Mitochondrial alterations in the cochlea of Cdk5rap1-knockout mice with age-related hearing loss

Abstract

Previous studies have revealed that age-related hearing loss (AHL) in Cdk5 regulatory subunit-associated protein 1 (Cdk5rap1)-knockout mice is associated with pathology in the cochlea. Here, we aimed to identify mitochondrial alterations in the cochlea of Cdk5rap1-knockout mice with AHL. Mitochondria in the spiral ganglion neurons (SGNs) and hair cells (HCs) were normal despite senescence; however, the mitochondria of types I, II, and IV spiral ligament fibrocytes were ballooned, damaged, and ballooned, respectively, in the stria vascularis. Our results suggest that the accumulation of dysfunctional mitochondria in the lateral wall, rather than the loss of HCs and SGNs, leads to the onset of AHL. Our results provide valuable information regarding the underlying mechanisms of AHL and the relationship between aberrant tRNA modification-induced hearing loss and mitochondrial dysfunction.

Keywords: Cdk5rap1; age-related hearing loss; microstructural findings; mitochondria; mitochondrial tRNA; transmission electron microscopy.

Fig. 1

Cochlear senescence in Cdk5rap1‐KO mice. An illustration of a typical adult cochlea (cross‐section) shows the senescent cells in blue and degeneration (daggers) in Cdk5rap1‐KO mice compared with those in control (CNT) mice. The cochlea of an adult mammal is divided into three compartments: the tympani, media, and scala vestibuli. The organ of Corti is located in the scala media, which can be observed in this cross‐sectional image. Scale bar = 100 μm. OC, organ of Corti; SC, supporting cell.

Fig. 2

Normal mitochondria observed in the OHCs of Cdk5rap1‐KO mice. (A) A cross‐sectional image of the organ of Corti illustrates the distribution of senescent cells in blue in Cdk5rap1‐KO mice in contrast with control (CNT) mice. The arrow indicates the investigated cells. (B) TEM images of the OHCs from the cochlear middle turn. The arrow indicates the investigated cells. Scale bar = 100 μm. (C) TEM and magnified images of the third row of OHCs from the middle turn of the cochlea and mitochondrial cristae structure in Cdk5rap1‐KO or littermate CNT mice at various ages. Upper: CNT mice; lower: KO mice; mt: mitochondria; N: nuclei. Scale bar = 1 μm. Daggers denote mitochondrial cristae loss. In magnified images, scale bars = 100 nm. (D) Damaged mitochondria in OHCs of CNT and Cdk5rap1‐KO mice. (E) Mitochondrial size in OHCs in CNT and Cdk5rap1‐KO mice of various ages. Blue: CNT mice; red: Cdk5rap1‐KO mice; ns: not significant.

Fig. 3

Normal mitochondria present in the SGNs of Cdk5rap1‐KO mice. (A) A cross‐sectional image of the SGN illustrates the distribution of the senescent cells in blue in Cdk5rap1‐KO mice compared with that in control (CNT) mice. Arrow indicates the investigated cells. (B) TEM images of SGNs from the cochlear middle turn. The arrow indicates the investigated cells. Scale bar = 100 μm. (C) TEM and magnified images of SGNs from the cochlear middle turn. The mitochondrial cristae structure of CNT or Cdk5rap1‐KO mice at various ages. Upper: CNT mice; lower: Cdk5rap1‐KO mice; mt: mitochondria; N: nuclei. Scale bar = 1 μm. Daggers indicate the absence of cristae of mitochondria. In magnified images, scale bars = 100 nm. (D) Damaged mitochondria in SGNs of CNT and Cdk5rap1‐KO mice. (E) Size of mitochondria in SGNs in CNT and Cdk5rap1‐KO mice of various ages. Blue: CNT mice; red: Cdk5rap1‐KO mice; ns: not significant.

Fig. 4

Ballooning of mitochondria observed in the SV of Cdk5rap1‐KO mice. (A) A cross‐sectional image of the SV illustrates the distribution of the senescent cells (blue) in Cdk5rap1‐KO mice versus that in control (CNT) mice. Arrow indicates the investigated cells. (B) TEM images of the SV from the cochlear middle turn. Arrow indicates the investigated cells. Scale bar = 100 μm. (C) TEM and magnified images of SV marginal cells from the cochlear middle turn. Age‐related changes in the mitochondrial cristae structure of Cdk5rap1‐KO or CNT mice. Upper: CNT mice; lower: KO; mt: mitochondria; N: nuclei. Scale bar = 1 μm. Daggers indicate mitochondrial cristae loss. In magnified images, scale bars = 100 nm. (D) Damaged mitochondria in the SVs of CNT and Cdk5rap1‐KO mice. (E) Size of mitochondria in the SVs of Cdk5rap1‐KO and CNT mice at various ages. Blue: CNT mice; red: Cdk5rap1‐KO; *P < 0.05; **P < 0.01; ns, nonsignificant.

Fig. 5

Disruptions of mitochondria are present in fibrocyte types I, II, and IV of the SLi in Cdk5rap1‐KO mice, with ballooning of mitochondria observed in type II and IV fibrocytes of the SLi in Cdk5rap1‐KO mice. (A) A cross‐sectional image of the cochlear lateral wall illustrates the distribution of senescent cells in blue in Cdk5rap1‐KO mice contrasted with that in control (CNT) mice. The arrow and arrowhead indicate the investigated cells. (B) TEM images of the SLi from the cochlear middle turn. Arrow and arrowhead indicate the investigated cells. Scale bar = 100 μm. (C) TEM and magnified images of type I fibrocytes of the SLi from the middle cochlear turn. In the cristae structure of the mitochondria, there were age‐related differences between Cdk5rap1‐KO and littermate CNT mice. Upper: CNT mice; lower: Cdk5rap1‐KO mice; mt: mitochondria; N: nuclei. Scale bar = 1 μm. Daggers denote mitochondrial cristae loss. In magnified images, scale bars = 100 nm. (D) Damaged mitochondria in type I fibrocyte of the SLi in CNT and Cdk5rap1‐KO mice. (E) Size of mitochondria in type I fibrocytes of the SLi in Cdk5rap1‐KO and CNT mice at various ages. (F) TEM and magnified images of fibrocytes (type II and IV) from the middle cochlear turn SLi. The cristae structure in the mitochondria of littermate CNT or Cdk5rap1‐KO mice at various ages. Upper: CNT mice; lower: Cdk5rap1‐KO; mt: mitochondria; N: nuclei. Scale bar = 1 μm. Daggers indicate mitochondrial cristae loss. In magnified images, scale bars = 100 nm. (G) Damaged mitochondria in fibrocytes (type II and IV) in Cdk5rap1‐KO and CNT mice SLis. (H) Size of mitochondrial in fibrocytes (type II and IV) of the SLi in Cdk5rap1‐KO and CNT mice at different ages. Blue: CNT mice; red: Cdk5rap1‐KO mice; *P < 0.05; **P < 0.01; ***P < 0.001; ns, nonsignificant.