Genetic Hearing Loss Associated With Autoinflammation

Abstract

Sensorineural hearing loss can result from dysfunction of the inner ear, auditory nerve, or auditory pathways in the central nervous system. Sensorineural hearing loss can be associated with age, exposure to ototoxic drugs or noise, or mutations in nuclear or mitochondrial genes. However, it is idiopathic in some patients. Although these disorders are mainly caused by dysfunction of the inner ear, little of the pathophysiology in sensorineural hearing loss is known due to inaccessibility of the living human inner ear for biopsy and pathological analysis. The inner ear has previously been thought of as an immune-privileged organ. We recently showed that a missense mutation of the NLRP3 gene is associated with autosomal-dominant sensorineural hearing loss with cochlear autoinflammation in two unrelated families. NLRP3 encodes the NLRP3 protein, a key component of the NLRP3 inflammasome that is expressed in immune cells, including monocytes and macrophages. Gain-of-function mutations of NLRP3 cause abnormal activation of the NLRP3 inflammasome leading to IL-1β secretion in a spectrum of autosomal dominant systemic autoinflammatory phenotypes termed cryopyrin-associated periodic syndromes. The affected subjects of our two families demonstrated atypical phenotypes compared with those reported for subjects with cryopyrin-associated periodic syndromes. These observations led us to test the hypothesis that macrophage/monocyte-like cells in the cochlea can mediate local autoinflammation via activation of the NLRP3 inflammasome. The inflammasome can indeed be activated in macrophage/monocyte-like cells of the mouse cochlea, with secretion of IL-1β. The macrophage/monocyte-like cells in the cochlea were also found to be associated with hearing loss in a Slc26a4-insufficient mouse model of human deafness. This review addresses our understanding of genetic hearing loss mediated by autoinflammation and macrophage/monocyte-like cells in the cochlea.

Keywords: NLRP3; Pendred syndrome; SLC26A4; cryopyrin-associated periodic syndromes; hearing loss; interleukin-1β; macrophage.

Figure 1

NLRP3 inflammasome activation. Stimulation of Toll-like receptors (TLRs), IL-1 receptor (IL-1R), or tumor necrosis factor receptor (TNFR) increases NLRP3 and pro–IL-1β protein expression (priming step). The activation step involves efflux of K⁺, which can be initiated by pore-forming toxin or ATP-triggered channel P2X7, or Ca²⁺ influx. When the NLRP3 inflammasome is activated, the PYD domain mediates recruitment of ASC and procaspase-1 to form a complex that cleaves inactive procaspase-1 to generate active caspase-1. Caspase-1 can process pro–IL-1β to mature IL-1β, which is secreted. Reproduced from Figure S1, Nakanishi et al. (6).

Figure 2

Resident macrophage-like cells in mouse cochlea. (A) P4 Cx3cr1^GFP/+ cochleae have GFP⁺ cells scattered throughout the cochlea, including the auditory nerve (AN), spiral ganglion (SG), basilar membrane (BM), stria vascularis (SV), and spiral ligament (SL). Phalloidin (red) and DAPI (blue) label F-actin and nuclei, respectively. (B) Lateral wall of a P4 Cx3cr1^GFP/+ cochlea with GFP⁺ cells mainly localized around blood vessels (blue, labeled with anti-CD34 antibody). SV, stria vascularis; SP, spiral prominence; OS, outer sulcus; scale bars, 100 μm. Reproduced from Figure 7, Nakanishi et al. (6). GFP⁺ cells are similarly distributed in all parts of the cochlea at P30 (6).

Figure 3

Cultured lateral wall of a P4 Cx3cr1^GFP/+ cochlea stained with anti-IL-1β antibody (red). Pro-IL-1β immunoreactivity is present in a subset of GFP⁺ cells from cultured cochleae stimulated with LPS, whereas no immunoreactivity is detected without stimulation. Scale bars, 50 μm. Reproduced from Figure 10, Nakanishi et al. (6).

Figure 4

Whole-mount preparation of stria vascularis at 1 month of age stained with anti-CD68 (green) and anti-CD34 antibodies (blue). Anti-CD68 antibodies stain cells of the macrophage–monocyte lineage in control (Slc26a4^+/+; left), Slc26a4-insufficient (Tg[E];Tg[R];Slc26a4^Δ/Δ; middle) and Slc26a4-null (Slc26a4^Δ/Δ; right) stria vascularis. The area of anti-CD68 staining was higher in Slc26a4-insufficient ears compared to control ears, and the area of staining was even higher in Slc26a4-null ears. Slc26a4-insufficient (Tg[E];Tg[R];Slc26a4^Δ/Δ) mice have the effector transgene (Tg[E]) and the responder transgene (Tg[R]). All Slc26a4 expression is under the control of doxycycline (33). Blood vessels were stained with anti-CD34 antibodies (blue). Scale bar, 40 μm. Reproduced from Figure 7, Ito et al. (32).

Figure 5

Pigmentation granules and macrophages in the stria vascularis. (A) Whole mount preparation of P30 Slc26a4-insufficient (Tg[E];Tg[R];Slc26a4^Δ/Δ) stria vascularis stained with anti-CD68 antibody (green). Many pigmentations granules of variable size and shape were observed, some of which were associated with macrophages labeled by CD68 antibody (arrows). Macrophages appear to phagocytose parts of aggregated pigmentation and degenerated intermediate cells. (B) P30 Slc26a4-insufficient cochlear section stained with toluidine blue. Pigmentation is localized in the intermediate cell layer. Slc26a4-insufficient (Tg[E];Tg[R];Slc26a4^Δ/Δ) mice have the effector transgene (Tg[E]) and the responder transgene (Tg[R]). All Slc26a4 expression is under the control of doxycycline (33). Reproduced from Figure 9, Ito et al. (32).