You Can Recognize the Process of Pinocytosis When _____

Specialized Phagocytic Cells Can Ingest Large Particles

Phagocytosis is a special form of endocytosis in which large particles such as microorganisms and dead cells are ingested via large endocytic vesicles chosen phagosomes. In protozoa, phagocytosis is a course of feeding: large particles taken upward into phagosomes end upward in lysosomes, and the products of the subsequent digestive processes pass into the cytosol to exist utilized every bit food. However, few cells in multicellular organisms are able to ingest such large particles efficiently. In the gut of animals, for example, the particles of food are cleaved down extracellularly and their hydrolysis products are imported into cells.

Phagocytosis is important in most animals for purposes other than nutrition, and information technology is mainly carried out by specialized cells—and then-called
professional phagocytes.
In mammals, three classes of white blood cells act as professional phagocytes—macrophages, neutrophils, and dendritic cells. These cells all develop from hemopoietic stem cells (discussed in Chapter 22), and they defend us against infection by ingesting invading microorganisms. Macrophages also have an important function in scavenging senescent cells and cells that have died past apoptosis (discussed in Affiliate 17). In quantitative terms, the latter part is by far the most important: our macrophages phagocytose more than x^eleven
senescent red blood cells in each of us every day, for instance.

Whereas the endocytic vesicles involved in pinocytosis are small-scale and uniform, phagosomes have diameters that are adamant past the size of the ingested particle, and they tin be most as big equally the phagocytic prison cell itself (Figure xiii-39). The phagosomes fuse with lysosomes inside the cell, and the ingested fabric is then degraded. Any indigestible substances will remain in lysosomes, forming
residuum bodies.
Some of the internalized plasma membrane components never reach the lysosome, because they are retrieved from the phagosome in ship vesicles and returned to the plasma membrane.

To be phagocytosed, particles must start demark to the surface of the phagocyte. Withal, non all particles that bind are ingested. Phagocytes have a variety of specialized surface receptors that are functionally linked to the phagocytic mechanism of the cell. Unlike pinocytosis, which is a constitutive procedure that occurs continuously, phagocytosis is a triggered procedure, requiring that receptors be activated that transmit signals to the jail cell interior and initiate the response. The best-characterized triggers are antibodies, which protect us by bounden to the surface of infectious microorganisms to form a coat in which the tail region of each antibody molecule, called the Fc region, is exposed on the exterior (discussed in Chapter 24). This antibody glaze is recognized by specific
Fc receptors
on the surface of macrophages and neutrophils, whose binding induces the phagocytic cell to extend pseudopods that engulf the particle and fuse at their tips to class a phagosome (Figure 13-twoscore).

Several other classes of receptors that promote phagocytosis have been characterized. Some recognize
complement
components, which collaborate with antibodies in targeting microbes for devastation (discussed in Chapter 25). Others directly recognize oligosaccharides on the surface of certain microorganisms. Still others recognize cells that take died past apoptosis. Apoptotic cells lose the asymmetric distribution of phospholipids in their plasma membrane. As a event, negatively charged phosphatidylserine, which is unremarkably confined to the cytosolic leaflet of the lipid bilayer, is at present exposed on the outside of the cell, where it triggers the phagocytosis of the dead prison cell.

Remarkably, macrophages will also phagocytose a variety of inanimate particles—such equally glass, latex beads, or asbestos fibers—even so they do not phagocytose live animal cells. It seems that living animal cells display “don’t-eat-me” signals in the form of prison cell-surface proteins that demark to inhibiting receptors on the surface of macrophages. The inhibitory receptors recruit tyrosine phosphatases that antagonize the intracellular signaling events required to initiate phagocytosis, thereby locally inhibiting the phagocytic process. Thus phagocytosis, like many other cell processes, depends on a balance betwixt positive signals that activate the process and negative signals that inhibit it.

