ABSTRACT The biogenesis of small extracellular vesicles (sEVs) is only partially understood. Our recent findings provide evidence that a newly described sEV secretion pathway, the amphiectosome release followed by the sEV discharge by the ‘torn bag mechanism’ are present in all tested cell lines and mouse organs. Surprisingly, in in situ fixed steady‐state cells, transmission electron microscopy did not reveal sEV release via exocytosis of multivesicular endosomes (MVEs). In the current study, we extended our previous analysis to additional mouse organs and confirmed the presence of secreted amphiectosomes in all of them. Furthermore, we investigated which parameters influence the activation of the distinct sEV release mechanisms in HEK cells. Our results show that under stress conditions (such as Ca2+ ionophore‐induced membrane stress or metabolic stress, induced by serum starvation), exocytosis of MVEs is activated, while this process is absent in steady‐state conditions. By silencing ATG5 (a key regulator of autophagy) and RAB27a (an essential small GTPase for MVE exocytosis), we selectively modulated these two mechanisms, respectively. Amphiectosome release depended on both autophagy and ATG5, while exocytosis of MVE was autophagy‐independent but RAB27a‐dependent. Our findings suggest that sEV release via the ‘torn bag mechanism’ is a general and essential secretion pathway in non‐stressed, steady‐state mammalian cells, while stress conditions induce the sEV release via MVE exocytosis.

Microglia are unique damage sensors of the central nervous system, and their homeostatic roles are increasingly recognized. Purinergic signaling through the P2Y12 receptor (P2Y12R) is indispensable for directed process movement of microglia in response to danger-related ATP release. P2Y12R has also been shown to modulate microglial communication with neurovascular elements in the brain and to profoundly influence outcomes in experimental models of brain injury. However, the exact role of P2Y12R in shaping microglial phenotypes and interactions under physiological conditions remains unresolved due to disagreements between ex vivo and in vivo observations. Using in vivo 3D two-photon imaging and high-resolution anatomy we show that P2Y12Rs are essential regulators of microglial physiology, fundamentally shaping homeostatic microglial surveillance activity and direct contacts with other cell types. Genetic deletion or acute pharmacological blockade of P2Y12R function leads to altered surveillance activity, microglial morphology and P2Y12R nanoclustering, resulting in changes of direct microglial contacts with neuronal cell bodies, smooth muscle-bearing blood vessels and oligodendrocyte processes in the somatosensory cortex of mice. Furthermore, molecular anatomy of P2Y12R expression shows correlation with disease severity and altered microglia-neuron interactions in human epilepsy. Thus, our results identify P2Y12Rs as major participants in microglial physiology whose dysfunction could impact defined cell-cell interactions in different neurological states.

Recent studies showed an unexpected complexity of extracellular vesicle (EV) biogenesis pathways. We previously found evidence that human colorectal cancer cells in vivo release large multivesicular body-like structures en bloc. Here, we tested whether this large EV type is unique to colorectal cancer cells. We found that all cell types we studied (including different cell lines and cells in their original tissue environment) released multivesicular large EVs (MV-lEVs). We also demonstrated that upon spontaneous rupture of the limiting membrane of the MV-lEVs, their intraluminal vesicles (ILVs) escaped to the extracellular environment by a ‘torn bag mechanism’. We proved that the MV-lEVs were released by ectocytosis of amphisomes (hence, we termed them amphiectosomes). Both ILVs of amphiectosomes and small EVs separated from conditioned media were either exclusively CD63 or LC3B positive. According to our model, upon fusion of multivesicular bodies with autophagosomes, fragments of the autophagosomal inner membrane curl up to form LC3B positive ILVs of amphisomes, while CD63 positive small EVs are of multivesicular body origin. Our data suggest a novel common release mechanism for small EVs, distinct from the exocytosis of multivesicular bodies or amphisomes, as well as the small ectosome release pathway.

