A Critical Perspective on the Synaptic Pruning Hypothesis of Schizophrenia Pathogenesis

CSMD1 regulates brain complement activity and circuit development

Complement proteins facilitate synaptic elimination during neurodevelopmental pruning, but neural complement regulation is not well understood. CUB and Sushi Multiple Domains 1 (CSMD1) can regulate complement activity in vitro, is expressed in the brain, and is associated with increased schizophrenia risk. Beyond this, little is known about CSMD1 including whether it regulates complement activity in the brain or otherwise plays a role in neurodevelopment. We used biochemical, immunohistochemical, and proteomic techniques to examine the regional, cellular, and subcellular distribution as well as protein interactions of CSMD1 in the brain. To evaluate whether CSMD1 is involved in complement-mediated synapse elimination, we examined Csmd1-knockout mice and CSMD1-knockout human stem cell-derived neurons. We interrogated synapse and circuit development of the mouse visual thalamus, a process that involves complement pathway activity. We also quantified complement deposition on synapses in mouse visual thalamus and on cultured human neurons. Finally, we assessed uptake of synaptosomes by cultured microglia. We found that CSMD1 is present at synapses and interacts with complement proteins in the brain. Mice lacking Csmd1 displayed increased levels of complement component C3, an increased colocalization of C3 with presynaptic terminals, fewer retinogeniculate synapses, and aberrant segregation of eye-specific retinal inputs to the visual thalamus during the critical period of complement-dependent refinement of this circuit. Loss of CSMD1 in vivo enhanced synaptosome engulfment by microglia in vitro, and this effect was dependent on activity of the microglial complement receptor, CR3. Finally, human stem cell-derived neurons lacking CSMD1 were more vulnerable to complement deposition. These data suggest that CSMD1 can function as a regulator of complement-mediated synapse elimination in the brain during development.

Cortical gene expression architecture links healthy neurodevelopment to the imaging, transcriptomics and genetics of autism and schizophrenia

Human brain organization involves the coordinated expression of thousands of genes. For example, the first principal component (C1) of cortical transcription identifies a hierarchy from sensorimotor to association regions. In this study, optimized processing of the Allen Human Brain Atlas revealed two new components of cortical gene expression architecture, C2 and C3, which are distinctively enriched for neuronal, metabolic and immune processes, specific cell types and cytoarchitectonics, and genetic variants associated with intelligence. Using additional datasets (PsychENCODE, Allen Cell Atlas and BrainSpan), we found that C1-C3 represent generalizable transcriptional programs that are coordinated within cells and differentially phased during fetal and postnatal development. Autism spectrum disorder and schizophrenia were specifically associated with C1/C2 and C3, respectively, across neuroimaging, differential expression and genome-wide association studies. Evidence converged especially in support of C3 as a normative transcriptional program for adolescent brain development, which can lead to atypical supragranular cortical connectivity in people at high genetic risk for schizophrenia.

Lower complement C1q levels in first-episode psychosis and in schizophrenia

Recent evidence has implicated complement component (C) 4A in excessive elimination of synapses in schizophrenia. C4A is believed to contribute to physiological synapse removal through signaling within the C1q initiated classical activation axis of the complement system. So far, a potential involvement of C1q in the pathophysiology of schizophrenia remains unclear. In this study, we first utilized large-scale gene expression datasets (n = 586 patients with schizophrenia and n = 986 controls) to observe lower C1QA mRNA expression in prefrontal cortex tissue of individuals with schizophrenia (P = 4.8x10-05), while C1QA seeded co-expression networks displayed no enrichment for schizophrenia risk variants beyond C4A. We then used targeted liquid chromatography-mass spectrometry (LS-MS) to measure cerebrospinal fluid (CSF) levels of C1qA in 113 individuals with first-episode psychosis (FEP), among which 66 individuals was later diagnosed with schizophrenia, and 87 healthy controls. CSF concentrations of C1qA were lower in individuals diagnosed with FEP (P = 0.0001), also after removing subjects with a short-term prescription of an antipsychotic agent (P = 0.0005). We conclude that C1q mRNA and protein levels are lower in schizophrenia and that further experimental studies are needed to understand the functional implications.

