The Protective Effects of Human Embryonic Stem Cell-Derived Mesenchymal Stem Cells in Noise-Induced Hearing Loss of Rats

A few prior animal studies have suggested the transplantation or protective effects of mesenchymal stem cells (MSCs) in noise-induced hearing loss. This study intended to evaluate the fates of administered MSCs in the inner ears and the otoprotective effects of MSCs in the noise-induced hearing loss of rats. Human embryonic stem cell-derived MSCs (ES-MSCs) were systematically administered via the tail vein in adult rats. Eight-week-old Sprague-Dawley rats were randomly allocated to the control (n = 8), ES-MSC (n = 4), noise (n = 8), and ES-MSC+noise (n = 10) groups. In ES-MSC and ES-MSC+noise rats, 5 × 105 ES-MSCs were injected via the tail vein. In noise and ES-MSC+noise rats, broadband noise with 115 dB SPL was exposed for 3 h daily for 5 days. The hearing levels were measured using auditory brainstem response (ABR) at 4, 8, 16, and 32 kHz. Cochlear histology was examined using H&E staining and cochlear whole mount immunofluorescence. The presence of human DNA was examined using Sry PCR, and the presence of human cytoplasmic protein was examined using STEM121 immunofluorescence staining. The protein expression levels of heat shock protein 70 (HSP70), apoptosis-inducing factor (AIF), poly (ADP-ribose) (PAR), PAR polymerase (PARP), caspase 3, and cleaved caspase 3 were estimated. The ES-MSC rats did not show changes in ABR thresholds following the administration of ES-MSCs. The ES-MSC+ noise rats demonstrated lower ABR thresholds at 4, 8, and 16 kHz than the noise rats. Cochlear spiral ganglial cells and outer hair cells were more preserved in the ES-MSC+ noise rats than in the noise rats. The Sry PCR bands were highly detected in lung tissue and less in cochlear tissue of ES-MSC+noise rats. Only a few STEM121-positivities were observed in the spiral ganglial cell area of ES-MSC and ES-MSC+noise rats. The protein levels of AIF, PAR, PARP, caspase 3, and cleaved caspase 3 were lower in the ES-MSC+noise rats than in the noise rats. The systemic injection of ES-MSCs preserved hearing levels and attenuated parthanatos and apoptosis in rats with noise-induced hearing loss. In addition, a tiny number of transplanted ES-MSCs were observed in the spiral ganglial areas.

Purification of Fibroblasts From the Spiral Ganglion

Using cultures of freshly isolated spiral ganglion cells (SGC) is common to investigate the effect of substances on spiral ganglion neurons (SGN) in vitro. As these cultures contain more cell types than just neurons, and it might be beneficial to have cochlear fibroblasts available to further investigate approaches to reduce the growth of fibrous tissue around the electrode array after cochlear implantation, we aimed at the purification of fibroblasts from the spiral ganglion in the current study. Subcultivation of the primary SGC culture removed the neurons from the culture and increased the fibroblast to glial cell ratio in the preparations, which was revealed by staining for vimentin, the S100B-protein, and the 200-kD neurofilament. We performed direct immunolabeling for the Thy1-glycoprotein and the p75NGFR-enabled fluorescence-based cell sorting. This procedure resulted in a cell culture of cochlear fibroblasts with a purity of more than 99%. The received fibroblasts can be subcultivated for up to 10 passages before proliferation rates drop. Additionally, 80% of the cells survived the first attempt of cryopreservation and exhibited a fibroblast-specific morphology. Using the described approach provides a purified preparation of cochlear fibroblasts, which can now be used in vitro for further investigations.

