Early Developmental Origins of Cortical Disorders Modeled in Human Neural Stem Cells.

The implications of the early phases of human telencephalic development, involving neural stem cells (NSCs), in the etiology of cortical disorders remain elusive. Here, we explored the expression dynamics of cortical and neuropsychiatric disorder-associated genes in datasets generated from human NSCs across telencephalic fate transitions in vitro and in vivo. We identified risk genes expressed in brain organizers and sequential gene regulatory networks across corticogenesis revealing disease-specific critical phases, when NSCs are more vulnerable to gene dysfunctions, and converging signaling across multiple diseases. Moreover, we simulated the impact of risk transcription factor (TF) depletions on different neural cell types spanning the developing human neocortex and observed a spatiotemporal-dependent effect for each perturbation. Finally, single-cell transcriptomics of newly generated autism-affected patient-derived NSCs in vitro revealed recurrent alterations of TFs orchestrating brain patterning and NSC lineage commitment. This work opens new perspectives to explore human brain dysfunctions at the early phases of development.

Molecular and cellular evolution of the primate dorsolateral prefrontal cortex.

The granular dorsolateral prefrontal cortex (dlPFC) is an evolutionary specialization of primates that is centrally involved in cognition. We assessed more than 600,000 single-nucleus transcriptomes from adult human, chimpanzee, macaque, and marmoset dlPFC. Although most cell subtypes defined transcriptomically are conserved, we detected several that exist only in a subset of species as well as substantial species-specific molecular differences across homologous neuronal, glial, and non-neural subtypes. The latter are exemplified by human-specific switching between expression of the neuropeptide somatostatin and tyrosine hydroxylase, the rate-limiting enzyme in dopamine production in certain interneurons. The above molecular differences are also illustrated by expression of the neuropsychiatric risk gene FOXP2, which is human-specific in microglia and primate-specific in layer 4 granular neurons. We generated a comprehensive survey of the dlPFC cellular repertoire and its shared and divergent features in anthropoid primates.

Transcriptomic taxonomy and neurogenic trajectories of adult human, macaque, and pig hippocampal and entorhinal cells.

The hippocampal-entorhinal system supports cognitive functions, has lifelong neurogenic capabilities in many species, and is selectively vulnerable to Alzheimer's disease. To investigate neurogenic potential and cellular diversity, we profiled single-nucleus transcriptomes in five hippocampal-entorhinal subregions in humans, macaques, and pigs. Integrated cross-species analysis revealed robust transcriptomic and histologic signatures of neurogenesis in the adult mouse, pig, and macaque but not humans. Doublecortin (DCX), a widely accepted marker of newly generated granule cells, was detected in diverse human neurons, but it did not define immature neuron populations. To explore species differences in cellular diversity and implications for disease, we characterized subregion-specific, transcriptomically defined cell types and transitional changes from the three-layered archicortex to the six-layered neocortex. Notably, METTL7B defined subregion-specific excitatory neurons and astrocytes in primates, associated with endoplasmic reticulum and lipid droplet proteins, including Alzheimer's disease-related proteins. This resource reveals cell-type- and species-specific properties shaping hippocampal-entorhinal neurogenesis and function.

Whole-Genome and RNA Sequencing Reveal Variation and Transcriptomic Coordination in the Developing Human Prefrontal Cortex.

Gene expression levels vary across developmental stage, cell type, and region in the brain. Genomic variants also contribute to the variation in expression, and some neuropsychiatric disorder loci may exert their effects through this mechanism. To investigate these relationships, we present BrainVar, a unique resource of paired whole-genome and bulk tissue RNA sequencing from the dorsolateral prefrontal cortex of 176 individuals across prenatal and postnatal development. Here we identify common variants that alter gene expression (expression quantitative trait loci [eQTLs]) constantly across development or predominantly during prenatal or postnatal stages. Both "constant" and "temporal-predominant" eQTLs are enriched for loci associated with neuropsychiatric traits and disorders and colocalize with specific variants. Expression levels of more than 12,000 genes rise or fall in a concerted late-fetal transition, with the transitional genes enriched for cell-type-specific genes and neuropsychiatric risk loci, underscoring the importance of cataloging developmental trajectories in understanding cortical physiology and pathology.

