Inorganic Phosphate 

There are many different components of our body that build parts of our tiniest organelles. One of those components is known as inorganic phosphate. Inorganic phosphate, also known as Pi, is a free phosphate ion within a solution. What separates them from organic phosphate is that organic phosphate is known as an ester which are molecules that have an organic molecule replacing a hydrogen atom in some kind of acid¹. Inorganic phosphate is found bound to three oxygen atoms when it is by itself. However, Pi can engage with other structures to make molecules that aid in our metabolism¹. Inorganic phosphates have many different roles overall but two that aid in cellular metabolism are the regulation of energy metabolism by glucose storage and by adapting energy metabolites².

Inorganic phosphate plays a role in metabolism by its function in adenosine triphosphate (ATP) which functions as a key factor in the regulation of energy metabolism. ATP consists of a nitrogenous base, a ribose sugar, and three phosphate groups that bind together to create energy for our cells³. The ATP ends up being used to fuel processes such as for our brain and muscles, and all of this occurs because of inorganic phosphate being available within our bodies. ATP also has the ability to change into different forms of energy such as ADP and even back to inorganic phosphate when it has been broken down to where molecules are separated². When people go on runs or exercise extensively, their bodies run out of energy pretty fast. This is due to our ATP storage quickly depleting, and our body realizes that we need to get this energy back immediately. How our body gains more energy is by breaking down glycogen in a process known as glycogenolysis². During this process, our body is able to get pyruvates needed for oxidative phosphorylation to occur, which will aid in the production of more ATP in mitochondria cells. Another way we receive more phosphate into our body is by eating foods that are high in phosphorus which are metabolites². These metabolites will get taken up by other cells within our bodies and recycled for future usage; possibly for more ATP to be made!

One of the structures that inorganic phosphate can form into are phospholipids. These structures are made from a phosphate group, two fatty acids, and a glycerol. They all come together to form what is known as a phospholipid. This structure acts almost like a gateway where it allows certain ions and small particles to be able to go through it without needing the help from secondary structures⁴. Particles that are known to be able to pass through the bilayers are water and nonpolar molecules such as oxygen and carbon dioxide⁵. While larger molecules such as sodium or potassium ions require assistance from larger structures that may need ATP to fuel them up. Some phospholipids can have multiple layers which form molecules called multilamellar bodies. Originally I had thought that these structures were used as a great way to prevent certain particles from leaving or penetrating the cell since there are numerous layers however, the main functions for these types of structures are lipid storage and secretion⁶.  

Without phospholipids, our cells would not be able to function since they would have all of their organelles floating about rather than within an enclosed area, similar to a gate. The following sections will review some of the characteristics and functions of inorganic phosphate with PXo bodies.

In figure 2 of the literacy paper, researchers used several images to review size and possible traits of inorganic phosphate bodies by using various stains. When looking at the shape of these phosphate bodies, most appeared circular in nature; some happened to have an oval shape as well, similar to bacillus bacteria. The size of the phosphate bodies increased depending on the environment that they were in. For instance, in the glycosylation and the phospholipid tracer stains, the phosphate bodies looked larger than the other stains examining other characteristics of inorganic phosphate⁷. Regarding the actual characteristics of inorganic phosphate bodies there were seven stains, three images for each stain, that looked at potential traits they may possess. If the top stain showed any yellow fluorescent lighting, that demonstrated that there was a certain trait present in the stain. If there was no yellow present, then that displayed that that trait was not present within the PXo bodies. In the first stain, image ‘e’, it demonstrates that the phosphate bodies are acidic due to the markers appearing yellow. In the second stain, image ‘f’, the phosphate bodies were tested to see if they had lysosomes or lysosome functions. Since there was no yellow within the stains, the phosphate bodies do not possess any lysosome characteristics. In image ‘g’, the inorganic phosphate bodies were tested on whether they were associated with lipids. Given that inorganic phosphate are main components of a cell’s bilayer, they should be associated with lipids, and from the stain we can see that that was the case. The next image showed testing for whether phosphate bodies are part of golgi structures, and it appeared that they were not given that no yellow showed up in the stain⁷. The following images displayed that there are glycosylation functions present within phosphate bodies and that they have phospholipid properties within them. Finally, within the last image, it displayed tiny areas where there could be slight endocytosis which could demonstrate that some of the PXo bodies are capable of moving substances or materials within the cell.

