Changes in metabolism of inorganic polyphosphate in rat tissues and human cells during development and apoptosis

Amyloid Accelerator Polyphosphate Implicated as the Mystery Density in α-Synuclein Fibrils

Aberrant aggregation of α-Synuclein is the pathological hallmark of a set of neurodegenerative diseases termed synucleinopathies. Recent advances in cryo-electron microscopy have led to the structural determination of the first synucleinopathy-derived α-Synuclein fibrils, which contain a non-proteinaceous, "mystery density" at the core of the protofilaments, hypothesized to be highly negatively charged. Guided by previous studies that demonstrated that polyphosphate (polyP), a universally conserved polyanion, significantly accelerates α-Synuclein fibril formation, we conducted blind docking and molecular dynamics simulation experiments to model the polyP binding site in α-Synuclein fibrils. Here we demonstrate that our models uniformly place polyP into the lysine-rich pocket, which coordinates the mystery density in patient-derived fibrils. Subsequent in vitro studies and experiments in cells revealed that substitution of the two critical lysine residues K43 and K45 leads to a loss of all previously reported effects of polyP binding on α-Synuclein, including stimulation of fibril formation, change in filament conformation and stability as well as alleviation of cytotoxicity. In summary, our study demonstrates that polyP fits the unknown electron density present in in vivo α-Synuclein fibrils and suggests that polyP exerts its functions by neutralizing charge repulsion between neighboring lysine residues.

Polyphosphate Nanoparticles: Balancing Energy Requirements in Tissue Regeneration Processes

Nanoparticles of a particular, evolutionarily old inorganic polymer found across the biological kingdoms have attracted increasing interest in recent years not only because of their crucial role in metabolism but also their potential medical applicability: it is inorganic polyphosphate (polyP). This ubiquitous linear polymer is composed of 10-1000 phosphate residues linked by high-energy anhydride bonds. PolyP causes induction of gene activity, provides phosphate for bone mineralization, and serves as an energy supplier through enzymatic cleavage of its acid anhydride bonds and subsequent ATP formation. The biomedical breakthrough of polyP came with the development of a successful fabrication process, in depot form, as Ca- or Mg-polyP nanoparticles, or as the directly effective polymer, as soluble Na-polyP, for regenerative repair and healing processes, especially in tissue areas with insufficient blood supply. Physiologically, the platelets are the main vehicles for polyP nanoparticles in the circulating blood. To be biomedically active, these particles undergo coacervation. This review provides an overview of the properties of polyP and polyP nanoparticles for applications in the regeneration and repair of bone, cartilage, and skin. In addition to studies on animal models, the first successful proof-of-concept studies on humans for the healing of chronic wounds are outlined.

Inorganic polyphosphate: from basic research to diagnostic and therapeutic opportunities in ALS/FTD

Inorganic polyphosphate (polyP) is a simple, negatively charged biopolymer with chain lengths ranging from just a few to over a thousand ortho-phosphate (Pi) residues. polyP is detected in every cell type across all organisms in nature thus far analyzed. Despite its structural simplicity, polyP has been shown to play important roles in a remarkably broad spectrum of biological processes, including blood coagulation, bone mineralization and inflammation. Furthermore, polyP has been implicated in brain function and the neurodegenerative diseases amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Alzheimer's disease and Parkinson's disease. In this review, we first address the challenges associated with identifying mammalian polyP metabolizing enzymes, such as Nudt3, and quantifying polyP levels in brain tissue, cultured neural cells and cerebrospinal fluid. Subsequently, we focus on recent studies that unveil how the excessive release of polyP by human and mouse ALS/FTD astrocytes contributes to these devastating diseases by inducing hyperexcitability, leading to motoneuron death. Potential implications of elevated polyP levels in ALS/FTD patients for innovative diagnostic and therapeutic approaches are explored. It is emphasized, however, that caution is required in targeting polyP in the brain due to its diverse physiological functions, serving as an energy source, a chelator for divalent cations and a scaffold for amyloidogenic proteins. Reducing polyP levels, especially in neurons, might thus have adverse effects in brain functioning. Finally, we discuss how activated mast cells and platelets also can significantly contribute to ALS progression, as they can massively release polyP.

The Protein Scaffolding Functions of Polyphosphate

Inorganic polyphosphate (polyP), one of the first high-energy compound on earth, defies its extreme compositional and structural simplicity with an astoundingly wide array of biological activities across all domains of life. However, the underlying mechanism of such functional pleiotropy remains largely elusive. In this review, we will summarize recent studies demonstrating that this simple polyanion stabilizes protein folding intermediates and scaffolds select native proteins. These functions allow polyP to act as molecular chaperone that protects cells against protein aggregation, as pro-amyloidogenic factor that accelerates both physiological and disease-associated amyloid formation, and as a modulator of liquid-liquid phase separation processes. These activities help to explain polyP's known roles in bacterial stress responses and pathogenicity, provide the mechanistic foundation for its potential role in human neurodegenerative diseases, and open a new direction regarding its influence on gene expression through condensate formation. We will highlight critical unanswered questions and point out potential directions that will help to further understand the pleiotropic functions of this ancient and ubiquitous biopolymer.

Inorganic Polyphosphate—Regulator of Cellular Metabolism in Homeostasis and Disease

Inorganic polyphosphate (polyP), a simple anionic polymer consisting of even hundreds of orthophosphate units, is a universal molecule present in both simple and complex organisms. PolyP controls homeostatic processes in animals, such as blood coagulation, tissue regeneration, and energy metabolism. Furthermore, this polymer is a potent regulator of inflammation and influences host immune response in bacterial and viral infections. Disturbed polyP systems have been related to several pathological conditions, including neurodegeneration, cardiovascular disorders, and cancer, but we lack a full understanding of polyP biogenesis and mechanistic insights into the pathways through which polyP may act. This review summarizes recent studies that describe the role of polyP in cell homeostasis and show how disturbances in polyP levels may lead to disease. Based on the collected findings, we highlight the possible usage of this polymer as a promising therapeutic tool in multiple pathologies.

Biomimetic Polyphosphate Materials: Toward Application in Regenerative Medicine

In recent years, inorganic polyphosphate (polyP) has attracted increasing attention as a biomedical polymer or biomaterial with a great potential for application in regenerative medicine, in particular in the fields of tissue engineering and repair. The interest in polyP is based on two properties of this physiological polymer that make polyP stand out from other polymers: polyP has morphogenetic activity by inducing cell differentiation through specific gene expression, and it functions as an energy store and donor of metabolic energy, especially in the extracellular matrix or in the extracellular space. No other biopolymer applicable in tissue regeneration/repair is known that is endowed with this combination of properties. In addition, polyP can be fabricated both in the form of a biologically active coacervate and as biomimetic amorphous polyP nano/microparticles, which are stable and are activated by transformation into the coacervate phase after contact with protein/body fluids. PolyP can be used in the form of various metal salts and in combination with various hydrogel-forming polymers, whereby (even printable) hybrid materials with defined porosities and mechanical and biological properties can be produced, which can even be loaded with cells for 3D cell printing or with drugs and support the growth and differentiation of (stem) cells as well as cell migration/microvascularization. Potential applications in therapy of bone, cartilage and eye disorders/injuries and wound healing are summarized and possible mechanisms are discussed.

