Glyoxylate cycle in the epiphyseal growth plate: isocitrate lyase and malate synthase identified in mammalian cartilage

Metabolomic and transcriptional profiling reveals bioenergetic stress and activation of cell death and inflammatory pathways in vivo after neuronal deletion of NAMPT

Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD+ salvage pathway. Our previous study demonstrated that deletion of NAMPT gene in projection neurons using Thy1-NAMPT-/- conditional knockout (cKO) mice causes neuronal degeneration, muscle atrophy, neuromuscular junction abnormalities, paralysis and eventually death. Here we conducted a combined metabolomic and transcriptional profiling study in vivo in an attempt to further investigate the mechanism of neuronal degeneration at metabolite and mRNA levels after NAMPT deletion. Here using steady-state metabolomics, we demonstrate that deletion of NAMPT causes a significant decrease of NAD+ metabolome and bioenergetics, a buildup of metabolic intermediates upstream of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in glycolysis, and an increase of oxidative stress. RNA-seq shows that NAMPT deletion leads to the increase of mRNA levels of enzymes in NAD metabolism, in particular PARP family of NAD+ consumption enzymes, as well as glycolytic genes Glut1, Hk2 and PFBFK3 before GAPDH. GO, KEGG and GSEA analyses show the activations of apoptosis, inflammation and immune responsive pathways and the inhibition of neuronal/synaptic function in the cKO mice. The current study suggests that increased oxidative stress, apoptosis and neuroinflammation contribute to neurodegeneration and mouse death as a direct consequence of bioenergetic stress after NAMPT deletion.

Immunohistochemical Localization of Peroxisomal Enzymes During Rat Embryonic Development

Peroxisomes are cytoplasmic organelles involved in a variety of metabolic pathways. Thus far, the morphological and biochemical features of peroxisomes have been extensively characterized in adult tissues. However, the existence of congenital peroxisomal disorders, primarily affecting tissue differentiation, emphasizes the importance of these organelles in the early stages of organogenesis. We investigated the occurrence and tissue distribution of three peroxisomal enzymes in rat embryos at various developmental stages. By means of a highly sensitive biotinyl-tyramide protocol, catalase, acyl-CoA oxidase, and ketoacyl-CoA thiolase were detected in embryonic tissues where peroxisomes had not thus far been recognized, i.e., adrenal and pancreatic parenchyma, choroid plexus, neuroblasts of cranial and spinal ganglia and myenteric plexus, and chondroblasts of certain skeletal structures. In other tissues, i.e., gut epithelium and neuroblasts of some CNS areas, they were identified earlier than previously. In select CNS areas, ultrastructural catalase cytochemistry allowed identification of actively proliferating organelles at early developmental stages in several cell types. Our data show that in most organs maturation of peroxisomes parallels the acquirement of specific functions, mainly related to lipid metabolism, thus supporting an involvement of the organelles in tissue differentiation.

Molecular characterization and partial cDNA cloning of facilitative glucose transporters expressed in human articular chondrocytes; stimulation of 2-deoxyglucose uptake by IGF-I and elevated MMP-2 secretion by glucose deprivation

OBJECTIVE

Recent evidence suggests that human chondrocytes express several facilitative glucose transporter (GLUT) isoforms and also that 2-deoxyglucose transport is accelerated by cytokine stimulation. The aim of the present investigation was to determine if human articular chondrocytes express any of the recently identified members of the GLUT/SLC2A gene family and to examine the effects of endocrine factors, such as insulin and IGF-I on the capacity of human chondrocytes for transporting 2-deoxyglucose.

DESIGN/METHODS

PCR, cloning and immunohistochemistry were employed to study the expression of GLUT/SLC2A transporters in normal human articular cartilage. The uptake of 2-deoxyglucose was examined in monolayer cultured immortalized human chondrocytes following stimulation with TNF-alpha, insulin and IGF-I. Levels of MMP-2 were assessed by gelatin zymography following glucose deprivation of alginate cultures.

RESULTS

Using PCR we detected transcripts for eight glucose transporter isoforms (GLUTs 1, 3, 6, 8, 9, 10, 11 and 12) and for a fructose transporter (GLUT5) in human articular cartilage. Expression of GLUT1, GLUT3 and GLUT9 proteins in normal human articular cartilage was confirmed by immunohistochemistry. The uptake of 2-deoxyglucose was dependent on time and temperature, inhibited by cytochalasin B and phloretin, and significantly accelerated in chondrocyte cultures stimulated with IGF-I. However, 2-deoxyglucose uptake was unaffected by short and long-term insulin treatment, which ruled out a functional role for insulin-sensitive GLUT4-mediated glucose transport. Furthermore, secretion of MMP-2 was increased in alginate cultures deprived of glucose.

