Unit IV: ECM, Epithelia, & Cilia Flashcards
4 classes of molecules in the ECM
Glycosaminoglycans (GAGs), usually covalently linked to proteins to form proteoglycans
Fibrous proteins; ex: Collagen and Elastin
Proteins: Multidomain adaptor proteins (i.e. Fibronectin, Laminin) + enzymes, proteases, etc.
Water & solutes
GAG structure
Glycosaminoglycans (GAGs) are unbranched polysaccharide chains composed of disaccharide repeats, usually an amino sugar and a uronic acid; these sugars can be sulfated differently, creating functional diversity
GAGs are usually linked to proteins to form proteoglycans, except for Hyaluronan
6 Properties of GAGs & Function
Highly negatively charged Sulfated Hydrated Extended conformation Fill large volumes relative to their mass Readily form gels through salt bridges
GAGs function to create the hydrogel backbone of the ECM and to regulate signaling molecules in different tissues
Structure of proteoglycans
PGs are comprised of GAGs covalently linked to proteins via a special serine-tetrasaccharide linkage; PGs contain at least one GAG chain but may contain many; they may be membrane-bound, anchored via a TM core protein domain or GPI linkage
Collagen
Collagen is the most abundant protein in mammals (25% of protein mass); there are 20 different types of collagen
Collagen I is the most common form, found in connective tissues
Collagen IV is found in the basal lamina
Fibronectin
A type of multi-domain adapter protein found in the ECM; fibronectin is a dimeric glycoprotein that binds to integrins via a Fibronectin Type III repeat region containing an Arg-Gly-Asp (RGD) sequence; other domains bind collagen and self-associate to form insoluble fibronectin fibrils
Laminin
A type of multi-domain adapter protein found in the basal lamina; comprised of an alpha, beta, and gamma subunit; contains binding sites for integrins and other ECM proteins, including collagen IV
Metalloproteases (MMP)
Secreted, extracellular proteases which function to turn over the ECM; MMPs are secreted in in a pro-form and cleaved into their active form; important in tissue remodeling during development and cellular migration
Cell Adhesion Molecules (CAMs)
Cadherins
Ig Super Family (IgSF)
Integrins
Cadherins
Transmembrane glycoproteins with an extracellular domain that binds to cadherin molecules expressed on other cells (homophilic binding), stabilized by Ca2+; cytosolic C-terminal tails interact with alpha and beta catenin, which bind to actin, anchoring cadherin to the cytoskeleton
B-catenin is frequently mutated in epithelial to mesenchymal transition, leading to loss of cell-cell adhesion and invasion/metastasis
Ig Superfamily (IgSF) CAMs
Single pass TM glycoproteins that engage in homophilic binding mediated by multiple Ig domains
Integrins
Transmembrane glycoprotein composed of alpha/beta heterodimers that dimerize in different combinations to bind different ECM molecules (laminin, fibronectin, collagen, etc.); integrin cytosolic domains anchor to actin microfilaments
CAM-associated cytoplasmic proteins
The cytoplasmic tails of CAMs are linked to the cytoskeleton via proteins such as alpha and Beta catenin; mutation in these CAM-associated cytoplasmic proteins can cause loss of cell-cell adhesion and contact inhibition
Collagen Formation & Structure
Pro-collagen triple helix monomers are made within the ER; they are glycosylated and hydroxylated prior to secretion; in the ECM, cleavage of the N and C termini by procollagen peptidase allows for assembly and cross-linking of long collagen fibers
Cleavage of termini produces N-telo-peptides, which can be detected in blood & urine as a marker for bone disease
Role of CAMs in cells signaling
Ligand-bound integrin recruits pro-survival kinases (i.e. src) to initiate cell survival pathways
In suspension, cells die because cell survival pathways are not activated AND apoptosis pathways are activated via Caspase 8
