Up-regulation of stromal cell-derived factor-1 enhances migration of transplanted neural stem cells to injury region following degeneration of spiral ganglion neurons in the adult rat inner ear

(Re)building the nervous system: A review of neuron-glia interactions from development to disease

Neuron-glia interactions are fundamental to the development and function of the nervous system. During development, glia, including astrocytes, microglia, and oligodendrocytes, influence neuronal differentiation and migration, synapse formation and refinement, and myelination. In the mature brain, glia are crucial for maintaining neural homeostasis, modulating synaptic activity, and supporting metabolic functions. Neurons, inherently vulnerable to various stressors, rely on glia for protection and repair. However, glia, in their reactive state, can also promote neuronal damage, which contributes to neurodegenerative and neuropsychiatric diseases. Understanding the dual role of glia-as both protectors and potential aggressors-sheds light on their complex contributions to disease etiology and pathology. By appropriately modulating glial activity, it may be possible to mitigate neurodegeneration and restore neuronal function. In this review, which originated from the International Society for Neurochemistry (ISN) Advanced School in 2019 held in Montreal, Canada, we first describe the critical importance of glia in the development and maintenance of a healthy nervous system as well as their contributions to neuronal damage and neurological disorders. We then discuss potential strategies to modulate glial activity during disease to protect and promote a properly functioning nervous system. We propose that targeting glial cells presents a promising therapeutic avenue for rebuilding the nervous system.

RIP3-mediated necroptosis was essential for spiral ganglion neuron damage

To identify the role of RIP3 in ouabain-induced necroptosis and offer clinical implications to prevent spiral ganglion neurons (SGNs) from death, ouabain was applied in SGNs derived from fetal rats and injected into Sprague-Dawley rats to construct injury model in vitro and in vivo, respectively. The necroptosis rate of SGNs was determined by flow cytometry and MTT assays. The protein levels and phosphorylation of RIP3 were evaluated using western blotting and immunofluorescence. SGNs injury was observed using H&E staining and immunofluorescence. The hearing function of rats was evaluated by the auditory brainstem response (ABR) and Distortion Product Otoacoustic Emissions (DPOAE) methods. Ouabain caused dose-dependent necroptosis in SGNs and significant loss of SGNs of the cochlear axis in vivo. RIP3 and pRIP3 were upregulated with SGNs injury promoted, and RIP3 overexpression promoted ouabain-induced necroptosis in SGNs in vitro, which could be suppressed by necrostatin-1. RIP3 knockdown inhibited ouabain-induced necroptosis and reduced the phosphorylation of MLKL but no RIP3-dependent effect on the level of MLKL. RIP3 inhibition in vivo protected rats from ouabain-induced hearing damage with reducing ABR threshold shifts and promoting DPOAE amplitudes, while overexpression of RIP3 enhanced ouabain-induced injury that could be partially reversed by necrostatin-1. A decrease of SGNs density and an upregulation of pRIP3 were observed with RIP3 overexpression, which was in contrast when RIP3 was silenced. Therefore, RIP3 was essential for mediating necroptosis in ouabain-induced SGNs damage. Targeting RIP3 may prevent SGNs from death in clinical practice, and finally help the treatment of sensorineural hearing loss.

Endothelial cell secreted metalloproteinase-2 enhances neural stem cell N-cadherin expression, clustering, and migration

Neuroblasts have a clustered phenotype critical for their unidirectional migration, which in part is dependent on signaling from microvascular endothelial cells (EC) and pericytes (PC). Diffusible signals secreted by vascular cells have been demonstrated to increase survival, proliferation, and differentiation of subventricular zone resident neural stem cells (NSC); however, the signals that promote the necessary initiating step of NSC clustering are undefined. To investigate the role of vascular cells in promoting NSC clustering and directing migration, we created a 3-D hydrogel that mimics the biomechanics, biochemistry, and architectural complexity of brain tissue. We demonstrate that EC, and not PC, have a crucial role in NSC clustering and migration, further verified through microfluidic chamber systems and traction force microscopy. Ablation of the extended NSC aggregate arm halts aggregate movement, suggesting that clustering is a prerequisite for migration. When cultured with EC, NSC clustering occurs and NSC coincidentally increase their expression of N-cadherin, as compared to NSC cultured alone. NSC-presented N-cadherin expression was increased following exposure to EC secreted metalloproteinase-2 (MMP2). We demonstrate that inhibition of MMP2 prevented NSC N-cadherin surface expression and subsequent NSC clustering, even when NSC were in direct contact with EC. Furthermore, with exogenous activation of EGFR, which serves as a downstream activator of N-cadherin cleavage, NSC form clusters. Our results suggest that EC secretion of MMP2 promotes NSC clustering through N-cadherin expression. The insight gained about the mechanisms by which EC promote NSC migration may enhance NSC therapeutic response to sites of injury.

