Serping1/C1 Inhibitor Affects Cortical Development in a Cell Autonomous and Non-cell Autonomous Manner

Secretome analysis of oligodendrocytes and precursors reveals their roles as contributors to the extracellular matrix and potential regulators of inflammation

Oligodendrocytes form myelin that ensheaths axons and accelerates the speed of action potential propagation. Oligodendrocyte progenitor cells (OPCs) proliferate and replenish oligodendrocytes. While the myelin-forming role of oligodendrocytes and OPCs is well-established, potential additional roles of these cells are yet to be fully explored. Here, we analyzed the secreted proteome of oligodendrocytes and OPCs in vitro to determine whether these cell types are major sources of secreted proteins in the central nervous system (CNS). Interestingly, we found that both oligodendrocytes and OPCs secret various extracellular matrix proteins. Considering the critical role of neuroinflammation in neurological disorders, we evaluated the responses and potential contributions of oligodendrocytes and OPCs to this process. By characterizing the secreted proteomes of these cells after pro-inflammatory cytokine treatment, we discovered the secretion of immunoregulators such as C2 and B2m. This finding sheds new light on the hitherto underappreciated role of oligodendrocytes and OPCs in actively modulating neuroinflammation. Our study provides a comprehensive and unbiased proteomic dataset of proteins secreted by oligodendrocyte and OPC under both physiological and inflammatory conditions. It revealed the potential of these cells to secrete matrix and signaling molecules, highlighting their multifaceted function beyond their conventional myelin-forming roles.

Roles of complement system in psychiatric disorders

The complement system is an important part of the innate immune system, including more than 50 secretory proteins and membrane-bound proteins, and it contributes to the clearance of apoptotic cells and invading pathogens to limit inflammatory immune responses and maintaining brain homeostasis. Complement activity is strictly regulated to protect cells from random attacks or to prevent the deposition of complement proteins in physiological cases. However, overactivation or abnormal regulation of the complement cascade in the brain can lead to neuronal damage and brain dysfunction. Recent studies have pointed out that changes in complement molecules exist in patients with psychiatric diseases and play an important role in the occurrence and development of diseases by regulating the function of neurons and glial cells. Therefore, summarizing the latest research progress of complement system in psychiatric diseases such as schizophrenia, autism spectrum disorder, major depression, bipolar disorder and anxiety disorder can provide new ideas for preventing and controlling psychiatric diseases caused by abnormal activation of complement system.

Proteome profiling of the prefrontal cortex of Fmr1 knockout mouse reveals enhancement of complement and coagulation cascades

Fragile X mental retardation protein (FMRP) deficit resulted from mutations in its encoded fragile X mental retardation 1 (Fmr1) gene is a common inherited cause of Fragile X syndrome (FXS) characterized by intellectual disability and autism spectrum disorder (ASD). The FMRP absence-induced altered gene expression in prefrontal cortex (PFC) are associated with autistic behaviors. However, there lacks a large-scale protein profiling in the PFC upon loss of FMRP. This study used a TMT-labeled proteomic analysis to identify a protein profile of the PFC in the Fmr1 knockout mouse. A total of 5886 proteins were identified in the PFC with 100 differentially abundant proteins (DAPs) in response to FMRP deficiency. Bioinformatical analyses showed that these DAPs were mostly enriched in immune system, extracellular part and complement and coagulation cascades. The complement and coagulation cascades include 6 upregulated proteins (SERPING1, C1QA, C3, FGA, FGB and FGG), which are associated with fibrin degradation, cell lysis, degranulation chemotaxis and phagocytosis linked to activation of immune and inflammatory responses. Thus, our data provide an altered protein profile upon loss of FMRP in the PFC, and suggest that the enhancement of complement and coagulation cascades might contribute to etiological and pathogenic roles of ASD in FXS. SIGNIFICANCE: The etiology of autism spectrum disorder (ASD), a group of neurobiological disorders characterized by deficits in social interaction barriers and other abnormal behaviors, is still elusive. Autistic-like phenotypes are present in both Fragile X syndrome (FXS) patients and FMRP-deficiency FXS models. Given that prefrontal cortex is a critical brain area for social interaction, the FMRP absence induced-changes of a subset of proteins might contribute to ASD in FXS. Using a comprehensive proteomic analysis, this study provides a prefrontal protein profile of the FMRP-absent mouse with a total of 100 differentially abundant proteins (DAPs). Bioinformatic analyses suggest that these DAPs are mainly involved in the regulations of immune system and complement and coagulation cascades. We also show that 6 upregulated proteins (SERPING1, C1QA, C3, FGA, FGB and FGG) in the complement and coagulation cascades are associated with fibrin degradation, cell lysis, degranulation chemotaxis and phagocytosis regarding dysregulation of immune and inflammatory responses in the prefrontal cortex. Therefore, this study suggests that these FMRP-deficient DAPs in the prefrontal cortex might contribute to the etiology and pathogenesis of ASD in FXS.

