Netrin 1 mediates protective effects exerted by insulin-like growth factor 1 on cochlear hair cells

Netrin-1 co-cross-linked hydrogel accelerates diabetic wound healing in situ by modulating macrophage heterogeneity and promoting angiogenesis

Diabetic wounds, characterized by prolonged inflammation and impaired vascularization, are a serious complication of diabetes. This study aimed to design a gelatin methacrylate (GelMA) hydrogel for the sustained release of netrin-1 and evaluate its potential as a scaffold to promote diabetic wound healing. The results showed that netrin-1 was highly expressed during the inflammation and proliferation phases of normal wounds, whereas it synchronously exhibited aberrantly low expression in diabetic wounds. Neutralization of netrin-1 inhibited normal wound healing, and the topical application of netrin-1 accelerated diabetic wound healing. Mechanistic studies demonstrated that netrin-1 regulated macrophage heterogeneity via the A2bR/STAT/PPARγ signaling pathway and promoted the function of endothelial cells, thus accelerating diabetic wound healing. These data suggest that netrin-1 is a potential therapeutic target for diabetic wounds.

Signaling pathways regulating the immune function of cochlear supporting cells and their involvement in cochlear pathophysiology

The inner ear, including the cochlea, used to be regarded as an immune-privileged site because of its immunologically isolated environment caused by the blood-labyrinthine barrier. Cochlear resident macrophages, which originate from the yolk sac or fetal liver during the embryonic stage and are maintained after birth, are distributed throughout various regions of the cochlear duct. Intriguingly, these cells are absent in the organ of Corti, where hair cells (HCs) and supporting cells (SCs) are located, except for a limited number of ionized calcium-binding adapter molecule 1 (Iba1)-positive cells. Instead, SCs exert glial functions varying from a quiescent to an emergency state. Notably, SCs acquire the nature of macrophages and begin to secrete inflammatory cytokines during viral infection in the organ of Corti, which is ostensibly unprotected owing to the lack of general resident macrophages. This review provides an overview of both positive and negative functions of SCs enabled to acquire macrophage phenotypes upon viral infection focusing on the signaling pathways that regulate these functions. The former function protects HCs from viral infection by inducting type I interferons, and the latter function induces HC death by necroptosis, leading to sensorineural hearing loss. Thus, SCs play contradictory roles as immune cells with acquired macrophage phenotypes; thereby, they are favorable and unfavorable to HCs, which play a pivotal role in hearing function.

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.

Ultrasound Microbubbles Enhance the Efficacy of Insulin-Like Growth Factor-1 Therapy for the Treatment of Noise-Induced Hearing Loss

The application of insulin-like growth factor 1 (IGF-1) to the round window membrane (RWM) is an emerging treatment for inner ear diseases. RWM permeability is the key factor for efficient IGF-1 delivery. Ultrasound microbubbles (USMBs) can increase drug permeation through the RWM. In the present study, the enhancing effect of USMBs on the efficacy of IGF-1 application and the treatment effect of USMB-mediated IGF-1 delivery for noise-induced hearing loss (NIHL) were investigated. Forty-seven guinea pigs were assigned to three groups: the USM group, which received local application of recombinant human IGF-1 (rhIGF-1, 10 µg/µL) following application of USMBs to the RWM; the RWS group, which received IGF-1 application alone; and the saline-treated group. The perilymphatic concentration of rhIGF-1 in the USM group was 1.95- and 1.67- fold of that in the RWS group, 2 and 24 h after treatment, respectively. After 5 h of 118 dB SPL noise exposure, the USM group had the lowest threshold shift in auditory brainstem response, least loss of cochlear outer hair cells, and least reduction in the number of synaptic ribbons on postexposure day 28 among the three groups. The combination of USMB and IGF-1 led to a better therapeutic response to NIHL. Two hours after treatment, the USM group had significantly higher levels of Akt1 and Mapk3 gene expression than the other two groups. The most intense immunostaining for phosphor-AKT and phospho-ERK1/2 was detected in the cochlea in the USM group. These results suggested that USMB can be applied to enhance the efficacy of IGF-1 therapy in the treatment of inner ear diseases.

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.

