The state of senescence is also known as the zombie state. Swollen cells in a state of senescence have lost their original features, function and affect nearby cells. This state is irreversible and there’s currently no effective reversal. Various cellular stresses are reported as the main cause inducing cellular senescence, including telomere exhaustion, oncogene expression and DNA damage. Cellular senescence has been recognized not only as a hallmark of in vitro cell culture decay but also as an important contributor to age-related disorders in a wide variety of tissues and organs.
An increase of senescent cells in culture reduces proliferation capacity and gradually decays the culture. Despite this, the zombie cells can affect other cells through a specific secretome: the senescence‐associated secretory phenotype (SASP). This includes chemokines, as well as inflammatory, pro-angiogenesis and growth regulators, serving as communication signals between cells (paracrine and autocrine signaling). The paracrine signaling of the SASP extends the malfunction of senescent cells from intracellular to tissue level, leading to changes in intracellular microenvironment.
Senescent cells generally have an enlarged and flattened cell morphology. However the appearance can differ between cell types. To more precisely verify a state of senescence, there are multiple markers that can be utilized to identify these zombie cells in vitro and in vivo. The commonly used markers include γ-H2AX, cyclin dependent kinase (CDK) inhibitors (Cdkn1a, Cdkn2a and Cdkn2d), nuclear Lamin B1 protein and lysosome abundance detected by senescence associated-β-galactosidase (SA-β-gal) staining. However, none of these markers can provide evidence the senescent state on its own. All these markers have their own cellular characteristics and participate in other biological pathways. Therefore, the zombie state needs to be evaluated by multiple markers in parallel to formulate a comprehensive judgment of the cell senescence.
For primary cultures, manufacturer protocols optimize culture confluency, reduces reactive oxygen species (ROS) propagation and therefore can minimize cellular senescence (following a well-designed instruction is important!). However primary cells have their limitation in vitro. The zombies can eventually appear with high cell passage. As all organs go through aging, utilizing senescent culture and exploring the field of senescence are becoming the latest research trend, eg. selective senescent cells removal through pharmaceutical approaches (fight the zombies!).
Written by
Ming
Scientific Support Specialist