COVID-19 is associated with diverse neurological abnormalities, but the underlying mechanisms are unclear. We hypothesized that microglia, the resident immune cells of the brain, are centrally involved in this process. To study this, we developed an autopsy platform allowing the integration of molecular anatomy, protein and mRNA datasets in postmortem mirror blocks of brain and peripheral organ samples from cases of COVID-19. We observed focal loss of microglial P2Y12R, CX3CR1-CX3CL1 axis deficits and metabolic failure at sites of virus-associated vascular inflammation in severely affected medullary autonomic nuclei and other brain areas. Microglial dysfunction is linked to mitochondrial injury at sites of excessive synapse and myelin phagocytosis and loss of glutamatergic terminals, in line with proteomic changes of synapse assembly, metabolism and neuronal injury. Furthermore, regionally heterogeneous microglial changes are associated with viral load and central and systemic inflammation related to interleukin (IL)-1 or IL-6 via virus-sensing pattern recognition receptors and inflammasomes. Thus, SARS-CoV-2-induced inflammation might lead to a primarily gliovascular failure in the brain, which could be a common contributor to diverse COVID-19-related neuropathologies.

Microglia are resident immune cells of the central nervous system (CNS) that dynamically adapt to their microenvironment to achieve multiple housekeeping roles. While ex vivo and in vitro models are instrumental tools to study microglial function, the slicing or culturing process inherently leads to markedly altered microglial phenotypes. Understanding the nature of these limitations and developing better ex vivo and in vitro models are crucial for enhancing the utility of these methods. In this review, we discuss recent developments in ex vivo and in vitro microglia models, from cell cultures to brain slices, focusing on the mechanisms that may need to be considered when using these tools and interpreting the obtained results. We suggest that limitations of ex vivo and in vitro models also provide opportunities to better understand the mechanisms driving microglial phenotype changes in various disease states.

The biogenesis of small extracellular vesicles (sEVs) is only partially understood. Our recent findings provide evidence that a newly described sEV secretion pathway, the amphiectosome release and the “torn bag mechanism”, is present in all tested cell lines and in mouse liver and kidney. Surprisingly, in in situ fixed steady-state cells, transmission electron microscopy did not reveal the classical exosome secretion route, the sEV release via exocytosis of multivesicular endosomes (MVEs). In the current study, we investigated which parameters influence the activation of the two distinct sEV release mechanisms. Our results show that under stress conditions (such as Ca²⁺ ionophore-induced membrane stress or metabolic stress-induced by serum starvation), exocytosis of MVEs is activated, while this process is absent in steady-state conditions. By silencing ATG5 (a key regulator of autophagy) and RAB27a (essential small GTPase for MVE exocytosis), we selectively modulated these two mechanisms. Amphiectosome release depended on both autophagy and ATG5, while exocytosis of MVE was autophagy-independent but RAB27a-dependent. Our findings suggest that sEV release via the “torn bag mechanism” is a general and essential secretion pathway in non-stressed, steady-state mammalian cells, while stress conditions induce the sEV release via MVE exocytosis. ### Competing Interest Statement EIB is member of the Scientific Advisor Board of the Ludwig Boltzmann Institute for Nanovesicular Precision Medicine, Austria. TV is a scientific advisor of Bioxol Ltd (Hungary). AC and ZM are employees of Brain Vision Center Research Institute and Competence Centre Ltd. (Hungary) and participated in the development of the BioxML dye family for Bioxol Ltd. (Hungary). Hungarian National Research, Development and Innovation Office (NKFIH) NKFIH, 2019-2.1.7-ERA-NET-2021-00015, EKOP-25 European Union’s Horizon 2020 Research and Innovation Programme, 739593 Hungarian Academy of Sciences, ‘Momentum’ research grant LP2022-5/2022, Hungarian Brain Research Program (grant no. NAP2022-I-1/2022)

Microglia represent the main immunocompetent cell type in the parenchyma of the brain and the spinal cord, with roles extending way beyond their immune functions. While emerging data show the pivotal role of microglia in brain development, brain health and brain diseases, the exact mechanisms through which microglia contribute to complex neuroimmune interactions are still largely unclear. Understanding the communication between microglia and other cells represents an important cornerstone of these interactions, which may provide novel opportunities for therapeutic interventions in neurological or psychiatric disorders. As such, in line with studying the effects of the numerous soluble mediators that influence neuroimmune processes, attention on physical interactions between microglia and other cells in the CNS has increased substantially in recent years. In this chapter, we briefly summarize the latest literature on "microglial contactomics" and its functional implications in health and disease.

