a Investigating sodium-hydrogen exchanger 1 (NHE1) inhibition as a potential therapy for Duchenne muscular dystrophy

Abstract

The absence of dystrophin in Duchenne muscular dystrophy (DMD) muscle cells results in increased membrane permeability and subsequent intracellular calcium (Ca2+) overload. We postulated that the dysregulation of Ca2+ homeostasis is exacerbated by the increased activity of the sodium-hydrogen exchanger 1 (NHE1). NHE1 over-activity leads to an increased influx of sodium (Na+), which in turn switches the sodium-calcium exchanger (NCX) into reverse mode, resulting in an increased Ca2+ influx. Selective NHE1 inhibitors can be used to reduce the Na+ influx and thereby revert the NCX to normal mode with a subsequent decrease in the cellular Ca2+ load. This observation has led to the hypothesis that the use of specific NHE1 inhibitors could improve the Ca2+ homeostasis and alleviate pathology in DMD muscle. The current study investigated the efficacy of a specific NHE1 inhibitor, KR-33028, that has a good safety and potency profile in several pre-clinical studies. In order for the efficacy of the drug to be determined, dystrophin-deficient mdx mice were treated chronically via drug in chow. The Ca2+ dynamics in both skeletal and cardiac muscles were studied using manganese-enhanced magnetic resonance imaging (MEMRI). Additionally, cine cardiac MRI was carried out to assess the development and progression of cardiomyopathy. Interestingly, four-limb functional grip strength tests that were carried out throughout the treatment demonstrated a significant increase of the grip strength of the drug-treated mice, in comparison to the vehicle-treated ones; and histological analysis demonstrated a significant decrease of fibrosis in the diaphragm of treated mice. Additionally, proteomic analysis showed that chronic treatment with KR- 33028 resulted in a significant change of abundance of key skeletal muscle proteins and impacted on the Ca2+ machinery, further providing evidence to support our theory. The proposed studies with a first prototype NHE1 inhibitor are an important step towards potential clinical trials for dystrophinopathies with this class of compounds.