In the article, “A Phosphate- Sensing Organelle Regulates Phosphate and Tissue Homeostasis” a study is being conducted to determine if these researchers have found a new organelle. Through a series of tests, they express different characteristics, functions, regulations, and formations that this organelle expresses. After reviewing this research, there are several components that are important to analyze to determine whether or not this is a new organelle that has a distinct function in the cell or not. 

In figure 2 of the article, there is a series of images that are used to demonstrate the different characteristics of the PXo bodies in the study. This is done by the use of fluorescent imaging.¹  A green fluorescent protein which is attached to the head of a protein and an added HA tag attached to the tail end of a protein glowing red is used in this test.¹ To determine the characteristics of the PXo bodies, one must investigate the coloring of the images. If the image illuminates yellow, that means that the function is prevalent and occurs in the organelle, but if there are separate colors of red and green, then it does not. After reviewing panels E, F, G, H, I, J, and K, it was discovered that the PXo bodies inhibit several characteristics. In panel E, it is measuring the acidity through Lysotracker Red, and when overlapped, there are some yellow fluorescents in the image. This shows that these PXo bodies have some acidity to them. In panel G, the use of Nile Red is used to determine if the PXo bodies are associated with lipids, with the results showing a few yellow fluorescents, so it can be concluded that it does. Panel I uses ConA which can express if the PXo body is glycosylated and the images show that they are. In panel J a phospholipid tracker, P-Cho is used and concludes that the PXo bodies are phospholipids. Panels F, H, and K did not have any yellow fluorescents showing in the images meaning they did not inhibit the characteristics of being a lysosome, being a part of the golgi, and taking part in endocytosis. Other characteristics examined in figure 2 were in panels A, B, C, and D. The images show an oval shaped organelle, with a blue DAPI stain which located the nucleus. All of these characteristics will help determine what type of organelle this is classified as and what functions it contributes. 

The next test is demonstrated through figure 3 of the article. The technique used is fluorescent residents energy transfer, also known as FRET. This test uses two fluorescent molecules, one of the molecules (blue), will be in an excited state and is capable of passing its excited energy state to the second molecule (yellow). A binding protein for inorganic phosphate will be placed between the two molecules to determine the levels of FRET. If the blue fluorescent is prominent, then that means there is low FRET ratio, meaning there are high concentrations in inorganic phosphate in the cytoplasm. As for when the yellow fluorescent is more prominent, the FRET ratio is high, and there are low levels of inorganic phosphate in the cytoplasm.¹ In panels D and E, a heat map is used. A correlating data chart in panel F compares the average FRET ratio with the supplemental phosphate FRET ratio. From these results gathered, we can determine that since the heat map of the PXo inhibited bodies is blue, and there is a decrease in the FRET ratio, then PXo regulates the levels of inorganic phosphate in the cytoplasm.

To determine how we know that the formation of PXo bodies depends on the availability of inorganic phosphate, we must review figure 4 of the article. In panels A, B, and C we are able to evaluate the size of the organelles. The first image shows us a normal PXo body. The next image demonstrates PXo bodies with inhibitor PFA, which inhibits cellular phosphate uptake. It is seen that these PXo bodies are smaller than the normal presented ones. The last image we see is the PXo inhibitor, which is RNA based. These PXo bodies showed larger than the normal and PFA organelles. Panel D is also an indicator of the sizes of each PXo body and shows it in a data chart. The next set of data allows for us to see how many phosphates were present in the different environments. The PFA inhibitor showed much less, while the PXo inhibitor had much more phosphate. From here it is evident that through the addition of inorganic phosphate, the PXo bodies have more in number and are larger in size than without extra Pi. 

Inorganic phosphate is a vital nutrient in living organisms. This macronutrient is one of the most important due to its role in biosynthesis of cellular components (ATP, phospholipids, and proteins) and because it has functions in metabolic pathways (energy transfer and protein activation). It also plays an important role that is used for cell function and skeletal mineralization, also known as biological processes. Skeletal mineralization is the process in which the skeleton gets its strength or structure through minerals. Inorganic phosphate also plays an important role in cellular metabolism, which is how living organisms maintain their life through chemical reactions that occur in the cells of the body. One way that inorganic phosphate plays a role in cellular metabolism is that cellular metabolism depends on the concentration of inorganic phosphate. In order for cellular metabolism to work, inorganic phosphate has starvation response genes to help facilitate the process. Another way inorganic phosphate plays a role in cellular metabolism is how it works in the different processes, including synthesis of lipids, ATP formation, and nucleic acids. Overall, inorganic phosphate plays an important role in the metabolism process within the body. 

Another important part of data to observe was in figure 5. These charts were used to express the differences in the types of phospholipids found in the PXo bodies when phosphate levels drop in the cytoplasm. One difference seen between the control and the PFA was that the control had 90.6% phospholipids, and the PFA consisted of 84.2%. In each of these PXo bodies, the two mostly concentrated phospholipids were phosphatidylcholine and phosphatidylethanolamine. The distribution of the phospholipids did not differ by much with the drop in levels of phosphate in the cytoplasm. Some of them actually stay the same including PE, DG, and PA. PS, PI, Cer, TG only fluctuate by 1% between the two. The major difference seen is in PC where the control, (or the PXo body with more phospholipids), has a concentration of 6% more. 

After reviewing all of the figures from the data in the article I do not think that this is PXo bodies which form distinct organelles with a unique biochemical function in the cell. When looking at its characteristics it shared a lot with others. Also after reviewing the size and amounts with the addition of phosphate it is prevalent that inorganic phosphates play a role in these PXo bodies. I believe these PXo bodies are organelles that have already been discovered since there is not much difference from these organelles to the others.      

Citations

Xu, C., Xu, J., Tang, H.W., Ericsson, M., Weng, J.H., DiRusso, J., Hu, Y., Ma, W., Asara, J.M., and Perrimon, 

N. A phosphate-sensing organelle regulates phosphate and tissue homeostasis. Nature. 2023; 617, 798-806. 

10.1038/s41586-023-06039-y.