Abdulkhalek Dakhel: What they do in the shadows: Uncovering hidden roles of astrocytes in the neurodegenerative brain

Abstract

The brain is the most complex and least understood organ in the human body. Its complexity stems from the high plasticity and the diverse functions of its components. Astrocytes are highly dynamic cells that modulate neuronal function and maintain brain homeostasis by conducting structural, defensive, communicative, and metabolic functions. Despite the growing interest in glial cells, much remains unknown about how astrocytes function in health and disease. This PhD aimed to explore understudied mechanisms regarding astrocytes’ role in neurodegenerative disease. To achieve that, experiments were conducted using hiPSC-derived in vitro models of Alzheimer’s disease (AD) and Parkinson’s disease (PD). In addition, many of the results were validated using patient-derived samples. Analysis was mainly performed using microscopy techniques supplemented by proteomic and immunologic methods.

Paper I reports the discovery of a novel form of astrocytic couriers named zombosomes. Zombosomes are nucleus-free, astrocyte-derived cellular fragments that retain motility, adhesion, and carry cellular organelles. They express all the common astrocytic markers, with vimentin being the most prominent. Importantly, zombosomes can act as mobile disease carriers, spreading α-synuclein between cells, potentially driving the progression of PD.

Paper II shows that astrocytes carrying α-synuclein, as well as Aβ and tau, can successfully infiltrate cerebral organoids. However, spreading following direct addition of fibrils proved to be more efficient than astrocyte-mediated propagation. In both exposure modes, the ability of aggregates to seed tau pathology was limited, underscoring some important caveats in the organoid model.

Paper III explores the diversity of brain vimentin using the 84-1 antibody clone, which detects a distinct pool of the protein. Interestingly, 84-1 identified intracellular vimentin deposits in the AD and PD brain sections that were absent from controls. Proteomic analysis revealed that this modified vimentin consists mainly of an N-terminal cleaved proteoform that can be released into the cerebrospinal fluid in response to AD and PD pathology.

Finally, Paper IV shows that zombosomes provide critical insights into the role of astrocytes in the formation of corpora amylacea (CA). CA and astrocytic zombosomes have comparable structural patterns and share key molecular components, including the characteristic PAS-positive polysaccharides. Moreover, chronic exposure to zombosomes led to the formation of CA-like structures in cultured astrocytes, suggesting a potential role of zombosomes in the accumulation of CA bodies.

Taken together, the results of this PhD thesis reveal new perspectives on the impact of astrocytes in neurodegenerative disease spreading and suggest an important role of vimentin in both homeostatic and disease-associated molecular pathways.