Histology / Cell Biology Flashcards
(110 cards)
1
Q
What is angiogenesis?
A
The growth of new blood vessels from preexsiting vessels
2
Q
Tumor cells ___ ___invade normal tissue and capture existing blood vessels, rather tumor cells ________ normal endothelial cells to form new blood vessels.
A
Tumors DO NOT invade normal tissue and capture existing blood vessels, rather tumors recruit normal endothelial cells to form new blood vessels
3
Q
Describe the general process of angiogenesis
A
Cells typically monitor Oxygen (O2) tension. Under hypoxic conditions cells release angiogenic factors such as VEGF (vascular endothelial growth factors). VEFG binds to tyrosine kinase receptors on the surface of endothelial cells stimulating their proliferation. Endothelial cells migrate towards areas of higher VEGF concentration. In this way, VEGF can act as a chemoattract.
4
Q
What is the angiogenic “switch”?
A
- Normal conditions prevent cells from triggering angiogensis
- As tumors progress they gain the capacity to promote angiogenesis
- Thus, overcoming the normal inhibition of angiogenesis is a step or switch, in tumor progression
5
Q
What are the differences between normal capillaries and tumor capillaries?
A
Tumor capillaries are:
- Larger - 3 times the diameter
- Disorganized layout
- Loosely associated pericytes
- Gaps of several microns between cells
- Permeable - Walls up to 10 times more permeable
- High Pressure - Higher interstitial fluid pressure
- Poor Drainage - Poor lymphatic drainage
6
Q
Name some angiogenic growth factors
A
- VEGF (Vascular Endothelial Growth Factor)
- TGF-beta (Transforming Growth Factor Beta)
- bFGF (Basic Fibroblast Growth Factor)
- Interleukin-8
- Angiopoietin
- Angiogenin
- PDGF (Platelet Derived Growth Factor)
7
Q
How do tumor cells induce angiogenesis
A
- Tumor cells can attract stromal cell types, which in turn can promote angiogenesis
- Tumor cells may release enzymes (like MMPs) which degrade ECM and the basement membrane
- Sometimes other angiogenic growth factors are present in ECM. When it is degraded those are released leading to additional angiogenesis.
- Tumor cells may attract local endothelial cells, but can also attract precursor cells from distant sites like the bone marrow.
8
Q
Name some Anti-Angiogenic growth factors
A
Angiogenic Inhibitors
- Thrombospondin-1: ECM glycoprotein. Stops endothelial cells from proliferating
- Arrestin: fragment of collagen type IV
- Tumstatin: fragment of collagen type IV
- Angiostatin: fragment of plasminogen
- Endostatin: fragment of collagen XVIII
- Fibulin: fragment of basement membrane
- Endorepellin: fragment of perlecan
- TIMP-2: inhibitor of matrix metaloproteinase-2
9
Q
What biological processes rely on angiogenesis?
A
- Embryonic development
- Implantation of the placenta
- Wound healing
- Disease processes
- Diabetic retinopathy, psoriasis, rheumatoid arthritis
- Tumorigenesis
10
Q
What are the stages of the cell cycle? What are their relative lengths?
A
- S Phase (8-12 hours)
- M Phase (1 hour)
- G1+G2 (11-13 hours)
Interphase is G1+S+G2
Rapidly proliferating cells go through the cell cycle faster
11
Q
Why are there checkpoint controls in the cell cycle?
A
Checkpoints provide an opportunity for a cell to determine if it is in a favorable condition to continue in the cell cycle. Is all the DNA replicated? Are all chromosomes attached to the spindle? Is the environment favorable?
Checkpoints allow for delays due to internal issues such as damaged DNA or poorly attached chromosomes
Checkpoints also allow for accumulation of microenvironmental signals such as growth factors indicating a general need to cell cycle progression in the tissue bed.
12
Q
Where are checkpoints located in the cell cycle? What happens at each checkpoint?
A
(1) Before entering S phase from G1– Is the environment favorable? Checkpoint prevents S phase entry by stimulating p53 that activates transcription of the CKI called p21 that inhibits both G1/S-Cdk and S-Cdk
(2) Before entering M phase from G2– is all the DNA replicated? Checkpoint prevents M phase entry by stimulating kinases that inactivate the Cdc25 phosphatase needed for M-Cdk activation
(3) Before cytokinesis in M phase– are all the chromosomes attached to the spindle? Mad2 binds unattached kinetochores and blocks Cdc20-APC induced destruction of securin by sequestering Cdc20
13
Q
How do checkpoints operate?
A
By NEGATIVE signals designed to stop the action
14
Q
How were cell cycle control genes initially predicted and validated? (list experiments)
A
Genetic approach: Temperature sensitive yeat mutants– cell cycle control genes were predicted by G1/S blockade. Essential kinases are rate0limiting for cell cycle progression, and if these kinases are non functional, cell cycling stops at point where the kinases are needed.
