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SUMMARYGroup AStreptococcus(GAS) infections can cause neuropsychiatric sequelae in children due to post-infectious encephalitis. Multiple GAS infections induce migration of Th17 lymphocytes from the nose into the brain, which are critical for microglial activation, blood-brain barrier (BBB) and neural circuit impairment in a mouse disease model. How endothelial cells (ECs) and microglia respond to GAS infections, and which Th17-derived cytokines are essential for these responses are unknown. Using single-cell RNA sequencing and spatial transcriptomics, we found that ECs downregulate BBB genes and microglia upregulate interferon-response, chemokine and antigen-presentation genes after GAS infections. Several microglial-derived chemokines were elevated in patient sera. Administration of a neutralizing antibody against interleukin-17A (IL-17A), but not ablation of granulocyte-macrophage colony-stimulating factor (GM-CSF) in T cells, partially rescued BBB dysfunction and microglial expression of chemokine genes. Thus, IL-17A is critical for neuropsychiatric sequelae of GAS infections and may be targeted to treat these disorders.

31 p.-12 fig.-1 tab. We dedicate this paper to Takashi Umekawa, who generated the HI model at the Karolinska Institute, and unfortunately passed away in 2018., Microglial phagocytosis of apoptotic debris prevents buildup damage of neighbor neurons and inflammatory responses. Whereas microglia are very competent phagocytes under physiological conditions, we report their dysfunction in mouse and preclinical monkey models of stroke (macaques and marmosets) by transient occlusion of the medial cerebral artery (tMCAo). By analyzing recently published bulk and single cell RNA sequencing databases, we show that the phagocytosis dysfunction was not explained by transcriptional changes. In contrast, we demonstrate that the impairment of both engulfment and degradation was related to energy depletion triggered by oxygen and nutrient deprivation (OND), which led to reduced process motility, lysosomal exhaustion, and the induction of a protective macroautophagy/autophagy response in microglia. Basal autophagy, in charge of removing and recycling intracellular elements, was critical to maintain microglial physiology, including survival and phagocytosis, as we determined both in vivo and in vitro using pharmacological and transgenic approaches. Notably, the autophagy inducer rapamycin partially prevented the phagocytosis impairment induced by tMCAo in vivo but not by OND in vitro, where it even had a detrimental effect on microglia, suggesting that modulating microglial autophagy to optimal levels may be a hard to achieve goal. Nonetheless, our results show that pharmacological interventions, acting directly on microglia or indirectly on the brain environment, have the potential to recover phagocytosis efficiency in the diseased brain. We propose that phagocytosis is a therapeutic target yet to be explored in stroke and other brain disorders and provide evidence that it can be modulated in vivo using rapamycin. Abbreviations: AIF1/IBA1: allograft inflammatory factor 1; AMBRA1: autophagy/beclin 1 regulator 1; ATG4B: autophagy related 4B, cysteine peptidase; ATP: adenosine triphosphate; BECN1: beclin 1, autophagy related; CASP3: caspase 3; CBF: cerebral blood flow; CCA: common carotid artery; CCR2: chemokine (C-C motif) receptor 2; CIR: cranial irradiation; Csf1r/v-fms: colony stimulating factor 1 receptor; CX3CR1: chemokine (C-X3-C motif) receptor 1; DAPI: 4’,6-diamidino-2-phenylindole; DG: dentate gyrus; GO: Gene Ontology; HBSS: Hanks’ balanced salt solution; HI: hypoxia-ischemia; LAMP1: lysosomal-associated membrane protein 1; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MCA: medial cerebral artery; MTOR: mechanistic target of rapamycin kinase; OND: oxygen and nutrient deprivation; Ph/A coupling: phagocytosis-apoptosis coupling; Ph capacity: phagocytic capacity; Ph index: phagocytic index; SQSTM1: sequestosome 1; RNA-Seq: RNA sequencing; TEM: transmission electron microscopy; tMCAo: transient medial cerebral artery occlusion; ULK1: unc-51 like kinase 1. © 2023 The Author(s)., This work was supported by grants from the Spanish Ministry of Science and Innovation Competitiveness MCIN/AEI/10.13039/501100011033 (https://www.ciencia.gob.es/) and ERDF “A way to make Europe” (RTI2018-099267-B-I00 and RYC-2013-12817 to AS; RTI2018–097948-A-100 and RYC-2016–20480 to OP), a Tatiana Foundation Award (P-048-FTPGB 2018) to AS a Basque Government Department of Education project (PIBA 2020_1_0030; http://www.euskadi.eus/basque-government/department-education/) to AS, a Basque Government Department of Economic development, Sustainability and environment (ELKARTEK KK-2020/00034; https://www.spri.eus/en/) to ECZ, and . SB is recipient of predoctoral fellowship from the Spanish Ministry of Economy and Competitiveness, and VST is recipient of predoctoral fellowship from the Basque Government.

