Further Studies of Amino Acid Transport by Embryonic Chick Bone

SLC1A5 provides glutamine and asparagine necessary for bone development in mice

Osteoblast differentiation is sequentially characterized by high rates of proliferation followed by increased protein and matrix synthesis, processes that require substantial amino acid acquisition and production. How osteoblasts obtain or maintain intracellular amino acid production is poorly understood. Here, we identify SLC1A5 as a critical amino acid transporter during bone development. Using a genetic and metabolomic approach, we show SLC1A5 acts cell autonomously to regulate protein synthesis and osteoblast differentiation. SLC1A5 provides both glutamine and asparagine which are essential for osteoblast differentiation. Mechanistically, glutamine and to a lesser extent asparagine support amino acid biosynthesis. Thus, osteoblasts depend on Slc1a5 to provide glutamine and asparagine, which are subsequently used to produce non-essential amino acids and support osteoblast differentiation and bone development.

The role of osteoblasts in energy homeostasis

Osteoblasts are specialized mesenchymal cells that synthesize bone matrix and coordinate the mineralization of the skeleton. These cells work in harmony with osteoclasts, which resorb bone, in a continuous cycle that occurs throughout life. The unique function of osteoblasts requires substantial amounts of energy production, particularly during states of new bone formation and remodelling. Over the last 15 years, studies have shown that osteoblasts secrete endocrine factors that integrate the metabolic requirements of bone formation with global energy balance through the regulation of insulin production, feeding behaviour and adipose tissue metabolism. In this article, we summarize the current understanding of three osteoblast-derived metabolic hormones (osteocalcin, lipocalin and sclerostin) and the clinical evidence that suggests the relevance of these pathways in humans, while also discussing the necessity of specific energy substrates (glucose, fatty acids and amino acids) to fuel bone formation and promote osteoblast differentiation.

Wnt signaling and cellular metabolism in osteoblasts

The adult human skeleton is a multifunctional organ undergoing continuous remodeling. Homeostasis of bone mass in a healthy adult requires an exquisite balance between bone resorption by osteoclasts and bone formation by osteoblasts; disturbance of such balance is the root cause for various bone disorders including osteoporosis. To develop effective and safe therapeutics to modulate bone formation, it is essential to elucidate the molecular mechanisms governing osteoblast differentiation and activity. Due to their specialized function in collagen synthesis and secretion, osteoblasts are expected to consume large amounts of nutrients. However, studies of bioenergetics and building blocks in osteoblasts have been lagging behind those of growth factors and transcription factors. Genetic studies in both humans and mice over the past 15 years have established Wnt signaling as a critical mechanism for stimulating osteoblast differentiation and activity. Importantly, recent studies have uncovered that Wnt signaling directly reprograms cellular metabolism by stimulating aerobic glycolysis, glutamine catabolism as well as fatty acid oxidation in osteoblast-lineage cells. Such findings therefore reveal an important regulatory axis between bone anabolic signals and cellular bioenergetics. A comprehensive understanding of osteoblast metabolism and its regulation is likely to reveal molecular targets for novel bone therapies.

Transport of neutral amino acids by adult articular cartilage

The transport of three neutral amino acids: alanine, serine and leucine and the non-metabolizable analogue aminoisobutyric acid (AIB) was characterized in bovine articular cartilage slices. Alanine and serine were strongly concentrated intracellularly by a factor of 4.5 and 5 respectively and were transported in a sodium dependent and pH sensitive manner. AIB was concentrated by a factor of 1.8 with respect to the extracellular medium and was also transported in a sodium dependent and pH sensitive manner. Leucine was concentrated weakly across the chondrocyte membrane (X1.1) and its transport was independent of sodium ion concentration and the lowering of the extracellular pH. The apparent Km of transport of alanine, serine, leucine and AIB was determined to be 0.46 mM, 0.69 mM, 0.93 mM and 0.66 mM respectively and the values of Vmax were 35.8, 52.1, 14.8 and 6.8 pmol/min per mg respectively. It was shown that both alanine and serine were transported predominantly by system ASC (63.8% and 67.4% respectively) with a small percentage of transport occurring via system A. AIB was transported mainly by system A (76.0%) and leucine was transported equally via systems L (48.4%) and ASC (45.0%).