Pinocytic Vesicles Grade from Coated Pits in the Plasma Membrane

Nearly all eucaryotic cells continually ingest bits of their plasma membrane in the form of pocket-sized pinocytic (endocytic) vesicles, which are later returned to the cell surface. The charge per unit at which plasma membrane is internalized in this procedure of pinocytosis varies between cell types, but it is usually surprisingly large. A macrophage, for example, ingests 25% of its ain volume of fluid each hour. This means that it must ingest three% of its plasma membrane each infinitesimal, or 100% in near half an hour. Fibroblasts endocytose at a somewhat lower charge per unit (1% per minute), whereas some amoebae ingest their plasma membrane even more than quickly. Since a cell’southward surface area and volume remain unchanged during this process, it is articulate that the same corporeality of membrane that is being removed past endocytosis is being added to the cell surface by
exocytosis, the converse process, as we discuss later. In this sense, endocytosis and exocytosis are linked processes that tin exist considered to constitute an
endocytic-exocytic wheel.

The endocytic part of the cycle often begins at clathrin-coated pits. These specialized regions typically occupy about two% of the total plasma membrane area. The lifetime of a clathrin-coated pit is short: within a infinitesimal or then of being formed, it invaginates into the cell and pinches off to form a clathrin-coated vesicle (Figure 13-41). It has been estimated that nigh 2500 clathrin-coated vesicles leave the plasma membrane of a cultured fibroblast every minute. The coated vesicles are even more transient than the coated pits: within seconds of being formed, they shed their coat and are able to fuse with early endosomes. Since extracellular fluid is trapped in clathrin-coated pits equally they invaginate to form coated vesicles, any substance dissolved in the extracellular fluid is internalized—a process chosen
fluid-phase endocytosis.

Non All Pinocytic Vesicles Are Clathrin-coated

In addition to clathrin-coated pits and vesicles, in that location are other, less well-understood mechanisms by which cells can form pinocytic vesicles. One of these pathways initiates at
caveolae
(from the Latin for “little cavities”), originally recognized by their ability to transport molecules across endothelial cells, which form the inner lining of blood vessels. Caveolae are present in the plasma membrane of near cell types, and in some of these they are seen as deeply invaginated flasks in the electron microscope (Figure xiii-42). They are thought to form from
lipid rafts,
which are patches of the plasma membrane that are peculiarly rich in cholesterol, glycosphingolipids, and GPI-anchored membrane proteins (meet Figure 12-57). The major structural protein in caveolae is
caveolin, a multipass integral membrane protein that is a member of a heterogeneous protein family.

In contrast to clathrin-coated and COPI- or COPII-coated vesicles, caveolae are thought to invaginate and collect cargo proteins by virtue of the lipid composition of the calveolar membrane, rather than by the assembly of a cytosolic protein coat. Caveolae pinch off from the plasma membrane and tin evangelize their contents either to endosome-like compartments or (in a procedure called transcytosis, which is discussed later) to the plasma membrane on the contrary side of a polarized cell. Some animal viruses also enter cells in vesicles derived from caveolae. The viruses are commencement delivered to an endosome-like compartment, from where they are moved to the ER. In the ER, they extrude their genome into the cytosol to start their infectious cycle. It remains a mystery how material endocytosed in caveolae-derived vesicles can end up in so many dissimilar locations in the cell.

Cells Import Selected Extracellular Macromolecules by Receptor-mediated Endocytosis

In most animal cells, clathrin-coated pits and vesicles provide an efficient pathway for taking upward specific macromolecules from the extracellular fluid. In this process, called receptor-mediated endocytosis, the macromolecules demark to complementary transmembrane receptor proteins, accumulate in coated pits, and then enter the cell as receptor-macromolecule complexes in clathrin-coated vesicles (run across Figure 13-41). Receptor-mediated endocytosis provides a selective concentrating machinery that increases the efficiency of internalization of detail ligands more than a hundredfold, and then that even pocket-sized components of the extracellular fluid tin be specifically taken upward in large amounts without taking in a correspondingly large volume of extracellular fluid. A particularly well-understood and physiologically important example is the process whereby mammalian cells take up cholesterol.

