An Interpretation of Given Data
Possible New, Phosphate-Sensing Organelle discovered in Fruit Flies
An Interpretation of Given Data
Jessica Clark
Biology Department, Old Dominion University
BIO 293: Cellular Biology
Dr. Christina Steele
Abstract
Inorganic phosphates are salts of phosphoric acid. They can occur naturally using elements such as sodium, potassium, and calcium, but are not limited to these. Inorganic phosphate participates in many cell functions, including phospholipid construction for cell membranes, DNA and RNA, signal production, and most importantly cell metabolism. Inorganic phosphate is used to create ATP, the energy cells require, through glycolysis. In phospholipids, a phosphate group is attached to the 3rd carbon of a glycerol, making up the hydrophilic head of the phospholipid. Their water-repelling properties help to maintain membranes by keeping surrounding aqueous solutions out, while keeping cytosol in. Inorganic phosphates can also store energy in this state, and when more energy is needed, the fatty acid tails of phospholipids can be removed and used to create ATP. To create ATP, a proton gradient across the inner mitochondrial membrane forces the synthesis of ATP from ADP and inorganic phosphate. Without inorganic phosphate, this process would not be possible, and cell functions would cease. The organelles responsible for making sure these phospholipids exist are multilamellar bodies. These organelles function in lipid storage and secretion.
Possible New, Phosphate-Sensing Organelle discovered in Fruit Flies
An Interpretation of Given Data
Scientists have found a potential new organelle in an animal species, the fruit fly. This organelle is thought to stockpile inorganic phosphate (Pi). Inorganic phosphate is a key component of cell membranes and other processes in the body. In Figure 2 (Xu, 2023, page 800), we see the processes used to identify structures in the cell. In graph A, we see green used to identify the N-terminal (or head) of the PXo bodies, while red is used to identify the C-terminal, or tail. By looking at the places of overlap, indicated by yellow, we can see that the bodies themselves tend to be spherical. Graphs B and C show that these bodies are located throughout the cell and around the nucleus, as indicated by the black, spherical dots. Graph E shows us that the organelles are acidic, like lysosomes, which explains why they have managed to go undetected so far. By looking at panel F though, we can see the differentiation of PXo and lysosomes, with PXo in green and lysosomes in red. We can see here that lysosomes are actually smaller than PXo bodies. In graph G, lipids are stained red, allowing us to see that the inorganic phosphates have become phospholipids. Figure H shows us that the PXo bodies appear outside of the Golgi complex, Graph I shows that sugars have been added to the phospholipids, while Graph J reinforces the phospholipid presence. Finally, Graph K shows us that PXo bodies are not a part of the endocytosis pathway.
In Figure 3 (Xu, 2023, page 801), Xu and his colleagues used FRET, Fluorescence Resonance Energy Transfer, to measure the amount of inorganic phosphate in the transfer portions of the amino acid sequences of PXo in the cytosol when RNA inhibition is used to lower levels of inorganic phosphate in the cytoplasm. When a specific wavelength of light is passed through the FRET molecule, it gets excited and then passes on that energy to a second molecule. Panels D, E, G, and H all show either low FRET ratios in blue, indicating high levels of inorganic phosphate in the cytoplasm, or high FRET ratios, indicated by yellow, meaning low levels of inorganic phosphate in the cytoplasm. Panel F puts these scenarios in scatter plot form, showing that with the RNA inhibition, the levels of inorganic phosphate in the cytoplasm increased, rather than decreased as expected. This tells us that the PXo bodies must be releasing inorganic phosphate into the cytoplasm. But why?
Figure 4 (Xu, 2023, page 801) further looks into this phenomenon by looking at whether the number and size of PXo bodies change in response to inorganic phosphate availability in the cell. Two methods of inhibition were used, phosphonoformic acid (PFA), which inhibits cellular Pi uptake, and an RNAi reagent, which also stimulates a change in Pi levels in the cytoplasm. Panel A shows a normal PXo body. Panels B and C show the same PXo bodies after introduction of either the PFA or the RNAi. After introduction, a definite decrease in size of PXo bodies is noted. Panel D puts the data into scatter-graph format, and allows us to see that with the introduction of PFA and PXo-i, the average size of PXo bodies did indeed decrease. However, with the introduction of additional inorganic phosphate, the size of PXo bodies increased slightly. Panels E, F, and G are stains which allow us to further see what is going on when inhibitor or extra Pi is used. Panel E is normal, and we see the PXo bodies in green. In panel F, we can see that the number of PXo bodies has decreased as well as the size as seen in panel D. Panel G is a visualization of extra Pi, showing numerous PXo bodies, as well as their increase in size , also seen in panel D. Panels J, K, and L show the same information, as well as shape, in vivid three-dimensional photos.
For Figure 5 (Xu, 2023, page 803), Xu and his team did a pie chart comparison of phospholipids contained in the control group and the PFA group. For the control group, 90.6% of the PXo body was composed of phospholipids. Those broke down into 45% phosphatidylcholine (PC), 35% phosphatidylethanolamine (PE), and 3% phosphatidylserine (PS). Six other phospholipids are present in the chart, but are not as important in the analysis. In comparison, the PFA bodies contained 39% PC, a 6% decrease, 35 % PE, no change, and 4% PS, a 1% decrease. In both groups, PE and PC were the most common phospholipids found, and accounted for most of the bodies’ make-up. While PE remained unchanged in both groups, PC and PS were directly impacted by the inhibitor, with PC showing the most downward change.
After looking over the data, the conclusions found in Xu’s experiment appear to be correct. PXo bodies are a “new” organelle in the cell, one whose purpose is to regulate inorganic phosphate in the cell. Their experiments testing the inorganic phosphate levels with multiple inhibitors showed that the “obvious” assumption, that Pi levels would remain unchanged, or increase, were in truth, proven incorrect, as Pi levels dropped instead. The findings showed that these organelles degrade in the absence of Pi in the cell, releasing their own
Pi to be used by the cell. The stain imaging in figures 3 and 4 showed the small, round appearance of these organelles, and that they were found throughout the cell. The importance of understanding inorganic phosphate uses in the cell and its connection to Cell Biology is clear. Learning about new structures in the cell allow us, as scientists, to understand our bodies and their mechanisms better for a multitude of reasons, including medicine and technology. Long live PXo bodies! On to the next!
References
Steel, C [Christina Steel]. (2024, March 31). PXo Organelles Walkthrough [Video]. YouTube. https://www.youtube.com/watch?v=sz7DdpoxE18
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.
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