Glycolysis, mitochondrial substrate oxidation, and the pentose phosphate pathway (PPP) are critical for neuronal bioenergetics and oxidation-reduction homeostasis, but quantitating their fluxes remains challenging, especially when processes such as hexose phosphate (i.e., glucose/fructose-6-phosphate) recycling in the PPP are considered. A hexose phosphate recycling model was developed which exploited the rates of glucose consumption, lactate production, and mitochondrial respiration to infer fluxes through the major glucose consuming pathways of adherent cerebellar granule neurons by replicating [13C]lactate labeling from metabolism of [1,2-13C2]glucose. Flux calculations were predicated on a steady-state system with reactions having known stoichiometries and carbon atom transitions. Non-oxidative PPP activity and consequent hexose phosphate recycling, as well as pyruvate production by cytoplasmic malic enzyme, were optimized by the model and found to account for 28±2 % and 7.7±0.2 % of hexose phosphate and pyruvate labeling, respectively. From the resulting fluxes, 52±6 % of glucose was metabolized by glycolysis, compared to 19±2 % by the combined oxidative/non-oxidative pentose cycle that allows for hexose phosphate recycling, and 29±8 % by the combined oxidative PPP/de novo nucleotide synthesis reactions. By extension, 62±6 % of glucose was converted to pyruvate, the metabolism of which resulted in 16±1 % of glucose oxidized by mitochondria and 46±6 % exported as lactate. The results indicate a surprisingly high proportion of glucose utilized by the pentose cycle and the reactions synthesizing nucleotides, and exported as lactate. While the in vitro conditions to which the neurons were exposed (high glucose, no lactate or other exogenous substrates) limit extrapolating these results to the in vivo state, the approach provides a means of assessing a number of metabolic fluxes within the context of hexose phosphate recycling in the PPP from a minimal set of measurements.
Traumatic brain injury (TBI) is a major public health concern and remains a leading cause of disability and socio-economic burden. To date, there is no proven therapy that promotes brain repair following an injury to the brain. In this study, we explored the role of an isoform of adenosine kinase expressed in the cell nucleus (ADK-L) as a potential regulator of neural stem cell proliferation in the brain. The rationale for this hypothesis is based on coordinated expression changes of ADK-L during foetal and postnatal murine and human brain development indicating a role in the regulation of cell proliferation and plasticity in the brain. We first tested whether the genetic disruption of ADK-L would increase neural stem cell proliferation after TBI. Three days after TBI, modelled by a controlled cortical impact, transgenic mice, which lack ADK-L (ADKΔneuron) in the dentate gyrus (DG) showed a significant increase in neural stem cell proliferation as evidenced by significant increases in doublecortin and Ki67-positive cells, whereas animals with transgenic overexpression of ADK-L in dorsal forebrain neurons (ADK-Ltg) showed an opposite effect of attenuated neural stem cell proliferation. Next, we translated those findings into a pharmacological approach to augment neural stem cell proliferation in the injured brain. Wild-type C57BL/6 mice were treated with the small molecule adenosine kinase inhibitor 5-iodotubercidin for 3 days after the induction of TBI. We demonstrate significantly enhanced neural stem cell proliferation in the DG of 5-iodotubercidin-treated mice compared to vehicle-treated injured animals. To rule out the possibility that blockade of ADK-L has any effects in non-injured animals, we quantified baseline neural stem cell proliferation in ADKΔneuron mice, which was not altered, whereas baseline neural stem cell proliferation in ADK-Ltg mice was enhanced. Together these findings demonstrate a novel function of ADK-L involved in the regulation of neural stem cell proliferation after TBI.
DNA methylation has a major role in cancer, and its inhibitors are used therapeutically. DNA methylation depends on methyl group flux through the transmethylation pathway, which forms adenosine. We hypothesized that an adenosine kinase isoform with nuclear expression (ADK-L) determines global DNA methylation in cancer cells. We quantified ADK-L expression (Western Blot) and global DNA methylation as percent 5-methyldeoxycytidine (5mdC, LC-MS/MS) in three cancer lines (HeLa, HepG2, and U373). ADK-L expression and global DNA methylation correlated positively with the highest levels in HeLa cells compared to U373 and HepG2 cells. To determine whether ADK increases global DNA methylation and to validate its potential therapeutics, we treated HeLa cells with potent ADK inhibitors MRS4203 and MRS4380 (IC50 88 and 140 nM, respectively). Both nucleosides, but not a structurally related poor ADK inhibitor, significantly reduced global DNA methylation in HeLa cells in a concentration-dependent manner. Thus, ADK-L is a potential target for the therapeutic manipulation of DNA methylation levels in cancer.
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