Microglia are crucial for maintaining brain health and neuron function. Here, we report that microglia establish connections with neurons using tunneling nanotubes (TNTs) in both physiological and pathological conditions. These TNTs facilitate the rapid exchange of organelles, vesicles, and proteins. In neurodegenerative diseases like Parkinson’s and Alzheimer’s disease, toxic aggregates of alpha-synuclein (α-syn) and tau accumulate within neurons. Our research demonstrates that microglia use TNTs to extract neurons from these aggregates, restoring neuronal health. Additionally, microglia share their healthy mitochondria with burdened neurons, reducing oxidative stress and normalizing gene expression. Disrupting mitochondrial function with antimycin A before TNT formation eliminates this neuroprotection. Moreover, co-culturing neurons with microglia and promoting TNT formation rescues suppressed neuronal activity caused by α-syn or tau aggregates. Notably, TNT-mediated aggregate transfer is compromised in microglia carrying Lrrk22(Gly2019Ser) or Trem2(T66M) and (R47H) mutations, suggesting a role in the pathology of these gene variants in neurodegenerative diseases.

Acute brain slices represent a workhorse model for studying the central nervous system (CNS) from nanoscale events to complex circuits. While slice preparation inherently involves tissue damage, it is unclear how microglia, the main immune cells and damage sensors of the CNS react to this injury and shape neuronal activity ex vivo. To this end, we investigated microglial phenotypes and contribution to network organization and functioning in acute brain slices. We reveal time-dependent microglial phenotype changes influenced by complex extracellular ATP dynamics through P2Y12R and CX3CR1 signalling, which is sustained for hours in ex vivo mouse brain slices. Downregulation of P2Y12R and changes of microglia-neuron interactions occur in line with alterations in the number of excitatory and inhibitory synapses over time. Importantly, functional microglia modulate synapse sprouting, while microglial dysfunction results in markedly impaired ripple activity both ex vivo and in vivo. Collectively, our data suggest that microglia are modulators of complex neuronal networks with important roles to maintain neuronal network integrity and activity. We suggest that slice preparation can be used to model time-dependent changes of microglia-neuron interactions to reveal how microglia shape neuronal circuits in physiological and pathological conditions. Microglia undergo rapid phenotype changes in response to tissue disturbances but the impact of these changes on neuronal networks is unclear. Here, the authors show that microglia contribute to neuronal synchrony in acute brain slices, while their marked phenotypic transformation is influenced by injury-related focal ATP events.

COVID-19 is associated with diverse neurological abnormalities, which predict poor outcome in patients. However, the mechanisms whereby infection-induced inflammation could affect complex neuropathologies in COVID-19 are unclear. We hypothesized that microglia, the resident immune cells of brain, are centrally involved in this process. To study this, we developed an autopsy platform allowing the integration of molecular anatomy-, protein- and mRNA data sets in post-mortem mirror blocks of brain and peripheral organ samples from COVID-19 cases. Nanoscale microscopy, single-cell RNA sequencing and analysis of inflammatory and metabolic signatures revealed distinct mechanisms of microglial dysfunction associated with cerebral SARS-CoV-2 infection. We observed focal loss of microglial P2Y12R at sites of virus-associated vascular inflammation together with dysregulated microglia-vascular-astrocyte interactions, Cx3Cr1-fractalkine axis deficits and mitochondrial failure in severely affected medullary autonomic nuclei and other brain areas. Microglial dysfunction occurs at sites of excessive synapse- and myelin phagocytosis and loss of glutamatergic terminals. While central and systemic viral load is strongly linked in individual patients, the regionally heterogenous microglial reactivity in the brain correlated with the extent of central and systemic inflammation related to IL-1 / IL-6 via virus-sensing pattern recognition receptors (PRRs) and inflammasome activation pathways. Thus, SARS-CoV-2-induced central and systemic inflammation might lead to a primarily glio-vascular failure in the brain, which could be a common contributor to diverse COVID-19-related neuropathologies.