The effects of substrate composition and topography on the characteristics and growth of cell cultures of cochlear fibrocytes

Spiral ligament fibrocytes of the cochlea play homoeostatic roles in hearing and their degeneration contributes to hearing loss. Culturing fibrocytes in vitro provides a way to evaluate their functional characteristics and study possible therapies for hearing loss. We investigated whether in vivo characteristics of fibrocytes could be recapitulated in vitro by modifying the culture substrates and carried out proof of concept studies for potential transplantation of culture cells into the inner ear. Fibrocytes cultured from 4 to 5-week old CD/1 mice were grown on 2D substrates coated with collagen I, II, V or IX and, after harvesting, onto or into 3D substrates (hydrogels) of collagen I alone or mixed collagen I and II at a 1:1 ratio. We also assessed magnetic nanoparticle (MNP) uptake. Cell counts, immunohistochemical and ultrastructural studies showed that fibrocytes grown on 2D substrates proliferated, formed both small spindle-shaped and large flat cells that avidly took up MNPs. Of the different collagen coatings, only collagen II had an effect, causing a reduced size of the larger cells. On hydrogels, the cells were plump/rounded with extended processes, resembling native cells. They formed networks over the surface and became incorporated into the gel. In all culture formats, the majority co-expressed caldesmon, aquaporin 1, S-100 and sodium potassium ATPase, indicating a mixed or uncharacterised phenotype. Time-course experiments showed a decrease to ∼50% of the starting population by 4d after seeding on collagen I hydrogels, but better survival (∼60%) was found on collagen I + II gels, whilst TEM revealed the presence of apoptotic cells. Cells grown within gels additionally showed necrosis. These results demonstrate that fibrocytes grown in 3D recapitulate in vivo morphology of native fibrocytes, but have poorer survival, compared with 2D. Therefore hydrogel cultures could be used to study fibrocyte function and might also offer avenues for cell-replacement therapies, but need more optimization for therapeutic use. Fibrocyte function could be modified using MNPs in combination, for example, with gene transfection.

Achievements and Challenges in Transplantation of Mesenchymal Stem Cells in Otorhinolaryngology

Otorhinolaryngology enrolls head and neck surgery in various tissues such as ear, nose, and throat (ENT) that govern different activities such as hearing, breathing, smelling, production of vocal sounds, the balance, deglutition, facial animation, air filtration and humidification, and articulation during speech, while absence of these functions can lead to high morbidity and even mortality. Conventional therapies for head and neck damaged tissues include grafts, transplants, and artificial materials, but grafts have limited availability and cause morbidity in the donor site. To improve these limitations, regenerative medicine, as a novel and rapidly growing field, has opened a new therapeutic window in otorhinolaryngology by using cell transplantation to target the healing and replacement of injured tissues. There is a high risk of rejection and tumor formation for transplantation of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs); mesenchymal stem cells (MSCs) lack these drawbacks. They have easy expansion and antiapoptotic properties with a wide range of healing and aesthetic functions that make them a novel candidate in otorhinolaryngology for craniofacial defects and diseases and hold immense promise for bone tissue healing; even the tissue sources and types of MSCs, the method of cell introduction and their preparation quality can influence the final outcome in the injured tissue. In this review, we demonstrated the anti-inflammatory and immunomodulatory properties of MSCs, from different sources, to be safely used for cell-based therapies in otorhinolaryngology, while their achievements and challenges have been described too.

Therapeutic Application of Mesenchymal Stem Cells for Cochlear Regeneration

Hearing loss is one of the major worldwide health problems that seriously affects human social and cognitive development. In the auditory system, three components outer ear, middle ear and inner ear are essential for the hearing mechanism. In the inner ear, sensory hair cells and ganglion neuronal cells are the essential supporters for hearing mechanism. Damage to these cells can be caused by long-term exposure of excessive noise, ototoxic drugs (aminoglycosides), ear tumors, infections, heredity and aging. Since mammalian cochlear hair cells do not regenerate naturally, some therapeutic interventions may be required to replace the damaged or lost cells. Cochlear implants and hearing aids are the temporary solutions for people suffering from severe hearing loss. The current discoveries in gene therapy may provide a deeper understanding in essential genes for the inner ear regeneration. Stem cell migration, survival and differentiation to supporting cells, cochlear hair cells and spiral ganglion neurons are the important foundation in understanding stem cell therapy. Moreover, mesenchymal stem cells (MSCs) from different sources (bone marrow, umbilical cord, adipose tissue and placenta) could be used in inner ear therapy. Transplanted MSCs in the inner ear can recruit homing factors at the damaged sites to induce transdifferentiation into inner hair cells and ganglion neurons or regeneration of sensory hair cells, thus enhancing the cochlear function. This review summarizes the potential application of mesenchymal stem cells in hearing restoration and combining stem cell and molecular therapeutic strategies can also be used in the recovery of cochlear function.

