Transcriptomic and metabolic analyses reveal salvage pathways in creatine-deficient AGAT(-/-) mice.

Skeletal muscles require energy either at constant low (e.g., standing and posture) or immediate high rates (e.g., exercise). To fulfill these requirements, myocytes utilize the phosphocreatine (PCr)/creatine (Cr) system as a fast energy buffer and shuttle. We have generated mice lacking L-arginine:glycine amidino transferase (AGAT), the first enzyme of creatine biosynthesis. These AGAT(-/-) (d/d) mice are devoid of the PCr/Cr system and reveal severely altered oxidative phosphorylation. In addition, they exhibit complete resistance to diet-induced obesity, which is associated with a chronic activation of AMP-activated protein kinase in muscle and white adipose tissue. The underlying metabolic rearrangements have not yet been further analyzed. Here, we performed gene expression analysis in skeletal muscle and a serum amino acid profile of d/d mice revealing transcriptomic and metabolic alterations in pyruvate and glucose pathways. Differential pyruvate tolerance tests demonstrated preferential conversion of pyruvate to alanine, which was supported by increased protein levels of enzymes involved in pyruvate and alanine metabolism. Pyruvate tolerance tests suggested severely impaired hepatic gluconeogenesis despite increased availability of pyruvate and alanine. Furthermore, enzymes of serine production and one-carbon metabolism were significantly up-regulated in d/d mice, indicating increased de novo formation of one-carbon units from carbohydrate metabolism linked to NAD(P)H production. Besides the well-established function of the PCr/Cr system in energy metabolism, our transcriptomic and metabolic analyses suggest that it plays a pivotal role in systemic one-carbon metabolism, oxidation/reduction, and biosynthetic processes. Therefore, the PCr/Cr system is not only an energy buffer and shuttle, but also a crucial component involved in numerous systemic metabolic processes.

L-arginine:glycine amidinotransferase deficiency protects from metabolic syndrome.

Phosphorylated creatine (Cr) serves as an energy buffer for ATP replenishment in organs with highly fluctuating energy demand. The central role of Cr in the brain and muscle is emphasized by severe neurometabolic disorders caused by Cr deficiency. Common symptoms of inborn errors of creatine synthesis or distribution include mental retardation and muscular weakness. Human mutations in l-arginine:glycine amidinotransferase (AGAT), the first enzyme of Cr synthesis, lead to severely reduced Cr and guanidinoacetate (GuA) levels. Here, we report the generation and metabolic characterization of AGAT-deficient mice that are devoid of Cr and its precursor GuA. AGAT-deficient mice exhibited decreased fat deposition, attenuated gluconeogenesis, reduced cholesterol levels and enhanced glucose tolerance. Furthermore, Cr deficiency completely protected from the development of metabolic syndrome caused by diet-induced obesity. Biochemical analyses revealed the chronic Cr-dependent activation of AMP-activated protein kinase (AMPK), which stimulates catabolic pathways in metabolically relevant tissues such as the brain, skeletal muscle, adipose tissue and liver, suggesting a mechanism underlying the metabolic phenotype. In summary, our results show marked metabolic effects of Cr deficiency via the chronic activation of AMPK in a first animal model of AGAT deficiency. In addition to insights into metabolic changes in Cr deficiency syndromes, our genetic model reveals a novel mechanism as a potential treatment option for obesity and type 2 diabetes mellitus.