Insulin-Like Growth Factor 1 on the Maintenance of Ribbon Synapses in Mouse Cochlear Explant Cultures

Hearing loss has become one of the most common disabilities worldwide. The synaptic connections between inner hair cells (IHCs) and spiral ganglion neurons have specialized synaptic constructions, termed ribbon synapses, which are important for auditory function. The ribbon synapses in the cochlea are quite vulnerable to various insults. As such, the maintenance of ribbon synapses is important for ensuring hearing function. Insulin-like growth factor 1 (IGF1) plays a critical role in the development and maintenance of the cochlea and has the potential to protect cochlear hair cells from various insults. In this study, we examined the role of IGF1 in the maintenance of ribbon synapses in cochlear explants of postnatal day four mice. We cultured cochlear explants with an IGF1 receptor antagonist, JB1, which is an IGF1 peptide analog. Results showed that exposure to JB1 for 24 h resulted in the loss of ribbon synapses. After an additional 24-h culture without JB1, the number of ribbon synapses spontaneously recovered. The application of exogenous IGF1 showed two different aspects of ribbon synapses. Low doses of exogenous IGF1 promoted the recovery of ribbon synapses, while it compromised the spontaneous recovery of ribbon synapses at high doses. Altogether, these results indicate that the paracrine or autocrine release of IGF1 in the cochlea plays a crucial role in the maintenance of cochlear ribbon synapses.

[New progress of IGF-1 and allosteroid injection in the treatment of sudden deafness complicated with type 2 diabetes]

Glucocorticoids are a recognized treatment for sudden deafness, and there has always been a contradiction between the control of blood glucose levels and the use of glucocorticoids. The systemic use of hormones may lead to a series of adverse events, which are dose-dependent. High doses can induce an increase in blood sugar, especially for patients with type 2 diabetes, which can aggravate their condition or cause complications. The systemic application of glucocorticoids has been largely replaced by local glucocorticoids treatment. Topical insulin-like growth factor （IGF-1） is used without increasing blood sugar, thus avoiding the possible complications. The author intends to compare the local IGF-1 treatment and local glucocorticoid treatment to systemic therapy. The efficacy of local IGF-1 therapy in treating corticosteroid-refractory sudden sensorineural hearing loss combined with type 2 diabetes is reviewed.

Overexpressed CES2 has prognostic value in CRC and knockdown CES2 reverses L-OHP-resistance in CRC cells by inhibition of the PI3K signaling pathway

CES-2 (carboxylesterase-2) belongs to the carboxylesterase gene family, which plays crucial roles in lipid mobilization and chemosensitivity to irinotecan. However, its role in chemosensitivity to oxaliplatin (L-OHP) remains unclear. Herein, L-OHP-resistant cells (HCT-116L and RKOL) were established by increasing the concentration of L-OHP. The results showed that CES2 expression was upregulated in L-OHP-resistant tissues and cells lines (both P < 0.01). Low expression of CES2 correlated with a better survival, and the results were further confirmed in the R2 platform: a biologist friendly web-based genomics analysis and visualization application. Downregulation of CES2 suppressed cell proliferation, induced apoptosis and reversed L-OHP resistance by medicating the PI3K signaling pathway in L-OHP-resistant cells. However, both PI3K inhibitor (LY294002) and activator (IGF-1) could not medicate CES2 expression. These findings indicated that CES2 may be utilized as a novel biomarker and therapeutic target for L-OHP resistance in CRC treatment.

The functional variant of NTN1 contributes to the risk of nonsyndromic cleft lip with or without cleft palate

Previous genome-wide association study of nonsyndromic cleft lip with or without cleft palate (NSCL/P) identified a susceptible variant (rs4791774). We hypothesized that the functional single nucleotide polymorphism (SNP) may be in linkage disequilibrium with this lead SNP. The potential functional SNP (rs4791331) was identified by bioinformatic analysis. A case–control study with 891 orofacial cleft cases and 830 controls was designed to investigate its association with orofacial cleft. The allele-specific DNA-protein binding preference was predicted by JASPAR database. Cell proliferation, cycle and apoptosis, luciferase activity and netrin-1 (NTN1) expression were examined after transfection with the rs4791331 C/T vector in HEK-293 and HEPM cell lines. Forty-six lip tissues of NSCL/P patients were collected to detect NTN1 expression. ntn1a knockout zebrafish models were generated by CRISPR/Cas9 and observed with micro-CT. In the case–control study, the rs4791331-T allele was associated with an increased risk of nonsyndromic orofacial cleft (OR = 1.41, 95% CI = 1.19–1.68), as well as the subgroups cleft lip only (OR = 1.46, 95% CI = 1.14–1.87) and cleft lip and palate (OR = 1.58, 95% CI = 1.27–1.96). The T allele of rs4791331 exhibited anti-apoptotic effects and promoted cell cycle progression at the G1/S transition. Decreased enhancer activity and reduced NTN1 expression following transfection of the T allele were observed. Carriers of the CT/TT genotypes showed significantly lower expression of NTN1 than CC carriers. The ntn1a^−/− zebrafish showed relatively wider intermaxillary fissures. These results indicate that rs4791331 (C > T) disrupted motif binding and led to abnormal expression of NTN1, which may be involved in the development of NSCL/P.

