Microglia monitor and protect neuronal function via specialized somatic purinergic junctions

2019. december 13.

Microglia are the main immunocompetent cells of the nervous system and their role in brain development and maintenance of proper neuronal function throughout life is widely recognized. These cells perform dynamic surveillance of their microenvironment via motile microglial processes that constantly interact with neurons. Still, the molecular mechanisms of bidirectional microglia-neuron communication are unclear. To date, the majority of studies have focused on the interactions between microglial processes and synaptic elements. However, neurons are extremely polarized cells with a high degree of functional independence concerning metabolism and signal integration in their dendritic and axonal compartments, therefore this type of interaction seemed unlikely to be accountable for all microglia-neuron interactions.
In this study, we identified a novel communication site between microglia and neurons: during their dynamic surveillance activity, microglial processes form and maintain contacts with neuronal somata for up to several hours. These contacts - termed somatic purinergic junctions - are present on virtually every neuron, independently of their neurochemical identity in both the mouse and the human brain. Somatic junctions possess a unique ultrastructure that is tailored for exocytosis of mitochondria- or other organelle-derived messenger molecules. Through these junctions microglia are reacting to physiological or pathological changes in neuronal state exerting homeostatic and possibly neuroprotective functions. A key element of this communication is the microglia-specific purinergic receptor P2Y12, since acute blockade or genetic elimination of this receptor shortens the lifetime of somatic junctions, prevents increased microglial process coverage in response to neuronal activation or ischemia, and results in disturbed neuronal calcium-dynamics and increased lesion size following acute brain injury.
Genome-wide association studies, clinical data and experimental research collectively identify both microglia and neuronal mitochondria as key effectors in common neurological diseases including dementia, psychiatric disorders, acute brain injury and different forms of neurodegeneration. Our data provide evidence-based explanation for the first time on how microglia are capable of monitoring mitochondrial function in neurons and influencing neuronal fate directly at neuronal cell bodies, where the majority of mitochondria critical for cell fate decisions are localized. Since these specialized somatic junctions accommodate presently unexplored molecular fingerprints and signaling pathways optimized for intercellular communication, and are fundamentally different from microglia-synapse interactions, we suggest that microglia-neuron interactions through these sites could show profound alterations in different forms of acute and chronic neuropathologies, indicating their remarkable therapeutic potential.

(A) 3D reconstruction of a confocal volume scan showing a microglial cell (yellow) establishing contacts with the cell body of a cortical pyramidal cell (magenta) as well as with GABAergic (red) and glutamatergic (cyan) synapses. Insert shows electron tomographic reconstruction of a somatic junction. Please note the specific ultrastructure on the neuronal side (membrane: magenta) with a nearby mitochondrion (turquoise), mitochondrion-associated membrane (pale-green), reticular (red) and vesicular (blue) organelles. Microglial membrane (yellow) is in direct contact with neuronal membrane and microglial P2Y12 receptors (white) are concentrated around the core of the somatic junction. (B) Electron microscopic image shows a microglial process (mic., yellow) establishing direct membrane-to-membrane contact with the somata of a neuron (neu., magenta). A mitochondrion (mito, turquoise) and anchored reticular and vesicular organelles are visible. Black grains mark anti-P2Y12 receptor immunogold particles. (C) Somatic junctions were also observed in post mortem human neocortex. (D) The prevalence of direct contacts at a given time point with different populations of nerve cells and synapses. Somatic junctions are highly abundant in both mice and human. (E) Outline of chemogenetic experiments. Mice were injected with a neuron-specific construct coding a Gq-coupled receptor that can be activated with clozapine-N-oxide (CNO). Acute P2Y12 receptor blockade was achieved by intracisternal injection of PSB0739. (F) Representative images show increased microglial process coverage (yellow) on the cell body of a neuron expressing the inducible receptor (red) and showing increased neuronal activity (cFos, cyan) in response to CNO. The increase in microglial process coverage is completely absent, when P2Y12 receptors are inhibited with PSB. (G) 3D reconstruction from electron tomographic volume shows elevated microglial coverage and fragmentation of neuronal mitochondria following acute ischemic stroke. (H) CLSM images of cortical neurons show that in parallel with the declustering of Kv2.1-channels (cyan), microglial coverage (yellow) is significantly increased after stroke in the penumbra. (I) Microglial coverage of neuronal cell bodies is robustly increased after stroke, while acute central blockade of P2Y12Rs or activation of mitochondrial ATP-sensitive potassium (KATP) channels completely abolishes the stroke-induced increase of coverage. (J) Stroke induces a 1.5-fold increase in somatic microglia coverage of human cortical neurons. (K) Infarct volume is increased after acute central P2Y12R-inhibition.

Csaba Cserép, Balázs Pósfai, Nikolett Lénárt, Rebeka Fekete, Zsófia I. László, Zsolt Lele, Barbara Orsolits, Gábor Molnár, Steffanie Heindl, Anett D. Schwarcz, Katinka Ujvári, Zsuzsanna Környei, Krisztina Tóth, Eszter Szabadits, Beáta Sperlágh, Mária Baranyi, László Csiba, Tibor Hortobágyi, Zsófia Maglóczky, Bernadett Martinecz, Gábor Szabó, Ferenc Erdélyi, Róbert Szipőcs, Michael M. Tamkun, Benno Gesierich, Marco Duering, István Katona, Arthur Liesz, Gábor Tamás, Ádám Dénes* Science 12 Dec 2019.

These authors contributed equally. * Corresponding author.
E-mail: denes.adam@koki.mta.hu