Strength of Excitatory Inputs to Layer 3 Pyramidal Neurons During Synaptic Pruning in the Monkey Prefrontal Cortex: Relevance for the Pathogenesis of Schizophrenia

BACKGROUND

In schizophrenia, layer 3 pyramidal neurons (L3PNs) of the dorsolateral prefrontal cortex exhibit deficits in markers of excitatory synaptic inputs that are thought to disrupt the patterns of neural network activity essential for cognitive function. These deficits are usually interpreted under Irwin Feinberg's hypothesis of altered synaptic pruning, which postulates that normal periadolescent pruning, thought to preferentially eliminate weak/immature synapses, is altered in schizophrenia. However, it remains unknown whether periadolescent pruning on L3PNs in the primate dorsolateral prefrontal cortex selectively eliminates weak excitatory synapses or uniformly eliminates excitatory synapses across the full distribution of synaptic strengths.

METHODS

To distinguish between these alternative models of synaptic pruning, we assessed the densities of dendritic spines, the site of most excitatory inputs to L3PNs, and the distributions of excitatory synaptic strengths in dorsolateral prefrontal cortex L3PNs from male and female monkeys across the periadolescent period of synaptic pruning. We used patch-clamp methods in acute brain slices to record miniature excitatory synaptic currents and intracellular filling with biocytin to quantify dendritic spines.

RESULTS

On L3PNs, dendritic spines exhibited the expected age-related decline in density, but mean synaptic strength and the shape of synaptic strength distributions remained stable with age.

CONCLUSIONS

The absence of age-related differences in mean synaptic strength and synaptic strength distributions supports the model of a uniform pattern of synaptic pruning across the full range of synaptic strengths. The implications of these findings for the pathogenesis and functional consequences of dendritic spine deficits in schizophrenia are discussed.

The multimodal Munich Clinical Deep Phenotyping study to bridge the translational gap in severe mental illness treatment research

Introduction

Treatment of severe mental illness (SMI) symptoms, especially negative symptoms and cognitive dysfunction in schizophrenia, remains a major unmet need. There is good evidence that SMIs have a strong genetic background and are characterized by multiple biological alterations, including disturbed brain circuits and connectivity, dysregulated neuronal excitation-inhibition, disturbed dopaminergic and glutamatergic pathways, and partially dysregulated inflammatory processes. The ways in which the dysregulated signaling pathways are interconnected remains largely unknown, in part because well-characterized clinical studies on comprehensive biomaterial are lacking. Furthermore, the development of drugs to treat SMIs such as schizophrenia is limited by the use of operationalized symptom-based clusters for diagnosis.

Methods

In line with the Research Domain Criteria initiative, the Clinical Deep Phenotyping (CDP) study is using a multimodal approach to reveal the neurobiological underpinnings of clinically relevant schizophrenia subgroups by performing broad transdiagnostic clinical characterization with standardized neurocognitive assessments, multimodal neuroimaging, electrophysiological assessments, retinal investigations, and omics-based analyzes of blood and cerebrospinal fluid. Moreover, to bridge the translational gap in biological psychiatry the study includes in vitro investigations on human-induced pluripotent stem cells, which are available from a subset of participants.

Results

Here, we report on the feasibility of this multimodal approach, which has been successfully initiated in the first participants in the CDP cohort; to date, the cohort comprises over 194 individuals with SMI and 187 age and gender matched healthy controls. In addition, we describe the applied research modalities and study objectives.

Discussion

The identification of cross-diagnostic and diagnosis-specific biotype-informed subgroups of patients and the translational dissection of those subgroups may help to pave the way toward precision medicine with artificial intelligence-supported tailored interventions and treatment. This aim is particularly important in psychiatry, a field where innovation is urgently needed because specific symptom domains, such as negative symptoms and cognitive dysfunction, and treatment-resistant symptoms in general are still difficult to treat.