Dysregulation of complement system in neuropsychiatric disorders: A mini review

Complement system is one of the most important defense mechanisms of the innate immune system. In addition to their roles in immune regulation, complement proteins are also involved in neurodevelopment and adult brain plasticity. Complement dysregulation has been shown in neurodevelopmental disorders including schizophrenia and autism spectrum disorder as well as in mood disorders. A number of clinical as well as genetic studies suggest the role of complement proteins in the cortical thinning and excessive synaptic pruning frequently associated with schizophrenia. The changes in complement proteins are also associated with the pathophysiology of autism spectrum disorder, major depressive disorder and bipolar disorder, but warrant further research. In addition, rodent models suggest a strong case for complement system in anxiety-like behavior. In this article, we review the recent findings on the role of complement system in neuropsychiatric disorders. The possible uses for future complement targeted therapies are also discussed.

Single-nuclei analysis reveals depot-specific transcriptional heterogeneity and depot-specific cell types in adipose tissue of dairy cows

Adipose tissue (AT) is an endocrine organ with a central role on whole-body energy metabolism and development of metabolic diseases. Single-cell and single-nuclei RNA sequencing (scRNA-seq and snRNA-seq, respectively) analyses in mice and human AT have revealed vast cell heterogeneity and functionally distinct subtypes that are potential therapeutic targets to metabolic disease. In periparturient dairy cows, AT goes through intensive remodeling and its dysfunction is associated with metabolic disease pathogenesis and decreased productive performance. The contributions of depot-specific cells and subtypes to the development of diseases in dairy cows remain to be studied. Our objective was to elucidate differences in cellular diversity of visceral (VAT) and subcutaneous (SAT) AT in dairy cows at the single-nuclei level. We collected matched SAT and VAT samples from three dairy cows and performed snRNA-seq analysis. We identified distinct cell types including four major mature adipocytes (AD) and three stem and progenitor cells (ASPC) subtypes, along with endothelial cells (EC), mesothelial cells (ME), immune cells, and pericytes and smooth muscle cells. All major cell types were present in both SAT and VAT, although a strong VAT-specificity was observed for ME, which were basically absent in SAT. One ASPC subtype was defined as adipogenic (PPARG+) while the other two had a fibro-adipogenic profile (PDGFRA+). We identified vascular and lymphatic EC subtypes, and different immune cell types and subtypes in both SAT and VAT, i.e., macrophages, monocytes, T cells, and natural killer cells. Not only did VAT show a greater proportion of immune cells, but these visceral immune cells had greater activation of pathways related to immune and inflammatory response, and complement cascade in comparison with SAT. There was a substantial contrast between depots for gene expression of complement cascade, which were greatly expressed by VAT cell subtypes compared to SAT, indicating a pro-inflammatory profile in VAT. Unprecedently, our study demonstrated cell-type and depot-specific heterogeneity in VAT and SAT of dairy cows. A better understanding of depot-specific molecular and cellular features of SAT and VAT will aid in the development of AT-targeted strategies to prevent and treat metabolic disease in dairy cows, especially during the periparturient period.