Interferon regulatory factor 8 (IRF8) is a transcription factor necessary for the maturation of microglia, as well as other peripheral immune cells. It also regulates the transition of microglia and other immune cells to a pro-inflammatory phenotype. Irf8 is also a known risk gene for multiple sclerosis and lupus, and it has recently been shown to be downregulated in schizophrenia. While most studies have focused on IRF8-dependent regulation of immune cell function, little is known about how it impacts neural circuits. Here, we show by RNAseq from Irf8(−/−) male and female mouse brains that several genes involved in regulation of neural activity are dysregulated. We then show that these molecular changes are reflected in heightened neural excitability and a profound increase in susceptibility to lethal seizures in male and female Irf8(−/−) mice. Finally, we identify that TNF-α is elevated specifically in microglia in the CNS, and genetic or acute pharmacological blockade of TNF-α in the Irf8(−/−) CNS rescued the seizure phenotype. These results provide important insights into the consequences of IRF8 signaling and TNF-α on neural circuits. Our data further suggest that neuronal function is impacted by loss of IRF8, a factor involved in neuropsychiatric and neurodegenerative diseases. SIGNIFICANCE STATEMENT Here, we identify a previously unknown and key role for interferon regulator factor 8 (IRF8) in regulating neural excitability and seizures. We further determine that these effects on neural circuits are through elevated TNF-α in the CNS. As IRF8 has most widely been studied in the context of regulating the development and inflammatory signaling in microglia and other immune cells, we have uncovered a novel function. Further, IRF8 is a risk gene for multiple sclerosis and lupus, IRF8 is dysregulated in schizophrenia, and elevated TNF-α has been identified in a multitude of neurologic conditions. Thus, elucidating these IRF8 and TNF-α-dependent effects on brain circuit function has profound implications for understanding underlying, therapeutically relevant mechanisms of disease.

The olfactory system’s ability to detect and discriminate between the vast array of chemicals present in the environment is critical for an animal’s survival. In mammals, the first step of this odor processing is executed by olfactory sensory neurons, which project their axons to a stereotyped location within the olfactory bulb (OB) to form glomeruli. The stereotyped positioning of glomeruli within the OB suggests an importance for this organization in odor perception. However, because the location of only a limited subset of glomeruli has been determined it has been challenging to determine the relationship between glomerular location and odor discrimination. Using a combination of single-cell RNA sequencing (scRNAseq), spatial transcriptomics, and machine learning, we have generated a map of the majority of glomerular positions within the mouse OB. These observations significantly extend earlier studies and suggest an overall organizational principle within the OB that may be used by the brain to assist in odor decoding.

Microglial phagocytosis is rapidly emerging as a therapeutic target in neurodegenerative and neurological disorders. An efficient removal of cellular debris is necessary to prevent buildup damage of neighbor neurons and the development of an inflammatory response. As the brain professional phagocytes, microglia are equipped with an array of mechanisms that enable them to recognize and degrade several types of cargo, including neurons undergoing apoptotic cell death. While microglia are very competent phagocytes of apoptotic cells under physiological conditions, here we report their dysfunction in mouse and monkey (Macaca fascicularis and Callithrix jacchus) models of stroke by transient occlusion of the medial cerebral artery (tMCAo). The impairment of both engulfment and degradation was related to energy depletion triggered by oxygen and nutrients deprivation (OND), which led to reduced process motility, lysosomal depletion, and the induction of a protective autophagy response in microglia. Basal autophagy, which is in charge of removing and recycling intracellular elements, was critical to maintain microglial physiology, including survival and phagocytosis, as we determined both in vivo and in vitro using knock-out models of autophagy genes and the autophagy inhibitor MRT68921. Notably, the autophagy inducer rapamycin partially prevented the phagocytosis impairment induced by tMCAo in vivo but not by OND in vitro. These results suggest a more complex role of microglia in stroke than previously acknowledged, classically related to the inflammatory response. In contrast, here we demonstrate the impairment of apoptotic cell phagocytosis, a microglial function critical for brain recovery. We propose that phagocytosis is a therapeutic target yet to be explored and provide evidence that it can be modulated in vivo using rapamycin, setting the stage for future therapies for stroke patients.