Biochemical approach: Xenopus frog egg cytoplasm combined with the nuclei from a frog sperm and ATP– caused cell-free mitotic cycle, providing evidence for proteins in the cytoplasm driving mitosis and causing chromosomes to replicate and condense
Cell biology approach: Imaging replicated DNA in single cells or cell populations provides kinetic and spatial information about S phase activity– can see DNA replication occuring via an image, and can see relative amounts of DNA increasing during S phase
15
Q
What are cyclins?
A
Cyclins are rate-limiting proteins that bind kinases (cyclin dependent kinases, or Cdk)
Cyclins undergo cycles of synthesis and degradation in each cycle
Cyclins and Cdk-activating kinase (CAK) are needed to activate Cdk
16
Q
How is Cdk activity modulated?
A
(1) Inhibitory phosphorylation, such as Wee1 kinase with M-Cdk. The Wee1 kinase adds an extra phosphate which causes the inactivation of M-Cdk. Cdc25 phosphatase removes the extra phosphate to activate M-Cdk
(2) Cyclin-dependent kinase inhibitor (CKI), an inhibitory protein which binds to the active Cdk and caused an inactive CKI-Cdk complex, such as with G1/S Cdk and p27 CKI
(3) Ubiquitin ligase, reserves of cyclin:Cdk complexes can also be deployed or destroyed through ubiquitinization
–Anaphase Promoting Complex (APC) ligates ubiquitin to M-cyclin to end M phase
–SCF adds ubiquitin to p27 CKI to activate G1/S Cdk
17
Q
Can the cytosol from an S phase nucleus trigger DNA replication in the G1 or G2 nucleus?
A
Only in G1 nucleus
18
Q
What are the 4 phases of the cell cycle and what happens during each one?
A
G1: gathering nutrients and synthesizing RNA and proteins necessary for DNA synthesis
S: DNA replication
G2: cell growth and reorganization of organelles
M: mitosis
19
Q
Describe the steps involved in replicating DNA before, during, and after S phase.
A
(1) Pre-replicative phase in G1: Cdc6 binds to the origin recognition complex (ORC) and MCM is recruited.
(2) S phase: S-Cdk phosphorylates Cdc6 causing its removal from the ORC and eventual degradation, MCM and phosphorylated ORC create replication fork.
(3) Upon the completion of DNA replication, M-Cdk phosphorylates MCM and Cdc6 creating a lack of the pre-replicative complex, so no more DNA can be replicated
20
Q
Which cyclin-Cdk complex regulates G1 phase progression?
A
Cyclin D and Cdk4/6
21
Q
What happens at the DNA replication checkpoint?
A
- Unfinished replication forks send a negative signal to M-Cdk
- Negative signal activates a kinase that inhibits the Cdc25 phosphatase and allows Wee1 to keep M-Cdk in inactive state
- M-Cdk is very important in driving mitosis, so inhibiting it will allow for replication forks to finish
22
Q
What does M-Cdk cause?
A
M-Cdk induces assembly of mitotic spindle, chromosome condensation, nuclear envelope breakdown, actin-myosin cytokinesis, and distribution of membranous organelles to daughter cells
23
Q
How are sister chromatids separated?
A
M-Cdk causes two actions:
(1) Promotes Cdc20 to bind to inactive APC (anaphase promoting complex) to make active APC–> active APC causes the degradation of securin and activation of separase
(2) Mitotic spindle formation and attachment to the sister chromatids, separase cleaves cohesins
Now chromosomes can move toward the poles
24
Q
How does the spindle-attachment checkpoint work?
A
Mitosis
- Sensor detects whether the sister chromatids are connected to spindle aparatus and moving to opposite pole
- If there is a failure in this, it will send a negative signal to block anaphase
- Mad2 binds unattached kinetochores and blocks Cdc20-APC induced destruction of securin by sequestering Cdc20
- M-Cdk and Polo kinase both release Cdc20 when all kinetochores become attached to the spindle, allowing for anaphase to proceed
25
How is M-Cdk degraded?
The APC-Cdc20 complex ubiquinates M-Cyclin
26
What happens to the Cdk activity in G1?
G1 phase = Cdk inactivity
CKIs accumulate to block G1/S-Cdk to prevent premature entry into S phase
APC creates new complex with Hct1 after Cdc20 removal-- this assures full destruction of remaining M-cyclin and Cdc20 removal
27
How to cells move out of G1 and into S phase?
- G1-Cdk activation necessary-- linkd to growth factor receptor activated kinases (MAP kinases such as Ras)
- Hct1-APC does not block G- or G1/S-cyclins
- G1-Cdk activates retinoblastoma protein (Rb) reducing its storage of E2F, a powerful transcriptional activator of genes needed for escape from G1 into S phase
28
How to mitogens affect exit from G1? What does Myc do?
