Introduction
Reduction in cerebral blood flow (CBF) is the first clinically detectable symptom of Alzheimer’s Disease (AD) and is sufficient to cause cognitive decline, highlighting its therapeutic potential. To study this, we focus on how neuronal activity regulates local blood flow in the brain capillary network and how this is perturbed in AD (PMID: 31221773). Biogenic amines are well established modulators of CBF, and serotonin (5HT) specifically is inversely correlated with Aβ deposition (PMID: 21873225). Hence, our initial aim was to establish the molecular mechanism by which 5HT regulates flow in physiology, and explore whether this pathway is perturbed in AD.
Methods
To address this, we follow a preclinical approach combining experiments on in vitro acute brain slices, with in vivo two-photon brain imaging of anaesthetised mice. To study amyloidosis, we are using the AppNL-G-F mouse model of AD, in which APP with a humanized Aβ region containing three AD-related mutations is knocked in to avoid artefacts associated with APP overexpression.
Results
Our preliminary results suggest that in healthy animals, 5HT increases calcium in astrocytes through 5HT2CR which in turn leads to pericyte contraction though the release of vasoactive lipids. To our surprise, AD mice not only show changes in 5HT axon density, but most importantly develop markedly enlarged neuronal processes termed axonal spheroids, whose effect on cortical interstitial 5HT level and circuit functions remains unknown. Similar structures can also be observed in cholinergic, noradrenergic and dopaminergic neurons, which differ dramatically from axonal spheroids in glutamatergic axons, with far more axon swelling closer to the plaque.
Conclusions
Conceivably, the peri-plaque environment is strongly dictated by the type of affected axons, and the clinically used acetylcholinesterase and monoamine reuptake inhibitors restore peri-plaque neurotransmitter concentrations in the cortex. We are currently investigating how the presence of cholinergic and monoaminergic axonal spheroids affects local circuit activity, microglial responses and blood flow.
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