Inner ear is a target for insulin signaling and insulin resistance: evidence from mice and auditory HEI-OC1 cells

Impact of physical activity on brain oxidative metabolism and intrinsic capacities in young swiss mice fed a high fat diet

Type 2 diabetes and obesity characterized by hallmarks of insulin resistance along with an imbalance in brain oxidative metabolism would impair intrinsic capacities (ICs), a new concept for assessing mental and physical functioning. Here, we explored the impact of physical activity on antioxidant responses and oxidative metabolism in discrete brain areas of HFD or standard diet (STD) fed mice but also its consequences on specific domains of ICs. 6-week-old Swiss male mice were exposed to a STD or a HFD for 16 weeks and half of the mice in each group had access to an activity wheel and the other half did not. As expected HFD mice displayed peripheral insulin resistance but also a persistent inhibition of aconitase activity in cortices revealing an increase in mitochondrial reactive oxygen species (ROS) production. Animals with access to the running wheel displayed an improvement of insulin sensitivity regardless of the diet factor whereas ROS production remained impaired. Moreover, although the access of the running wheel did not influence mitochondrial biomass, in the oxidative metabolism area, it produced a slight decrease in brain SOD1 and catalase expression notably in HFD fed mice. At the behavioural level, physical exercise produced anxiolytic/antidepressant-like responses and improved motor coordination in both STD and HFD fed mice. However, this non-pharmacological intervention failed to enhance cognitive performance. These findings paint a contrasting landscape about physical exercise as a non-pharmacological intervention for positively orienting the aging trajectory.

Betahistine prevents development of endolymphatic hydrops in a mouse model of insulin resistance and diabetes

BACKGROUND

Diabetes is associated with inner ear dysfunction. Furthermore, C57BL/6J mice fed high fat diet (HFD), a model for insulin resistance and diabetes, develop endolymphatic hydrops (EH).

AIM

Evaluate if betahistine, spironolactone (aldosterone antagonist) and empagliflozin (sodium -glucose cotransporter2 inhibitor) can prevent EH induced by HFD and explore potential mechanisms.

METHODS

C57BL/6J mice fed HFD were treated with respective drug. The size of the endolymphatic fluid compartment was measured using contrast enhanced MRI. Secondarily, mice treated with cilostamide, a phosphodiesterase3 inhibitor, to induce EH and HEI-OC1 auditory cells were used to study potential cellular mechanisms of betahistine.

RESULTS

HFD-induced EH was prevented by betahistine but not by spironolactone and empagliflozin. Betahistine induced phosphorylation of protein kinaseA substrates but did not prevent cilostamide-induced EH.

CONCLUSIONS

Betahistine prevents the development of EH in mice fed HFD, most likely not involving pathways downstream of phosphodiesterase3, an enzyme with implications for dysfunction in diabetes. The finding that spironolactone did not prevent HFD-induced EH suggests different mechanisms for EH induction/treatment since spironolactone prevents EH induced by vasopressin, as previously observed.

SIGNIFICANCE

This further demonstrates that independent mechanisms can cause hydropic inner ear diseases which suggests different therapeutic approaches and emphazises the need for personalized medicine.

Diabetes Mellitus and Hearing Loss: A Complex Relationship

Background and Objectives: Discussion is open about the relationship between diabetes (DM) and hearing loss (HL). There is a lot of evidence in the literature suggesting a causal link between these conditions, beyond being considered simple comorbidities. The difficulty in identifying populations free from confounding factors makes it difficult to reach definitive conclusions on the pathophysiological mechanisms at play. Nonetheless, there is numerous evidence that demonstrates how the population affected by DM is more affected by sensorineural HL (SNHL) and exhibit a higher prevalence of idiopathic sudden sensorineural HL (ISSNHL). Materials and Methods: Articles reporting potentially relevant information were reviewed, and the most significant results are discussed in this article. Starting from the possible mechanisms relating to auditory impairment in the diabetic condition, this article summarizes the studies on auditory evaluation in subjects with DM1 and DM2 and addresses the relationship between DM and ISSNHL. Results: DM is considered a risk factor for SNHL, although some studies have reported no relationship when the associations were adjusted for age, gender, and hypertension. Macro and microvascular insults that cause decreased blood flow, oxygen exchange, and ion transport are major complications of hypertension and DM and can have a direct effect on the sensory and support cells of the cochlea. Conclusions: Given the difficulty of carrying out studies on populations without confounding factors, new laboratory studies are strongly required to clarify which specific physiopathological mechanisms underlie the diabetic damage caused to the hearing organs and how pharmacological management may contribute to counteracting the pathophysiological effects of the diabetic condition on the auditory system.

