LIN28 binds messenger RNAs at GGAGA motifs and regulates splicing factor abundance.

LIN28 is a conserved RNA-binding protein implicated in pluripotency, reprogramming, and oncogenesis. It was previously shown to act primarily by blocking let-7 microRNA (miRNA) biogenesis, but here we elucidate distinct roles of LIN28 regulation via its direct messenger RNA (mRNA) targets. Through crosslinking and immunoprecipitation coupled with high-throughput sequencing (CLIP-seq) in human embryonic stem cells and somatic cells expressing exogenous LIN28, we have defined discrete LIN28-binding sites in a quarter of human transcripts. These sites revealed that LIN28 binds to GGAGA sequences enriched within loop structures in mRNAs, reminiscent of its interaction with let-7 miRNA precursors. Among LIN28 mRNA targets, we found evidence for LIN28 autoregulation and also direct but differing effects on the protein abundance of splicing regulators in somatic and pluripotent stem cells. Splicing-sensitive microarrays demonstrated that exogenous LIN28 expression causes widespread downstream alternative splicing changes. These findings identify important regulatory functions of LIN28 via direct mRNA interactions.

Differential L1 regulation in pluripotent stem cells of humans and apes.

Identifying cellular and molecular differences between human and non-human primates (NHPs) is essential to the basic understanding of the evolution and diversity of our own species. Until now, preserved tissues have been the main source for most comparative studies between humans, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). However, these tissue samples do not fairly represent the distinctive traits of live cell behaviour and are not amenable to genetic manipulation. We propose that induced pluripotent stem (iPS) cells could be a unique biological resource to determine relevant phenotypical differences between human and NHPs, and that those differences could have potential adaptation and speciation value. Here we describe the generation and initial characterization of iPS cells from chimpanzees and bonobos as new tools to explore factors that may have contributed to great ape evolution. Comparative gene expression analysis of human and NHP iPS cells revealed differences in the regulation of long interspersed element-1 (L1, also known as LINE-1) transposons. A force of change in mammalian evolution, L1 elements are retrotransposons that have remained active during primate evolution. Decreased levels of L1-restricting factors APOBEC3B (also known as A3B) and PIWIL2 (ref. 7) in NHP iPS cells correlated with increased L1 mobility and endogenous L1 messenger RNA levels. Moreover, results from the manipulation of A3B and PIWIL2 levels in iPS cells supported a causal inverse relationship between levels of these proteins and L1 retrotransposition. Finally, we found increased copy numbers of species-specific L1 elements in the genome of chimpanzees compared to humans, supporting the idea that increased L1 mobility in NHPs is not limited to iPS cells in culture and may have also occurred in the germ line or embryonic cells developmentally upstream to germline specification during primate evolution. We propose that differences in L1 mobility may have differentially shaped the genomes of humans and NHPs and could have continuing adaptive significance.

Evidence for premature aging due to oxidative stress in iPSCs from Cockayne syndrome.

Cockayne syndrome (CS) is a human premature aging disorder associated with neurological and developmental abnormalities, caused by mutations mainly in the CS group B gene (ERCC6). At the molecular level, CS is characterized by a deficiency in the transcription-couple DNA repair pathway. To understand the role of this molecular pathway in a pluripotent cell and the impact of CSB mutation during human cellular development, we generated induced pluripotent stem cells (iPSCs) from CSB skin fibroblasts (CSB-iPSC). Here, we showed that the lack of functional CSB does not represent a barrier to genetic reprogramming. However, iPSCs derived from CSB patient's fibroblasts exhibited elevated cell death rate and higher reactive oxygen species (ROS) production. Moreover, these cellular phenotypes were accompanied by an up-regulation of TXNIP and TP53 transcriptional expression. Our findings suggest that CSB modulates cell viability in pluripotent stem cells, regulating the expression of TP53 and TXNIP and ROS production.

Large-scale tethered function assays identify factors that regulate mRNA stability and translation.

The molecular functions of the majority of RNA-binding proteins (RBPs) remain unclear, highlighting a major bottleneck to a full understanding of gene expression regulation. Here, we develop a plasmid resource of 690 human RBPs that we subject to luciferase-based 3'-untranslated-region tethered function assays to pinpoint RBPs that regulate RNA stability or translation. Enhanced UV-cross-linking and immunoprecipitation of these RBPs identifies thousands of endogenous mRNA targets that respond to changes in RBP level, recapitulating effects observed in tethered function assays. Among these RBPs, the ubiquitin-associated protein 2-like (UBAP2L) protein interacts with RNA via its RGG domain and cross-links to mRNA and rRNA. Fusion of UBAP2L to RNA-targeting CRISPR-Cas9 demonstrates programmable translational enhancement. Polysome profiling indicates that UBAP2L promotes translation of target mRNAs, particularly global regulators of translation. Our tethering survey allows rapid assignment of the molecular activity of proteins, such as UBAP2L, to specific steps of mRNA metabolism.