The 7q11.23 Protein DNAJC30 Interacts with ATP Synthase and Links Mitochondria to Brain Development.

Despite the known causality of copy-number variations (CNVs) to human neurodevelopmental disorders, the mechanisms behind each gene's contribution to the constellation of neural phenotypes remain elusive. Here, we investigated the 7q11.23 CNV, whose hemideletion causes Williams syndrome (WS), and uncovered that mitochondrial dysfunction participates in WS pathogenesis. Dysfunction is facilitated in part by the 7q11.23 protein DNAJC30, which interacts with mitochondrial ATP-synthase machinery. Removal of Dnajc30 in mice resulted in hypofunctional mitochondria, diminished morphological features of neocortical pyramidal neurons, and altered behaviors reminiscent of WS. The mitochondrial features are consistent with our observations of decreased integrity of oxidative phosphorylation supercomplexes and ATP-synthase dimers in WS. Thus, we identify DNAJC30 as an auxiliary component of ATP-synthase machinery and reveal mitochondrial maladies as underlying certain defects in brain development and function associated with WS.

Molecular and cellular reorganization of neural circuits in the human lineage.

To better understand the molecular and cellular differences in brain organization between human and nonhuman primates, we performed transcriptome sequencing of 16 regions of adult human, chimpanzee, and macaque brains. Integration with human single-cell transcriptomic data revealed global, regional, and cell-type-specific species expression differences in genes representing distinct functional categories. We validated and further characterized the human specificity of genes enriched in distinct cell types through histological and functional analyses, including rare subpallial-derived interneurons expressing dopamine biosynthesis genes enriched in the human striatum and absent in the nonhuman African ape neocortex. Our integrated analysis of the generated data revealed diverse molecular and cellular features of the phylogenetic reorganization of the human brain across multiple levels, with relevance for brain function and disease.

Zika Virus Disrupts Phospho-TBK1 Localization and Mitosis in Human Neuroepithelial Stem Cells and Radial Glia.

The mechanisms underlying Zika virus (ZIKV)-related microcephaly and other neurodevelopment defects remain poorly understood. Here, we describe the derivation and characterization, including single-cell RNA-seq, of neocortical and spinal cord neuroepithelial stem (NES) cells to model early human neurodevelopment and ZIKV-related neuropathogenesis. By analyzing human NES cells, organotypic fetal brain slices, and a ZIKV-infected micrencephalic brain, we show that ZIKV infects both neocortical and spinal NES cells as well as their fetal homolog, radial glial cells (RGCs), causing disrupted mitoses, supernumerary centrosomes, structural disorganization, and cell death. ZIKV infection of NES cells and RGCs causes centrosomal depletion and mitochondrial sequestration of phospho-TBK1 during mitosis. We also found that nucleoside analogs inhibit ZIKV replication in NES cells, protecting them from ZIKV-induced pTBK1 relocalization and cell death. We established a model system of human neural stem cells to reveal cellular and molecular mechanisms underlying neurodevelopmental defects associated with ZIKV infection and its potential treatment.

Experimental mutagenesis of huntingtin to map cleavage sites: different outcomes in cell and mouse models.