Age-Related Hearing Loss: The Link between Inflammaging, Immunosenescence, and Gut Dysbiosis

This article provides a theoretical overview of the association between age-related hearing loss (ARHL), immune system ageing (immunosenescence), and chronic inflammation. ARHL, or presbyacusis, is the most common sensory disability that significantly reduces the quality of life and has a high economic impact. This disorder is linked to genetic risk factors but is also influenced by a lifelong cumulative effect of environmental stressors, such as noise, otological diseases, or ototoxic drugs. Age-related hearing loss and other age-related disorders share common mechanisms which often converge on low-grade chronic inflammation known as “inflammaging”. Various stimuli can sustain inflammaging, including pathogens, cell debris, nutrients, and gut microbiota. As a result of ageing, the immune system can become defective, leading to the accumulation of unresolved inflammatory processes in the body. Gut microbiota plays a central role in inflammaging because it can release inflammatory mediators and crosstalk with other organ systems. A proinflammatory gut environment associated with ageing could result in a leaky gut and the translocation of bacterial metabolites and inflammatory mediators to distant organs via the systemic circulation. Here, we postulate that inflammaging, as a result of immunosenescence and gut dysbiosis, accelerates age-related cochlear degeneration, contributing to the development of ARHL. Age-dependent gut dysbiosis was included as a hypothetical link that should receive more attention in future studies.

Non-invasive ways of administering insulin

BACKGROUND AND AIMS

Insulin is crucial in the management of diabetes. However, requires injection which itself comes with some challenges. Alternative delivery routes have been investigated that are needle-free, with enhanced absorption and bioavailability. This review presents novel non-invasive insulin administration approaches that overcome some hurdles, as well as delineating their advantages and disadvantages.

METHODS

Information was primarily gathered by employing various PubMed scholarly articles for real-world examples in addition to data extraction from supplementary manuscripts. Articles were evaluated between 1958 and 2022. An introductive approach was used to identify matters related to the concept of different ways of administering insulin.

RESULTS

Approaches aim to administer insulin in a safe, stable, and easy to use form, whether via oral, buccal, intranasal, oral inhalation, transdermal, ocular, rectal, or vaginal routes. Some have been shown to clinically improve blood glucose levels, while others are still in the investigational stage.

CONCLUSION

Many approaches have been taken in an attempt to overcome physical barriers of insulin delivery. Some of these systems discussed may reach the market in the future and assist the millions of people who currently take subcutaneous injections of insulin.

Insulin regulates Nedd4-2 via a PKB-dependent mechanism in HEI-OC1 auditory cells-crosstalks with sphingolipid and cAMP signaling

BACKGROUND

The mechanisms of association between diabetes and inner ear dysfunction are unknown, although endolymphatic hydrops may be involved. We have previously shown that insulin signaling components are expressed in human saccule and that insulin signaling takes place in HEI-OC1 auditory cells.

AIM

To explore Nedd4-2 as a target for insulin signaling.

MATERIALS AND METHODS

Effects of insulin were analyzed using western blot and confocal microscopy in HEI-OC1 auditory cells.

RESULTS

Insulin induced phosphorylation of Nedd4-2 and increased the amount of ENaC at the plasma membrane. Also, protein kinase B (PKB) and NDRG1, a substrate for SGK1 (serum and glucocorticoid stimulated kinase), were phosphorylated in response to insulin. The SGK1 inhibitor GSK650394 prevented insulin-induced phosphorylation of NRDG1, but not of PKB and Nedd4-2, whereas the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin and the PKB inhibitor MK2206 inhibited phosphorylation of all components. Ceramides prevented insulin-induced phosphorylation of PKB and NDRG1, but not of Nedd4-2. The ceramide metabolite sphingosine 1-phosphate induced phosphorylation of Nedd4-2.

CONCLUSIONS

Insulin induces phosphorylation of Nedd4-2, most likely involving PI3K/PKB signaling. Sphingosine 1-phosphate might protect Nedd4-2 against ceramide-induced insulin resistance.

SIGNIFICANCE

Insulin-mediated regulation of Nedd4-2 might impact on inner ear sodium homeostasis with implications for diabetes-induced inner ear damage.