CONCLUSIONS

The data supports a critical role for glucose transport and metabolism in the synthesis and degradation of cartilage.

Quantitative structural organization of normal adult human articular cartilage

OBJECTIVE

Data pertaining to the quantitative structural features and organization of normal articular cartilage are of great importance in understanding its biomechanical properties and in attempting to establish this tissue's counterpart by engineering in vitro. A comprehensive set of such baseline data is, however, not available for humans. It was the purpose of the present study to furnish the necessary information.

DESIGN

The articular cartilage layer covering the medial femoral condyle of deceased persons aged between 23 and 49 years was chosen for the morphometric analysis of cell parameters using confocal microscopy in conjunction with unbiased stereological methods. The height of the hyaline articular cartilage layer, as well as that of the calcified cartilage layer and the subchondral bone plate, were also measured.

RESULTS

The mean height of the hyaline articular cartilage layer was found to be 2.4mm, the volume density of chondrocytes therein being 1.65%, the number of cells per mm(3) of tissue 9626 and the mean cell diameter 13 microm. Other estimators (including matrix mass per cell and cell profile density) were also determined.

CONCLUSIONS

A comparison of these normal human quantitative data with those published for experimental animals commonly used in orthopaedic research reveals substantial differences, consideration of which in tissue engineering strategies destined for human application are of paramount importance for successful repair.

Alcohol consumption in the absence of adequate nutrition may lead to activation of the glyoxylate cycle in man

The consumption of alcohol prior to food intake results in alcohol metabolism occurring in the liver, and the liver is often damaged in chronic alcoholics. This paper highlights the possibility that alcohol consumption in the absence of adequate nutrition after an extended period of time may lead to activation of the glyoxylate cycle, an energy pathway associated with the conversion of fat into carbohydrate which until recently was thought to only exist in plants and bacteria.

Induction of glyoxylate cycle enzymes in rat liver upon food starvation

The key enzymes of the glyoxylate cycle, isocitrate lyase and malate synthase, have been detected in liver of food-starved rats. Activities became measurable 3 days and peaked 5 days after the beginning of starvation. Both enzymes were found in the peroxisomal cell fraction after organelle fractionation by isopycnic centrifugation. Isocitrate lyase was purified 112-fold by ammonium sulfate precipitation, and chromotography on DEAE-cellulose and Toyopearl HW-65. The specific activity of the purified enzyme was 9.0 units per mg protein. The K(m)(isocitrate) was 68 microM and the pH optimum was at pH 7.4. Malate synthase was enriched 4-fold by ammonium sulfate precipitation. The enzyme had a K(m)(acetyl-CoA) of 0.2 microM, a K(m)(glyoxylate) of 3 mM and a pH optimum of 7.6.

Mitochondrial and peroxisomal fractions derived from human white adipocytes

1. A procedure is described for the separation of intact peroxisomes from human white adipocytes using a linear metrizamide gradient (20-50% w/v). 2. Peroxisomes were found in the high density region of the gradient in an intact form. 3. Mitochondria were distributed in the high density and low density regions of the gradient. 4. Lysosomes separated well from the peroxisomes, occurring only in the low density region of the gradient. 5. Low levels of glyoxylate cycle enzyme activities (isocitrate lyase and malate synthase) were detected within the light and heavy mitochondrial pellet fractions.

The absence of glyoxylate cycle enzymes in rodent and embryonic chick liver

There have been several reports over the past decade of the presence of the glyoxylate cycle enzymes, isocitrate lyase and malate synthase, in animal tissues. Reaction products in these assays have been measured principally by chromatographic separation of isotopes or by colorimetric procedures. In this report more sensitive and accurate HPLC and HPCE analyses were used to detect enzymatic activity. Reversed phase HPLC revealed the absence of detectable isocitrate lyase activity in guinea pig, rat and chick embryonic liver. The formation of several other alpha-keto acids was detected and this may account for the previously reported activities. Using HPCE to monitor malate formation malate synthase activity was not detected in these tissues. These results indicate that when assaying enzyme activities in crude tissue homogenates specific methods for the identification of end products are required.

