Betaine-homocysteine S-methyltransferases (BHMTs) are methionine cycle enzymes that remethylate homocysteine; hence, their malfunction leads to hyperhomocysteinemia. Epidemiologic and experimental studies have revealed a correlation between hyperhomocysteinemia and hearing loss. Here, we have studied the expression of methionine cycle genes in the mouse cochlea and the impact of knocking out the Bhmt gene in the auditory receptor. We evaluated age-related changes in mouse hearing by recording auditory brainstem responses before and following exposure to noise. Also, we measured cochlear cytoarchitecture, gene expression by RNA-arrays and quantitative RT-PCR, and metabolite levels in liver and plasma by HPLC. Our results indicate that there is an age-dependent strain-specific expression of methionine cycle genes in the mouse cochlea and a further regulation during the response to noise damage. Loss of Bhmt did not cause an evident impact in the hearing acuity of young mice, but it produced higher threshold shifts and poorer recovery following noise challenge. Hearing loss was associated with increased cochlear injury, outer hair cell loss, altered expression of cochlear methionine cycle genes, and hyperhomocysteinemia. Our results suggest that BHMT plays a central role in the homeostasis of cochlear methionine metabolism and that Bhmt2 up-regulation could carry out a compensatory role in cochlear protection against noise injury in the absence of BHMT.—Partearroyo, T., Murillo-Cuesta, S., Vallecillo, N., Bermúdez-Muñoz, J. M., Rodríguez-de la Rosa, L., Mandruzzato, G., Celaya, A. M., Zeisel, S. H., Pajares, M. A., Varela-Moreiras, G., Varela-Nieto, I. Betaine-homocysteine S-methyltransferase deficiency causes increased susceptibility to noise-induced hearing loss associated to plasma hyperhomocysteinemia., This work was supported by the Spanish Ministerio de Economía y Competitividad (MINECO)/
FEDER SAF2017-86107-R to I.V.-N., U.S. National Institutes of Health (NIH), National Institute of Diabetes and Digestive and Kidney Diseases (Grant DK056350 to S.H.Z.), CEU-Banco Santander precompetitive project (MUSPB047) and CEUBanco Santander consolidation project (MBS18C12) to T.P., and Puleva BioFoods (to I.V.-N., G.V.-M., and M.A.P.). G.M. was supported by TARGEAR (FP7 PEOPLE 2013 IAPP612261)., Peer reviewed

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., This work has been supported by FP7-IAPP2013-TARGEAR to IVN, Spanish MINECO/FEDER SAF2017-86107-R to IVN and MM and SAF2015-65960-P to IP., Peer reviewed

Embryonic development requires the coordinated regulation of apoptosis, survival, autophagy, proliferation and differentiation programs. Senescence has recently joined the cellular processes required to master development, in addition to its well-described roles in cancer and ageing. Here, we show that senescent cells are present in a highly regulated temporal pattern in the developing vertebrate inner ear, first, surrounding the otic pore and, later, in the otocyst at the endolymphatic duct. Cellular senescence is associated with areas of increased apoptosis and reduced proliferation consistent with the induction of the process when the endolymphatic duct is being formed. Modulation of senescence disrupts otic vesicle morphology. Transforming growth factor beta (TGFβ) signaling interacts with signaling pathways elicited by insulin-like growth factor type 1 (IGF-1) to jointly coordinate cellular dynamics required for morphogenesis and differentiation. Taken together, these results show that senescence is a natural occurring process essential for early inner ear development., This work was supported by the European Commission FP7-PEOPLE-2013-IAPP TARGEAR to IVN and by the Spanish MINECO/FEDER SAF2017-86107-R to IVN and MM. SP was supported by a FPI predoctoral fellowship (BES-2015-071311) funded by MINECO/European Social Fund., Peer reviewed

