Reducing monocarboxylate transporter MCT1 worsens experimental diabetic peripheral neuropathy

Abstract

Diabetic peripheral neuropathy (DPN) is one of the most common complications in diabetic patients. Though the exact mechanism for DPN is unknown, it clearly involves metabolic dysfunction and energy failure in multiple cells within the peripheral nervous system. Lactate is an alternate source of metabolic energy that is increasingly recognized for its role in supporting neurons. The primary transporter for lactate in the nervous system, monocarboxylate transporter-1 (MCT1), has been shown to be critical for peripheral nerve regeneration and metabolic support to neurons/axons. In this study, MCT1 was reduced in both sciatic nerve and dorsal root ganglia in wild-type mice treated with streptozotocin (STZ), a common model of type-1 diabetes. Heterozygous MCT1 null mice that developed hyperglycemia following STZ treatment developed a more severe DPN compared to wild-type mice, as measured by greater axonal demyelination, decreased peripheral nerve function, and increased numbness to innocuous low-threshold mechanical stimulation. Given that MCT1 inhibitors are being developed as both immunosuppressive and chemotherapeutic medications, our results suggest that clinical development in patients with diabetes should proceed with caution. Collectively, our findings uncover an important role for MCT1 in DPN and provide a potential lead toward developing novel treatments for this currently untreatable disease.

Keywords: Diabetic peripheral neuropathy; Dorsal root ganglion; Metabolism; Monocarboxylate transporter; Peripheral nerve.

Figure 1.. Expression of MCT1 in sciatic nerve and DRG of diabetic mice.

The relative expression of MCT1 mRNA in sciatic nerve (A) and DRG (B) after 2 and 6 weeks of STZ treatment, depicted as fold change compared with wild-type mice normalized to their corresponding GAPDH mRNA levels. Mean ± SEM, n = 3–5 per group, *p < 0.05, ***p < 0.001; ns = not significant, one-way ANOVA with Bonferroni's multiple comparisons test.

Figure 2.. Blood glucose levels and body weight pre- and post-STZ and insulin treatment.

Blood glucose levels (A) and body weight (B) were measure before and 3 days after STZ administration in Het MCT1-null mice and Wild-type littermates. Both groups of mice showed identical extent of hyperglycemia and body weight pre- and post-STZ treatment, n = 6–8 per group. Blood glucose levels (C) were measured in Het MCT1-null mice and Wild-type littermates pre- and post-STZ, as well as following treatment with insulin pellets. n= 4-5 per group. Mean ± SEM, ns = not significant, two-way ANOVA with Bonferroni's multiple comparisons test.

Figure 3.. Impact of MCT1 deficiency on nerve conductions after diabetes induction.

Sensory (A) and motor (C) nerve conduction velocities and SNAP (B) and CMAP (D) amplitudes in Het MCT1-null mice and control littermates before and after STZ administration with hyperglycemia, n = 7–15 per group. Sensory (E) and motor (G) nerve conduction velocities and SNAP (F) and CMAP (H) amplitudes in Het MCT1-null mice and control littermates before and after STZ administration without hyperglycemia, n = 5-8 per group. Mean ± SEM, *p < 0.05, **p < 0.01; two-way ANOVA with Bonferroni's multiple comparisons test. CMAP, compound muscle action potential; NCV, nerve conduction velocity; SNAP, sensory nerve action potential; N1 Glucose, STZ administration without hyperglycemia.

Figure 4.. Impact of MCT1 deficiency on sensory behavioral testing after diabetes induction.

Paw withdrawal frequency to mechanical stimulation by calibrated von Frey monofilaments of forces 0.07 g (A) and 0.45 g (B) and paw withdrawal latency (C) to thermal stimulation by radiant paw-heating assay in Het MCT1-null mice and Wild-type littermates before and after STZ administration with hyperglycemia, n = 6–13 per group. Paw withdrawal frequency to mechanical stimulation by calibrated von Frey monofilaments of forces 0.07 g (D) and 0.45 g (E) and paw withdrawal latency (F) to thermal stimulation by radiant paw-heating assay in Het MCT1-null mice and Wild-type littermates before and after STZ administration without hyperglycemia, n = 5-8 per group. Current set at baseline level: 20%, 10–12 s; cut off time; 30 s. Mean ± SEM, *p < 0.05; **p <0 .01; ***p <0 .001; ****p <0 .0001, two-way ANOVA with Bonferroni's multiple comparisons test. N1 Glucose, STZ administration without hyperglycemia

Figure 5.. Impact of reduced MCT1 on sural nerve myelination and integrity in diabetes.

(A) Light microscope photomicrographs of toluidine blue-stained sections of sural nerves from Het MCT1-null mice and Wild-type littermates with hyperglycemia 10 weeks after STZ treatment. These images were analyzed for g ratio (B), scatter plot graph displaying g ratio (y-axis) in relation to axon diameter (x-axis) of individual fiber (C), myelin thickness (D), and the diameter (E) and number (F) of myelinated axons. Mean ± SEM, n = 3 per group, **p < 0.01; ***p < .001; ns = not significant, unpaired t test. Scale bar, 20 μm. The g ratio between wild-type (blue line) and Het MCT1-null (red line) mice (C) was significantly different (p < 0.0001; t = 30.75, df = 3,177, unpaired t test). (H) IENFD obtained from the footpads of control or diabetic mice at 10 weeks after STZ administration following immunohistochemical staining for PGP9.5. Mean ± SEM, n = 4–7 per group, ns = not significant, two-way ANOVA with Bonferroni's multiple comparisons test. IENFD, intraepidermal nerve fiber density.