Immunohistochemical identification of superoxide dismutases, catalase, and glutathione-S-transferases in rat femora

We used light microscopic immunohistochemistry to locate manganese superoxide dismutase, copper zinc superoxide dismutase, catalase, and glutathione-S-transferases in demineralized femora from rats of 4-14 weeks of age. Immunoblots confirmed the specificity of the polyclonal antibodies for the rat proteins of interest. Each of the enzymes exhibited a unique staining pattern. Copper-zinc superoxide dismutase was detected within some articular and epiphyseal chondrocytes of younger animals. Manganese superoxide dismutase was detected within some articular and epiphyseal chondrocytes, within some osteoprogenitor cells and osteoblasts, within many osteoclasts, and within some vascular smooth muscle cells. Catalase was identified within articular chondrocytes, epiphyseal chondrocytes, and osteocytes, whereas staining at the periphery of hypertrophic chondrocytes suggested extracellular and/or cell membrane-associted catalase. Glutathione-S-transferases were detected within and at the periphery of epiphyseal and articular chondrocytes and less prominently within cortical osteocytes. There were no major age-related changes in antioxidant enzyme distribution.

Identification of glyoxylate cycle enzymes in chick liver--the effect of vitamin D3: cytochemistry and biochemistry

Information regarding the presence of the glyoxylate cycle in chick liver was sought. This metabolic pathway has long been thought to be absent from vertebrate tissues. Previous studies in other tissues have shown that, when present, this pathway is sensitive to vitamin-D. Thus, the effect of long-term vitamin-D deficiency and subsequent vitamin-D replacement on liver structure was studied by light microscopy. In addition, specific biochemical assays for the presence of glyoxylate cycle enzymes were performed. Light microscopy of lipid extracted tissues, light microscopic histochemistry, and quantitative histochemistry showed that the hepatocytes from vitamin-D-deficient animals contained primarily lipid. Hepatocytes from normal and vitamin-D-replete livers contained primarily carbohydrate as judged by their staining with periodic acid-Schiff (PAS). Also, malate synthase positive peroxisomes were seen in hepatocytes from normal and vitamin-D-treated chicks. Structures positive for this glyoxylate cycle enzyme were rarely seen in the hepatocytes from vitamin-D-deficient animals. Biochemical analyses showed the presence of the two unique glyoxylate cycle enzymes, isocitrate lyase and malate synthase, in chick hepatocytes. The activity of these enzymes was markedly increased in the vitamin-D-replete livers. In addition, chick hepatocytes demonstrated the capacity to oxidize fatty acid in the presence of cyanide. This activity, which is characteristic of peroxisomal B-oxidation rather than mitochondrial, was stimulated by vitamin-D treatment. Lastly, incubation of chick liver in the presence of a fatty acid substrate (palmitate) led to higher tissue glycogen content. The latter was further increased in liver from vitamin-D-replete animals. These data show the presence of glyoxylate cycle enzymes in a higher vertebrate and indicate that this tissue is endowed with the capacity to convert lipid to carbohydrate.

Absence of cartilage canals in the long bone extremities of four species of skeletally immature marsupials

The cartilaginous epiphyses and physes from the bone extremities of four species of skeletally immature marsupials were studied. The microscopic and ultrastructural features of the marsupial tissues were compared with similar samples from a neonatal lamb and a 1-day-old chick. Chondrocyte differentiation and endochondral ossification appeared similar in physes from the marsupials, foetal lamb, and 1-day-old chick. However, unlike the lamb and chick, which both contained cartilage canals, there were no cartilage canals in the epiphyseal or physeal cartilage from the marsupials. Many of the epiphyseal chondrocytes from the marsupial specimens contained large lipid droplets. It is suggested that the lipids in marsupial chondrocytes may be utilized in metabolic pathways. Despite hypertrophy of chondrocytes, there were no epiphyseal ossification centers in the femoral heads of the marsupial specimens; this was possibly due to the absence of cartilage canals, which are considered a source of osteoproginator cells. This study indicates that physeal and epiphyseal cartilage in marsupials is viable and functions in an avascular environment; this may be due to unique metabolic properties of the chondrocytes.

Hibernation activates glyoxylate cycle and gluconeogenesis in black bear brown adipose tissue

Biochemical studies on brown adipose tissue removed from a hibernating black bear and a non-hibernating control animal demonstrate that this tissue: (1) can carry out cyanide-insensitive fatty acid oxidation, and (2) possesses catalase activity and the enzyme activities unique to the glyoxylate cycle, isocitrate lyase and malate synthase. These activities are all markedly increased in brown fat obtained from the hibernating animal. Additionally, hibernation enhances the ability of the tissue to synthesize glycogen in the presence of a fatty acid substrate. The glyoxylate cycle enzymes and the ability to convert fatty acid carbons to glucose have been generally regarded as being absent from vertebrate cells and tissues.