Polyphosphate degradation by Nudt3-Zn2+ mediates oxidative stress response

Polyphosphate (polyP) is a polymer of hundreds of phosphate residues present in all organisms. In mammals, polyP is involved in crucial physiological processes, including coagulation, inflammation, and stress response. However, after decades of research, the metabolic enzymes are still unknown. Here, we purify and identify Nudt3, a NUDIX family member, as the enzyme responsible for polyP phosphatase activity in mammalian cells. We show that Nudt3 shifts its substrate specificity depending on the cation; specifically, Nudt3 is active on polyP when Zn²⁺ is present. Nudt3 has in vivo polyP phosphatase activity in human cells, and importantly, we show that cells with altered polyP levels by modifying Nudt3 protein amount present reduced viability upon oxidative stress and increased DNA damage, suggesting that polyP and Nudt3 play a role in oxidative stress protection. Finally, we show that Nudt3 is involved in the early stages of embryo development in zebrafish.

Regulation of inorganic polyphosphate is required for proper vacuolar proteolysis in fission yeast

Regulation of cellular proliferation and quiescence is a central issue in biology that has been studied using model unicellular eukaryotes, such as the fission yeast Schizosaccharomyces pombe. We previously reported that the ubiquitin/proteasome pathway and autophagy are essential to maintain quiescence induced by nitrogen deprivation in S. pombe; however, specific ubiquitin ligases that maintain quiescence are not fully understood. Here we investigated the SPX-RING-type ubiquitin ligase Pqr1, identified as required for quiescence in a genetic screen. Pqr1 is found to be crucial for vacuolar proteolysis, the final step of autophagy, through proper regulation of phosphate and its polymer polyphosphate. Pqr1 restricts phosphate uptake into the cell through ubiquitination and subsequent degradation of phosphate transporters on plasma membranes. We hypothesized that Pqr1 may act as the central regulator for phosphate control in S. pombe, through the function of the SPX domain involved in phosphate sensing. Deletion of pqr1⁺ resulted in hyperaccumulation of intracellular phosphate and polyphosphate and in improper autophagy-dependent proteolysis under conditions of nitrogen starvation. Polyphosphate hyperaccumulation in pqr1⁺-deficient cells was mediated by the polyphosphate synthase VTC complex in vacuoles. Simultaneous deletion of VTC complex subunits rescued Pqr1 mutant phenotypes, including defects in proteolysis and loss of viability during quiescence. We conclude that excess polyphosphate may interfere with proteolysis in vacuoles by mechanisms that as yet remain unknown. The present results demonstrate a connection between polyphosphate metabolism and vacuolar functions for proper autophagy-dependent proteolysis, and we propose that polyphosphate homeostasis contributes to maintenance of cellular viability during quiescence.

Inorganic polyphosphate is produced and hydrolyzed in F0F1-ATP synthase of mammalian mitochondria

Inorganic polyphosphate (polyP) is a polymer present in all living organisms. Although polyP is found to be involved in a variety of functions in cells of higher organisms, the enzyme responsible for polyP production and consumption has not yet been identified. Here, we studied the effect of polyP on mitochondrial respiration, oxidative phosphorylation and activity of F₀F₁-ATPsynthase. We have found that polyP activates mitochondrial respiration which does not coupled with ATP production (V₂) but inhibits ADP-dependent respiration (V₃). Moreover, PolyP can stimulate F₀F₁-ATPase activity in the presence of ATP and, importantly, can be hydrolyzed in this enzyme instead of ATP. Furthermore, PolyP can be produced in mitochondria in the presence of substrates for respiration and phosphate by the F₀F₁-ATPsynthase. Thus, polyP is an energy molecule in mammalian cells which can be produced and hydrolyzed in the mitochondrial F₀F₁-ATPsynthase.

Inorganic polyphosphate controls cyclophilin B-mediated collagen folding in osteoblast-like cells

Evidence is emerging that inorganic polyphosphate (polyP) is a fundamental molecule involved in a wide range of biological processes. In higher eukaryotes, polyP is abundant in osteoblasts but questions remain as to its functions. Here, we find that polyP is particularly enriched in endoplasmic reticulum (ER) where it colocalizes with cyclophilin B (CypB) using osteoblastic SaOS-2 model cell line. PolyP binds directly and specifically to CypB, inhibiting its peptidyl-prolyl cis-trans isomerase activity which is critical for collagen folding. PolyP sequestration by spermine and ER-specific polyP reduction by polyphosphatase expression in cells reduced collagen misfolding and confirmed that endogenous polyP acts as a molecular control of CypB-mediated collagen folding. We propose that polyP is a previously unrecognized critical regulator of protein homeostasis in ER.

Inorganic Polyphosphates As Storage for and Generator of Metabolic Energy in the Extracellular Matrix

Inorganic polyphosphates (polyP) consist of linear chains of orthophosphate residues, linked by high-energy phosphoanhydride bonds. They are evolutionarily old biopolymers that are present from bacteria to man. No other molecule concentrates as much (bio)chemically usable energy as polyP. However, the function and metabolism of this long-neglected polymer are scarcely known, especially in higher eukaryotes. In recent years, interest in polyP experienced a renaissance, beginning with the discovery of polyP as phosphate source in bone mineralization. Later, two discoveries placed polyP into the focus of regenerative medicine applications. First, polyP shows morphogenetic activity, i.e., induces cell differentiation via gene induction, and, second, acts as an energy storage and donor in the extracellular space. Studies on acidocalcisomes and mitochondria provided first insights into the enzymatic basis of eukaryotic polyP formation. In addition, a concerted action of alkaline phosphatase and adenylate kinase proved crucial for ADP/ATP generation from polyP. PolyP added extracellularly to mammalian cells resulted in a 3-fold increase of ATP. The importance and mechanism of this phosphotransfer reaction for energy-consuming processes in the extracellular matrix are discussed. This review aims to give a critical overview about the formation and function of this unique polymer that is capable of storing (bio)chemically useful energy.

Mechanistic insights into the protective roles of polyphosphate against amyloid cytotoxicity

This study provides novel insights into the mechanisms by which presence of polyP alters the formation, structural properties, and cytotoxic effects of α-synuclein fibers.

The universally abundant polyphosphate (polyP) accelerates fibril formation of disease-related amyloids and protects against amyloid cytotoxicity. To gain insights into the mechanism(s) by which polyP exerts these effects, we focused on α-synuclein, a well-studied amyloid protein, which constitutes the major component of Lewy bodies found in Parkinson’s disease. Here, we demonstrate that polyP is unable to accelerate the rate-limiting step of α-synuclein fibril formation but effectively nucleates fibril assembly once α-synuclein oligomers are formed. Binding of polyP to α-synuclein either during fibril formation or upon fibril maturation substantially alters fibril morphology and effectively reduces the ability of α-synuclein fibrils to interact with cell membranes. The effect of polyP appears to be α-synuclein fibril specific and successfully prevents the uptake of fibrils into neuronal cells. These results suggest that altering the polyP levels in the extracellular space might be a potential therapeutic strategy to prevent the spreading of the disease.

Oral Polyphosphate Suppresses Bacterial Collagenase Production and Prevents Anastomotic Leak Due to Serratia marcescens and Pseudomonas aeruginosa

OBJECTIVE

The objective of this study was to determine the effect of polyphosphate on intestinal bacterial collagenase production and anastomotic leak in mice undergoing colon surgery.

BACKGROUND

We have previously shown that anastomotic leak can be caused by intestinal pathogens that produce collagenase. Because bacteria harbor sensory systems to detect the extracellular concentration of phosphate which controls their virulence, we tested whether local phosphate administration in the form of polyphosphate could attenuate pathogen virulence and prevent leak without affecting bacterial growth.

