The endoplasmic reticulum (ER) is a network of membrane-enclosed tubules and sacs (cisternae) that extends from the nuclear membrane throughout the cytoplasm. The entire endoplasmic reticulum is enclosed by a continuous membrane and is the largest organelle of most eukaryotic cells.
The cytoplasmic matrix is traversed by a complex network of inter-connecting membrane-bound vacuoles or cavities. These vacuoles or cavities often remain concentrated in the endoplasmic portion of the cytoplasm; therefore, known as endoplasmic reticulum, a name derived from the fact that in the light microscope it looks like a “net in the cytoplasm.” (Eighteenth-century European ladies carried purses of netting called reticules).
The name “endoplasmic reticulum” was coined in 1953 by Porter, who in 1945 had observed it in electron micrographs of liver cells. Fawcett and Ito (1958), Thiery (1958), and Rose and Pomerat (1960) have made various important contributions to the endoplasmic reticulum.
The occurrence of The Endoplasmic reticulum.
The occurrence of the endoplasmic reticulum varies from cell to cell. The erythrocytes (RBC), egg and embryonic cells lack in the endoplasmic reticulum. (Note. In the reticulocytes (immature red blood cells) which produce only proteins to be retained in the cytoplasmic matrix (cytosol) (e.g., hemoglobin), the ER is poorly developed or non-existent, although the cell may contain many ribosomes).
The spermatocytes have poorly developed endoplasmic reticulum. The adipose tissues, brown fat cells, and adrenocortical cells, interstitial cells of testes and cells of corpus luteum of ovaries, sebaceous cells, and retinal pigment cells contain only smooth endoplasmic reticulum (SER).
The cells of those organs which are actively engaged in the synthesis of proteins such as acinar cells of the pancreas, plasma cells, goblet cells, and cells of some endocrine glands are found to contain rough endoplasmic reticulum (RER) which is highly developed. The presence of both SER and RER in the hepatocytes (liver cells) is reflective of the variety of the roles played by the liver in metabolism.
Structure of Endoplasmic Reticulum
Morphologically, the endoplasmic reticulum may occur in the following three forms:
- Lamellar form or cisternae (A closed, fluid-filled sac, vesicle or cavity is called cisternae);
- vesicular form or vesicle and
- tubular form or tubules.
The cisternae are long, flattened, sac-like, unbranched tubules having a diameter of 40 to 50 μm. They remain arranged parallelly in bundles or stakes. RER usually exists as cisternae which occur in those cells which have synthetic roles as the cells of the pancreas, notochord, and brain.
The vesicles are oval, membrane-bound vacuolar structures having a diameter of 25 to 500 μm. They often remain isolated in the cytoplasm and occur in most cells but are especially abundant in the SER.
The tubules are branched structures forming the reticular system along with the cisternae and vesicles. They usually have a diameter from 50 to 190 μm and occur almost in all the cells. The tubular form of ER is often found in SER and is dynamic in nature, i.e., it is associated with membrane movements, fission, and fusion between membranes of cytocavity network.
Ultrastructure of Endoplasmic Reticulum
The cavities of cisternae, vesicles, and tubules of the endoplasmic reticulum are bounded by a thin membrane of 50 to 60 Aº thickness. The membrane of the endoplasmic reticulum is fluid-mosaic like the unit membrane of the plasma membrane, nucleus, Golgi apparatus, etc.
The membrane, thus, is composed of a bimolecular layer of phospholipids in which ‘float’ proteins of various sorts. The membrane of the endoplasmic reticulum remains continuous with the membranes of the plasma membrane, nuclear membrane, and Golgi apparatus.
The cavity of the endoplasmic reticulum is well developed and acts as a passage for the secretory products. Palade (1956) has observed secretory granules in the cavity of the endoplasmic reticulum.
Sometimes, the cavity of RER is very narrow with two membranes closely apposed and is much distended in certain cells that are actively engaged in protein synthesis (e.g., acinar cells, plasma cells, and goblet cells). Weibel et al., 1969, have calculated that the total surface of ER contained in 1ml of liver tissue is about 11 square meters, two-third of which is of rough type (i.e., RER).
