Absence of Survival and Motor Deficits in 500 Repeat C9ORF72 BAC Mice.

A hexanucleotide repeat expansion at C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis (ALS)/frontotemporal dementia (FTD). Initial studies of bacterial artificial chromosome (BAC) transgenic mice harboring this expansion described an absence of motor and survival phenotypes. However, a recent study by Liu and colleagues described transgenic mice harboring a large repeat expansion (C9-500) and reported decreased survival and progressive motor phenotypes. To determine the utility of the C9-500 animals for understanding degenerative mechanisms, we validated and established two independent colonies of transgene carriers. However, extended studies of these animals for up to 1 year revealed no reproducible abnormalities in survival, motor function, or neurodegeneration. Here, we propose several potential explanations for the disparate nature of our findings from those of Liu and colleagues. Resolving the discrepancies we identify will be essential to settle the translational utility of C9-500 mice. This Matters Arising paper is in response to Liu et al. (2016), published in Neuron. See also the response by Nguyen et al. (2020), published in this issue.

Reducing monocarboxylate transporter MCT1 worsens experimental diabetic peripheral neuropathy.

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.

Monocarboxylate transporter 1 in Schwann cells contributes to maintenance of sensory nerve myelination during aging.

Schwann cell (SC)-specific monocarboxylate transporter 1 (MCT1) knockout mice were generated by mating MCT1 f/f mice with myelin protein zero (P0)-Cre mice. P0-Cre+/- , MCT1 f/f mice have no detectable early developmental defects, but develop hypomyelination and reduced conduction velocity in sensory, but not motor, peripheral nerves during maturation and aging. Furthermore, reduced mechanical sensitivity is evident in aged P0-Cre+/- , MCT1 f/f mice. MCT1 deletion in SCs impairs both their glycolytic and mitochondrial functions, leading to altered lipid metabolism of triacylglycerides, diacylglycerides, and sphingomyelin, decreased expression of myelin-associated glycoprotein, and increased expression of c-Jun and p75-neurotrophin receptor, suggesting a regression of SCs to a less mature developmental state. Taken together, our results define the contribution of SC MCT1 to both SC metabolism and peripheral nerve maturation and aging.