Ecto- and cytosolic 5′-nucleotidases in normal and AMP deaminase-deficient human skeletal muscle

Proteinuria in AMPD2-deficient mice

The AMPD2 gene, a member of the AMPD gene family encoding AMP deaminase, is widely expressed in nonmuscle tissues including kidney, although its functions have not been fully elucidated. In this study, we studied the function of the AMPD2 gene by establishing AMPD2-deficient model animal. We established AMPD2 knockout mice by using gene transfer and homologous recombination in murine ES cells and studied phenotypes and functions in the kidneys of these animals. AMPD activity was decreased from 22.9 mIU/mg protein to 2.5 mIU/mg protein in the kidneys of AMPD knockout mice. In addition to changes in nucleotide metabolism in the kidneys, proteinuria was found in 3-week-old AMPD2 knockout mice, followed by a further increment up to a peak level at 6 weeks old (up to 0.6 g/dL). The major protein component in the urine of AMPD2 knockout mice was found to be albumin, indicating that AMPD2 may have a key role in glomerular filtration. Indeed, an ultrastructure study of glomerulus specimens from these mice showed effacement of the podocyte foot processes, resembling minimal-change nephropathy in humans. Based on our results, we concluded that AMPD2 deficiency induces imbalanced nucleotide metabolism and proteinuria, probably due to podocyte dysfunction.

Suppression of 5'-nucleotidase enzymes promotes AMP-activated protein kinase (AMPK) phosphorylation and metabolism in human and mouse skeletal muscle

The 5'-nucleotidase (NT5) family of enzyme dephosphorylates non-cyclic nucleoside monophosphates to produce nucleosides and inorganic phosphates. We hypothesized that gene silencing of NT5 enzymes to increase the intracellular availability of AMP would increase AMP-activated protein kinase (AMPK) activity and metabolism. We determined the role of cytosolic NT5 in metabolic responses linked to the development of insulin resistance in obesity and type 2 diabetes. Using siRNA to silence NT5C2 expression in cultured human myotubes, we observed a 2-fold increase in the AMP/ATP ratio, a 2.4-fold increase in AMPK phosphorylation (Thr(172)), and a 2.8-fold increase in acetyl-CoA carboxylase phosphorylation (Ser(79)) (p < 0.05). siRNA silencing of NT5C2 expression increased palmitate oxidation by 2-fold in the absence and by 8-fold in the presence of 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside. This was paralleled by an increase in glucose transport and a decrease in glucose oxidation, incorporation into glycogen, and lactate release from NT5C2-depleted myotubes. Gene silencing of NT5C1A by shRNA injection and electroporation in mouse tibialis anterior muscle reduced protein content (60%; p < 0.05) and increased phosphorylation of AMPK (60%; p < 0.05) and acetyl-CoA carboxylase (50%; p < 0.05) and glucose uptake (20%; p < 0.05). Endogenous expression of NT5C enzymes inhibited basal lipid oxidation and glucose transport in skeletal muscle. Reduction of 5'-nucleotidase expression or activity may promote metabolic flexibility in type 2 diabetes.