Cell Transplantation to Restore Lost Auditory Nerve Function is a Realistic Clinical Opportunity

Hearing is one of our most important means of communication. Disabling hearing loss (DHL) is a long-standing, unmet problem in medicine, and in many elderly people, it leads to social isolation, depression, and even dementia. Traditionally, major efforts to cure DHL have focused on hair cells (HCs). However, the auditory nerve is also important because it transmits electrical signals generated by HCs to the brainstem. Its function is critical for the success of cochlear implants as well as for future therapies for HC regeneration. Over the past two decades, cell transplantation has emerged as a promising therapeutic option for restoring lost auditory nerve function, and two independent studies on animal models show that cell transplantation can lead to functional recovery. In this article, we consider the approaches most likely to achieve success in the clinic. We conclude that the structure and biochemical integrity of the auditory nerve is critical and that it is important to preserve the remaining neural scaffold, and in particular the glial scar, for the functional integration of donor cells. To exploit the natural, autologous cell scaffold and to minimize the deleterious effects of surgery, donor cells can be placed relatively easily on the surface of the nerve endoscopically. In this context, the selection of donor cells is a critical issue. Nevertheless, there is now a very realistic possibility for clinical application of cell transplantation for several different types of hearing loss.

Hearing regeneration and regenerative medicine: present and future approaches

More than 5% of the world population lives with a hearing impairment. The main factors responsible for hearing degeneration are ototoxic drugs, aging, continued exposure to excessive noise and infections. The pool of adult stem cells in the inner ear drops dramatically after birth, and therefore an endogenous cellular source for regeneration is absent. Hearing loss can emerge after the degeneration of different cochlear components, so there are multiple targets to be reached, such as hair cells (HCs), spiral ganglion neurons (SGNs), supporting cells (SCs) and ribbon synapses. Important discoveries in the hearing regeneration field have been reported regarding stem cell transplantation, migration and survival; genetic systems for cell fate monitoring; and stem cell differentiation to HCs, SGNs and SCs using adult stem cells, embryonic stem cells and induced pluripotent stem cells. Moreover, some molecular mediators that affect the establishment of functional synapses have been identified. In this review, we will focus on reporting the state of the art in the regenerative medicine field for hearing recovery. Stem cell research has enabled remarkable advances in regeneration, particularly in neuronal cells and synapses. Despite the progress achieved, there are certain issues that need a deeper development to improve the results already obtained, or to develop new approaches aiming for the clinical application.

Stem Cells: A New Hope for Hearing Loss Therapy

Permanent hearing loss was considered which cannot be cured since cochlear hair cells and primary afferent neurons cannot be regenerated. In recent years, due to the in-depth study of stem cell and its therapeutic potential, regenerating auditory sensory cells is made possible. By using two strategies of endogenous stem cell activation and exogenous stem cell transplantation, researchers hope to find methods to restore hearing function. However, there are complex factors that need to be considered in the in vivo application of stem cell therapy, such as stem cell-type choice, signaling pathway regulations, transplantation approaches, internal environment of the cochlea, and external stimulation. After years of investigations, some theoretic progress has been made in the treatment of hearing loss using stem cells, but there are also many problems which limited its application that need to be solved. Understanding the future perspective of stem cell therapy in hearing loss, solving the encountered problems, and promoting its development are the common goals of audiological researchers. In this review, we present critical experimental findings of stem cell therapy on treatment of hearing loss and intend to bring hope to researchers and patients.

