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

Abstract

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.

Keywords: auditory nerve; cell transplantation; glial scar; nerve regeneration; scaffold.

Figure 1.

Auditory neurons and their degeneration patterns. (A) The auditory nerve is a bundle of bipolar auditory neurons. The peripheral processes of auditory neurons form synapses with HCs and the central processes with CNs in the brainstem. HCs provide much of the trophic support required for the maintenance and survival of auditory neurons, including BDNF and NT-3. Auditory neurons synthesize the high-affinity tyrosine receptor kinases, TrkB and TrkC. The interface between the PNS and CNS is called the TZ, which is distal to the IAM. Myelin sheaths are formed by oligodendrocytes centrally from the TZ, and the surrounding milieu is astrocytic. Peripheral to the TZ, the myelin sheaths are formed by Schwann cells that are enveloped in endoneurium. The interface is penetrated only by axons. (B) The onset of anterograde (Wallerian) (૜), trans-neuronal (૝), and retrograde degeneration (૞) of the auditory nerve depends on the initial site of injury (x). In HC damage, neurodegeneration involves the auditory neuron entirely (૟) and neurodegeneration proceeds to higher-level neurons including the CNs (૝). Shaded arrows indicate the progression of degeneration, and dotted arrows indicate transneuronal degeneration. BDNF, brain-derived neurotrophic factor; CNS, central nervous system; CNs, cochlear nucleus cells; HC, hair cells; IAM, internal auditory meatus; NT-3, neurotrophin 3; PNS, peripheral nervous system; TZ, transitional zone.

Figure 2.

Reported cell delivery methods to restore AuN function. (A) Reported cell delivery methods in Table 1 are shown with arrows in the upper panel. Dark shaded parts of each arrow indicate intracochlear or intraneural portions of each route. Arrows show each route conceptually and do not trace each anatomical route precisely. The dotted rectangle is enlarged to illustrate intracochlear structures in detail. (B) Surface transplantation of DCs on degenerated AuN. DCs transplanted onto the surface of degenerated AuN autonomously enter the nerve, differentiate (*) and form functional synapses with HCs and CNs (#). In degenerated AuN, the AO and SC columns form a continuous, “naturally occurring autologous cell bridge”, the AO–SC complex (a part is shown here), which acts as an anatomical scaffold for DC migration to connect between the PNS and the CNS (see the text). Note regenerating axons run parallel with the AO–SC complex. Studies using systemic delivery of donor cells are not shown here. AO, astrocyte outgrowth; AuN, auditory nerve; CNS, central nervous system; CN, cochlear nucleus cell; CPA, cerebellopontine angle; DC, donor cells; HC, hair cell; IAC, internal auditory canal; IAM, internal auditory meatus; IHC, inner hair cell; OHC, outer hair cell; MLI, membranous labyrinth injured; MLP, membranous labyrinth preserved; PNS, peripheral nervous system; RC, Rosenthal’s canal; ScM, the scala media; SC, Schwann cell; ScT, the scala tympani; ScV, the scala vestibuli; SuC, supporting cell; TZ, the transitional zone.

Figure 3.

Transitional zone and the astrocyte outgrowth following auditory nerve mechanical compression. (A) Normal TZ. The CNS portion of the AuN extends peripherally, with a dome-like shape (arrowheads with dotted line). Rosenthal’s canals are densely packed with auditory SGC (arrows). Rat, Hematoxylin and Eosin stain, Scale bar, 200 µm. Cited from Sekiya et al. (2007) with publisher’s permission. (B) Gliotic AuN after compression. A glial scar is induced following mechanical compression applied to the CNS portion of the auditory nerve in the cerebellopontine angle (arrowheads). Marked AO is indicated by double arrows. Most auditory SGCs degenerate following sustained compression (single arrows in dotted circle, Rosenthal’s canal). With GFAP antibody, an astrocyte marker, the glial scar is also stained because it contains many reactive astrocytes. An antibody Tuj1 against beta-tubulin stains neurons and neurites, including SGCs. The curved dotted line indicates the default position of the TZ. (Inset) Normal rat AuN. The TZ (arrows) is clearly observed as a peripherally convex, dome-like shape. The PNS portion of the nerve (outlined by dotted line) is GFAP-negative because astrocytes exist only in the CNS. Scale bars, 200 μm. AO, astrocyte outgrowth; AuN, auditory nerve; CNS, central nervous system; GFAP, glial fibrillary acidic protein; IAM, internal auditory meatus; PNS, peripheral nervous system; SGCs, spiral ganglion cells; TZ, transitional zone.

