Delivery of gene therapy through a cerebrospinal fluid conduit to rescue hearing in adult mice

Pharmacological Approaches to Hearing Loss

Hearing disorders pose significant challenges to individuals experiencing them and their overall quality of life, emphasizing the critical need for advanced pharmacological approaches to address these conditions. Current treatment options often focus on amplification devices, cochlear implants, or other rehabilitative therapies, leaving a substantial gap regarding effective pharmacological interventions. Advancements in our understanding of the molecular and cellular mechanisms involved in hearing disorders induced by noise, aging, and ototoxicity have opened new avenues for drug development, some of which have led to numerous clinical trials, with promising results. The development of optimal drug delivery solutions in animals and humans can also enhance the targeted delivery of medications to the ear. Moreover, large genome studies contributing to a genetic understanding of hearing loss in humans combined with advanced molecular technologies in animal studies have shown a great potential to increase our understanding of the etiologies of hearing loss. The auditory system exhibits circadian rhythms and temporal variations in its physiology, its vulnerability to auditory insults, and its responsiveness to drug treatments. The cochlear clock rhythms are under the control of the glucocorticoid system, and preclinical evidence suggests that the risk/benefit profile of hearing disorder treatments using chronopharmacological approaches would be beneficial. If translatable to the bedside, such approaches may improve the outcome of clinical trials. Ongoing research into the molecular and genetic basis of auditory disorders, coupled with advancements in drug formulation and delivery as well as optimized timing of drug administration, holds great promise of more effective treatments. SIGNIFICANCE STATEMENT: Hearing disorders pose significant challenges to individuals and their overall quality of life, emphasizing the critical need for advanced pharmacological approaches to address these conditions. Ongoing research into the molecular and genetic basis of auditory disorders, coupled with advancements in drug delivery procedures and optimized timing of drug administration, holds the promise of more effective treatments.

Cochlear Aqueduct Post-Natal Growth: A Computed Tomography Study

The cochlear aqueduct (CA) is a bony canal located at the base of the scala tympani of the cochlea. It connects the inner ear perilymph fluid to the cerebrospinal fluid of the posterior cerebral fossa. Its function is not well understood, as it seems to be patent in only a fraction of adult patients. Indirect observations argue in favor of the CA being more patent in children. To study the CA morphology in children, we performed a retrospective single-center study of 85 high-resolution temporal bone computed tomography (hrCT) scans of children with a mean age of 3.23 ± 3.07 years (13 days of life up to 18 years), and compared them with a group of 22 adult hrCT (mean age of 24.01 ± 3.58 years). The CA morphology measurements included its total length, its funnel (wider intracranial portion) length and width and its type (indicating its radiological patency), according to a previously published classification. The dimensions of the CA were significantly smaller in children compared with adults for the axial length (10.37 ± 2.58 versus 14.63 ± 2.40 mm, respectively, p < 0,001) and the funnel length (3.94 ± 1.59 versus 6.01 ± 1.77 mm, respectively, p < 0,001). The funnel width tended to be smaller but the difference was not significant: 3.49 ± 1,33 versus 3.89 ± 1.07 mm, p = 0,22. The repartition of types of CA was also statistically different. The CA appeared to be more identifiable in the children population. Type 1 (CA visible along its entire course) accounted for 42% (36/85) of children and only 5% (1/22) of adults, type 2 (visible in the medial two thirds) for 30% (25/85) versus 31% (7/22), type 3 (not visible completely along the medial two thirds) for 27% (23/85) versus 50% (11/22). Finally, type 4 (undetectable) was found in only 1% (1/85) of children and 14% (3/22) of adults (p < 0,001). Our study showed significant postnatal growth of the length of the CA, which was more rapid before the age of 2, and slowed after 6 years of age. Its width increased less, with children older than 2 years presenting a similar width to adults. The CA was more identifiable in hrCT in children, arguing for a more permeable tract. The number of completely ossified CA was significantly lower in the children population. These findings highlight the differences between the CA morphology in adults and children and raise the question of differences in function. Moreover, these differences may impact the pharmacodynamics of drugs or vectors delivered into the pediatric inner ear. Further studies are required, both on the anatomy of temporal bones and on the function of the CA in children.

