3. INTRODUCTION
Endocytosisandexocytosisaretheprocessesbywhichcellsmovematerialsin
to or out of the cell that are too large to directly pass through the lipid
bilayerof the
cellmembrane.Largemolecules,microorganismsandwasteproductsareso
me of the substances moved through the cell membrane via exocytosis
and endocytosis
Endocytosisandexocytosisaretheprocessesbywhichcellsmovematerialsin
to or out of the cell tht are tool argetodirectly passthroughthe
lipidbilayerof the
cellmembrane.Largemolecules,microorganismsandwasteproductsareso
me of the substances moved through the cell membrane via
exocytosisandendocytosis Cell membranes are semi-permeable,
meaning they allow certain small
moleculesandionstopassivelydiffusethroughthem.Othersmallmoleculesar
e abletomaketheirwayintooroutof thecell
throughcarrierproteinsorchannels.
4. ENDOCYTOSIS
Cell membranes are semi permeable, meaning that certain small
molecules and ions are able to pass through them passively. Other
small molecules are able to enter or exit the cell through carrier
proteins or channels. Endocytosis is when a part of the cell membrane
encloses extracellular fluids and various molecules or microorganisms
and breaks off into a vesicle, which is then transported within the cell.
5. Endocytosis serves many purposes, including:Taking innutrients for
cellular growth, function and repair: Cells needmaterials like proteins
and lipids to function. Capturing pathogens or other unknown
substances that may endanger the organism:When pathogens like
bacteria are identified by the immunesystem, they are engulfed by
immune cells to be destroyed. Disposing of old or damaged cells:
Cellsmustbesafelydisposedofwhentheystop functioningproperly
toprevent damage toother cells. These cells are eliminated
throughendocytosis
6. TYPES OF ENDOCYTOSIS
1. Phagocytosis
Phagocytosis is the process by which cells engulf large particles
(e.g., bacteria, cell debris, or even whole intact cells). When the
particle binds to receptors on the surface of the phosphorylated cell,
it triggers the extension of the pseudopodia, an actin-based
translocation of the cell surface (see next section for receptor
mediated endocytosis). The pseudopodia eventually surround the
particle, and their membranes fuse together to form a large (~ 0.25
μm) intracellular vase (phagosome). The phagosomes then bind to
lysosomes to form phagolysomes, in which the ingested material
undergoes digestion by lysosomal acidic and hydrolase enzymes.
During maturation, part of the internalized membrane proteins is
recycled back to the plasma membrane.
7. The ingestion of large particles by phagocytosis plays distinct roles in different
kinds of
Cells. Phagocytosis is a mechanism used by many amoebas to collect food
particles, such as bacteria or other protozoans. Among multicellular
organisms, phagocytosis plays key roles
are to rid the body of aging or damaged cells and to act as a barrier against
incoming microbes. The two main types of white blood cells that perform
phagocytosis in mammals are neutrophils and macrophages, which are
commonly referred to as "professional phagocytes." In the body's defense
mechanisms, neutrophils and macrophages both play vital roles in removing
microorganisms from contaminated tissues. Furthermore, macrophages
remove aging or dead cells from bodily tissues. This activity's scope is
strikingly demonstrated by the macrophages.
8. 2. Pinocytosis :
Pinocytosis is a form of endocytosis involving fluids containing many
solutes. In humans, this process occurs in cells lining the small intestine
and is used primarily for absorption of fat droplets. In endocytosis the cell
plasma membrane extends and folds around desired extracellular
material, forming a pouch that pinches off creating an internalized vesicle.
9. Pinocytosis, which translates as "to drink," is the mechanism by which the
cell absorbs the liquids and any dissolved tiny molecules. The cell
membrane folds during this process, forming tiny pockets that are used to
hold dissolved materials and cellular fluid. After that, the cell membrane
contracts to create vesicles that will hold the liquid and tiny molecules
inside the cell. When a lysosome and such a vesicle combine, the
molecules are broken down by the digestive enzymes, allowing the vesicle
to be recycled.
10. 3. Receptor –Mediated Endocytosis :
In contrast to phagocytosis, which plays only specialized roles, pinocytosis
is common among eukaryotic cells. The best-characterized form of this
process is receptor-mediated endocytosis, which provides a mechanism
for the selective uptake of specific macromolecules. The macromolecules
to be internalized first bind to specific cell surface receptors. These
receptors are concentrated in specialized regions of the plasma
membrane, called clathrin-coated pits. These pits bud from the membrane
to form small clathrin-coated vesicles containing the receptors and their
bound macromolecules (ligands). The clathrin-coated vesicles then fuse
with early endosomes, in which their contents are sorted for transport to
lysosomes or recycling to the plasmamembrane.