Mitogen-activated protein kinases (MAPK) such as p38 and the c-Jun N-terminal kinases (JNKs) are activated during the cellular response to stress signals. Their activity is regulated by the MAPK-phosphatase 1 (DUSP1), a key component of the anti-inflammatory response. Stress kinases are well-described elements of the response to otic injury and the otoprotective potential of JNK inhibitors is being tested in clinical trials. By contrast, there are no studies exploring the role of DUSP1 in hearing and hearing loss. Here we show that Dusp1 expression is age-regulated in the mouse cochlea. Dusp1 gene knock-out caused premature progressive hearing loss, as confirmed by auditory evoked responses in Dusp1-/- mice. Hearing loss correlated with cell death in hair cells, degeneration of spiral neurons and increased macrophage infiltration. Dusp1-/- mouse cochleae showed imbalanced redox status and dysregulated expression of cytokines. These data suggest that DUSP1 is essential for cochlear homeostasis in the response to stress during ageing., This work has been supported by Spanish MINECO/FEDER SAF2017-86107-R to IVN and P17-01401 (Fondo de Investigaciones Sanitarias, Instituto de Salud Carlos III, Spain) supported by FEDER funds to RP and IS-P. SM, and LR hold CIBER ISCIII researcher contracts. AMC was supported by contracts from FP7-PEOPLE-2013-IAPP TARGEAR and FEDER/CIBERER., Peer reviewed

© 2019 by the authors., Cisplatin is a chemotherapeutic agent that causes the irreversible death of auditory sensory cells, leading to hearing loss. Local administration of cytoprotective drugs is a potentially better option co-therapy for cisplatin, but there are strong limitations due to the difficulty of accessing the inner ear. The use of nanocarriers for the efficient delivery of drugs to auditory cells is a novel approach for this problem. Solid lipid nanoparticles (SLNs) are biodegradable and biocompatible nanocarriers with low solubility in aqueous media. We show here that stearic acid-based SLNs have the adequate particle size, polydispersity index and ζ-potential, to be considered optimal nanocarriers for drug delivery. Stearic acid-based SLNs were loaded with the fluorescent probe rhodamine to show that they are efficiently incorporated by auditory HEI-OC1 (House Ear Institute-Organ of Corti 1) cells. SLNs were not ototoxic over a wide dose range. Glucocorticoids are used to decrease cisplatin-induced ototoxicity. Therefore, to test SLNs’ drug delivery efficiency, dexamethasone and hydrocortisone were tested either alone or loaded into SLNs and tested in a cisplatin-induced ototoxicity in vitro assay. Our results indicate that the encapsulation in SLNs increases the protective effect of low doses of hydrocortisone and lengthens the survival of HEI-OC1 cells treated with cisplatin., B.C. was supported by an MSC postdoctoral contract, FP7-PEOPLE2013-IAPP TARGEAR. This work was supported by grants MINECO/FEDER (SAF2017-86107-R) and Comunidad de Madrid/FEDER MULTITARGET&VIEW-CM (B2017/BMD-3688) to I.V.N. and ELKARTEK (2017 KK-2017/00008) to I.A., Peer reviewed

© 2019 Lassaletta, Calvino, Morales-Puebla, Lapunzina, Rodriguez-de la Rosa, Varela-Nieto and Martinez-Glez., Vestibular schwannomas (VSs) are benign tumors composed of differentiated neoplastic Schwann cells. They can be classified into two groups: sporadic VS and those associated with neurofibromatosis type 2 (NF2). VSs usually grow slowly, initially causing unilateral sensorineural hearing loss (HL) and tinnitus. These tumors cause HL both due to compression of the auditory nerve or the labyrinthine artery and due to the secretion of different substances potentially toxic to the inner ear or the cochlear nerve. As more and more patients are diagnosed and need to be managed, we are more than ever in need of searching for biomarkers associated with these tumors. Owing to an unknown toxic substance generated by the tumor, HL in VS may be linked to a high protein amount of perilymph. Previous studies have identified perilymph proteins correlated with tumor-associated HL, including μ-Crystallin (CRYM), low density lipoprotein receptor-related protein 2 (LRP2), immunoglobulin (Ig) γ-4 chain C region, Ig κ-chain C region, complement C3, and immunoglobulin heavy constant γ 3. Besides, the presence of specific subtypes of heat shock protein 70 has been suggested to be associated with preservation of residual hearing. It has been recently demonstrated that chemokine receptor-4 (CXCR4) is overexpressed in sporadic VS as well as in NF2 tumors and that hearing disability and CXCR4 expression may be correlated. Further, the genetic profile of VS and its relationship with poor hearing has also been studied, including DNA methylation, deregulated genes, growth factors, and NF2 gene mutations. The knowledge of biomarkers associated with VS would be of significant value to maximize outcomes of hearing preservation in these patients., This work was supported by a FEDER/Ministerio de Economía y Competitividad grant (SAF2017-86107-R) and by a Comunidad Autónoma de Madrid grant (B2017/BMD-3688).