Many animals cells have up cholesterol through receptor-mediated endocytosis and, in this way, larn nearly of the cholesterol they require to brand new membrane. If the uptake is blocked, cholesterol accumulates in the blood and tin contribute to the formation in blood vessel walls of
atherosclerotic plaques,
deposits of lipid and gristly tissue that can cause strokes and heart attacks by blocking claret menstruation. In fact, it was through a study of humans with a strong genetic predisposition for
atherosclerosis
that the mechanism of receptor-mediated endocytosis was first clearly revealed.

Near cholesterol is transported in the blood as cholesteryl esters in the form of lipid-protein particles known every bit
low-density lipoproteins (LDL)
(Figure thirteen-43). When a prison cell needs cholesterol for membrane synthesis, it makes transmembrane receptor proteins for LDL and inserts them into its plasma membrane. One time in the plasma membrane, the LDL receptors diffuse until they acquaintance with clathrin-coated pits that are in the procedure of forming (Figure xiii-44A). Since coated pits constantly pinch off to class coated vesicles, any LDL particles bound to LDL receptors in the coated pits are rapidly internalized in coated vesicles. Later on shedding their clathrin coats, the vesicles deliver their contents to early on endosomes, which are located virtually the jail cell periphery. Once the LDL and LDL receptors encounter the low pH in the endosomes, LDL is released from its receptor and is delivered via late endosomes to lysosomes. There the cholesteryl esters in the LDL particles are hydrolyzed to free cholesterol, which is now available to the cell for new membrane synthesis. If too much free cholesterol accumulates in a cell, the jail cell shuts off both its ain cholesterol synthesis and the synthesis of LDL receptor proteins, so that it ceases either to make or to take up cholesterol.

This regulated pathway for the uptake of cholesterol is disrupted in individuals who inherit defective genes encoding LDL receptor proteins. The resulting high levels of claret cholesterol predispose these individuals to develop atherosclerosis prematurely, and many die at an early age of heart attacks resulting from coronary artery disease. In some cases, the receptor is defective birthday. In others, the receptors are defective—in either the extracellular binding site for LDL or the intracellular bounden site that attaches the receptor to the glaze of a clathrin-coated pit (run into Figure 13-44B). In the latter case, normal numbers of LDL-binding receptor proteins are nowadays, but they fail to become localized in the clathrin-coated regions of the plasma membrane. Although LDL binds to the surface of these mutant cells, it is not internalized, directly demonstrating the importance of clathrin-coated pits in the receptor-mediated endocytosis of cholesterol.

More than than 25 unlike receptors are known to participate in receptor-mediated endocytosis of different types of molecules, and they all apparently use the aforementioned clathrin-coated-pit pathway. Many of these receptors, like the LDL receptor, enter coated pits irrespective of whether they take bound their specific ligands. Others enter preferentially when bound to a specific ligand, suggesting that a ligand-induced conformational modify is required for them to activate the signal sequence that guides them into the pits. Since most plasma membrane proteins fail to become full-bodied in clathrin-coated pits, the pits must function as molecular filters, preferentially collecting certain plasma membrane proteins (receptors) over others.

Point peptides guide transmembrane proteins into clathrin-coated pits by binding to the adaptins. Despite a mutual function, their amino acid sequences vary. A common endocytosis signal consists of only four amino acids Y-Ten-X-Ψ, where Y is tyrosine, 10 any polar amino acid, and Ψ a hydrophobic amino acid. This curt peptide, which is shared by many receptors, binds directly to one of the adaptins in clathrin-coated pits. By dissimilarity, the cytosolic tail of the LDL receptor contains a unique signal (Asn-Pro-Val-Tyr) that apparently binds to the same adaptin protein.

Electron-microscope studies of cultured cells exposed simultaneously to different labeled ligands demonstrate that many kinds of receptors can cluster in the aforementioned coated pit. The plasma membrane of ane clathrin-coated pit tin can probably accommodate up to thou receptors of contrasted varieties. Although all of the receptor-ligand complexes that use this endocytic pathway are plain delivered to the same endosomal compartment, the subsequent fates of the endocytosed molecules vary, equally we hash out next.

