cell physiology Flashcards
MITOCHONDRIA STRUCTURE
Majority of cells contain many mitochondria that can occupy up to 25% of cell volume. Mitochondria has two membranes (see figure). The outer membrane is semi-permeable. The inner membrane is much less-permeable and contains most of the machinery required for oxidative phsophorylation. The surface area of inner membrane is greatly increased by a large number of infoldings known as cristae. The central space of mitochondria is known as matrix. Mitochondria have their own DNA that is located in the matrix. Since majority of mitochondrial proteins are encoded in nucleus, these proteins are synthesized in cytosol and imported via TOM (translocase of outer membrane) and TIM (translocase of inner membrane) complexes. Transport through TOM is passive, while transport through TIM is ATP-dependent. Mitochondria are very dynamic organelles that undergo constant fusion and fission. Fusion plays a key role in repairing damaged mitochondria. Fission is required for mitophagy. Fussion and fission are both dependent on cellular GTPases: Mfn and OPA1 (fusion), as well as Fis1 and Drp (fission).
TIM (translocase of inner membrane)
a complex of proteins found in the inner mitochondrial membrane of the mitochondria. Components of the TIM complex facilitate the translocation of proteins across the inner membrane and into the matrix (biology). to get to inner membrane you have TIM23 and 22. Both are highly gated. Proteins need MT targeting seq to open port. protein plugs port as is passes through then it closes immediately after. The port is small but this means that folded proteins cant fit. MT forces protein through with mtHsp 70 (diff than hsp 70) are ATPases binds to mt targeting seq, creates kink and pulls protein through hole, if doesnt fit
it denatures protein as it pulls it through. MT has a lot of heat shock proteins that refold proteins.
TOM (translocase of outer membrane)
a complex of proteins found in the outer mitochondrial membrane of the mitochondria. Its function is to allow movement of proteins through this barrier and into the intermembrane space of the mitochondrion. Most of the proteins needed for mitochondrial function are encoded by the nucleus of the cell. The outer membrane of the mitochondrion is impermeable to large molecules greater than 5000 Daltons. GIP recongizes proteins and shuttle it. this is not gated always open
OPA1
Dynamin-related GTPase required for mitochondrial fusion and regulation of apoptosis. May form a diffusion barrier for proteins stored in mitochondrial cristae. Proteolytic processing in response to intrinsic apoptotic signals may lead to disassembly of OPA1 oligomers and release of the caspase activator cytochrome C (CYCS) into the mitochondrial intermembrane space. Mutations in this gene can cause autosomal dominant optic atrophy
Mfn
Mfn plays a role in MT fusion. Has a coiled-coild domain that causes fusion similiarl to Snares. Mutations in this gene can cause Charcot-Marie-Tooth neuropathy 2A.
Fis1
Promotes fission
Drp
Dynamin related protein is a key component in mitochondrial fission and is required to promoted MT fission and apoptosis. The structural and functional similarities between dynamin and Drp1 suggest that Drp1 wraps around the constriction points of dividing mitochondria, analogous to dynamin collars at the necks of budding vesicles.
ELECTRON TRANSPORT AND GENERATION OF PROTON GRADIENT
Most of the free energy released during oxidation of glucose is retained in the reducing coenzymes such as NADH (nicotinamide adenine dinucleotide). During respiration in mitochondria electrons are released from NADH and transferred to O2 to form H2O. All of this electron transfer occurs across inner mitochondria membrane and is achieved by a four major protein complexes that are embedded in the inner mitochondria membrane. During electron transfer process, protons from mitochondrial matrix are pumped across the inner membrane. This transport generates a proton concentration gradient. In addition, proton transport also generates electric potential across the inner mitochondria membrane (more negative in matrix as compared to outside of mitochondria). Thus, the energy released from NADH is stored both as an electric potential and a proton concentration gradient in mitochondria.
ATP synthase
The ATP synthesis from ADP and Pi (inorganic phosphate) coupled to electron transfer from NADH is a major source of ATP (and energy) in animal cells. Proton gradient and electric potential is directly used to make ATP. That is achieved by the inner membrane embedded protein complex known as ATP synthase. ATP synthase consists from two main parts: F1 and F0. F0 protein complex spans the inner mitochondria membrane and forms a proton channel. F1 protein complex is bound to F0 and is an actual enzyme that makes ATP. F0 uses the energy of proton movement through the channel to generate ATP. Three protons are needed to generate one ATP molecule. Once made, ATP is transported out of mitochondria via ATP-ADP antiporter.
MITOCHOMNDRIA AND CELL DEATH
In addition to production of ATP, mitochondria is also involved in regulating cell death. Cell damage induces Bak/Bax-dependent permeabilization of outer mitochondria membrane, leading to cytochrome c release. Cytochrome c then binds to several cytpolasmic proteins forming protein complex known as apoptosome. Apoptosome activates caspases, thus intiating apoptosis (regulated cell death). During ischemic injury, mitochondria also promotes necrotic cell death. Ischemic injury results in MPTP-dependent permeabilization of inner and outer mitochondria membranes, resulting in cytochrome release and elimination of proton gradient. Lack of protein gradient blocks ATP production. Furthermore, in the absence of proton gradient, ATP synthase is converted into ATPase, thus using up available ATP. That leads to ATP depletion and necrosis.
reactive oxygen (ROS)
Damaged mitochondria not only is uncapable of producing ATP, but also generate excessive amounts of reactive oxygen (ROS). ROS causes cell damage and senescence by oxidating various cellular proteins, lipids and DNA. As the result, the mitochondria quality s strictly controlled at three levels. First, several mitochondrial proteases, such as mAAA, iAAA and Lon are responsible for recognizing and degrading misfolded proteins. Second, damage mitochondria can be “fixed” by fusing with healthy mitochondria, or can be eliminated by mitophagy. Finally, if mitochondria damage is extensive, mitochondria induces apoptotic cell death.