METHODS

Groups of mice underwent a colorectal anastomosis which was then exposed to collagenolytic strains of either Serratia marcescens or Pseudomonas aeruginosa via enema. Mice were then randomly assigned to drink water or water supplemented with a 6-mer of polyphosphate (PPi-6). All mice were sacrificed on postoperative day 10 and anastomoses assessed for leakage, the presence of collagenolytic bacteria, and anastomotic PPi-6 concentration.

RESULTS

PPi-6 markedly attenuated collagenase and biofilm production, and also swimming and swarming motility in both S. marcescens and P. aeruginosa while supporting their normal growth. Mice drinking PPi-6 demonstrated increased levels of PPi-6 and decreased colonization of S. marcescens and P. aeruginosa, and collagenase activity at anastomotic tissues. PPi-6 prevented anastomotic abscess formation and leak in mice after anastomotic exposure to S. marcescens and P. aeruginosa.

CONCLUSIONS

Polyphosphate administration may be an alternative approach to prevent anastomotic leak induced by collagenolytic bacteria with the advantage of preserving the intestinal microbiome and its colonization resistance.

Sustained Release of Phosphates From Hydrogel Nanoparticles Suppresses Bacterial Collagenase and Biofilm Formation in vitro

Intestinal disease or surgical intervention results in local changes in tissue and host-derived factors triggering bacterial virulence. A key phenotype involved in impaired tissue healing is increased bacterial collagenase expression which degrades intestinal collagen. Antibiotic administration is ineffective in addressing this issue as it inadvertently eliminates normal flora while allowing pathogenic bacteria to "bloom" and acquire antibiotic resistance. Compounds that could attenuate collagenase production while allowing commensal bacteria to proliferate normally would offer major advantages without the risk of the emergence of resistance. We have previously shown that intestinal phosphate depletion in the surgically stressed host is a major cue that triggers P. aeruginosa virulence which is suppressed under phosphate abundant conditions. Recent findings indicate that orally administered polyphosphate, hexametaphosphate, (PPi) suppresses collagenase, and biofilm production of P. aeruginosa and S. marcescens in animal models of intestinal injury but does not attenuate E. faecalis induced collagenolytic activity (Hyoju et al., 2017). Systemic administration of phosphates, however, is susceptible to rapid clearance. Given the diversity of collagenase producing bacteria and the variation of phosphate metabolism among microbial species, a combination therapy involving different phosphate compounds may be required to attenuate pathogenic phenotypes. To address these barriers, we present a drug delivery approach for sustained release of phosphates from poly(ethylene) glycol (PEG) hydrogel nanoparticles. The efficacy of monophosphate (Pi)- and PPi-loaded NPs (NP-Pi and NP-PPi, respectively) and a combination treatment (NP-Pi + NP-PPi) in mitigating collagenase and biofilm production of gram-positive and gram-negative pathogens expressing high collagenolytic activity was investigated. NP-PPi was found to significantly decrease collagenase and biofilm production of S. marcescens and P. aeruginosa. Treatment with either NP-Pi or NP-Pi + NP-PPi resulted in more prominent decreases in E. faecalis collagenase compared to NP-PPi alone. The combination treatment was also found to significantly reduce P. aeruginosa collagenase production. Finally, significant attenuation in biofilm dispersal was observed with NP-PPi or NP-Pi + NP-PPi treatment across all test pathogens. These findings suggest that sustained release of different forms of phosphate confers protection against gram-positive and gram-negative pathogens, thereby providing a promising treatment to attenuate expression of tissue-disruptive bacterial phenotypes without eradicating protective flora over the course of intestinal healing.

Inorganic Polyphosphate Regulates AMPA and NMDA Receptors and Protects Against Glutamate Excitotoxicity via Activation of P2Y Receptors

Glutamate is one of the most important neurotransmitters in the process of signal transduction in the CNS. Excessive amounts of this neurotransmitter lead to glutamate excitotoxicity, which is accountable for neuronal death in acute neurological disorders, including stroke and trauma, and in neurodegenerative diseases. Inorganic polyphosphate (PolyP) plays multiple roles in the mammalian brain, including function as a calcium-dependent gliotransmitter mediating communication between astrocytes, while its role in the regulation of neuronal activity is unknown. Here we studied the effect of PolyP on glutamate-induced calcium signal in primary rat neurons in both physiological and pathological conditions. We found that preincubation of primary neurons with PolyP reduced glutamate-induced and AMPA-induced but not the NMDA-induced calcium signal. However, in rat hippocampal acute slices, PolyP reduced ion flux through NMDA and AMPA receptors in native neurons. The effect of PolyP on glutamate and specifically on the AMPA receptors was dependent on the presence of P2Y1 but not of P2X receptor inhibitors and also could be mimicked by P2Y1 agonist 2MeSADP. Preincubation of cortical neurons with PolyP significantly reduced the initial calcium peak as well as the number of neurons with delayed calcium deregulation in response to high concentrations of glutamate and resulted in protection of neurons against glutamate-induced cell death. As a result, activation of P2Y1 receptors by PolyP reduced calcium signal acting through AMPA receptors, thus protecting neurons against glutamate excitotoxicity by reduction of the calcium overload and restoration of mitochondrial function.SIGNIFICANCE STATEMENT One of the oldest polymers in the evolution of living matter is the inorganic polyphosphate (PolyP). It is shown to play a role of gliotransmitter in the brain; however, the role of polyphosphate in neuronal signaling is not clear. Here we demonstrate that inorganic polyphosphate is able to reduce calcium signaling induced by physiological or high concentrations of glutamate. The effect of polyphosphate on glutamate-induced calcium signal in neurons is due to the effect of this polymer on the AMPA receptors. The effect of PolyP on glutamate-induced and AMPA-induced calcium signal is dependent on P2Y receptor antagonist. The ability of PolyP to restrict the glutamate-induced calcium signal lies in the basis of its protection of neurons against glutamate excitotoxicity.

Role of Polyphosphate in Amyloidogenic Processes

Polyphosphate (polyP), an extremely simple polyanion, has long been known to be involved in a variety of different cellular processes, ranging from stress resistance, biofilm formation, and virulence in bacteria to bone mineralization, blood clotting, and mammalian target of rapamycin (mTOR) signaling in mammalian organisms. Our laboratory recently discovered a completely unexpected role of polyP as a stabilizing scaffold for β-sheet-containing protein-folding intermediates. This realization led us to investigate the effects of polyP on amyloidogenic processes and the novel concept that polyP might play a role in neurodegenerative diseases. In this review, we will summarize recent results that show that polyP is a physiological modifier that accelerates amyloid fiber formation, alters fiber morphology, and protects cells against amyloid toxicity. We will review the current knowledge on the distribution, levels, and roles of polyP in the mammalian brain, and discuss potential mechanisms by which polyP might ameliorate amyloid toxicity.

Inorganic polyphosphate, a multifunctional polyanionic protein scaffold

Polyphosphate (polyP) consists of a linear arrangement of inorganic phosphates and defies its structural simplicity with an astounding number of different activities in the cell. Already well known for its ability to partake in phosphate, calcium, and energy metabolism, polyP recently gained a new functional dimension with the discovery that it serves as a stabilizing scaffold for protein-folding intermediates. In this review, we summarize and discuss the recent studies that have identified polyP not only as a potent protein-like chaperone that protects cells against stress-induced protein aggregation, but also as a robust modifier of amyloidogenic processes that shields neuronal cells from amyloid toxicity. We consider some of the most pressing questions in the field, the obstacles faced, and the potential avenues to take to provide a complete picture about the working mechanism and physiological relevance of this intriguing biopolymer.