Summary Early prenatal inflammatory conditions are thought to be a risk factor for different neurodevelopmental disorders. Maternal interleukin-6 (IL-6) elevation during pregnancy causes abnormal behavior in offspring, but whether these defects result from altered synaptic developmental trajectories remains unclear. Here we showed that transient IL-6 elevation via injection into pregnant mice or developing embryos enhanced glutamatergic synapses and led to overall brain hyperconnectivity in offspring into adulthood. IL-6 activated synaptogenesis gene programs in glutamatergic neurons and required the transcription factor STAT3 and expression of the RGS4 gene. The STAT3-RGS4 pathway was also activated in neonatal brains during poly(I:C)-induced maternal immune activation, which mimics viral infection during pregnancy. These findings indicate that IL-6 elevation at early developmental stages is sufficient to exert a long-lasting effect on glutamatergic synaptogenesis and brain connectivity, providing a mechanistic framework for the association between prenatal inflammatory events and brain neurodevelopmental disorders., Graphical abstract, Highlights • Prenatal IL-6 causes increases in excitatory synapses and brain connectivity in adults • IL-6 activates genetic programs of synaptogenesis in developing neurons • Transcription factor STAT3 is activated in neurons upon IL-6 elevation • The STAT3 downstream gene Rgs4 is responsible for the increase in excitatory synapses, Prenatal inflammation is a risk factor for different neurodevelopmental disorders, but the mechanisms by which brain connectivity is affected remain unclear. Mirabella et al. demonstrate that transient maternal elevation of IL-6 induces an abnormal, long-lasting increase of excitatory synapses and brain connectivity in the offspring, providing a mechanistic link between maternal immune activation and defects in newborn brain development.

Nearly 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this time, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here, we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synaptic components, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programs may go awry in neurodevelopmental disorders of the human CNS with a focus on autism spectrum disorders (ASD) and schizophrenia (SZ). Together, we intend to give an overview of how the field of activity-dependent pruning has evolved, has led to exciting new questions, and has guided the identification of new, therapeutically-relevant mechanisms by which circuit development can go awry in neurodevelopmental disorders.

Mutations in the astrocyte-enriched enzyme aldehyde dehydrogenase 7a1 (ALDH7A1) cause a neonatal epilepsy accompanied by treatment-resistant, inter-ictal neuropsychiatric symptoms. Nevertheless, the mechanistic impact of ALDH7A1 dysfunction in the brain remains elusive. We generated ALDH7A1 knockout mice and report that constitutive global ALDH7A1 depletion increases chemoconvulsant sensitivity and altered mood-associated behaviors. However, contrary to our expectation, astrocyte-specific ALDH7A1 depletion only affects mood-associated behaviors. Accordingly, in astrocyte-specific ALDH7A1 knockout mice, we show enhanced redox-sensitive microdomain Ca2+signaling in astrocytes and both elevated synaptic inhibitory tone and increased dendritic spine density in prelimbic pyramidal neurons. Sulforaphane (SFN), an indirect antioxidant and dietary supplement, has been explored as a possible treatment to ameliorate neuropsychiatric manifestations in autism and schizophrenia, at least at the clinical levels, but its mechanism in the brain is unclear. Here we show that SFN rescues both the physiological and behavioral changes by targeting astrocytic redox imbalance in ALDH7A1 knockout mice, implicating astrocyte redox changes in creating cortical excitatory-inhibitory imbalance and mood alteration.