MITOCHONDRIAL DISEASES
Accumulation of mitochondria damage and increase in ROS is related to senescence and increased sensitivity to neuronal degeneration. In addition, several mutations of the proteins in mitochondria quality control pathways result in various neuropathys. Mutations in mitochondria fusion machinery causes autosomal dominant optic atrophy (OPA1 gene) and Charcot-Marie-Tooth neuropathy type 2A (Mfn2 gene). Mutation in mAAA protease causes hereditary spastic paraplegia.
arsenic
works by inhibiting oxidative phosphorylation and inhibiting ATP production.
Mitochondria Functions
1) Generation of ATP 2) Apoptosis 3) Regulation of intracellular Ca ions
autosomal dominant optic atrophy
a neuro-ophthalmic condition characterized by a bilateral degeneration of the optic nerves, causing insidious visual loss, typically starting during the first decade of life. The disease affects primary the retinal ganglion cells (RGC) and their axons forming the optic nerve, which transfer the visual information from the photoreceptors to the lateral geniculus in the brain. Two genes (OPA1, OPA3) encoding inner mitochondrial membrane proteins and three loci (OPA4, OPA5, OPA8) are currently known for DOA. Additional loci and genes (OPA2, OPA6 and OPA7) are responsible for X-linked or recessive optic atrophy. All OPA genes yet identified encode mitochondrial proteins embedded in the inner membrane and ubiquitously expressed, as are the proteins mutated in the Leber Hereditary Optic Neuropathy. OPA1 mutations affect mitochondrial fusion, energy metabolism, control of apoptosis, calcium clearance and maintenance of mitochondrial genome integrity. OPA3 mutations only affect the energy metabolism and the control of apoptosis.
Nicotinamide adenine dinucleotide (NAD)
a coenzyme found in all living cells. The compound is a dinucleotide, because it consists of two nucleotides joined through their phosphate groups. Nicotinamide adenine dinucleotide, like all dinucleotides, consists of two nucleotides joined by a pair of bridging phosphate groups. The nucleotides consist of ribose rings, one with adenine attached to the first carbon atom (the 1’ position) and the other with nicotinamide at this position. The nicotinamide is what is reduced.
oxidative phosphorylation
Glucose- 6P, goes to glycolases cycle leading to two pyruvate. (2ATP). not dependent on o2. Pyruvate- 3c goes to oxidatize phosphorylation when there is o2. without o2 just keep making pyruvate, which gets converted to lactic acid through reductions. Yeast can convert pyruvate into CO2 and ethanol (2c). Pyruvate is transported in to inner mem and enters citric cycle makeing actely coA and 3 nadh 1fadh
ATP synthesis
synthase part with three alpha and three beta gamma makes it spin in place. As it spins it goes through three phases making conformation shift of alpha subunit. 1) binds to ADP and Pi 2) combines to make ATP 3) low affinity to ATP than goes back to 1st conformation spinning is reduced by proton passing through
Cytochrome c
Cytochrome c is a component of the electron transport chain in mitochondria. Cytochrome c is also an intermediate in apoptosis, a controlled form of cell death used to kill cells in the process of development or in response to infection or DNA damage. cytocrome c is in mt. with damage, pore is made in MT and cytrochrome c is leaked out. Cell can still retrake at this point by reducing it and inactivating it. This is important in hypoxia for heart cells. Maybe you can save these cells by restoring blood flow.
Mitochondrial Permeability Transition, or MPT
The MPT pore is a protein pore that is formed in the inner membrane of the mitochondria under certain pathological conditions such as traumatic brain injury and stroke. Induction of the permeability transition pore can lead to mitochondrial swelling and cell death through apoptosis or necrosis depending on the particular biological setting. In some mitochondria, such as those in the central nervous system, high levels of Ca2+ within mitochondria can cause the MPT pore to open. Reactive oxygen species (ROS) are also produced as a result of opening the MPT pore. MPT can allow antioxidant molecules such as glutathione to exit mitochondria, reducing the organelles’ ability to neutralize ROS. In addition, the electron transport chain (ETC) may produce more free radicals due to loss of components of the ETC, such as cytochrome c, through the MPTP. Loss of ETC components can lead to escape of electrons from the chain, which can then reduce molecules and form free radicals. This event may lead mitochondria to swell and may cause the outer membrane to rupture, releasing cytochrome c.[34] Cytochrome c can in turn cause the cell to go through apoptosis (“commit suicide”) by activating pro-apoptotic factors. In cell damage resulting from conditions such as neurodegenerative diseases and head injury, opening of the mitochondrial permeability transition pore can greatly reduce ATP production, and can cause ATP synthase to begin hydrolysing, rather than producing, ATP, which can also lead to induction of MPTP
i-AAA
Catalytic subunit of the mitochondrial inner membrane i-AAA protease supercomplex required for mitochondrial inner membrane protein turnover. The protease is probably ATP-dependent. Important to maintain the integrity of the mitochondrial compartment.
m-AAA
m-AAA proteases are ATP-dependent proteolytic machines in the inner membrane of mitochondria which are crucial for the maintenance of mitochondrial activities. mutations in this protein can cause hereditary spastic paraplegia
Lon
In molecular biology, the Lon protease family is a family of proteases. In the eukaryotes the majority of the Lon proteases are located in the mitochondrial matrix.
