Real-Time Tracking of BODIPY-C12 Long-Chain Fatty Acid in Human Term Placenta Reveals Unique Lipid Dynamics in Cytotrophoblast Cells

While the human placenta must provide selected long-chain fatty acids to support the developing fetal brain, little is known about the mechanisms underlying the transport process. We tracked the movement of the fluorescently labeled long-chain fatty acid analogue, BODIPY-C₁₂, across the cell layers of living explants of human term placenta. Although all layers took up the fatty acid, rapid esterification of long-chain fatty acids and incorporation into lipid droplets was exclusive to the inner layer cytotrophoblast cells rather than the expected outer syncytiotrophoblast layer. Cytotrophoblast is a progenitor cell layer previously relegated to a repair role. As isolated cytotrophoblasts differentiated into syncytialized cells in culture, they weakened their lipid processing capacity. Syncytializing cells suppress previously active genes that regulate fatty-acid uptake (SLC27A2/FATP2, FABP4, ACSL5) and lipid metabolism (GPAT3, LPCAT3). We speculate that cytotrophoblast performs a previously unrecognized role in regulating placental fatty acid uptake and metabolism.

Preliminary Examination of Olanzapine and Diet Interactions On Metabolism in a Female Macaque

Clinical data suggest that atypical antipsychotics such as olanzapine (OLZ) induce significant metabolic changes that are serious side effects of their primary use. Since controlled human studies are problematic and rodent data may be poorly translatable, we have initiated development of a macaque model of OLZ-induced metabolic disease. In this preliminary feasibility study, we examined some metabolic effects of OLZ in a female macaque in the context of a standard low-calorie/fat monkey chow diet followed by a high-fat/sugar Western-style diet (WSD). A female Japanese macaque was administered OLZ (1.25 mg/day) for 6 months, with dietary changes at 2-month intervals as follows: OLZ+Restricted chow, OLZ+Unrestricted chow, OLZ+WSD, and placebo+WSD. Weight was assessed weekly. Glucose tolerance tests (GTT) and Dexascans were performed at baseline and every 2 months. Omental (OM) and subcutaneous (SQ) adipose tissue biopsies were obtained at baseline, after OLZ+Unrestricted chow and after OLZ+WSD to evaluate adipocyte size, lipolysis and insulin-stimulated free fatty acid uptake (FFA). A separate trial was conducted on 2 monkeys with 5 days of OLZ- or no-treatment followed by RT-PCR on rostral and medial basal hypothalamus. Weight increased on OLZ+Restricted chow and stabilized on OLZ+Unrestricted chow. OLZ+WSD did not significantly change the weight plateau. Weight declined upon withdrawal of OLZ with continued WSD. Body fat increased from 14% at baseline to 22%, 30%, 28% and 19% at 2, 4, 6 and 8 mo, respectively, indicating that body fat was elevated on OLZ regardless of diet and declined upon OLZ removal. Glucose tolerance and the insulin response during GTT were normal with OLZ+Restricted chow or OLZ+Unrestricted chow. Addition of WSD with OLZ impaired glucose clearance during GTT. Insulin remained in the normal range, but first phase insulin secretion was reduced. After removal of OLZ, but continued WSD, glucose clearance returned to normal, but this was associated with hyperinsulinemia. Adipocyte diameter was increased in OM and SQ fat by OLZ+chow and OLZ+WSD to a similar extent. (p<0.01, 2-way ANOVA). In OM, isoproterenol-stimulated lipolysis occurred at baseline. In both depots, isoproterenol-stimulated lipolysis occurred with OLZ+chow, but it was significantly blunted by addition of WSD (ANOVA p<0.0001; posthoc p<0.05). Insulin increased FFA uptake at baseline. OLZ +chow or OLZ+WSD increased basal FFA uptake and insulin-induced FFA uptake was blunted in both depots (posthoc p<0.05). There was a marked decrease in POMC gene expression, and increased AgRP and NPY expression in the hypothalamus. There was also a clear increase in serotonin (5HT) 2C, melanocortin (MCR4), and Leptin (LepR) receptor gene expression. These data support the hypotheses that OLZ acts on peripheral tissues as well as in the CNS; that changes in hypothalamic gene expression occur very rapidly and precede increased fat accumulation; that adipose tissue exhibits insulin resistance prior to alterations in GTT; that addition of WSD to OLZ precipitates hyperglycemia without an obvious insulin response; and that removal of OLZ and continued WSD resulted in normalized glucose clearance and elevated insulin. These data suggest complex and early responses to OLZ that may be exacerbated by WSD.

