You order take-out at your favorite restaurant over the phone. You then drive to this restaurant, park your car, go inside and get your food. But haven't you wished that all restaurants had the option of a drive-through window? You save time, and it is indeed way more efficient.
Similarly, tiny cells in our body have their own version of drive-through windows, specialized structures known as 'porosomes.' However, unlike restaurants, every cell has several porosomes embedded on their outer covering, the cell membrane. The porosomes are tiny and cup-shaped, in the size range of nanometers, you could fit at least a 1000 porosomes on the tip of a single hair strand! The word 'porosome' literally translates to pore-forming bodies. They open and close, aiding in proficient cell secretion, just like a restaurant drive-through!
Wait…but what is secretion?...
To understand secretion, we need to know what cells are, and for that, we need to understand how animals are organized! Throwback to our school days, we learned that animals are made up of organs like the heart, brain, lungs, and many others. Each organ is made of tissues, and every tissue is built out of cells, so technically speaking, cells are the basic unit of life. Cells need to communicate with each other and their surroundings to keep an animal functional. They do so by releasing chemical messengers into their surroundings, and this process is known as 'secretion.'
Secretion is a universal phenomenon; almost all cells secrete at a basal level. But certain types of cells are specialized for secretion. These cells are critical for simple life processes such as thinking, digestion, building muscles, and energy production. Porosomes are thus present on cells that undergo secretion as their primary function. Furthermore, defective secretion by cells can lead to diseases; for example, Diabetes may be caused when β- cells of the pancreas fail to secrete the hormone, insulin.
So, how do porosomes know that a cell wants to secrete?
An extremely complex process turns on a switch within cells that causes them to secrete. Once the switch is on, it sets the 'secretory granules' in motion.Secretory granules are spherical bags within cells that carry chemical messengers such as insulin. During secretion, the secretory granules travel from the center of the cell towards the cell membrane, on which the porosomes are located.
The base of porosomes has a dedicated machinery that anchors secretory granules until they have released their contents. Once a granule latches on to a porosome, the porosome opens up, facilitating the release of chemical messengers. The porosomes, therefore, establish a connection between the granules and the cell exterior, similar to a drive-through window.
Cool! What else do we know about these porosomes?
Porosomes were discovered pretty recently in the early 1990s, and there is an interesting story to their discovery. Conventionally, it was believed that granules fuse to the cell membrane during secretion. But this couldn't explain some crucial observations made by scientists. Firstly, high-resolution microscopic images of cells after secretion show that granules still have chemical messengers left inside them. Secondly, imagine, if all the secretory granules fuse with the cell membrane, then cell size and volume must increase, which wasn't witnessed. Lastly and importantly, the secretory granules are very similar to soap bubbles. They are spherical, have a high surface tension, and pop as soon as they come in contact with a surface. Therefore, granules may find it difficult to merge with the cell membrane without exploding. So, a porosome structure was suggested and later discovered that prevents granule collapse.
Neurons of the brain, cells of lungs, and β- cells of the pancreas are all equipped with porosomes for secretion. Although similarly shaped, every cell type has porosomes that are distinct in size, make, and design. For instance, porosomes from brain cells may be unable to secrete insulin.
Recently, porosomes were fished out from β-cells of mouse pancreas and inserted into living cells of the same type. The newly engineered β- cells had extra sets of porosomes embedded on their cell membranes. These cells were able to release more insulin than usual. Although further research is required, this could be a potential therapy for mitigating, not just Diabetes but many other secretory diseases.
Another secretory disease is cystic fibrosis (CF), which occurs when lung cells have faulty fluid secretion due to the sub-optimal function of the CFTR protein. Interestingly, when separated from lung cells, CFTR was found attached to the porosome suggesting that reconstitution could be a potential treatment for CF patients too.
Porosomes, although tiny, have made a mighty shift in the way we now comprehend cell secretion.