Mitochondria Quality Control
MT produces more and more ROS as it lives longer, this is why fusion and fissionis important to mix up damaged proteins or you can fragment it and degrade it.
Epithelia Overview
Epithelia are the tissues that line body surfaces, body cavities, and the surfaces of internal tubes, ducts, and other spaces in organs. They comprise the main functional units of glands and several organs. In general, these tissues and their cells tend to share several properties: Epithelial cells are adherent to one another. The cells are arranged in one to several layers or sheets. Most epithelia are polarized (asymmetric): One side of an epithelium usually has a free outer or apical surface; plasma membranes of apical cells are directly exposed to fluids or the environment. The inner surface of an epithelium, called the basal or basolateral surface, is connected to underlying connective tissue. Many epithelial cells are themselves polarized, especially those in single-layer epithelia. A basal lamina (a sheet of extracellular material) lines and is attached to the basal surface and is also attached to elements of the underlying connective tissue. Epithelial tissues undergo turn-over or renewal: cells die and must be replaced by cell division and differentiation, processes that are driven by epithelial stem cells. Epithelial tissues are avascular (no direct blood supply). Nutrients and oxygen must diffuse through connective tissue and through the basal lamina to reach epithelial cells. Epithelial tissues are highly diverse; there are many different types. Moreover, within any given epithelium, there can be several different cell types.
functions of epithelia
A barrier that protects internal tissues against abrasion, infection, harmful chemicals or radiation (e.g. sunlight). Selective absorption and transport of various molecules from the environment. Selective secretion of various molecules and fluids. Movement of particles and mucous through passage ways. Biochemical modification of molecules (e.g. liver). Communication to (and from) other tissues and organs. Reception of sensory stimuli (e.g. smell, taste and touch). Different epithelia have distinct functions: Some carry out a very specific function (e.g. cells of the anterior pituitary gland secrete pituitary hormones) while some carry out many of the functions listed above (e.g. the intestinal epithelium protects, absorbs, secretes, and sends molecular signals to other organs/tissues).
endothelium
A tissue that faces blood and lymph, made of endothelial cells. Like other epithelia, these tissues are made of tightly adherent cells, have a free surface facing blood and lymph (or body cavities), and (usually) rest on a basal lamina.
mesothelium
the sheets of cells that line the enclosed internal spaces of the body cavities
Gross Anatomy of Epithelia
The most obvious epithelia at the gross level is the outer layer of the skin and hair (epidermis). However, epithelia are found in and around most organs and body structures. The moist linings of the internal passage ways (e.g. mouth, nose, GI tract, reproductive organs, etc.) are generally each called a mucosa (aka mucous membrane): The surface layer of all mucosae is an epithelium (more on this below). In addition, several organs are composed mostly of epithelial cells (e.g. liver, pancreas, kidney), where epithelia are the primary functional units.
Developmental Origins
Epithelia are derived from all three primary germ layers (ectoderm, mesoderm, endoderm), by a variety of mechanisms. During early development, many epithelia form, disassemble, and reform in new patterns until body morphology and organ morphogenesis are achieved. In particular, embryonic epithelia often disassemble and move into the mesenchymal (connective) tissues; there they may migrate to other locations to form new epithelia, or they may transform into distinct non-epithelial cell lineages that give rise to other tissues. This process is known as the epithelial to mesenchymal transition. Some embryonic epithelia serve as crucial signaling centers that organize pattern formation.
Orientations of epithelia to other tissues
Epithelial tissues line all surfaces that face the outside world (e.g. the air and the passageways of the gastrointestinal tract) and they form the surfaces of internal tubes, ducts, and secretory tissues. Therefore, throughout the body, they have a common general relationship to each of the other major tissue types; connective tissues (CT), blood vessels (and blood), muscles, and nerves. The apical side faces the space or outside world, the basal side is attached to the basal lamina, which in turn is attached to the underlying connective tissue (CT) (all by a series of multiple protein:protein interactions; described below). Blood vessels run through (CT), as do many nerves. Because epithelia are avascular, they must get O2 and nutrients from and communicate with blood within blood vessels (and lymph in lymphatic vessels) that course through the CT. Therefore, the CT directly underlying epithelia often houses many of the smallest blood vessels (capillaries) and lymphatic vessels (discussed in blood vessel lecture). Capillaries are connected to larger vessels that are typically deeper (more internal) within the CT. Muscles (both striated and smooth) are also imbedded within connective tissue layers that are also typical deeper (more internal) relative to the epithelial surface, although some small muscle tissue can be very close to the epithelial surface. These other tissue types (muscle, blood vessels, nerves) typically are surrounded by their own basal laminae that attach to their own neighboring connective tissues. Therefore, epithelia directly attach to connective tissues, and are separated from but attach to blood vessels, muscles, and many nerve endings by these connective tissues and by the different basal laminae that surround each tissue type. Of course there are exceptions to this general orientation: For example, some specific sensory nerve cells make intimate contacts with specific epithelial cells specialized for sensory input (e.g. taste buds in the tongue). Other exceptions are specialized immune system cells called dendritic cells, which infiltrate epithelia and can migrate out (and in), enter the connective tissue, and get into blood or lymph.
mucosae
many moist internal linings (mouth, nose, throat, GI tract, reproductive systems, etc.) that separate “outside” from “inside”. A mucosa is composed of two layers: (1) the outer epithelium and (2) the CT directly underneath, which is typically (unfortunately) called the lamina propria.
