Nucleotide sugars, nucleotide sulfate, and ATP transporters of the endoplasmic reticulum and Golgi apparatus

The Ion Channel and GPCR Toolkit of Brain Capillary Pericytes

Brain pericytes reside on the abluminal surface of capillaries, and their processes cover ~90% of the length of the capillary bed. These cells were first described almost 150 years ago (Eberth, 1871; Rouget, 1873) and have been the subject of intense experimental scrutiny in recent years, but their physiological roles remain uncertain and little is known of the complement of signaling elements that they employ to carry out their functions. In this review, we synthesize functional data with single-cell RNAseq screens to explore the ion channel and G protein-coupled receptor (GPCR) toolkit of mesh and thin-strand pericytes of the brain, with the aim of providing a framework for deeper explorations of the molecular mechanisms that govern pericyte physiology. We argue that their complement of channels and receptors ideally positions capillary pericytes to play a central role in adapting blood flow to meet the challenge of satisfying neuronal energy requirements from deep within the capillary bed, by enabling dynamic regulation of their membrane potential to influence the electrical output of the cell. In particular, we outline how genetic and functional evidence suggest an important role for G_s-coupled GPCRs and ATP-sensitive potassium (K_ATP) channels in this context. We put forth a predictive model for long-range hyperpolarizing electrical signaling from pericytes to upstream arterioles, and detail the TRP and Ca²⁺ channels and G_q, G_i/o, and G_12/13 signaling processes that counterbalance this. We underscore critical questions that need to be addressed to further advance our understanding of the signaling topology of capillary pericytes, and how this contributes to their physiological roles and their dysfunction in disease.

Role of UDP-Sugar Receptor P2Y14 in Murine Osteoblasts

The purinergic (P2) receptor P2Y₁₄ is the only P2 receptor that is stimulated by uridine diphosphate (UDP)-sugars and its role in bone formation is unknown. We confirmed P2Y₁₄ expression in primary murine osteoblasts (CB-Ob) and the C2C12-BMP2 osteoblastic cell line (C2-Ob). UDP-glucose (UDPG) had undiscernible effects on cAMP levels, however, induced dose-dependent elevations in the cytosolic free calcium concentration ([Ca²⁺]_i) in CB-Ob, but not C2-Ob cells. To antagonize the P2Y₁₄ function, we used the P2Y₁₄ inhibitor PPTN or generated CRISPR-Cas9-mediated P2Y₁₄ knockout C2-Ob clones (Y14_KO). P2Y₁₄ inhibition facilitated calcium signalling and altered basal cAMP levels in both models of osteoblasts. Importantly, P2Y₁₄ inhibition augmented Ca²⁺ signalling in response to ATP, ADP and mechanical stimulation. P2Y₁₄ knockout or inhibition reduced osteoblast proliferation and decreased ERK1/2 phosphorylation and increased AMPKα phosphorylation. During in vitro osteogenic differentiation, P2Y₁₄ inhibition modulated the timing of osteogenic gene expression, collagen deposition, and mineralization, but did not significantly affect differentiation status by day 28. Of interest, while P2ry14^-/- mice from the International Mouse Phenotyping Consortium were similar to wild-type controls in bone mineral density, their tibia length was significantly increased. We conclude that P2Y₁₄ in osteoblasts reduces cell responsiveness to mechanical stimulation and mechanotransductive signalling and modulates osteoblast differentiation.

Empowering human cardiac progenitor cells by P2Y14 nucleotide receptor overexpression

KEY POINTS

Autologous cardiac progenitor cell (CPC) therapy is a promising approach for treatment of heart failure (HF). There is an unmet need to identify inherent deficits in aged/diseased human CPCs (hCPCs) derived from HF patients in the attempts to augment their regenerative capacity prior to use in the clinical setting. Here we report significant functional correlations between phenotypic properties of hCPCs isolated from cardiac biopsies of HF patients, clinical parameters of patients and expression of the P2Y₁₄ purinergic receptor (P2Y₁₄ R), a crucial detector for extracellular UDP-sugars released during injury/stress. P2Y₁₄ R is downregulated in hCPCs derived from HF patients with lower ejection fraction or diagnosed with diabetes. Augmenting P2Y₁₄ R expression levels in aged/diseased hCPCs antagonizes senescence and improves functional responses. This study introduces purinergic signalling modulation as a potential strategy to rejuvenate and improve phenotypic characteristics of aged/functionally compromised hCPCs prior to transplantation in HF patients.

