Diabetes regulates mitochondrial biogenesis and fission in mouse neurons

Abstract

Aims/hypothesis: Normal mitochondrial activity is a critical component of neuronal metabolism and function. Disruption of mitochondrial activity by altered mitochondrial fission and fusion is the root cause of both neurodegenerative disorders and Charcot-Marie-Tooth type 2A inherited neuropathy. This study addressed the role of mitochondrial fission in the pathogenesis of diabetic neuropathy.

Methods: Mitochondrial biogenesis and fission were assayed in both in vivo and in vitro models of diabetic neuropathy. Gene, protein, mitochondrial DNA and ultrastructural analyses were used to assess mitochondrial biogenesis and fission.

Results: There was greater mitochondrial biogenesis in dorsal root ganglion neurons from diabetic compared with non-diabetic mice. An essential step in mitochondrial biogenesis is mitochondrial fission, regulated by the mitochondrial fission protein dynamin-related protein 1 (DRP1). Evaluation of diabetic neurons in vivo indicated small, fragmented mitochondria, suggesting increased fission. In vitro studies revealed that short-term hyperglycaemic exposure increased levels of DRP1 protein. The influence of hyperglycaemia-mediated mitochondrial fission on cell viability was evaluated by knockdown of Drp1 (also known as Dnm1l). Knockdown of Drp1 resulted in decreased susceptibility to hyperglycaemic damage.

Conclusions/interpretation: We propose that: (1) mitochondria undergo biogenesis in response to hyperglycaemia, but the increased biogenesis is insufficient to accommodate the metabolic load; (2) hyperglycaemia causes an excess of mitochondrial fission, creating small, damaged mitochondria; and (3) reduction of aberrant mitochondrial fission increases neuronal survival and indicates an important role for the fission-fusion equilibrium in the pathogenesis of diabetic neuropathy.

Fig. 1. Mitochondrial DNA levels increase in diabetic mice

Mitochondrial (Mt) DNA levels were determined by measuring cytochrome c DNA levels. Genomic DNA isolated from control (db/+) and diabetic (db/db) DRG neurons at 5 and 24 wk was subjected to real-time (RT) PCR using primers listed in table 1 (n = 4). * p < 0.05.

Fig. 2. Transcription of genes involved in mitochondria biogenesis is significantly increased in response to an acute response to hyperglycemia

RNA was extracted from DRG of control (db/+) and diabetic (db/db) mice at 5 and 24 wk of age. RT-PCR was used to analyze mRNA levels of genes involved in Mt biogenesis (PGC1-α, PGC1-β), Mt fission (Drp1 and Fis1), Mt fusion (MFN2) and Mt mass (COX-IV).

Fig. 3. Prolonged hyperglycemia significantly increases protein levels of PGC1-α, COX-IV, and Drp1

Immunoblot of mitochondrial proteins from DRG neurons of 5 wk (A) and 24 wk (B) control (db/+) and diabetic (db/db) mice. The blots were analyzed using antibodies against PGC1-α, Drp1, COX-IV and actin. Each lane represents DRG lysates from different animals. Numbers beneath bands indicate density relative to actin.

Fig. 4. Hyperglycemia leads to altered mitochondria dynamics and morphology in DRG neurons in vivo

Transmision electron micrographs of Mt from control (A) and diabetic neurons (B, C and D). Arrows indicate mitochondria. (D) Mitochondria from DRG neuron from diabetic (db/db) mice show slightly dilated intracristal space while the matrix is still dense. n, nucleus. Scale bar: A, B and C = 1 μm; D = 180 nm. (E) Quantitative analysis of mitochondria in DRG neurons from 24 wk old mice showed an increase in the number of Mt in the diabetic (db/db) compared to control (db/+) mice; p < 0.0001. (F) Mitochondria from diabetic (db/db) mice showed reduced diameters compared to control (db/+); p < 0.0001. n=2

Fig. 5. Hyperglycemia increases mitochondrial biogenesis and fission in DRG neurons in vitro

(A) DRG. Dissected and dissociated rat E15 DRGs were incubated in the presence of control (25 mM) or high glucose media (45 mM) for 6 h. Quantitation of cytochorome b DNA was used as a marker for MtDNA. n=3 (B) In vitro analysis of BrdU incorporation into MtDNA of DRG neurons under hyperglycemia. DRG neurons were cultured in the presence (or absence, ctrl no BrdU) of BrdU and normal (25 mM) or high glucose (45 mM) for 6 h. BrdU incorporation into MtDNA (arrows) was visualized by immunocytochemstry with tyramide amplified AlexaFluor 488 green signal (top panels) and merged with nuclear staining (DAPI, blue) and differential interference contrast (DIC) images (lower panels). Cells were visualized using an Olympus FluoView 500 laser scanning confocal microscope. Bar = 10 μm. (C) Quantitation of MtDNA was done by identifying BrdU-positive DRG soma. In vitro cultures were incubated in control or high glucose media for 6 h. n=10 per group (D) Western blot analysis of in vitro cultures DRG after control or treatment media exposure. Protein lysates were subjected to SDS-PAGE and analyzed with antibodies against COX-IV, Drp1, and actin (internal control).

Fig. 6. Drp1 promotes hyperglycemia-induced cell death in sensory neurons

Cultured Rat E15 DRG neurons were lentiviral infected (1:20 or 1:40 dilution) with non-specific miRNA (scrambled), Drp1 specific miRNA (mouse), or non-infected (control). Neurons were treated with high glucose (+) or control (-) media. Cell extracts were subjected to SDS-PAGE and analyzed by western blotting with antibodies for cleaved Caspase-3, Drp1 and actin (internal control).