Endocytosed Materials That Are Non Retrieved From Endosomes End Upwards in Lysosomes

The endosomal compartments of a jail cell can be complex. They can be fabricated visible in the electron microscope by calculation a readily detectable tracer molecule, such as the enzyme peroxidase, to the extracellular medium and leaving the cells for various lengths of fourth dimension to have it up by endocytosis. The distribution of the molecule after its uptake reveals the endosomal compartments as a set of heterogeneous, membrane-enclosed tubes extending from the periphery of the cell to the perinuclear region, where it is often close to the Golgi apparatus. Two sequential sets of endosomes can be readily distinguished in such labeling experiments. The tracer molecule appears within a minute or and then in early endosomes, only beneath the plasma membrane. After 5–xv minutes, it moves to late endosomes, close to the Golgi apparatus and virtually the nucleus. Early and late endosomes differ in their protein compositions; they are associated with different Rab proteins, for example.

As mentioned earlier, the interior of the endosomal compartment is kept acidic (pH ~six) by a vacuolar H⁺
ATPase in the endosomal membrane that pumps H⁺
into the lumen from the cytosol. In general, later endosomes are more than acidic than early endosomes. This acidic surround has a crucial role in the function of these organelles.

We take already seen how endocytosed materials that attain the belatedly endosomes become mixed with newly synthesized acid hydrolases and end upward being degraded in lysosomes. Many molecules, even so, are specifically diverted from this journey to devastation. They are recycled instead from the early endosomes back to the plasma membrane via transport vesicles. Just molecules that are not retrieved from endosomes in this fashion are delivered to lysosomes for degradation.

Specific Proteins Are Removed from Early Endosomes and Returned to the Plasma Membrane

The
early endosomes
course a compartment that acts as the main sorting station in the endocytic pathway, merely as the
cis
and
trans
Golgi networks serve this function in the biosynthetic-secretory pathway. In the acidic environment of the early endosome, many internalized receptor proteins change their conformation and release their ligand, merely equally the M6P receptors unload their cargo of acid hydrolases in the even more acidic late endosomes. Those endocytosed ligands that dissociate from their receptors in the early on endosome are usually doomed to destruction in lysosomes, along with the other soluble contents of the endosome. Some other endocytosed ligands, nonetheless, remain jump to their receptors, and thereby share the fate of the receptors.

The fates of the receptor proteins—and of whatever ligands remaining spring to them—vary according to the specific type of receptor. (one) Most receptors are recycled and return to the same plasma membrane domain from which they came; (2) some proceed to a different domain of the plasma membrane, thereby mediating a process called
transcytosis; and (3) some progress to lysosomes, where they are degraded (Figure 13-45).

The LDL receptor follows the beginning pathway. It dissociates from its ligand LDL in the early endosome and is recycled to the plasma membrane for reuse, leaving the discharged LDL to be carried to lysosomes (Figure 13-46). The recycling vesicles bud from long, narrow tubules that extend from the early endosomes. It is likely that the geometry of these tubules helps the sorting process. Considering tubules have a large membrane expanse enclosing a small volume, membrane proteins tend to accumulate in that location. Ship vesicles that render material to the plasma membrane begin budding from the tubules, just tubular portions of the early endosome also compression off and fuse with one another to form
recycling endosomes,
a way-station for the traffic between the early endosomes and the plasma membrane. During this process, the tubules and and so the recycling endosome continuously shed vesicles that return to the plasma membrane.

The
transferrin receptor
follows a similar recycling pathway, just it also recycles its ligand. Transferrin is a soluble protein that carries atomic number 26 in the blood. Cell-surface transferrin receptors deliver transferrin with its bound atomic number 26 to early endosomes past receptor-mediated endocytosis. The depression pH in the endosome induces transferrin to release its jump atomic number 26, but the iron-free transferrin itself (chosen apotransferrin) remains spring to its receptor. The receptor-apotransferrin complex enters the tubular extensions of the early endosome and from there is recycled dorsum to the plasma membrane (Figure xiii-47). When the apotransferrin returns to the neutral pH of the extracellular fluid, information technology dissociates from the receptor and is thereby freed to selection upwardly more iron and begin the cycle again. Thus, transferrin shuttles dorsum and along between the extracellular fluid and the endosomal compartment, avoiding lysosomes and delivering the iron that cells need to grow to the cell interior.