Mutations in TOP2B cause autosomal-dominant hereditary hearing loss via inhibition of the PI3K-Akt signalling pathway

Hereditary hearing impairment is a clinically and genetically heterogeneous disease. Whole-exome sequencing was performed on seven affected and six unaffected members in a large Chinese family with autosomal-dominant nonsyndromic hearing loss. The pathogenic variant of the gene encoding human topoisomerase IIβ TOP2B (c.G4837C:p.D1613H) was cosegregated with hearing loss in this pedigree and another two variants of TOP2B were detected in 66 sporadic patients with hearing loss. top2b knockdown led to significant defects in zebrafish inner ears and caused downregulation of akt which resulted in inactivation of PI3K-Akt signalling. As a result, supporting cell and hair cell numbers were reduced through inhibition of the PI3K-Akt pathway. Therefore, we hypothesized that mutations in TOP2B can cause autosomal-dominant nonsyndromic hearing impairment through inhibition of the PI3K-Akt signalling pathway. DATABASE: The whole-exome sequence data in the study are available at the Sequence Read Archive database (NCBI) under the accession numbers SRR9050868, SRR9050867, SRR90508676, SRR90508675, SRR90508674, SRR90508673, SRR90508672, SRR90508671, SRR90508679, SRR90508670, SRR9050859. SRR9050858 and SRR9050857, respectively.

Pregnancy-associated plasma protein-aa supports hair cell survival by regulating mitochondrial function

To support cell survival, mitochondria must balance energy production with oxidative stress. Inner ear hair cells are particularly vulnerable to oxidative stress; thus require tight mitochondrial regulation. We identified a novel molecular regulator of the hair cells’ mitochondria and survival: Pregnancy-associated plasma protein-aa (Pappaa). Hair cells in zebrafish pappaa mutants exhibit mitochondrial defects, including elevated mitochondrial calcium, transmembrane potential, and reactive oxygen species (ROS) production and reduced antioxidant expression. In pappaa mutants, hair cell death is enhanced by stimulation of mitochondrial calcium or ROS production and suppressed by a mitochondrial ROS scavenger. As a secreted metalloprotease, Pappaa stimulates extracellular insulin-like growth factor 1 (IGF1) bioavailability. We found that the pappaa mutants’ enhanced hair cell loss can be suppressed by stimulation of IGF1 availability and that Pappaa-IGF1 signaling acts post-developmentally to support hair cell survival. These results reveal Pappaa as an extracellular regulator of hair cell survival and essential mitochondrial function.

Advances in understanding of presbycusis

The pathophysiology of age-related hearing loss (ARHL), or presbycusis, involves a complex interplay between environmental and genetic factors. The fundamental biomolecular mechanisms of ARHL have been well described, including the roles of membrane transport, reactive oxygen species, cochlear synaptopathy, vascular insults, hormones, and microRNA, to name a few. The genetic basis underlying these mechanisms remains under-investigated and poorly understood. The emergence of genome-wide association studies has allowed for the identification of specific groups of genes involved in ARHL. This review highlights recent advances in understanding of the pathogenesis of ARHL, the genetic basis underlying these processes and suggests future directions for research and potential therapeutic avenues.

Insulin-like growth factor 1 promotes cochlear synapse regeneration after excitotoxic trauma in vitro

In the context of acquired sensorineural hearing loss (SNHL), cochlear hair cells have long been thought to be among the most vulnerable elements in mammalian cochleae. However, recent studies have indicated that the synaptic connection between inner hair cells (IHC) and spiral ganglion neurons (SGN) can be an important target for the treatment of SNHL. Our previous studies in patients with sudden SNHL demonstrated delayed and gradual hearing recovery following topical application of insulin-like growth factor 1 (IGF-1), suggesting that not only protective but also regenerative mechanisms may account for hearing recovery after treatment with IGF-1. We then hypothesized that IGF-1 has the potential to drive the regeneration of IHC-SGN synapses. To test this hypothesis, we investigated the effects of IGF-1 on IHC-SGN synapses using cochlear explant cultures from postnatal day 2 mice that had been damaged by exposure to the excitatory amino acids N-methyl-d-aspartate and kainate. Cochlear explants that lost IHC-SGN synapses upon exposure to excitatory amino acids were cultured with exogenous IGF-1 for an additional 48 h. We observed increased numbers of IHC-SGN synapses after exogenous IGF-1 application. Pharmacological inhibition of the IGF-1 receptor attenuated the restoration of IHC-SGN synapses by exogenous IGF-1. These findings indicated that IGF-1 induces regeneration of IHC-SGN synapses in cochlear explant cultures from postnatal day 2 mice. Therefore, in a future study we will perform in vivo experiments using adult mice to ascertain the effects of IGF-1 on the regeneration of IHC-SGN synapses.

