Investigating cochlear cellular dynamics in neurofibromatosis type 2-associated schwannomatosis: a histopathological study

Abstract

Sensorineural hearing loss (SNHL) is a hallmark symptom in patients with neurofibromatosis type 2-associated schwannomatosis (NF2-SWN), a genetic condition caused by mutations in the Neurofibromin II gene that encodes the tumor suppressor protein Moesin-Ezrin-Radixin-Like Protein (Merlin; also known as schwannomin). These mutations lead to the development of various tumors, including schwannomas, ependymomas and meningiomas along the vestibular nerve and the cerebellopontine angle. Original theories attributed SNHL in NF2-SWN to the mechanical compression of the vestibulocochlear nerve from the tumor itself, in addition to secretion of toxic tumor byproducts. However, the observation that SNHL can progress independently of tumor size and growth dynamics challenges this view and reveals a critical gap in our understanding of its underlying etiology. To better define cochlear changes associated with hearing loss in NF2-SWN, immunohistochemical cell type markers were used on archival postmortem temporal bone samples from both NF2-SWN patients and healthy controls and quantified the number and cellular density of neural (TUJ1), glial (SOX10), and immune cells (IBA1) within apical, middle, and basal turns of the cochlea. Our findings demonstrated a significant loss of spiral ganglion neurons, a slight increase of Schwann cells, and marked activation of cochlear macrophages in NF2-SWN cases. These findings indicate the contribution of cochlear macrophage-mediated inflammation and Schwann cell dysregulation in the pathophysiology of SNHL in NF2-SWN.

Keywords: NF2-SWN; Schwann cells; macrophages; neurofibromatosis type 2; sensorineural hearing loss; spiral ganglion neurons.

Figure 1

Cochlear anatomy, case selection and audiometric outcomes. (A) (i) illustration of the inner ear, including the cochlea and vestibular apparatus. (ii) Cross section of a mid-modiolar cochlea section with central axons feeding into the cochlear nerve (yellow). (iii) Cross section through one turn of the cochlea, with SGN cell bodies inside Rosenthal’s canal (white arrowhead), organ of Corti (black arrow) containing cochlear hair cells (purple, pink), scala vestibuli (star), scala media (asterisk) and scala tympani (black arrowhead). Created in BioRender. Kempfle, J. (37) https://BioRender.com/gmjutz7. (B) Audiometric data with worse hearing across all frequencies in NF2-SWN compared to age matched controls (Control).

Figure 2

Characterization and quantification of neurons and Schwann cells in control and NF2-SWN samples. (A,B) Control and NF2-SWN spiral ganglia, respectively, in a mid-modiolar section. Staining: Neurons, TUJ, green; Schwann cells, SOX10+, greyscale; DAPI nuclear counterstain, blue. (i) Middle turn Rosenthal’s canal at 20X magnification. (ii) Apical turn Rosenthal’s canal at 20X magnification. (iii) Basal turn Rosenthal’s canal at 63X magnification. (C) Quantification demonstrating significantly decreased neurons in all cochlear turns in NF2-SWN as compared to controls. (D) Quantification of Schwann cells (SC) with significantly increased cells in middle and basal cochlear turns in NF2-SWN as compared to controls. (ns represents p ≥ 0.05, * represents p < 0.05; ** represents p < 0.01, *** represents p < 0.001, **** represents p < 0.0001).

Figure 3

Characterization and quantification of macrophages and ramification indices in normal hearing control and NF2-SWN ears. (A,B) Control and NF2-SWN spiral ganglia, respectively, in a mid-modiolar section. Staining: Neurons, TUJ, green; Macrophages, IBA1+, greyscale; DAPI nuclear counterstain, blue. (i) Middle turn Rosenthal’s canal at 20X magnification. (ii) Apical turn Rosenthal’s canal at 20X magnification. (iii) Middle turn Rosenthal’s canal at 189X magnification. (C) Quantification of macrophages with no significant difference in any cochlear turns in NF2-SWN as compared to controls. (D) Comparison of macrophage ramification index with significantly lower ramification indices in NF2-SWN as compared to controls in apical and middle turns. (ns represents p ≥ 0.05, * represents p < 0.05; ** represents p < 0.01, **** represents p < 0.001).

Figure 4

Relative abundance of neurons, Schwann cells and macrophages in control and NF2-SWN cochleae. Neurons were found to have a significantly lower proportion in NF2-SWN cochlea versus control, while Schwann cells had a significantly higher proportion. Macrophages maintained a near stable proportion in both disease and control states.