BACKGROUND: N-terminal cleavage products of mutant huntingtin (htt) generate pathologic neuronal inclusion bodies. The precise length of the htt fragment, termed Cp-A/1, that produces HD pathologic inclusions is unknown. OBJECTIVE: We sought to elucidate the protein sequence elements within the N-terminus of htt that mediate its proteolysis based on a model in which engineered htt fragments terminating at residue 171 are cleaved to produce Cp-A/1 fragments. METHODS: We expressed htt N171 cDNAs harboring a series of experimental mutations in the presumptive cleavage site that generates Cp-A/1 in cells to identify cleavage resistant mutants of htt N171. One of these constructs was expressed in mice, followed by analysis using immunoblots of brain extracts and immunohistochemistry of transgenic mouse brain tissues. RESULTS: Using the HEK293 cell model, mutagenesis studies mapped the cleavage site in htt N171 to sequences between residues 105-114. Mutation of 8 positively charged residues (H, K, R) located between residues 88 and 114 to alanine to neutralize the charge also blocked the generation of Cp-A/1 like fragments. Transgenic mice expressing this latter construct, termed N171-82Q-N8, developed phenotypes similar to previously characterized N171-82Q transgenic mice, including rotarod deficiency, intranuclear inclusions, and premature death. Surprisingly, the N171-82Q-N8 protein was efficiently cleaved in vivo to produce Cp-A/1 fragments that accumulated as insoluble inclusions. CONCLUSION: Mutagenesis of htt to identify critical amino acids that direct its cleavage predicted a role for charged residues in the sequence flanking the presumptive cleavage site. However, the role for these residues could not be confirmed in vivo. The basis for the discrepancy between predicted outcomes in HEK293 cells and the mouse models remain unresolved, but the data provide another validation of the hypothesis that Cp-A/1 fragments of mutant htt can induce HD-like phenotypes.

The developmental transcriptome of the human brain: implications for neurodevelopmental disorders.

PURPOSE OF REVIEW: Recent characterizations of the transcriptome of the developing human brain by several groups have generated comprehensive datasets on coding and noncoding RNAs that will be instrumental for illuminating the underlying biology of complex neurodevelopmental disorders. This review summarizes recent studies successfully utilizing these data to increase our understanding of the molecular mechanisms of pathogenesis. RECENT FINDINGS: Several approaches have successfully integrated developmental transcriptome data with gene discovery to generate testable hypotheses about when and where in the developing human brain disease-associated genes converge. Specifically, these include the projection neurons in the prefrontal and primary motor--somatosensory cortex during mid-fetal development in autism spectrum disorder and the frontal cortex during fetal development in schizophrenia. SUMMARY: Developmental transcriptome data is a key to interpreting disease-associated mutations and transcriptional changes. Novel approaches integrating the spatial and temporal dimensions of these data have increased our understanding of when and where disease occurs. Refinement of spatial and temporal properties and expanding these findings to other neurodevelopmental disorders will provide critical insights for understanding disease biology.

Analysis of proteolytic processes and enzymatic activities in the generation of huntingtin n-terminal fragments in an HEK293 cell model.

BACKGROUND: N-terminal fragments of mutant huntingtin (htt) that terminate between residues 90-115, termed cleavage product A or 1 (cp-A/1), form intracellular and intranuclear inclusion bodies in the brains of patients with Huntington's disease (HD). These fragments appear to be proteolytic products of the full-length protein. Here, we use an HEK293 cell culture model to investigate huntingtin proteolytic processing; previous studies of these cells have demonstrated cleavage of htt to cp-A/1 like htt fragments. RESULTS: Recombinant N-terminal htt fragments, terminating at residue 171 (also referred to as cp-B/2 like), were efficiently cleaved to produce cp-A/1 whereas fragments representing endogenous caspase, calpain, and metalloproteinase cleavage products, terminating between residues 400-600, were inefficiently cleaved. Using cysteine-labeling techniques and antibody binding mapping, we localized the C-terminus of the cp-A/1 fragments produced by HEK293 cells to sequences minimally limited by cysteine 105 and an antibody epitope composed of residues 115-124. A combination of genetic and pharmacologic approaches to inhibit potential proteases, including γ-secretase and calpain, proved ineffective in preventing production of cp-A/1. CONCLUSIONS: Our findings indicate that HEK293 cells express a protease that is capable of efficiently cleaving cp-B/2 like fragments of htt with normal or expanded glutamine repeats. For reasons that remain unclear, this protease cleaves longer htt fragments, with normal or expanded glutamine expansions, much less efficiently. The protease in HEK293 cells that is capable of generating a cp-A/1 like htt fragment may be a novel protease with a high preference for a cp-B/2-like htt fragment as substrate.

