What happens when a neuron does not have a supply of NGF?
-if don’t have supply of NGF will go into apoptosis and die, once they get past that stage they don’t die by apoptosis but wither away, for healthy neuron function= needs throughout its entire life a supply of neurotrophin from a cell that it is innervating, both in the CNS and PNS
-cell with Trk-A if it doesn’t get NGF, will go into apoptosis during emrbyogenesis, if this happens in an adult it will start losing function, dediferrentiate, will make less neurotransmitter, withdraw its connections, look less like neuron, debate about if cell death occurs if the cell doesn’t get the neurotrophin
-in alzheimer’s it is the ACh neurons that stop working the first
-there are many other growth factors in the body!
-TrK B= if expressed can survive if it find BDNF or NT-4
How were neurotrophins first discovered?
-NGF first, BDNF second, then NT-3 (neurotrophin 3) and NT-4 (neurotrophin 4) in mammals only 4 calssical neurotrophins, homologous to each other and each have their receptors, these are tyrosine kinase receptors
What are the actions of neurotrophins?
2. Neuronal Survival
-the 4 neurotrophins are homologous, look similar and similar functions, main function is neuronal maintenance
What happens in mature neurons when withdrawal of neurotrophins happens?
-Neurotrophins provide Neuronal Maintenance in “Adult neurons”
-“Mature Neuron” is Probably a Better Term
-In Mature Neurons, Withdrawal of Neurotrophin causes:
1. decreased transmitter production
2. decreased transmitter release
3. retraction of axonal branches
4. reduction in cell body size
5. gradual de-differentiation (if withdrawal is prolonged)
What are the neurotrophin receptors like?
-most receptors are tyrosine kinases, their cytoplasmic domain has enzymatic module which is tyrosine kinase, so it phosphorylates kinase resides on other proteins, all tyrosine kinase are specific, phosphorylate a specific protein
-they work by dimerisation, bind the neurotrophin to the receptor, induces homodimerisation for trk b, trk c its nt 3
-NT3 is promiscuous, can somewhat activate trk b a little bit
-binding of neurotrphin to the receptor induces homodimerisation, in rest the receptors exist as monodimers and only upon binding do they come to be dimers!
What neurotrophin binds to which receptor?
What is p75 receptor?
- a receptor that cooperates with trk A to make a high affinity NGF receptor, it was thought that there were to types of receptors one high and one low affinity it was discovered that the low affinity is the p75 is the low affinity and binds to all the neurotrophins, the high affinity receptor is the mix of p75 and TrK A, likely that the case is the same with Trk B etc but not as well proven
-Trk alone has medium to high affinity but with p75 it is high affinity
-homodimerisation after binding of ligand and initiation o signalling sequence to the nucleus of the cell, in development it detrmines survival and death, in adult it determines function (how healthy they are)
-ACh neurons are the only NGF dependent neurons in the brain!
-if don’t have any Trks, and have p75 alone then have a different function= acts as a death signal
-p75 amplifies the dependence of a cell on the growth factor
What are the p75 functions?
-The p75 neurotrophin receptor binds to all 4 neurotrophins with equal affinity.
-p75’s functions in conjunction with each TrK receptor, increases specificity and sensitivity
-in the absence of Trk receptors it causes cell death
What is the low and high affinity NGF receptors?
– Low affinity Receptor Kd = 10-9 M
– High affinity Receptor Kd = 10-11 M
– The low affinity NGF receptor has a molecular weight of 75 kD and was known as the p75NGFR It is now known as the p75NTR.
– The high affinity receptor consists of a high MW protein complex of TrkA and p75NTR
-the higher the numbe rthe higher the affinity
-the neurotrophins are present in minute amounts as can respond
What is this?
-in gestation there is period where sensory neurons are dependent on NGF in terms of survival, later just dependent
Do neurotrophins have distinct effects on different target neurons?
-yes -different neurons respond to different neurotrohins
-merkel disk= touch dependent BDNF
What are the target populations of NGF?
-Nociceptive sensory neurons
-Sympathetic ganglia neurons (all)
-Cholinergic neurons of septum
What are the target populations of BDNF?
-Light touch sensory neurons
-Some motor neurons
-Dopaminergic neurons of midbrain
What are the target populations of NT-3?
-proprioceptive sensory neurons
What are the target populations of NT-4?
-Some sensory neurons
-? some motor neurons
What is the structure of the NGF?
– Consists of two identical 118 amino acid monomers
– The two monomers are not joined by disulfide or other covalent bond
(-bonded non covalently, but high affinity, very stable when dimerise, back to back structure one faces one way and the other the other way)
– Each monomer has a receptor (TrkA) binding site – Molecular weight of dimer is 26 kD
How does NGF bring receptors together?
-Each NGF Monomer has a Binding Site For the Receptor (TrkA)
-NGF binding brings two receptors together
-one dimer, binds two receptors, this is how they cause homodimerisation
-onceNGF bidns to one TRk A the other one quickly comes along and dimerisation happens (via diffusion)
When are receptor tyrosine kinases monomers and when dimers and how does the shift occur?
-Monomers in the unliganded state
-Dimers when liganded
-they cross phosphorylate, one of the monomers phosphorylates the other and vice versa
-now activatef TrkA and will binds to proteins that bind to phoshpotyrosines taht will signall the cell
What are the properties of receptor tyrosine kinases?
• They are often receptors for cell growth factors
• High affinity for ligand
• Dimerise when bound
• Dimerisation induces cross- phosphorylation on tyrosine residues
• Phosphorylated tyrosines bind adapters with “src” domains
What is the Ras-Erk pathway?
-The effects of Ras- Erk Pathway Stimulation depend on cell- type
-end up of phsophoryation of Erk , that is the key endpoints of trk signalling and when phosphorylated goes from the cytoplasm to the nucleus and stimulates transcription, changes gene expression
-this is when the receptors are dimerised and neurotrophin binds
What does ERK do?
• It is a kinase, normally resident in the cytoplasm
• When phosphorylated, it moves into the cell nucleus.
• In the nucleus, it phosphorylates certain transcription factors (transcription factors are proteins within the nucleus that control the transcription of specific genes)
What does the activation of TrkA set off?
• Fully activated TrkA has 4 critical phosphotyrosines
• Phosphotyrosines bind specific adaptor proteins eg proteins with “src” domains
• Each of the 4 phosphotyrosines in TrkA binds to different adaptors and thus may initiate different signalling processes
• TrkA initiates at least 3 signalling processes
What is this?
-Signaling via neurotrophins and their receptors
-don’t need the detial just the dimerisation, phosphorylation, signalling activation and tarcnsription
-Remember that the substrate for both PI3K and PLC is PIP2 PI3K: PIP2 → PIP3 PLC: PIP2 → DAG + IP3
What is generally true of neurotrophic signalling?
– Neurotrophins bind to specific receptors on nerve terminals – Endocytosis: Internalisation of bound receptors
– Local signalling at axon terminal
– Retrograde transport to cell body
– Signalling to cell nucleus
-via dinein go back, called signalling endosome, get signalling events locally, get immediate effects at the terminus and then later in the cell body via erk, the neurotriphin signalling initiates event on the terminal and also later in the cell body