Tissue-Wide Effects Override Cell-Intrinsic Gene Function in Radial Neuron Migration

The mammalian neocortex is composed of diverse neuronal and glial cell classes that broadly arrange in six distinct laminae. Cortical layers emerge during development and defects in the developmental programs that orchestrate cortical lamination are associated with neurodevelopmental diseases. The developmental principle of cortical layer formation depends on concerted radial projection neuron migration, from their birthplace to their final target position. Radial migration occurs in defined sequential steps, regulated by a large array of signaling pathways. However, based on genetic loss-of-function experiments, most studies have thus far focused on the role of cell-autonomous gene function. Yet, cortical neuron migration in situ is a complex process and migrating neurons traverse along diverse cellular compartments and environments. The role of tissue-wide properties and genetic state in radial neuron migration is however not clear. Here we utilized mosaic analysis with double markers (MADM) technology to either sparsely or globally delete gene function, followed by quantitative single-cell phenotyping. The MADM-based gene ablation paradigms in combination with computational modeling demonstrated that global tissue-wide effects predominate cell-autonomous gene function albeit in a gene-specific manner. Our results thus suggest that the genetic landscape in a tissue critically affects the overall migration phenotype of individual cortical projection neurons. In a broader context, our findings imply that global tissue-wide effects represent an essential component of the underlying etiology associated with focal malformations of cortical development in particular, and neurological diseases in general.

Characterization of Microfragmented Adipose Tissue Architecture, Mesenchymal Stromal Cell Content and Release of Paracrine Mediators

The use of microfragmented adipose tissue (µFAT) for the treatment of musculoskeletal disorders, especially osteoarthritis (OA), is gaining popularity, following positive results reported in recent case series and clinical trials. Although these outcomes were postulated to rely on paracrine signals, to date, a thorough fingerprint of released molecules is largely missing. The purpose of this study was to first characterize both structure and cell content of unprocessed lipoaspirate (LA) and µFAT, and further identify and frame the array of signaling factors in the context of OA disease, by means of high throughput qRT-PCR for extracellular-vesicle (EV) embedded miRNAs and proteomics for tissue and secreted factors. Cell count showed reduction of blood cells in µFAT, confirmed by histological and flow cytometry analyses, that also showed a conserved presence of structural, endothelial and stromal components and pericytes. In the secretome, 376 and 381 EV-miRNAs in LA and µFAT, respectively, were identified. In particular, most abundant and µFAT upregulated EV-miRNAs were mainly recapitulating those already reported as ASC-EVs-specific, with crucial roles in cartilage protection and M2 macrophage polarization, while only a scarce presence of those related to blood cells emerged. Furthermore, secretome proteomic analysis revealed reduction in µFAT of acute phase factors driving OA progression. Taken together, these results suggest that processing of LA into µFAT allows for removal of blood elements and maintenance of tissue structure and stromal cell populations, and possibly the increase of OA-protective molecular features. Thus, microfragmentation represents a safe and efficient method for the application of adipose tissue properties in the frame of musculoskeletal disorders.

The Complement System in the Central Nervous System: From Neurodevelopment to Neurodegeneration

The functions of the complement system to both innate and adaptive immunity through opsonization, cell lysis, and inflammatory activities are well known. In contrast, the role of complement in the central nervous system (CNS) which extends beyond immunity, is only beginning to be recognized as important to neurodevelopment and neurodegeneration. In addition to protecting the brain against invasive pathogens, appropriate activation of the complement system is pivotal to the maintenance of normal brain function. Moreover, overactivation or dysregulation may cause synaptic dysfunction and promote excessive pro-inflammatory responses. Recent studies have provided insights into the various responses of complement components in different neurological diseases and the regulatory mechanisms involved in their pathophysiology, as well as a glimpse into targeting complement factors as a potential therapeutic modality. However, there remain significant knowledge gaps in the relationship between the complement system and different brain disorders. This review summarizes recent key findings regarding the role of different components of the complement system in health and pathology of the CNS and discusses the therapeutic potential of anti-complement strategies for the treatment of neurodegenerative conditions.

