4. Cell signalling Flashcards
Why is signalling important for multicellular organisms?
Signalling allows cells to coordinate - emergent properties
What are the components of cell signalling process?
- signalling cell
- signalling molecule
- target cell - receptor for signalling molecule
- signal output
How can cell signalling be quantified?
Cell signalling can be quantified by counting signalling genes (signalling proteins) - gives measure of cell’s signalling capacity
Compare unicellular vs multicellular signalling capacity
Unicellular:
- less signalling genes
Multicellular:
- x10 more genes for signalling
- needed for cell communication
What are the possible types of signallling molecules?
- proteins (insulin, growth factors)
- small hydrophobic molecules (animal steroid hormones)
- small hydrophillic molecules (plant auxins)
- gas (ethylene, nitric oxide)
- electrical (nerve impulses)
What is signalling range?
Signalling range - the distance between the signalling cell and target cell
What are the signalling ranges?
- long distance
- intermediate distance
- short distance
Explain long range signalling
Long range signalling:
- endocrine signalling
- transport through bloodstream / plant sap / nervous system
Explain intermediate range signalling
Intermediate range signalling:
- paracrine signalling
- releases signal into local environment - only signalling molecule comes in contact with the target cell (not the signalling cell)
Explain short range signalling
Short range signalling:
- juxtacrine signalling
- signalling and target CELLS come in contact
What are the examples of long range signalling?
- Male and female hormones in sexual dimorphism
- Flowering in plants triggered by daylength
- The nervous system
Explain long range signalling: male and female hormones in sexual dimorphism
Male and female hormones in sexual dimorphism:
- gonads secrete hormone cocktails - signalling molecules
- sensed by cells in the body
- target cells develop ‘male’ / ‘female’ appearance
Explain long range signalling: flowering in plants triggered by daylength
Flowering in plants triggered by daylength:
- sunlight - signal
- sunlight sensor (receptor) - CO protein in leaf - CO accumulates when days are long
- high CO levels promote FT protein synthesis
- high FT levels - moves through sap into leaf shoot
- FT stimulates flowers to form at the shoot
Allows same species to synchronise flowering; important for cross pollination - flowers need other flowers for pollination
What is cell signalling compentence?
Cell signalling competence - ability to respond to a specific signalling molecule
Competent cells - signal responsive cells
Incompetent cells - signal unresponsive cells
Why is cell signalling competence important in long range cell signalling?
In long range signalling - signalling molecule exposed to many different cells - only target cells must respond to the signal - responsive cells - cell signalling competence
Explain long range signalling: in mature / developing nervous system
Mature nervous system:
- long range signalling (neurons - elaborate shapes + connect to form functional circuits - nerve signal travels long distance)
- signalling within the brain + from the brain to periphery
- Signal travels long distance but signalling is paracrine: pre-synaptic (signalling) and post-synaptic (target cell) contact each other at synapse - neurotransmitter secreted into synaptic cleft (close but no contact) – at the end of motor neurons - signal transmitted into muscle cells
Developing nervous system:
- short range signalling in nervous system developement
How do nerve cells develop (synapsis development)?
Nerve cells must connect to each other to form synapsis:
- nerve cells grow out axons to the target nerve cell:
- axon navigation: part of neuron - growth cone navigates
- the growth direction sensed by growth cone from ‘guidepost’ cells - secrete guidepost cues in the tissue
- complex trajectories form by breaking long journies into several steps
Explain axon navigation
Axon navigation:
- axon growth cones navigate growth direction by ‘guidepost’ cell secreted ‘guidepost’ cues
- attractive / repulsive signals from ‘guidepost’ cells
Explain how neurons grow into complex trajectories
Long axon growth path broken down into several steps - each step is guided by ‘guidepost’ cells - secrete guidance cues - attractive / repullsive signals
What range signalling do axon growth cones receive?