ABSTRACT

Autologous cardiac progenitor cell therapy is a promising alternative approach to current inefficient therapies for heart failure (HF). However, ex vivo expansion and pharmacological/genetic modification of human cardiac progenitor cells (hCPCs) are necessary interventions to rejuvenate aged/diseased cells and improve their regenerative capacities. This study was designed to assess the potential of improving hCPC functional capacity by targeting the P2Y₁₄ purinergic receptor (P2Y₁₄ R), which has been previously reported to induce regenerative and anti-senescence responses in a variety of experimental models. c-Kit⁺ hCPCs were isolated from cardiac biopsies of multiple HF patients undergoing left ventricular assist device implantation surgery. Significant correlations existed between the expression of P2Y₁₄ R in hCPCs and clinical parameters of HF patients. P2Y₁₄ R was downregulated in hCPCs derived from patients with a relatively lower ejection fraction and patients diagnosed with diabetes. hCPC lines with lower P2Y₁₄ R expression did not respond to P2Y₁₄ R agonist UDP-glucose (UDP-Glu) while hCPCs with higher P2Y₁₄ R expression showed enhanced proliferation in response to UDP-Glu stimulation. Mechanistically, UDP-Glu stimulation enhanced the activation of canonical growth signalling pathways ERK1/2 and AKT. Restoring P2Y₁₄ R expression levels in functionally compromised hCPCs via lentiviral-mediated overexpression improved proliferation, migration and survival under stress stimuli. Additionally, P2Y₁₄ R overexpression reversed senescence-associated morphology and reduced levels of molecular markers of senescence p16^INK4a , p53, p21 and mitochondrial reactive oxygen species. Findings from this study unveil novel biological roles of the UDP-sugar receptor P2Y₁₄ in hCPCs and suggest purinergic signalling modulation as a promising strategy to improve phenotypic properties of functionally impaired hCPCs.

Heparan sulfate proteoglycans as key regulators of the mesenchymal niche of hematopoietic stem cells

The complex microenvironment that surrounds hematopoietic stem cells (HSCs) in the bone marrow niche involves different coordinated signaling pathways. The stem cells establish permanent interactions with distinct cell types such as mesenchymal stromal cells, osteoblasts, osteoclasts or endothelial cells and with secreted regulators such as growth factors, cytokines, chemokines and their receptors. These interactions are mediated through adhesion to extracellular matrix compounds also. All these signaling pathways are important for stem cell fates such as self-renewal, proliferation or differentiation, homing and mobilization, as well as for remodeling of the niche. Among these complex molecular cues, this review focuses on heparan sulfate (HS) structures and functions and on the role of enzymes involved in their biosynthesis and turnover. HS associated to core protein, constitute the superfamily of heparan sulfate proteoglycans (HSPGs) present on the cell surface and in the extracellular matrix of all tissues. The key regulatory effects of major medullar HSPGs are described, focusing on their roles in the interactions between hematopoietic stem cells and their endosteal niche, and on their ability to interact with Heparin Binding Proteins (HBPs). Finally, according to the relevance of HS moieties effects on this complex medullar niche, we describe recent data that identify HS mimetics or sulfated HS signatures as new glycanic tools and targets, respectively, for hematopoietic and mesenchymal stem cell based therapeutic applications.

