Mitochondrial and bioenergetic dysfunction in trauma-induced painful peripheral neuropathy

Abstract

Background: Mitochondrial dysfunction is observed in various neuropathic pain phenotypes, such as chemotherapy induced neuropathy, diabetic neuropathy, HIV-associated neuropathy, and in Charcot-Marie-Tooth neuropathy. To investigate whether mitochondrial dysfunction is present in trauma-induced painful mononeuropathy, a time-course of mitochondrial function and bioenergetics was characterized in the mouse partial sciatic nerve ligation model.

Results: Traumatic nerve injury induces increased metabolic indices of the nerve, resulting in increased oxygen consumption and increased glycolysis. Increased metabolic needs of the nerve are concomitant with bioenergetic and mitochondrial dysfunction. Mitochondrial dysfunction is characterized by reduced ATP synthase activity, reduced electron transport chain activity, and increased futile proton cycling. Bioenergetic dysfunction is characterized by reduced glycolytic reserve, reduced glycolytic capacity, and increased non-glycolytic acidification.

Conclusion: Traumatic peripheral nerve injury induces persistent mitochondrial and bioenergetic dysfunction which implies that pharmacological agents which seek to normalize mitochondrial and bioenergetic dysfunction could be expected to be beneficial for pain treatment. Increases in both glycolytic acidification and non-glycolytic acidification suggest that pH sensitive drugs which preferentially act on acidic tissue will have the ability to preferential act on injured nerves without affecting healthy tissues.

Fig. 1

The bioenergetic profile of mouse sciatic nerves can be assessed by Seahorse metabolic assay. Sciatic nerves from mice were isolated and cut into small 1 mm hemi-segments. Tissue from a 3 mm long segment of nerve was placed into a single well. a Oxygen consumption rate was measured, and the oxygen consumption rate in response to oligomycin, FCCP, and antimycin A with rotenone was determined. This allowed ex vivo measurement of oxygen consumption specific to total cellular respiration, mitochondrial respiration, mitochondrial ATP production, mitochondrial proton leak, mitochondrial maximal respiration, mitochondrial spare respiratory capacity, and non-mitochondrial respiration. b Extracellular acidification rate can also be measured by this method. Ex vivo measurement of the extracellular acidification rate corresponding to total extracellular acidification, glycolysis, glycolytic capacity, glycolytic reserve, and non-glycolytic acidification can be measured following administration of oligomycin and 2-deoxyglucose to the tissue culture medium

Fig. 2

Partial sciatic nerve ligation causes persistently increased metabolic needs of injured nerves. a Total cellular oxygen consumption is persistently increased following nerve injury. This demonstrates that under ex vivo conditionsf b The oxygen consumption specific to mitochondrial respiration is also persistently increased following nerve injury. c Total extracellular acidification levels are persistently increased following nerve injury, demonstrating that injured nerves produce more acid than uninjured nerves. d Glycolysis specific extracellular acidification is also increased

Fig. 3

Partial sciatic nerve ligation induces persistent mitochondrial dysfunction in injured nerves. a The proportion of the oxygen consumption rate specific to mitochondrial ATP production is persistently reduced following partial sciatic nerve ligation, demonstrating dysfunction in mitochondrial ATP production. b Additionally, the mitochondria of injured nerves have no spare capacity. When stimulated pharmacologically with a proton uncoupling agent, mitochondria are unable to consume any more oxygen after nerve injury. This suggests that the mitochondria in injured nerves are under bioenergetic stress and are already operating at their maximum capabilities for ATP production. c Maximal mitochondrial respiration is persistently reduced following nerve injury. This demonstrates that nerve injury reduces the ability of mitochondria to perform oxidative processes. d Futile proton cycling, or proton leak, is increased in mitochondria following nerve injury. Mitochondria undergo changes to allow more protons to passively dissipate the proton motive force after nerve injury. This avails fewer protons for ATP synthesis

Fig. 4

Partial sciatic nerve ligation induces persistent glycolytic dysfunction in injured nerves. a The proportion of the extracellular acidification rate specific to glycolysis is persistently reduced following partial sciatic nerve ligation. This demonstrates that the ability of the nerve to increase glycolytic activity in response to bioenergic stress is reduced following nerve injury. b Glycolytic capacity is also persistently reduced following partial sciatic nerve ligation. A reduction in glycolytic capacity suggests injured nerve tissue is less capable of producing ATP by glycolytic mechanisms

Fig. 5

Nerve injury does not affect non-mitochondrial respiration, but increases non-glycolytic acidification. a Nerve injury induces no proportionate change in non-mitochondrial respiration. This suggests that oxygen consumption of non-mitochondrial processes such as cellular oxidases do not undergo alterations following nerve injury. b Nerve injury induces a persistent increase in non-glycolytic acidification. This suggests that non-glycolytic processes which produce acids are increased following nerve injury. Increased acidification may have undesirable effects on sensory neuron excitability