# An Update on Polyphosphate In Vivo Activities
## 7 Extracellular PolyP  
PolyP has both intracellular and extracellular activities (Table 2). In bacteria and unicellular eukaryotes, polyP is an intracellular polymer. However, when cells disintegrate, the polymer might be exposed to the extracellular environment. In contrast, some eukaryotic cells, including mast cells, astrocytes, and thrombocytes, can actively release the polymer from their secretory storage organelles [30; 90]. PolyP release and exposure at the cell surface have been identified in thrombocytes [91]. Thrombocytes contain polyP within their dense granules, which appear dark in electron microscopy due to their high Ca2+ and P1 content [92]. The dense granules share similarities with acidocalcisomes in unicellular eukaryotes [93]. However, unlike intracellular acidocalcisomes, dense granules fuse with the cell membrane upon activation and release intracellular polyP to the extracellular compartment [92]. Elegant real-time microscopy studies have visualised polyP \"secretion\" [20]. In contrast to released soluble mediators from dense granules such as ATP, platelet polyP is a mostly insoluble polymer. It is retained on the surface of procoagulant platelets as Ca2+/Zn2+-rich microparticles [20]. Only small amounts of relatively short-chain polymers of ~70-80 P1 residues are soluble and found in the plasma compartment. However, it is uncertain whether these polymers are in a solution or if they stick to extracellular vesicles or exosomes [94; 95]. Polyanion-exchanger-based polyP extraction from platelets showed that platelets, similar to other mammalian cells, mostly contain long-chain polyP with chain lengths of several hundreds of units. However, a small amount of total polyP is in the range of ~80 P1 units [96]. Isolation of platelet polyP using the phenol/chloroform-based extraction method selects for these soluble polymers and results in the purification of short-chain ~80 P1 long polymers [14]. The regulation of platelet polyP remains to be fully elucidated. Nevertheless, intracellular P1 levels have been identified as critical regulators of \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline
**Extracellular Functions** & **Intracellular Functions** \\ \hline Regulation of cell motility & Phosphate and energy storage \\ \hline Contact activation of Factor XII and, consecutively, the kallikrein-kinin system and the intrinsic coagulation pathway & Regulation of Ca2+ homeostasis and signalling \\ \hline pH regulation & Activating of mTOR-dependent pathways \\ \hline Cation binding & Regulation of growth, differentiation, and apoptosis \\ \hline Biofilm formation & Complexation of toxic heavy metal ions \\ \hline Storage of Ca2+ and other ions in the ECM & Regulation of mitochondria permeability \\ \hline \end{tabular}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline  & Interference with oxidative stress \\ \hline  & pH regulation \\ \hline  & Osmoregulation \\ \hline  & Regulation of enzyme activities \\ \hline \end{tabular}
\end{table} 
Table 2: Gross functions of extracellular and intracellular polyP. cellular polymer content [97], and the conditional ablation of platelet XPR1 increased polyP levels and augmented platelet-driven thrombosis with no bleeding abnormalities [85]. Synthetic and cell-purified polyP has modulated an array of reactions in plasma ex vivo [98; 99]. However, the in vivo activities of the polymer seem to be mediated chiefly, if not exclusively, via the factor (FXII)-activated plasma contact system [100]. FXII initiates the plasma contact system that drives the formation of bradykinin (BK) via the kallikrein-kinin system and the clots via the intrinsic coagulation pathway, respectively [101]. Binding to negatively charged polyP microparticles on cell surfaces induces a conformational change in FXII zymogen, resulting in an active serine protease, activated FXII (FXIIa) [102; 103]. Supporting a direct polyP-mediated FXII activation, immunofluorescence microscopy colocalised FXIIa and polyP on surfaces of cancer cells and cancer-derived exosomes [90].