- Mitogens bind to mitogen receptor, causing Ras/MAP kinase activation of gene regulatory protein and transcription of myc gene
- Myc protein causes:
- increased cyclin D--\>G1-Cdk activation--\> Rb phosphorylation
- increased p27 degradation--\> G1/S-Cdk activation--\> Rb phosphorylation
- Increased E2F synthesis
All of these cause increased E2F activity and entry into S phase
29
How does p53 work at the G1 checkpoint?
If DNA is ok, p53 is degraded
If there in an activating event (such as UV or X-rays), p53 is stabilized and binds to regulatory region of p21 gene, causing the creation of p21
p21 binds to G1/S-Cdk and S-Cdk preventing the cell's progression into S phase
30
Describe the main features of Rb structure
The Rb gene codes for a large protein, retinoblastoma protein or Rb, (~150,000 daltons) that functions as a growth suppressor by binding to and sequestering a family of transcription factors known as E2F.
_Phosphorylation_ of Rb by Cdk4/cyclin D and Cdk2/cyclin E results in hyperphosphorylation and release of the E2F transcription factors. Release and function of the E2F transcription factors allows cells to progress through the restriction point of the cell cycle.
31
What are three primary mechanisms that convert a proto-oncogene to an oncogene?
(1) Mutation
(2) Gene amplification (multiple copies of the gene)
(3) Chromosome rearragement (translocation or transposition, the gene is moved to a new locus under new promoter controls)
32
What is an oncogene?
A gene, which upon mutation, produces a protein that is over pexpressed or hyperactive resulting in excessive cell proliferation
Typically components of signal transduction pathways
Generally, a mutation if only one copy of a proto-oncogene is sufficient to disrupt growth regulation. Oncogenic mutations are thought to be dominant, and tumorigenesis results from the gain of function in the mutated oncogene.
33
What are possible consequences of proviral insertion?
When a retrovirus inserts DNA, it can cause retroviral transformation.
Proviral insertion may result in:
(1) Placement of long terminal repeat enhances sequences in proximity of a cellular proto-oncogene resulting in disruption of normal regulation of the proto-oncogene
(2) Insertion withing the coding sequence of a cellular proto-oncogene resulting in the production of an altered protein product that may have lost critical regulatory regions
(3) Insertion into regions of the gene that regulate proto-oncogene mRNA stability or protein stability, resulting in longer lived and/or greater amounts of protein
34
How can proto-oncogenes be activated by point mutations?
A mutation that inactivates a gene that has antiproliferative function can also lead to excessive proliferation.
In this case, the normal gene is referred to as a tumor-suppressor gene. In contrast to proto-oncogenes, usually both copies of a tumor-suppressor gene must be mutated or inactivated for an effect on growth regulation to become apparent. Tumorigenesis thus may also result from a loss of function of a tumor-suppressor gene.
35
What is retinoblastoma (Rb) protein?
Several roles:
(1) Functions as a growth suppressor by binding to and sequestering a family of transcription factors known as E2F. Regulates passage through G1/S restriction point of cell cycle. Phosphorylation of Rb by Cdk4/cyclin D and Cdk2/cyclin E results in hyperphosphorylation and release of the E2F transcription factors. E2F allows cells to progress through the restriction point of the cell cycle.
(2) Rb can capture SKP2/SCF and thus prevent the degradation of p27 CKI. The continued availability of p27 continues the inhibition of Cdks and keeps the cell in G1.
(3) Assists with the maintenance of chromatin structure during G1 and M phase
Mutations in Rb cause a wide variety of cancers.
36
What is Rb protein called the "master regulator"?
The restriction point monitored by Rb protein blocks passage through the remainder of the cell cycle
Passage through the restriction point normally allows cells to progress though the remainder of the cell cycle and divide once.
Other cell cycle checkpoints monitor cell cycle progression and, if activated, delay progression or repair or trigger apoptosis if damage is too great.
37
How does Myc modulate Rb phosphorylation?
Myc causes transcription of cyclin D gene, yielding increased cyclin D protein, causing G1-Cdk activation (cyclin D-Cdk4), which phosphorylates Rb
Myc increases transcription of SCF which degrades p27. Decreasing p27 allos for G1/S-Cdk (cyclin E-Cdk2) activation, which phosphorylates Rb
38
Describe the events in the cell after Rb phosphorylation.
Hyperphosphorylation of Rb protein (by G1-Cdk) inactivates Rb and allows for the release of E2F protein.
Active E2F protein increases S-phase transcription (which in turn activates more E2F protein. G1/S-cyclin (cyclin E) and S-cyclin (cyclin A) are transcriped. Newly activated G1/S-Cdk and S-Cdk positively reinforce S-phase transition by phsophorylating more Rb protein.
The cell exits G1 phase and enters S phase.
39
How does E2F function?