Cell-autonomous heterogeneity of nutrient uptake in white adipose tissue of rhesus macaques

Phenotypic diversity may play an adaptive role by providing graded biological responses to fluctuations in environmental stimuli. We used single-cell imaging of the metabolizable fluorescent fatty acid analog 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-C12 and fluorescent 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-2-deoxy-D-glucose (2-NBDG) to explore cellular heterogeneity in nutrient uptake in white adipose tissue (WAT) explants of rhesus macaques. Surprisingly, WAT displayed a striking cell size-independent mosaic pattern, in that adjacent adipocytes varied with respect to insulin-stimulated BODIPY-C12 and 2-NBDG uptake. Relative free fatty acid (FFA) transport activity correlated with the cellular levels of FFA transporter protein-1 and the scavenger receptor CD36 in individual adipocytes. In vitro incubation of WAT explants for 24 hours caused partial desynchronization of cellular responses, suggesting that adipocytes may slowly alter their differential nutrient uptake activity. In vitro-differentiated human adipocytes also exhibited a mosaic pattern of BODIPY-C12 uptake. WAT from animals containing a homogeneous population of large adipocytes was nonmosaic, in that every adipocyte exhibited a similar level of BODIPY-C12 fluorescence, suggesting that the development of obesity is associated with the loss of heterogeneity in WAT. Hence, for the first time, we demonstrate an intrinsic heterogeneity in FFA and glucose transport activity in WAT.

Spatiotemporal dynamics of triglyceride storage in unilocular adipocytes

Real-time fluorescence microscopy is used to investigate the trafficking of metabolizable fluorescent fatty acid in unilocular adipocytes from adipose tissue of nonhuman primates. The study reveals novel cell biological features that may contribute to the mechanism of adipocyte hypertrophy.

The spatiotemporal dynamics of triglyceride (TG) storage in unilocular adipocytes are not well understood. Here we applied ex vivo technology to study trafficking and metabolism of fluorescent fatty acids in adipose tissue explants. Live imaging revealed multiple cytoplasmic nodules surrounding the large central lipid droplet (cLD) of unilocular adipocytes. Each cytoplasmic nodule harbors a series of closely associated cellular organelles, including micro–lipid droplets (mLDs), mitochondria, and the endoplasmic reticulum. Exogenously added free fatty acids are rapidly adsorbed by mLDs and concurrently get esterified to TG. This process is greatly accelerated by insulin. mLDs transfer their content to the cLD, serving as intermediates that mediate packaging of newly synthesized TG in the large interior of a unilocular adipocyte. This study reveals novel cell biological features that may contribute to the mechanism of adipocyte hypertrophy.

Autistic children display elevated urine levels of bovine casomorphin-7 immunoreactivity

Elevated concentrations of circulating casomorphins (CM), the exogenous opioid peptides from milk casein, may contribute to the pathogenesis of autism in children. Because several mass spectrometry studies failed to detect casomorphins in autistic children, it was questioned whether these peptides can be detected in body fluids by mass spec. Here we demonstrated, using a novel high sensitivity ELISA method, that autistic children have significantly higher levels of urine CM-7 than control children. The severity of autistic symptoms correlated with concentrations of CM-7 in the urine. Because CMs interact with opioid and serotonin receptors, the known modulators of synaptogenesis, we suggest that chronic exposure to elevated levels of bovine CMs may impair early child development, setting the stage for autistic disorders.