Comprehensive review of amino acid transporters as therapeutic targets

The solute carrier (SLC) family, with more than 400 membrane-bound proteins, facilitates the transport of a wide array of substrates such as nutrients, ions, metabolites, and drugs across biological membranes. Amino acid transporters (AATs) are membrane transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs participate in many important physiological functions including nutrient supply, metabolic transformation, energy homeostasis, redox regulation, and neurological regulation. Several AATs have been found to significantly impact the progression of human malignancies, and dysregulation of AATs results in metabolic reprogramming affecting tumor growth and progression. However, current clinical therapies that directly target AATs have not been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, the molecular mechanisms in human diseases such as tumors, kidney diseases, and emerging therapeutic strategies for targeting AATs.

Complete Restoration of Hearing Loss and Cochlear Synaptopathy via Minimally Invasive, Single-Dose, and Controllable Middle Ear Delivery of Brain-Derived Neurotrophic Factor-Poly(dl-lactic acid-co-glycolic acid)-Loaded Hydrogel

Noise-induced hearing loss (NIHL) often accompanies cochlear synaptopathy, which can be potentially reversed to restore hearing. However, there has been little success in achieving complete recovery of sensorineural deafness using nearly noninvasive middle ear drug delivery before. Here, we present a study demonstrating the efficacy of a middle ear delivery system employing brain-derived neurotrophic factor (BDNF)-poly-(dl-lactic acid-co-glycolic acid) (PLGA)-loaded hydrogel in reversing synaptopathy and restoring hearing function in a mouse model with NIHL. The mouse model achieved using the single noise exposure (NE, 115 dBL, 4 h) exhibited an average 20 dBL elevation of hearing thresholds with intact cochlear hair cells but a loss of ribbon synapses as the primary cause of hearing impairment. We developed a BDNF-PLGA-loaded thermosensitive hydrogel, which was administered via a single controllable injection into the tympanic cavity of noise-exposed mice, allowing its presence in the middle ear for a duration of 2 weeks. This intervention resulted in complete restoration of NIHL at frequencies of click, 4, 8, 16, and 32 kHz. Moreover, the cochlear ribbon synapses exhibited significant recovery, whereas other cochlear components (hair cells and auditory nerves) remained unchanged. Additionally, the cochlea of NE treated mice revealed activation of tropomyosin receptor kinase B (TRKB) signaling upon exposure to BDNF. These findings demonstrate a controllable and minimally invasive therapeutic approach that utilizes a BDNF-PLGA-loaded hydrogel to restore NIHL by specifically repairing cochlear synaptopathy. This tailored middle ear delivery system holds great promise for achieving ideal clinical outcomes in the treatment of NIHL and cochlear synaptopathy.

Oberg, Clark RD, Wenman C, Loughlin S, Saad R, Ashraf T, Male A, Tadros S, Boostani R, Abdel-Salam GM, Zaki M, Abdalla E, Manzini MC, Pehlivan D, Posey JE, Gibbs RA, Houlden H, Alkuraya FS, Bujakowska K, Maroofian R, Lupski JR, Nguyen LN. Biallelic variation in the choline and ethanolamine transporter FLVCR1 underlies a pleiotropic disease spectrum from adult neurodegeneration to severe developmental disorders

FLVCR1 encodes Feline leukemia virus subgroup C receptor 1 (FLVCR1), a solute carrier (SLC) transporter within the Major Facilitator Superfamily. FLVCR1 is a widely expressed transmembrane protein with plasma membrane and mitochondrial isoforms implicated in heme, choline, and ethanolamine transport. While Flvcr1 knockout mice die in utero with skeletal malformations and defective erythropoiesis reminiscent of Diamond-Blackfan anemia, rare biallelic pathogenic FLVCR1 variants are linked to childhood or adult-onset neurodegeneration of the retina, spinal cord, and peripheral nervous system. We ascertained from research and clinical exome sequencing 27 individuals from 20 unrelated families with biallelic ultra-rare missense and predicted loss-of-function (pLoF) FLVCR1 variant alleles. We characterize an expansive FLVCR1 phenotypic spectrum ranging from adult-onset retinitis pigmentosa to severe developmental disorders with microcephaly, reduced brain volume, epilepsy, spasticity, and premature death. The most severely affected individuals, including three individuals with homozygous pLoF variants, share traits with Flvcr1 knockout mice and Diamond-Blackfan anemia including macrocytic anemia and congenital skeletal malformations. Pathogenic FLVCR1 missense variants primarily lie within transmembrane domains and reduce choline and ethanolamine transport activity compared with wild-type FLVCR1 with minimal impact on FLVCR1 stability or subcellular localization. Several variants disrupt splicing in a mini-gene assay which may contribute to genotype-phenotype correlations. Taken together, these data support an allele-specific gene dosage model in which phenotypic severity reflects residual FLVCR1 activity. This study expands our understanding of Mendelian disorders of choline and ethanolamine transport and demonstrates the importance of choline and ethanolamine in neurodevelopment and neuronal homeostasis.