Dual role of inorganic polyphosphate in cardiac myocytes: The importance of polyP chain length for energy metabolism and mPTP activation

We have previously demonstrated that inorganic polyphosphate (polyP) is a potent activator of the mitochondrial permeability transition pore (mPTP) in cardiac myocytes. PolyP depletion protected against Ca²⁺-induced mPTP opening, however it did not prevent and even exacerbated cell death during ischemia-reperfusion (I/R). The central goal of this study was to investigate potential molecular mechanisms underlying these dichotomous effects of polyP on mitochondrial function. We utilized a Langendorff-perfused heart model of I/R to monitor changes in polyP size and chain length at baseline, 20 min no-flow ischemia, and 15 min reperfusion. Freshly isolated cardiac myocytes and mitochondria from C57BL/6J (WT) and cyclophilin D knock-out (CypD KO) mice were used to measure polyP uptake, mPTP activity, mitochondrial membrane potential, respiration and ATP generation. We found that I/R induced a significant decrease in polyP chain length. We, therefore, tested, the ability of synthetic polyPs with different chain length to accumulate in mitochondria and induce mPTP. Both short and long chain polyPs accumulated in mitochondria in oligomycin-sensitive manner implicating potential involvement of mitochondrial ATP synthase in polyP transport. Notably, only short-chain polyP activated mPTP in WT myocytes, and this effect was prevented by mPTP inhibitor cyclosprorin A and absent in CypD KO myocytes. To the contrary, long-chain polyP suppressed mPTP activation, and enhanced ADP-linked respiration and ATP production. Our data indicate that 1) effect of polyP on cardiac function strongly depends on polymer chain length; and 2) short-chain polyPs (as increased in ischemia-reperfusion) induce mPTP and mitochondrial uncoupling, while long-chain polyPs contribute to energy generation and cell metabolism.

Long-chain polyphosphate in osteoblast matrix vesicles: Enrichment and inhibition of mineralization

BACKGROUND

Inorganic polyphosphate (polyP) is a fundamental and ubiquitous molecule in prokaryotes and eukaryotes. PolyP has been found in mammalian tissues with particularly high levels of long-chain polyP in bone and cartilage where critical questions remain as to its localization and function. Here, we investigated polyP presence and function in osteoblast-like SaOS-2 cells and cell-derived matrix vesicles (MVs), the initial sites of bone mineral formation.

METHODS

PolyP was quantified by 4',6-diamidino-2-phenylindole (DAPI) fluorescence and characterized by enzymatic methods coupled to urea polyacrylamide gel electrophoresis. Transmission electron microscopy and confocal microscopy were used to investigate polyP localization. A chicken embryo cartilage model was used to investigate the effect of polyP on mineralization.

RESULTS

PolyP increased in concentration as SaOS-2 cells matured and mineralized. Particularly high levels of polyP were observed in MVs. The average length of MV polyP was determined to be longer than 196 P_i residues by gel chromatography. Electron micrographs of MVs, stained by two polyP-specific staining approaches, revealed polyP localization in the vicinity of the MV membrane. Additional extracellular polyP binds to MVs and inhibits MV-induced hydroxyapatite formation.

CONCLUSION

PolyP is highly enriched in matrix vesicles and can inhibit apatite formation. PolyP may be hydrolysed to phosphate for further mineralization in the extracellular matrix.

GENERAL SIGNIFICANCE

PolyP is a unique yet underappreciated macromolecule which plays a critical role in extracellular mineralization in matrix vesicles.

Rebalancing β-Amyloid-Induced Decrease of ATP Level by Amorphous Nano/Micro Polyphosphate: Suppression of the Neurotoxic Effect of Amyloid β-Protein Fragment 25-35

Morbus Alzheimer neuropathology is characterized by an impaired energy homeostasis of brain tissue. We present an approach towards a potential therapy of Alzheimer disease based on the high-energy polymer inorganic polyphosphate (polyP), which physiologically occurs both in the extracellular and in the intracellular space. Rat pheochromocytoma (PC) 12 cells, as well as rat primary cortical neurons were exposed to the Alzheimer peptide Aβ25-35. They were incubated in vitro with polyphosphate (polyP); ortho-phosphate was used as a control. The polymer remained as Na⁺ salt; or complexed in a stoichiometric ratio to Ca²⁺ (Na-polyP[Ca²⁺]); or was processed as amorphous Ca-polyP microparticles (Ca-polyP-MP). Ortho-phosphate was fabricated as crystalline Ca-phosphate nanoparticles (Ca-phosphate-NP). We show that the pre-incubation of PC12 cells and primary cortical neurons with polyP protects the cells against the neurotoxic effect of the Alzheimer peptide Aβ25-35. The strongest effect was observed with amorphous polyP microparticles (Ca-polyP-MP). The effect of the soluble sodium salt; Na-polyP (Na-polyP[Ca²⁺]) was lower; while crystalline orthophosphate nanoparticles (Ca-phosphate-NP) were ineffective. Ca-polyP-MP microparticles and Na-polyP[Ca²⁺] were found to markedly enhance the intracellular ATP level. Pre-incubation of Aβ25-35 during aggregate formation, with the polyP preparation before exposure of the cells, had a small effect on neurotoxicity. We conclude that recovery of the compromised energy status in neuronal cells by administration of nontoxic biodegradable Ca-salts of polyP reverse the β-amyloid-induced decrease of adenosine triphosphate (ATP) level. This study contributes to a new routes for a potential therapeutic intervention in Alzheimer’s disease pathophysiology.

Polyphosphate is a key factor for cell survival after DNA damage in eukaryotic cells

Cells require extra amounts of dNTPs to repair DNA after damage. Polyphosphate (polyP) is an evolutionary conserved linear polymer of up to several hundred inorganic phosphate (Pi) residues that is involved in many functions, including Pi storage. In the present article, we report on findings demonstrating that polyP functions as a source of Pi when required to sustain the dNTP increment essential for DNA repair after damage. We show that mutant yeast cells without polyP produce less dNTPs upon DNA damage and that their survival is compromised. In contrast, when polyP levels are ectopically increased, yeast cells become more resistant to DNA damage. More importantly, we show that when polyP is reduced in HEK293 mammalian cell line cells and in human dermal primary fibroblasts (HDFa), these cells become more sensitive to DNA damage, suggesting that the protective role of polyP against DNA damage is evolutionary conserved. In conclusion, we present polyP as a molecule involved in resistance to DNA damage and suggest that polyP may be a putative target for new approaches in cancer treatment or prevention.