UDP-Sugars as Extracellular Signaling Molecules: Cellular and Physiologic Consequences of P2Y14 Receptor Activation

UDP-sugars, which are indispensable for protein glycosylation reactions in cellular secretory pathways, also act as important extracellular signaling molecules. We discuss here the broadly expressed P2Y14 receptor, a G-protein-coupled receptor targeted by UDP sugars, and the increasingly diverse set of physiologic responses discovered recently functioning downstream of this receptor in many epithelia as well as in immune, inflammatory, and other cells.

Evidence that adrenal hexose-6-phosphate dehydrogenase can effect microsomal P450 cytochrome steroidogenic enzymes

The role of adrenal hexose-6-phosphate dehydrogenase in providing reducing equivalents to P450 cytochrome steroidogenic enzymes in the endoplasmic reticulum is uncertain. Hexose-6-phosphate dehydrogenase resides in the endoplasmic reticulum lumen and co-localizes with the bidirectional enzyme 11β-hydroxysteroid dehydrogenase 1. Hexose-6-phosphate dehydrogenase likely provides 11β-hydroxysteroid dehydrogenase 1 with NADPH electrons via channeling. Intracellularly, two compartmentalized reactions generate NADPH upon oxidation of glucose-6-phosphate: cytosolic glucose-6-phosphate dehydrogenase and microsomal hexose-6-phosphate dehydrogenase. Because some endoplasmic reticulum enzymes require an electron donor (NADPH), it is conceivable that hexose-6-phosphate dehydrogenase serves in this capacity for these pathways. Besides 11β-hydroxysteroid dehydrogenase 1, we examined whether hexose-6-phosphate dehydrogenase generates reduced pyridine nucleotide for pivotal adrenal microsomal P450 enzymes. 21-hydroxylase activity was increased with glucose-6-phosphate and, also, glucose and glucosamine-6-phosphate. The latter two substrates are only metabolized by hexose-6-phosphate dehydrogenase, indicating that requisite NADPH for 21-hydroxylase activity was not via glucose-6-phosphate dehydrogenase. Moreover, dihydroepiandrostenedione, a non-competitive inhibitor of glucose-6-phosphate dehydrogenase, but not hexose-6-phosphate dehydrogenase, did not curtail activation by glucose-6-phosphate. Finally, the most compelling observation was that the microsomal glucose-6-phosphate transport inhibitor, chlorogenic acid, blunted the activation by glucose-6-phosphate of both 21-hydroxylase and 17-hydroxylase indicating that luminal hexose-6-phosphate dehydrogenase can supply NADPH for these enzymes. Analogous kinetic observations were found with microsomal 17-hydroxylase. These findings indicate that hexose-6-phosphate dehydrogenase can be a source, but not exclusively so, of NADPH for several adrenal P450 enzymes in the steroid pathway. Although the reduced pyridine nucleotides are produced intra-luminally, these compounds may also slowly transverse the endoplasmic reticulum membrane by unknown mechanisms.

Engineering mammalian mucin-type O-glycosylation in plants

Mucin-type O-glycosylation is an important post-translational modification that confers a variety of biological properties and functions to proteins. This post-translational modification has a particularly complex and differentially regulated biosynthesis rendering prediction and control of where O-glycans are attached to proteins, and which structures are formed, difficult. Because plants are devoid of GalNAc-type O-glycosylation, we have assessed requirements for establishing human GalNAc O-glycosylation de novo in plants with the aim of developing cell systems with custom-designed O-glycosylation capacity. Transient expression of a Pseudomonas aeruginosa Glc(NAc) C4-epimerase and a human polypeptide GalNAc-transferase in leaves of Nicotiana benthamiana resulted in GalNAc O-glycosylation of co-expressed human O-glycoprotein substrates. A chimeric YFP construct containing a 3.5 tandem repeat sequence of MUC1 was glycosylated with up to three and five GalNAc residues when co-expressed with GalNAc-T2 and a combination of GalNAc-T2 and GalNAc-T4, respectively, as determined by mass spectrometry. O-Glycosylation was furthermore demonstrated on a tandem repeat of MUC16 and interferon α2b. In plants, prolines in certain classes of proteins are hydroxylated and further substituted with plant-specific O-glycosylation; unsubstituted hydroxyprolines were identified in our MUC1 construct. In summary, this study demonstrates that mammalian type O-glycosylation can be established in plants and that plants may serve as a host cell for production of recombinant O-glycoproteins with custom-designed O-glycosylation. The observed hydroxyproline modifications, however, call for additional future engineering efforts.