Transgenic mice expressing caspase-6-derived N-terminal fragments of mutant huntingtin develop neurologic abnormalities with predominant cytoplasmic inclusion pathology composed largely of a smaller proteolytic derivative.

Recent studies have implicated an N-terminal caspase-6 cleavage product of mutant huntingtin (htt) as an important mediator of toxicity in Huntington's disease (HD). To directly assess the consequences of such fragments on neurologic function, we produced transgenic mice that express a caspase-6 length N-terminal fragment of mutant htt (N586) with both normal (23Q) and disease (82Q) length glutamine repeats. In contrast to mice expressing N586-23Q, mice expressing N586-82Q accumulate large cytoplasmic inclusion bodies that can be visualized with antibodies to epitopes throughout the N586 protein. However, biochemical analyses of aggregated mutant huntingtin in these mice demonstrated that the inclusion bodies are composed largely of a much smaller htt fragment (terminating before residue 115), with lesser amounts of full-length N586-82Q fragments. Mice expressing the N586-82Q fragment show symptoms typical of previously generated mice expressing mutant huntingtin fragments, including failure to maintain weight, small brain weight and reductions in specific mRNAs in the striatum. Uniquely, these N586-82Q mice develop a progressive movement disorder that includes dramatic deficits in motor performance on the rotarod and ataxia. Our findings suggest that caspase-6-derived fragments of mutant htt are capable of inducing novel HD-related phenotypes, but these fragments are not terminal cleavage products as they are subject to further proteolysis. In this scenario, mutant htt fragments derived from caspase 6, or possibly other proteases, could mediate HD pathogenesis via a 'hit and run' type of mechanism in which caspase-6, or other larger N-terminal fragments, mediate a neurotoxic process before being cleaved to a smaller fragment that accumulates pathologically.

Premature death and neurologic abnormalities in transgenic mice expressing a mutant huntingtin exon-2 fragment.

Huntington's disease (HD) is a fatal neurodegenerative disease characterized pathologically by aggregates composed of N-terminal fragments of the mutant form of the protein huntingtin (htt). The role of these N-terminal fragments in disease pathogenesis has been questioned based in part on studies in transgenic mice. In one important example, mice that express an N-terminal fragment of mutant htt terminating at the C-terminus of exon 2 (termed the Shortstop mouse) were reported to develop robust inclusion pathology without developing phenotypic abnormalities seen in the R6/2 or N171-82Q models of HD, which are also based on expression of mutant N-terminal htt fragments. To further explore the capacity of mutant exon-2 htt fragments to produce neurologic abnormalities (N-terminal 118 amino acids; N118), we generated transgenic mice expressing cDNA that encodes htt N118-82Q with the mouse prion promoter vector. In mice generated in this manner, we demonstrate robust inclusion pathology accompanied by early death and failure to gain weight. These phenotypes are the most robust abnormalities identified in the R6/2 and N171-82Q models. We conclude that the lack of an overt phenotype in the initial Shortstop mice cannot be completely explained by the properties of mutant htt N118 fragments.

Analysis of chaperone mRNA expression in the adult mouse brain by meta analysis of the Allen Brain Atlas.

The pathology of many neurodegenerative diseases is characterized by the accumulation of misfolded and aggregated proteins in various cell types and regional substructures throughout the central and peripheral nervous systems. The accumulation of these aggregated proteins signals dysfunction of cellular protein homeostatic mechanisms such as the ubiquitin/proteasome system, autophagy, and the chaperone network. Although there are several published studies in which transcriptional profiling has been used to examine gene expression in various tissues, including tissues of neurodegenerative disease models, there has not been a report that focuses exclusively on expression of the chaperone network. In the present study, we used the Allen Brain Atlas online database to analyze chaperone expression levels. This database utilizes a quantitative in situ hybridization approach and provides data on 270 chaperone genes within many substructures of the adult mouse brain. We determined that 256 of these chaperone genes are expressed at some level. Surprisingly, relatively few genes, only 30, showed significant variations in levels of mRNA across different substructures of the brain. The greatest degree of variability was exhibited by genes of the DnaJ co-chaperone, Tetratricopeptide repeat, and the HSPH families. Our analysis provides a valuable resource towards determining how variations in chaperone gene expression may modulate the vulnerability of specific neuronal populations of mammalian brain.

