Studies on caspase signaling in microglia

Abstract

The aim of this thesis is to investigate the roles of caspase-8 and caspase-3 in microglia and in brain disorders, beyond their function in apoptosis.

Microglia are resident immune cells of the central nervous system and act as the first line of defense against invading pathogens and other types of brain insults. They are important for normal brain homeostasis and can get rapidly activated upon infection or damage in the brain. However, dysregulated and overactivated microglia have also been reported to cause neurotoxicity and can further contribute to the damage in neurodegenerative diseases.

Caspases are a family of proteases that are important for signaling pathways controlling cell death, inflammation, proliferation and differentiation. Caspase-8 and -3 are well known for their functions in apoptosis, but can also be involved in other processes. We have earlier shown that upon pro-inflammatory stimuli, microglia activate caspase-8 and -3/7 which regulate their activation through a PKCδ-dependent pathway, without triggering of cell death.

In the first paper we investigate why caspase-3 under some circumstances kills the cells, but under others, like activation of microglia, does not. We show that upon pro-inflammatory stimuli of microglia, cIAP2 regulates the processing of caspase-3 so that it remains in the cytoplasm. Under cell death conditions caspase-3 can be fully processed and thereby enter the nucleus and exert its apoptotic functions.

In order to provide genetic evidence of caspase control of microglia activation, we have created a model in which caspase-8 is conditionally knocked out in microglia. The CASP8 knock-out mice showed a reduction in pro-inflammatory activated microglia, which were associated with reduced neurotoxicity in a mouse model of Parkinson´s disease. We are also presenting data on the temporal and spatial activation of caspase-8 and -3 in microglia cells in human subjects who suffered a stroke. In the last paper we screened for genes involved in the metabolism of amyloid precursor protein in patients with late-onset Alzheimer´s disease. Significant association was found for rare variants of CASP8 and late-onset of the disease. For two of the variants we performed functional studies, which indicated an altered caspase-8 protein expression and reduced enzymatic activity.

In summary, we have provided genetic evidence for caspase regulation of microglia proinflammatory activation and described a mechanism for how caspases prevent killing of cells during activation. We also describe the expression pattern of caspases in microglia cells after stroke, and provide evidence of rare mutations in caspase-8 associated with late-onset Alzheimer´s disease.