14C-Labelled amino acids and glucose in rat brain and liver after injection of [2-14C]propionate

Apoptosome-deficient cells lose cytochrome c through proteasomal degradation but survive by autophagy-dependent glycolysis

Cytochrome c release from mitochondria promotes apoptosome formation and caspase activation. The question as to whether mitochondrial permeabilization kills cells via a caspase-independent pathway when caspase activation is prevented is still open. Here we report that proneural cells of embryonic origin, when induced to die but rescued by apoptosome inactivation are deprived of cytosolic cytochrome c through proteasomal degradation. We also show that, in this context, those cells keep generating ATP by glycolysis for a long period of time and that they keep their mitochondria in a depolarized state that can be reverted. Moreover, under these conditions, such apoptosome-deficient cells activate a Beclin 1-dependent autophagy pathway to sustain glycolytic-dependent ATP production. Our findings contribute to elucidating what the point-of-no-return in apoptosis is. They also help in clarifying the issue of survival of apoptosome-deficient proneural cells under stress conditions. Unraveling this issue could be highly relevant for pharmacological intervention and for therapies based on neural stem cell transfer in the treatment of neurological disorders.

Propionate increases neuronal histone acetylation, but is metabolized oxidatively by glia. Relevance for propionic acidemia

In propionic acidemia, propionate acts as a metabolic toxin in liver cells by accumulating in mitochondria as propionyl-CoA and its derivative, methylcitrate, two tricarboxylic acid cycle inhibitors. Little is known about the cerebral metabolism of propionate, although clinical effects of propionic acidemia are largely neurological. We found that propionate was metabolized oxidatively by glia: [3-(14)C]propionate injected into mouse striatum or cortex, gave a specific activity of glutamine that was 5-6 times that of glutamate, indicating metabolism in cells that express glutamine synthetase, i.e., glia. Further, cultured cerebellar astrocytes metabolized [3-(14)C]propionate; cultured neurons did not. However, both cultured cerebellar neurons and astrocytes took up [3H]propionate, and propionate exposure increased histone acetylation in cultured neurons and astrocytes as well as in hippocampal CA3 pyramidal neurons of wake mice. The inability of neurons to metabolize propionate may be due to lack of mitochondrial propionyl-CoA synthetase activity or transport of propionyl residues into mitochondria, as cultured neurons expressed propionyl-CoA carboxylase, a mitochondrial matrix enzyme, and oxidized isoleucine, which becomes converted into propionyl-CoA intramitochondrially. The glial metabolism of propionate suggests astrocytic vulnerability in propionic acidemia when intramitochondrial propionyl-CoA may accumulate. Propionic acidemia may alter both neuronal and glial gene expression by affecting histone acetylation.

Amphibolic role of the Krebs cycle in the insulin-stimulated protein synthesis

It has been a generally held view that insulin does not significantly affect the incorporation of amino acids into liver protein. This interpretation was based on data obtained from studies using the branched chain amino acids, which are poorly metabolized by the hepatic tissue. The effect of insulin on 14CO2 formation and protein incorporation of several 1-14C-labeled or U-14C-labeled amino acids was studied in isolated rat hepatocytes and diaphragm pieces. It was shown that insulin enhanced 14CO2 formation and protein incorporation primarily of those carbons of amino acids which are metabolized through the mitochondrial Krebs cycle. Using aminooxyacetic acid (0.5 mM), a potent inhibitor of the transamination reaction, it was shown that there exists an "insulin-sensitive" pool of glutamate which is preferentially utilized for protein synthesis in the presence of insulin. The insulin effect on protein incorporation of 14C-labeled glutamate generated in the Krebs cycle was abolished in the presence of aminooxyacetic acid. We interpret these results to signify that mitochondrial transamination of alpha-ketoglutarate to glutamate is essential for insulin stimulation of 14C incorporation into hepatocyte protein.

