W4 Cell determination/senescence Flashcards
Positive feedback as memory
Signal causes A to be made (or activated)
A causes B to be made (or activated)
B causes A to be made (or activated)
Signal not needed anymore as continous cycle of +ve feedback is initiated
Example:
Melanocytes, MITF
Skeletal muscle, MYOD1 family
Melanocytes, MITF +ve feedback
MSH-MC1R signal
cAMP to MITF (via CREB activation)
MITF to cAMP (via MC1R transcription as protein which keeps cAMP pathway going)
MC1R also has some basal activity (without ligand MSH), so once MC1R is present, some cAMP and MITF continue to be made even if MSH not present
Melanocyte differentiation can thus be switched on by MSH, and stabilized even without it
MITF (Microphthalmia-associated Transcription Factor)
Master gene regulator for melanocytes
Severe mutations in mouse Mitf gene, when homozygous, cause loss of all melanocytes in the body (skin, hair, eyes)
Eyes become small - loss of pigmented retina.(microphthalmia = micro-ophthalmia)
So MITF is needed for all normal melanocyte development
Waardenburg syndrome 2
Mutation of 1 copy of the MITF gene in humans
Deafness and congenital patchy loss of pigment – in skin, and can include irises
So MITF also needed for human pigmentation
MSH-MC1R-cAMP signalling in melanocytes: effect on MITF
MC1R receptor for MSH becomes active when phosphorylated
Gs unit activates Ad cyclase
Converts ATP to cAMP
cAMP into PKA
PKA phosphorylates CREB
CREB (activated) binds to CRE (cAMP responsive element) which stimulates transcription of genes including MITF
MC1R signalling (through cAMP) increases [MITF] in melanocytes via transcription
Myogenic factors
Four interacting master gene regulators in skeletal muscle differentiation MYOD1 (=MYOD), MYF5, MYOG and MRF4
E proteins
Widely expressed transcription factors Myogenic factors normally work as dimers with E proteins
ID1
(Inhibitor of differentiation 1)
A protein in myoblasts (muscle precursor cells), which can strongly bind E proteins but not DNA
The MYOD family, E proteins in normal SM
MYOD1 + E bound together and bound to binding site on DNA in gene promoters
MYOD1, MYF5 etc bind & activate muscle gene promoters, working as dimers with E proteins
The MYOD family, E proteins and ID1 in myoblasts
ID1 binds strongly to E proteins, and prevents activation. ID1 has no DNA-binding domain
So ID1 inhibits differentiation
MYOD1 unable to bind DNA
The MYOD family in skeletal muscle differentiation in embryo
In migrating myoblasts ID1 bound to E to prevent differentiation
Low IGF + FGF at start of differentiation
ID1 destabilized
MYF5, MYOD1, MYOG + MRF4 promote each others transcription
All form active complexes w/E to activate muscle genes (myosin, actin, muscle creatine kinase, desmin AChR etc)
System of stable muscle differentiation gets switched on
Cell sensescence
A major defence against cancer
Strongly implicated in symptoms of ageing
Permanent cell growth arrest, following extended cell proliferation
At growth arrest cells split but do not divide
Molecular markers for senescent cells
Lysosomal b-galactosidase
All cells have some lysosomes, but senescent cells have many
Sometimes called “senescence-associated” b-galactosidase (SABG), but it is the same enzyme
Protein p16, a cell-cycle inhibitor - mechanism for stopping cells dividing
Telomeres and cell senescence
Telomeres: 1000’s of repeats of a hexamer sequence (TTAGGG) at chromosome ends
3’ ends of linear DNA can’t be replicated normally, because an RNA primer has to bind beyond the part to be replicated
So the enzyme telomerase is needed to maintain telomere length
Telomerase
The enzyme telomerase, a protein-RNA complex, can replicate telomeric DNA by reverse-transcribing DNA hexamers (TTAGGG) from its own RNA sequence, and joining them to the chromosome end
Telomerase activity is highest in germ cells (among normal cells), which thus have the longest telomeres (~15 kb in human)
TERT: telomerase reverse transcriptase (the protein part, = catalytic subunit).
TERC (or TR): telomerase RNA component
TERT
In humans, most somatic cells express no TERT, so no telomerase activity, so telomeres shorten as cells divide
Cancer cell lines in culture nearly all (~90%) express TERT, so they are immortal
Replicative senescence is triggered in normal cells when telomere(s) get quite short. (About 1-5 short telomeres sufficient.)
Established effector pathways of cell senescence
Telomere shortening > p53 > p21 (stops cell cycle, CDK1/2 + CDK4/6)
Radiation, oxidative stress + DNA damage > p16
Advanced cancer cells abnormalities
Expression of TERT
p53 defects
p16 defects
Cell senescence and ageing
Telomere length (measured in blood cells), variable, but on average falls with age. Typically very short in people aged >100.
p16 and other senescence-associated proteins are expressed increasingly in ageing tissues.
Telomere length at birth varies between people: genetically linked to age at death.
Defective genes for telomerase subunits give syndromes with premature ageing and early death.
p16 (CDKN2A) locus also genetically associated with human senile defects – cardiovascular disease, frailty, type II diabetes, neurodegeneration, cancer.
Stem cell
A cell that is capable of both self-replication (division into more cells like itself), and differentiation into one or more kinds of specialized, functional cells
Totipotent
Can form all functional cell types including placenta
The zygote is totipotent but is not normally considered to be a stem cell (it doesn’t divide to make more zygotes
Only in embryos
Examples of connections between cell senescence (including stem cell senescence) and ageing symptoms
Bone marrow: Older people show decreased immunity, increased bone marrow failure, decreased success rate as bone marrow donors. Reduced proliferative ability of marrow stem cells. Can’t divide much
Hair greying linked to decreased melanocyte stem cell maintenance in hair follicles (data from mice).
Reduced healing ability of skin with age, increased risk of skin ulcers. Proposed to be due to senescence in dermal fibroblasts.
However epidermal stem cells have very little telomere shortening & remain able to divide throughout life.