On the Role of Fibrocytes and the Extracellular Matrix in the Physiology and Pathophysiology of the Spiral Ligament

The spiral ligament in the cochlea has been suggested to play a significant role in the pathophysiology of different etiologies of strial hearing loss. Spiral ligament fibrocytes (SLFs), the main cell type in the lateral wall, are crucial in maintaining the endocochlear potential and regulating blood flow. SLF dysfunction can therefore cause cochlear dysfunction and thus hearing impairment. Recent studies have highlighted the role of SLFs in the immune response of the cochlea. In contrast to sensory cells in the inner ear, SLFs (more specifically type III fibrocytes) have also demonstrated the ability to regenerate after different types of trauma such as drug toxicity and noise. SLFs are responsible for producing proteins, such as collagen and cochlin, that create an adequate extracellular matrix to thrive in. Any dysfunction of SLFs or structural changes to the extracellular matrix can significantly impact hearing function. However, SLFs may prove useful in restoring hearing by their potential to regenerate cells in the spiral ligament.

Three-Dimensional Distribution of Cochlear Macrophages in the Lateral Wall of Cleared Cochlea

Objectives

Resident macrophages are well known to be present in the cochlea, but the exact patterns thereof in spiral ligaments have not been discussed in previous studies. We sought to document the distribution of macrophages in intact cochleae using three-dimensional imaging.

Methods

Cochleae were obtained from C-X3-C motif chemokine receptor 1^+/GFP mice, and organ clearing was performed. Three-dimensional images of cleared intact cochleae were reconstructed using two-photon microscopy. The locations of individual macrophages were investigated using 100-μm stacked images to reduce bias. Cochlear inflammation was then induced by lipopolysaccharide (LPS) inoculation into the middle ear through the tympanic membrane. Four days after inoculation, three-dimensional images were obtained.

Results

Macrophages were scarce in areas adjacent to the stria vascularis, particularly the area just beneath it even though many have suspected macrophages to be abundant in this area. This finding remained consistent upon LPS-induced cochlear inflammation, despite a significant increase in the number of macrophages, compared to non-treated cochlea.

Conclusion

Resident macrophages in spiral ligaments are scarce in areas adjacent to the stria vascularis.

The olfactory mucosa: a potential source of stem cells for hearing regeneration

The olfactory mucosa contains cells that enable it to generate new neurons and other supporting cells throughout life, allowing it to replace cells of the mucosa that have been damaged by exposure to various insults. In this article, we discuss the different types of stem cell found within the olfactory mucosa and their properties. In particular, the mesenchymal-like cells found within the lamina propria will be reviewed in detail. In addition, we discuss potential applications of olfactory-derived stem cells toward hearing regeneration secondary to either inner hair cell loss or primary or secondary auditory nerve degeneration.

Characterization of slow-cycling cells in the mouse cochlear lateral wall

Cochlear spiral ligament fibrocytes (SLFs) play essential roles in the physiology of hearing including ion recycling and the generation of endocochlear potential. In adult animals, SLFs can repopulate after damages, yet little is known about the characteristics of proliferating cells that support SLFs' self-renewal. Here we report in detail about the characteristics of cycling cells in the spiral ligament (SL). Fifteen P6 mice and six noise-exposed P28 mice were injected with 5-bromo-2'-deoxyuridine (BrdU) for 7 days and we chased BrdU retaining cells for as long as 60 days. Immunohistochemistry revealed that the BrdU positive IB4 (an endotherial marker) negative cells expressed an early SLF marker Pou3f4 but negative for cleaved-Caspase 3. Marker studies revealed that type 3 SLFs displayed significantly higher percentage of BrdU+ cells compared to other subtypes. Notably, the cells retained BrdU until P72, demonstrating they were dividing slowly. In the noise-damaged mice, in contrast to the loss of the other types, the number of type 3 SLFs did not altered and the BrdU incorporating- phosphorylated Histone H3 positive type 3 cells were increased from day 1 to 14 after noise exposure. Furthermore, the cells repopulating type 1 area, where the cells diminished profoundly after damage, were positive for the type 3 SLF markers. Collectively, in the latral wall of the cochlea, type 3 SLFs have the stem cell capacity and may contribute to the endogenous regeneration of lateral wall spiral ligament. Manipulating type 3 cells may be employed for potential regenerative therapies.

