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Brain Cells Release Calreticulin That Attracts and Activates Microglia, and Inhibits Amyloid Beta Aggregation and
Kyle M Reid1, Emily J A Kitchener1, Claire A Butler1
1Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom.
Abstract:
Calreticulin is a chaperone, normally found in the endoplasmic reticulum, but can be released by macrophages into the extracellular medium. It is also found in cerebrospinal fluid bound to amyloid beta (Aβ). We investigated whether brain cells release calreticulin, and whether extracellular calreticulin had any effects on microglia and neurons relevant to neuroinflammation and neurodegeneration. We found that microglia release nanomolar levels of calreticulin when inflammatory-activated with lipopolysaccharide, when endoplasmic reticulum stress was induced by tunicamycin, or when cell death was induced by staurosporine, and that neurons release calreticulin when crushed. Addition of nanomolar levels of extracellular calreticulin was found to chemoattract microglia, and activate microglia to release cytokines TNF-α, IL-6 and IL-1β, as well as chemokine (C-C motif) ligand 2. Calreticulin blocked Aβ fibrillization and modified Aβ oligomerization, as measured by thioflavin T fluorescence and transmission electron microscopy. Extracellular calreticulin also altered microglial morphology and proliferation, and prevented Aβ-induced neuronal loss in primary neuron-glial cultures. Thus, calreticulin is released by microglia and neurons, and acts: as an alarmin to recruit and activate microglia, as an extracellular chaperone to prevent Aβ aggregation, and as a neuroprotectant against Aβ neurotoxicity.
Insights
Brain cells release calreticulin, an endoplasmic reticulum chaperone, which acts as an alarmin to recruit and activate microglia. It also prevents amyloid beta aggregation and protects neurons from toxicity.
Area of Science:
- Neuroscience
- Immunology
- Cell Biology
Background:
- Calreticulin is an endoplasmic reticulum chaperone found extracellularly, including in cerebrospinal fluid bound to amyloid beta (Aβ).
- The release and function of calreticulin by brain cells in neuroinflammation and neurodegeneration remain largely uncharacterized.
Purpose of the Study:
- To investigate calreticulin release by brain cells (microglia and neurons).
- To determine the effects of extracellular calreticulin on microglia and neurons in the context of neuroinflammation and neurodegeneration.
Main Methods:
- Assessing calreticulin release from microglia and neurons under various stress conditions (LPS, tunicamycin, staurosporine, crushing).
- Evaluating the impact of extracellular calreticulin on microglial recruitment, activation, cytokine release, morphology, and proliferation.
- Analyzing calreticulin's effect on amyloid beta (Aβ) fibrillization and oligomerization using thioflavin T fluorescence and transmission electron microscopy.
- Assessing neuroprotection against Aβ-induced neuronal loss in primary neuron-glial cultures.
Main Results:
- Microglia release nanomolar levels of calreticulin upon inflammatory activation, ER stress, or cell death.
- Neurons release calreticulin when subjected to mechanical stress (crushing).
- Extracellular calreticulin chemoattracts and activates microglia, leading to the release of pro-inflammatory cytokines (TNF-α, IL-6, IL-1β) and chemokines (CCL2).
- Calreticulin inhibits Aβ fibrillization and modifies Aβ oligomerization.
- Extracellular calreticulin alters microglial morphology and proliferation and confers neuroprotection against Aβ toxicity.
Conclusions:
- Calreticulin is released by both microglia and neurons.
- Extracellular calreticulin functions as an alarmin, recruiting and activating microglia.
- Calreticulin acts as an extracellular chaperone, preventing Aβ aggregation, and exhibits neuroprotective properties against Aβ-induced neurotoxicity.