Real-time detection of SNARE complex assembly with FRET using the tetracysteine system

Small tetracysteine insertions are more suitable for fluorescence resonance energy transfer (FRET) studies of protein folding and small complex assembly than bulky GFP-based fluorophores. Here, we describe a procedure for expression, purification, and fluorescent labeling of a FRET-based probe, called CSNAC that can track the conformational changes undergone by SNAP-25 as it folds in the exocytic complex. The fluorescent protein Cerulean was attached to the N-terminus and served as a FRET donor. The biarsenical dye FlAsH, served as a FRET acceptor, was bound to a short tetracysteine motif positioned in the linker domain of SNAP-25. CSNAC can report real-time FRET changes when the Syntaxin soluble domain is added in vitro.

Sex-specific differences in lipid and glucose metabolism

Energy metabolism in humans is tuned to distinct sex-specific functions that potentially reflect the unique requirements in females for gestation and lactation, whereas male metabolism may represent a default state. These differences are the consequence of the action of sex chromosomes and sex-specific hormones, including estrogens and progesterone in females and androgens in males. In humans, sex-specific specialization is associated with distinct body-fat distribution and energy substrate-utilization patterns; i.e., females store more lipids and have higher whole-body insulin sensitivity than males, while males tend to oxidize more lipids than females. These patterns are influenced by the menstrual phase in females, and by nutritional status and exercise intensity in both sexes. This minireview focuses on sex-specific mechanisms in lipid and glucose metabolism and their regulation by sex hormones, with a primary emphasis on studies in humans and the most relevant pre-clinical model of human physiology, non-human primates.

Ovarian cycle-specific regulation of adipose tissue lipid storage by testosterone in female nonhuman primates

Previous studies in rodents and humans suggest that hyperandrogenemia causes white adipose tissue (WAT) dysfunction in females, although the underlying mechanisms are poorly understood. In light of the differences in the length of the ovarian cycle between humans and rodents, we used a nonhuman primate model to elucidate the effects of chronic hyperandrogenemia on WAT function in vivo. Female rhesus macaques implanted with testosterone capsules developed insulin resistance and altered leptin secretion on a high-fat, Western-style diet. In control visceral WAT, lipolysis and hormone-sensitive lipase expression were upregulated during the luteal phase compared with the early follicular (menses) phase of the ovarian cycle. Hyperandrogenemia attenuated elevated lipolysis and hormone-sensitive lipase activity in visceral WAT during the luteal phase but not during menses. Under control conditions, insulin-stimulated Akt and Erk activation and fatty acid uptake in WAT were not significantly affected by the ovarian cycle. In contrast, testosterone treatment preferentially increased fatty acid uptake and insulin signaling at menses. The fatty acid synthase and glucose transporter-4 genes were upregulated by testosterone during the luteal phase. In summary, this study reveals ovarian stage-specific fluctuations in adipocyte lipolysis and suggests that male sex hormones increase and female sex hormones decrease lipid storage in female WAT.

Selective control of SNARE recycling by Golgi retention

Two distinct sets of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) catalyze membrane fusion in the cis-Golgi and trans-Golgi. The mechanism that controls Golgi localization of SNAREs remains largely unknown. Here we tested three potential mechanisms, including vesicle recycling between the Golgi and the endoplasmic reticulum, partitioning in Golgi lipid microdomains, and selective intra-Golgi retention. Recycling rates showed a linear relationship with intra-Golgi mobility of SNAREs. The cis-Golgi SNAREs had higher mobility than intra-Golgi SNAREs, whereas vesicle SNAREs had higher mobility than target membrane SNAREs. The differences in SNARE mobility were not due to preferential partitioning into detergent-resistant membrane microdomains. We propose that intra-Golgi retention precludes entropy-driven redistribution of SNAREs to the endoplasmic reticulum and endocytic compartments.

