So what are we to make of this? My take is that this experiment wasn't designed to test anything we are interested in and that what they showed was that, perhaps, the RDA for mice is about 30 mg/Kg chow of NR which also seem to be roughly what it is for rats for Niacin as shown here: [/font]http://ratfanclub.org/nutreq.html (20mg/KG chow)
It was absolutely designed to test things we are interested in — including addressing questions that none of us stopped to think about, not only on dose but also on genetics of human translatability.
The adipocytes are clearly smaller in the 900NR case. Normally larger adipocytes are a sign of obesity.
The 900NR group has fewer large-sized adipocytes, but also fewer small-sized ones — and more medium-sized ones; add it up, and "No differences were found in body weight, cumulative feed intake, lean mass or fat mass, when comparing mice with 5, 15, 30, 180 or 900 mg NR/kg diet".
In any case, the key question here is the functional effects on metabolism, not the distribution of adipocyte sizes — and here they found that the higher doses did not improve metabolic function, but instead made it worse. Fasting metabolic flexibility, maxΔRERCHO1→FAO, was better at 30 NR than at higher or lower doses, although only statistically significantly better as compared to 5NR; "Refeeding metabolic flexibility, maxΔRERFAO→CHO2, was significantly greater in 30NR than in 5NR, 15NR, or 900NR ... No differences were seen in blood glucose, serum TG or NEFA among NR doses (Fig. 3A-C). Serum insulin, leptin, adiponectin, leptin/adiponectin ratio and HOMA-IR index exhibited a tendency towards a dose-response curve, without reaching statistical significance (Fig. 3D-H). In all cases, except adiponectin which shows opposite behaviour, the measured value decreased and then increased, with 30NR being the turning point" — that is, the dose-response curve on markers of insulin resistance were U-shaped, with the nominal insulin resistance lowest at 30 NR and then rising again at higher doses. The suggestion in the trend is that high-dose NR caused the animals to pump out more insulin just to maintain the same glucose level.
Stefan wrote: The nnt note may not be the full story as the mice used here show slightly more weight gain than the ones with the nnt defect.
But that just reinforces the point. The whole reason everyone is watching mouse studies on the effects of NR is on the hypothesis that they'll translate to humans. Normal humans have an intact NNT gene, or SNPs that are by comparison functionally minor; the mutation of NNT in C57BL/6 mice is so severe that no mature NNT protein can be detected in their cells.
Stefan wrote: In any case I would think that the nnt gene only impacts beta cells.
No: NNT is needed to transfer reducing equivalents from NADH to NADP+, to generate the NADPH needed to detoxify ROS in the mitochondria, both directly and by regenerating thioredoxin and glutathione: it's necessary to maintain redox homeostasis and efficient ATP synthesis in every single cell in the body. "NNT expression differs between cell types, being highest in the heart and kidney. Approximately half of the mitochondrial NADPH in the brain is believed to depend on the action of the NNT, and its inhibition causes significant oxidative stress."(1)
The effects in beta-cells are important to the mutation's effects on metabolism and susceptibility to weight gain, but it does a lot of other less-than-obvious things as well:
The key role of the NNT and the network of interactions taking place in cytosolic glucose metabolism highlight that the pathways involved in maintenance of the NAD and NADP pools in their separate redox states are highly interconnected. Indeed, in addition to the malate-asparate and citrate-α-ketoglutarate shuttles providing separate transmission of NAD and NADP redox state between cytosol and mitochondria, a pyruvate-malate shuttle in which the redox state of the cytosolic NADP pool is coupled to that of the mitochondrial NAD pool has also been observed {109}.
Additional complexity in these redox networks also arises from the reversibility of a number of the reactions. For example, during ischaemia, the citric acid cycle may reverse and consume NADH {39}, the NNT may oxidise NADPH to produce NADH when the membrane potential is collapsed {104} or lactate dehydrogenase may reverse, using lactate as a metabolic substrate, producing NADH in the cytosol alongside pyruvate for aerobic ATP production {110}. Indeed, it has been suggested that lactate secreted by astrocytes may serve as the primary energy source for neurons in the brain {111}. Thus, the highly contrasting intracellular roles of the NAD and NADP pools and their separate redox states are supported by a complex and interconnected network of pathways.
Because of all of this, when you knock out the mitochondrial form of SOD, most mouse strains use NNT to partially make up for it; because they lack functional NNT, giving C57Bl/6 the same MnSOD mutation leads to much more severe effects. The more you then back-cross them with strains of mouse with functional NNT, the more they're able to cope with the MnSOD mutation:
congenic Sod2−/− mice on a C57BL/6J background (B6 Sod2−/−) develop a fetal form of dilated cardiomyopathy, and most of them die about day 15 (ED15) of gestation. On the other hand, Sod2−/− mice generated on a DBA/2J (D2 Sod2−/−) background develop normally through gestation and do not have dilated cardiomyopathy. However, these mice develop severe metabolic acidosis and have an average lifespan of 8 days. F1 mice (B6D2F1 Sod2−/−) generated from the two parental strains have a cardiac phenotype similar to that of D2 Sod2−/− mice, but with a milder form of metabolic acidosis. Consequently, these mice are able to survive for up to three weeks without any pharmacological intervention (13). Consistent with our observation, a different Sod2 mutant strain (SOD2m1BCM) generated on a B6/129 mixed genetic background was shown to survive for up to 3 weeks after birth and had a phenotype similar to that of B6D2F1 Sod2−/−tm1Cje (14).(2)
Reference
1: Blacker TS, Duchen MR. Investigating mitochondrial redox state using NADH and NADPH autofluorescence. Free Radic Biol Med. 2016 Nov;100:53-65. doi: 10.1016/j.freeradbiomed.2016.08.010. Epub 2016 Aug 9. Review. PubMed PMID: 27519271; PubMed Central PMCID: PMC5145803.
2: Huang TT, Naeemuddin M, Elchuri S, Yamaguchi M, Kozy HM, Carlson EJ, Epstein CJ. Genetic modifiers of the phenotype of mice deficient in mitochondrial superoxide dismutase. Hum Mol Genet. 2006 Apr 1;15(7):1187-94. Epub 2006 Feb 23. PubMed PMID: 16497723.
Edited by Michael, 19 April 2017 - 05:10 PM.