Dense granules (also known as dense bodies or delta granules) are specialized

secretory Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical mec ...

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s. Dense granules are found only in

platelet Platelets or thrombocytes () are a part of blood whose function (along with the coagulation#Coagulation factors, coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a thrombus, blood clot. Platelets have no ...

s and are smaller than alpha granules.Michelson, A. D. (2013). ''Platelets'' (Vol. 3rd ed). Amsterdam: Academic Press. The origin of these dense granules is still unknown, however, it is thought that may come from the mechanism involving the endocytotic pathway.Ambrosio, A. L., Boyle, J. A., & Di Pietro, S. M. (2012). Mechanism of platelet dense granule biogenesis: study of cargo transport and function of Rab32 and Rab38 in a model system. ''Blood'', ''120''(19), 4072–4081. Dense granules are a sub group of

lysosome A lysosome () is a membrane-bound organelle that is found in all mammalian cells, with the exception of red blood cells (erythrocytes). There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation cent ...

-related organelles (LRO). There are about three to eight of these in a normal human platelet.McNicol, A., & Israels, S. J. (1999). ''Platelet dense granules: Structure, function and implications for haemostasis''

In unicellular organisms

They are found in animals and in unicellular organisms including

Apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...

protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

ns. They are also found in ''

Entamoeba ''Entamoeba'' is a genus of Amoebozoa found as internal parasites or commensals of animals. In 1875, Fedor Lösch described the first proven case of amoebic dysentery in St. Petersburg, Russia. He referred to the amoeba he observed microscopica ...

''. Dense granules play a major role in

Toxoplasma gondii ''Toxoplasma gondii'' () is a species of parasitic alveolate that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but members of the cat family (felidae) are the only known d ...

. When the parasite invades it releases its dense granules which help to create the

parasitophorous vacuole The parasitophorous vacuole (PV) is a structure produced by apicomplexan parasites in the cells of its host. The PV allows the parasite to develop while protected from the phagolysosomes of the host cell. The PV is a bubble-like compartment made ...

''Toxoplasma gondii''

''T. gondii'' contains organelles called unique organelles including dense granules. Dense granules, along with other secretory vesicles such as a

microneme Micronemes are secretory organelles, possessed by parasitic apicomplexans. Micronemes are located on the apical third of the protozoan body. They are surrounded by a typical unit membrane. On electron microscopy they have an electron-dense ma ...

and

rhoptry A rhoptry is a specialized secretory organelle. They are club-shaped organelles connected by thin necks to the extreme apical pole of the parasite. These organelles, like micronemes, are characteristic of the motile stages of Apicomplexa protozoa ...

secrete proteins involved in the gliding motility, invasion, and parasitophorous vacuole formation of Toxoplasma gondii. Dense granules specifically secrete their contents several minutes after parasite invasion and localization into the parasitophorous vacuole. Proteins released from these specialized organelles are critical to adapting to the intracellular environment of the invaded host cell and contribute to parasitophorous vacuolar structure and maintenance.

Structure and Biogenesis

Dense granules in ''T. gondii'' are spherical, electron dense bodies that resemble secretory

vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features ...

s in mammalian cells about 200 nm in diameter and most likely form from budding off the trans-golgi network.Cesbron-Delauw, M. -. (1994). Dense-granule organelles of Toxoplasma gondii: Their role in the host-parasite relationship. Parasitology Today, 10(8), 293–296. Dense granule protein aggregation and retention is vital to maintaining dense granule biogenesis. This process is thought to follow the sorting-by-retention model in higher eukaryotes due to the morphological similarities of ''T. gondii''’s dense granule and higher eukaryotes’ dense core granules. The proposition includes the accumulation of secretory proteins within the granules that prevents their escape from maturing dense granules in the trans-golgi network by constitutive vesicles budding.Venugopal, K., & Marion, S. (2018). Secretory organelle trafficking in Toxoplasma gondii: A long story for a short travel. International Journal of Medical Microbiology, 308(7), 751–760. Additionally, dense granule formation follows a

clathrin Clathrin is a protein that plays a role in the formation of coated vesicles. Clathrin was first isolated by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When the triskel ...

dependent matter at the trans-golgi network. ''T. gondii'' expresses one clathrin heavy chain (CHC1) important for forming micronemes and rhoptries in T. gondii, but the adaptor important for targeting CHC1 to dense granules remains unknown. After clathrin is recruited, the mature dense granules bud off the golgi apparatus and are shuttled to plasma membrane release sites in order to secrete their contents.

Trafficking and Secretion of Dense Granule Proteins (GRAs)

The majority of GRA proteins contain an N-terminal ER-targeting signal peptide and enter the secretory pathway via synthesis and translocation at the

rough endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for "little n ...