The Link between Gut Dysbiosis Caused by a High-Fat Diet and Hearing Loss

This review aims to provide a conceptual and theoretical overview of the association between gut dysbiosis and hearing loss. Hearing loss is a global health issue; the World Health Organisation (WHO) estimates that 2.5 billion people will be living with some degree of hearing loss by 2050. The aetiology of sensorineural hearing loss (SNHL) is complex and multifactorial, arising from congenital and acquired causes. Recent evidence suggests that impaired gut health may also be a risk factor for SNHL. Inflammatory bowel disease (IBD), type 2 diabetes, diet-induced obesity (DIO), and high-fat diet (HFD) all show links to hearing loss. Previous studies have shown that a HFD can result in microangiopathy, impaired insulin signalling, and oxidative stress in the inner ear. A HFD can also induce pathological shifts in gut microbiota and affect intestinal barrier (IB) integrity, leading to a leaky gut. A leaky gut can result in chronic systemic inflammation, which may affect extraintestinal organs. Here, we postulate that changes in gut microbiota resulting from a chronic HFD and DIO may cause a systemic inflammatory response that can compromise the permeability of the blood-labyrinth barrier (BLB) in the inner ear, thus inducing cochlear inflammation and hearing deficits.

Hearing Impairment in a Mouse Model of Diabetes Is Associated with Mitochondrial Dysfunction, Synaptopathy, and Activation of the Intrinsic Apoptosis Pathway

Although previous studies continuously report an increased risk of hearing loss in diabetes patients, the impact of the disease on the inner ear remains unexplored. Herein, we examine the pathophysiology of diabetes-associated hearing impairment and cochlear synaptopathy in a mouse model of diabetes. Male B6.BKS(D)-Lepr^db/J (db/db, diabetes) and heterozygote (db/+, control) mice were assigned into each experimental group (control vs. diabetes) based on the genotype and tested for hearing sensitivity every week from 6 weeks of age. Each cochlea was collected for histological and biological assays at 14 weeks of age. The diabetic mice exerted impaired hearing and a reduction in cochlear blood flow and C-terminal-binding protein 2 (CtBP2, a presynaptic ribbon marker) expression. Ultrastructural images revealed severely damaged mitochondria from diabetic cochlea accompanied by a reduction in Cytochrome c oxidase subunit 4 (COX4) and CR6-interacting factor 1 (CRIF1). The diabetic mice presented significantly decreased levels of platelet endothelial cell adhesion molecule (PECAM-1), B-cell lymphoma 2 (BCL-2), and procaspase-9, but not procaspase-8. Importantly, significant changes were not found in necroptotic programmed cell death markers (receptor-interacting serine/threonine-protein kinase 1, RIPK1; RIPK3; and mixed lineage kinase domain-like pseudokinase, MLKL) between the groups. Taken together, diabetic hearing loss is accompanied by synaptopathy, microangiopathy, damage to the mitochondrial structure/function, and activation of the intrinsic apoptosis pathway. Our results imply that mitochondrial dysfunction is deeply involved in diabetic hearing loss, and further suggests the potential benefits of therapeutic strategies targeting mitochondria.

Translational implications of the interactions between hormones and age-related hearing loss

Provocative research has revealed both positive and negative effects of hormones on hearing as we age; with in some cases, mis-regulation of hormonal levels in instances of medical comorbidities linked to aging, lying at the heart of the problem. Animal model studies have discovered that hormonal fluctuations can sharpen hearing for improved communication and processing of mating calls during reproductive seasons. Sex hormones sometimes have positive effects on auditory processing, as is often the case with estrogen, whereas combinations of estrogen and progesterone, and testosterone, can have negative effects on hearing abilities, particularly in aging subjects. Too much or too little of some hormones can be detrimental, as is the case for aldosterone and thyroid hormones, which generally decline in older individuals. Too little insulin, as in Type 1 diabetics, or poor regulation of insulin, as in Type 2 diabetics, is also harmful to hearing in our aged population. In terms of clinical translational possibilities, hormone therapies can be problematic due to systemic side effects, as has happened for estrogen/progestin combination hormone replacement therapy (HRT) in older women, where the HRT induces a hearing loss. As hormone therapy approaches are further developed, it may be possible to lower needed doses of hormones by combining them with supplements, such as antioxidants. Another option will be to take advantage of emerging technologies for local drug delivery to the inner ear, including biodegradeable, sustained-release hydrogels and micro-pumps which can be implanted in the middle ear near the round window. In closing, exciting research completed to date, summarized in the present report bodes well for emerging biomedical therapies to prevent or treat age-related hearing loss utilizing hormonal strategies.