Types of Endoplasmic Reticulum
Two types of endoplasmic reticulum have been observed in the same or different types of cells which are as follows:
1. Agranular or Smooth Endoplasmic Reticulum
This type of endoplasmic reticulum possesses smooth walls because the ribosomes are not attached to its membranes. The smooth type of endoplasmic reticulum occurs mostly in those cells, which are involved in the metabolism of lipids (including steroids) and glycogen.
The smooth endoplasmic reticulum is generally found in adipose cells, interstitial cells, glycogen storing cells of the liver, conduction fibers of the heart, spermatocytes, and leucocytes. The muscle cells are also rich in a smooth type of endoplasmic reticulum and here it is known as sarcoplasmic reticulum. In the pigmented retinal cells, it exists in the form of tightly packed vesicles and tubes known as myeloid bodies.
Although the SER forms a continuous system with RER, it has different morphology. For example, liver cells, consist of a tubular network that pervades a major portion of the cytoplasmic matrix. These fine tubules are present in regions rich in glycogen and can be observed as dense particles, called glycosomes, in the matrix.
Glycosomes measure 50 to 200 nm in diameter and contain glycogen along with enzymes involved in the synthesis of glycogen. Many glycosomes attached to the membranes of SER have been observed by electron microscopy in the liver and conduction fiber of the heart.
2. Granular or Rough Endoplasmic Reticulum
The granular or rough type of endoplasmic reticulum possesses rough walls because the ribosomes remain attached with its membranes. Ribosomes play a vital role in the process of protein synthesis.
The granular or rough type of endoplasmic reticulum is found abundantly in those cells which are active in protein syntheses such as pancreatic cells, plasma cells, goblet cells, and liver cells. The granular type of endoplasmic reticulum takes basiophilic stain due to its RNA contents of ribosomes.
The region of the matrix containing a granular type of endoplasmic reticulum takes basiophilic stain and is named ergastoplasm, basiophilic bodies, chromophilic substances, or Nissl bodies by early cytologists.
In RER, ribosomes are often present as polysomes held together by mRNA and are arranged in typical “rosettes” or spirals. RER contains two transmembrane glycoproteins (called ribophorins I and II of 65,000 and 64,000 dalton MW, respectively), to which are attached the ribosomes by their 60S subunits.
Usually, the endoplasmic reticulum has no pores or annuli in it but in certain cases, the pores or annuli have been reported, e.g., ER of invertebrates, ovocytes, and spermatocytes of the vertebrates. These annuli resemble the pores or annuli of the nuclear membranes.
Like the annuli of nuclear membranes, it contains a diaphragm across it and possesses octagonal symmetry. The annulate lamellae (pores) of the ER arise by the evagination from the nuclear envelope and have their association with the ribosomes.
Function of Endoplasmic Reticulum.
The endoplasmic reticulum acts as a secretory, storage, circulatory and nervous system for the cell. It performs the following important functions:
A. Common Functions of smooth and Rough Endoplasmic Reticulum
1. The endoplasmic reticulum provides an ultrastructural skeletal framework to the cell and gives mechanical support to the colloidal cytoplasmic matrix.
2. The exchange of molecules by the process of osmosis, diffusion, and active transport occurs through the membranes of the endoplasmic reticulum. Like the plasma membrane, the ER membrane has permeases and carriers.
3. The endoplasmic membranes contain many enzymes which perform various synthetic and metabolic activities. Further, the endoplasmic reticulum provides an increased surface for various enzymatic reactions.
4. The endoplasmic reticulum acts as an intracellular circulatory or transporting system. Various secretory products of granular endoplasmic reticulum are transported to various organelles as follows: Granular ER→ agranular ER → Golgi membrane→ lysosomes, transport vesicles, or secretory granules.
Membrane flow may also be an important mechanism for carrying particles, molecules, and ions into and out of the cells. Export of RNA and nucleoproteins from the nucleus to cytoplasm may also occur by this type of flow.
5. The ER membranes are found to conduct intra-cellular impulses. For example, the sarcoplasmic reticulum transmits impulses from the surface membrane into the deep region of the muscle fibers.