Paracrine (intermediate):
- growth cone reacts to signals in the local environment (no contact between cells)
Juxtacrine (in contact) signalling:
- growth cone reacts to signals in contact with the signaling cell
What type of signalling does the brain use?
Endocrine: hormone signalling - travels in the blood (pituitary, gonads)
Paracrine: nervous system - synapsis (short distance but no contact)
What are the primary sex characteristics?
Reproductive organs (capacity fo the reproductive gland):
- gonads (ovaries / testes)
- gametes (eggs / sperm)
What is sexual dimorphism?
Sexual dimorphism - distinct difference in size / appearance between the sexes in addition to sexual organs (feathers, size, body differences) - secondary sexual characteristics
What are the examples of secondary sexual characteristics?
- size (usually females are larger)
- bodily hair
- body forms
- fur / feather colours
How is sex determined in mammals?
Sex in mammals is determined genetically at fertilisation - XX and XY - sex depends on sex chromosomes
How does the sex develop in a mammalian fertilised egg?
- Sex neutral development (both male and female structures)
- Signalling from Y chromosome (Sry) for male gonad development / no extra signalling for females -> sex specific development
What is Sry?
Sry - a TF encoded by Y chromosome - regulates gene expression for testes differentiation
Explain male sex determination in XY
- In sex neutral development both male and female structures present (indifferent state)
- Sry expressed - Mullerian inhibiting substance secreted - inhibits oviduct (Mullerian duct) formation (default development is female)
- Testosterone signals Wolffian duct to develop into vas deferans + secondary male characteristics
Explain long range testosterone signalling
Testosterone (hydrophobic, steroid hormone) secreted by gonads - acts on distant target cells (long range signalling) via blood - binds to cell receptors - regulates gene expression in targeted cells
Explain female sex determination in XX
- In sex neutral development both male and female structures present (indifferent state)
- Sry absent - gonads develop into ovaries - Wolffian duct disappears - oviduct develops
- No testosterone made - no Mullerian inhibiting substance - no male structures develop - ovaries secrete female hormones
What is the default sex of a mammalian embryo?
Default sex - female - no change unless Sry (male) triggered
What kind of signalling is used when gonads secrete hormones for sexual dimorphism?
Long range signalling - endocrine: hormones secreted by gonads and transported in blood around the body - only target cells contain receptors for sexual hormones - induced changes - sexual dimorphism
What is the disorder making XY human unresponsive to sex hormones?
CAIS (complete androgen insensitivity syndrome): some XY lack receptors for androgens and have internal testes - develop as females at puberty => response to androgen signals from testes is essential for male characteristic development (because default is female)
What are androgens?
Androgen - a male sex hormone
How is genetic sex determination in birds different to humans?
Also use unequally sized chromosomes - W and Z: ZZ male (homozygous), ZW female (heterozygous)
Is sex development in bird embryo same as in human?
Yes, initially embryos are sexually indifferent (sex nuetral development) -> sex chromosomes signal -> sex specific development
Can birds have both male and female features at the same time?
Yes but disorder:
- gynandromorphs - have both male and female dimorphisms - mixture of ZZ and ZW cells
- although all cells are exposed to same hormones - have intrinsic sex identity (hormone interpretation is questioned) - CELL AUTONOMOUS sex determination
=> primary component driving sex differentiation in birds is not long range hormone signalling
How can a mix of genetically distinct cells arise in an organism?
- Mosaic: due to genetic change in cell lineage derived from single zygote
-
Chimera: due to fusion of genetically distinct embryos
=> the result/effect is the same mixture fo cells
What are the three main types of sex determination methods?
- genetic determination (humans)
- cell autonomous sex determination (birds)
- temperature dependent determination (alligators)
Explain how sex determination occurs in alligators?
Sex determination - temperature dependent (not genetically):
- lay eggs outside of body - incubate
- incubation temperature determines sex phenotype
- warm >34 - male; cold <30 - female
- temperature acts as a signal
What other organism than alligator can use temperature dependent sex determination (TSD) mechanism?