Factors that influence adult neurogenesis as potential therapy

Adult neurogenesis involves persistent proliferative neuroprogenitor populations that reside within distinct regions of the brain. This phenomenon was first described over 50 years ago and it is now firmly established that new neurons are continually generated in distinct regions of the adult brain. The potential of enhancing the neurogenic process lies in improved brain cognition and neuronal plasticity particularly in the context of neuronal injury and neurodegenerative disorders. In addition, adult neurogenesis might also play a role in mood and affective disorders. The factors that regulate adult neurogenesis have been broadly studied. However, the underlying molecular mechanisms of regulating neurogenesis are still not fully defined. In this review, we will provide critical analysis of our current understanding of the factors and molecular mechanisms that determine neurogenesis. We will further discuss pre-clinical and clinical studies that have investigated the potential of modulating neurogenesis as therapeutic intervention in neurodegeneration.

Critical role of SDF-1/CXCR4 signaling pathway in stem cell homing in the deafened rat cochlea after acoustic trauma

Previous animal studies have shown that stromal cell-derived factor-1 (SDF-1)/CXC chemokine receptor-4 (CXCR4) signaling pathway plays an important role in the targeted migration of bone marrow-derived mesenchymal stem cells (BMSCs) to the injured area. In the present study, we aimed to investigate the potential role of chemotactic SDF-1/CXCR4 signaling pathway in the homing of transplanted BMSCs to the injured cochlea after noise-induced hearing loss (NIHL) in a rat model. White noise exposure (110 dB) paradigm was used for hearing loss induction in male rats for 6 hours in 5 days. Distortion-product otoacoustic emission (DPOAE) responses were recorded before the experiment and post noise exposure. Hoechst 33342-labeled BMSCs and CXCR4 antagonist (AMD3100)-treated BMSCs were injected into the rat cochlea through the round window. SDF-1 protein expression in the cochlear tissue was assayed using western blot assay. The number of labeled BMSCs reaching the endolymph was determined after 24 hours. SDF-1 was significantly increased in the cochlear tissue of rats in the noise exposure group than in the control group. The number of Hoechst 33342-labeled BMSCs reaching the endolymph of the cochlea was significantly smaller in the AMD3100-treated BMSCs group than in the normal BMSCs group. Our present findings suggest that the SDF-1/CXCR4 signaling pathway has a critical role in BMSCs migration to the injured cochlea in a rat model of noise-induced hearing loss.

Advances in translational inner ear stem cell research

Stem cell research is expanding our understanding of developmental biology as well as promising the development of new therapies for a range of different diseases. Within hearing research, the use of stem cells has focused mainly on cell replacement. Stem cells however have a broad range of other potential applications that are just beginning to be explored in the ear. Mesenchymal stem cells are an adult derived stem cell population that have been shown to produce growth factors, modulate the immune system and can differentiate into a wide variety of tissue types. Potential advantages of mesenchymal/adult stem cells are that they have no ethical constraints on their use. However, appropriate regulatory oversight seems necessary in order to protect patients from side effects. Disadvantages may be the lack of efficacy in many preclinical studies. But if proven safe and efficacious, they are easily translatable to clinical trials. The current review will focus on the potential application on mesenchymal stem cells for the treatment of inner ear disorders.

Aminoglycoside Increases Permeability of Osseous Spiral Laminae of Cochlea by Interrupting MMP-2 and MMP-9 Balance

The spiral ganglion neurons (SGNs) located in the Rosenthal's canal of cochlea are essential target for cochlear implant. Previous studies found that the canaliculi perforantes, small pores on the surface of the osseous spiral lamina (OSL) of the scala tympanic (ST) of cochlea, may provide communication between the cochlear perilymph and SGNs. In this study, we found that chronic treatment of aminoglycosides antibiotics, which is well known to cause sensory cell damage in the cochlea, induced significant damage of bone lining cells on the OSLs and increased the permeability of the Rosenthal's canal. The pores among the bone lining cells became significantly wider after chronic treatment of amikacin (100 mg/kg/day for 3-7 days). Injection of Evans Blue in the ST resulted in significant increase in its migration in the modulus in the amikacin-treated cochlea compared to the control ears, suggesting increased permeability of these passages. Treatment of amikacin with oxytetracycline, an inhibitor of matrix metalloproteases (MMPs), significantly reduced the amount of dye migrated from the ST to the modiolus. These results suggest that amikacin enhanced the permeability between the ST and SGNs by increasing MMPs. Aggregating the permeability of the bone lining cells on the OSLs may benefit gene and stem cell delivery to the SGNs in the cochlea.