- Transcription factor
- Five members: E2F1-5
- Trans-activate cell cycle associated genes
- Form heteromeric complexes needed for induction of gene transcription
- Regulated by direct interaction with pocket proteins
40
How does the Rb pocket work?
When Rb is not phosphorylated, E2F sits inside the Rb pocket.
When Rb is phosphorylated, it causes Rb to change shape, and E2F can no longer fit in the pocket, and is thus released.
41
What are chromatin regulators?
Chromatin regulators are a variety of co-regulatory proteins that interact with Rb protein and facilitate conformation changes needed to release its negative control of E2F transcription factor activity.
Chromatin regulators may utilize the LXCXE pocket to allow for E2F dissociation.
42
What is an E2F independent mechanism for Rb-induced cell cycle arrest?
Rb can capture S-phase kinase associated protein 2 (SKP2/SCF) that adds ubituitin to the p27 cyclin-kinase inhibitor (CKI) needed for p27 turnover. p27 degradation is needed for progression through G1.
43
What are cohesins? And condensins?
Cohesins are multisubunit proteins that are deposited along the length of replicated chromatin to link sisters together.
Condensin proteins cause replicated chromatin to condense about 50-fold to produce visible chromosomes.
M-Ckd activates condensins which use ATP to drive coiling proces.
44
How do Rb mutations lead to genomic instability independent from cell-cycle progression?
In addition to the cell cycle restriction point functions, Rb protein also assists with the maintenance of chromatin structure during G1 and M phase
- Rb mutations can lead to defects in loading of condensin along prometaphase chromosomes and cohesin at the centromere.
- Condensin promotes condensation and cohesin holds the sister chromatids together
- Unregulated E2F causes high expression of MAD2 that prevents timely attachment of chromosomes to spindle microtubules.
- Lack of Rb and E2F excess results in tangled chromosomes and aneuploidy.
45
What are the 2 checkpoints in G1 and their roles?
1. Restriction point- cell self-evaluates its replicative potential, mediated by interactions between Rb and E2F
2. DNA damage checkpoint- monitors integrity of newly replicated DNA, if DNA is damaged, high levels of p53 do not let cell enter S phase
46
What is the checkpoint in S phase and its role?
DNA damage checkpoint- monitors quality of replicating DNA
47
What are the 2 checkpoints in G2 and their roles?
1. DNA-damage checkpoint
2. Unreplicated-DNA checkpoint- prevents progression into mitosis before DNA synthesis is complete
48
What cyclin/Cdk complex regulates G1 progression?
Cyclin D
Cdk4/6
49
What cyclin/Cdk complex regulates S phase entry?
Cyclin E
Cdk2
50
What cyclin/Cdk complex regulates S phase progression?
Cyclin A
Cdk2
51
What cyclin/Cdk complex regulates S phase through G2 phase and M phase entry?
Cyclin A
Cdk1
52
What cyclin/Cdk complex regulates M phase progression?
Cyclin E
Cdk1
53
What is p53 and how does it work?
Tumor suppressor protein that senses DNA damage at 2 checkpoints (G1/S, G2/M) and suspends cell cycle transit until damage is repaired
Low levels are normally maintained by ubiquitin --\> activated/phosphorylated by protein kinases that are activated by DNA damage
When phosphorylated p53 dissociates from MDM2 and translocates to nucleus to promote cell cycle arrest by stimulating CKI p21WAF
If DNA repair fails, p53 activates apoptosis
54
What are the 4 phases of mitosis?
1. Prophase- chromosomes condense, nuclear envelope disintegrates, kinetochores appear on satellite DNA
2. Metaphase- mitotic spindle formation, chromosomes move to metaphase plate
3. Anaphase- cohesins holding sister chromatids break, sister chromatids separate, chromatids pulled towards opposite poles by dyneins
4. Telophase- cytokinesis, nuclear envelope reforms, chromosomes uncoil
55
What are the 2 checkpoints in M and their roles?
1. Spindle-assembly checkpoint
2. Chromosome segregation checkpoint- prevents cytokinesis until all chromosomes have been correctly separated
56
Where is loose connective tissue located?
Beneath epithelia that cover body surfaces and line internal surfaces of body, associated w/ epithelium of glands, surrounds smallest blood vessels
57
What are the structural/cellular features of loose connective tissue?
- Thin, sparse collagen fibers (type III)
- Abundant viscous/gel-like ground substance that plays important role in diffusion of O2 and nutrients
- Abundant cells of various types (lots of nuclei visible), cells migrate from vessels in response to stimuli (site of inflammatory and immune rxns)
58
Where is dense irregular connective tissue located?
Submucosa of hollow organs has fiber bundles in varying planes, allowing organs to resist excessive stretching and distension
59
What is the function of dense irregular connective tissue?
Significant strength, resistance to stretching/tearing
60
What are the cellular components of dense irregular connective tissue?
