Dietary lysine restriction induces lipid accumulation in skeletal muscle through an increase in serum threonine levels in rats.

We previously reported that dietary amino acid restriction induces the accumulation of triglycerides (TAG) in the liver of growing rats. However, differences in TAG accumulation in individual cell types or other tissues were not examined. In this study, we show that TAG also accumulates in the muscle and adipose tissues of rats fed a low amino acid (low-AA) diet. In addition, dietary lysine restriction (low-Lys) induces lipid accumulation in muscle and adipose tissues. In adjusting the nitrogen content to that of the control diet, we found that glutamic acid supplementation to the low-AA diet blocked lipid accumulation, but supplementation with the low-Lys diet did not, suggesting that a shortage of nitrogen caused lipids to accumulate in the skeletal muscle in the rats fed a low-AA diet. Serum amino acid measurement revealed that, in rats fed a low-Lys diet, serum lysine levels were decreased, while serum threonine levels were significantly increased compared with the control rats. When the threonine content was restricted in the low-Lys diet, TAG accumulation induced by the low-Lys diet was completely abolished in skeletal muscle. Moreover, in L6 myotubes cultured in medium containing high threonine and low lysine, fatty acid uptake was enhanced compared with that in cells cultured in control medium. These findings suggest that the increased serum threonine in rats fed a low-Lys diet resulted in lipid incorporation into skeletal muscle, leading to the formation of fatty muscle tissue. Collectively, we propose conceptual hypothesis that "amino-acid signal" based on lysine and threonine regulates lipid metabolism.

A novel amino acid signaling process governs glucose-6-phosphatase transcription.

Emerging evidence has shown that amino acids act as metabolic regulatory signals. Here, we showed that glucose-6-phosphatase (G6Pase) mRNA levels in cultured hepatocyte models were downregulated in an amino-acid-depleted medium. Inversely, stimulation with amino acids increased G6Pase mRNA levels, demonstrating that G6Pase mRNA level is directly controlled by amino acids in a reversible manner. Promoter assay revealed that these amino-acid-mediated changes in G6Pase mRNA levels were attributable to transcriptional regulation, independent of canonical hormone signaling pathways. Metabolomic analysis revealed that amino acid starvation induces a defect in the urea cycle, decreasing ornithine, a major intermediate, and supplementation of ornithine in an amino-acid-depleted medium fully rescued G6Pase mRNA transcription, similar to the effects of amino acid stimulation. This pathway was also independent of established mammalian target of rapamycin complex 1 pathway. Collectively, we present a hypothetical concept of "metabolic regulatory amino acid signal," possibly mediated by ornithine.

Regional differences in genes and variants causing retinitis pigmentosa in Japan.

PURPOSE: To investigate the regional differences in the genes and variants causing retinitis pigmentosa (RP) in Japan STUDY DESIGN: Retrospective multicenter study METHODS: In total, 1204 probands of each pedigree clinically diagnosed with nonsyndromic RP were enrolled from 5 Japanese facilities. The regions were divided into the Tohoku region, the Kanto and Chubu regions, and the Kyushu region according to the location of the hospitals where the participants were enrolled. We compared the proportions of the causative genes and the distributions of the pathogenic variants among these 3 regions. RESULTS: The proportions of genetically solved cases were 29.4% in the Tohoku region (n = 500), 29.6% in the Kanto and Chubu regions (n = 196), and 29.7% in the Kyushu region (n = 508), which did not differ statistically (P = .99). No significant regional differences in the proportions of each causative gene in genetically solved patients were observed after correction by multiple testing. Among the 29 pathogenic variants detected in all 3 regions, only p.(Pro347Leu) in RHO was an autosomal dominant variant; the remaining 28 variants were found in autosomal recessive genes. Conversely, 78.6% (275/350) of the pathogenic variants were detected only in a single region, and 6 pathogenic variants (p.[Asn3062fs] in EYS, p.[Ala315fs] in EYS, p.[Arg872fs] in RP1, p.[Ala126Val] in RDH12, p.[Arg41Trp] in CRX, and p.[Gly381fs] in PRPF31) were frequently found in ≥ 4 patients in the single region. CONCLUSION: We observed region-specific pathogenic variants in the Japanese population. Further investigations of causative genes in multiple regions in Japan will contribute to the expansion of the catalog of genetic variants causing RP.

Genetic characteristics of retinitis pigmentosa in 1204 Japanese patients.

BACKGROUND: The genetic profile of retinitis pigmentosa (RP) in East Asian populations has not been well characterised. Therefore, we conducted a large-scale sequencing study to investigate the genes and variants causing RP in a Japanese population. METHODS: A total of 1209 Japanese patients diagnosed with typical RP were enrolled. We performed deep resequencing of 83 known causative genes of RP using next-generation sequencing. We defined pathogenic variants as those that were putatively deleterious or registered as pathogenic in the Human Gene Mutation Database or ClinVar database and had a minor allele frequency in any ethnic population of ≤0.5% for recessive genes or ≤0.01% for dominant genes as determined using population-based databases. RESULTS: We successfully sequenced 1204 patients with RP and determined 200 pathogenic variants in 38 genes as the cause of RP in 356 patients (29.6%). Variants in six genes (EYS, USH2A, RP1L1, RHO, RP1 and RPGR) caused RP in 65.4% (233/356) of those patients. Among autosomal recessive genes, two known founder variants in EYS [p.(Ser1653fs) and p.(Tyr2935*)] and four East Asian-specific variants [p.(Gly2752Arg) in USH2A, p.(Arg658*) in RP1L1, p.(Gly2186Glu) in EYS and p.(Ile535Asn) in PDE6B] and p.(Cys934Trp) in USH2A were found in ≥10 patients. Among autosomal dominant genes, four pathogenic variants [p.(Pro347Leu) in RHO, p.(Arg872fs) in RP1, p.(Arg41Trp) in CRX and p.(Gly381fs) in PRPF31] were found in ≥4 patients, while these variants were unreported or extremely rare in both East Asian and non-East Asian population-based databases. CONCLUSIONS: East Asian-specific variants in causative genes were the major causes of RP in the Japanese population.

Importance of Serum Amino Acid Profile for Induction of Hepatic Steatosis under Protein Malnutrition.

We previously reported that a low-protein diet caused animals to develop fatty liver containing a high level of triglycerides (TG), similar to the human nutritional disorder "kwashiorkor". To investigate the underlying mechanisms, we cultured hepatocytes in amino acid-sufficient or deficient medium. Surprisingly, the intracellular TG level was increased by amino acid deficiency without addition of any lipids or hormones, accompanied by enhanced lipid synthesis, indicating that hepatocytes themselves monitored the extracellular amino acid concentrations to induce lipid accumulation in a cell-autonomous manner. We then confirmed that a low-amino acid diet also resulted in the development of fatty liver, and supplementation of the low-amino acid diet with glutamic acid to compensate the loss of nitrogen source did not completely suppress the hepatic TG accumulation. Only a dietary arginine or threonine deficiency was sufficient to induce hepatic TG accumulation. However, supplementation of a low-amino acid diet with arginine or threonine failed to reverse it. In silico analysis succeeded in predicting liver TG level from the serum amino acid profile. Based on these results, we conclude that dietary amino acid composition dynamically affects the serum amino acid profile, which is sensed by hepatocytes and lipid synthesis was activated cell-autonomously, leading to hepatic steatosis.