Biocalcite and Carbonic Acid Activators

Based on evolution of biomineralizing systems and energetic considerations, there is now compelling evidence that enzymes play a driving role in the formation of the inorganic skeletons from the simplest animals, the sponges, up to humans. Focusing on skeletons based on calcium minerals, the principle enzymes involved are the carbonic anhydrase (formation of the calcium carbonate-based skeletons of many invertebrates like the calcareous sponges, as well as deposition of the calcium carbonate bioseeds during human bone formation) and the alkaline phosphatase (providing the phosphate for bone calcium phosphate-hydroxyapatite formation). These two enzymes, both being involved in human bone formation, open novel not yet exploited targets for pharmacological intervention of human bone diseases like osteoporosis, using compounds that act as activators of these enzymes. This chapter focuses on carbonic anhydrases of biomedical interest and the search for potential activators of these enzymes, was well as the interplay between carbonic anhydrase-mediated calcium carbonate bioseed synthesis and metabolism of energy-rich inorganic polyphosphates. Beyond that, the combination of the two metabolic products, calcium carbonate and calcium-polyphosphate, if applied in an amorphous form, turned out to provide the basis for a new generation of scaffold materials for bone tissue engineering and repair that are, for the first time, morphogenetically active.

New Target Sites for Treatment of Osteoporosis

In the last few years, much progress has been achieved in the discovery of new drug target sites for treatment of osteoporotic disorders, one of the main challenging diseases with a large burden for the public health systems. Among these new agents promoting bone formation, shifting the impaired equilibrium between bone anabolism and bone catabolism in the direction of bone synthesis are inorganic polymers, in particular inorganic polyphosphates that show strong stimulatory effects on the expression of bone anabolic marker proteins and hydroxyapatite formation. The bone-forming activity of these polymers can even be enhanced by combination with certain small molecules like quercetin, or if given as functionally active particles with certain divalent cations like strontium ions even showing by itself biological activity. This chapter summarizes recent developments in the search and development of novel anti-osteoporotic agents, with a particular focus on therapeutic approaches based on the potential application of inorganic polymers and combinations.

Inorganic polyphosphate in cardiac myocytes: from bioenergetics to the permeability transition pore and cell survival

Inorganic polyphosphate (polyP) is a linear polymer of Pi residues linked together by high-energy phosphoanhydride bonds as in ATP. PolyP is present in all living organisms ranging from bacteria to human and possibly even predating life of this planet. The length of polyP chain can vary from just a few phosphates to several thousand phosphate units long, depending on the organism and the tissue in which it is synthesized. PolyP was extensively studied in prokaryotes and unicellular eukaryotes by Kulaev's group in the Russian Academy of Sciences and by the Nobel Prize Laureate Arthur Kornberg at Stanford University. Recently, we reported that mitochondria of cardiac ventricular myocytes contain significant amounts (280±60 pmol/mg of protein) of polyP with an average length of 25 Pi and that polyP is involved in Ca(2+)-dependent activation of the mitochondrial permeability transition pore (mPTP). Enzymatic polyP depletion prevented Ca(2+)-induced mPTP opening during ischaemia; however, it did not affect reactive oxygen species (ROS)-mediated mPTP opening during reperfusion and even enhanced cell death in cardiac myocytes. We found that ROS generation was actually enhanced in polyP-depleted cells demonstrating that polyP protects cardiac myocytes against enhanced ROS formation. Furthermore, polyP concentration was dynamically changed during activation of the mitochondrial respiratory chain and stress conditions such as ischaemia/reperfusion (I/R) and heart failure (HF) indicating that polyP is required for the normal heart metabolism. This review discusses the current literature on the roles of polyP in cardiovascular health and disease.

Polyphosphate: A Conserved Modifier of Amyloidogenic Processes

Polyphosphate (polyP), a several billion-year-old biopolymer, is produced in every cell, tissue, and organism studied. Structurally extremely simple, polyP consists of long chains of covalently linked inorganic phosphate groups. We report here the surprising discovery that polyP shows a remarkable efficacy in accelerating amyloid fibril formation. We found that polyP serves as an effective nucleation source for various different amyloid proteins, ranging from bacterial CsgA to human α-synuclein, Aβ1-40/42, and Tau. polyP-associated α-synuclein fibrils show distinct differences in seeding behavior, morphology, and fibril stability compared with fibrils formed in the absence of polyP. In vivo, the amyloid-stimulating and fibril-stabilizing effects of polyP have wide-reaching consequences, increasing the rate of biofilm formation in pathogenic bacteria and mitigating amyloid toxicity in differentiated neuroblastoma cells and C. elegans strains that serve as models for human folding diseases. These results suggest that we have discovered a conserved cytoprotective modifier of amyloidogenic processes.

Modulation of mitochondrial ion transport by inorganic polyphosphate - essential role in mitochondrial permeability transition pore

Inorganic polyphosphate (polyP) is a biopolymer of phosphoanhydride-linked orthophosphate residues. PolyP is involved in multiple cellular processes including mitochondrial metabolism and cell death. We used artificial membranes and isolated mitochondria to investigate the role of the polyP in mitochondrial ion transport and in activation of PTP. Here, we found that polyP can modify ion permeability of de-energised mitochondrial membranes but not artificial membranes. This permeability was selective for Ba²⁺ and Ca²⁺ but not for other monovalent and bivalent cations and can be blocked by inhibitors of the permeability transition pore – cyclosporine A or ADP. Lower concentrations of polyP modulate calcium dependent permeability transition pore opening. Increase in polyP concentrations and elongation chain length of the polymer causes calcium independent swelling in energized conditions. Physiologically relevant concentrations of inorganic polyP can regulate calcium dependent as well calcium independent mitochondrial permeability transition pore opening. This raises the possibility that cytoplasmic polyP can be an important contributor towards regulation of the cell death.

Isoquercitrin and polyphosphate co-enhance mineralization of human osteoblast-like SaOS-2 cells via separate activation of two RUNX2 cofactors AFT6 and Ets1

Isoquercitrin, a dietary phytoestrogen, is a potential stimulator of bone mineralization used for prophylaxis of osteoporotic disorders. Here we studied the combined effects of isoquercitrin, a cell membrane permeable 3-O-glucoside of quercetin, and polyphosphate [polyP], a naturally occurring inorganic polymer inducing bone formation, on mineralization of human osteoblast-like SaOS-2 cells. Both compounds isoquercitrin and polyP induce at non-toxic concentrations the mineralization process of SaOS-2 cells. Co-incubation experiments revealed that isoquercitrin (at 0.1 and 0.3μM), if given simultaneously with polyP (as Ca(2+) salt; at 3, 10, 30 and 100μM) amplifies the mineralization-enhancing effect of the inorganic polymer. The biomineralization process induced by isoquercitrin and polyP is based on two different modes of action. After incubation of the cells with isoquercitrin or polyP the expression of the Runt-related transcription factor 2 [RUNX2] is significantly upregulated. In addition, isoquercitrin causes a strong increase of the steady-state-levels of the two co-activators of RUNX2, the activating transcription factor 6 [ATF6] and the Ets oncogene homolog 1 [Ets1]. The activating effect of isoquercitrin occurs via a signal transduction pathway involving ATF6, and by that, is independent from the induction cascade initiated by polyP. This conclusion is supported by the finding that isoquercitrin upregulates the expression of the gene encoding for osteocalcin, while polyP strongly increases the expression of the Ets1 gene and of the alkaline phosphatase. We show that the two compounds, polyP and isoquercitrin, have a co-enhancing effect on bone mineral formation and in turn might be of potential therapeutic value for prevention/treatment of osteoporosis.