Cellular senescence has classically been associated with aging. Intriguingly, recent studies have also unravelled key roles for senescence in embryonic development, regeneration, and reprogramming. Developmental senescence has been reported during embryonic development in different organisms and structures, such as the endolymphatic duct during inner ear development of mammals and birds. However, there is no study addressing the possible role of senescence on otic neurogenesis. TGFβ/SMAD is the best-known pathway linked to induction of developmental programmed cell senescence. Here, we studied if TGFβ2 induces cellular senescence during acoustic-vestibular-ganglion (AVG) formation. Using organotypic cultures of AVG, and characterizing different stages of otic neurogenesis in the presence of TGFβ2 and a selective TGF-β receptor type-I inhibitor, we show that TGFβ2 exerts a powerful action in inner ear neurogenesis but, contrary to what we recently observed during endolymphatic duct development, these actions are independent of cellular senescence. We show that TGFβ2 reduces proliferation, and induces differentiation and neuritogenesis of neuroblasts, without altering cell death. Our studies highlight the roles of TGFβ2 and cellular senescence in the precise regulation of cell fate within the developing inner ear and its different cell types, being their mechanisms of action highly cell-type dependent., This work has been supported by Spanish MINECO/FEDER SAF2017-86107-R to IV-N and MM., Peer reviewed

[Introduction]: Excessive exposure to noise is a common occurrence that contributes to approximately 50% of the non-genetic hearing loss cases. Researchers need to develop standardized preclinical models and identify molecular targets to effectively develop prevention and curative therapies., [Areas covered]: In this review, the authors discuss the many facets of human noise-induced pathology, and the primary experimental models for studying the basic mechanisms of noise-induced damage, making connections and inferences among basic science studies, preclinical proofs of concept and clinical trials., [Expert opinion]: Whilst experimental research in animal models has helped to unravel the mechanisms of noise-induced hearing loss, there are often methodological variations and conflicting results between animal and human studies which make it difficult to integrate data and translate basic outcomes to clinical practice. Standardization of exposure paradigms and application of -omic technologies will contribute to improving the effectiveness of transferring newly gained knowledge to clinical practice., This work was supported by the grants Multi Target and View FEDER/CM-B2017/BMD-3688 from the Consejería de Educación e Investigación, Comunidad de Madrid and MINECO/FEDER SAF2017-86107-R from the Ministerio de Economía, Industria y Competitividad, Gobierno de España., Peer reviewed