, and the signal sequence is thought to be cleaved off though not proven.Griffith, M. B., Pearce, C. S., & Heaslip, A. T. (2022). Dense granule biogenesis, secretion, and function in Toxoplasma gondii. Journal of Eukaryotic Microbiology, 69(6), e12904. Many GRA proteins contain a single transmembrane domain, meaning that the proteins are translocated across the endoplasmic reticulum lumen, exported to and shuttled through the

golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...

, and eventually secreted from the parasite into the vacuolar space or parasitophorous vacuolar membrane.

SNARE protein SNARE proteins – " SNAP REceptors" – are a large protein family consisting of at least 24 members in yeasts and more than 60 members in mammalian and plant cells. The primary role of SNARE proteins is to mediate the fusion of vesicles ...

complexes drive the transport and docking of vesicles with proteins from the endoplasmic reticulum to the golgi body and vice versa. Once the dense granule organelle is fully matured, the organelle appears to directly fuse with the

plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

between gaps of the parasite's inner membrane complex. Dense granule secretion contains characteristics of both regulated and constitutive secretory pathways. Despite GRA proteins accumulating rapidly as a “burst” after a few minutes into invasion to help facilitate the newly formed parasitophorous vacuole similarly to a regulated secretory event, secretion of GRA proteins is a constitutive process independent of calcium occurring throughout the parasite's life cycle both intracellularly and extracellularly.

Functions

According to the stage of infection, the number of dense granules present in a parasite may vary from approximately 15 in tachyzoites and sporozoites, 8–10 in bradyzoites and 3–6 in merozoites. After invading a host cell and setting up the parasitophorous vacuole, dense granules secrete their dense granule proteins (GRAs) into the vacuolar space where the proteins will insert into the vacuole's membrane, stay in the vacuole lumen, interact with the intravacuolar network or be secreted into the host cell. In order to scavenge and sequester host cells’ nutrients and lipids, the intravacuolar network (IVN) must first be formed by ''T. gondii''. This network of membranous tubules is involved in acquiring nutrients, modulating immune response, and facilitating cyst development. Then, GRA proteins can associate with the IVN, allowing for nutrient acquisition. For nutrient acquisition, GRA proteins have been implicated in scavenging host lipids, such as sphingolipids from host Golgi derived vesicles, cholesterol from host endosomal or lysosomal vesicles, and other lipids from host neutral lipid droplets. GRA proteins also are involved in host protein uptake into the parasite. GRA proteins secreted into the host cell cytosol are implicated in altering host cell gene expression and immune responses. Altering these responses allows parasite replication, proper growth, and egress of parasites. Other things GRA proteins secreted into the host cell are implicated in include host

cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...

arrest and control, host cell immune responses including NF-kB,

IFN-γ Interferon gamma (IFNG or IFN-γ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. ...

, and

p38 mitogen-activated protein kinases p38 mitogen-activated protein kinases are a class of mitogen-activated protein kinases (MAPKs) that are responsive to stress stimuli, such as cytokines, ultraviolet irradiation, heat shock, and osmotic shock, and are involved in cell diffe ...

pathways, and host cell antigen presentation. While not all of the T. gondii GRA proteins have been characterized, some novel GRA proteins are important for parasite replication, virulence, and cyst formation. Other new GRA proteins have been implicated in parasite egress and calcium homeostasis. Further research is needed to characterize the function and prevalence of all GRA proteins.

In multicellular organisms

Components

The dense granules of human

s contain

adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbon ...

(ADP),

adenosine triphosphate Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...

(ATP), ionized

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

(which is necessary for several steps of the

coagulation cascade Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of coagulat ...

), and

serotonin Serotonin (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...

. Dense granules are similar to

s with an acidic pH and even some lysosomal proteins like

CD63 CD63 antigen is a protein that, in humans, is encoded by the ''CD63'' gene. CD63 is mainly associated with membranes of intracellular vesicles, although cell surface expression may be induced. Function The protein encoded by this gene is a me ...

.Sharda, A., & Flaumenhaft, R. (2018). The life cycle of platelet granules. ''F1000Research'', ''7'', 236. There is a granular adenine nucleotide pool within the dense granule. It is thought that it is made up of system of insoluble calcium. This pool is likely to be different than that of the cytoplasmic nucleotides. In some animals it has been shown that the platelets contain

histamine Histamine is an organic nitrogenous compound involved in local immune responses communication, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Discovered in 19 ...

. During exocytosis, the pool of ATP within the dense granule is released. Serotonin is picked up by the dense granules where it interacts with ATP and calcium. The serotonin that is then released by the dense granule, recruits other platelets and helps play a major role in stopping the loss of blood at the injury. The calcium from a dense granule accounts for the majority of the calcium within the platelets and plays a role in the binding of different proteins.