Cerebral ischemia is a devastating condition that results in impaired blood flow in the brain leading to acute brain injury. As the most common form of stroke, occlusion of cerebral arteries leads to a characteristic sequence of pathophysiological changes in the brain tissue. The mechanisms involved, and comorbidities that determine outcome after an ischemic event appear to be highly heterogeneous. On their own, the processes leading to neuronal injury in the absence of sufficient blood supply to meet the metabolic demand of the cells are complex and manifest at different temporal and spatial scales. While the contribution of non-neuronal cells to stroke pathophysiology is increasingly recognized, recent data show that microglia, the main immune cells of the central nervous system parenchyma, play previously unrecognized roles in basic physiological processes beyond their inflammatory functions, which markedly change during ischemic conditions. In this review, we aim to discuss some of the known microglia-neuron-vascular interactions assumed to contribute to the acute and delayed pathologies after cerebral ischemia. Because the mechanisms of neuronal injury have been extensively discussed in several excellent previous reviews, here we focus on some recently explored pathways that may directly or indirectly shape neuronal injury through microglia-related actions. These discoveries suggest that modulating gliovascular processes in different forms of stroke and other neurological disorders might have presently unexplored therapeutic potential in combination with neuroprotective and flow restoration strategies.

Microglia, the main immunocompetent cells of the brain, regulate neuronal function, but their contribution to cerebral blood flow (CBF) regulation has remained elusive. Here, we identify microglia as important modulators of CBF both under physiological conditions and during hypoperfusion. Microglia establish direct, dynamic purinergic contacts with cells in the neurovascular unit that shape CBF in both mice and humans. Surprisingly, the absence of microglia or blockade of microglial P2Y12 receptor (P2Y12R) substantially impairs neurovascular coupling in mice, which is reiterated by chemogenetically induced microglial dysfunction associated with impaired ATP sensitivity. Hypercapnia induces rapid microglial calcium changes, P2Y12R-mediated formation of perivascular phylopodia, and microglial adenosine production, while depletion of microglia reduces brain pH and impairs hypercapnia-induced vasodilation. Microglial actions modulate vascular cyclic GMP levels but are partially independent of nitric oxide. Finally, microglial dysfunction markedly impairs P2Y12R-mediated cerebrovascular adaptation to common carotid artery occlusion resulting in hypoperfusion. Thus, our data reveal a previously unrecognized role for microglia in CBF regulation, with broad implications for common neurological diseases.

The NKCC1 ion transporter contributes to the pathophysiology of common neurological disorders, but its function in microglia, the main inflammatory cells of the brain, has remained unclear to date. Therefore, we generated a novel transgenic mouse line in which microglial NKCC1 was deleted. We show that microglial NKCC1 shapes both baseline and reactive microglia morphology, process recruitment to the site of injury, and adaptation to changes in cellular volume in a cell-autonomous manner via regulating membrane conductance. In addition, microglial NKCC1 deficiency results in NLRP3 inflammasome priming and increased production of interleukin-1β (IL-1β), rendering microglia prone to exaggerated inflammatory responses. In line with this, central (intracortical) administration of the NKCC1 blocker, bumetanide, potentiated intracortical lipopolysaccharide (LPS)-induced cytokine levels. In contrast, systemic bumetanide application decreased inflammation in the brain. Microglial NKCC1 KO animals exposed to experimental stroke showed significantly increased brain injury, inflammation, cerebral edema and worse neurological outcome. Thus, NKCC1 emerges as an important player in controlling microglial ion homeostasis and inflammatory responses through which microglia modulate brain injury. The contribution of microglia to central NKCC1 actions is likely to be relevant for common neurological disorders.