Contains mostly collagen fibers (type I) arranged in bundles and oriented in various directions, sparse in cells (typically just fibroblasts), relatively little ground substance
61
Where is dense regular connective tissue located?
Reticular layer (dermis) provides resistance to tearing due to stretching
Tendons- parallel bundles of collagen fibers w/ fibroblasts
Ligaments- parallel bundles of collagen fibers, less regularly arranged than tendon
Aponeuroses- fibers arranged in multiple layers, arranged at 90 deg
62
What is the function of dense regular connective tissue?
Provide maximum strength
63
What are the cellular components of dense regular connective tissue?
Lots of collagen fibers (type I) arranged in parallel array and densely packed, cells that produce/maintain fibers are packed and aligned between fiber bundles, little ECM
64
What is the amino acid sequence of collagen and the significance on structure of collagen fibril?
X-Gly-Y (X is usually hydroxyproline or hydroxylysine, Y is usually Pro)
Essential for triple helical formation (hydrogen bonding)
65
What is the composition of Type I collagen? Type II collagen? Type III collagen?
Type I: heterotrimeric, 2 a1 chains + 1 a2 chain = [a1(I)]2a2(I)
Type II and Type III: homotrimeric, 3 a1 chains = [a1(II/III)]3
66
What is the sequence of events during INTRACELLULAR synthesis of type I collagen?
1. Collagen alpha chains synthesized in rER as long precursors (pro-a chains or preprocollagen molecules)
2. Post translational modifications in cisternae of rER
3. Folded procollagen passes to Golgi apparatus to begin to associate into small bundles
4. Free and small aggregates of procollagen are packaged into secretory vesicles and transported to cell surface
67
What is the role of vitamin C in the process of type I collagen formation? What happens in vitamin C deficiency?
Vitamin C is a cofactor for enzymes prolylhydroxylase and lysylhydroxylase, which hydroxylate Pro and Lys residues
Vitamin C deficiency = scurvy (H-bonds essential to final structure of collagen can't form) --\> wounds fail to heal and bone formation is impaired
68
What are the mechanisms involved in the post translational modification of the procollagen molecule?
1. N-terminus cleaved
2. Pro and Lys residues hydroxylated
3. Sugar groups added
4. Globular structure formed at C-terminus, stabilized by disulfide bonds (allows correct alignment of 3 a chains during triple helix formation)
5. Triple helix formed by 3 alpha chains
6. Intrachain and interchain H and disulfide bonds form --\> influence shape of molecule
7. Triple helix stabilized by binding of chaperone protein hsp47 (prevents premature aggregation of trimers in cell)
69
How does a collagen fibril form (extracellularly)?
1. Procollagen peptidase associated w/ cell membrane cleaves uncoiled ends of procollagen to make mature collagen molecule
2. Aggregated collagen molecules align together in coves to form collagen fibrils, cross-linked by covalent bonds formed between lysine and hydroxylysine aldehyde groups
70
Whace cells synthesize collagen? How is collagen synthesis regulated?
FIbroblasts, epithelial cells (produce collagen of basement membrane)
Stimulated by TGF-B and PDGF
Inhibitede by glucocorticoids
71
What are the two mechanisms and the molecules involved in the degradation of collagen?
* *Proteolytic degradation** occurs outside cells by MMPs
- Collagenases- degrade type I, II, III, and X collagen
- Gelatinases- degrade most types of denatured collagens, laminin, fibronectin, elastin
- Stromelysins- degrade proteoglycans, fibronectin, denatured colalgens
- Matrilysins- degrade type IV collagen and proteoglycans
- Membrane-type MMPs- produced by cancer cells, potent pericellular fibrinolytic activity
- Macrophage metalloelastases- degrade elastin, type IV collagen, laminin
**Phagocytic degradation** occurs intracellularly, involves macrophages that remove components of ECM
72
What disease is caused by a defect in type I collagen? What are the clinical features of this disease?
Osteogenesis Imperfecta type I
Clinical features: repeated fractures after minor trauma, brittle bones, abnormal teeth, thin skin, weak tendons, blue sclerae, progressive hearing loss (fractures and fusions of bone in middle ear)
73
What disease is caused by a defect in type III collagen? What are the clinical features of this disease?
Ehlers-Danlos Syndrome type IV
Clinical features: hypermobility of joints of digits, pale thin skin, severe bruisability, bleeding diathesis, early morbidity and mortality resulting from rupture of vessels and internal organs
74
How does the number and shape of microvilli influence a cell’s absorptive capacity?
Cells that transport fluid or absorb metabolites have closely packed, tall microvilli (eg. striated border in intestine, brush border in kidney tubules)
Cells that transport less have smaller, irregularly shaped microvilli
75
What comprises the core of a microvillus and the terminal web to which they are connected?
