Mitochondrial fission is an acute and adaptive response in injured motor neurons

Abstract

Successful recovery from neuronal damage requires a huge energy supply, which is provided by mitochondria. However, the physiological relevance of mitochondrial dynamics in damaged neurons in vivo is poorly understood. To address this issue, we established unique bacterial artificial chromosome transgenic (BAC Tg) mice, which develop and function normally, but in which neuronal injury induces labelling of mitochondria with green fluorescent protein (GFP) and expression of cre recombinase. GFP-labelled mitochondria in BAC Tg mice appear shorter in regenerating motor axons soon after nerve injury compared with mitochondria in non-injured axons, suggesting the importance of increased mitochondrial fission during the early phase of nerve regeneration. Crossing the BAC Tg mice with mice carrying a floxed dynamin-related protein 1 gene (Drp1), which is necessary for mitochondrial fission, ablates mitochondrial fission specifically in injured neurons. Injury-induced Drp1-deficient motor neurons show elongated or abnormally gigantic mitochondria, which have impaired membrane potential and axonal transport velocity during the early phase after injury, and eventually promote neuronal death. Our in vivo data suggest that acute and prominent mitochondrial fission during the early stage after nerve injury is an adaptive response and is involved in the maintenance of mitochondrial and neuronal integrity to prevent neurodegeneration.

Damage-induced neuronal endopeptidase (DINE) enhances axonal regeneration potential of retinal ganglion cells after optic nerve injury

“…25 The Atf3 :BAC Tg mouse expresses GFP specifically in injured neurons, which mimics transcriptional regulation of Atf 3. Because the mitochondrial targeting signal is attached to GFP in the mouse, the GFP is imported into mitochondria immediately after synthesis, resides in the soma, and is transported into axonal tips and dendrites.…”

The regulation of mitochondrial dynamics in neurite outgrowth by retinoic acid receptor β signaling

“…2 A – E ). Increased fission is a behavior associated with mitochondrial biogenesis (21) and previously observed in regenerating axons (22, 23). …”

Three‐dimensional analysis of somatic mitochondrial dynamics in fission‐deficient injured motor neurons using FIB/SEM

“…As described above, these intriguing morphological changes of mitochondria in Drp1 CKO injured neurons were seen at 1wk after injury. The previous reports using the same Drp1‐deficient model (Kiryu‐Seo et al, ) showed decreased mitochondria integrity, including reduced membrane‐potential detected in injured axon with marker TMRM, and in injured soma with accumulation of Parkin, a maker for dysfunctional mitochondria (Narendra, Tanaka, Suen, & Youle, ). Therefore, it is likely that the mitochondrial morphological changes of Drp1 CKO mice shown in this study were correlating with the mitochondrial function to some extent.…”

“…The present FIB/SEM technology allows for the reconstruction of 3D images of organelles; the images of several hundreds of serial sections can be obtained by SEM and the resolution of these images are comparable to those obtained with TEM. After examining 3D morphologies of somatic mitochondria of motor neurons before and after nerve injury using FIB/SEM, we found no significant changes in morphology of somatic mitochondria, although the size of axonal mitochondria became smaller (Kiryu‐Seo et al, ). Drp1 deficiency in injured motor neurons, however, induced drastic changes of somatic mitochondrial morphology as well as the degeneration of mitochondria demonstrating huge spheroidal morphology with local or total cristae collapse, and mitophagy.…”

“…Multiple studies have revealed that neuronal damage induces mitochondrial fragmentation, especially in axons (Cho et al, ; Kiryu‐Seo, Ohno, Kidd, Komuro, & Trapp, ; Song, Song, Kincaid, Bossy, & Bossy‐Wetzel, ; Wang et al, ). Recently, we found that the size of mitochondria in axons is reduced in a nerve injury model compared with normal conditions, with faster transportation enabling the effective delivery of mitochondria to the damaged or regenerating axonal tip (Kiryu‐Seo et al, ). In the same study, we successfully ablated the fission regulatory protein, dynamin‐related protein 1 (Drp1), specifically in injured motor neurons using injury‐inducible Cre driver mice, Atf3 :BAC Tg mice, because the Atf3 promoter is specifically activated in response to nerve injury (Nakagomi, Suzuki, Namikawa, Kiryu‐Seo, & Kiyama, ; Kiryu‐Seo & Kiyama, ; Kiryu‐Seo et al, ).…”