Microglia, the main immune cells of the central nervous system (CNS) have long been known for their remarkable sensitivity to tissue disturbance or injury, but its implications to the interpretation of results from ex vivo models of the CNS have remained largely unclear to date. To this end, we have followed the course of microglial phenotype changes and contribution to neuronal network organisation and functioning in acute brain slices prepared from mice, widely used to study the physiology of the brain from nanoscale events to complex circuits. We found that upon acute slice preparation, microglial cell bodies dislocate and migrate towards the surface of slices, alongside with rapidly progressing morphological changes and altered interactions with neurons. This is accompanied by gradual depolarization and downregulation of P2Y12 receptors, which are instrumental for microglia-neuron communication. Quantitative post-embedding immunofluorescent labelling reveals time-dependent increase in the number of excitatory and inhibitory synapses upon slice preparation in the cerebral cortex, which are markedly influenced by microglia. In line with this, the absence of microglia diminishes the incidence, amplitude and frequency of sharp wave-ripple activity in hippocampal slices. Collectively, our data suggest that microglia are not only inherent modulators of complex neuronal networks, but their specific actions on network reorganisation and functioning must be taken into account when learning lessons from ex vivo models of the CNS.### Competing Interest StatementThe authors have declared no competing interest.

Microglia, the resident immune cells of the brain, play important roles during development. Although bi-direc-tional communication between microglia and neuronal progenitors or immature neurons has been demon-strated, the main sites of interaction and the underlying mechanisms remain elusive. By using advanced methods, here we provide evidence that microglial processes form specialized contacts with the cell bodies of developing neurons throughout embryonic, early postnatal, and adult neurogenesis. These early develop-mental contacts are highly reminiscent of somatic purinergic junctions that are instrumental for microglia-neuron communication in the adult brain. The formation and maintenance of these junctions is regulated by functional microglial P2Y12 receptors, and deletion of P2Y12Rs disturbs proliferation of neuronal precur-sors and leads to aberrant cortical cytoarchitecture during development and in adulthood. We propose that early developmental formation of somatic purinergic junctions represents an important interface for microglia to monitor the status of immature neurons and control neurodevelopment.

1 Momentum Laboratory of Neuroimmunology, Institute of Experimental Medicine, Budapest, H-1083 Hungary 2 János Szentágothai Doctoral School of Neuroscience, Semmelweis University, Budapest, H-1083 Hungary 3 Department of Physiology, Faculty of Medicine, University of Debrecen, Debrecen, H-4012 Hungary 4 Doctoral School of Molecular Cellular and Immune Biology, Faculty of Medicine, University of Debrecen, Debrecen, H-4012 Hungary

SummaryMicroglia are the resident immune cells of the brain with multiple homeostatic and regulatory roles. Emerging evidence also highlights the fundamental transformative role of microglia in brain development. While tightly controlled, bi-directional communication between microglia and neuronal progenitors or immature neurons has been postulated, the main sites of interaction and the underlying mechanisms remain elusive. By using correlated light and electron microscopy together with super-resolution imaging, here we provide evidence that microglial processes form specialized nanoscale contacts with the cell bodies of developing and immature neurons throughout embryonic, early postnatal and adult neurogenesis. These early developmental contacts are highly reminiscent to somatic purinergic junctions that are instrumental for microglia-neuron communication in the adult brain. We propose that early developmental formation of somatic purinergic junctions represents an ideal interface for microglia to monitor the status of developing neurons and to direct prenatal, early postnatal and adult neurogenesis.

Authors: 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. Tamkun11, Benno Gesierich4, Marco Duering4,13, István Katona2, Arthur Liesz4,13, Gábor Tamás, Ádám Dénes*

Microglia, the main immunocompetent cells of the brain regulate neuronal function in health and disease, but their contribution to cerebral blood flow (CBF) remained elusive. Here we identify microglia as important modulators of CBF both under physiological conditions and during hypoperfusion. We show that microglia establish direct purinergic contacts with cells in the neurovascular unit that shape cerebral perfusion in both mice and humans. Surprisingly, the absence of microglia or blockade of microglial P2Y12 receptor (P2Y12R) substantially impairs neurovascular coupling in the barrel cortex after whisker stimulation. We also reveal that hypercapnia, which is associated with acidification, induces microglial adenosine production, while depletion of microglia reduces brain pH and impairs hypercapnia-induced vasodilation. Furthermore, the absence or dysfunction of microglia markedly impairs adaptation to hypoperfusion via P2Y12R after transient unilateral common carotid artery occlusion, which is also influenced by CX3CR1-mediated actions. Thus, our data reveal a previously unrecognized role for microglia in CBF regulation with broad implications for common neurological diseases.