11. Protein Trafficking In Endocytosis :
Clathrin-coated vesicles quickly shed their coats after internalizing and
combine with early endosomes, which are tubular-tipped vesicles that are
found at the cell's edge. The specificity of fusion of endocytic vesicles with
endosomes is determined by
interactionsbetween complementary pairs of transmembrane proteins of
the vesicle and
targetmembranes (v-SNAREs and t-SNAREs) and by Rab GTP-
bindingproteins.
Early endosomes function as a sorting compartment where molecules that
are taken up by endocytosis are either sent to lysosomes for degradation
or recycled to the plasma membrane. Furthermore, endocytosed proteins
can be transferred by the early endosomes of polarized cells between
distinct plasma membrane domains, such as between the basolateral and
apical domains of epithelial cells.
12. An important feature of early endosomes is that they maintain an acidic internal pH as the
result of the action of a membrane H+ pump. This acidic pH leads to the dissociation of many
ligands from their receptors within the early endosome compartment. Following this
uncoupling, the receptors and their ligands can be transported to different intracellular
destinations. A classic example is provided by LDL, which dissociates from its receptor within
early endosomes. The receptor is then returned to the plasma membrane via transport
vesicles that bud from the tubular extensions of endosomes. In contrast, LDL is transported
(along with other soluble contents of the endosome) to lysosomes, where its degradation
releases cholesterol. Recycling to the plasma membrane is the major fate of membrane
proteins taken up by receptor-mediated endocytosis, with many receptors (like the LDL
receptor) being returned to the plasma membrane following dissociation of their bound ligands
in early endosomes. The recycling of these receptors results in the continuous internalization
of their ligands. Each LDL receptor, for example, makes a round-trip from the plasma
membrane to endosomes and back approximately every 10 minutes. The importance of the
recycling pathway is further emphasized by the magnitude of membrane traffic resulting from
endocytosis. As already noted, approximately 50% of the plasma membrane is internalized by
receptor-mediated endocytosis every hour and must therefore be replaced at an equivalent
rate. Most of this replacement is the result of receptor recycling; only about 5% of the cell
surface is newly synthesized per hour.
13. EXOCYTOSIS
Exocytosis is the process by which secretory vesicles fuse with the plasma
membrane, releasing their contents into the extracellular space and allowing
fresh proteins and lipids to be incorporated into the membrane. All cells have
the ability to undergo constitutive exocytosis, or it can be regulated in
specialized cells like neurons, endocrine, and exocrine cells.
Usually, but not always, a rise in the cytosolic free Ca2+ concentration
initiates regulated exocytosis. In response to cell stimulation, a tiny
percentage of regulated secretory vesicles in neurons and endocrine cells
are prepared to fuse with the plasma membrane; however, the majority are
held in reserve for future stimulation by attachment to a filamentous network
of synapsins (in neurons) or actin (in endocrine cells). The kinetics of
regulated exocytosis vary widely, and
14. It is likely that several different Ca(2+)-binding proteins are involved in regulated
exocytosis, with synaptotagminapparently essential for fast exocytosis at
synapses. GTP-binding proteins of both the monomeric and heterotrimeric forms
are involved in exocytosis, although their precise role is unclear. Intense current
interest focuses on the idea that the molecular mechanism of vesicle docking and
fusion is conserved from yeast to mammalian brain. The SNARE hypothesis
postulates that vesicle SNAREs (synaptobrevinand homologues) mediate
docking by binding to target SNAREs (syntaxin/SNAP-25 and homologues),
whereupon SNAPs and NSF bind to elicit membranefusion.
15. LDL TRANSPORT (LOW DENSITY
LYPOPROTEIN)
Exocytosis is the process by which secretory vesicles fuse with the plasma
membrane, releasing their contents into the extracellular space and allowing
fresh proteins and lipids to be incorporated into the membrane. Exocytosis can
be constitutive (all cells)
orregulated (specialized cells such as neurons, endocrine and exocrine cells).