Auditory stimulation modulates CXCL12/CXCR4 expression in postnatal development of the newborn rat cochlea

Sensorineural hearing loss is one of the most common sensory deficits. Recently, inner-ear stem cell therapy has been proposed for auditory afferent rehabilitation. CXCR4 is the primary physiologic receptor for CXC chemokine ligand 12 (CXCL12) and the CXCL12-CXCR4 pathway has been implicated in the process of migration, differentiation, and maturation of vertebrate neural stem cells. In this study, we examined changes in the auditory brainstem response and CXCL12/CXCR4 expression in newborn rat cochleae under different acoustic environments by quantitative real-time PCR, western blot, enzyme-linked immunosorbent assay, immunohistochemistry, and immunofluorescence analyses. Rats were divided randomly into three groups: the augmented acoustic environment (AAE) group, the auditory deprivation (AD) group, and the control group. Auditory brainstem response thresholds were markedly increased in the AAE group and in the AD group. Compared with postnatal day 1, the expression of CXCL12/CXCR4 mRNA and protein under normal acoustic conditions was increased on postnatal day 14 and then decreased on postnatal day 28 in the cochlea. However, on postnatal day 28, CXCL12/CXCR4 expression, as well as its spatiotemporal distribution as detected by immunohistochemistry and immunofluorescence assays, was augmented by AAE treatment and inhibited by AD treatment. Therefore, our results confirmed that auditory stimulation influenced the spatiotemporal expression of CXCL12/CXCR4 in newborn rat cochlea, which might help to unravel the role of the CXCL12-CXCR4 pathway in the synaptic contacts and hearing function establishment in rat cochlea development.

CXCL12/CXCR4 signaling pathway regulates cochlear development in neonatal mice

Chemotactic cytokines (chemokines) are a highly conserved class of secreted signaling molecules that are important in various cellular processes. CXC chemokine ligand 12 (CXCL12) and its receptor, CXC chemokine receptor 4 (CXCR4) have been previously reported to be crucial for the establishment of neural networks in different neuronal systems. However, it is unclear whether the CXCL12/CXCR4 signaling pathway regulates the development of the cochlea. The current study investigated the effects of the CXCL12/CXCR4 signaling pathway on cochlear development in neonatal mice. The expression levels of CXCL12 and CXCR4 were detected using immunofluorescence, reverse transcription‑quantitative polymerase chain reaction and western blot analysis demonstrating that CXCL12 and CXCR4 expression were significantly increased during cochlear development in neonatal mice. Treatment of spiral ganglion neurons with CXCL12 significantly decreased the protein expression levels of caspase‑3 and cleaved caspase‑3, indicating that CXCL12/CXCR4 signaling increased cell survival of spiral ganglion neurons. Furthermore, CXCL12 treatment significantly increased the number and length of neurites extending from spiral ganglion neurons. By contrast, the in vitro effects of CXCL12 were significantly abrogated by AMD100, a CXCR4 antagonist. Additionally, inhibiting CXCL12/CXCR4 signaling in neonatal mice significantly reduced the cell number and altered the morphology of spiral ganglion neurons in vivo. Thus, the present study indicates that the CXCL12/CXCR4 signaling pathway is important during the development of cochleae in neonatal mice.

Effects of genetic correction on the differentiation of hair cell-like cells from iPSCs with MYO15A mutation

Deafness or hearing loss is a major issue in human health. Inner ear hair cells are the main sensory receptors responsible for hearing. Defects in hair cells are one of the major causes of deafness. A combination of induced pluripotent stem cell (iPSC) technology with genome-editing technology may provide an attractive cell-based strategy to regenerate hair cells and treat hereditary deafness in humans. Here, we report the generation of iPSCs from members of a Chinese family carrying MYO15A c.4642G>A and c.8374G>A mutations and the induction of hair cell-like cells from those iPSCs. The compound heterozygous MYO15A mutations resulted in abnormal morphology and dysfunction of the derived hair cell-like cells. We used a CRISPR/Cas9 approach to genetically correct the MYO15A mutation in the iPSCs and rescued the morphology and function of the derived hair cell-like cells. Our data demonstrate the feasibility of generating inner ear hair cells from human iPSCs and the functional rescue of gene mutation-based deafness by using genetic correction.