Polyphosphate-mediated inhibition of tartrate-resistant acid phosphatase and suppression of bone resorption of osteoclasts

Inorganic polyphosphate (poly(P)) has recently been found to play an important role in bone formation. In this study, we found that tartrate-resistant acid phosphatase (TRAP), which is abundantly expressed in osteoclasts, has polyphosphatase activity that degrades poly(P) and yields Pi as well as shorter poly(P) chains. Since the TRAP protein that coprecipitated with anti-TRAP monoclonal antibodies exhibited both polyphosphatase and the original phosphatase activity, poly(P) degradation activity is dependent on TRAP and not on other contaminating enzymes. The ferrous chelator α, α’-bipyridyl, which inhibits the TRAP-mediated production of reactive oxygen species (ROS), had no effect on such poly(P) degradation, suggesting that the degradation is not dependent on ROS. In addition, shorter chain length poly(P) molecules were better substrates than longer chains for TRAP, and poly(P) inhibited the phosphatase activity of TRAP depending on its chain length. The IC50 of poly(P) against the original phosphatase activity of TRAP was 9.8 µM with an average chain length more than 300 phosphate residues, whereas the IC50 of poly(P) with a shorter average chain length of 15 phosphate residues was 8.3 mM. Finally, the pit formation activity of cultured rat osteoclasts differentiated by RANKL and M-CSF were markedly inhibited by poly(P), while no obvious decrease in cell number or differentiation efficiency was observed for poly(P). In particular, the inhibition of pit formation by long chain poly(P) with 300 phosphate residues was stronger than that of shorter chain poly(P). Thus, poly(P) may play an important regulatory role in osteoclastic bone resorption by inhibiting TRAP activity, which is dependent on its chain length.

Inorganic polyphosphates regulate hexokinase activity and reactive oxygen species generation in mitochondria of Rhipicephalus (Boophilus) microplus embryo

The physiological roles of polyphosphates (poly P) recently found in arthropod mitochondria remain obscure. Here, the possible involvement of poly P with reactive oxygen species generation in mitochondria of Rhipicephalus microplus embryos was investigated. Mitochondrial hexokinase and scavenger antioxidant enzymes, such as superoxide dismutase, catalase, and glutathione reductase were assayed during embryogenesis of R. microplus. The influence of poly P₃ and poly P₁₅ were analyzed during the period of higher enzymatic activity during embryogenesis. Both poly Ps inhibited hexokinase activity by up to 90% and, interestingly, the mitochondrial membrane exopolyphosphatase activity was stimulated by the hexokinase reaction product, glucose-6-phosphate. Poly P increased hydrogen peroxide generation in mitochondria in a situation where mitochondrial hexokinase is also active. The superoxide dismutase, catalase and glutathione reductase activities were higher during embryo cellularization, at the end of embryogenesis and during embryo segmentation, respectively. All of the enzymes were stimulated by poly P₃. However, superoxide dismutase was not affected by poly P₁₅, catalase activity was stimulated only at high concentrations and glutathione reductase was the only enzyme that was stimulated in the same way by both poly Ps. Altogether, our results indicate that inorganic polyphosphate and mitochondrial membrane exopolyphosphatase regulation can be correlated with the generation of reactive oxygen species in the mitochondria of R. microplus embryos.

Induction of carbonic anhydrase in SaOS-2 cells, exposed to bicarbonate and consequences for calcium phosphate crystal formation

Ca-phosphate/hydroxyapatite crystals constitute the mineralic matrix of vertebrate bones, while Ca-carbonate dominates the inorganic matrix of otoliths. In addition, Ca-carbonate has been identified in lower percentage in apatite crystals. By using the human osteogenic SaOS-2 cells it could be shown that after exposure of the cells to Ca-bicarbonate in vitro, at concentrations between 1 and 10 mm, a significant increase of Ca-deposit formation results. The crystallite nodules formed on the surfaces of SaOS-2 cells become denser and larger in the presence of bicarbonate if simultaneously added together with the mineralization activation cocktail (β-glycerophosphate/ascorbic acid/dexamethasone). In parallel, with the increase of Ca-deposit formation, the expression of the carbonic anhydrase-II (CA-II) gene becomes upregulated. This effect, measured on transcriptional level is also substantiated by immunohistological studies. The stimulatory effect of bicarbonate on Ca-deposit formation is prevented if the carbonic anhydrase inhibitor acetazolamide is added to the cultures. Mapping the surface of the Ca-deposit producing SaOS-2 cells by scanning electron microscopy coupled with energy-dispersive X-ray analysis revealed an accumulation of the signals for the element carbon and, as expected, also for phosphorus. Finally, it is shown that ortho-phosphate and hydrolysis products of polyphosphate inhibit CA-II activity, suggesting a feedback regulatory system between the CA-driven Ca-carbonate deposition and a subsequent inactivation of this process by ortho-phosphate. Based on the presented data we suggest that Ca-carbonate deposits act as bioseeds for a downstream Ca-phosphate deposition process. We propose that activators for CA, especially for CA-II, might be beneficial for the treatment of bone deficiency diseases.

Polyphosphates inhibit extracellular matrix mineralization in MC3T3-E1 osteoblast cultures

Studies on various compounds of inorganic phosphate, as well as on organic phosphate added by post-translational phosphorylation of proteins, all demonstrate a central role for phosphate in biomineralization processes. Inorganic polyphosphates are chains of orthophosphates linked by phosphoanhydride bonds that can be up to hundreds of orthophosphates in length. The role of polyphosphates in mammalian systems, where they are ubiquitous in cells, tissues and bodily fluids, and are at particularly high levels in osteoblasts, is not well understood. In cell-free systems, polyphosphates inhibit hydroxyapatite nucleation, crystal formation and growth, and solubility. In animal studies, polyphosphate injections inhibit induced ectopic calcification. While recent work has proposed an integrated view of polyphosphate function in bone, little experimental data for bone are available. Here we demonstrate in osteoblast cultures producing an abundant collagenous matrix that normally show robust mineralization, that two polyphosphates (PolyP5 and PolyP65, polyphosphates of 5 and 65 phosphate residues in length) are potent mineralization inhibitors. Twelve-day MC3T3-E1 osteoblast cultures with added ascorbic acid (for collagen matrix assembly) and β-glycerophosphate (a source of phosphate for mineralization) were treated with either PolyP5 or PolyP65. Von Kossa staining and calcium quantification revealed that mineralization was inhibited in a dose-dependent manner by both polyphosphates, with complete mineralization inhibition at 10μM. Cell proliferation and collagen assembly were unaffected by polyphosphate treatment, indicating that polyphosphate inhibition of mineralization results not from cell and matrix effects but from direct inhibition of mineralization. This was confirmed by showing that PolyP5 and PolyP65 bound to synthetic hydroxyapatite in a concentration-dependent manner. Tissue-nonspecific alkaline phosphatase (TNAP, ALPL) efficiently hydrolyzed the two PolyPs as measured by Pi release. Importantly, at the concentrations of polyphosphates used in this study which inhibited bone cell culture mineralization, the polyphosphates competitively saturated TNAP, thus potentially interfering with its ability to hydrolyze mineralization-inhibiting pyrophosphate (PPi) and mineralizing-promoting β-glycerophosphate (in cell culture). In the biological setting, polyphosphates may regulate mineralization by shielding the essential inhibitory substrate pyrophosphate from TNAP degradation, and in the same process, delay the release of phosphate from this source. In conclusion, the inhibition of mineralization by polyphosphates is shown to occur via direct binding to apatitic mineral and by mixed inhibition of TNAP.

