MicroRNA-eQTLs in the developing human neocortex link miR-4707-3p expression to brain size.

Expression quantitative trait loci (eQTL) data have proven important for linking non-coding loci to protein-coding genes. But eQTL studies rarely measure microRNAs (miRNAs), small non-coding RNAs known to play a role in human brain development and neurogenesis. Here, we performed small-RNA sequencing across 212 mid-gestation human neocortical tissue samples, measured 907 expressed miRNAs, discovering 111 of which were novel, and identified 85 local-miRNA-eQTLs. Colocalization of miRNA-eQTLs with GWAS summary statistics yielded one robust colocalization of miR-4707-3p expression with educational attainment and brain size phenotypes, where the miRNA expression increasing allele was associated with decreased brain size. Exogenous expression of miR-4707-3p in primary human neural progenitor cells decreased expression of predicted targets and increased cell proliferation, indicating miR-4707-3p modulates progenitor gene regulation and cell fate decisions. Integrating miRNA-eQTLs with existing GWAS yielded evidence of a miRNA that may influence human brain size and function via modulation of neocortical brain development.

Cellular Genome-wide Association Study Identifies Common Genetic Variation Influencing Lithium-Induced Neural Progenitor Proliferation.

BACKGROUND: Bipolar disorder is a highly heritable neuropsychiatric condition affecting more than 1% of the human population. Lithium salts are commonly prescribed as a mood stabilizer for individuals with bipolar disorder. Lithium is clinically effective in approximately half of treated individuals, and their genetic backgrounds are known to influence treatment outcomes. While the mechanism of lithium's therapeutic action is unclear, it stimulates adult neural progenitor cell proliferation, similar to some antidepressant drugs. METHODS: To identify common genetic variants that modulate lithium-induced proliferation, we conducted an EdU incorporation assay in a library of 80 genotyped human neural progenitor cells treated with lithium. These data were used to perform a genome-wide association study to identify common genetic variants that influence lithium-induced neural progenitor cell proliferation. We manipulated the expression of a putatively causal gene using CRISPRi/a (clustered regularly interspaced short palindromic repeats interference/activation) constructs to experimentally verify lithium-induced proliferation effects. RESULTS: We identified a locus on chr3p21.1 associated with lithium-induced proliferation. This locus is also associated with bipolar disorder risk, schizophrenia risk, and interindividual differences in intelligence. We identified a single gene, GNL3, whose expression temporally increased in an allele-specific fashion following lithium treatment. Experimentally increasing the expression of GNL3 led to increased proliferation under baseline conditions, while experimentally decreasing GNL3 expression suppressed lithium-induced proliferation. CONCLUSIONS: Our experiments reveal that common genetic variation modulates lithium-induced neural progenitor proliferation and that GNL3 expression is necessary for the full proliferation-stimulating effects of lithium. These results suggest that performing genome-wide associations in genetically diverse human cell lines is a useful approach to discover context-specific pharmacogenomic effects.

Brain-trait-associated variants impact cell-type-specific gene regulation during neurogenesis.

Interpretation of the function of non-coding risk loci for neuropsychiatric disorders and brain-relevant traits via gene expression and alternative splicing quantitative trait locus (e/sQTL) analyses is generally performed in bulk post-mortem adult tissue. However, genetic risk loci are enriched in regulatory elements active during neocortical differentiation, and regulatory effects of risk variants may be masked by heterogeneity in bulk tissue. Here, we map e/sQTLs, and allele-specific expression in cultured cells representing two major developmental stages, primary human neural progenitors (n = 85) and their sorted neuronal progeny (n = 74), identifying numerous loci not detected in either bulk developing cortical wall or adult cortex. Using colocalization and genetic imputation via transcriptome-wide association, we uncover cell-type-specific regulatory mechanisms underlying risk for brain-relevant traits that are active during neocortical differentiation. Specifically, we identified a progenitor-specific eQTL for CENPW co-localized with common variant associations for cortical surface area and educational attainment.

Cell-type-specific effects of genetic variation on chromatin accessibility during human neuronal differentiation.

Common genetic risk for neuropsychiatric disorders is enriched in regulatory elements active during cortical neurogenesis. However, it remains poorly understood as to how these variants influence gene regulation. To model the functional impact of common genetic variation on the noncoding genome during human cortical development, we performed the assay for transposase accessible chromatin using sequencing (ATAC-seq) and analyzed chromatin accessibility quantitative trait loci (QTL) in cultured human neural progenitor cells and their differentiated neuronal progeny from 87 donors. We identified significant genetic effects on 988/1,839 neuron/progenitor regulatory elements, with highly cell-type and temporally specific effects. A subset (roughly 30%) of chromatin accessibility-QTL were also associated with changes in gene expression. Motif-disrupting alleles of transcriptional activators generally led to decreases in chromatin accessibility, whereas motif-disrupting alleles of repressors led to increases in chromatin accessibility. By integrating cell-type-specific chromatin accessibility-QTL and brain-relevant genome-wide association data, we were able to fine-map and identify regulatory mechanisms underlying noncoding neuropsychiatric disorder risk loci.