Amino acid transport in the rat exocrine pancreas. I. Transport of neutral amino acids and their utilization in protein synthesis

The transport and utilization of three neutral amino acids in protein synthesis was studied using isolated pancreatic lobules in vitro. The significance of the extracellular and intracellular amino acid pool in this process was analyzed removing the extracellular pool (labeled by inulin) by a cold-wash procedure. This was especially useful in short-term experiments. Double-labeling experiments indicated a predominant utilization of the intracellular amino acid pool during protein synthesis. The advantage of isolated pancreatic lobule preparations compared to tissue fragments or slices was emphasized by fine structural studies. Using freeze-etching techniques on the same preparations, differences in the distribution of membrane particles between luminal and lateral plasma membranes described earlier were confirmed, as well as the abundant occurrence of gap junctions on both membrane faces.

Adaptive regulation of amino acid transport across the cell membrane in avian and mammalian tissues

The regulation of amino acid transport across the cell membrane by adaptive mechanisms has been studied in a variety of mesenchymal and epithelial cells and tissues of avian and mammalian origin. Changes in transport activity as a function of time under various in vitro conditions (amino acid dependence, active and inhibited protein synthesis) have been evaluated by measurements of initial entry rates with representative amino acids. Results and conclusions based on the adopted experimental approach include the following. (1) An adaptive control mechanism for the transport of neutral amino acids corresponding to the typical substrates of the A mediation is operative in (a) mesenchymal cells (fibroblasts, chondroblasts, osteoblasts and myoblasts) from embryonic tissues of avian (chick embryo) origin and (b) mesenchymal cells from immature rat uterus (fibroblasts and smooth muscle cells) and other mammalian tissues (cardiac cells from newborn mouse and rat heart). (2) Adaptive regulation is restricted to a discrete subgroup of amino acids (L-proline, glycine and the analogue alpha-aminoisobutyric acid) in rat peritoneal macrophages and thymic lymphocytes. (3) Adaptive regulation is absent in erythroid cells (human erythrocytes, rabbit erythrocytes and reticulocytes, avian erythrocytes) which lack the A mediation and are incapable of active gene transcription. (4) Adaptive regulation is absent in the epithelial kidney cortex tissue and possibly absent in the epithelial component of liver tissue from adult rats; it is fully operative in the chick embryo crystalline lens, i.e. an epithelial preparation of embryonic origin. (5) These observations indicate that adaptive control mechanisms of amino acid transport across the cell membrane are quite common among tissues and species and emphasize their broad biological significance in eukaryotes.

Stimulation of cartilage amino acid uptake by growth hormone-dependent factors in serum. Mediation by adenosine 3':5'-monophosphate

The effects of growth hormone-dependent serum factors on amino acid transport and on cartilage cyclic AMP levels in embryonic chicken cartilage were studied in vitro. Cartilages incubated in medium containing rat serum showed a significantly greater uptake of alpha-amino [1-14C] isobutyrate or [1-14C] cycloleucine than control cartilages incubated in medium alone. Normal rat serum (5%) added to the incubation medium also caused an increase in cartilage cyclic AMP content (from as little as 23% to as much as 109%). The factors in serum which increase cartilage cyclic AMP and amino acid uptake are growth hormone dependent, since neither growth hormone itself nor serum from hypophysectomized rats restores these serum factors. Studies comparing the ability of sera with varying amounts of growth hormone-dependent factors to stimulate amino-aminoisobutyrate transport and to increase cartilage cyclic AMP show a striking linear correlation between the two effects (r=0.977). Theophylline and prostaglandin E1, WHICH RAISE CARTILAGE CYCLIC AMP also increase amino-aminoisobutyrate transport. Exogenous cyclic AMP, N6-monobutyryl cyclic AMP and n6, 02'-dibutyryl cyclic AMP increase cartilage amino-aminoisobutyrate transport. The data are compatible with the thesis that growth hormone-dependent serum factors increase cartilage amino acid transport by elevating cartilage cyclic AMP.