The role of Espin in the stereocilia regeneration and protection in Atoh1-overexpressed cochlear epithelium

Promotion of Cx26 mutants located in TM4 region for membrane translocation successfully rescued hearing loss

Rationale: The GJB2 gene, which encodes connexin 26 (Cx26), is recognized as the leading cause of non-syndromic hereditary hearing loss. In clinical settings, a total of 131 Cx26 mutations have been identified in association with hearing loss. Certain Cx26 mutants display normal structural and functional properties but fail to translocate to the plasma membrane. Enhancing the membrane localization of these mutants may provide a promising strategy for rescuing hearing loss and hair cell degeneration. Methods: This study investigated the membrane localization of Cx26 using in vitro cell lines, cultured cochlear explants, and in vivo murine models. Key proteins involved in the membrane localization of Cx26 were identified and validated through immunoprecipitation-mass spectrometry (IP-MS) and co-immunoprecipitation (Co-IP). Additionally, cell lines and murine models harboring Cx26 mutants were developed to evaluate the effects of Narciclasine on enhancing the membrane localization of these mutants, as well as its potential to rescue hearing loss. Results: The membrane localization of Cx26 was dependent on the integrity of the intracellular transport network consisting of microtubules, actin microfilaments, and the Golgi apparatus. Additionally, SPTBN1 played a significant role in this process. The transmembrane domain 4 (TM4) region exhibited a strong association with the membrane localization of Cx26, and Cx26 mutants located in TM4 region retained in the cytoplasm. Narciclasine promoted cytoskeletal development, thereby enhancing the membrane localization of Cx26 mutants retained in the cytoplasm. This process helped to reconstruct the inner ear gap junction network and rescue hearing loss and hair cell degeneration. Conclusion: These findings present that enhancing the membrane localization of Cx26 mutants can significantly improve auditory function. This strategy offers a potential therapeutic approach for addressing hereditary sensorineural hearing loss associated with GJB2 mutations.

Co-overexpression of Atoh1, Pou4f3, and Gfi1 enhances the transdifferentiation of supporting cells into hair cells in the neonatal mouse utricle

Hair cells (HCs) are essential for vestibular function, and irreversible damage to vestibular HCs in mammals is closely associated with vertigo. The stimulation of HC regeneration through exogenous gene delivery represents an ideal therapeutic approach for restoring vestibular function. Overexpression of Atoh1, Pou4f3, and Gfi1 (collectively referred to as APG) has demonstrated efficacy in promoting HC regeneration in the cochlea. However, the effects of APG on vestibular HC regeneration remain unclear. Here, we used adeno-associated virus-inner ear (AAVie) as a carrier to deliver APG to the utricles of neonatal mice and assessed the morphology and number of HCs and supporting cells (SCs) by immunofluorescence staining. GLASTCreERT;Rosa26tdTomato mouse line was used to trace SCs. The results showed that APG overexpression resulted in substantial SC transdifferentiation into HCs in the neonatal mouse utricle. Furthermore, APG overexpression maintained SC number by facilitating SC proliferation. Continuous Atoh1 overexpression caused stereocilia damage, which was alleviated by APG overexpression. This study highlights the potential of regulating multiple transcription factors to promote vestibular HC regeneration.

Development of the inner ear and regeneration of hair cells after hearing impairment

Hearing loss, as a sensory disorder, is the most common occurrence among humans, which has received increasing attention from society. It is mainly caused by the damage of inner ear hair cells (HCs) or the degeneration of spiral ganglion neurons. In mammals, cochlear HCs cannot regenerate naturally after injury, leading to irreversible hearing loss. Therefore, HCs are essential for hearing protection. In recent years, the protection of drug-related ototoxicity, inner ear stem cells, gene therapy, new materials, and signal regulation have become important ways to develop regeneration strategies of HCs. An in-depth study of the causes of the occurrence and development of hearing impairment and the regeneration of hearing loss for effective prevention, discovery, and treatment of deafness has great significance. This review aimed to analyze the development of the inner ear and summarize the related factors leading to HCs injury and the research progress of regeneration after injury.

Origin and evolution of microvilli

BACKGROUND INFORMATION

Microvilli are finger-like, straight, and stable cellular protrusions that are filled with F-actin and present a stereotypical length. They are present in a broad range of cell types across the animal tree of life and mediate several fundamental functions, including nutrient absorption, photosensation, and mechanosensation. Therefore, understanding the origin and evolution of microvilli is key to reconstructing the evolution of animal cellular form and function. Here, we review the current state of knowledge on microvilli evolution and perform a bioinformatic survey of the conservation of genes encoding microvillar proteins in animals and their unicellular relatives.

RESULTS

We first present a detailed description of mammalian microvilli based on two well-studied examples, the brush border microvilli of enterocytes and the stereocilia of hair cells. We also survey the broader diversity of microvilli and discuss similarities and differences between microvilli and filopodia. Based on our bioinformatic survey coupled with carefully reconstructed molecular phylogenies, we reconstitute the order of evolutionary appearance of microvillar proteins. We document the stepwise evolutionary assembly of the "molecular microvillar toolkit" with notable bursts of innovation at two key nodes: the last common filozoan ancestor (correlated with the evolution of microvilli distinct from filopodia) and the last common choanozoan ancestor (correlated with the emergence of inter-microvillar adhesions).