Androgen effects on adipose tissue architecture and function in nonhuman primates

The differential association of hypoandrogenism in men and hyperandrogenism in women with insulin resistance and obesity suggests that androgens may exert sex-specific effects on adipose and other tissues, although the underlying mechanisms remain poorly understood. Moreover, recent studies also suggest that rodents and humans may respond differently to androgen imbalance. To achieve better insight into clinically relevant sex-specific mechanisms of androgen action, we used nonhuman primates to investigate the direct effects of gonadectomy and hormone replacement on white adipose tissue. We also employed a novel ex vivo approach that provides a convenient framework for understanding of adipose tissue physiology under a controlled tissue culture environment. In vivo androgen deprivation of males did not result in overt obesity or insulin resistance but did induce the appearance of very small, multilocular white adipocytes. Testosterone replacement restored normal cell size and a unilocular phenotype and stimulated adipogenic gene transcription and improved insulin sensitivity of male adipose tissue. Ex vivo studies demonstrated sex-specific effects of androgens on adipocyte function. Female adipose tissue treated with androgens displayed elevated basal but reduced insulin-dependent fatty acid uptake. Androgen-stimulated basal uptake was greater in adipose tissue of ovariectomized females than in adipose tissue of intact females and ovariectomized females replaced with estrogen and progesterone in vivo. Collectively, these data demonstrate that androgens are essential for normal adipogenesis in males and can impair essential adipocyte functions in females, thus strengthening the experimental basis for sex-specific effects of androgens in adipose tissue.

Hypoxia-induced inflammatory cytokine secretion in human adipose tissue stromovascular cells

AIMS

Hypoxia has been implicated as a cause of adipose tissue inflammation in obesity, although the inflammatory response of human adipose tissue to hypoxia is not well understood. The goal of this study was to define in vitro inflammatory responses of human adipose tissue to hypoxia and identify molecular mechanisms of hypoxia-induced inflammation.

METHODS

The inflammatory milieu and responses of visceral (VAT) and subcutaneous (SAT) adipose tissue explants and purified stromovascular cells (SVFs) from obese and lean humans were studied in an in vitro hypoxic culture system using quantitative real-time PCR, ELISA, western blotting, immunofluorescence microscopy, flow cytometry and immunohistochemistry.

RESULTS

Human adipose tissue in obesity demonstrates an increased leucocyte infiltrate that is greater in VAT than SAT and involves macrophages, T cells and natural killer (NK) cells. Hypoxic culture regulates inflammatory cytokine secretion and transcription of metabolic stress response genes in human adipose tissue SVF. Adipocyte diameter is increased and adipose tissue capillary density is decreased in obese participants. Inhibition of c-Jun terminal kinase (JNK) or p38 significantly attenuates hypoxia-induced SVF inflammatory responses. Hypoxia induces phosphorylation of p38 in adipose tissue.

CONCLUSIONS

Human adipose tissue in obesity is characterised by a depot-specific inflammatory cell infiltrate that involves not only macrophages, but also T cells and NK cells. Hypoxia induces inflammatory cytokine secretion by human adipose tissue SVF, the primary source of which is adipose tissue macrophages. These data implicate p38 in the regulation of hypoxia-induced inflammation and suggest that alterations in adipocyte diameter and adipose tissue capillary density may be potential underlying causes of adipose tissue hypoxia.

Live-cell imaging demonstrates rapid cargo exchange between lipid droplets in adipocytes

Lipid droplets form the storage reservoirs for lipids in adipocytes, and their stable appearance suggests a static nature of lipid storage. A stable lipid store, however, may be maintained through the dynamic recycling of lipid cargo between the cytoplasmic compartment and the lipid droplet. In this study, we applied live-cell microscopy to follow intracellular transport steps of fluorescently labeled fatty acids in differentiated 3T3-L1 adipocytes. We demonstrate that intracellular lipids continuously exit and re-enter lipid droplets, and that individual lipid droplets exchange their content on a timescale of minutes. These data demonstrate a surprisingly high rate of intracellular lipid turnover in adipocytes and support the novel concept that lipid storage is achieved by dynamic recycling rather than static retention.