Partial depletion of CREB-binding protein reduces life expectancy in a mouse model of Huntington disease.

Previous studies have reported that mutant huntingtin (htt) interferes with cyclic AMP response element binding protein binding protein (CBP)-mediated transcription, possibly by inhibiting the acetylation of histones. In Drosophila models that express fragments of mutant htt, histone deacetylase inhibitors reverse deficits in histone acetylation, rescue photoreceptor degeneration, and prolong their survival. These compounds also improve motor deficits in a transgenic mouse model of Huntington disease (HD). To determine whether endogenous CBP depletion contributes to HD pathogenesis, we crossed HD-N171-82Q transgenic mice with mice harboring a disrupted CBP gene and produced mice with partial (50%) depletion of CBP. This reduction of CBP levels decreased the life expectancy of the HD-N171-82Q Line 6 mouse model. The loss of CBP had no obvious impact on the severity of motor impairment, degeneration of the striatum, mutant htt inclusion formation, or global levels of acetylated histones H3 or H4 in brain. In cell models, we confirmed that mutant htt inclusions recruit human CBP but found no evidence for interactions between soluble forms of mutant htt and CBP. Although we identified no neurological explanation for the decreased life expectancy of HD-N171-82Q mice with partial depletion of CBP, the data are consistent with the notion that CBP function mitigates mutant htt toxicity by a currently unidentified mechanism.

Protein aggregate characterization in models of neurodegenerative disease.

A pathological hallmark of many neurodegenerative diseases is the presence of protein aggregates. Transgenic mice that recapitulate this pathology are a valuable resource to isolate these proteins for detailed study. One aspect of our research program is to characterize and quantify aggregates beta-amyloid (Abeta) peptides, superoxide dismutase 1 (SOD1), and huntingtin (htt) that comprise pathologic lesions found in Alzheimer's disease, familial amyotrophic lateral sclerosis, and Huntington's disease, respectively. In this chapter, we describe methods, based on sequential detergent extraction and ultracentrifugation, to isolate and analyze these protein aggregates. These methods have been applied to human tissues to some extent, but have been highly useful in studies involving transgenic mouse models of these diseases.

Characterization of huntingtin pathologic fragments in human Huntington disease, transgenic mice, and cell models.

Huntington disease (HD) is caused by the expansion of a glutamine (Q) repeat near the N terminus of huntingtin (htt), resulting in altered conformation of the mutant protein to produce, most prominently in brain neurons, nuclear and cytoplasmic inclusion pathology. The inclusions and associated diffuse accumulation of mutant htt in nuclei are composed of N-terminal fragments of mutant protein. Here, we used a panel of peptide antibodies to characterize the htt protein pathologies in brain tissues from human HD, and a transgenic mouse model created by expressing the first 171 amino acids of human htt with 82Q (htt-N171-82Q). In tissues from both sources, htt pathologic features in nuclei were detected by antibodies to htt peptides 1-17 and 81-90 but not 115-129 (wild-type huntingtin numbering with 23 repeats). Human HEK 293 cells transfected with expression vectors that encode either the N-terminal 233 amino acids of human htt (htt-N233-82Q) or htt-N171-18Q accumulated smaller N-terminal fragments with antibody-binding characteristics identical to those of pathologic peptides. We conclude that the mutant htt peptides that accumulate in pathologic structures of human HD and httN171-82Q in mice are produced by similar, yet to be defined, proteolytic events in a region of the protein near or within amino acids 90-115.