6. The ER membranes form the new nuclear envelope after each nuclear division.
7. The sarcoplasmic reticulum plays a role in releasing calcium when the muscle is stimulated and actively transporting calcium back into the sarcoplasmic reticulum when the stimulation stops and the muscle must be relaxed.
B. Functions of Smooth Endoplasmic Reticulum
Smooth ER performs the following functions of the cell:
1. Synthesis of lipids.
SER performs the synthesis of lipids (e.g., phospholipids, cholesterol, etc.) and lipoproteins. Studies with radioactive precursors have indicated that the newly synthesized phospholipids are rapidly transferred to other cellular membranes with the help of specific cytosolic enzymes, called phospholipid exchange proteins.
2. Glycogenolysis and blood glucose homeostasis.
The process of glycogen synthesis (glycogenesis) occurs in the cytosol (in glycosomes). The enzyme UDPG-glycogen transferase, which is directly involved in the synthesis of glycogen by the addition of uridine diphosphate glucose (UDPG) to primer glycogen is bound to the glycogen particles or glycosomes.
SER is found related to glycogenolysis or breakdown of glycogen. An enzyme, called glucose- 6- phosphatase (a marker enzyme) exists as an integral protein of the membrane of SER (e.g., liver cell).
Generally, this enzyme acts as a glucogenic phosphohydrolase that catalyzes the release of free glucose molecules in the lumen of SER from its phosphorylated form in the liver. Thus, this process operates to maintain homeostatic levels of glucose in the blood for the maintenance of functions of red blood cells and nerve tissues.
3. Sterol metabolism.
The SER contains several key enzymes that catalyze the synthesis of cholesterol which is also a precursor substance for the biosynthesis of two types of compounds— the steroid hormones and bile acids:
(i) Cholesterol biosynthesis.
The cholesterol is synthesized from the acetate and its entire biosynthetic pathway involves about 20 steps; each step is catalyzed by an enzyme.
Out of these twenty enzymes, eleven enzymes are bounded to SER membranes, rest nine enzymes are the soluble enzymes located in the cytosol and mitochondria. Examples of SER-bound enzymes include HMG-Co A reductase and squalene synthetase.
(ii) Bile acid synthesis.
The biosynthesis of the bile acids represents a very complex pattern of enzymes and products. Enzymes involved in the biosynthetic pathway of bile acids are hydroxylases, mono-oxygenases, dehydrogenases, isomerases, and reductases.
For example, with the help of the enzyme cholesterol 7α-hydroxylase, the cholesterol is first converted into 7α- hydroxyl cholesterol, which is then converted into bile acids by the help of hydroxylase enzymes. The latter reaction requires NADPH and molecular oxygen and depends on the enzymes of Electron transport chains of SER such as cytochrome P-450 and NADPH-cytochrome-c-reductase.
(iii) Steroid hormone biosynthesis.
Steroid hormones are synthesized in the cells of various organs such as the cortex of the adrenal gland, the ovaries, the testes, and the placenta. For example, cholesterol is the precursor for both types of sex hormones—estrogen and testosterone—made in the reproductive tissues, and the adrenocorticoids (e.g., corticosterone, aldosterone, and cortisol) formed in the adrenal glands.
Many enzymes (e.g., dehydrogenases, isomerases, and hydroxylases) are involved in the biosynthetic pathway of steroid hormones, some of which are located in SER membranes and some occur in the mitochondria. This biosynthetic pathway has the following steps:
Protectively, the ER chemically modifies xenobiotics (toxic materials of both endogenous and exogenous origin), making them more hydrophilic, hence, more readily excreted. Among these materials are drugs, aspirin (acetyl-salicylic acid), insecticides, anesthetics, petroleum products, pollutants, and carcinogens (i.e., inducers of cancer; e.g., 3-4- benzopyrene and 3-methyl cholanthrene).
The enzymes involved in the detoxification of aromatic hydrocarbons are aryl hydroxylases. It is now known that benzopyrene (found in charcoal-broiled meat) is not carcinogenic, but under the action of the aryl hydroxylase enzyme in the liver, it is converted into 5, 6-epoxide, which is a powerful carcinogen.