Approximating Isotope Distributions of Biomolecule Fragments.

In mass spectrometry (MS)-based proteomics, protein and peptide sequences are determined by the isolation and subsequent fragmentation of precursor ions. When an isolation window captures only part of a precursor's isotopic distribution, the isotope distributions of the fragments depend on the subset of isolated precursor isotopes. Approximation of the expected isotope distributions of these fragments prior to sequence determination enables MS2 deisotoping, monoisotopic mass calculation, charge assignment of fragment peaks, and deconvolution of chimeric spectra. However, currently such methods do not exist, and precursor isotope distributions are often used as a proxy. Here, we present methods that approximate the isotope distribution of a biomolecule's fragment given its monoisotopic mass, the monoisotopic mass of its precursor, the set of isolated precursor isotopes, and optionally sulfur atom content. Our methods use either the Averagine model or splines, the latter of which have similar accuracy to the Averagine approach, but are 20 times faster to compute. Theoretical and approximated isotope distributions are consistent for fragments of in silico digested peptides. Furthermore, mass spectrometry experiments with the angiotensin I peptide and HeLa cell lysate demonstrate that the fragment methods match isotope peaks in MS2 spectra more accurately than the precursor Averagine approach. The algorithms for the approximation of fragment isotope distributions have been added to the OpenMS software library. By providing the means for analyzing fragment isotopic distributions, these methods will improve MS2 spectra interpretation.

We FRET so You Don't Have To: New Models of the Lipoprotein Lipase Dimer.

Lipoprotein lipase (LPL) is a dimeric enzyme that is responsible for clearing triglyceride-rich lipoproteins from the blood. Although LPL plays a key role in cardiovascular health, an experimentally derived three-dimensional structure has not been determined. Such a structure would aid in understanding mutations in LPL that cause familial LPL deficiency in patients and help in the development of therapeutic strategies to target LPL. A major obstacle to structural studies of LPL is that LPL is an unstable protein that is difficult to produce in the quantities needed for nuclear magnetic resonance or crystallography. We present updated LPL structural models generated by combining disulfide mapping, computational modeling, and data derived from single-molecule Förster resonance energy transfer (smFRET). We pioneer the technique of smFRET for use with LPL by developing conditions for imaging active LPL and identifying positions in LPL for the attachment of fluorophores. Using this approach, we measure LPL-LPL intermolecular interactions to generate experimental constraints that inform new computational models of the LPL dimer structure. These models suggest that LPL may dimerize using an interface that is different from the dimerization interface suggested by crystal packing contacts seen in structures of pancreatic lipase.

Biochemical Analysis of the Lipoprotein Lipase Truncation Variant, LPLS447X, Reveals Increased Lipoprotein Uptake.

Lipoprotein lipase (LPL) is responsible for the hydrolysis of triglycerides from circulating lipoproteins. Whereas most identified mutations in the LPL gene are deleterious, one mutation, LPLS447X, causes a gain of function. This mutation truncates two amino acids from LPL's C-terminus. Carriers of LPLS447X have decreased VLDL levels and increased HDL levels, a cardioprotective phenotype. LPLS447X is used in Alipogene tiparvovec, the gene therapy product for individuals with familial LPL deficiency. It is unclear why LPLS447X results in a serum lipid profile more favorable than that of LPL. In vitro reports vary as to whether LPLS447X is more active than LPL. We report a comprehensive, biochemical comparison of purified LPLS447X and LPL dimers. We found no difference in specific activity on synthetic and natural substrates. We also did not observe a difference in the Ki for ANGPTL4 inhibition of LPLS447X relative to that of LPL. Finally, we analyzed LPL-mediated uptake of fluorescently labeled lipoprotein particles and found that LPLS447X enhanced lipoprotein uptake to a greater degree than LPL did. An LPL structural model suggests that the LPLS447X truncation exposes residues implicated in LPL binding to uptake receptors.

Angiopoietin-like protein 4 inhibition of lipoprotein lipase: evidence for reversible complex formation.

Elevated triglycerides are associated with an increased risk of cardiovascular disease, and lipoprotein lipase (LPL) is the rate-limiting enzyme for the hydrolysis of triglycerides from circulating lipoproteins. The N-terminal domain of angiopoietin-like protein 4 (ANGPTL4) inhibits LPL activity. ANGPTL4 was previously described as an unfolding molecular chaperone of LPL that catalytically converts active LPL dimers into inactive monomers. Our studies show that ANGPTL4 is more accurately described as a reversible, noncompetitive inhibitor of LPL. We find that inhibited LPL is in a complex with ANGPTL4, and upon dissociation, LPL regains lipase activity. Furthermore, we have generated a variant of ANGPTL4 that is dependent on divalent cations for its ability to inhibit LPL. We show that LPL inactivation by this regulatable variant of ANGPTL4 is fully reversible after treatment with a chelator.