CONCLUSION AND SIGNIFICANCE

We conclude with a scenario for the evolution of microvilli from filopodia-like ancestral structures in unicellular precursors of animals.

Recent Progress in Generation of Inner Ear Organoid

Inner ear organoids play a crucial role in hearing research. In comparison to other animal models and 2D cell culture systems, inner ear organoids offer significant advantages for studying the mechanisms of inner ear development and exploring novel approaches to disease treatment. Inner ear organoids derived from human cells are more closely resemble normal human organs in development and function. The 3D culture system of the inner ear organoid enhances cell-cell interactions and mimics the internal environment. In this review, the progress and limitations of organoid culture methods derived from tissue-specific progenitors and pluripotent stem cells (PSCs) are summarized, which may offer new insights into generating organoids that closely resemble the inner ear in terms of morphology and function.

AAV-mediated Gpm6b expression supports hair cell reprogramming

Irreversible damage to hair cells (HCs) in the cochlea leads to hearing loss. Cochlear supporting cells (SCs) in the murine cochlea have the potential to differentiate into HCs. Neuron membrane glycoprotein M6B (Gpm6b) as a four-transmembrane protein is a potential regulator of HC regeneration according to our previous research. In this study, we found that AAV-ie-mediated Gpm6b overexpression promoted SC-derived organoid expansion. Enhanced Gpm6b prevented the normal decrease in SC plasticity as the cochlea develops by supporting cells re-entry cell cycle and facilitating the SC-to-HC transformation. Also, overexpression of Gpm6b in the organ of Corti through the round window membrane injection facilitated the trans-differentiation of Lgr5+ SCs into HCs. In conclusion, our results suggest that Gpm6b overexpression promotes HC regeneration and highlights a promising target for hearing repair using the inner ear stem cells combined with AAV.

lncRNAs regulate cell stemness in physiology and pathology during differentiation and development

Long non-coding RNA (lncRNA) are an important class of ubiquitous genes involved in diverse biological functions. lncRNAs, defined as noncoding RNAs with a length exceeding 200 nucleotides, are abundantly expressed throughout cells; however, their precise functions remain largely elusive. From embryonic stem cell proliferation and differentiation to cancer cell proliferation and invasion, lncRNAs play multifaceted regulatory roles across various cellular stages. Moreover, lncRNAs participate in the regulation of differentiation and regeneration during cellular development processes while also playing a pivotal role in maintaining and regulating cell stemness. In this article, we comprehensively review the current knowledge regarding lncRNAs in this field, discussing their biological functions and mechanisms underlying stemness regulation along with the factors implicated in these processes. We emphasize the growing evidence supporting the significance of lncRNAs in governing cell stemness while indicating that disruptions or mutations within them may serve as fundamental causes for certain developmental disorders.

Gene expression analysis of oxidative stress-related genes in the apical, middle, and basal turns of the cochlea

It can be observed from aminoglycoside-induced hair cell damage that the cochlea basal turn is more susceptible to trauma than the apex. Drug-induced hearing loss is closely related to oxidative damage. The basilar membrane directly exposed to these ototoxic drugs exhibits differences in damage, indicating that there is an inherent difference in the sensitivity to oxidative damage from the apex to the base of the cochlea. It has been reported that the morphology and characteristics of the cochlea vary from the apex to the base. Therefore, we investigated oxidative stress-related gene expression profiles in the apical, middle, and basal turns of the cochlea. The Oxidative Stress RT2 Profiler™ PCR Array revealed that three of the 84 genes (Mb, Mpo, and Ncf1) were upregulated in the middle turn compared to their level in the apical turn. Moreover, eight genes (Mb, Duox1, Ncf1, Ngb, Fmo2, Gpx3, Mpo, and Gstk1) were upregulated in the basal turn compared to their level in the apical turn. The qPCR verification data were similar to that of the PCR Array. We found that MPO was expressed in the rat cochlea and protected against gentamicin-induced hair cell death. This study summarized the data for the gradient of expression of oxidative stress-related genes in the cochlea and found potential candidate targets for prevention of ototoxic deafness, which may provide new insights for cochlear pathology.

Stem Cell-Based Hair Cell Regeneration and Therapy in the Inner Ear

Hearing loss has become increasingly prevalent and causes considerable disability, thus gravely burdening the global economy. Irreversible loss of hair cells is a main cause of sensorineural hearing loss, and currently, the only relatively effective clinical treatments are limited to digital hearing equipment like cochlear implants and hearing aids, but these are of limited benefit in patients. It is therefore urgent to understand the mechanisms of damage repair in order to develop new neuroprotective strategies. At present, how to promote the regeneration of functional hair cells is a key scientific question in the field of hearing research. Multiple signaling pathways and transcriptional factors trigger the activation of hair cell progenitors and ensure the maturation of newborn hair cells, and in this article, we first review the principal mechanisms underlying hair cell reproduction. We then further discuss therapeutic strategies involving the co-regulation of multiple signaling pathways in order to induce effective functional hair cell regeneration after degeneration, and we summarize current achievements in hair cell regeneration. Lastly, we discuss potential future approaches, such as small molecule drugs and gene therapy, which might be applied for regenerating functional hair cells in the clinic.