The Role of Insulin-Like Growth Factor 1 in the Progression of Age-Related Hearing Loss

Aging is associated with impairment of sensorial functions and with the onset of neurodegenerative diseases. As pari passu circulating insulin-like growth factor 1 (IGF-1) bioavailability progressively decreases, we see a direct correlation with sensory impairment and cognitive performance in older humans. Age-related sensory loss is typically caused by the irreversible death of highly differentiated neurons and sensory receptor cells. Among sensory deficits, age-related hearing loss (ARHL), also named presbycusis, affects one third of the population over 65 years of age and is a major factor in the progression of cognitive problems in the elderly. The genetic and molecular bases of ARHL are largely unknown and only a few genes related to susceptibility to oxidative stress, excitotoxicity, and cell death have been identified. IGF-1 is known to be a neuroprotective agent that maintains cellular metabolism, activates growth, proliferation and differentiation, and limits cell death. Inborn IGF-1 deficiency leads to profound sensorineural hearing loss both in humans and mice. IGF-1 haploinsufficiency has also been shown to correlate with ARHL. There is not much information available on the effect of IGF-1 deficiency on other human sensory systems, but experimental models show a long-term impact on the retina. A secondary action of IGF-1 is the control of oxidative stress and inflammation, thus helping to resolve damage situations, acute or made chronic by aging. Here we will review the primary actions of IGF-1 in the auditory system and the underlying molecular mechanisms.

An update on drug design strategies to prevent acquired sensorineural hearing loss

INTRODUCTION

Acute sensorineural hearing loss is a dramatic event for the patient. Different pathologies might result in acute sensorineural hearing loss, such as sudden hearing loss, exposure to medications/drugs or loud sound. Current therapeutic approaches include steroids and hyperbaric oxygen in addition to other methods. Research activities of the past have shed light on the molecular mechanisms involved in damage to hair cells, the synapses at the hair cell spiral ganglion junction and the stria vascularis. Molecular events and signaling pathways which underlie damage to these structures have been discovered. Areas covered: This paper summarizes current research efforts involved in investigating the molecular mechanisms involved in acute sensorineural hearing loss. Expert opinion: While progress has been made in unraveling basic mechanisms involved in acute sensorineural hearing loss, it is difficult to translate basic concepts to the clinic. There are often conflicting data in animal and human studies on the effect of a given intervention. There is also a lack of high quality clinical trials (double blind, placebo controlled and high powered). However, this author is confident that research efforts will pay out and that some of these efforts will translate into new therapeutic options for patients with acute hearing loss.

Resolution of Cochlear Inflammation: Novel Target for Preventing or Ameliorating Drug-, Noise- and Age-related Hearing Loss

A significant number of studies support the idea that inflammatory responses are intimately associated with drug-, noise- and age-related hearing loss (DRHL, NRHL and ARHL). Consequently, several clinical strategies aimed at reducing auditory dysfunction by preventing inflammation are currently under intense scrutiny. Inflammation, however, is a normal adaptive response aimed at restoring tissue functionality and homeostasis after infection, tissue injury and even stress under sterile conditions, and suppressing it could have unintended negative consequences. Therefore, an appropriate approach to prevent or ameliorate DRHL, NRHL and ARHL should involve improving the resolution of the inflammatory process in the cochlea rather than inhibiting this phenomenon. The resolution of inflammation is not a passive response but rather an active, highly controlled and coordinated process. Inflammation by itself produces specialized pro-resolving mediators with critical functions, including essential fatty acid derivatives (lipoxins, resolvins, protectins and maresins), proteins and peptides such as annexin A1 and galectins, purines (adenosine), gaseous mediators (NO, H₂S and CO), as well as neuromodulators like acetylcholine and netrin-1. In this review article, we describe recent advances in the understanding of the resolution phase of inflammation and highlight therapeutic strategies that might be useful in preventing inflammation-induced cochlear damage. In particular, we emphasize beneficial approaches that have been tested in pre-clinical models of inflammatory responses induced by recognized ototoxic drugs such as cisplatin and aminoglycoside antibiotics. Since these studies suggest that improving the resolution process could be useful for the prevention of inflammation-associated diseases in humans, we discuss the potential application of similar strategies to prevent or mitigate DRHL, NRHL and ARHL.