Fish also use TSD - have come and gone through evolution
What type of signalling is used by morphogens/development signalling molecules in organism development?
Paracrine (intermediate range) signalling (<1mm range: 1-100 cell diameters)
Suggest an example to study paracrine signalling during development? Why useful?
Pentadactyl limb development - 5 digit structure conserved between tetrapods - can compare between species + easier experimentally (no humans used) - paracrine signalling from growth plates to limb ends - in the local environment
How is pentadactyl limb development similar/ different between species?
**Early development - conserved **between species (very similar structures) - as development progresses - developing structures diverge into different shapes and sizes
What is ‘windowing’ technique in chick eggs?
“Windowing” - surgical manipulatino of the shell to view the embryo on the surface of the yolk sac
Explain limb formation in the chick embryo
- Limb formation starts 3 days after fertilisation
- Anterior limb bud (wing bud) and posterior limb bud (leg bud)
- signalling over <1mm can aid to develop the whole limb - morphogen gradient
- signalling molecule presence determines the developing limb structure at a particular place
What are the different body axes?
- anterior-posterior
- dorsal-ventral
- proximal-distal
What are the limb axes?
- Proximo-distal axis: proximal wrist (close to the body) - distal fingertips (far from the body)
- Anterio-posterior axis: anterior thumb (to the front) - posterior little finger (to the back)
How do chick wing and human hand digits vary?
Human hand: pentadactyl developed into 5 digits
Chick wing: pentadactyl limb lost digits 1 and 5 in evolution (maybe for lighter wings)
How is cell fate decided along the three body axes?
Paracrine signalling:
- patterning of the embryo
- growth (cell division) within petterning
- cell signalling
Explain limb development in proximo-distal axis
Proximo-distal axis (near-far the body):
- apical ectoderm ridge (AER) at the tip of limb bud - important for proximo-distal development
- AER is important over the whole limb development period
- AER contains FGF4 (fibroblast growth factor) signalling molecule used for proximo-distal limb development
- proximo-distal limb development not autonomous - responds to the external signal of FGF4 (tested in FGF4 injection in wrong cells)
Explain AER removal experiment
- AER (apical ectoderm ridge) surgically removed from wing bud at different development stages
- development allowed to proceed
=> the later the AER was removed, the more distal structure formed
Explain how was the molecular AER mechanism discovered?
- AER (apical ectoderm ridge) surgically removed
- signalling molecules (FGF4) introduced into tissues by FGF4 soaked beads -> proximo-distal limb still developed
=> AER needed in development because releases signalling molecules
How was it determined that FGF4 is sufficient for proximo-distal limb development?
- FGF4 soaked beads were applied to a wrong location on chick embryo prodcued extra limb bud (ectopic limb bud) - induced leg
What is an ectopic limb bud?
An extra limb bud which development was induced artificially
What is the disorder in Dachshund leg development?
Short leg phenotype due to abnormal expression of FGF4 in developing limbs
Explain limb development in anterior-posterior axis
Anterior-posterior axis:
- driven by morphogen gradients - different gradient thresholds of same morphogen for different structures (French flag model)
- source of morphogen is the posterior end: high threshold for posterior structures, low threshold for anterior structures - zone of polarising activity (ZPA)
- ZPA secretes sonic hedgehog (Shh) morphogen for anterior-posterior limb development in different gradients
- cells are dependent on external signal to develop (not autonomous) and all cells can respond to Shh (grafting experiment) - Shh is master regulator of organogenesis
What’s the french-flag model in development
Different gradients of the same morphogen drive development of different structures - secreted from one place - as morphogen diffuses - different gradients created - different thresholds needed for different structures to develop
Explain ZPA grafting experiment
Second ZPA grafted into anterior position in embryo - mirror image limb formed (axis of symmetry)
How was it investigated if Shh is sufficient for anterior-posterior limb development?
