Periodic Reporting for period 3 - MicroSynCom (Mechanisms of Microglia Synapse Communication)
Okres sprawozdawczy: 2023-09-01 do 2025-02-28
Microglia are innate immune cells of the brain, responsible for the immediate defense against pathogens. In the past, microglia were thought to be sessile cells serving as glue for neurons in the brain. Their name microglia originated from the two words “micro” and “glue”. Almost 20 years ago, researchers found that microglia are highly motile cells constantly scanning the brain parenchyma with their fine processes. A novel microscopy technique called “two-photon imaging” enabled to record movies of cells in the brain of living animals and led to this discovery. Although microglia seemed to have a primary role in defense and ingestion of leftover cell material, novel results pointed at a role at synapse stability and regulation of neuronal activity. We think that microglia are not just garbage collectors, they are also sensitive cells that are creators of new connections between neurons – the so-called synapses. The project MicroSynCom tries to reveal the mechanisms how microglia mediate the formation of new synapses. This would assign a novel, so far unknown, function to microglia. If we understand the mechanisms how microglia mediate the formation of new synapses, we may use this knowledge to initiate this function via drugs. There are several diseases, especially neurodegenerative and neurologic diseases that are characterized by a dramatic synapse loss. If we would find drugs that are able to trigger the formation of new synapses via microglia, these diseases may be ultimately treatable. Therefore, we aim to establish microglia as mediators of new synapse formation.
From the beginning of the project until the end of the reporting period, we have been working on objective 1. The goal was to show that microglia sense neurotransmitters at synapses in the hippocampal CA1 region. We tested different promotors in capsid-modified AAV6 viruses to express fluorescent sensors for the neurotransmitter acetylcholine and glutamate. Testing the serotype unfortunately revealed that the viral expression was not microglia-specific. However, we also tested transgenic mice that express the Ca2+-sensor GCaMP in microglia. These experiments turned out to be successful and decided to use Ca2+-imaging as a proxy for detecting neurotransmitter release. Furthermore, other groups developed further improved microglia-specific AAV-serotypes that we established in the lab. Although not initially planned, we established another novel fluorescent sensor for serotonin together with Olivia Massek. We could show that the sensor is suitable for in vivo imaging, which will lead to an improved spatio-temporal and longitudinal recording of serotonin in the brain of living animal models.
We also established several transgenic mousselines necessary for objective1. We analyzed Cx3cr1-CreERT2::a7nAChRfl/fl::Rosa26-tdTomatofl/fl mice to investigate sensing of acetylcholine by microglia. Microglia showed reduced fine processes motility upon tamoxifen-induced deletion of the a7-nAChR in microglia. Furthermore, we optogenetically modulated the activity of CA3 neurons. With activating or inactivating DREADDs, we switched CA3 neurons either ON or OFF. Since CA3 neurons send axons, the Schaffer collaterals, to the dendrites of CA1 neurons, we were able to activate or stop the release of glutamate in dorsal CA1 region of the hippocampus. We were able to show that microglial motility correlated with release of glutamate at these synapses. Furthermore, the stability of dendritic spines correlated with microglia motility supporting our hypothesis that microglia sense neurotransmitters and mediate formation and elimination of post-synapses.
We also established several transgenic mousselines necessary for objective1. We analyzed Cx3cr1-CreERT2::a7nAChRfl/fl::Rosa26-tdTomatofl/fl mice to investigate sensing of acetylcholine by microglia. Microglia showed reduced fine processes motility upon tamoxifen-induced deletion of the a7-nAChR in microglia. Furthermore, we optogenetically modulated the activity of CA3 neurons. With activating or inactivating DREADDs, we switched CA3 neurons either ON or OFF. Since CA3 neurons send axons, the Schaffer collaterals, to the dendrites of CA1 neurons, we were able to activate or stop the release of glutamate in dorsal CA1 region of the hippocampus. We were able to show that microglial motility correlated with release of glutamate at these synapses. Furthermore, the stability of dendritic spines correlated with microglia motility supporting our hypothesis that microglia sense neurotransmitters and mediate formation and elimination of post-synapses.
We established two-photon 3D-STED imaging that increases the axial resolution of our 2P-STED microscope up to 150 nm. We also performed these measurements with two fluorescent dyes, which has not been achieved before. This enabled us to simultaneously visualize microglia and dendritic spines at super-resolution in vivo. Furthermore, we established a novel fluorescent sensor for serotonin that will enable the measurement of the neurotransmitter in the brain of living model animals longitudinally. We were also able to establish that microglia motility correlates with release of glutamate from projections of CA3 neurons a long-standing question, that we solved. Furthermore, microglia contact rates of dendritic spines are pivotal for synapse formation and elimination.