Precision medicine: a new era for inner ear diseases

The inner ear is the organ responsible for hearing and balance. Inner ear dysfunction can be the result of infection, trauma, ototoxic drugs, genetic mutation or predisposition. Often, like for Ménière disease, the cause is unknown. Due to the complex access to the inner ear as a fluid-filled cavity within the temporal bone of the skull, effective diagnosis of inner ear pathologies and targeted drug delivery pose significant challenges. Samples of inner ear fluids can only be collected during surgery because the available procedures damage the tiny and fragile structures of the inner ear. Concerning drug administration, the final dose, kinetics, and targets cannot be controlled. Overcoming these limitations is crucial for successful inner ear precision medicine. Recently, notable advancements in microneedle technologies offer the potential for safe sampling of inner ear fluids and local treatment. Ultrasharp microneedles can reach the inner ear fluids with minimal damage to the organ, collect μl amounts of perilymph, and deliver therapeutic agents in loco. This review highlights the potential of ultrasharp microneedles, combined with nano vectors and gene therapy, to effectively treat inner ear diseases of different etiology on an individual basis. Though further research is necessary to translate these innovative approaches into clinical practice, these technologies may represent a true breakthrough in the clinical approach to inner ear diseases, ushering in a new era of personalized medicine.

Advancements and future prospects of adeno-associated virus-mediated gene therapy for sensorineural hearing loss

Sensorineural hearing loss (SNHL), a highly prevalent sensory impairment, results from a multifaceted interaction of genetic and environmental factors. As we continually gain insights into the molecular basis of auditory development and the growing compendium of deafness genes identified, research on gene therapy for SNHL has significantly deepened. Adeno-associated virus (AAV), considered a relatively secure vector for gene therapy in clinical trials, can deliver various transgenes based on gene therapy strategies such as gene replacement, gene silencing, gene editing, or gene addition to alleviate diverse types of SNHL. This review delved into the preclinical advances in AAV-based gene therapy for SNHL, spanning hereditary and acquired types. Particular focus is placed on the dual-AAV construction method and its application, the vector delivery route of mouse inner ear models (local, systemic, fetal, and cerebrospinal fluid administration), and the significant considerations in transforming from AAV-based animal model inner ear gene therapy to clinical implementation.

Structural characterization of SLYM-a 4th meningeal membrane

Traditionally, the meninges are described as 3 distinct layers, dura, arachnoid and pia. Yet, the classification of the connective meningeal membranes surrounding the brain is based on postmortem macroscopic examination. Ultrastructural and single cell transcriptome analyses have documented that the 3 meningeal layers can be subdivided into several distinct layers based on cellular characteristics. We here re-examined the existence of a 4th meningeal membrane, Subarachnoid Lymphatic-like Membrane or SLYM in Prox1-eGFP reporter mice. Imaging of freshly resected whole brains showed that SLYM covers the entire brain and brain stem and forms a roof shielding the subarachnoid cerebrospinal fluid (CSF)-filled cisterns and the pia-adjacent vasculature. Thus, SLYM is strategically positioned to facilitate periarterial influx of freshly produced CSF and thereby support unidirectional glymphatic CSF transport. Histological analysis showed that, in spinal cord and parts of dorsal cortex, SLYM fused with the arachnoid barrier layer, while in the basal brain stem typically formed a 1-3 cell layered membrane subdividing the subarachnoid space into two compartments. However, great care should be taken when interpreting the organization of the delicate leptomeningeal membranes in tissue sections. We show that hyperosmotic fixatives dehydrate the tissue with the risk of shrinkage and dislocation of these fragile membranes in postmortem preparations.