Generally speaking, but not always, a rise in the cytosolic free Ca2+
concentration initiates regulated exocytosis. The majority of regulated secretory
vesicles in neurons and endocrine cells are stored in reserve for future
stimulation by attachment to a filamentous network of synapsins (in neurons) or
actin (in endocrine cells). A tiny percentage of these vesicles are ready to fuse
with the plasma membrane in response to cell stimulation. Kin's regulated
exocytosis differs significantly. Fat
16. The endocytic vesicle encloses an extracellular ligand that is transported to
intracellular locations, usually lysosomes, where it is broken down. The
receptor's role in one type of receptor-mediated endocytosis is to internalize
low-density lipoprotein (LDL), or plasma LDL.
Mammalian cell membranes require cholesterol as a constituent because it
controls the biophysical characteristics of the membrane and the actions of
proteins that are attached to the membrane.
Between membranes, cholesterol is unevenly distributed, with the plasma
membrane having a higher concentration of cholesterol. Furthermore, the
trans-Golgi network (TGN) and endocytic recycling endosomes both contain a
large amount of cholesterol.
17. The LDL is delivered to lysosomes where it is degraded and its
cholesterol is released for use in the synthesis of membranes, steroid
hormones and bile acids. Three recent advances in the LDL receptor
system are reviewed:
(1) The development of a method for purifying the receptor to
apparent homogeneity and the demonstration that the LDL-binding
site is contained within a glycoprotein of relative molecular mass
164000 and an acidic isoelectric point of 4.6.
(2) The production of monoclonal antibodies directed against the
receptor and the use of these antibodies as probes for receptor-
mediated endocytosis .
(3) The use of monovalent carboxylic ionophores (such as monensin)
to demonstrate by immunofluorescence that the LDL receptor
enters the cell together with LDL, after which it
recyclestothesurface.
18. Niemann-Pick disease type C (NPC) is characterized by lysosomal storage of cholesterol and
gangliosides, which results from defects in intracellular lipid trafficking. Most studies of NPC1 have
focused on its role in intracellular cholesterol movement. Our hypothesis is that NPC1 facilitates the
egress of cholesterol from late endosomes, which are where active NPC1 is located. When NPC1
is defective, cholesterol does not exit late endosomes; instead, it is carried along to lysosomal
storage bodies, where it accumulates. In this study, we addressed whether cholesterol is
transported from endosomes to the plasma membrane before reaching NPC1-containing late
endosomes. Our study was conducted in Chinese hamster ovary cell lines that display the classical
NPC biochemical phenotype and belong to the NPC1 complementation group. We used three
approaches to test whether low density lipoprotein (LDL)-derived cholesterol enroute to NPC1-
containing organelles passes through the plasma membrane. First, we used cyclodextrins to
measure the arrival of LDL cholesterol at the plasma membrane and found that the arrival of LDL
cholesterol in a cyclodextrin-accessible pool was significantly delayed in NPC1 cells. Second, the
movement of LDL cholesterol to NPC1-containing late endosomes was assessed and found to be
normal in Chinese hamster ovary mutant 3-6, which exhibits defective movement of plasma
membrane cholesterol to intracellular membranes. Third, we examined the movement of plasma
membrane cholesterol to the endoplasmic reticulum and found that this pathway is intact in NPC1
cells, i.e. it does not pass through NPC1-containing late endosomes. Our data suggest that in
NPC1 cells LDL cholesterol traffics directly through endosomes to lysosomes, bypassing the
plasma membrane, and is trapped there because of dysfunctionalNPC1
19. Endocytic vesicles carry the extracellular ligand, which is broken down at intracellular
locations, usually lysosomes. The receptor's role in receptor-mediated endocytosis is
to internalize low-density lipoprotein (LDL) in the case of the plasma LDL system.
An essential component of mammalian cell membranes, cholesterol controls the
biophysical characteristics of the membrane as well as the actions of proteins that are
attached to the membrane.
The plasma membrane is where cholesterol is enriched and is found in a
heterogeneous distribution across membranes. Furthermore, there is a lot of
cholesterol in the trans-Golgi network (TGN) and endocytic recycling endosomes[1].
The sterol regulatory element-binding protein-SREBPcleavage-activating protein
system is the primary regulatory machinery for detecting and regulating cellular
cholesterol levels, and it is located here because the cholesterol content of the ER is
low [2]. Reduced ER cholesterol content is attained by esterification and rapid
exchange to more cholesterol-rich membranes. toafattyacidbyacyl-CoA
cholesterol acyl transferase, a.k.a. sterol O-acyl transferase, followed by
depositionof theproducedcholesteryl estersinlipiddroplets
[3]. Aside from the liver and central nervous system cells, which are effective in
synthesising cholesterol,