* *Microvillus:** core of actin filaments anchored to villin at the tip of microvillus
- Cross-linked by actin-bundling proteins (fascin, espin, fimbrin) to provide support and rigidity
- Myosin I binds actin filaments to plasma membrane of microvillus
* *Terminal web:** actin filaments
- Stabilized by spectrin
- Myosin II and tropomyosin give contractility
76
What is the composition and function of sterocilia in the epididymis?
Composition: apical cell protrusions connected by cytoplasmic bridges, made of actin filaments cross-linked by fimbrin and ezrin, also contains alpha-actinin
Function: facilitates absorption
77
What is the composition and function of sterocilia in the inner ear?
Components: actin filaments cross-linked by espin, but no ezrin or alpha-actinin, treadmilling effect (actin monomers constantly added to tips and removed at base of stereocilia)
Function: sensory mechanoreceptors
78
What is the location, structure, and function of motile cilia?
Location and function: tracheobroncial tree (sweep mucus and trapped particulates toward oropharynx to be swallowed), oviducts (help transport ova and fluid toward uterus), brain ependyma, olfactory epithelium, sperm cells (as flagella)
Structure: 9 + 2 axonemal organization w/ microtubule-associated motor proteins to induce ACTIVE motility
79
What is the location, structure, and function of primary cilia?
Location: solitary projects on almost all eukaryotic cells
Structure: 9 + 0 axonemal arrangement
Function: immotile (passive movement), function as chemosensors, osmosensors, mechanosensors and mediate light sensation, odorant, and sound perception in multiple organs
80
What is the location, structure, and functionf of nodal cilia?
Location: embryo on bilaminar embryonic disc at gastrulation, concentrated in area that surrounds primitive node
Structure: 9 + 0 axonemal arrangement w/ associatted motor proteins
Function: perform passive rotational movement, important in early embryonic development (generates left-right asymmetry of internal organs)
81
What is the 9 + 2 axonemal pattern of arrangement?
Internal core of microtubules formed by 9 pairs (doublets) of circularly arranged microtubules surrounding 2 central microtubules
Dynein (microtubule-associated motor protein) "arms" on each doublet, uses ATP hydrolysis to move along surface of adjacent microtubule --\> creates active movement
82
What are basal bodies?
Modified centrioles that functions as microtubule organizing centers (MTOCs)
Contains basal foot (coordinates ciliary movement by rotating basal bodies to desired position)
Alar sheet tethers basal body to apical plasma membrane
83
What alteration leads to primary ciliary dyskinesia?
Immotile cilia caused by structural abnormality that results in absence of dynein arms (Kartagener's syndrome) or malformation of radial spokes and dynein arms (Young's syndrome)
Most prominent symptoms: chronic resp distress (incl bronchitis and sinusitis), otitis media (inflammation of middle ear cavity), persistent cough, asthma, sterility (in males)
Also causes situs inversus (organs of visceral are transposed through sagittal plane) due to abnormal function of nodal cilia
84
What is the defect in primary cilia that can lead to polycystic kidney disease?
Normal function: primary cilia in glomerulus and tubular cells of kidney function as mechanoreceptors (fluid flows through renal corpuscle and tubules cause them to bend, initiates influx of Ca2+ into cell)
Polycystic kidney disease (PKD)- autosomal recessive, mutation in genes ADPKD1 and ADPKD2 encode polycystin-1 and polycystin-2 (essential for formation of calcium channels associated w/ primary cilia) --\> affects development of primary cilia --\> multiple expanding cysts in both kidneys --\> destroys renal cortex --\> renal failure
Clinical features: cysts in pancreas and liver, enlargement and dilatation of biliary tree system, retinitis pigmentosa (abnormalities of the retina that cause progressive vision loss), sensorineural hearing loss, diabetes, learning disabilities
85
What is the molecular structure and function of the zonula occludens?
Structure: narrow region where plasma membrane of adjoining cells comes in close contact to seal off intercellular space
- Series of focal fusions created by transmembrane proteins of adjoining cells (occludins, claudins, JAMs)
- Cytoplasmic portions of proteins have domains that attract regulatory and signaling proteins (ZO-1, 2, 3)
Function: selective passage of substances from 1 side of epithelium to the other
- Transcellular pathway- across apical plasma membrane of epithelial cells, into cytoplasm and across lateral membrane below level of occluding junction into intercellular compartment
- Paracellular pathway- occurs across zonula occludens between 2 epithelial cells, permability depends on composition and tightness of zonula occludens (eg. number of zonula occludens strands, number of active channels formed by claudin molecules)
86
What is the target of the bacteria Clostridium perfringens (causes food poisoning)?
Enterotoxin binds to **claudin molecules** and forms pores in apical domain of plasma membrane --\> leads to dehydration as a result of massive movement of fluids into lumen of intestines
87
What is the target of the bacteria H. pylori?