Aging of the auditory system is associated with the incremental production of reactive oxygen species (ROS) and the accumulation of oxidative damage in macromolecules, which contributes to cellular malfunction, compromises cell viability, and, ultimately, leads to functional decline. Cellular detoxification relies in part on the production of NADPH, which is an important cofactor for major cellular antioxidant systems. NADPH is produced principally by the housekeeping enzyme glucose‐6‐phosphate dehydrogenase (G6PD), which catalyzes the rate‐limiting step in the pentose phosphate pathway. We show here that G6PD transgenic mice (G6PD‐Tg), which show enhanced constitutive G6PD activity and NADPH production along life, have lower auditory thresholds than wild‐type mice during aging, together with preserved inner hair cell (IHC) and outer hair cell (OHC), OHC innervation, and a conserved number of synapses per IHC. Gene expression of antioxidant enzymes was higher in 3‐month‐old G6PD‐Tg mice than in wild‐type counterparts, whereas the levels of pro‐apoptotic proteins were lower. Consequently, nitration of proteins, mitochondrial damage, and TUNEL+ apoptotic cells were all lower in 9‐month‐old G6PD‐Tg than in wild‐type counterparts. Unexpectedly, G6PD overexpression triggered low‐grade inflammation that was effectively resolved in young mice, as shown by the absence of cochlear cellular damage and macrophage infiltration. Our results lead us to propose that NADPH overproduction from an early stage is an efficient mechanism to maintain the balance between the production of ROS and cellular detoxification power along aging and thus prevents hearing loss progression., This work was funded by Multi Target and View FEDER/CM‐B2017/BMD‐3688 and MINECO/FEDER SAF2017‐86107‐R to IVN. JMBM was supported by a PhD CSIC contract., Peer reviewed

Sphingolipids are bioactive lipid components of cell membranes with important signal transduction functions in health and disease. Ceramide is the central building block for sphingolipid biosynthesis and is processed to form structurally and functionally distinct sphingolipids. Ceramide can be phosphorylated by ceramide kinase (CERK) to generate ceramide-1-phosphate, a cytoprotective signaling molecule that has been widely studied in multiple tissues and organs, including the developing otocyst. However, little is known about ceramide kinase regulation during inner ear development. Using chicken otocysts, we show that genes for CERK and other enzymes of ceramide metabolism are expressed during the early stages of inner ear development and that CERK is developmentally regulated at the otic vesicle stage. To explore its role in inner ear morphogenesis, we blocked CERK activity in organotypic cultures of otic vesicles with a specific inhibitor. Inhibition of CERK activity impaired proliferation and promoted apoptosis of epithelial otic progenitors. CERK inhibition also compromised neurogenesis of the acoustic-vestibular ganglion. Insulin-like growth factor-1 (IGF-1) is a key factor for proliferation, survival and differentiation in the chicken otocyst. CERK inhibition decreased IGF-1-induced AKT phosphorylation and blocked IGF-1-induced cell survival. Overall, our data suggest that CERK is activated as a central element in the network of anti-apoptotic pro-survival pathways elicited by IGF-1 during early inner ear development., This work was supported by the Spanish Ministerio de Economia y Competitividad, project FEDER/SAF2017-86107-R to IVN and MM., Peer reviewed

© 2021 The Authors, Cis-diammine-dichloroplatinum[II] (cisplatin) is a potent and widely used chemotherapeutic agent with significant efficacy against several forms of cancer in adults and children. Unfortunately, cisplatin has multiple adverse effects and causes irreversible hearing loss (HL) in a large proportion of patients, which severely impacts their quality of life.1 Here, we confirm that treatment with LPT99, a small molecule inhibitor of the apoptosome, preserves hearing levels in a rat model of cisplatin-induced HL and prevents apoptosis in cisplatin-exposed HEI-OC1 cells. These data are encouraging and support the potential of LPT99 for the prevention of the secondary effects of cisplatin for HL., This work was supported by a SPIRALTHCIBERER ER17PE12 grant to SMC. SMC and LRR hold CIBERER contracts. BC, AMC, and JMB contracts were supported in part by projects from FP7-PEOPLE-2013-IAPP-TARGEAR and from MINECO/FEDER SAF2017- 86107-R to IVN.

