First discovered in trypanosomes, acidocalcisomes are lysosome-related organelles that store calcium, high amounts of phosphorous (pyrophosphate, orthophosphate and polyphosphate) as well as a few other cations.
In various species (e.g. trypanosomatids), acidocalcisomes have also been associated with such functions as phosphate metabolism and homeostasis processes that allow the organism to adapt to environmental changes.
Acidocalcisome share the following characteristics with lysosome-related organelles (e.g. lytic granules, MHC class II compartments, and melanosomes):
Similar general morphology
* The main difference between acidocalcisomes and lysosomes is that acidocalcisomes do not accumulate endocytic tracers.
A Brief Look at the Origin of Acidocalcisomes
Although they were first identified in eukaryotic organisms (trypanosomatids) and later in human beings (in the dense granules of human platelets), new studies revealed their presence in bacteria. This was evidence that acidocalcisomes are a few of the organelles that have been conserved over the course of evolution (from simply bacterial cells to humans).
* Acidocalcisomes are a few of the membrane-bound organelles in bacteria (they have a lipid-based membrane).
Through new studies, the organelles have also been found to be present in such domains as Archaea. For a good number of scientists, this is sufficient evidence of a common ancestor. While there are various similarities between acidocalcisomes found in single-celled organisms like trypanosomes and lysosome-related organelles, studies have also found a number of differences.
Whereas adaptor protein 3 is involved in the transportation of membrane proteins to lysosome-related organelles in cells of mammals, it's involved in biogenesis of the organelle in other organisms (e.g. Leishmania major and T. brucei).
Although this may suggest different origins between various organisms, some scientists attribute such differences to evolutionally adaptions over time.
Characteristics of Acidocalcisomes in Different Types of Organisms:
As already mentioned, acidocalcisomes are a few of the membrane-bound organelles found in bacteria. Although it was initially assumed that they lack a membrane, the use of electron microscopy and staining procedures helped reveal a limiting membrane.
Some of the other characteristics of bacterial acidocalcisomes revealed include:
Are electron-dense bodies
Spherical in shape
Range from 15 to 200nm in diameter depending on the species
Depending on the cell, the number of acidocalcisomes per cell may range from 1 to 3
Are very small and take up about 1 percent of the total cell volume
* The acidocalcisome membrane contains several pumps, one channel, and several exchanges.
Calcium pumps - Ca2+-ATPase
Proton pumps - vacuolar-type H+-ATPase and H+-pyrophosphatase
Na+/H+ and Ca2+/H+ exchangers
Aquaporins - Water channel
Based on microscopic studies, acidocalcisomes found in protists have a number of similarities to those found in bacteria. For instance, like acidocalcisomes found in bacteria, those found in protists are electron-dense and give an empty appearance when viewed under an electron microscope. They are also spherically shaped with a diameter ranging between 0.2 and 0.6 micrometers.
Compared to bacterial acidocalcisomes, those found in protists have been suggested to present a polymorphic appearance.
Some of the other characteristics of acidocalcisomes found in protists include:
Contain high amounts of short-chain phosphates (P3, P4, P5)
Make up between 1 and 2 percent of the total cell volume
Contain several enzymes including exopolyphosphatase, metacaspase, inorganic pyrophosphatase, and olyphosphate kinase
Membrane contains a number of pumps and antiporters
Acidocalcisomes have also been identified in the eggs of chicken, sea urchins and insects. While the amount of calcium and other cations in the acidocalcisomes of these eggs remain unknown, studies have shown them to be similar in morphology and composition.
Moreover, they have been shown to contain short poly phosphorous chains that form complexes with various cations (sodium, magnesium, and calcium in the eggs of sea urchin; sodium, zinc, calcium, and magnesium in the eggs of chicken and iron, potassium, sodium, magnesium, and calcium in insect eggs).
Acidocalcisomes of Mammalian Cells
In mammals, lysosome-related organelles that resemble acidocalcisomes include lytic granules, basophilic granules, platelet dense granules, and melanosomes among others. Like acidocalcisomes found in protists and other organisms, these organelles are both acidic in nature and contain high amounts of calcium.
As compared to some of the other lysosome-related organelles, platelet dense granules are more similar to acidocalcisomes. As such, they are electron-dense and contain pyrophosphate (PPi) and polyphosphate (poly P). Moreover, they contain high amounts of calcium.
General Structural Characteristics of Acidocalcisomes
In bacteria and eukaryotic organisms (single-celled eukaryotes), acidocalcisomes appear as spherical bodies ranging from 0.2 to 0.5 micrometers in diameter. However, polymorphic morphologies have been identified under the microscope.
Within these cells, their position is random and can, therefore, be seen at different locations when viewed under the microscope. Under an electron microscope acidocalcisomes may appear empty or consisting of a thin layer of dense material adhering to the inner surface of the surrounding membrane.
They are also characterized by large amounts of calcium and phosphorus with such cations as potassium and iron etc being present in lower amounts.
Some of the other characteristics associated with acidocalcisomes of bacteria and eukaryotic microbes include:
Contain large amounts of pyrophosphate, about 50 Pi short chains and between 50 and 800 Pi long chains.
Within the granules, the concentration of polyphosphates may reach molar levels
Contain basic amino acids
Contain a few soluble enzymes in their matrix
As mentioned, acidocalcisomes serve as storage bodies for phosphorous compounds and a number of cations. By storing these compounds and other cations, acidocalcisomes are also able to perform a number of other functions. For instance, the storage of poly P in acidocalcisomes helps reduce the overall osmotic effect caused by a large amount of the compound and cations.
Apart from its osmotic function, some of the other functions of the compound (poly P) include acting as an energy source, as a building block of the membrane, regulating gene transcription and enzyme activities as well as being a component of the structure of channels and pumps located on the surface of the organelle.
In protists, poly P has been associated with the development, sporulation and stress adaption of different species among other functions. For some of the parasites, lower amounts of poly P has been shown to result in low virulence.
In higher animals, the compound serves as a potent modulator of blood coagulation when it is released from platelet dense granules. In the process, it affects such processes as vascular permeability as well as affecting the intrinsic pathway, etc.
In such single-celled organisms as T. brucei, acidocalcisomes have also been shown to regulate intracellular pH (through the generation of hydrogen ions from the hydrolysis of poly P) as well as osmoregulation. In S. cerevisiae, the regulation of intracellular pH helps in the neutralization of pH changes.
On the other hand, osmoregulatory functions of the organelle are particularly important for such parasites as trypanosomatids given that they allow the organisms to adapt to different osmotic environmental conditions as they change from one form to another in their life cycle (in vertebrate and invertebrate hosts).
Electron microscopy is one of the methods used to observe acidocalcisomes. In the event that unfixed whole cells are used, the cells are first washed in PBS (pH 7.2). The droplets are then applied to 100-mesh Formvar-coated copper grips and allowed to adhere. After 10 minutes, they are gently blotted before being observed in an energy-filtering electron microscope.
When viewed under an electron microscope, acidocalcisome appear as empty vacuoles (usually spherical in shape). However, when visualized under an energy-filtered transmission electron microscope, they appear as electron-dense spots in unfixed cells.
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