The evidence of the past decades, and particularly the past seven years, strongly supports the idea that the accumulation of senescent cells is a root cause of aging. Cells become senescent in large numbers day in and day out, a normal end of life state for somatic cells that have reached the Hayflick limit. Cells also become senescent as the result of damage, or a toxic environment, and there is ever more of that with advancing age. Near all of these cells are destroyed quite quickly after they enter a senescent state, but enough linger to ensure that a few percent of all cells are senescent in old age. These problem cells secrete a potent mix of signals that induces chronic inflammation, degrades tissue structure, and alters the behavior of normal cells for the worse.
Senescent cells are not the only component of aging, but given enough time senescent cells alone would be able to kill you. Senescent cells are a prominent cause of fibrosis and declining function of organs such as the lungs, kidneys, liver, and heart. They cause arthritis. Ever more immune cells are senescent in later life. The list goes on, and scientists are adding to it with each passing month, as ever more is discovered of the role of senescent cells in specific age-related conditions. The research I'll point out today is an example of the type, in this case early evidence that indicates senescent microglia in the brain are a contributing cause of synucleinopathies such as Parkinson's disease.
Synucleinopathies are associated with the aggregation of solid deposits of α-synuclein in the aging brain. Neurons are harmed by the halo of surrounding biochemistry that arrives alongside the presence of these protein aggregates. This is a similar story to that related to amyloid-β and tau: deposits in the brain; an associated collection of molecules and interactions that harm neurons; the association with age-related neurodegeneration. It will be most interesting to see how the exploration of cellular senescence in the supporting cells of the brain plays out in this context over the years ahead. How much of this protein aggregation in aging is driven by the secretions of senescent cells, and how greatly can the onset of these conditions be delayed by targeted destruction of those senescent cells?
An example of the changing environment in the aging brain is the changes in the supporting cells in the brain, including microglia. Healthy microglia monitor their environment, phagocytosing debris, and releasing numerous molecules that can impact other cells. Activated microglia can act as antigen presenting cells and activate T-cells. After an infection has been dealt with microglia can recruit cells that are involved in neuronal repair and secrete anti-inflammatory cytokines. The idea of aging microglia stems from histological observations of healthy aged brains where the cells often develop dystrophic phenotypic characteristics. Dystrophic microglia have also been associated with the increased release of toxic reactive oxygen species and inflammatory cytokines and impaired phagocytic ability. However, one of the most unique changes observed in dystrophic microglia in the aging brain is the very high accumulation of iron, which is found to be stored in proteins, such as ferritin.
The presence of healthy glial cells is critically important to neuronal wellbeing. Microglia maintain homeostasis in the healthy brain and fight infection, when it is present, through a complicated system of signalling molecules. The importance of microglia to neurons is supported by higher incidence of dystrophic microglia and microglial apoptosis in Alzheimer's disease. The inflammation of the nervous system in neurodegenerative disease was thought to be due to activated microglia. However, low, but sustained, release of inflammatory factors and impaired neuroprotective ability of microglia seen in neurodegeneration could be due to dystrophic changes instead.
The cytosolic protein alpha-synuclein (α-syn) is associated with a range of neurodegenerative diseases, including Parkinson's disease (PD). In PD there has been discussion of the possible involvement of microglia and experiments with rodent PD models have shown that microglial activation can cause PD-like symptoms. In the current work, we establish iron overload as a mechanism to switch microglial phenotype to one that has many of the characteristics of senescent microglia. Iron overload was achieved by growing microglia in high concentrations of iron. We also show that iron overloaded (iron-fed) microglia release factors, including increased levels of the cytokine TNFα that caused an increased expression of α-syn, altered its activity, and increased its aggregation.
Developing a model of senescent/dystrophic microglia in vitro has numerous issues. Chief among these is the lack of clarity in defining dystrophic microglia. There is currently no single molecular marker that would define a dystrophic or senescent microglial cell. Proteomics/transcriptomics based studies comparing microglia from old and young brains have been carried out for both mouse and human but have yielded conflicting results. However, there is a general cellular senescence signature that all cells show and this is no different for microglia, which also show characteristics aligning with a senescence-associated secretory phenotype.
Synucleinopathies are associated with the aggregation of α-syn in cells and this is believed to stem from two causative processes. The first and most well recognized is an increased expression of α-syn, resulting in molecular crowding. Using conditioned medium from our model dystrophic microglia, we were able to induce increased α-syn expression and increased aggregation in SH-SY5Y cells. Thus, by the incorporation of an aspect of brain aging we were able to induce several aspects of the disease state in neuronal cells. For this reason, we believe that we have developed a simple and valuable tool for the exploration of the molecular mechanisms behind synuclein related diseases and possibly other neurodegenerative diseases.