Molecular mechanisms of purine and pyrimidine nucleotide release

Given the widespread importance of purinergic receptor-evoked signaling, understanding how ATP and other nucleotides are released from cells in a regulated manner is an essential physiological question. Nonlytic release of ATP, UTP, UDP-glucose, and other nucleotides occurs in all cell types and tissues via both constitutive mechanisms, that is, in the absence of external stimuli, and to a greater extent in response to biochemical or mechanical/physical stimuli. However, a molecular understanding of the processes regulating nucleotide release has only recently begun to emerge. It is generally accepted that nucleotide release occurs in two different scenarios, exocytotic release from the secretory pathway or via conductive/transport mechanisms, and a critical review of our current understanding of these mechanisms is presented in this chapter.

Translocon pores in the endoplasmic reticulum are permeable to small anions

Contribution of translocon peptide channels to the permeation of low molecular mass anions was investigated in rat liver microsomes. Puromycin, which purges translocon pores of nascent polypeptides, creating additional empty pores, raised the microsomal uptake of radiolabeled UDP-glucuronic acid, while it did not increase the uptake of glucose-6-phosphate or glutathione. The role of translocon pores in the transport of small anions was also investigated by measuring the effect of puromycin on the activity of microsomal enzymes with intraluminal active sites. The mannose-6-phosphatase activity of glucose-6-phosphatase and the activity of UDP-glucuronosyltransferase were elevated upon addition of puromycin, but glucose-6-phosphatase and beta-glucuronidase activities were not changed. The increase in enzyme activities was due to a better access of the substrates to the luminal compartment rather than to activation of the enzymes. Antibody against Sec61 translocon component decreased the activity of UDP-glucuronosyltransferase and antagonized the effect of puromycin. Similarly, the addition of the puromycin antagonist anisomycin or treatments of microsomes, resulting in the release of attached ribosomes, prevented the puromycin-dependent increase in the activity. Mannose-6-phosphatase and UDP-glucuronosyltransferase activities of smooth microsomal vesicles showed higher basal latencies that were not affected by puromycin. In conclusion, translationally inactive, ribosome-bound translocons allow small anions to cross the endoplasmic reticulum membrane. This pathway can contribute to the nonspecific substrate supply of enzymes with intraluminal active centers.

A mammalian lysosomal membrane protein confers multidrug resistance upon expression in Saccharomyces cerevisiae

Mouse transporter protein (MTP) is a highly conserved polytopic membrane protein present in mammalian lysosomes and endosomes. The role of MTP in regulating the in vivo subcellular distribution of numerous structurally distinct small molecules has been examined in this study by its expression in a drug-sensitive strain of the yeast Saccharomyces cerevisiae. Surprisingly, the expression of MTP in membranes of an intracellular compartment resulted in a cellular resistance or hypersensitivity to a range of drugs that included nucleoside and nucleobase analogs, antibiotics, anthracyclines, ionophores, and steroid hormones. The intracellular bioavailability of steroid hormones was altered by MTP, as determined using an in vivo glucocorticoid receptor-driven reporter assay in yeast, suggesting that the MTP-regulated drug sensitivity arose due to a change in the subcellular compartmentalization of steroid hormones and other drugs. MTP-regulated drug sensitivity in yeast was blocked to varying degrees by compounds that inhibit lysosomal function, interfere with intracellular cholesterol transport, or modulate the multidrug resistance phenotype of mammalian cells. These results indicate that MTP is involved in the subcellular compartmentalization of diverse hydrophobic small molecules and contributes to the inherent drug sensitivity or resistance of the mammalian cell.