Figure 4.

Comparison between intraneural and surface transplantation of cells. (A) Intraneural transplantation of DCs. ABRs before compression (1), 5 weeks after compression before cell transplantation (2), and 3 months after (3). Arrowhead in panel 3, monophasic positive potential indicating electrical failure of nerve impulse transmission. I–V, ABR wave I–V. (B) Surface transplantation of donor cells. ABRs before compression (1), 5 weeks after compression (2), and 3 months after surface transplantation (3). Note a significant improvement of ABRs 3 months after surface transplantation (see Sekiya et al., 2015 for more details). (C) Schematic drawing of fate of intraneurally injected cells. (1) Cell debris mainly in the site of cell transplantation (large arrow), and a few cells are seemingly stuck in the gliotic auditory nerve tissue (small arrows). (2) Large arrow indicates cavity formation (asterisk) in the nerve due to infusion pressure during injection and the infused cell mass. Small arrow indicates cell debris around the cavity. (see Sekiya et al., 2015 for original images). Scale bars: (1), 200 μm; (2) 50 μm. (D) Schematic drawing of various modes of cell migration of donor cells transplanted on the surface of the auditory nerve (see Sekiya et al., 2015 for original images). (1) The DCs autonomously enter the AuN in a chain formation (hollow arrows). CSF, cerebrospinal fluid in the cerebellopontine angle subarachnoid space. (2) Within a gliotic auditory nerve, a transplanted cell is intimately associated with a GFAP+ process (black arrow) derived from the glial scar and migrated (hollow arrow). (3) three migrating donor cells (hollow arrows) form chains within GFAP+ sheaths (2 pairs of black arrows). (4) Migrating transplanted cells (hollow arrows) associated with neurons (black arrow), possibly for guidance. Scale bars: (1, 2, 4), 20 μm; (3) 10 μm. Cited from Sekiya et al. (2015) with publisher’s permission. ABR, auditory evoked brainstem responses; AuN, auditory nerve; BS, brainstem; CSF, cerebrospinal fluid; DCs, donor cells; GFAP, glial fibrillary acidic protein.

Figure 5.

Endoscopic surface transplantation of donor cells. (A) In patients with non-tumorous auditory neuropathic hearing loss, an endoscope can be introduced in the cerebellopontine angle cistern and DCs placed onto the AuN. Normal facial nerve (VII) and vestibulocochlear nerve (VIII) are shown in the left upper corner of the panel. (B) In open surgery for larger VS, following tumor removal (left), donor cells can be placed on the surface of the AuN. The posterior wall of the internal auditory meatus is drilled to expose the tumor entirely. (C) In radiotherapy for small to medium-sized VS (arrows), a similar approach shown in A can be undertaken immediately after treatment. DCs are placed both on the distal side of the tumor through the internal auditory meatus (shown in this figure) and on its medial side if possible (not shown here). The dotted line indicates tumor shrinkage after radiotherapy. Regenerated bipolar neurons (dotted line) are shown in the nerve in each panel. AuN, auditory nerve; CN, cochlear nucleus cells; Cr, cerebellum; DC, donor cell; En, endoscope; HC, hair cell; IAM, internal auditory meatus; kh, keyhole; Rt, retractor; VS, vestibular schwannoma.