Scientists have discovered a new organelle called PXo bodies in the intestine of fruit flies (Drosophila melanogaster).
But what are organelles and why are they important?
An organelle is defined as being a specialized subunit present within a cell that carries out a specific function and is usually enclosed within its membrane. In 1833, Brown discovered the nucleus, which helped form the basis of Schleiden’s cell theory in 1838, where he proposed that all plant tissues are composed of nucleated cells. Since then, many scientists have used progressions in electron microscopy and subcellular fractionation to discover 18 common eukaryotic cell organelles, as well as some other less common ones, such as the ‘frodosome’ (unofficial name) which was discovered in 2019 and plays a role in bone metastasis.
The most common organelles tend to be morphologically similar and perform similar functions in all eukaryotic cells but some can be more specialised in certain cells. Different cells often have different combinations of organelles based on their function – for example animal cells don’t have chloroplasts and the cells of higher plants don’t have centrioles. Each organelle is specialised to perform a specific function for the cell. They communicate through vesicular trafficking pathways and membrane contact sites in order to work together to allow the cell to perform its function within the tissue and maintain homeostasis within the cell. This makes them incredibly important for all eukaryotic life, meaning the discovery of a brand new organelle, PXo bodies, is very exciting for biologists, especially as little is known about intracellular Pi metabolism and signalling in animal tissues.
The discovery of PXo bodies - why fruit flies?
Scientists have been studying Drosophila melanogaster for over a century, with the first documented use of this model organism in the laboratory being at Harvard in 1901. Fruit flies are low cost and regenerate rapidly making them instrumental in many large discoveries, especially in those of developmental biology as they were used in the discovery of homeobox genes, and have led to multiple Nobel prizes in different fields of biology.
If Drosophila are so well researched, how did biologists not spot PXo bodies sooner?
The discovery of PXo bodies was actually accidental!
A scientist named Chiwei Xu and his colleagues were studying phosphate absorption in the digestive epithelium in the Drosophila melanogaster’s intestine and how this affected proliferation of these cells. They determined that chronic Pi starvation, from feeding the flies meals low in phosphate or giving them a drug that inhibits Pi absorption, causes hyperproliferation of their digestive epithelial cells. This was surprising as the cells in their midgut had little to no phosphate but were still able to divide rapidly. They found the same results when they suppressed a Pi transporter protein (PXo). This rapid cell renewal may be a way for the Drosophila to increase phosphate absorption when there is a deficit, due to the presence of PXo bodies in the intestinal cells that are proliferating, however, this is yet to have been proved.
They fused the PXo protein with a florescent protein and used a fluorescence microscope to analyse the proteins ultrastructure further, which is when they discovered that this protein marks oval shaped structures in the cell. They used further immunostains for known organelles to try and identify these structures but when none of the stains worked, they realised that they had discovered an entirely new organelle!
What do PXo bodies do?
Researchers found, through electron microscopy, that the PXo bodies have whorls in their membrane structure which are studded with PXo proteins. These proteins help the PXo bodies store Pi by transporting phosphate molecules from the cytoplasm into the organelle to create an intracellular Pi reserve. The PXo bodies also require PXo proteins to undergo biogenesis after Pi starvation. Additionally, PXo proteins are involved in a PXo-Cka-JNK signalling cascade within the cells of the Drosophila’s gut making them a key component in maintaining tissue homeostasis.
The study found PXo bodies are an important regulator of cytosolic Pi concentrations and could control phosphate levels in the entire Drosophila melanogaster organism. There are many other proteins that interact with PXo proteins which scientists plan on studying, in order to work out their roles in phosphate absorption in PXo bodies and therefore, gain a better overall understanding of intracellular Pi metabolism and signalling in animal tissues. There is also exciting potential for the PXo bodies to exist in other eukaryotic cells meaning this rare discovery could lead to many more in the future.
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