The 2nd pathway that endocytosed receptors can follow from endosomes is taken both by opioid receptors (see Figure 13-47) and by the receptor that binds
epidermal growth factor (EGF).
EGF is a small, extracellular point poly peptide that stimulates epidermal and various other cells to divide. Unlike LDL receptors, EGF receptors accumulate in clathrin-coated pits only subsequently bounden EGF, and well-nigh of them practice not recycle but are degraded in lysosomes, forth with the ingested EGF. EGF binding therefore starting time activates intracellular signaling pathways and so leads to a decrease in the concentration of EGF receptors on the cell surface, a process called
receptor down-regulation
that reduces the cell’southward subsequent sensitivity to EGF (discussed in Affiliate 15).

Multivesicular Bodies Form on the Pathway to Tardily Endosomes

It is still uncertain how endocytosed molecules movement from the early to the late endosomal compartment and so as to cease upwardly in lysosomes. A electric current view is that portions of the early on endosomes migrate slowly along microtubules toward the cell interior, shedding tubules of cloth to exist recycled to the plasma membrane. The migrating endosomes enclose big amounts of invaginated membrane and internally pinched-off vesicles and are therefore called
multivesicular bodies
(Figure 13-48). It is unknown whether multivesicular bodies somewhen fuse with a belatedly endosomal compartment or if they fuse instead with each other to become tardily endosomes. At the stop of this pathway, the late endosomes are converted to lysosomes as a result of their fusion with hydrolase-bearing transport vesicles from the
trans
Golgi network and their increased acidification (Figure xiii-49).

The multivesicular bodies carry specific endocytosed membrane proteins that are to be degraded just exclude others that are to be recycled. As function of the protein-sorting process, specific proteins—for case, the occupied EGF receptor described previously—selectively partition to the invaginating membrane of the multivesicular bodies (Figure 13-50). In this mode, the receptors, likewise as any signaling proteins strongly bound to them, are rendered fully accessible to the digestive enzymes that volition degrade them (see Effigy 13-50).

Membrane proteins that are sorted into the internal membrane vesicles of a multivesicular body are first covalently modified with the small poly peptide ubiquitin. Unlike multi-ubiquitylation which typically targets substrate proteins for degradation in proteasomes (discussed in Chapter 6), ubiquitin tagging for sorting into the internal membrane vesicles of a multivesicular trunk requires the addition of simply a single ubiquitin molecule that is added to activated receptors while still at the plasma membrane. The ubiquitin tag facilitates the uptake of the receptors into endocytic vesicles and is then recognized again by proteins that mediate the sorting process into the internal membrane vesicles of multivesicular bodies. In addition, membrane invagination in multivesicular bodies is regulated past a lipid kinase that phosphorylates phosphatidylinositol. The phosphorylated caput groups of these lipids are idea to serve as docking sites for the proteins that mediate the invagination process. Local modification of lipid molecules is thus another way in which specific membrane patches tin be induced to modify shape and destiny.

In addition to endocytosed membrane proteins, multivesicular bodies also comprise most of the soluble content of early endosomes destined for digestion in lysosomes.

Macromolecules Tin can Exist Transferred Across Epithelial Cell Sheets past Transcytosis

Some receptors on the surface of polarized epithelial cells transfer specific macromolecules from one extracellular space to another by transcytosis (Figure thirteen-51). These receptors are endocytosed and then follow a pathway from endosomes to a different plasma membrane domain (see Figure 13-46). A newborn rat, for example, obtains antibodies from its mother’southward milk (which aid protect it against infection) by transporting them across the epithelium of its gut. The lumen of the gut is acidic, and, at this low pH, the antibodies in the milk bind to specific receptors on the apical (absorptive) surface of the gut epithelial cells. The receptor-antibiotic complexes are internalized via clathrin-coated pits and vesicles and are delivered to early on endosomes. The complexes remain intact and are retrieved in transport vesicles that bud from the early endosome and subsequently fuse with the basolateral domain of the plasma membrane. On exposure to the neutral pH of the extracellular fluid that bathes the basolateral surface of the cells, the antibodies dissociate from their receptors and somewhen enter the newborn’s bloodstream.