Tumor-propagating side population cells are a dynamic subpopulation in undifferentiated pleomorphic sarcoma

Sarcomas contain a subpopulation of tumor-propagating cells (TPCs) with enhanced tumor-initiating and self-renewal properties. However, it is unclear whether the TPC phenotype in sarcomas is stable or a dynamic cell state that can derive from non-TPCs. In this study, we utilized a mouse model of undifferentiated pleomorphic sarcoma (UPS) to trace the lineage relationship between sarcoma side population (SP) cells that are enriched for TPCs and non-SP cells. By cotransplanting SP and non-SP cells expressing different endogenous fluorescent reporters, we show that non-SP cells can give rise to SP cells with enhanced tumor-propagating potential in vivo. Lineage trajectory analysis using single-cell RNA sequencing from SP and non-SP cells supports the notion that non-SP cells can assume the SP cell phenotype de novo. To test the effect of eradicating SP cells on tumor growth and self-renewal, we generated mouse sarcomas in which the diphtheria toxin receptor is expressed in the SP cells and their progeny. Ablation of the SP population using diphtheria toxin did not impede tumor growth or self-renewal. Altogether, we show that the sarcoma SP represent a dynamic cell state and targeting TPCs alone is insufficient to eliminate tumor progression.

Elevated blood urea nitrogen alters the transcriptome of equine embryos

High blood urea nitrogen (BUN) in cows and ewes has a negative effect on embryo development; however, no comparable studies have been published in mares. The aims of the present study were to evaluate the effects of high BUN on blastocoele fluid, systemic progesterone and Day 14 equine embryos. When a follicle with a mean (±s.e.m.) diameter of 25±3mm was detected, mares were administered urea (0.4g kg-1) with sweet feed and molasses (n=9) or sweet feed and molasses alone (control; n=10). Blood samples were collected every other day. Mares were subjected to AI and the day ovulation was detected was designated as Day 0. Embryos were collected on Day 14 (urea-treated, n=5 embryos; control, n=7 embryos). There was an increase in systemic BUN in the urea-treated group compared with control (P<0.05), with no difference in progesterone concentrations. There were no differences between the two groups in embryo recovery or embryo size. Urea concentrations in the blastocoele fluid tended to be higher in the urea-treated mares, with a strong correlation with plasma BUN. However, there was no difference in the osmolality or pH of the blastocoele fluid between the two groups. Differentially expressed genes in Day 14 embryos from urea-treated mares analysed by RNA sequencing were involved in neurological development, urea transport, vascular remodelling and adhesion. In conclusion, oral urea treatment in mares increased BUN and induced transcriptome changes in Day 14 equine embryos of genes important in normal embryo development.

Complement C4 Is Reduced in iPSC-Derived Astrocytes of Autism Spectrum Disorder Subjects

In recent years, accumulating evidence has shown that the innate immune complement system is involved in several aspects of normal brain development and in neurodevelopmental disorders, including autism spectrum disorder (ASD). Although abnormal expression of complement components was observed in post-mortem brain samples from individuals with ASD, little is known about the expression patterns of complement molecules in distinct cell types in the developing autistic brain. In the present study, we characterized the mRNA and protein expression profiles of a wide range of complement system components, receptors and regulators in induced pluripotent stem cell (iPSC)-derived neural progenitor cells, neurons and astrocytes of individuals with ASD and neurotypical controls, which constitute in vitro cellular models that recapitulate certain features of both human brain development and ASD pathophysiology. We observed that all the analyzed cell lines constitutively express several key complement molecules. Interestingly, using different quantification strategies, we found that complement C4 mRNA and protein are expressed in significantly lower levels by astrocytes derived from ASD individuals compared to control astrocytes. As astrocytes participate in synapse elimination, and diminished C4 levels have been linked to defective synaptic pruning, our findings may contribute to an increased understanding of the atypically enhanced brain connectivity in ASD.