The role of RIP3 mediated necroptosis in ouabain-induced spiral ganglion neurons injuries

Spiral ganglion neuron (SGN) injury is a generally accepted precursor of auditory neuropathy. Receptor-interacting protein 3 (RIP3) has been reported as an important necroptosis pathway mediator that can be blocked by necrostatin-1 (Nec-1). In our study, we sought to identify whether necroptosis participated in SGN injury. Ouabain was applied to establish an SGN injury model. We measured the auditory brain-stem response (ABR) threshold shift as an indicator of the auditory conditions. Positive β3-tubulin immunofluorescence staining indicated the surviving SGNs. RIP3 expression was evaluated using immunofluorescence, quantitative real-time polymerase chain reaction and western blot. SGN injury promoted an increase in RIP3 expression that could be suppressed by application of the necroptosis inhibitor Nec-1. A decreased ABR threshold shift and increased SGN density were observed when Nec-1 was administered with apoptosis inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD). These results demonstrated that necroptosis is an indispensable pathway separately from apoptosis leading to SGN death pathway, in which RIP3 plays an important role.

Acoustical stimulus changes the expression of stromal cell-derived factor-1 in the spiral ganglion neurons of the rat cochlea

Neural stem cell (NSC) transplantation into the cochlea has been tested as a treatment for spiral ganglion neuron (SGN) degenerative disease and injury in various animal models. A recent study has shown evidence of functional recovery after transplantation of the stem cells into a degenerated-SGN model. Chemokine stromal cell-derived factor-1 (SDF-1, or known as CXC chemokine ligand-12, CXCL-12) signaling through CXCR4 has previously been identified as a key step in the homing of the stem cells within the injury areas; meanwhile, studies have revealed that the SDF-1/CXCR4 axis is also involved in axon guidance and pathfinding. A study found that transplanted neural precursor cells can migrate to the root of the auditory nerve when animals are subjected to an augmented acoustic environment (AAE). In accordance with these studies, we hypothesize that AAE will up-regulate the expression of SDF-1 in acoustic nerves. We tested our hypothesis by examining the expression of SDF-1 in different acoustic environments, and the results were confirmed by the auditory brainstem response (ABR), immunohistochemical and RT-PCR analyses. The results showed that SDF-1 was expressed at a relatively low level in the SGNs under normal animal unit acoustic conditions (40-50 dB). Moreover, it was significantly up-regulated in the SGNs under the 75 dB (augmented physiological process without hearing loss) and 90 dB AAE (pathological process with light hearing loss) conditions; however, under the 115 dB AAE (pathological process with severe hearing loss) condition, the expression of SDF-1 was not up-regulated. The results confirmed that appropriately augmented acoustical stimuli lead to the up-regulation of SDF-1, which may assist in the migration of the transplanted cells and the subsequent establishment of essential synaptic contacts between the exogenous cells and the host auditory pathway.

Overexpression of Rho-GDP-dissociation inhibitor-γ inhibits migration of neural stem cells

Neural stem cell (NSC) migration relies heavily on the regulation of actin and microtubule cytoskeletons by Rho GTPases, which are critical regulators of key steps during NSC migration. However, the migration mechanism remains unclear. Rho-GDP-dissociation inhibitor-γ (Rho-GDIγ) was identified as an important downregulator of the Rho family of GTPases, because of its ability to prevent nucleotide exchange and thus membrane association. This study investigates the role of Rho-GDIγ in neural stem cells migration. Our results indicate that the overexpression of Rho-GDIγ maintains NSCs in the stem cell state, meanwhile preventing NSC migration through inhibition of Rac1 expression, one of the Rho-family GTPases. This study provides the basis for further study of the molecular mechanism of NSC migration.