Schizophrenia (SZ) is a devastating psychiatric condition affecting numerous brain systems. Recent studies have identified genetic factors that confer an increased risk of SZ and participate in the disease etiopathogenesis. In parallel to such bottom-up approaches, other studies have extensively reported biological changes in patients by brain imaging, neurochemical and pharmacological approaches. This review highlights the molecular substrates identified through studies with SZ patients, namely those using top-down approaches, while also referring to the fruitful outcomes of recent genetic studies. We have subclassified the molecular substrates by system, focusing on elements of neurotransmission, targets in white matter-associated connectivity, immune/inflammatory and oxidative stress-related substrates, and molecules in endocrine and metabolic cascades. We further touch on cross-talk among these systems and comment on the utility of animal models in charting the developmental progression and interaction of these substrates. Based on this comprehensive information, we propose a framework for SZ research based on the hypothesis of an imbalance in homeostatic signaling from immune/inflammatory, oxidative stress, endocrine and metabolic cascades that, at least in part, underlies deficits in neural connectivity relevant to SZ. Thus, this review aims to provide information that is translationally useful and complementary to pathogenic hypotheses that have emerged from genetic studies. Based on such advances in SZ research, it is highly expected that we will discover biomarkers that may help in the early intervention, diagnosis or treatment of SZ.

Mitochondria play a crucial role in neuronal function, especially in energy production, generation of reactive oxygen species, and calcium signaling. Multiple lines of evidence have suggested possible involvement of mitochondrial deficits in major psychiatric disorders, such as schizophrenia and bipolar disorder. In the first half, this review will outline the current understanding of the physiological role of mitochondria and their dysfunction under pathological conditions, particularly in psychiatric disorders. Nevertheless, the current knowledge about mitochondrial deficits in these disorders is somewhat limited, due to the lack of effective methods to dissect dynamic changes in the functional deficits that are directly associated with psychiatric conditions. Human neuronal cell model systems, which have been dramatically developed in recent years through the use of stem cell technology, may be key tools for overcoming this dilemma and improving our understanding of the dynamic changes in the mitochondrial deficits in patients with psychiatric disorders. We introduce recent discoveries from new experimental models and conclude the discussion by referring to future perspectives. In the perspectives, we emphasize the significance of combining studies of human neuronal cell models with those of other experimental systems, including animal models.

Changes in inflammatory cascades have been implicated in the underlying pathophysiology of psychosis. Translocator protein 18 kDa (TSPO) has been used to assess neuroinflammatory processes in psychotic disorders. Nonetheless, it is unclear whether TSPO, a mitochondrial protein, can be interpreted as a general marker for inflammation in diseases involving psychosis. To address this question, we investigated TSPO signaling in representative mouse models for psychosis with inflammatory disturbances. The maternal immune activation and cuprizone short-term exposure models show different TSPO signaling. Furthermore, we observed similarities and differences in their respective stress pathways including stress hormone signaling and oxidative stress that are functionally interconnected with the inflammatory responses. We propose that more careful studies of TSPO distribution in neuroinflammation and other stress cascades associated with psychotic symptoms will allow us to understand the biological mechanisms underlying psychosis-related behaviors.

Neuregulin-1 (NRG1) and its receptor ErbB4 influence several processes of neurodevelopment, but the mechanisms regulating this signalling in the mature brain are not well known. DISC1 is a multifunctional scaffold protein that mediates many cellular processes. Here we present a functional relationship between DISC1 and NRG1-ErbB4 signalling in mature cortical interneurons. By cell type-specific gene modulation in vitro and in vivo including in a mutant DISC1 mouse model, we demonstrate that DISC1 inhibits NRG1-induced ErbB4 activation and signalling. This effect is likely mediated by competitive inhibition of binding of ErbB4 to PSD95. Finally, we show that interneuronal DISC1 affects NRG1-ErbB4-mediated phenotypes in the fast spiking interneuron-pyramidal neuron circuit. Post-mortem brain analyses and some genetic studies have reported interneuronal deficits and involvement of the DISC1, NRG1 and ErbB4 genes in schizophrenia, respectively. Our results suggest a mechanism by which cross-talk between DISC1 and NRG1-ErbB4 signalling may contribute to these deficits., Neuregulin-1 and DISC1 signalling pathways have both been linked to neurodevelopment and schizophrenia. Here, Seshadri et al. demonstrate that DISC1 negatively regulates NRG1-induced ErbB4 signalling in adult cortical interneurons both in vitro and in vivo, possibly via competitive binding to PSD95.