Shh expressing cells were grafted into anterior side of limb bud - full mirror image limb structures developed => Shh is sufficient to drive anterior-posterior limb axis development
Define what is a morphogen
Morphogen - substance active in pattern formation whose concentration varies in space and to which cells respond differently at different thresholds (ex: Shh but NOT FGF4 - on/off mechanism)
What is an organiser?
Organiser - a signalling center that directs development of the whole / part of an embryo (ex: ZPA, AER)
What is lateral inhibition?
Lateral inhibition - inihibitory signalling at close range to organise cell structures (ex: neurons, cell spacing)
What is a plant trichomes?
Plant trichomes - ‘hairs’ on plant stems / leaves - trident structure
What are the functions of plant trichomes?
Trichome functions:
-** stinging hairs** to protect from predators
- insect trapping
- seed dispersal
Explain how plant trichomes differentiate
- Differentiate from epidermal precursor cells - all the same
- One cells becomes selected for trichome - trichome precursor -> differentiation into trichome
- Lateral inhibition - trichomes spaced out
What properties must be regulated for plant trichome to function properly?
- Trichome spacing (pattern formation)
- Trichome anatomy (differentiation of individual trichomes)
Explain trichome spacing
Trichomes at different densities in different plants - spacing needed - mechanism for spacing - must allow variable spacing:
- each epidermal cell has chance for trichome - non-random pattern - trichomes never touch - depending on signal from surrounding cells (non-cell autonomous)
- lateral inhibition - inhibits neighbours not to become trichome - tryptochon gene
- higher/lower trichome densities achieved by signal gradient
Explain tryptochon mutation in plant trichome development
Tryptochon mutants - break no touching rule in trichome spacing - disruption of signalling => gene tryptochon ensures trichome spacing by lateral inhibition
What are the biological examples of cell spacing based on juxtacrine signalling?
- Plant trichomes (lateral inhibition of adjacent cells)
- Hair spacing on insect surface (lateral inhibition of adjacent cells)
- Tree spacing in forests (resource depletion - irregular patern)
What is the function of hairs on insect surface?
Hairs - sensory bristles on body surface - sense the environment - each bristle connected to sensory neuron
How are insect sensory bristles arranged at a regular pattern?
Juxtacrine (ell-cell contact) lateral inhibition - where bristle formed also sensory neuron - bristle neurons never formed close - Notch inhibitory signalling pathway
Explain laser ablation experiment on insect bristle spacing
Laser ablation experiment:
- Laser used to kill bristle precursor cells only on the right embryo side - left was control
- Single bristle formed nearby the destroyed - all cells have the potential to become bristle cells - when one becomes - others inhibited - lateral inhibition
How regeneration of gut villi is maintained?
Gut villi in eroding env. - cells shed from the tip - need to be replaced - stem cell differentiation - all cells move up - restore lost cells
Repulsive signalling between EphB and ephrinB restricts stem cell movement out of the crypt
Explain how cell-cell contact repulsion helps to maintain gut villi development? How stem cells don’t escape the crypt?
Cell-cell contact helps in inwardly buckled crypt structure development - uses EphB / EphrinB repulsion:
- mutual repulsion by signalling between EphB (proliferating stem cells) and ephrinB (flanking cells) - transmembrane proteins
- cells separate - minimise area where cells with EphB and ephrinB come in contact => stem cells proliferate but can’t escape the crypt because it would involve mixing with flanking cells
What are the mechanisms where Eph/Ephrin contact dependent repulsion is used?
- in gut villi constant renewal
- in axon growth cone navigation
Explain autocrine signalling
Autocrine - same cell signalling and targeted - responds to own signals
Signalling lectures summary
What TF regulates trichome formation?
Glabra1 (GL1) regulates trichome formation and development in plants, expressed in trichome precursors