In figure 3, researchers looked at how phosphate bodies regulated inorganic phosphate levels within a cell. In image letter ‘e’, most of the cell is blue which means that there are increased levels of inorganic phosphate present⁷. This is due to the inorganic phosphate being able to bind between two proteins and increase blue light within the image In image letter ‘h’, there are little patches of blue which displays that there are decreased levels of inorganic phosphate being able to bind between the proteins which increases FRET and makes more yellow and red show up in the stain⁷. This is also in result to an inhibitor of phosphate bodies, PXo-HA, being present which will prevent the phosphate bodies from being able to let inorganic phosphate into them and the cytoplasm; therefore decreasing inorganic phosphate levels. 

Looking at figure 4, images shown there displayed how phosphate bodies responded to increased levels of inorganic phosphate vs when there are inhibitors around. In some image stains, phosphate bodies actually decreased in size when inhibitors were present as compared to their normal size⁷. In contrast, when additional inorganic phosphate was present, the size of the phosphate bodies increased as a result. When looking at how the number of phosphate bodies would respond to inhibitors or additional Pi present, there were similar patterns here as well. With the presence of inhibitors, the number of phosphate bodies decreased; however, when additional Pi was present, the number of them increased drastically⁷. Without inorganic phosphate, the phosphate bodies form in smaller sizes and in less quantity.

Lastly, in figure 5 researchers observed certain characterizations of phosphate bodies such as proteomes and lipidomic characterizations. With decreased levels of inorganic phosphate, the presence of phosphatidylcholine decreases within phosphate bodies but PE, a different kind of phospholipid, stays the same⁷. This shows that inorganic phosphate influences not just size and shape of phosphate bodies, but also the levels of proteomic and lipidomic characterizations within these bodies. So when levels of inorganic phosphate are low, many of the types of phospholipids will decrease in number as a result. When thinking about the structure of types of phospholipids, one of the simplest forms are phosphatidic acids. These lipids are the simplest phospholipids consisting of one phosphate group, a glycerol unit, and two fatty tails⁸. If these lipids were present and experienced low levels of Pi, the number of these phospholipids would decrease given that there would not be much inorganic phosphate necessary to make functional units of phosphatic acids.

In conclusion, PXo bodies are unique in the sense that they have various distinctions and characteristics that depend mainly on whether inorganic phosphate is present or if it is at low levels. Many phospholipid structures are decreased in the absence of Pi, Pxo bodies have been shown to decrease in size and quantity because of low levels of inorganic phosphate which compromises with the phosphate bodies ability to move inorganic phosphate into the cell. In addition, with the findings of figure 2, characteristics of the PXo bodies were seen. Some being that they are phospholipids that are able to go through glycosylation, and slight signs of possible endocytosis.

References

1. Dou, M., & Wang, L. A review on organophosphate esters: Physiochemical properties, applications, and toxicities as well as occurrence and human exposure in dust environment. Journal of environmental management. 2023; 325(Pt B), 116601. 
2.  Blaszczyk, J. Metabolites of life: Phosphate. Metabolites. 2023;13(7), 860. 
3. Dunn, J., and Grider, M.H. Physiology, adenosine triphosphate. StatPearls – NCBI Bookshelf. 2023; https://www.ncbi.nlm.nih.gov/books/NBK553175/#:~:text=The%20structure%20of%20ATP%20is,second%20and%20third%20phosphate%20groups.
4. Dai, Y., Tang, H., & Pang, S.  The Crucial Roles of Phospholipids in Aging and Lifespan Regulation. Frontiers in physiology. 2021; 12, 775648.
5. Cooper, GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates. 2000; https://www.ncbi.nlm.nih.gov/books/NBK9928/
6. Hariri, M., Millane, G., Guimond, M. P., Guay, G., Dennis, J. W., & Nabi, I. R. Biogenesis of multilamellar bodies via autophagy. Molecular biology of the cell, 2000; 11(1), 255–268. 
7. Xu, C., Xu, J., Tang, H. W., Ericsson, M., Weng, J. H., DiRusso, J., Hu, Y., Ma, W., Asara, J. M., & Perrimon, N. A phosphate-sensing organelle regulates phosphate and tissue homeostasis. Nature. 2023; 617(7962), 798–806.
8. Tzur, R., & Shapiro, B. Phosphatidic acid metabolism in rat liver microsomes. European journal of biochemistry. 1976; 64(1), 301–305