Clinical significance

A deficiency of

can be associated with

Hermansky–Pudlak syndrome Heřmanský–Pudlák syndrome (often written Hermansky–Pudlak syndrome or abbreviated HPS) is an extremely rare autosomal recessive disorder which results in Albinism, oculocutaneous albinism (decreased pigmentation), bleeding problems due to a ...

. The patients with this disease show signs of abnormal dense granules and

melanosome A melanosome is an organelle found in animal cells and is the site for synthesis, storage and transport of melanin, the most common light-absorbing pigment found in the animal kingdom. Melanosomes are responsible for color and photoprotectio ...

s which can cause prolonged bleeding and

albinism Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and reddish pink or blue eyes. Individuals with the condition are referred to as albinos. Varied use and interpretation of ...

. Chediak-Higashi syndrome is an autosomal recessive disorder where patients platelets have a deficient amount of dense granules. CHS is very similar to beige mouse.

Biogenesis

The dense granule is very important in the coagulation cascade because of the bleeding disorders caused by a dense granule deficiency. However, the exact details of how it created is unknown. It has been observed that they are produced in

bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...

megakaryocyte A megakaryocyte () is a large bone marrow cell with a lobation, lobated nucleus that produces blood platelets (thrombocytes), which are necessary for normal blood coagulation, clotting. In humans, megakaryocytes usually account for 1 out of 10,00 ...

s. Within the megakaryocytes it is thought that their production has something to do with the endocytotic pathway. Dense granules have their components sent to maturing dense granules using vesicular nucleotide transporters. This is what is thought to cause the build up of ADP/ATP in dense granules. This mechanism is also responsible for the build up of MRP4 which picks up

cAMP Camp may refer to: Areas of confinement, imprisonment, or for execution * Concentration camp, an internment camp for political prisoners or politically targeted demographics, such as members of national or minority ethnic groups * Extermination ...

for the dense granule. Mice with MRP4-/- will have dysfunctional platelets from cAMP not being takin up from the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

and placed into the dense granule.

Membrane

There are a number of proteins that are within the dense granule membrane. To maintain the low pH within the granule, there is a Hydrogen ion pumping ATPase. Ral has been found within the granule's membrane. There are several adhesive receptors that have luminal binding domains and are expressed post

exocytosis Exocytosis is a term for the active transport process that transports large molecules from cell to the extracellular area. Hormones, proteins and neurotransmitters are examples of large molecules that can be transported out of the cell. Exocytosis ...

. These adhesive receptors help the adhesive receptors on the surface of the platelets. One of these receptors is

GPIb Glycoprotein Ib (GPIb), also known as CD42, is a component of the GPIb-V-IX complex on platelets. The GPIb-V-IX complex binds von Willebrand factor, allowing platelet adhesion and platelet plug formation at sites of vascular injury. Glycoprotei ...

. GPIb is one of the more important receptors within platelets.

Function

The true function of a dense granule is still unknown. However, the secretion of dense granules occurs along with platelet activation. Both, ADP and collagen can cause the secretion of dense granules. Patients and mice with dense granule deficiency have a harder time forming a

hemostatic plug Vascular closure devices (VCDs) are medical devices used to achieve hemostasis of the small hole in the artery after a cardiovascular procedure of endovascular surgery requiring a catheterization. Cardiovascular procedures requiring catheterizatio ...

and therefore have a longer bleed time.

Detection

The dense granules' matrix is dense with electrons that allow them to be detected through whole mount

electron microscopy An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing i ...

. The calcium levels within the dense granule allows for no extra staining when viewing the dense granule with an electron microscope. When observed by using

transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...

(TEM), these granules are

osmophilic An osmophile is an extremophile microorganism adapted to environments generating high osmotic pressures, such as aqueous solutions with high salt or sugar concentrations (''e.g.'', brines or sirups). Osmophiles are similar to halophiles (salt-lovi ...

. The secretion of dense granules can be detected by seeing how much ATP/ADP is being released with luciferase-based luminescence. The relationship to ATP/ADP released can be used to then determine the secretion of dense granules. Another option is to observe the amount of serotonin being released from a platelet with a large amount of serotonin already on it. Another way to detect the secretion of dense granules is through

flow cytometry Flow cytometry (FC) is a technique used to detect and measure the physical and chemical characteristics of a population of cells or particles. In this process, a sample containing cells or particles is suspended in a fluid and injected into the ...

. Since dense granules have surface membrane proteins, the activation of

and LAMP-2 can be observed with flow cytometry.

References

{{Authority control Organelles