Lamina propriae
Lamina propriae typically contain alot of immune system cells and small blood vessels, because they survey and extract foreign or ingested materials, cells, and molecules from the environment and must efficiently transport and monitor. Deeper layers of CT are directly continuous with lamina propria CT, but will often have different properties and house other tissues (bigger vessels and muscles, nerve axon bundles, etc.). This deeper CT tissue is typically called the submucosa.
epidermis
The external skin also has it’s own nomenclature, but reflects analagous relationships: the epithelium of skin
dermis
directly underlying CT; deeper CT is called hypodermis and like submucosae has distinct composition and function (more in Dermatology). However, many tubes and ducts emanate from (and to) all of these epithelial surfaces, and can penetrate deep into the deep layers of CT (submucosa or hypodermis) leading to epithelial glands and organs, including large internal organs like the liver and pancreas. The outer surface is DIRECTLY contiguous with the spaces of most of these tube and ducts, and ALL of tube duct surfaces are lined by epithelia. Thus, epithelial sheets exist in all of these layers. However, even within these deep lying organs and epithelial tissues, the basic relationships hold: Space(lumen)-Epithelia-epithelial basal lamina-CT-other CT embedded tissues (blood vessels, muscle, nerves…..all usually with their own basal laminae that connects them with the CT).
Simple epithelia
have all cells arranged in a single layer or sheet.
Stratified epithelia
have more than one layer of cells in which cells of the outer layers do not directly contact the basal lamina. Stratified epithelia are named according to their outermost layer (e.g. stratified squamous epithelia have a squamous outer layer, though inner cells are often cuboidal). Further subclassification is sometimes used reflecting surface specializations of the outer layer of epithelial cells.
Pseudostratified epithelia
are a special case where some cells do not reach the free surface (giving a stratified appearance), but all directly rest on the basal lamina.
Squamous cells
are flattened cells,
cuboidal cells
are cube-like,
columnar
are taller than they are wide.
Transitional epithelia
(found around the bladder) are a special case: these epithelia are stratified, but when stretched change their shape from cuboidal to squamous, and appear to decrease the layering: this is indicative of a tightly adherent epithelium that is very resilient and stretchable.
Tight junctions
(or the zonula occludens): provide a highly selective barrier that limits or prevents diffusion of substances between epithelial cells. These also limit/control diffusion of membrane proteins through the plasma membrane bilayer. Key core proteins of tight junctions are occludins and claudins: In some epithelia the “tightness” of this barrier is regulated. Tight barriers ensure that substances absorbed or secreted must pass through the epithelial cell by specific transport pathways.link neighboring epithelial cells together tightly, forming a seal that separates apical and basolateral components. Proteins that compose tight junctions include occludins and claudins. They limit the movement of molecules between cells (or block it all together). This forces transport to go through the epithelia instead of between them. They are also important for membrane polarity.
Adherence junctions
(zonula adherens): Promote attachment, but also polarity, morphological organization and stem cell behavior within the epithelial sheet. Adherence junctions contain specific cadherins that link to actin filaments and other adapter/signaling proteins in the cytoplasm. Cadherins are transmembrane proteins with extracellular domains that interact with each other, and cytoplasmic tails that bind adapters and actin filaments. Some cadherin-associated proteins (e.g. beta catenin and protein kinases) control various aspects of epithelial polarity, development, and function. Important for the structural integrity of epithelia. Also control physiology and biology of the cells through signalling.
Desmosomes
(macula adherens): Promote mechanical strength and resist shearing forces and promote the structural organization of the epithelial sheet. Core components of these junctions include a different class of cadherins that link to intermediate filaments and other adapter proteins. a type of e-cadherin that interacts with intermediate filaments. They are prominent in skin epithelia
Gap junctions
Promote rapid communication between epithelial cells, through diffusion of ions and small molecules. exist in cardiac muscle cells form channels that allow passage via diffusion of small molecules between cells.