Single-cell analysis of insulin-regulated fatty acid uptake in adipocytes

Increased body fat correlates with the enlargement of average fat cell size and reduced adipose tissue insulin sensitivity. It is currently unclear whether adipocytes, as they accumulate more triglycerides and grow in size, gradually become less insulin sensitive or whether obesity-related factors independently cause both the enlargement of adipocyte size and reduced adipose tissue insulin sensitivity. In the first instance, large and small adipocytes in the same tissue would exhibit differences in insulin sensitivity, whereas, in the second instance, adipocyte size per se would not necessarily correlate with insulin response. To analyze the effect of adipocyte size on insulin sensitivity, we employed a new single-cell imaging assay that resolves fatty acid uptake and insulin response in single adipocytes in subcutaneous adipose tissue explants. Here, we report that subcutaneous adipocytes are heterogeneous in size and intrinsic insulin sensitivity. Whereas smaller adipocytes respond to insulin by increasing lipid uptake, adipocytes with cell diameters larger than 80-100 microm are insulin resistant. We propose that, when cell size approaches a critical boundary, adipocytes lose insulin-dependent fatty acid transport. This negative feedback mechanism may protect adipocytes from lipid overload and restrict further expansion of adipose tissue, which leads to obesity and metabolic complications.

Dynamic transport of SNARE proteins in the Golgi apparatus

Localization of a membrane protein in a subcellular compartment can be achieved by its retention in the compartment or by its continuous transport toward this compartment. Previous results have suggested that specific enzymes are localized in the Golgi apparatus at least in part by selective retention and exclusion from transport vesicles. However, the function of some Golgi SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins is not compatible with their exclusion from transport vesicles. To help understand the mechanism accounting for the localization of SNARE proteins in the Golgi apparatus, we analyzed their lateral distribution in the Golgi cisternae and their incorporation into transport vesicles. According to our results, all SNARE proteins are efficiently incorporated into transport vesicles, indicating that the localization of SNARE proteins in the Golgi apparatus is not based on a static retention mechanism. Detailed analysis suggested that incorporation into transport vesicles was more efficient for SNARE proteins restricted to the cis face of the Golgi as compared with SNAREs present at the trans face. Furthermore, overexpression of a cis-Golgi SNARE protein altered concomitantly its incorporation in transport vesicles and its intra-Golgi localization. These observations suggest that, contrary to resident Golgi enzymes, SNARE proteins are localized in the Golgi apparatus as the result of a dynamic transport equilibrium.

Localization and activity of the SNARE Ykt6 determined by its regulatory domain and palmitoylation

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) catalyze compartment-specific membrane fusion. Whereas most SNAREs are bona fide type II membrane proteins, Ykt6 lacks a proteinaceous membrane anchor but contains a prenylation consensus motif (CAAX box) and exists in an inactive cytosolic and an active membrane-bound form. We demonstrate that both forms are farnesylated at the carboxyl-terminal cysteine of the CCAIM sequence. Farnesylation is the prerequisite for subsequent palmitoylation of the upstream cysteine, which permits stable membrane association of Ykt6. The double-lipid modification and membrane association is crucial for intra-Golgi transport in vitro and cell homeostasis/survival in vivo. The membrane recruitment and palmitoylation is controlled by the N-terminal domain of Ykt6, which interacts with the SNARE motif, keeping it in an inactive closed conformation. Together, these results suggest that conformational changes control the lipid modification and function of Ykt6. Considering the essential and central role of Ykt6 in the secretory pathway, this spatial and functional cycle might provide a mechanism to regulate the rate of intracellular membrane flow.