Animal models are invaluable for biomedical research, especially in the context of rare diseases, which have a very low prevalence and are often complex. Concretely mouse models provide key information on rare disease mechanisms and therapeutic strategies that cannot be obtained by using only alternative methods, and greatly contribute to accelerate the development of new therapeutic options for rare diseases. Despite this, the use of experimental animals remains controversial. The combination of respectful management, ethical laws and transparency regarding animal experimentation contributes to improve society's opinion about biomedical research and positively impacts on research quality, which eventually also benefits patients. Here we present examples of current advances in preclinical research in rare diseases using mouse models, together with our perspective on future directions and challenges., This work was supported by a grant from the Center for Biomedical Research on Rare Diseases (CIBERER) (ER18GDT761) to SM-C. Additional support came from: Multi Target and View FEDER/CM-B2017/BMD-3688 and MINECO/FEDER SAF2017-86107-R grants to SM-C and IV-N; CERCA Program/Generalitat de Catalunya, Autonomous Government of Catalonia (2017SGR1206), and CIBERER (ACCI18-27) to AP; Miguel Servet program CPII16/00016 (Instituto de Salud Carlos III co-funded by European Social Fund ESF) to SF; CNIC (ISCIII, Ministerio de Ciencia, Innovación y Universidades), Pro CNIC Foundation, Severo Ochoa Center of Excellence (SEV-2015-0505) to BI and EO; Generalitat de Catalunya (Grups consolidats 2017 SGR 926) to MD. EO was beneficiary of a grant from “Programa de Atracción de Talento” of Comunidad de Madrid (2017-T1/BMD-5185). SM-C holds a ISCiii CIBER postdoctoral researcher contract. The work at CRG was supported by grants from the Spanish Ministry of Science and Innovation to the EMBL partnership, the Centro de Excelencia Severo Ochoa and the CERCA Programme/Generalitat de Catalunya.

Animal models are invaluable for biomedical research, especially in the context of rare diseases, which have a very low prevalence and are often complex. Concretely mouse models provide key information on rare disease mechanisms and therapeutic strategies that cannot be obtained by using only alternative methods, and greatly contribute to accelerate the development of new therapeutic options for rare diseases. Despite this, the use of experimental animals remains controversial. The combination of respectful management, ethical laws and transparency regarding animal experimentation contributes to improve society's opinion about biomedical research and positively impacts on research quality, which eventually also benefits patients. Here we present examples of current advances in preclinical research in rare diseases using mouse models, together with our perspective on future directions and challenges., This work was supported by a grant from the Center for Biomedical Research on Rare Diseases (CIBERER) (ER18GDT761) to SM-C. Additional support came from: Multi Target and View FEDER/CM-B2017/BMD-3688 and MINECO/FEDER SAF2017-86107-R grants to SM-C and IV-N; CERCA Program/Generalitat de Catalunya, Autonomous Government of Catalonia (2017SGR1206), and CIBERER (ACCI18-27) to AP; Miguel Servet program CPII16/00016 (Instituto de Salud Carlos III co-funded by European Social Fund ESF) to SF; CNIC (ISCIII, Ministerio de Ciencia, Innovación y Universidades), Pro CNIC Foundation, Severo Ochoa Center of Excellence (SEV-2015-0505) to BI and EO; Generalitat de Catalunya (Grups consolidats 2017 SGR 926) to MD. EO was beneficiary of a grant from “Programa de Atracción de Talento” of Comunidad de Madrid (2017-T1/BMD-5185). SM-C holds a ISCiii CIBER postdoctoral researcher contract. The work at CRG was supported by grants from the Spanish Ministry of Science and Innovation to the EMBL partnership, the Centro de Excelencia Severo Ochoa and the CERCA Programme/Generalitat de Catalunya.

Animal models are invaluable for biomedical research, especially in the context of rare diseases, which have a very low prevalence and are often complex. Concretely mouse models provide key information on rare disease mechanisms and therapeutic strategies that cannot be obtained by using only alternative methods, and greatly contribute to accelerate the development of new therapeutic options for rare diseases. Despite this, the use of experimental animals remains controversial. The combination of respectful management, ethical laws and transparency regarding animal experimentation contributes to improve society's opinion about biomedical research and positively impacts on research quality, which eventually also benefits patients. Here we present examples of current advances in preclinical research in rare diseases using mouse models, together with our perspective on future directions and challenges.