Epithelial Cell Biology
Epithelia cells adhere to and communicate with one another through various cell junctions.Various types of cell junctions exist to mediate and control cell attachments. These cell junctions are present on the lateral surfaces of epithelial cells, generally towards the apical side, and are essential for several purposes. Most epithelial cells, particularly those in simple epithelia, are polarized containing distinct regions of plasma membrane and cytoplasm. Polarization leads to formation of an outer domain, the apical domain, that faces the free surface, and an “inner” basal or basolateral domain that faces the basal lamina.
aspects of epithelial cell polarity
There are two general but key aspects of polarity: (1). The plasma membrane composition is locally segregated into domains: The plasma membrane in the apical domain contains distinct membrane proteins and a distinct phospholipid content compared to the membrane in the basal domains. Key membrane proteins that localize to specific domains include: transporter enzymes, ion channels, receptors for exocytosis/endocytosis, signaling receptors and effectors, and proteins that mediate cell-cell and cell-lamina attachments. The tight junction complexes tend to reside near the apical surface in these cells. Therefore, the membrane surrounding the lateral and basal side is often called the basal-lateral (or basolateral) membrane because the protein and lipid content is similar on both the lateral and basal sides. However, in many epithelial cells (e.g. columnar and cuboidal cells), there are differences between the lateral and basal membranes too. (2) The cytoplasm is polarized: The cytoskeleton (particularly microtubules) is asymmetric or polar in orientation. Organelles are distributed in a precise polarized pattern; secretory vesicles in the apical domain are usually different from those in the basal domain, and they move in specific directions.
Functions of Polarity
Epithelial cell polarity is crucial to allow unidirectional secretion and/or absorption of molecules to or from one side of the epithelium. Polarity is also necessary for “trans-epithelial” transport of ions and macromolecules from the apical to basal surfaces (or visa versa). The endocytosis of substances from one membrane region, followed by trans-cellular transport of the vesicles and their exocytosis from another membrane region is called transcytosis. In addition, polarity is important for localizing and orienting intercellular signaling either among epithelial cells, or between epithelial cells and other cell types.
Cell surface specializations of epithelial cells
The cell surface of epithelial cells, mostly the apical surface, can be highly modified in terms of structure and function. The modifications of interest include: apical surface modifications (Microvilli, Cilia). Basolateral surface modifications.
Microvilli
Cell surface extensions (protrusions) that contain actin bundles connected to cytoskeletal elements in the cell interior. The primary function of microvilli is to increase surface area, which greatly increases the rate/efficiency of membrane transport and secretion. The size and abundance of microvilli varies with cell type.
Cilia
Microtubule-containing extensions (protrusions). There are at least three kinds of cilia: (1) A primary cilium is a single (one/cell) non-motile microtubule-based extension found on many different epithelial cell types. Primary cilia organize and promote signal transduction systems (receptors and effectors) that control epithelial cell division, fate (differentiated state), and function. (2) Motile cilia are related microtubule extensions that move, and are found only on specific epithelial cell types; these cilia wave like a boat oar to move mucous and other materials along passageways. Thus they tend to be found in epithelia of the respiratory tract (to move junk and mucous out of the airways), and in the oviduct (to assist in moving ova towards the uterus). (3) Some highly specialized non-motile sensory cilia are not motile and appear to function in sensory reception (e.g. in vestibular hair cells of the ear). These sensory cilia are likely specific variations of primary cilia, specialized to house sensory (touch, taste) systems that are connected to the central nervous system. Mutations in proteins common to cilia or their support structures result in a large set of diseases called ciliopathies.
Basolateral surface modifications
infolds and outfolds of the basolateral membrane are also found in many epithelial cells, though they seem to lack the structural organization of the apical microvilli and cilia. Again, these folds increase surface area and are likely to be seen in cells that transport heavily to or from their basolateral surface.
stereocilia
An unusual type of microvillus, unfortunately called stereocilia, are found in the epididymis and in sensory cells in the ear. These are extremely long, actin-filled microvilli, and are not related to cilia. In the ear, stereocilia function in the reception of sound.
basal lamina
A basal lamina is a thin sheet of extracellular material that underlies the basal surface of each epithelial tissue. Basal laminae also surround many other cell and tissue types as well; including the endothelium of blood vessels, muscle and nerve tissue. Basal laminae are formed by a special type of network-forming collagen, a fibrous protein. This type of collagen forms sheets of thin fibers that are interwoven with a variety of other extracellular glycoproteins. Network-forming collagen (especially “type IV”) and some other glycoproteins including laminins and entactin are common to most basal laminae. However, many other glycoproteins are specific to different basal laminae. Thus, the molecular structures of basal laminae in different epithelial tissues are quite diverse. It is thought that most of the basal lamina components are actually synthesized and secreted by epithelial cells. However, some extracellular components of the connective tissue (made by fibroblasts) bind to and possibly organize elements in the basal lamina. Basal laminae both separate epithelial cells from the underlying connective tissue, and attach epithelial cells to the extracellular matrix of the connective tissue. Epithelial cells directly connect to basal laminae by attachment of hemidesmosomes and focal adhesions on the basal surface of the epithelial cell to components of the basal lamina. The key class of proteins that form these connections are the integrins (distinct proteins from the cadherins that mediate cell-cell linkage).