The deep-sea natural products, biogenic polyphosphate (Bio-PolyP) and biogenic silica (Bio-Silica), as biomimetic scaffolds for bone tissue engineering: fabrication of a morphogenetically-active polymer

Bone defects in human, caused by fractures/nonunions or trauma, gain increasing impact and have become a medical challenge in the present-day aging population. Frequently, those fractures require surgical intervention which ideally relies on autografts or suboptimally on allografts. Therefore, it is pressing and likewise challenging to develop bone substitution materials to heal bone defects. During the differentiation of osteoblasts from their mesenchymal progenitor/stem cells and of osteoclasts from their hemopoietic precursor cells, a lineage-specific release of growth factors and a trans-lineage homeostatic cross-talk via signaling molecules take place. Hence, the major hurdle is to fabricate a template that is functioning in a way mimicking the morphogenetic, inductive role(s) of the native extracellular matrix. In the last few years, two naturally occurring polymers that are produced by deep-sea sponges, the biogenic polyphosphate (bio-polyP) and biogenic silica (bio-silica) have also been identified as promoting morphogenetic on both osteoblasts and osteoclasts. These polymers elicit cytokines that affect bone mineralization (hydroxyapatite formation). In this manner, bio-silica and bio-polyP cause an increased release of BMP-2, the key mediator activating the anabolic arm of the hydroxyapatite forming cells, and of RANKL. In addition, bio-polyP inhibits the progression of the pre-osteoclasts to functionally active osteoclasts. Based on these findings, new bioinspired strategies for the fabrication of bone biomimetic templates have been developed applying 3D-printing techniques. Finally, a strategy is outlined by which these two morphogenetically active polymers might be used to develop a novel functionally active polymer.

Inorganic polyphosphates: biologically active biopolymers for biomedical applications

Inorganic polyphosphate (polyP) is a widely occurring but only rarely investigated biopolymer which exists in both prokaryotic and eukaryotic organisms. Only in the last few years, this polymer has been identified to cause morphogenetic activity on cells involved in human bone formation. The calcium complex of polyP was found to display a dual effect on bone-forming osteoblasts and bone-resorbing osteoclasts. Exposure of these cells to polyP (Ca(2+) complex) elicits the expression of cytokines that promote the mineralization process by osteoblasts and suppress the differentiation of osteoclast precursor cells to the functionally active mature osteoclasts dissolving bone minerals. The effect of polyP on bone formation is associated with an increased release of the bone morphogenetic protein 2 (BMP-2), a key mediator that activates the anabolic processes leading to bone formation. In addition, polyP has been shown to act as a hemostatic regulator that displays various effects on blood coagulation and fibrinolysis and might play an important role in platelet-dependent proinflammatory and procoagulant disorders.

Inorganic polyphosphate stimulates cartilage tissue formation

Clinical utilization of tissue-engineered cartilage constructs has been limited by their inferior mechanical properties compared to native articular cartilage. A number of strategies have been investigated to increase the accumulation of major extracellular matrix components within in vitro-formed cartilage, including the administration of growth factors and mechanical stimulation. In this study, the anabolic effect of inorganic polyphosphates, a linear polymer of orthophosphate residues linked by phosphoanhydride bonds, was demonstrated in both chondrocyte cultures and native articular cartilage cultured ex vivo. Compared to untreated controls, polyphosphate treatment of three-dimensional primary chondrocyte cultures induced increased glycosaminoglycan and collagen accumulation in a concentration- and chain length-dependent manner. This effect was transient, because chondrocytes express exopolyphosphatases that hydrolyze polyphosphate. The anabolic effect of polyphosphates was accompanied by a lower rate of DNA increase within the chondrocyte cultures treated with inorganic polyphosphate. Inorganic polyphosphate enhances cartilage matrix accumulation and is a promising approach to improve the quality of tissue-engineered cartilage constructs.

Dual effect of inorganic polymeric phosphate/polyphosphate on osteoblasts and osteoclasts in vitro

Inorganic polymeric phosphate/polyphosphate (polyP) is a natural polymer existing in both pro- and eukaryotic systems. In the present study the effect of polyP as well as of polyP supplied in a stoichiometric ratio of 2 m polyP:1 m CaCl2 [polyP (Ca(2+) complex)] on the osteoblast-like SaOS-2 cells and the osteoclast-like RAW 264.7 cells was determined. Both polymers are non-toxic for these cells up to a concentration of 100 µm. In contrast to polyP, polyP (Ca(2+) complex) significantly induced hydroxyapatite formation at a concentration > 10 µm, as documented by alizarin red S staining and scanning electron microscopic (SEM) inspection. Furthermore, polyP (Ca(2+) complex) triggered in SaOS-2 cells transcription of BMP2 (bone morphogenetic protein 2), a cytokine involved in maturation of hydroxyapatite-forming cells. An additional activity of polyP (Ca(2+) complex) is described by showing that this polymer impairs osteoclastogenesis. At concentrations > 10 µm polyP (Ca(2+) complex) slows down the progression of RAW 264.7 cells to functional osteoclasts, as measured by the expression of TRAP (tartrate-resistant acid phosphatase). Finally, it is shown that 10-100 µm polyP (Ca(2+) complex) inhibited phosphorylation of IκBα by the respective kinase in RAW 264.7 cells. We concluded that polyP (Ca(2+) complex) displays a dual effect on bone metabolizing cells. It promotes hydroxyapatite formation in SaOS-2 cells (osteoblasts) and impairs maturation of the osteoclast-related RAW 264.7 cells.

Bio-silica and bio-polyphosphate: applications in biomedicine (bone formation)

Bio-silica represents the main mineral component of the sponge skeletal elements (siliceous spicules), while bio-polyphosphate (bio-polyP), a multifunctional polymer existing in microorganisms and animals acts, among others, as reinforcement for pores in cell membranes. These natural inorganic bio-polymers, which can be readily prepared, either by recombinant enzymes (bio-silica and bio-polyP) or chemically (polyP), are promising materials/substances for the amelioration and/or treatment of human bone diseases and dysfunctions. It has been demonstrated that bio-silica causes in vitro a differential effect on the expression of the genes OPG and RANKL, encoding two mediators that control the tuned interaction of the anabolic (osteoblasts) and catabolic (osteoclasts) pathways in human bone cells. Since bio-silica and bio-polyP also induce the expression of the key mediator BMP2 which directs the differentiation of bone-forming progenitor cells to mature osteoblasts and in parallel inhibits the function of osteoclasts, they are promising candidates for treatment of osteoporosis.

A mitochondrial membrane exopolyphosphatase is modulated by, and plays a role in, the energy metabolism of hard tick Rhipicephalus (Boophilus) microplus embryos

The physiological roles of polyphosphates (polyP) recently found in arthropod mitochondria remain obscure. Here, the relationship between the mitochondrial membrane exopolyphosphatase (PPX) and the energy metabolism of hard tick Rhipicephalus microplus embryos are investigated. Mitochondrial respiration was activated by adenosine diphosphate using polyP as the only source of inorganic phosphate (P_i) and this activation was much greater using polyP₃ than polyP₁₅. After mitochondrial subfractionation, most of the PPX activity was recovered in the membrane fraction and its kinetic analysis revealed that the affinity for polyP₃ was 10 times stronger than that for polyP₁₅. Membrane PPX activity was also increased in the presence of the respiratory substrate pyruvic acid and after addition of the protonophore carbonyl cyanide-p-trifluoromethoxyphenylhydrazone. Furthermore, these stimulatory effects disappeared upon addition of the cytochrome oxidase inhibitor potassium cyanide and the activity was completely inhibited by 20 μg/mL heparin. The activity was either increased or decreased by 50% upon addition of dithiothreitol or hydrogen peroxide, respectively, suggesting redox regulation. These results indicate a PPX activity that is regulated during mitochondrial respiration and that plays a role in adenosine-5′-triphosphate synthesis in hard tick embryos.

