Investigating the secretion of small heat shock proteins in reactive astrocytes in Alzheimer’s disease

Molecular chaperones have protective functions in neurodegeneration by preventing misfolding of aggregation-prone proteins. These functions have been mostly studied in neurons. However, members of the family of chaperones known as small heat shock proteins (sHSPs), including HSPB1, CRYAB, and HSPB8, are specifically expressed in glial cells rather than in neurons, and their levels increase in astrocytes in human post-mortem Alzheimer's disease (AD) brain. While traditionally considered intracellular proteins, novel extracellular functions have been attributed to chaperones in cancer or the immune system. Interestingly, HSPB1 is also found in senile plaques suggesting these can also be extracellular in the brain. We hypothesize that sHSP can be extracellular and aim to investigate whether sHSPs are secreted from astrocytes in Alzheimer's disease brain.We used human post-mortem brain sections from Alzheimer's disease patients to investigate changes in extracellular/intracellular localization of HSPB1. To further explore sHSPs secretion, we used primary mouse astrocytes. Reactive astrocytes were induced by TNFα and IL-1α, mimicking the local environment in the diseased brain. We investigated changes in astrocyte cell lysates and the astrocyte conditioned media by Western blot. To investigate the secretion mechanisms, we also use size exclusion chromatography (SEC) and nanoparticle tracking analysis (NTA) RESULT: Although sHSPs are intracellular chaperones, Our analysis of AD patients post-mortem brain sections suggested that extracellular HSPB1 might increase around astrocytes that are close to amyloid plaques. In primary mouse astrocytes, we also found an increase in sHSPs in the extracellular media when astrocytes become reactive upon treatment with TNFα and IL-1α. We further investigated their mechanism of secretion by fractionating the astrocyte conditioned media using SEC. NTA and proteinase K protection assays show that the sHSPs were not present in extracellular vesicle fractions, but rather free in the media.Our study suggests that HSPB1 is secreted from astrocytes near to plaques in AD brain, as well as from reactive astrocytes in culture, and this secretion is not dependent on extracellular vesicles. The mechanisms behind how sHSPs are secreted and their extracellular functions still need to be explored.