mediated by intracellular signaling networks, RAF, AKT, and p38 MAPK protein kinases modulate the expression and activity of transcription factors, as AP1, MEF2, FoxM1, and FoxP3, leading to the regulation of cell cycle and metabolism. Therapy with rhIGF-I has been approved in humans for the treatment of poor linear growth and certain neurodegenerative diseases. This review will discuss these findings and their implications in new IGF-I-based treatments for the protection or repair of hearing loss.

Decrease of prestin expression by increased potassium concentration in organotypic cultures of the organ of Corti of newborn rats

Prestin is the motor protein of the outer hair cells of the organ of Corti and a key factor in ensuring a high sensitivity level of mammalian hearing. In the present study, we examined the effects of increased extracellular potassium (K(+)) concentration on the expression of prestin mRNA and the transcription factors Gata-3 and Carf in the organotypic culture of the organ of Corti of newborn rats. Mannitol and NaCl were used to analyze possible effects of hyperosmotic stress or ion-specific changes, respectively. An increase in prestin expression by a factor of 1.5-2.0 was seen in cultures grown in the presence of 5mM K(+). Potassium concentration of 35 and 55 mM induced a parallel decrease in prestin and Carf expression, but Gata-3 expression increased. Mannitol had no effect on gene expression whereas increased NaCl concentrations decreased prestin, but not Carf expression. The data suggest that chronic depolarization might decrease the prestin expression and possibly contribute to hearing loss and tinnitus.

Expression of genes implicated in oxidative stress in the cochlea of newborn rats

Oxidative stress is an important mechanism inducing ototoxicity-, age- and noise-induced hearing loss. To better understand this phenomenon, we examined cochlear tissues for the expression of following genes involved directly or indirectly in the oxidative stress response: glyceraldehyde-3-phosphate dehydrogenase (Gapdh); solute carrier family-2 (facilitated glucose transporter), member-1 (Slc2a1); heme oxygenase-1 (Hmox1); heme oxygenase-2 (Hmox2); inducible nitric oxide synthase-2 (Nos2); transferrin (Tf); transferrin receptor (Tfrc); glutathione S-transferase A3 (Gsta3) and metallothionein-1a (Mt1a). Cochlear tissues were dissected from the p3-p5 Wistar rats, divided into the organ of Corti (OC), modiolus (MOD) and stria vascularis together with spiral ligament (SV + SL) and processed immediately or cultured under normoxic conditions or a short-term, mild hypoxia followed by re-oxygenation. After 24 h, explants were collected and total RNA isolated, transcribed and amplified in the real time RT-PCR. We found all genes listed above expressed in the freshly isolated cochlear tissues. In the OC and MOD, Slc2a1, Tf, and Mt1a were expressed on a lower level than in the SV + SL. In the OC, Hmox1 was expressed on a lower level than in the MOD and SV + SL. Hypoxic and normoxic cultures increased the transcript number of Gapdh, Slc2a1 and Hmox1 in all cochlear tissues. The expression of Nos2, Tf, Gsta3 and Mt1a increased in a tissue-specific manner. In the SV + SL, Mt1a expression decreased after normoxic and hypoxic conditions. Taken together, using real time RT-PCR, our results imply that oxidative stress may be an important component of cochlear injury during the developing period. In spite of the immaturity of the tissue, a differential response of antioxidant enzymes/proteins with respect to the pathway, the expression levels and regions was observed.