Lecture 7 - Cell communication I

Question 1

Contact dependent

Answer 1

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

Question 2

Paracrine

Answer 2

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)

Question 3

Autocrine

Answer 3

Signalling between nearby cells of the same type

Specificity is achieved by receptor expression and rapid ligand degradation (therefore cannot travel far)

Question 4

Synaptic

Answer 4

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

Question 5

Endocrine

Answer 5

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

Question 6

Hormone targeting

Answer 6

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

Question 7

Regulation of target cell sensitivity

Answer 7

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)

Question 8

Up-regulation

Answer 8

Up-regulation (synthesis>degradation) increases the number of receptors expressed to increase sensitivity

Question 9

Down-regulation

Answer 9

Down-regulation (synthesis< degradation)

Question 10

Synergy

Answer 10

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.

Question 11

What is a hormone?

Answer 11

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

Question 12

Chemical classifications of hormones

Answer 12

Peptides - from three amino acids to large proteins

Amines- derivatives of tyrosine (amino acid)

Steroids - synthesised from cholesterol

Question 13

Sex steroid hormone synthesis

Answer 13

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

Question 14

Amines

Answer 14

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

Question 15

Differences between neurotransmitters and hormones

Answer 15

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.

Question 16

Lipid soluble

Answer 16

Steroid hormones - synthesised when required

Question 17

Peptide synthesis and secretion

Answer 17

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

Question 18

Intracellular receptors

Answer 18

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

Question 19

Cell surface receptors

Answer 19

For lipophobic molecules that cannot pass through the plasma membrane
Usually activates second messenger pathway

Question 20

Second messengers

Answer 20

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

Question 21

Gene transcription

Answer 21

Second messenger transcription activation

Question 22

How activation of a small number of receptors can cause large responses in target cells …

Answer 22

Gene transcription and/or second messengers