Binds to extracellular domains of zonula occludens proteins --\> targets **ZO-1 and JAM proteins** --\> zonula occludens barrier becomes disrupted --\> capacity for tyrosine kinase signaling diminishes --\> cytoskeletal rearrangements --\> injury to protective barrier of stomach --\> gastric ulcers and gastric carcinomas
88
What is the target of RNA viruses responsible for infant enteritis?
Attachment to **JAM molecule** activates NFKB --\> NFKB migrates to nucleus and triggers cascade of cellular events leading to apoptosis
89
What is the target of oncogenic adenovirus and papillomavirus?
Target zonula occludens-associated proteins **ZO-2 and multi-PDZ-containing protein-1 (MUPP-1)** --\> sequestration and degradation of zonula occludens and tumor-suppressor proteins associated w/ viruses
90
What is the target of dermatophagoides pteronyssinus parasite?
Fecal pellets inhaled w/ dust particles --\> serine and cysteine peptidases in pellets cleave **occluding and ZO-1** proteins --\> breakdown of zonula occludens junctions in resp epithelium --\> inhaled allergens initiate immune response that can lead to severe asthma attacks
91
How do occludins and claudins differ?
**Occludin:** helps maintain barrier between adjacent cells and between apical and lateral domains, present in most occluding junctions (but not all)
**Claudin:** forms backbone of each zonula occludens strand, certain claudins can form extracellular aqueous channels for paracellular passage of ions and small molecules, helps determine tightness and selectivity of seal between cells
92
What are the four types of cell adhesion molecules?
cadherins, integrins, selectins, and immunoglobulin super family
93
What is the structure and function of cadherins?
Structure: transmembrane Ca2+-dependent cell adhesion molecules localized in zonula adherens, linked to actin filaments via catenins (intracellular proteins)
Function: homotypic interactions with neighboring cells --\> convey signals that regulate growth and differentiation, control cell-to-cell interactions, participate in cell recognition and embryonic cell migration
NOTE: E-cadherin- maintains zonula adherens junction between epithelial cells, important suppressor of epithelial tumor cells
94
What is the structure and function of integrins?
Structure: 2 transmembrane glycoprotein subunits
Function: heterotypic interactions with neighboring cells, interacts w/ ECM molecules (collagen, laminin, fibronectin) and w/ actin and intermediate filaments --\> regulate cell adhesion, control cell movement and shape, participate in cell growth and differentiation
95
What is the structure and function of selectins?
Structure: expressed on WBCs and endothelial cells
Function: mediate neutrophil-endothelial cell recognition (heterotypic interaction) --\> initiates neutrophil migration through endothelium of blood vessels into ECM, involved in directing lymphocytes to accumulations of lymphatic tissue (homing procedure)
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What is the structure and function of the immunoglobulin super family?
Structure: molecules not involved in immune functions, but that share a common precursor element w/ other molecules involved in immune rxns
Function: mediate homotypic cell-to-cell adhesions, ICAM, C-CAM, VCAM, DSCAM, PECAM, JAM play key roles in cell adhesion and differentiation, cancer and tumor metastasis, angiogenesis, inflammation, immune responses, microbial attachment
97
What is the molecular structure and function of the zonula adherens?
Structure: transmembrane cell adhesion molecule (E-cadherin) provides lateral adhesion in continuous band/beltlike configuration around cell
Function: interacts w/ network of actin filaments in cell, regulates cell adhesion, polarity, differentiation, migration, proliferation, survival of epithelial cells
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What is the molecular structure and function of the macula adherens?
Structure: localized on lateral domain of cell, found in conjunction w/ zonula occludens and zonula adherens, but not a continuous structure around cell (like zonula adherens is)
- Desmosomal attachment plaque- disc-shaped structure of dense material on cytoplasmic side, anchors intermediate filaments and dissipates physical forces, composed of desmoplakin and plakoglobin proteins
- Intermediate line- dense medial band of extracellular portion of transmembrane glycoproteins desmogleins and desmocollins, homotypic binding in presence of Ca2+ = cadherin zipper
Function: mediate direct cell-to-cell contact by providing anchoring sites for intermediate filaments (like spot welding), participates in tissue morphogenesis and differentiation
99
What is the molecular structure and function of gap junctions?
Structure: groups of tightly packed channels, each formed by 2 connexons ("half-channels"), embedded in facing membranes, bridge extracellular space between adjacent cells
- Each connexon contains 6 symmetrical subunits of connexin integral membrane protein
- Subunits arranged around cylindrical transmembrane channel
Function: form low-resistance junctions, allows cells to exchange ions, regulatory molecules, and small metabolites through pores
- Important in tissues in which activity of adjacent cells must be coordinated (eg. fluid and electrolyte transport) , present in epithelia, smooth and cardiac muscle, nerves, number of pores varies widely
100
What is the physiologic significance of plicae on the lateral walls of epithelial cells?