The transcytotic pathway from the early endosome to the plasma membrane is not direct. The receptors commencement move from the early endosome to an intermediate endosomal compartment, the
recycling endosome
described previously (run across Figure 13-51). The multifariousness of pathways that different receptors follow from endosomes implies that, in addition to binding sites for their ligands and binding sites for coated pits, many receptors too possess sorting signals that guide them into the advisable type of transport vesicle leaving the endosome and thereby to the appropriate target membrane in the cell.

A unique property of a recycling endosomes is that the exit of membrane proteins from the compartment tin can be regulated. Thus, cells tin can conform the flux of proteins through the transcytotic pathway co-ordinate to need. Although the mechanism of regulation is uncertain, information technology allows recycling endosomes an important part in adjusting the concentration of specific plasma membrane proteins. Fat cells and muscle cells, for example, contain big intracellular pools of the glucose transporters that are responsible for the uptake of glucose beyond the plasma membrane. These proteins are stored in specialized recycling endosomes until the cell is stimulated by the hormone insulin to increase its rate of glucose uptake. And so transport vesicles bud from the recycling endosome and deliver large numbers of glucose transporters to the plasma membrane, thereby greatly increasing the rate of glucose uptake into the cell (Effigy xiii-52).

Epithelial Cells Have 2 Distinct Early Endosomal Compartments But a Common Late Endosomal Compartment

In polarized epithelial cells, endocytosis occurs from both the
basolateral domain
and the
apical domain
of the plasma membrane. Cloth endocytosed from either domain first enters an early endosomal compartment that is unique to that domain. This arrangement allows endocytosed receptors to be recycled back to their original membrane domain, unless they comprise signals that mark them for transcytosis to the other domain. Molecules endocytosed from either plasma membrane domain that are non retrieved from the early endosomes end up in a common belatedly endosomal compartment almost the jail cell eye and are eventually degraded in lysosomes (Figure 13-53).

Whether cells contain a few continued or many unconnected endosomal compartments seems to depend on the jail cell type and the physiological land of the jail cell. Like many other membrane-enclosed organelles, endosomes of the aforementioned blazon can readily fuse with ane another (an example of homotypic fusion, discussed earlier) to create large continuous endosomes.

Summary

Cells ingest fluid, molecules, and particles by endocytosis, in which localized regions of the plasma membrane invaginate and pinch off to form endocytic vesicles. Many of the endocytosed molecules and particles end up in lysosomes, where they are degraded. Endocytosis occurs both constitutively and as a triggered response to extracellular signals. Endocytosis is so all-encompassing in many cells that a large fraction of the plasma membrane is internalized every hour. To brand this possible, most of the plasma membrane components (proteins and lipid) that are endocytosed are continually returned to the cell surface by exocytosis. This large-scale endocytic-exocytic bicycle is mediated largely by clathrin-coated pits and vesicles.

Many cell-surface receptors that bind specific extracellular macromolecules become localized in clathrin-coated pits. As a issue, they and their ligands are efficiently internalized in clathrin-coated vesicles, a process called receptor-mediated endocytosis. The coated endocytic vesicles speedily shed their clathrin coats and fuse with early endosomes.

Nigh of the ligands dissociate from their receptors in the acidic surroundings of the endosome and eventually terminate up in lysosomes, while about of the receptors are recycled via transport vesicles back to the cell surface for reuse. Simply receptor-ligand complexes can follow other pathways from the endosomal compartment. In some cases, both the receptor and the ligand cease upwards being degraded in lysosomes, resulting in receptor down-regulation. In other cases, both are transferred to a different plasma membrane domain, and the ligand is thereby released by exocytosis at a surface of the cell unlike from that where it originated, a process called transcytosis. The transcytosis pathway includes recycling endosomes, where endocytosed plasma membrane proteins can be stored until they are needed.