A wide variety of drugs (e.g., phenobarbital), when administrated to animals, they bring about the proliferation of the ER membranes (first RER and then SER) and/ or enhanced activity of enzymes related to detoxification.
5. Other synthetic functions.
SER plays a role in the synthesis of triglycerides in intestinal absorptive cells and of visual pigments from vitamin A by a pigmented epithelial cell of the retina. In-plant cells, SER forms the surface where cellulose cell walls are being formed.
C. Functions of Rough Endoplasmic Reticulum
The major function of the rough ER is the synthesis of protein. It has long been assumed that proteins destined for secretion (i.e., export) from the cell or proteins to be used in the synthesis of cellular membranes are synthesized on rough ER-bound ribosomes, while cytoplasmic proteins are translated for the most part on free ribosomes.
In fact, the array of the rough endoplasmic reticulum provides extensive surface area for the association of metabolically active enzymes, amino acids, and ribosomes. There is more efficient functioning of these materials to synthesize proteins when oriented on a membrane surface than when they are simply in solution, mainly because chemical combinations between molecules can be accomplished in specific geometric patterns.
The membrane-bound ribosomes are attached with specific binding sites or receptors of the rough ER membrane by their large 60S subunit, with small or 40S subunit sitting on top like a cap. These receptors are membrane proteins that extend well into and possibly through the lipid bilayer.
The receptor proteins with bound ribosomes can float laterally like other membrane proteins and may facilitate the formation of the polysome and probably translation which requires that mRNA and ribosome move with respect to each other.
Further, the secretory proteins, instead of passing into the cytoplasm, appear to pass instead into the cisternae of the rough ER and are, thus, protected from protease enzymes of the cytoplasm.
It is calculated that about 40 amino acid residues long segment at the— COOH end of the nascent protein remains protected inside the tunnel of ‘free’ or ‘bound’ ribosomes and the rest of the chain, with—NH2 end is protected by the lumen of RER.
The passage of nascent polypeptide chain into the ER cisterna takes place during translation leaving only a small segment exposed to the cytoplasm at any one time.
How the polypeptide chain gets through the lipid bilayer is not so clear, but it is quite reasonable to propose that the membrane proteins serving as ribosomal receptors also have a very fine channel through its core that opens into the cisterna of the rough ER.
The chain may have great flexibility, permitting the amino acids to snake their way single file through the proposed pore. As soon as the growing polypeptide chain reaches the cisterna, it folds into its secondary and tertiary structures and is thus trapped in the cisterna of the rough ER.
The covalent addition of sugars to the secretory proteins (i.e., glycosylation) is one of the major biosynthetic functions of rough ER. Most of the proteins that are isolated in the lumen of RER before being transported to the Golgi apparatus, lysosomes, plasma membrane, or extracellular space, are glycoproteins (a notable exception is albumin).
In contrast, very few proteins in the cytosol (cytoplasmic matrix) are glycosylated and those that carry them have a different sugar modification. The process of protein glycosylation in the RER lumen is one of the most well-understood cell biological phenomena.
During this process, a single species of oligosaccharide (which comprises N-acetyl- glucosamine, mannose, and glucose, containing a total of 14 sugar residues) is transferred to proteins in the ER. Because it is always transferred to the NH2 group on the side chain of an asparagine residue of the protein, this oligosaccharide is said to be N-linked or asparagine-linked.
The transfer is catalyzed by a membrane-bound enzyme (i.e., glycosyltransferase) with its active site exposed on the luminal surface of the ER membrane. The preformed precursor oligosaccharide is transferred en bloc to the target asparagine residue in a single enzymatic step almost as soon as that residue emerges in the lumen of ER during protein translocation.
Since most proteins are co-translationally imported into the ER, N-linked oligosaccharides are almost always added during protein synthesis, ensuring maximum access to the target asparagine residues, which are present in the sequences–Asn-X-Ser or Asn-X-Thr (where X is amino acid except proline). These two sequences, thus, function as signals for N-linked glycosylation.