The Sez6 Family Inhibits Complement by Facilitating Factor I Cleavage of C3b and Accelerating the Decay of C3 Convertases

The Sez6 family consists of Sez6, Sez6L, and Sez6L2. Its members are expressed throughout the brain and have been shown to influence synapse numbers and dendritic morphology. They are also linked to various neurological and psychiatric disorders. All Sez6 family members contain 2-3 CUB domains and 5 complement control protein (CCP) domains, suggesting that they may be involved in complement regulation. We show that Sez6 family members inhibit C3b/iC3b opsonization by the classical and alternative pathways with varying degrees of efficacy. For the classical pathway, Sez6 is a strong inhibitor, Sez6L2 is a moderate inhibitor, and Sez6L is a weak inhibitor. For the alternative pathway, the complement inhibitory activity of Sez6, Sez6L, and Sez6L2 all equaled or exceeded the activity of the known complement regulator MCP. Using Sez6L2 as the representative family member, we show that it specifically accelerates the dissociation of C3 convertases. Sez6L2 also functions as a cofactor for Factor I to facilitate the cleavage of C3b; however, Sez6L2 has no cofactor activity toward C4b. In summary, the Sez6 family are novel complement regulators that inhibit C3 convertases and promote C3b degradation.

Complement cascade functions during brain development and neurodegeneration

The complement system, an essential tightly regulated innate immune system, is a key regulator of normal central nervous system (CNS) development and function. However, aberrant complement component expression and activation in the brain may culminate into marked neuroinflammatory response, neurodegenerative processes and cognitive impairment. Over the years, complement-mediated neuroinflammatory responses and complement-driven neurodegeneration have been increasingly implicated in the pathogenesis of a wide spectrum of CNS disorders. This review describes how complement system contributes to normal brain development and function. We also discuss how pathologic insults such as misfolded proteins, lipid droplet/lipid droplet-associated protein or glycosaminoglycan accumulation could trigger complement-mediated neuroinflammatory responses and neurodegenerative process in neurodegenerative proteinopathies, age-related macular degeneration and neurodegenerative lysosomal storage disorders.

Toxoplasma Infection Induces Sustained Up-Regulation of Complement Factor B and C5a Receptor in the Mouse Brain via Microglial Activation: Implication for the Alternative Complement Pathway Activation and Anaphylatoxin Signaling in Cerebral Toxoplasmosis

Toxoplasma gondii is a neurotropic protozoan parasite, which is linked to neurological manifestations in immunocompromised individuals as well as severe neurodevelopmental sequelae in congenital toxoplasmosis. While the complement system is the first line of host defense that plays a significant role in the prevention of parasite dissemination, Toxoplasma artfully evades complement-mediated clearance via recruiting complement regulatory proteins to their surface. On the other hand, the details of Toxoplasma and the complement system interaction in the brain parenchyma remain elusive. In this study, infection-induced changes in the mRNA levels of complement components were analyzed by quantitative PCR using a murine Toxoplasma infection model in vivo and primary glial cells in vitro. In addition to the core components C3 and C1q, anaphylatoxin C3a and C5a receptors (C3aR and C5aR1), as well as alternative complement pathway components properdin (CFP) and factor B (CFB), were significantly upregulated 2 weeks after inoculation. Two months post-infection, CFB, C3, C3aR, and C5aR1 expression remained higher than in controls, while CFP upregulation was transient. Furthermore, Toxoplasma infection induced significant increase in CFP, CFB, C3, and C5aR1 in mixed glial culture, which was abrogated when microglial activation was inhibited by pre-treatment with minocycline. This study sheds new light on the roles for the complement system in the brain parenchyma during Toxoplasma infection, which may lead to the development of novel therapeutic approaches to Toxoplasma infection-induced neurological disorders.

Language Impairment with a Partial Duplication of DOCK8

Duplications of the distal region of the short arm of chromosome 9 are rare, but are associated with learning disabilities and behavioral disturbances. We report in detail the cognitive and language features of a child with a duplication in the 9p24.3 region, arr[hg19] 9p24.3(266,045-459,076)×3. The proband exhibits marked expressive and receptive problems, which affect both structural and functional aspects of language. These problems might result from a severe underlying deficit in working memory. Regarding the molecular causes of the observed symptoms, they might result from the altered expression of selected genes involved in procedural learning, particularly some of components of the SLIT/ROBO/FOXP2 network, strongly related to the development and evolution of language. Dysregulation of specific components of this network can result in turn from an altered interaction between DOCK8, affected by the microduplication, and CDC42, acting as the hub component of the network encompassing language-related genes.