Countercurrent distribution of two distinct SNARE complexes mediating transport within the Golgi stack

Genetic and biochemical evidence has established that a SNARE complex consisting of syntaxin 5 (Sed5)-mYkt6 (Ykt6)-GOS28 (Gos1)-GS15 (Sft1) is required for transport of proteins across the Golgi stack in animals (yeast). We have utilized quantitative immunogold labeling to establish the cis-trans distribution of the v-SNARE GS15 and the t-SNARE subunits GOS28 and syntaxin 5. Whereas the distribution of the t-SNARE is nearly even across the Golgi stack from the cis to the trans side, the v-SNARE GS15 is present in a gradient of increasing concentration toward the trans face of the stack. This contrasts with a second distinct SNARE complex, also required for intra-Golgi transport, consisting of syntaxin 5 (Sed5)-membrin (Bos1)-ERS24 (Sec22)-rBet1 (Bet1), whose v-(rBet1) and t-SNARE subunits (membrin and ERS24), progressively decrease in concentration toward the trans face. Transport within the stack therefore appears to utilize countercurrent gradients of two Golgi SNAREpins and may involve a mechanism akin to homotypic fusion.

i-SNAREs: inhibitory SNAREs that fine-tune the specificity of membrane fusion

A new functional class of SNAREs, designated inhibitory SNAREs (i-SNAREs), is described here. An i-SNARE inhibits fusion by substituting for or binding to a subunit of a fusogenic SNAREpin to form a nonfusogenic complex. Golgi-localized SNAREs were tested for i-SNARE activity by adding them as a fifth SNARE together with four other SNAREs that mediate Golgi fusion reactions. A striking pattern emerges in which certain subunits of the cis-Golgi SNAREpin function as i-SNAREs that inhibit fusion mediated by the trans-Golgi SNAREpin, and vice versa. Although the opposing distributions of the cis- and trans-Golgi SNAREs themselves could provide for a countercurrent fusion pattern in the Golgi stack, the gradients involved would be strongly sharpened by the complementary countercurrent distributions of the i-SNAREs.

A SNARE required for retrograde transport to the endoplasmic reticulum

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) are central components of the machinery mediating membrane fusion in all eukaryotic cells. Sequence analysis of the yeast genome revealed a previously uncharacterized SNARE, SNARE-like tail-anchored protein 1 (Slt1). Slt1 is an essential protein localized in the endoplasmic reticulum (ER). It forms a SNARE complex with Sec22 and the ER syntaxin Ufe1. Down-regulation of Slt1 levels leads to improper secretion of proteins normally resident in the ER. We suggest that Slt1 is a component of the SNAREpin required for retrograde traffic to the ER. Based on the previously reported association with Ufe1 and Sec22, Sec20 likely contributes the fourth SNARE to the SNAREpin.

Analysis of the carboxypeptidase D cytoplasmic domain: Implications in intracellular trafficking

Metallocarboxypeptidase D (CPD) is a type 1 transmembrane protein that functions in the processing of proteins that transit the secretory pathway. Previously, CPD was found to be enriched in the trans Golgi network (TGN) and to cycle between this compartment and the cell surface. In the present study, the roles of specific regions of the CPD cytosolic tail in intracellular trafficking were investigated in the AtT-20 cell line. When the CPD transmembrane region and cytosolic tail are attached to the C-terminus of albumin, this protein is retained in the TGN and cycles to the cell surface. Deletion analysis indicates that a C-terminal region functions in TGN-retention; removal of 10 amino acids from the C-terminus greatly increases the amount of fusion protein that enters nascent vesicles, which bud from the Golgi, but does not affect the half-life of the fusion protein or the ability of cell surface protein to return to the TGN. Because the 10-residue deletion disrupts a casein kinase 2 (CK2) consensus site, the two Thr in this site (TDT) were mutated to either Ala (ADA) or Glu (EDE). Neither mutation has an increased rate of budding from the TGN, although the ADA mutant has a shorter half-life than either the wild type sequence or the EDE mutant. Adaptor protein-1 and -2 bind to most of the deletion mutants, the EDE point mutant, and the CK2-phosphorylated CPD tail, but not to the wild type tail. Taken together, these results suggest that CPD localization to the TGN requires both static retention involving the C-terminal domain and phosphorylation at a CK2 site, which regulates the binding of adaptor proteins.