Propionate absorption and metabolism in the rabbit hindgut

Propionate disappearance from the loops of the hindgut in the rabbit was evaluated by measuring variations in the concentration of propionate in caecocolonic loops and differences in the arterial and venous plasma. In vivo metabolism in gut and liver tissues was studied after introduction of (1-14C) propionate into the caecocolonic loops. The rate of disappearance from the loops was always quantitatively significant but was greater in the proximal colon. Hindgut tissue metabolised propionate and the intensity of the metabolism varied with the segment studied; the proximal colon showed by far the highest propionate consumption. Radioactivity was found in a certain number of free amino acids, organic acids, sugars, lipid soluble substances and proteins. Propionate is an efficient respiratory fuel for the colonocyte and a good precursor for gluconeogenesis.

Absorption and metabolism of the volatile fatty acids in the hind-gut of the rabbit

Volatile fatty acids (VFA) absorption in the large intestine of the anaesthetized rabbit was evaluated by measuring variations in the concentration of VFA in intestinal loops and plasma arteriovenous differences. Metabolic conversions were studied using [1-14C]acetate, [1-14C]propionate and [3,4-14C]butyrate. The hind-gut tissues metabolized the three VFA, although this metabolism varied with the segment studied. Butyrate was the best respiratory fuel for the colonic wall, followed by propionate; acetate participated also, but it was mainly converted to glutamate. The liver was the main organ metabolizing absorbed propionate and butyrate; acetate was available for extrahepatic tissue metabolism. For the rabbit, VFA represented about 40% of the maintenance energy requirement.

The role of chloride in acetylcholine metabolism

The chloride dependence of acetylcholine (ACh) synthesis and release and of choline uptake was studied in synaptosomal preparations from rat brain. The substitution of propionate for chloride, in the presence of 35 mM-potassium, lowered the ACh content of the synaptosomes. However, in the presence of 5 mM-potassium, the ACh level in synaptosomes was reduced, but significantly less so. Propionate had no effect on choline acetyltransferase (EC 2.3.1.6) activity when measured in a standard chloride-containing medium. In the presence of propionate, the spontaneous release of ACh was unchanged, but potassium-stimulated release of ACh was markedly reduced as compared with a chloride-containing medium. The synthesis of ACh, as measured by the net increase in the amount of ACh in the synaptosomes and that released to the medium, was reduced with propionate at 5 mM-potassium and was totally inhibited when the potassium concentration was increased to 35 mM. Choline uptake studies revealed that with propionate only a low-affinity component of the choline transport system existed. Further, the Vmax was markedly reduced when the potassium concentration was increased to 35 mM. The results suggest that under certain conditions choline transported by a low-affinity system might provide a substantial source of choline for ACh synthesis.

Metabolism of sodium [1-14C] and [3-14C] propionate in the non-deficient and biotin deficient rat

The fixation and elimination kinetics of [1-14C] propionate were studied using autoradiographic technique in the non-deficient and biotin deficient mouse. To quantitate these results, radioactivity was evaluated in the principal organs and excreta over 24 hours from non-deficient and biotin deficient rats, after intravenous injection of sodium [1-14C] and [3-14C] propionate. Radioactivity was eliminated principally via the lungs in the non-deficient rat. Fecal and urinary elimination was weak. Tissular fixation of the radioactivity 24 hours after [14C] propionate injection was carried out principally on the glandular tissues. Almost identical results were obtained in the biotin deficient animal, in which the hepatic propionyl CoA carboxylase activity is decreased by 80 per cent.

Interrelated changes of amino acids in the retina and optic tectum of a marine fish with alterations of illuminating conditions

Examination of interrelated changes in the concentrations of 'clusters' of amino acids provides evidence that individual amino acid levels in the tissues cannot be considered to vary independently. Such variations are observed when the CNS at 'rest' is stimulated. Alterations in taurine cause changes in glutamic acid and also modify its anatomically related compartmentalized metabolism. Similar interdependent modifications may be indicated for the concentrations of 3 biochemically related neutral amino acids: threonine, serine and glycine. With specific reference to the fish optic tectum and retina, certain differences and coincidences in the regulation of GABA and glycine have also been uncovered. Finally, many of the findings presented here are closely analogous to results obtained when comparing normal and epileptic mammalian tissues. Possibly, this analogy can be attributed to the fact that the physiological states so far investigated, from the most simplistic point of view, represent extreme variations of nervous tissue excitation.