Constitutive activation of src in cancer allows cellular proliferation independent of adhesion signaling
Role of collagen in ECM
Fibrillar collagen (I) forms thick, long fibers that are linked to each other and to other proteins by thin, fibril-associated collagen; networking collagens (IV) form thin sheets the basal lamina
Elastin
Protein component of the ECM; elastin monomers form randomly coiled higher order structures through covalent linkages; they are able to stretch and recoil
Main components of the cilium
Outer sheath Transporters Axoneme Linkage Domain Base anchor
Motile Cilium Axoneme Structure
Formed by a circular array of 9 microtubule doublets (A-B tubules) anchored within the basal body at the (-) end with the (+) end projecting into the cilia tip; also contains a central pair of microtubules - this is a 9 + 2 structure; motile cilia also contain axonemal dynein arms which beat between the double microtubules
Primary Cilum Axoneme Structure
9 + 0 axoneme; formed by a circular array of 9 microtubule doublets (A-B microtubules) anchored within the basal body at the (-) end; generally lacks the central pair of microtubules as well as dynein arms
Primary Ciliogenesis
Centriole duplication occurs during the G1 to S-phase transition and the older centriole becomes the mother basal body in the subsequent G1 phase, during which ciliogenesis occurs; the distal end of the basal body is capped by a ciliary vesicle and microtubules assemble into the ciliary vesicle before the entire structure fuses with the plasma membrane of the cell
Basal Body Structure & Function
Basal bodies are the core anchors from which cilia are formed; basal bodies are microtubule-rich cylinders formed from nine triplet microtubules (A-B-C tubules)
Transition Zone
Links the basal body to the axoneme; this is the “gatekeeper” region that limits diffusion of membrane and soluble proteins into and out of the cilum, maintaining the unique identity of the ciliary membrane
Intraflagellar Transport (IFT)
Cargo needed for the assembly and maintenance of cilia is transported along the axoneme; kinesin and IFT-B complexes direct anterograde transport; retrograde transport is mediated by dynein and IFT-A.
General function of cilia in cell signaling
Cilia concentrate signals with a high receptor surface : volume ratio, and also localizes the signal within a discreet domain of the cell; cilia sense physical stimuli (mechanical, temperature, gravity), light, and chemical stimuli.
Role of cilia in left-right axis establishment
During embryogenesis, an invagination of ciliated cells forms near the midplate, after anterior-posterior positioning is established - this is the “ciliary node”; nodal cilia possess a 9 + 0 organization yet they beat in a rotary fashion to produce a net leftward flow of signaling molecules, which are detected by non-motile sensory cilia to signal organogenesis specific to the future left axis of the body
2 Examples of ciliopathies
- Bardet-Biedl Syndrome (BBS) - a highly pleiotropic disorder characterized by visual loss, polydactyly, anosmia, and renal abnormalities
- Polycystic Kidney Disease
Properties of Epithelial Cells
Cell-cell adhesion
Arranged in layers or sheets
Polarity
Turn-over and regeneration by epithelial stem cells
Avascular - nutrients and oxygen diffuse through the basal lamina
Functions of epithelia
Barrier to protect internal tissues
Selective absorbption, transport, and secretion of substances
Movement of particles & mucous
Biochemical modifications of molecules (i.e. liver)
Communication between other tissues (i.e. endocrine cells)
Reception of sensory stimuli
Endothelium
An epithelial tissue that faces blood or lymph
Mesothelium
An epithelial tissue that lines the enclosed internal spaces of body cavities
Epithelial to mesenchymal transition
During development, embryonic epithelia disassemble and move into the mesenchymal tissues where they transform into distinct, non-epithelial cell lineages (connective tissue, blood vessels, etc.)