Trajectory of change in brain complement factors from neonatal to young adult humans

Immune system components also regulate synapse formation and refinement in neurodevelopment. The complement pathway, associated with cell lysis and phagocytosis, is implicated in synaptic elimination. Aberrant adolescent synaptic pruning may underpin schizophrenia onset; thus, changes in cortical complement activity during human development are of major interest. Complement is genetically linked to schizophrenia via increased C4 copy number variants, but the developmental trajectory of complement expression in the human brain is undetermined. As complement increases during periods of active synaptic engulfment in rodents, we hypothesized that complement expression would increase during postnatal development in humans, particularly during adolescence. Using human postmortem prefrontal cortex, we observed that complement activator (C1QB and C3) transcripts peaked in early neurodevelopment, and were highest in toddlers, declining in teenagers (all ANCOVAs between F = 2.41 -3.325, p = .01-0.05). We found that C4 protein was higher at 1-5 years (H = 16.378, p = .012), whereas C3 protein levels were unchanged with age. The microglial complement receptor subunit CD11b increased in mRNA early in life and peaked in the toddler brain (ANCOVA: pH, F = 4.186, p = .003). Complement inhibitors (CD46 and CD55) increased at school age, but failed to decrease like complement activators (both ANCOVAs, F > 4.4, p < .01). These data suggest the activation of complement in the human prefrontal cortex occurs between 1 and 5 years. We did not find evidence of induction of complement factors during adolescence and instead found increased or sustained levels of complement inhibitor mRNA at maturation. Dysregulation of these typical patterns of complement may predispose the brain to neurodevelopmental disorders such as autism or schizophrenia.

Neuroprotective versus Neuroinflammatory Roles of Complement: From Development to Disease

Complement proteins are ancient components of innate immunity that have emerged as crucial regulators of neural networks. We discuss these roles in the context of the CNS development, acute CNS viral infections, and post-infectious and noninfectious CNS disorders, with an emphasis on microglia-mediated loss of synapses. Despite extensive examples that implicate classical complement proteins and their receptors in CNS dysfunction, recent data suggest that they exert neuroprotective roles in CNS homeostasis through continued refinement of synaptic connections. Thorough understanding of the mechanisms involved in these processes may lead to novel targets for the treatment of CNS diseases involving aberrant complement-mediated synapse loss.

Non-Cell-Autonomous Mechanisms in Radial Projection Neuron Migration in the Developing Cerebral Cortex

Concerted radial migration of newly born cortical projection neurons, from their birthplace to their final target lamina, is a key step in the assembly of the cerebral cortex. The cellular and molecular mechanisms regulating the specific sequential steps of radial neuronal migration in vivo are however still unclear, let alone the effects and interactions with the extracellular environment. In any in vivo context, cells will always be exposed to a complex extracellular environment consisting of (1) secreted factors acting as potential signaling cues, (2) the extracellular matrix, and (3) other cells providing cell–cell interaction through receptors and/or direct physical stimuli. Most studies so far have described and focused mainly on intrinsic cell-autonomous gene functions in neuronal migration but there is accumulating evidence that non-cell-autonomous-, local-, systemic-, and/or whole tissue-wide effects substantially contribute to the regulation of radial neuronal migration. These non-cell-autonomous effects may differentially affect cortical neuron migration in distinct cellular environments. However, the cellular and molecular natures of such non-cell-autonomous mechanisms are mostly unknown. Furthermore, physical forces due to collective migration and/or community effects (i.e., interactions with surrounding cells) may play important roles in neocortical projection neuron migration. In this concise review, we first outline distinct models of non-cell-autonomous interactions of cortical projection neurons along their radial migration trajectory during development. We then summarize experimental assays and platforms that can be utilized to visualize and potentially probe non-cell-autonomous mechanisms. Lastly, we define key questions to address in the future.

Essential Role of Complement in Pregnancy: From Implantation to Parturition and Beyond

The complement cascade was identified over 100 years ago, yet investigation of its role in pregnancy remains an area of intense research. Complement inhibitors at the maternal-fetal interface prevent inappropriate complement activation to protect the fetus. However, this versatile proteolytic cascade also favorably influences numerous stages of pregnancy, including implantation, fetal development, and labor. Inappropriate complement activation in pregnancy can have adverse lifelong sequelae for both mother and child. This review summarizes the current understanding of complement activation during all stages of pregnancy. In addition, consequences of complement dysregulation during adverse pregnancy outcomes from miscarriage, preeclampsia, and pre-term birth are examined. Finally, future research directions into complement activation during pregnancy are considered.