Basal laminae functions
- They mediate attachment of epithelia to the underlying connective tissue. 2. Basal laminae often contribute to selective filtration of substances diffusing to or from the epithelia. 3. They are necessary for the establishment and maintenance of epithelial cell polarity. 4. They can serve as specific “highways” for the migration of cells through connective tissue. 5. They provide a barrier to movement of invading microbes or cancerous cell into other tissues. 6. They control the gene expression of cells to affect their proliferation or development. 7. They control the development, morphogenesis, and organization of epithelial cells, providing a sort of “tissue scaffolding” function. Thus, they are critical to the repair of epithelial tissue following damage by injury or disease.
hemidesmosomes
Anchors intermediate filaments in a cell to the basal lamina. core protein is integrin, interacts with intermediate filaments
focal adhesions
Focal adhesions serve as the mechanical linkages to the ECM, and as a biochemical signaling hub to concentrate and direct numerous signaling proteins at sites of integrin binding and clustering. Focal adhesions regulate epithelial polarity and function through signaling mechanisms, and probably are particularly important during re-establishment of an epithelium following injury or disease, or during the normal process of cell replacement (turn-over). like adherins junctions. Organize signalling complexes controlling polarity, gene expression, transport, etc within epithelial cells
integrins
Integrins couple the ECM outside a cell to the cytoskeleton (in particular, the microfilaments) inside the cell. Which ligand in the ECM the integrin can bind to is defined by which α and β subunits the integrin is made of. Among the ligands of integrins are fibronectin, vitronectin, collagen, and laminin. The connection between the cell and the ECM may help the cell to endure pulling forces without being ripped out of the ECM. Specific integrins of hemidesmosomes connect to intermediate filaments in the epithelial cell and provide strength to the epithelial-connective tissue attachment. Different integrins of focal adhesions connect to actin filaments inside the epithelial cell.
stem cells
Epithelial tissues undergo substantial turn-over throughout adult life. Consequently, epithelia generally contain stem cells that are capable of cell division, and that serve to (i) self renew: regeneration of stem cells with each division, and (ii) produce differentiated cell types specific to each epithelia. In essence, these are the same processes that occur during early development, and in fact many of the same regulatory pathways critical to development are re-used to control stem cell-driven tissue regulation. Like all stem cells, epithelial stem cells replace cells that die and can flexibly regulate (change) the form and function of tissues. Stem cells produce differentiated cell types by cell division coupled with specific pathways of cell specification imposed on some of their daughter cells. In most stem cell-tissue systems, cell division and specification goes through several steps.
transit amplifying cells
Many stem cells produce daughters that also proliferate themselves (undergo cell division cycles), often at faster rates; these transitional intermediates are called transit amplifying cells. These transit amplifying cells then produce differentiated cells, either directly or through several steps of specification. At each step, specific changes in protein expression patterns and activities occur.
cell lineage
Many epithelia are composed of multiple cell types arranged in precise ways. The developmental state of stem cells determines the constellation of cell types, their relative proportions, and their arrangements. A specific stem cell type, its intermediate progeny, and their differentiated progeny are collectively called a cell lineage.
epithelial division and differentiation
Both division and differentiation in stem cell lineages are tightly regulated by cell to cell communication and cell signaling pathways, which are activated or inhibited by physiological and developmental cues. Because of tight regulation, stem cells typically divide very slowly, or infrequently in some tissues. Consequently, they are usually much less abundant than their differentiated offspring. Loss of this regulation is generally at the heart of the many diseases of stem cell systems. A typical cell signaling pathway consists of (i) an extracellular ligand (signal) secreted by signaling cells, (ii) a receptor in receiving cells that binds and is activated/inactivated by the ligand, (iii) downstream effector proteins in the cytoplasm and nucleus, (iv) numerous modulator proteins that promote or suppress pathway components. Cell signaling of epithelial stem cells may be local involving ligands secreted by cells within the same epithelia or by cells in neighboring connective tissue. Epithelial stem cells and/or their progeny respond to these ligands, and may also secrete their own ligands. The structural organizations described above serve to constrain, spatially orient, and control these signaling events. In some cases, however, epithelial turn-over may also be controlled by long-distance signaling between tissues. Long distance signals are typically secreted from endocrine glands into the blood stream (described below), or may be produced by the nervous system or other tissues.
epithelial signaling pathways
Some of the most important signaling systems that control epithelial stem cell renewal and differentiation are the same or very similar to the core group of signaling pathways that control development of many tissues during embryogenesis, fetal development, and childhood. These include the Wnt, Sonic Hedgehog, TGFβ, Notch, and receptor tyrosine kinase family (RTK; for example the EGF receptors), and FGF receptor pathways. Some key principles of these signaling pathways are important to understand: i. Each pathway is used by multiple, very distinct stem cell systems in different organs/tissues. Although some pathway proteins may be unique to certain stem cell subsets, most pathway components are shared by several distinct stem cell lineages (stem cell lineages=stem cells and all their daughters, granddaughters, etc). ii. A single signaling pathway often triggers different developmental outcomes in different stem cell lineages. These differences result from combinations of: (a) Different developmental histories of each stem cell type (cells can inherit and maintain unique differences in nuclear (chromatin) and cytoplasmic states that are generated earlier in development, which dramatically impact how a cell responds to a ligand) (b) Different environments stem cells reside in (or to put another way, the different combinations of signaling inputs a cell is exposed to). (c) Differences in the levels of the extracellular ligands, receptors, and/or downstream components. Defects in the regulation, expression, or structure of signaling components (ligands, receptors, effectors) lead to disease, particularly cancer. Thus, these pathways are prime targets for drug therapies. Indeed, drugs that specifically target several components of these pathways are now in use as anti-tumor agents (one example: Tarceva (erlotinib) inactivates the EGF receptor and is commonly used to treat lung and pancreatic cancers).