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.

Pharmacological inhibition of cochlear mitochondrial respiratory chain induces secondary inflammation in the lateral wall: a potential therapeutic target for sensorineural hearing loss

Cochlear lateral wall has recently been reported as a common site of inflammation, yet precise molecular mechanisms of the inflammatory responses remain elucidated. The present study examined the inflammatory responses in the lateral wall following acute mitochondrial dysfunction induced by a mitochondrial toxin, 3-nitropropionic acid (3-NP). Reverse-transcription (RT)-PCR revealed increases in the expression of the proinflammatory cytokines interleukin (IL)-1β and IL-6. Immunohistochemistry showed an increase in the number of activated cochlear macrophages in the lateral wall, which were in close proximity to IL-6-expressing cells. A genome-wide DNA microarray analysis of the lateral wall revealed that 35% and 60% of the genes showing >2-fold upregulation at 1 d and 3 d post-3-NP administration, respectively, were inflammatory genes, including CC- and CXC-type chemokine genes. High expression of CCL-1, 2, and 3 at 1 d, and of CCL-1, 2, 3, 4, and 5, CCR-2 and 5, and CX3CR1 at 3 d post-3-NP administration, coupled with no change in the level of CX3CL1 expression suggested that macrophages and monocytes may be involved in the inflammatory response to 3-NP-mediated injury. Quantitative (q)RT-PCR showed a transient induction of IL-1β and IL-6 expression within 24 h of 3-NP-mediated injury, followed by sustained expression of the chemoattractants, CCL-2, 4 and 5, up until 7 d after injury. The expression of CCL-2 and IL-6 was higher in animals showing permanent hearing impairment than in those showing temporary hearing impairment, suggesting that these inflammatory responses may be detrimental to hearing recovery. The present findings suggest that acute mitochondrial dysfunction induces secondary inflammatory responses in the lateral wall of the cochlear and that the IL-6/CCL-2 inflammatory pathway is involved in monocyte activation. Therefore, these secondary inflammatory responses may be a potential post-insult therapeutic target for treatments aimed at preventing the damage caused by acute mitochondrial dysfunction in the cochlear lateral wall.

The expression of nicotinic receptor alpha7 during cochlear development

Nicotinic acetylcholine receptor alpha7 expression was examined in the developing and adult auditory system using mice that were modified through homologous recombination to coexpress either GFP (alpha7GFP) or Cre (alpha7Cre), respectively. The expression of alpha7GFP is first detected at embryonic (E) day E13.5 in cells of the spiral prominence. By E14.5, sensory regions including the putative outer hair cells and Deiters' cells express alpha7GFP as do solitary efferent fibers. This pattern diminishes after E16.5 in a basal to apex progression, as Hensen's cells and cells of the spiral ligament acquire alpha7GFP expression. At birth and thereafter alpha7GFP also identifies a subset of spiral ganglion cells whose processes terminate on inner hair cells. Efferent fibers identified by peripherin or calcitonin gene-related protein do not coexpress alpha7GFP. In addition to cochlear structures, there is strong expression of alpha7GFP by cells of the central auditory pathways including the ventral posterior cochlear nucleus, lateral lemniscus, central inferior colliculus, and the medial geniculate nucleus. Our findings suggest that alpha7 expression by both neuronal and non-neuronal cells has the potential to impact multiple auditory functions through mechanisms that are not traditionally attributed to this receptor.