Inorganic polymeric phosphate/polyphosphate as an inducer of alkaline phosphatase and a modulator of intracellular Ca2+ level in osteoblasts (SaOS-2 cells) in vitro

Inorganic polymeric phosphate is a physiological polymer that accumulates in bone cells. In the present study osteoblast-like SaOS-2 cells were exposed to this polymer, complexed in a 2:1 stoichiometric ratio with Ca(2+), polyP (Ca(2+) salt). At a concentration of 100 μM, polyP (Ca(2+) salt) caused a strong increase in the activity of the alkaline phosphatase and also an induction of the steady-state expression of the gene encoding this enzyme. Comparative experiments showed that polyP (Ca(2+) salt) can efficiently replace β-glycerophosphate in the in vitro hydroxyapatite (HA) biomineralization assay. In the presence of polyP (Ca(2+) salt) the cells extensively form HA crystallites, which remain intimately associated with or covered by the plasma membrane. Only the tips of the crystallites are directly exposed to the extracellular space. Element mapping by scanning electron microscopy/energy-dispersive X-ray spectroscopy coupled to a silicon drift detector supported the finding that organic material was dispersed within the crystallites. Finally, polyP (Ca(2+) salt) was found to cause an increase in the intracellular Ca(2+) level, while polyP, as well as inorganic phosphate (P(i)) or Ca(2+) alone, had no effect at the concentrations used. These findings are compatible with the assumption that polyP (Ca(2+) salt) is locally, on the surface of the SaOS-2 cells, hydrolyzed to P(i) and Ca(2+). We conclude that the inorganic polymer polyP (Ca(2+) salt) in concert with a second inorganic, and physiologically occurring, polymer, biosilica, activates osteoblasts and impairs the maturation of osteoclasts.

Calcification of cartilage formed in vitro on calcium polyphosphate bone substitutes is regulated by inorganic polyphosphate

A major challenge to the successful clinical application of bioengineered cartilage remains its integration to surrounding tissues upon implantation. One way to address this consists of generating biphasic constructs composed of articular cartilage formed in vitro on the top surface and integrated with the porous sub-surface of a bone substitute material - in the case of this study, calcium polyphosphate (CPP). To improve the mechanical integrity of the cartilage-bone substitute interface, attempts have been made to generate a zone of calcified cartilage (ZCC) within the CPP-cartilage interface, thereby mimicking the native joint architecture. The purpose of this work was to establish the effects of the degradation products of CPP on cartilage calcification in order to explain the observed positioning of a ZCC away from the interface junction. It was determined that polyphosphate released from the CPP accumulates within in vitro-grown cartilage and inhibits cartilage calcification in a concentration and chain length (i.e. molecular weight) dependent manner. It was found that this effect is transient as chondrocytes express exopolyphosphatases which hydrolyze polyphosphate to release orthophosphate. Hence, the generation of biphasic constructs with a properly located ZCC will require tailoring of CPP substrates with lower degradation rates or the upregulation of exopolyphosphatases by chondrocytes.

Amorphous Calcium Polyphosphate Bone Regenerative Materials Based on Calcium Phosphate Glass

We developed new calcium phosphate bone substitute material, amorphous calcium polyphosphate. The new material is synthesized by a cement-like slif-setting reaction with calcium phosphate glass, basic materials and water. In this study, we prepared with CPG, Na2CO3 and NaOH solution. When they are mixed together, amorphous phase was precipitated. The precipitated amorphous phase consisted of polyphosphate chains condensed with Na ions released from Na2CO3 and NaOH. When the amorphous calcium polyphosphate dissolves, inorganic polyphosphates are released into the medium. The inorganic polyphosphates as the dissolution product inducted the calcification of the osteoblast cells. Therefore, in animal test, the new bone formation in rat calvarial defects treated with the new material was significantly higher than sham-surgery control group, especially in the initial stage. The amorphous calcium polyphosphate was biocompatible and bioresorbable and promoted the new bone formation.

A mitochondrial exopolyphosphatase activity modulated by phosphate demand in Rhipicephalus (Boophilus) microplus embryo

This study describes Exopolyphosphatases (PPX) activity in mitochondria of Rhipicephalus microplus embryos. Mitochondria were isolated by differential centrifugation and PPX activity was analyzed through the hydrolysis of the substrate Polyphosphate (Poly P(15)). We investigated the influence of NADH, NAD+, Pi and ADP in a concentration range of 0.1-2.0 mM. Poly P hydrolysis was stimulated about two-fold by NADH and strongly inhibited by Pi. The PPX activity also increased in the presence of the respiratory substrates pyruvic and succinic acids, and this stimulatory effect disappeared upon addition of KCN. Mitochondrial respiration was activated by ADP using poly P as the only source of Pi. Endogenous poly P content changed following PPX activity during embryogenesis from the first up to 18th day of development. The data describe exopoly P as being modulated by Pi demand and related to energy supply during embryogenesis of hard ticks.

Mammalian intestinal alkaline phosphatase acts as highly active exopolyphosphatase

Recent results revealed that inorganic polyphosphates (polyP), being energy-rich linear polymers of orthophosphate residues known from bacteria and yeast, also exist in higher eukaryotes. However, the enzymatic basis of their metabolism especially in mammalian cells is still uncertain. Here we demonstrate for the first time that alkaline phosphatase from calf intestine (CIAP) is able to cleave polyP molecules up to a chain length of about 800. The enzyme acts as an exopolyphosphatase degrading polyP in a processive manner. The pH optimum is in the alkaline range. Divalent cations are not required for catalytic activity but inhibit the degradation of polyP. The rate of hydrolysis of short-chain polyP by CIAP is comparable to that of the standard alkaline phosphatase (AP) substrate p-nitrophenyl phosphate. The specific activity of the enzyme decreases with increasing chain length of the polymer both in the alkaline and in the neutral pH range. The K(m) of the enzyme also decreases with increasing chain length. The mammalian tissue non-specific isoform of AP was not able to hydrolyze polyP under the conditions applied while the placental-type AP and the bacterial (Escherichia coli) AP displayed polyP-degrading activity.

Polyphosphate and phosphate pump

In microbial cells, inorganic polyphosphate (polyP) plays a significant role in increasing cell resistance to unfavorable environmental conditions and in regulating different biochemical processes. polyP is a polyfunctional compound. The most important of its functions are the following: phosphate and energy reservation, cation sequestration and storage, membrane channel formation, participation in phosphate transport, involvement in cell envelope formation and function, gene activity control, regulation of enzyme activities, and a vital role in stress response and stationary-phase adaptation. The functions of polyP have changed greatly during the evolution of living organisms. In prokaryotes, the most important functions are as an energy source and a phosphate reserve. In eukaryotic microorganisms, the regulatory functions predominate. Therefore, a great difference is observed between prokaryotes and eukaryotes in their polyP-metabolizing enzymes. Some key prokaryotic enzymes are not present in eukaryotes, and conversely, eukaryotes have developed new polyP-metabolizing enzymes that are not present in prokaryotes. The synthesis and degradation of polyP in each specialized organelle and compartment of eukaryotic cells are mediated by different sets of enzymes. This is consistent with the endosymbiotic hypothesis of eukaryotic cell origin.