Epithelial Glands
Glands of the body are derived from epithelial tissue. They are found in many sites of the body, often associated with the epithelia of body surfaces and cavities, and as components of specific organs. The primary function of glands is secretion of either specific bioactive molecules (e.g. hormones), complex fluids (e.g. mucous, sweat), or both. There are two major types of epithelial glands :Exocrine glands and Endocrine glands. Glands secrete their substances in one of two general ways. (1) Exocytosis: glands secreting by this familiar, and most common, mechanism are sometimes called merocrine or apocrine glands. (2) Total cell disintegration, which leads to the entire cellular contents becoming part of the secretion; glands of this sort are less common, and are sometimes called holocrine glands (examples include the secretion of oily sebum onto hair and skin).
Exocrine glands
secrete materials onto epithelia-lined surfaces or the outside world. Exocrine glands secrete materials onto the apical side of epithelial surfaces. They are generally multicellular. However, some epithelial cells in the lining of internal passageways function as unicellular glands secreting materials directly onto those surfaces (e.g. goblet cells). All multicellular exocrine glands have two main components: The Secretory units and Ducts. In general, exocrine glands of the body “tubes” (alimentary canal, respiratory, and urogenital systems) are of three general types: mucous: from which a viscous glycoprotein-rich fluid is produced, serous: from which a watery fluid, containing salts and some specific proteins, is produced, and mixed: from which both kinds of secretions are produced. The presence of mucous or serous secreting cells can be seen in histological sections. As might be expected, secretory epithelial cells of these glands secrete materials unidirectionally from their apical surface (with the exception of holocrine secretion).
Endocrine glands
secrete substances into the blood stream. Endocrine glands have no ducts and secrete substances to the blood stream. These glands produce mostly specific hormones that act over relatively long distances to control tissue function. Endocrine glands are generally organized as clumps or chords of cells that are embedded with and surrounded by connective tissue containing extensive capillary networks. Each clump is, of course, surrounded by a basal lamina. Blood capillaries are imbedded in connective tissue on the other side of the epithelial basal lamina. Therefore, hormone molecules must cross the basal surface and basal lamina of the epithelium (and finally the basal lamina and endothelial layer of the capillary) to reach the blood stream. For this reason, most endocrine cells secrete these molecules from the basolateral membrane.
The Secretory units
clumps of secretory epithelial cells, which produce and secrete the bulk of the secretion. Secretory units can be organized into bowl or flask-shaped lobules called alveoli or acini, and are called alveolar or acinar glands, or they may be organized into tubes, and are called tubular glands. Some glands have both tubular and alveolar character and are called tubuloalveolar. Some glands can have a single secretory unit or they may have multiple branched units.
Ducts
tubular structures that emanate from the secretory units. Ducts function as passageways to conduct secretions to their destinations, although their epithelia can also modify the secretion content by secretory and ion transport properties of duct cells. Glands that possess a single duct are classified as simple glands, whereas those with multiple branched ducts are called compound glands. Further subclassifications exist reflecting whether the glands is coiled, and whether the secretory units are multilobed (branched) or not.
Glandular secretions
The secretions from both exocrine and endocrine glands are generally regulated by the autonomic nervous system (i.e. by direct neuronal stimulation), by hormones from blood, or by both. Exocrine glands often secrete continuously at a low rate but can be triggered to greatly increase the volumes of secretion by these mechanisms. For example, the sight or smell of food stimulates serous secretions from salivary glands, and mucous and serous secretions from gastrointestinal glands. Endocrine secretions tend to be tightly regulated by hormonal or neuronal stimulation. Defects in this regulation lead to disease.
Epithelia and Medicine
Epithelial tissues are targets (direct or indirect) of a large variety of diseases. Just a few general examples include diabetes, various liver and kidney diseases, lung diseases (e.g. emphysema, cystic fibrosis), GI disease (e.g. gastric ulcers, inflammatory bowel diseases), skin diseases, hearing diseases, and ciliopathies. Examples in the gastrointestinal tract include ulcerative colitis, in which extensive ulceration and destruction of the absorptive epithelium occurs. A disease called pemphigus is an autoimmune disease in which antibodies are produced against components of desmosomes in skin, leading to extensive blistering. This broad clinical importance reflects the essential roles of epithelia in organ/tissue function, their relatively high rate of turn-over requiring stem cell-driven renewal, and their exposure to external damage (UV light, pathogens, chemicals). Epithelia are also important to consider in wound repair. Injuries to skin epithelia that do not extensively damage the basal epithelial stem cells and basal lamina can often be repaired by intrinsic mechanisms, but if the basal skin layers and basal laminae are severely compromised they are replaced by scar tissue; for example severe burns may require skin grafts to replace a damaged basal lamina and restore a population of basal skin epithelial stem cells.
carcinomas
Cancers of epithelial origin. The most common cancers by far are derived from epithelial tissue, probably because of damage exposure and their high stem-cell driven turnover rates. Tumors usually first develop within an epithelial sheet, but they can become invasive and spread (metastasize) to other parts of the body. Often, carcinomas retain some properties of their tissue of origin. Thus, the diagnosis and treatment of different carcinomas is aided by histological appearance of the tumor and knowledge of epithelial biology. It is now believed that carcinomas result from defects in regulatory pathways that control epithelial stem cells or their progeny during tissue development and maintenance. Targeted regulatory pathways include signaling systems that directly control development (e.g. Wnt, EGF or Notch signaling systems), internal cell cycle control factors, and factors that control DNA damage repair and apoptosis (programmed cell death).
