Reduced mitochondrial size in hippocampus and psychiatric behavioral changes in the mutant mice with homologous mutation of Timm8a1-I23fs49X

Background: Deafness-dystonia-optic neuronopathy (DDON) syndrome, a condition that predominantly affects males, is caused by mutations in translocase of mitochondrial inner membrane 8A (TIMM8A)/deafness dystonia protein 1 (DDP1) gene and characterized by progressive deafness coupled with other neurological abnormalities. In a previous study, we demonstrated the phenotype of male mice carrying the hemizygous mutation of Timm8a1-I23fs49X. In a follow-up to that study, this study aimed to observe the behavioral changes in the female mutant (MUT) mice with homologous mutation of Timm8a1 and to elucidate the underlying mechanism for the behavioral changes.

Materials and methods: Histological analysis, transmission electron microscopy (EM), Western blotting, hearing measurement by auditory brainstem response (ABR), and behavioral observation were compared between the MUT mice and wild-type (WT) littermates.

Results: The weight of the female MUT mice was less than that of the WT mice. Among MUT mice, both male and female mice showed hearing impairment, anxiety-like behavior by the elevated plus maze test, and cognitive deficit by the Morris water maze test. Furthermore, the female MUT mice exhibited coordination problems in the balance beam test. Although the general neuronal loss was not found in the hippocampus of the MUT genotype, EM assessment indicated that the mitochondrial size showing as aspect ratio and form factor in the hippocampus of the MUT strain was significantly reduced compared to that in the WT genotype. More importantly, this phenomenon was correlated with the upregulation of translation of mitochondrial fission process protein 1(Mtfp1)/mitochondrial 18 kDa protein (Mtp18), a key fission factor that is a positive regulator of mitochondrial fission and mitochondrial size. Interestingly, significant reductions in the size of the uterus and ovaries were noted in the female MUT mice, which contributed to significantly lower fertility in the MUT mice.

Conclusion: Together, a homologous mutation in the Timm8a1 gene caused the hearing impairment and psychiatric behavioral changes in the MUT mice; the latter phenotype might be related to a reduction in mitochondrial size regulated by MTP18.