Workshop 1 (Senescence, Apoptosis and Cellular Differentiation) Flashcards
What are the three mechanisms used by multi-cellular organisms to control cell number and development?
Quiescence, senescence and apoptosis.
What is quiescence?
Quiescence describes the reversible exit of a cell from the cell cycle and into a state of dormancy or inactivity. Quiescence is regulated by mitogen signalling.
What is senescence?
Senescence is the irreversible exit of a cell from the cell cycle. It was originally discovered in 1965 by Leonard Hayflick, who noted that some cells irreversibly exit the cell cycle after ~60 cell divisions - this is a phenomenon now known as replicative senescence. Senescence is a hallmark of ageing, and can be induced by telomere shortening or DNA damage. It acts as a protective mechanism against cancer through the blocking of cell division. Senescent cells are removed by macrophages.
How did Muñoz-Espín et al. describe senescence?
“In many ways, as a distinct cellular process, senescence can be equated to apoptosis, a tumor-suppressive mechanism that acts to functionally remove cells at risk of aberrant growth.”
How are senescent cells detected?
Senescent cells express beta-galactosidase activity; this can be detected by the addition of X-gal that stains senescent cells blue. This technique enables the simple discrimination of senescent cells in a population.
Describe the role of senescence in embryonic development.
Recent work has shown that senescence is also a developmentally-regulated program. It restricts growth of tissues and aids clearance of cells that are no longer required. Senescence requires expression of CDKI (mainly p21) proteins for cell cycle exit and cell clearance by macrophages.
Define ‘apoptosis’.
Apoptosis is a genetically-regulated process, occuring in specific tissues during development and disease, by which a cell destroys itself; the process is marked by the breakdown of most cellular components and a series of well-defined morphological changes.
Give examples of apoptosis in development.
Examples of apoptosis in development include the shortening of the tail of a tadpole as it matures into a frog, and in the development of claws in mice.
How is apoptosis identified?
Apoptosis can be identified by staining the mitochondrial protein cytochrome c, and then observing its movement within the cell as the mitochondrial membranes are disrupted. The same principle can be applied to the DNA in the nucleus of the cell, which is stained with ethidium bromide.
How is apoptosis regulated?
Apoptosis is regulated by a proteolytic cascade. Caspases are a family of proteins that are expressed in an inactive form and require proteolytic processing for activation. Initiator caspases respond to the pro-apoptotic stimulus and are first to be activated. Executioner caspases mediate the destruction of key cell components e.g. chromatin, proteins.
How are caspases processed?
Each caspase is initially synthesised as an inactive proenzyme (procaspase). Some procaspases are activated by proteolytic cleavage by an activated caspase in response to apoptotic signalling. Proteolysis of two procaspase molecules enables the association of two large and two small subunits into an active caspase tetramer, with the prodomains discarded.
Describe the structure of pro-apoptotic and anti-apoptotic proteins.
Why is the BH3 domain consistent throughout the different classes of pro- and anti-apoptotic proteins?
The BH3 domain mediates the interactions between the pro- and anti-apoptotic family members.
In the absence of an apoptotic stimulus, how do pro- and anti-apoptotic proteins interact?
In the absence of an apoptotic stimulus, Bcl2 proteins bind to, and inhibit, BH123 proteins on the outer membrane of the mitochondria and in the cytosol. This prevents the homodimerisation of BH123 and consequently inactivates the intrinsic apoptotic pathway.
In the presence of an apoptotic stimulus, how do pro- and anti-apoptotic proteins interact?
In the presence of an apoptotic stimulus, BH3-only protein are activated and bind to Bcl2 proteins, therefore repressing the inhibition of BH123. BH123 remains active, aggregates, and promotes the release of intermembrane mitochondrial proteins into the cytosol.
