Coenzyme Q10 prevents peripheral neuropathy and attenuates neuron loss in the db-/db- mouse, a type 2 diabetes model

Abstract

Diabetic peripheral neuropathy (DPN) is the most common complication in both type 1 and type 2 diabetes. Here we studied some phenotypic features of a well-established animal model of type 2 diabetes, the leptin receptor-deficient db(-)/db(-) mouse, and also the effect of long-term (6 mo) treatment with coenzyme Q10 (CoQ10), an endogenous antioxidant. Diabetic mice at 8 mo of age exhibited loss of sensation, hypoalgesia (an increase in mechanical threshold), and decreases in mechanical hyperalgesia, cold allodynia, and sciatic nerve conduction velocity. All these changes were virtually completely absent after the 6-mo, daily CoQ10 treatment in db(-)/db(-) mice when started at 7 wk of age. There was a 33% neuronal loss in the lumbar 5 dorsal root ganglia (DRGs) of the db(-)/db(-) mouse versus controls at 8 mo of age, which was significantly attenuated by CoQ10. There was no difference in neuron number in 5/6-wk-old mice between diabetic and control mice. We observed a strong down-regulation of phospholipase C (PLC) β3 in the DRGs of diabetic mice at 8 mo of age, a key molecule in pain signaling, and this effect was also blocked by the 6-mo CoQ10 treatment. Many of the phenotypic, neurochemical regulations encountered in lumbar DRGs in standard models of peripheral nerve injury were not observed in diabetic mice at 8 mo of age. These results suggest that reactive oxygen species and reduced PLCβ3 expression may contribute to the sensory deficits in the late-stage diabetic db(-)/db(-) mouse, and that early long-term administration of the antioxidant CoQ10 may represent a promising therapeutic strategy for type 2 diabetes neuropathy.

Fig. 1.

CoQ10 treatment prevents hypoalgesia in male db⁻/db⁻ mice. (A–C) Response to innoxious (von Frey filaments; A), noxious (pin prick; C), or cold (acetone; B) stimulations in db⁻/db⁻ and control groups at different weeks of age. (D–F) After CoQ10 treatment, response to mechanical (von Frey filaments; D), noxious (pin prick; F), or cold (acetone; E) stimulations in db⁻/db⁻ and control groups at 32/33 WoA. Q10⁻ and Q10⁺, without and with CoQ10 treatment, respectively. db⁻/db⁻, filled columns; control, open columns. *P < 0.05, **P < 0.01, ***P < 0.001 vs. control. All results are means ± SEM n = 5–20 mice.

Fig. 2.

CoQ10 treatment increases CV of the diabetic sciatic nerve at 32/33 WoA. (A) In vivo measurements. Superponated muscle responses of the gastrocnemius muscle to stimulation at the tibial (distal) and the sciatic (proximal) nerve sites, respectively (Materials and Methods). (B) In vitro measurements. Superponated compound action potentials from the sciatic nerve at increasing stimulation. Measurements at 24 °C. (C) CVs under in vivo conditions. (D) CVs under in vitro conditions. Con, control; ConQ10, CoQ10-treated control; db, diabetic; dbQ10, CoQ10-treated db⁻/db⁻ mice. Error bars indicate the SEM. Each data point in C and D is based on measurements from three to seven nerves (n = 21 in vivo and n = 23 in vitro) (Table S1). An ANOVA (Materials and Methods and SI Text) showed an interaction effect; CoQ10 treatment is beneficial for nerve CV in db⁻/db⁻ mice (P = 0.033 and P = 0.066 for in vivo and in vitro, respectively). (E) Relationship between CVs of fast (v₁) and slower (v₂) axons. Data from in vitro experiments.

Fig. 3.

CoQ10 attenuates neuron loss of DRG neurons in male db⁻/db⁻ mice. (A) Size distribution of DRG NPs of db⁻/db⁻ and control mice at 32/33 WoA. (B) Number of L5 DRG neurons in db⁻/db⁻ compared with control mice at 32/33 or 5/6 WoA. Q10⁻ and Q10⁺, without and with CoQ10 treatment, respectively. db⁻/db⁻, filled columns; control, open columns. *P < 0.05 vs. control. The results are means ± SEM (for NP) or SD (for number of neurons). n = 5–6 mice.

Fig. 4.

CoQ10 treatment prevents a reduction of PLCβ3 expression in diabetic DRGs of male mice. (A–E) Studies on DRGs of db⁻/db⁻ and control mice showing PLCβ3 mRNA (A–C) or protein expression (D and E) in db⁻/db⁻ (A and D) and control (B, C, and E) mice at 5/6 WoA, respectively. (F and G) PLCβ3 mRNA in db⁻/db⁻ (F) and control (G) mice at 32/33 WoA. (H–K) PLCβ3-LI in db⁻/db⁻ (H and J) and control (I and K) mice at 32/33 WoA without (H and I) or with CoQ10 (J and K) treatment. (L) Western blot shows PLCβ3 in db⁻/db⁻ mice (Left bands) and db⁻/db⁻ mice treated with CoQ10 (Right bands). (M and N) Intensity (M) or percentage (N) of PLCβ3-IR NPs in db⁻/db⁻ and control mice with or without CoQ10 treatment. **P < 0.01 vs. control. The results are means ± SEM db⁻/db⁻, filled columns; control, open columns. n = 5–10 mice. [Scale bars: 50 µm (A = B and C; D = E; F = G and H–K).]