adenocarcinomas
cancers derived from glandular epithelium
Cystic Fibrosis (CF)
an autosomal recessive condition that results in a syndrome of chronic sinopulmonary infections and nutritional abnormalities secondary to malabsorption. It is one of the most common lethal genetic diseases in the United States, with an incidence of approximately 1:3000 among Caucasians and 1:9,000 in Hispanic populations. Lung disease is the major cause of morbidity and mortality. Most individuals with CF develop obstructive lung disease associated with chronic infection that leads to progressive loss of pulmonary function and ultimately death from respiratory failure. Currently, the median life expectancy for people living with CF in the US is 37 years. CF is caused by a defect in an ATP-binding cassette transporter gene on chromosome 7 that encodes for the CF Transmembrane Conductance Regulator (CFTR) protein. The CFTR protein is an epithelial chloride channel that also has signaling effects on other membrane channels. The most common mutation is the F508del, although approximately 1500 other disease-causing mutations in the CF gene have been identified. Gene mutations lead to defects or deficiencies in CFTR, causing problems in salt and water movement across cell membranes, resulting in abnormally thick secretions in various organ systems and critically altering host defense in the lung. CF is now included in the newborn screening program for all states in the US, and many countries world-wide.
Typical features of CF
Greasy, bulky, malodorous stools; failure to thrive due to exocrine pancreatic insufficiency, Recurrent respiratory infections with opportunistic bacteria (e.g. Staphylococcus aureus, Pseudomonas aeruginosa and Burkholderia cepacia), Chronic sinus infections, Digital clubbing on examination. Bronchiectasis, Sweat chloride > 60 mmol/L.
Clinical Findings of Cystic Fibrosis
Due to widespread adoption of CF newborn screening in the past decade most children in the US with CF are identified soon after birth. However, older children born prior to newborn screening, and those missed by the screen (false negative rate ranges from 1-5%) may present with clinical symptoms later in life. The most common presentation of CF is failure to thrive. Children typically fail to gain weight despite good appetite and have frequent, bulky, foul-smelling, oily stools. These symptoms are the result of severe exocrine pancreatic insufficiency, the failure of the pancreas to produce sufficient digestive enzymes to allow breakdown and absorption of fats and protein. Pancreatic insufficiency occurs in more than 85% of persons with CF. Infants with undiagnosed CF may also present with hypoproteinemia with or without edema, anemia, and deficiency of the fat-soluble vitamins A, D, E, and K, because of ongoing steatorrhea. CF should also be considered in infants and children who present with severe dehydration and hypochloremic metabolic alkalosis. Other findings that should prompt a diagnostic evaluation for CF include unexplained bronchiectasis, rectal prolapse, nasal polyps, chronic sinusitis, and unexplained pancreatitis or cirrhosis. Approximately 15% of newborns with CF present at birth with meconium ileus, a severe intestinal obstruction resulting from inspissation of tenacious meconium in the terminal ileum. Infants with meconium ileus should be treated presumptively as having CF until a sweat test or genotyping can be obtained. From a respiratory standpoint, clinical manifestations include productive cough, wheezing, chronic bronchitis and recurrent pneumonias, progressive obstructive airways disease, exercise intolerance, dyspnea, and hemoptysis. Chronic airway infection with bacteria, including S aureus and H influenzae, often begins in the first few months of life, even in asymptomatic infants. Eventually, Pseudomonas aeruginosa becomes the predominant pathogen. Acquisition of the characteristic mucoid Pseudomonas is associated with a more rapid decline in pulmonary function. Chronic infection leads to airflow obstruction and progressive airway and lung destruction resulting in bronchiectasis. An acute change in respiratory signs and symptoms from the subject’s baseline is generically termed a pulmonary exacerbation. Clinically, an exacerbation is typically manifested by increased cough and sputum production, decreased exercise tolerance, malaise, and anorexia. These symptoms are usually associated with decreased measures of lung function, new chest radiographic findings, or both. Treatment for pulmonary exacerbations generally consists of antibiotics and augmented airway clearance and often requires hospitalization for 1-2 weeks.
CF treatment
Management of exocrine pancreatic insufficiency with enzyme supplementation, high calorie, high protein, and high fat diet, and fat soluble vitamin supplements. Daily salt supplement due to salt losses in sweat. Airway clearance treatment to loosen and remove thick mucus from airways: Daily percussive therapy (Chest PT, vest therapy); Recombinant human DNAse (Pulmozyme), an inhaled mucolytic agent; Inhaled hypertonic saline; and Bronchodilators. Antibiotic therapy targeting common CF related bacteria such as P. aeruginosa: Inhaled tobramycin (TOBI); Inhaled aztreonam (Cayston); and Oral or intravenous antibiotics. Anti-inflammatory treatments: Ibuprofen (high dose) and Chronic azithromycin. CFTR modulators: ivacaftor and lumacaftor
Ivacaftor (CFTR potentiator)
Only approved for people age 6 years or older with at least one G551D mutation or gating mutation
Lumacaftor/ Ivacaftor combination therapy
(CFTR corrector/potentiator) for people ages 12 years or older with 2 copies of the F508del mutation is currently undergoing review with the FDA.