Gene Therapy for Inherited Hearing Loss: Updates and Remaining Challenges

Hearing loss stands as the most prevalent sensory deficit among humans, posing a significant global health challenge. Projections indicate that by 2050, approximately 10% of the world's population will grapple with disabling hearing impairment. While approximately half of congenital hearing loss cases have a genetic etiology, traditional interventions such as hearing aids and cochlear implants do not completely restore normal hearing. The absence of biological treatment has prompted significant efforts in recent years, with a strong focus on gene therapy to address hereditary hearing loss. Although several studies have exhibited promising recovery from common forms of genetic deafness in mouse models, existing challenges must be overcome to make gene therapy applicable in the near future. Herein, we summarize the primary gene therapy strategies employed over past years, provide an overview of the recent achievements in preclinical studies for genetic hearing loss, and outline the current key obstacles to cochlear gene therapy.

Structural characterization of SLYM - a 4th meningeal membrane

Traditionally, the meninges are described as 3 distinct layers, dura, arachnoid and pia. Yet, the classification of the connective meningeal membranes surrounding the brain is based on postmortem macroscopic examination. Ultrastructural and single cell transcriptome analyses have documented that the 3 meningeal layers can be subdivided into several distinct layers based on cellular characteristics. We here re-examined the existence of a 4th meningeal membrane, Subarachnoid Lymphatic-like Membrane or SLYM in Prox1-eGFP reporter mice. Imaging of freshly resected whole brains showed that SLYM covers the entire brain and brain stem and forms a roof shielding the subarachnoid cerebrospinal fluid (CSF)-filled cisterns and the pia-adjacent vasculature. Thus, SLYM is strategically positioned to facilitate periarterial influx of freshly produced CSF and thereby support unidirectional glymphatic CSF transport. Histological analysis showed that, in spinal cord and parts of dorsal cortex, SLYM fused with the arachnoid barrier layer, while in the basal brain stem typically formed a 1-3 cell layered membrane subdividing the subarachnoid space into two compartments. However, great care should be taken when interpreting the organization of the delicate leptomeningeal membranes in tissue sections. We show that hyperosmotic fixatives dehydrate the tissue with the risk of shrinkage and dislocation of these fragile membranes in postmortem preparations.

Structural characterization of SLYM - a 4 th meningeal membrane

Traditionally, the meninges are described as 3 distinct layers, dura, arachnoid and pia. Yet, the classification of the connective meningeal membranes surrounding the brain is based on postmortem macroscopic examination. Ultrastructural and single cell transcriptome analyses have documented that the 3 meningeal layers can be subdivided into several distinct layers based on cellular characteristics. We here re-examined the existence of a 4 th meningeal membrane, S ubarachnoid Ly mphatic-like M embrane or SLYM in Prox1-eGFP reporter mice. Imaging of freshly resected whole brains showed that SLYM covers the entire brain and brain stem and forms a roof shielding the subarachnoid cerebrospinal fluid (CSF)-filled cisterns and the pia-adjacent vasculature. Thus, SLYM is strategically positioned to facilitate periarterial influx of freshly produced CSF and thereby support unidirectional glymphatic CSF transport. Histological analysis showed that, in spinal cord and parts of dorsal cortex, SLYM fused with the arachnoid barrier layer, while in the basal brain stem typically formed a 1-3 cell layered membrane subdividing the subarachnoid space into two compartments. However, great care should be taken when interpreting the organization of the delicate leptomeningeal membranes in tissue sections. We show that hyperosmotic fixatives dehydrate the tissue with the risk of shrinkage and dislocation of these fragile membranes in postmortem preparations.

Enhanced Cochlear Transduction by AAV9 with High-Concentration Sucrose

The inner ear is a primary lesion in sensorineural hearing loss and has been a target in gene therapy. The efficacy of gene therapy depends on achieving sufficient levels of transduction at a safe vector dose. Vectors derived from various adeno-associated viruses (AAVs) are predominantly used to deliver therapeutic genes to inner ear cells. AAV9 and its variants vector are attractive candidates for clinical applications since they can cross the mesothelial cell layer and transduce inner hair cells (IHCs), although this requires relatively high doses. In this study, we investigated the effects of sucrose on the transduction of a variant of the AAV9 vector for gene transfer in the inner ear. We found that high concentrations of sucrose increased gene transduction in House Ear Institute-Organ of Corti 1 (HEI-OC1) cells in vitro. In addition, we demonstrated that simultaneous administration of sucrose enhanced the transduction of mouse IHCs and spiral ligament cells using an AAV9 variant vector. The procedure did not increase the thresholds in the auditory brainstem response, suggesting that sucrose had no adverse effect on auditory function. This versatile method may be valuable in the development of novel gene therapies for adult-onset sensorineural hearing loss.