Plicae- infoldings on lateral surface along border of each cell w/ its neighbor --\> increases lateral surface area
- Important in cells engaged in fluid/electrolyte transport, esp intestinal and gallbladder epithelium)
Mechanism: Na/K ATPase in lateral plasma membrane pumps Na out, anions diffuse out to balance charge, water follows from cytoplasm to intercellular space --\> intercellular space distends --\> hydrostatic pressure builds and drives isotonic fluid from intercellular space into underlying connective tissue (occluding junction at apical end prevents fluid movement in opposite direction) --\> fluid replaced by diffusion across apical plasma membrane through microvilli
101
What are the cells that synthesize the components of the basal lamina?
Epithelial cells
102
What are the types of collagen that make up the basal lamina?
Type IV collagen- major component (~50%), forms scaffold of basal lamina, isoforms provide specificity to basal lamina in different tisuses
Type XV collagen- stabilizes structure of external lamina in skeletal and cardiac muscle cells
Type XVIII collagen- present in vascular and epithelial basal laminae, functions in angiogenesis
Type VII collagen- forms anchoring fibrils that link basal lamina to underlying reticular lamina
103
What is the role of proteoglycans in the assembly of the basal lamina?
Proteoglycans form the protein core to which heparan sulfate (eg. Perlecan, agrin), chondroitin sulfate (eg. Bamacan), or dermatan sulfate side chains are attached, highly anionic, extensively hydrated, carry high negative charge so they regulate passage of ions across basal lamina
- Perlecan- heparan sulfate proteoglycan, provides additional cross-links to basal lamina by binding laminin, type IV collagen, and entactin/nidogen
- Acrin- heparan sulfate proteoglycan found almost exclusively in glomerular basement membrane of kidney, major role in renal filtration and cell-to-ECM interactions
104
What is the role of laminins in the assembly of the basal lamina?
Cross-shaped glycoprotein molecules of 3 polypeptide chains that initiate assembly of basal lamina
- Possess binding sites for integrin receptors
- Involved in cell-to-ECM interactions
- Play a role in development, differentiation, remodeling of epithelium
105
What is the role of entactin and nidogen in the assembly of the basal lamina?
Small, rodlike sulfated glycoprotein that form a link between laminin and type IV collagen network
Distinct domains bind Ca2+, support cell adhesion, promote neutrophil chemotaxis and phagocytosis, interact w/ laminin, perlecan, fibronectin, and type IV collagen
106
What are the proteins that attach the basal lamina to the underlying connective tissue?
1. Anchoring fibrils (type VII collagen)- in close association w/ hemidesmosomes, extend from basal lamina to anchoring plaques in connective tissue matrix
- Dystrophic epidermolysis bullosa- mutations in collagen VII gene, results in inherited blistering skin disease where epithelium is detached below basement membrane
2. Fibrillin microfibrils- attach lamina densa to elastic fibers, have elastic properties
- Marfan's syndrome- mutation in fibrillin gene FBN1
3. Discrete projections of lamina densa on connective tissue side interact directly w/ reticular lamina to form additional binding sites w/ type III collagen
107
What are the functions of the basal lamina?
- Structural attachment- intermediary structure in attachment of cells to adjacent connective tissue
- Compartmentalization- separates/isolates connective tissue from epithelia, nerve, and muscle tissues
- Filtration- regulates movement of substances to and from connective tissue via ionic charges and integral spaces (eg. Kidney plasma filtrate must cross basal lamina of capillaries and epithelial cells to reach urinary space in renal corpuscle)
- Tissue scaffolding- guide or scaffold during tissue regeneration to help maintain original tissue architecture
- Regulation and signaling- molecules in basal lamina interact w/ cell surface receptors to influence epithelial behavior during morphogenesis, fetal development, wound healing
108
How do focal adhesions (integrins) facilitate cell movement?
Structure: cytoplasmic face binds actin filaments, extracellular face binds to proteins of ECM (laminin and fibronectin)
Function: anchors actin filaments of cytoskeleton to basement membrane
- Prominent role in dynamic changes in epithelial cells (eg. migration)
- Important sites of signal detection and transduction- detect contractile forces/mechanical changes in ECM and convert them to biochemical signals (mechanosensitivity) --\> affects migration, differentiation, growth
109
What is the molecular structure and function of the hemidesmosome?
Structure: found on basal cell surface
- Intracellular attachment plaque on cytoplasmic side of basal PM contains plectin, BP230, erbin
- Transmembrane proteins include a4B6 integrin, type XVII collagen, and CD151 glycoprotein
Function: anchor intermediate filaments of cytoskeleton to basement membrane, provide increased adhesion to basal lamina
- Found in epithelia subject to abrasion and mechanical shearing forces (cornea, skin, mucosa of oral cavity, esophagus, vagina)
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