*Also important in the formation of some cancers
Classification of epithelia
Simple - comprised of a single sheet of cells; may be squamous, cuboidal, or columnar
Stratified - comprised of multiple sheets of cells; may be squamous, cuboidal, or columnar based on the shape of the outer-most layer
Pseudo-stratified - all cells attach to basal lamina but not all cells are exposed to the apical space
Transitional - stratified epithelia that is flexible such that it becomes a single sheet when stretched (i.e. bladder)
Transcytosis
The process by which substances are endocytosed from one membrane region, shuttled across the cell via vesicular transport, and exocytosed from another membrane region
Tight Junctions (Zonula Occludens)
Linkages between epithelial cells formed by claudin-claudin protein interactions; provide a barrier to molecular flow between cells, forcing most transport to occur transcellularly; also limit diffusion of membrane proteins between different functional domains of the epithelial plasma membrane
Adherens Junctions (Zonula Adherens)
Epithelial cell-cell attachments formed by cadherin protein complexes that interact with actin filaments via their cytoplasmic tails
Desmosomes (Macula Adherens)
Epithelial cell-cell attachments formed by cadherin protein complexes that interact with intermediate filaments via their cytoplasmic tails
Gap Junctions
Channels that promote rapid communication between epithelial cells through diffusion of ions and small molecules
Basal Lamina Structure
Thin matrix of extracellular proteins that attaches to integrin proteins in the basolateral membrane of epithelial cells; often rich in collagen IV and glycoproteins including laminins and entactins
Focal Adhesions
Sites of attachment between epithelial cells basolateral membranes and the basal lamina; formed by integrins that attach to actin via their cytoplasmic tails
Hemidesmosomes
Sites of attachment between the basolateral membrane of epithelial cells and the basal lamina, formed by integrins that attach to intermediate filaments via their cytoplasmic tails
Exocrine glands
Arise from invaginations of epithelial sheets; cells become specialized to secrete mucous, serous, or mixed secretions from the apical surface directly onto the surface of the epithelial sheet
Endocrine glands
Arise from invaginations of epithelial sheets which bud off and detach from the epithelia during morphogenesis; they secrete hormones from the basolateral membrane into the blood stream through a series of specific transport steps
Exocrine gland structure
Secretory cells that make up the secretory unit are organized into lobules called alveoli or acini, or into tubes located in the base of the gland; simple epithelium lines the ducts and may modify the secretion as it moves along the epithelial surface
An exocrine gland may possess one duct (simple gland) or multiple ducts (compound gland)
Goblet Cells
Single cells imbedded within the gut epithelium that secrete mucus; basically single-cell exocrine glands
Properties of stem cells
Self-renewal - generation of more stem cells
Cell division
Produce differentiated progeny
Transit Amplifying Cells
Daughters of stem cells which also proliferate themselves, often at faster rates, to produce differentiated cells; intermediate cells in the stem-cell lineage
Wnts Pathway
Wnt signal interacts with the Frizzled receptor, which signals APC to de-inhibit B-catenin; B-catenin enters the nucleus where it promotes cell division and inhibits cell differentiation
Mutation in the APC gene causes colon cancer
Epitope
A specific region of a protein 10-15 AAs long that is recognized by an antibody
3 layers of mucosa
- Outer epithelium
- Lamina propria - underlying connective tissue, rich in immune cells and small blood vessels which facilitate response to ingested foreign materials
- Submucosa - deeper connective tissue
Layers of skin
- Epidermis - epithelia of the skin
- Dermis - Underlying connective tissue
- Hypodermis - Deeper connective tissue
Microvilli
Cell surface protrusions that contain linear actin filaments (FH2/Formin nucleated) connected to branched actin filaments (Arp2/3 nucleated) in the cell interior; microvilli function to increase surface area, improving the efficiency of membrane transport and secretion
Basal Lamina Functions
Attachment of epithelia to underlying connective tissue
Selective filtration of substances diffusing to or from epithelia
Establishment and maintenance of epithelial cell polarity
Guiding the migration of cells through connective tissues
Providing a barrier to movement of invading microbes
Control of development & organizing of epithelial cells following damage
Carcinoma nomenclature
Carcinomas are cancers derived from epithelial tissue
Adenocarcinomas are cancers derived from glandular epithelium