Distinct SNARE complexes mediating membrane fusion in Golgi transport based on combinatorial specificity

Syntaxin-5 (Sed5) is the only syntaxin needed for transport into and across the yeast Golgi, raising the question of how a single syntaxin species could mediate vesicle transport in both the anterograde and the retrograde direction within the stack. Sed5 is known to combine with two light chains (Bos1 and Sec22) to form the t-SNARE needed to receive vesicles from the endoplasmic reticulum. However, the yeast Golgi contains several other potential light chains with which Sed5 could potentially combine to form other t-SNAREs. To explore the degree of specificity in the choice of light chains by a t-SNARE, we undertook a comprehensive examination of the capacity of all 21 Sed5-based t-SNAREs that theoretically could assemble in the yeast Golgi to fuse with each of the 7 potential v-SNAREs also present in this organelle. Only one additional of these 147 combinations was fusogenic. This functional proteomic strategy thereby revealed a previously uncharacterized t-SNARE in which Sed5 is the heavy chain and Gos1 and Ykt6 are the light chains, and whose unique cognate v-SNARE is Sft1. Immunoprecipitation experiments confirmed the existence of this complex in vivo. Fusion mediated by this second Golgi SNAREpin is topologically restricted, and existing genetic and morphologic evidence implies that it is used for transport across the Golgi stack. From this study, together with the previous functional proteomic analyses which have tested 275 distinct quaternary SNARE combinations, it follows that the fusion potential and transport pathways of the yeast cell can be read out from its genome sequence according to the SNARE hypothesis with a predictive accuracy of about 99.6%.

Protein phosphatase 2A binds to the cytoplasmic tail of carboxypeptidase D and regulates post-trans-Golgi network trafficking

Carboxypeptidase D (CPD) is a transmembrane protein that processes proteins in the trans-Golgi network (TGN). A 20-residue region within the cytoplasmic tail of CPD binds protein phosphatase 2A (PP2A). PP2A also binds to the cytoplasmic tails of other secretory pathway proteins: peptidylglycine-(amino)-amidating mono-oxygenase, the cation-independent mannose-6-phosphate receptor and TGN38. The CPD tail is phosphorylated on Thr residues in the AtT-20 cell line. The CPD tail can also be phosphorylated by purified protein kinase A, protein kinase C and casein kinase II. Both the in vitro and the in vivo phosphorylated CPD tail can be dephosphorylated by purified PP2A. The binding of CPD tail peptide to PP2A does not influence phosphatase activity. The rate of transport of CPD from the TGN to the cell surface of AtT-20 cells is decreased 45% by okadaic acid, a PP2A inhibitor. Microinjection of the CPD tail into AtT-20 cells inhibits the transition of CPD from endosomal compartments to the TGN. However, okadaic acid does not affect the rate of budding of CPD from the TGN into nascent vesicles or the rate of uptake from the cell surface into endosomal compartments. These results are consistent with the model that PP2A is involved in the trafficking of proteins between a TGN recycling loop and a cell-surface recycling loop, but is not involved in the individual recycling loops.

Carboxypeptidase Z is present in the regulated secretory pathway and extracellular matrix in cultured cells and in human tissues

Carboxypeptidase Z (CPZ) is a newly reported member of the metallocarboxypeptidase gene family, but unlike other members of this family, CPZ contains an N-terminal domain that has amino acid sequence similarity to Wnt-binding proteins. In order to gain insights as to the potential function of CPZ, the intracellular localization of this protein was determined in cell culture and in human tissues. When expressed in the AtT-20 mouse pituitary cell line, CPZ protein is routed to the regulated secretory pathway and secreted upon stimulation. A fraction of the secreted CPZ remains associated with the extracellular matrix. Endogenous CPZ in the PC12 rat pheochromocytoma cell line is also associated with the extracellular matrix. In human placenta, CPZ is present within invasive trophoblasts and in the surrounding extracellular space, indicating an association with extracellular matrix. CPZ is also present in amnion cells, but is not readily apparent in the extracellular matrix of this cell type. A human adenocarcinoma of the colon shows expression of CPZ in the extracellular matrix adjacent to malignant cells. Taken together, CPZ appears to be a component of the extracellular matrix in some cell types, where it may function in the binding of Wnt.