The precursor oligosaccharide is held in the ER membrane by a special lipid molecule, dolicol (the carrier). The oligosaccharide is linked to the dolicol by a high-energy pyrophosphate bond which activates the oligosaccharide for its transfer from the lipid to an asparagine side chain (i.e., it provides activation energy for the glycosylation reaction).
The oligosaccharide is built up sugar by sugar on the membrane-bound dolicol (towards the cytosolic side) prior to its transfer to a protein. Sugars are first activated in the cytosol (cytoplasmic matrix) by the formation of nucleotide-sugar intermediates (e.g., UDP-glucose, UDP-N-acetylglucosamine, and GDP-mannose), which then donates their sugar (directly or indirectly) to the lipid in an orderly sequence.
At some step of this process, the lipid-linked oligosaccharide is flipped from the cytosolic to the luminal side of the ER membranes. Dolicol is long and very hydrophobic: its 22 five-carbon units can span the thickness of the lipid bilayer more than three times so that the attached oligosaccharide is firmly anchored to the membrane.
While still in the RER lumen, three glucose residues and one mannose residue are quickly removed from the oligosaccharides of most glycoproteins. Such oligosaccharide “trimming” or “processing” continues in the Golgi apparatus.
If a glycoprotein is to contain terminal glucose, fucose, or sialic acid, then those sugars are added to the Golgi apparatus where the appropriate sugar transferase enzymes are localized.
The signal hypothesis.
The proteins for the secretion, the lysosomes, and the membrane formation, are synthesized on the membrane-bound ribosomes. The free and bound ribosomes were found to be continuously interchanging and show no differences between them.
The signal hypothesis was proposed by Blobel and Sabatini (1971) to explain how the ribosomes which are meant for the biosynthesis of secretory type proteins get specifically attached to RER membranes. According to this hypothesis, the mRNA is able to recognize free or bound ribosomes.
It is postulated that the mRNA for secretory proteins contains a set of special signal codons localized after the initial codon AUG. Once the ribosome “recognizes” the signal the ribosome becomes attached to the membrane of ER and the polypeptide penetrates.
It is also postulated that at the luminal surface there is a signal peptidase enzyme that removes the signal peptide. Thus, the mRNA produces a preprotein of larger molecular weight than the final protein. This signal peptide has between 15 to 30 amino acids which are generally hydrophobic.
Such a signal peptide probably establishes the initial association of the ribosome with the membrane, but some protein factors are involved. A signal recognition protein (SRP) complex binds to the nascent signal peptide and stops the translation until it reaches the ER membrane.
It is suggested that an SRP receptor or docking protein which is a pore-containing integral membrane protein of ER, removes the SRP block, allowing for the translocation of the polypeptide into the lumen of RER.
Origin of Endoplasmic Reticulum
The exact process of the origin of the endoplasmic reticulum is still unknown. But because membranes of ER resemble the nuclear membrane and plasma membrane and also at the telophase stage the ER membranes are found to form the nuclear envelope.
Therefore, it is normally assumed that the ER has originated by evagination of the nuclear membranes. Seikevitz and Palade (1960) have reported that the granular type of ER has originated first and later it synthesizes the agranular or smooth type of endoplasmic reticulum. The synthesis of membranes of ER is found to proceed in the following direction: RER → SER.
In fact, membrane biogenesis is a multi-step process involving, first, the synthesis of a basic membrane of lipid and intrinsic proteins and thereafter the addition of other constituents such as enzymes, specific sugars, or lipids.
The process by which a membrane is modified chemically and structurally is called membrane differentiation. The ER (especially SER) is the organelle containing the main phospholipid synthesizing and translocating enzymes (i.e., there occurs an intense flip-flop of lipid components).
The insertion of proteins into ER membranes occurs at the level of RER. Most of these proteins are formed on membrane-bound ribosomes. However, some of these are synthesized by free ribosomes in the cytosol (cytoplasmic matrix) and then are inserted into the membrane.
For example, the enzyme NAD cytochrome-b5-reductase is synthesized in the cytosol (cytoplasmic matrix) and then becomes incorporated in various parts of the endomembrane system (i.e., RER, SER, and Golgi apparatus) and in the outer mitochondrial membrane.