Cognitive Impairment Associated with Schizophrenia: From Pathophysiology to Treatment

Cognitive impairment is a core feature of schizophrenia and a major contributor to poor functional outcomes. Methods for assessment of cognitive dysfunction in schizophrenia are now well established. In addition, there has been increasing appreciation in recent years of the additional role of social cognitive impairment in driving functional outcomes and of the contributions of sensory-level dysfunction to higher-order impairments. At the neurochemical level, acute administration of N-methyl-d-aspartate receptor (NMDAR) antagonists reproduces the pattern of neurocognitive dysfunction associated with schizophrenia, encouraging the development of treatments targeted at both NMDAR and its interactome. At the local-circuit level, an auditory neurophysiological measure, mismatch negativity, has emerged both as a veridical index of NMDAR dysfunction and excitatory/inhibitory imbalance in schizophrenia and as a critical biomarker for early-stage translational drug development. Although no compounds have yet been approved for treatment of cognitive impairment associated with schizophrenia, several candidates are showing promise in early-phase testing.

Using Nonhuman Primate Models to Reverse-Engineer Prefrontal Circuit Failure Underlying Cognitive Deficits in Schizophrenia

In this chapter, I review studies in nonhuman primates that emulate the circuit failure in prefrontal cortex responsible for working memory and cognitive control deficits in schizophrenia. These studies have characterized how synaptic malfunction, typically induced by blockade of NMDAR, disrupts neural function and computation in prefrontal networks to explain errors in cognitive tasks that are seen in schizophrenia. This work is finding causal relationships between pathogenic events of relevance to schizophrenia at vastly different levels of scale, from synapses, to neurons, local, circuits, distributed networks, computation, and behavior. Pharmacological manipulation, the dominant approach in primate models, has limited construct validity for schizophrenia pathogenesis, as the disease results from a complex interplay between environmental, developmental, and genetic factors. Genetic manipulation replicating schizophrenia risk is more advanced in rodent models. Nonetheless, gene manipulation in nonhuman primates is rapidly advancing, and primate developmental models have been established. Integration of large scale neural recording, genetic manipulation, and computational modeling in nonhuman primates holds considerable potential to provide a crucial schizophrenia model moving forward. Data generated by this approach is likely to fill several crucial gaps in our understanding of the causal sequence leading to schizophrenia in humans. This causal chain presents a vexing problem largely because it requires understanding how events at very different levels of scale relate to one another, from genes to circuits to cognition to social interactions. Nonhuman primate models excel here. They optimally enable discovery of causal relationships across levels of scale in the brain that are relevant to cognitive deficits in schizophrenia. The mechanistic understanding of prefrontal circuit failure they promise to provide may point the way to more effective therapeutic interventions to restore function to prefrontal networks in the disease.

Probing the molecular and cellular pathological mechanisms of schizophrenia using human induced pluripotent stem cell models

With recent advancements in psychiatric genomics, as a field, "stem cell-based disease modelers" were given the exciting yet daunting task of translating the extensive list of disease-associated risks into biologically and clinically relevant information in order to deliver therapeutically meaningful leads and insights. Despite their limitations, human induced pluripotent stem cell (iPSCs) based models have greatly aided our understanding of the molecular and cellular mechanisms underlying the complex etiology of brain disorders including schizophrenia (SCZ). In this review, we summarize the major findings from studies in the past decade which utilized iPSC models to investigate cell type-specific phenotypes relevant to idiopathic SCZ and disease penetrant alleles. Across cell type differences, several biological themes emerged, serving as potential neurodevelopmental mechanisms of SCZ, including oxidative stress and mitochondrial dysfunction, depletion of progenitor pools and insufficient differentiation potential of these progenitors, and structural and functional deficits of neurons and other supporting cells. Here, we discuss both the recent progress as well as challenges and improvements needed for future studies utilizing iPSCs as a model for SCZ and other neuropsychiatric disorders.