Complement System in Brain Architecture and Neurodevelopmental Disorders

Current evidence indicates that certain immune molecules such as components of the complement system are directly involved in neurobiological processes related to brain development, including neurogenesis, neuronal migration, synaptic remodeling, and response to prenatal or early postnatal brain insults. Consequently, complement system dysfunction has been increasingly implicated in disorders of neurodevelopmental origin, such as schizophrenia, autism spectrum disorder (ASD) and Rett syndrome. However, the mechanistic evidence for a causal relationship between impaired complement regulation and these disorders varies depending on the disease involved. Also, it is still unclear to what extent altered complement expression plays a role in these disorders through inflammation-independent or -dependent mechanisms. Furthermore, pathogenic mutations in specific complement components have been implicated in the etiology of 3MC syndrome, a rare autosomal recessive developmental disorder. The aims of this review are to discuss the current knowledge on the roles of the complement system in sculpting brain architecture and function during normal development as well as after specific inflammatory insults, such as maternal immune activation (MIA) during pregnancy, and to evaluate the existing evidence associating aberrant complement with developmental brain disorders.

Systems modeling of white matter microstructural abnormalities in Alzheimer's disease

INTRODUCTION

Microstructural abnormalities in white matter (WM) are often reported in Alzheimer's disease (AD). However, it is unclear which brain regions have the strongest WM changes in presymptomatic AD and what biological processes underlie WM abnormality during disease progression.

METHODS

We developed a systems biology framework to integrate matched diffusion tensor imaging (DTI), genetic and transcriptomic data to investigate regional vulnerability to AD and identify genetic risk factors and gene subnetworks underlying WM abnormality in AD.

RESULTS

We quantified regional WM abnormality and identified most vulnerable brain regions. A SNP rs2203712 in CELF1 was most significantly associated with several DTI-derived features in the hippocampus, the top ranked brain region. An immune response gene subnetwork in the blood was most correlated with DTI features across all the brain regions.

DISCUSSION

Incorporation of image analysis with gene network analysis enhances our understanding of disease progression and facilitates identification of novel therapeutic strategies for AD.

Complement dysregulation in the central nervous system during development and disease

The complement cascade is an important arm of the immune system that plays a key role in protecting the central nervous system (CNS) from infection. Recently, it has also become clear that complement proteins have fundamental roles in the developing and aging CNS that are distinct from their roles in immunity. During neurodevelopment, complement signalling is involved in diverse processes including neural tube closure, neural progenitor proliferation and differentiation, neuronal migration, and synaptic pruning. In acute neurotrauma and ischamic brain injury, complement drives inflammation and neuronal death, but also neuroprotection and regeneration. In diseases of the aging CNS including dementias and motor neuron disease, chronic complement activation is associated with glial activation, and synapse and neuron loss. Proper regulation of complement is thus essential to allow for an appropriately developed CNS and prevention of excessive damage following neurotrauma or during neurodegeneration. This review provides a comprehensive overview of the evidence for functional roles of complement in brain formation, and its dysregulation during acute and chronic disease. We also provide working models for how complement can lead to neurodevelopmental disorders such as schizophrenia and autism, and either protect, or propagate neurodegenerative diseases including Alzheimer's disease and amyotrophic lateral sclerosis.

Emerging Roles of Complement in Psychiatric Disorders

The complement system consists of more than 30 proteins that have long been known to participate to the immune defence against pathogens and to the removal of damaged cells. Their role, however, extends beyond immunity and clearance of altered "self" components in the periphery. In particular, complement proteins can be induced by all cell types in the brain. Recent discoveries highlight the role of complement in normal and pathological brain development. Specifically, the complement system mediates synaptic pruning, a developmental process whereby supernumerary synapses are eliminated in the immature brain. The complement system has been implicated in pathological synapse elimination in schizophrenia, West Nile virus infection, and lupus, all of which are associated with psychiatric manifestations. Complement also contributes to synapse loss in neurodegenerative conditions. This review provides a brief overview of the well-studied role of complement molecules in immunity. The contribution of complement to embryonic and adult neurogenesis, neuronal migration, and developmental synaptic elimination in the normal brain is reviewed. We discuss the role of complement in synapse loss in psychiatric and neurological diseases and evaluate the therapeutic potential of complement-targeting drugs for brain disorders.

