Lecture 7 - Cell communication I Flashcards
Contact dependent
Signalling between adjacent cells can also involve receptors
But unlike other receptor forms of receptor mediated cell to cell communication, the signalling message is not released
Specificity achieved by receptor expression and direct contact
Also known as juxtacrine signalling
Paracrine
Still local but it is from one cell type to another cell type
Specificity is achieved by receptor expression and rapid ligand metabolism (so when it travels further away it will not be available as it will be metabolised)
Autocrine
Signalling between nearby cells of the same type
Specificity is achieved by receptor expression and rapid ligand degradation (therefore cannot travel far)
Synaptic
One neuron sending a signal to another target such as muscle or endocrine cell
Specificity is achieved by precise contacts and rapid removal of neurotransmitter to prevent diffusion
Endocrine
Cell produces a signal called a hormone which travels through the blood to a distant target cell where it is detected via a specific receptor and then the target cell performs its task
Often involves signalling between distant cells in the body
Specificity is achieved by receptor expression being limited to target cells
Hormone targeting
Receptor expression is only on target cells therefore an effect will only occur on cells with the receptor when it binds the hormone that is compatible with it
The hormone does not come out of the secretory cell and just choose to fo to the cell with the correct receptor, it comes out of the secretory cell and just diffuses away in the local environment or in the synaptic cleft or in the blood stream and some of it by random change will reach the right receptor in order to induce an activation so although cell B and C might receive concentrations of the hormone they cannot actually see it because they do not have the right ‘eyes’ to see that message i.e. they do not have the right receptors. The blue square fits into the blue receptor and elicits a response in the cell that expresses that receptor. The target is self selecting, it is not the secretory cell that chooses the target, it is not the hormone that chooses the target, it is the target itself that chooses to be the target by the expression of receptors for that hormone
Regulation of target cell sensitivity
Sensitivity of a target cell depends on the number of receptors expressed
Receptors constantly degraded and synthesised to ensure efficiency
Up-regulation (synthesis>degradation) increases the number of receptors expressed to increase sensitivity
Down-regulation (synthesis < degradation)
Up-regulation
Up-regulation (synthesis>degradation) increases the number of receptors expressed to increase sensitivity
Down-regulation
Down-regulation (synthesis< degradation)
Synergy
Maximum combined response is bigger than addition of maximum individual responses
Synergism occurs when different entities working together and enhance the effect to an extent that cannot be produced singly.
Synergism occurs when two or more hormones combine to produce effects greater than the sum of their individual effects.
What is a hormone?
Hormones are chemical messengers that are secreted directly into the blood, which carries them to organs and tissues of the body to exert their functions.
Hormones are a molecule, they do not think, they just go anywhere so it is not the hormones themselves that find the target specifically
Widespread effects because hormones can go throughout the body so any cell in the body that has a receptor for that hormone is going to respond
Hormones are much slower at signalling than neurons because you might need multiple organ systems to be activated in order to respond for example stretch response which requires a whole body coordinated response by they are longer lasting and allow coordination across organ systems
Chemical classifications of hormones
Peptides - from three amino acids to large proteins
Amines- derivatives of tyrosine (amino acid)
Steroids - synthesised from cholesterol
Sex steroid hormone synthesis
Male sex steroid, testosterone is generated because the cells in the testes that make it express an enzyme called dehydrogenase which dehydrogenates the androstenedione to form testosterone
If that testosterone is inside a cell that also expresses another enzyme called aromatase, it will be converted into estradiol so the production of the steroids created are determined by the enzymes that are triggering the reactions
Amines
Thyroid hormones are also amines
L-Dopa undergoes further modification depending on the set of enzymes expressed in a particular cell
Dopamine, norepinephrine and epinephrine are the three signalling molecules (catecholamines which are packaged into vesicles). These can be neurotransmitters or hormones, it just depends on which particular cell and site or release you are looking at
Differences between neurotransmitters and hormones
Hormones: Hormones are produced in endocrine glands and are secreted into the blood stream. Neurotransmitters: Neurotransmitters are released by presynaptic nerve terminal into the synapse. Hormones: Hormones are transmitted through blood. Neurotransmitters: Neurotransmitters are transmitted across the synaptic cleft.
Lipid soluble
Steroid hormones - synthesised when required
Peptide synthesis and secretion
Synthesis at the rough endoplasmic reticulum
Packing at the Golgi apparatus
Storage in secretory vesicles (allows for the storage of the transmitter until it is needed)
Secretion by exocytosis
Intracellular receptors
For steroids and thyroid hormones
Can’t be stored in vesicles because they are lipophilic and then steroids are synthesised are therefore synthesised and then subsequently released across the membrane, the more you synthesise the more you secrete because you cannot store it, these hormones can diffuse across the membrane and into target cells
Transported on a carrier protein and it is usually a modified form of the receptor and it is at equilibrium so some of the hormone is attached to the carrier protein and some of it is free
Activates gene transcription and eventually leads to either increased protein synthesis or in some cases in can lead to gene inactivation and you end up with decreased protein synthesis
Thyroid hormones are also able to pass through the cell membrane like steroid hormones, thyroid hormones are derivatives of tyrosine
Cell surface receptors
For lipophobic molecules that cannot pass through the plasma membrane
Usually activates second messenger pathway
Second messengers
The first messenger is the thing that is released but he first cell (endocrine cell or the presynaptic neuron) and this carries the signal to the target cell and then within the target cell the second messenger system is activated which can allow you to have multiple effects in the cell
cAMP is a very common second messenger
There can be cAMP dependent protein kinase - kinases phosphorylate other molecules and the activation of these can cause multiple things to happen within a cell to coordinate that cells response to a single signal
Actual responses of the cell to the signal is very large despite it being one signalling molecule
Gene transcription
Second messenger transcription activation
How activation of a small number of receptors can cause large responses in target cells …
Gene transcription and/or second messengers