High-Throughput and Cost-Effective Characterization of Induced Pluripotent Stem Cells.

Reprogramming somatic cells to induced pluripotent stem cells (iPSCs) offers the possibility of studying the molecular mechanisms underlying human diseases in cell types difficult to extract from living patients, such as neurons and cardiomyocytes. To date, studies have been published that use small panels of iPSC-derived cell lines to study monogenic diseases. However, to study complex diseases, where the genetic variation underlying the disorder is unknown, a sizable number of patient-specific iPSC lines and controls need to be generated. Currently the methods for deriving and characterizing iPSCs are time consuming, expensive, and, in some cases, descriptive but not quantitative. Here we set out to develop a set of simple methods that reduce cost and increase throughput in the characterization of iPSC lines. Specifically, we outline methods for high-throughput quantification of surface markers, gene expression analysis of in vitro differentiation potential, and evaluation of karyotype with markedly reduced cost.

MicroRNA-101 Regulates Multiple Developmental Programs to Constrain Excitation in Adult Neural Networks.

A critical feature of neural networks is that they balance excitation and inhibition to prevent pathological dysfunction. How this is achieved is largely unknown, although deficits in the balance contribute to many neurological disorders. We show here that a microRNA (miR-101) is a key orchestrator of this essential feature, shaping the developing network to constrain excitation in the adult. Transient early blockade of miR-101 induces long-lasting hyper-excitability and persistent memory deficits. Using target site blockers in vivo, we identify multiple developmental programs regulated in parallel by miR-101 to achieve balanced networks. Repression of one target, NKCC1, initiates the switch in γ-aminobutyric acid (GABA) signaling, limits early spontaneous activity, and constrains dendritic growth. Kif1a and Ank2 are targeted to prevent excessive synapse formation. Simultaneous de-repression of these three targets completely phenocopies major dysfunctions produced by miR-101 blockade. Our results provide new mechanistic insight into brain development and suggest novel candidates for therapeutic intervention.

Two-photon imaging of calcium in virally transfected striate cortical neurons of behaving monkey.

Two-photon scanning microscopy has advanced our understanding of neural signaling in non-mammalian species and mammals. Various developments are needed to perform two-photon scanning microscopy over prolonged periods in non-human primates performing a behavioral task. In striate cortex in two macaque monkeys, cortical neurons were transfected with a genetically encoded fluorescent calcium sensor, memTNXL, using AAV1 as a viral vector. By constructing an extremely rigid and stable apparatus holding both the two-photon scanning microscope and the monkey's head, single neurons were imaged at high magnification for prolonged periods with minimal motion artifacts for up to ten months. Structural images of single neurons were obtained at high magnification. Changes in calcium during visual stimulation were measured as the monkeys performed a fixation task. Overall, functional responses and orientation tuning curves were obtained in 18.8% of the 234 labeled and imaged neurons. This demonstrated that the two-photon scanning microscopy can be successfully obtained in behaving primates.

Short Promoters in Viral Vectors Drive Selective Expression in Mammalian Inhibitory Neurons, but do not Restrict Activity to Specific Inhibitory Cell-Types.

Short cell-type specific promoter sequences are important for targeted gene therapy and studies of brain circuitry. We report on the ability of short promoter sequences to drive fluorescent protein expression in specific types of mammalian cortical inhibitory neurons using adeno-associated virus (AAV) and lentivirus (LV) vectors. We tested many gene regulatory sequences derived from fugu (Takifugu rubripes), mouse, human, and synthetic composite regulatory elements. All fugu compact promoters expressed in mouse cortex, with only the somatostatin (SST) and the neuropeptide Y (NPY) promoters largely restricting expression to GABAergic neurons. However these promoters did not control expression in inhibitory cells in a subtype specific manner. We also tested mammalian promoter sequences derived from genes putatively coexpressed or coregulated within three major inhibitory interneuron classes (PV, SST, VIP). In contrast to the fugu promoters, many of the mammalian sequences failed to express, and only the promoter from gene A930038C07Rik conferred restricted expression, although as in the case of the fugu sequences, this too was not inhibitory neuron subtype specific. Lastly and more promisingly, a synthetic sequence consisting of a composite regulatory element assembled with PAX6 E1.1 binding sites, NRSE and a minimal CMV promoter showed markedly restricted expression to a small subset of mostly inhibitory neurons, but whose commonalities are unknown.