New tricks for an ancient system: Physiological and pathological roles of complement in the CNS

While the mechanisms underlying the functions of the complement system in the central nervous system (CNS) and systemically, namely opsonization, chemotaxis, membrane lysis, and regulation of inflammation are the same, the plethora of functions that complement orchestrates in the central nervous system (CNS) is complex. Strictly controlled expression of complement effector molecules, regulators and receptors across the gamut of life stages (embryogenesis, development and maturation, aging and disease) dictate fascinating contributions for this ancient system. Furthermore, it is becoming apparent that complement functions differ widely across distinct brain regions. This review provides a comprehensive overview of the newly identified roles for complement in the brain, including its roles in CNS development and function, during aging and in the processes of neurodegeneration. The diversity and selectively of beneficial and detrimental activities of complement, while challenging, should lead to precision targeting of specific components to provide disease modifying treatments for devastating psychiatric and neurodegenerative disorders that are still without effective treatment.

Complement C3 Affects Rac1 Activity in the Developing Brain

The complement system, which is part of the innate immune response system, has been recently shown to participate in multiple key processes in the developing brain. Here we aimed to elucidate downstream signaling responses linking complement C3, a key molecule of the pathway, to small GTPases, known to affect the cytoskeleton. The expression pattern of the activated small GTPase Rac1 resembled that of complement C3. C3-deficient mice exhibited reduced Rac1 and elevated RhoA activity in comparison with control mice. The most pronounced reduction of Rac1 activity occurred at embryonic day 14. Rac1 has been implicated in neuronal migration as well as neuronal stem cell proliferation and differentiation. Consistent with the reduction in Rac1 activity, the expression of phospho-cofilin, decreased in migrating neurons. Reduced Rac1-GTP was also correlated with a decrease in the expression of progenitor markers (Nestin, Pax6 and Tbr2) and conversely the expression of neuronal markers (Dcx and NeuN) increased in C3 knockout (KO) cortices in comparison with wild-type (WT) cortices. More specifically, C3 deficiency resulted in a reduction in the number of the cells in S-phase and an elevation in the number of cells that precociously exited the cell cycle. Collectively, our findings suggest that C3 impacts the activity of small GTPases resulting in cell cycle defects and premature neuronal differentiation.

Complement: The Emerging Architect of the Developing Brain

Complement activation products have long been associated with roles in the innate immune system, linking the humoral and cellular responses. However, among their recently described non-inflammatory roles, complement proteins also have multiple emerging novel functions in brain development. Within this context, separate proteins and pathways of complement have carved out physiological niches in the formation, development, and refinement of neurons. They demonstrate actions that are both reminiscent of peripheral immune actions and removed from them. We review here three key roles for complement proteins in the developing brain: progenitor proliferation, neuronal migration, and synaptic pruning.

Complement in stem cells and development

From its discovery in the late nineteenth century, as a 'complement' to the cellular immune response, the complement system has been widely affirmed as a powerful controller of innate and adaptive immune responses. In recent decades however, new roles for complement have been discovered, with multiple complement proteins now known to function in a broad array of non-immune systems. This includes during development, where complement exerts control over stem cell populations from fertilization and implantation throughout embryogenesis and beyond post-natal development. It is involved in processes as diverse as cell localisation, tissue morphogenesis, and the growth and refinement of the brain. Such physiological actions of complement have also been described in adult stem cell populations, with roles in proliferation, differentiation, survival, and regeneration. With such a broad range of complement functions now described, it is likely that current research only describes a fraction of the full reach of complement proteins. Here, we review how complement control of physiological cell processes has been harnessed in stem cell populations throughout both development and in adult physiology.