The protocadherin alpha cluster is required for axon extension and myelination in the developing central nervous system

In adult mammals, axon regeneration after central nervous system injury is very poor, resulting in persistent functional loss. Enhancing the ability of axonal outgrowth may be a potential treatment strategy because mature neurons of the adult central nervous system may retain the intrinsic ability to regrow axons after injury. The protocadherin （Pcdh） clusters are thought to function in neuronal morphogenesis and in the assembly of neural circuitry in the brain. We cultured primary hippocampal neurons from E17.5 Pcdhα deletion （del-α） mouse embryos. After culture for 1 day, axon length was obviously shorter in del-α neurons compared with wild-type neurons. RNA sequencing of hippocampal E17.5 RNA showed that expression levels of BDNF, Fmod, Nrp2, OGN, and Sema3d, which are associated with axon extension, were significantly down-regulated in the absence of the Pcdhα gene cluster. Using transmission electron microscopy, the ratio of myelinated nerve fibers in the axons of del-α hippocampal neurons was significantly decreased; myelin sheaths of P21 Pcdhα-del mice showed lamellar disorder, discrete appearance, and vacuoles. These results indicate that the Pcdhα cluster can promote the growth and myelination of axons in the neurodevelopmental stage.

Many facades of CTCF unified by its coding for three-dimensional genome architecture

CCCTC-binding factor(CTCF)is a multifunctional zinc finger protein that is conserved in metazoan species.CTCF is consistently found to play an important role in many diverse biological processes.CTCF/cohesin-mediated active chromatin‘loop extrusion’architects three-dimensional(3D)genome folding.The 3D architectural role of CTCF underlies its multifarious functions,including developmental regulation of gene expression,protocadherin(Pcdh)promoter choice in the nervous system,immunoglobulin(Ig)and T-cell receptor(Tcr)V(D)J recombination in the immune system,homeobox(Hox)gene control during limb development,as well as many other aspects of biology.Here,we review the pleiotropic functions of CTCF from the perspective of its essential role in 3D genome architecture and topological promoter/enhancer selection.We envision the 3D genome as an enormous complex architecture,with tens of thousands of CTCF sites as connecting nodes and CTCF proteins as mysterious bonds that glue together genomic building parts with distinct articulation joints.In particular,we focus on the internal mechanisms by which CTCF controls higher order chromatin structures that manifest its many fa?ades of physiological and pathological functions.We also discuss the dichotomic role of CTCF sites as intriguing3D genome nodes for seemingly contradictory‘looping bridges’and‘topological insulators’to frame a beautiful magnificent house for a cell’s nuclear home.

Genetic evidence for asymmetric blocking of higher-order chromatin structure by CTCF/cohesin

Dear Editor,Similar to higher-order folding of polypeptide chains into functional proteins,linear DNA molecules are spatially folded in a hierarchical and dynamic manner into three-dimensional(3D)functional chromatin structures in eukaryotic nuclei(Huang and Wu,2016;Rowley and Corces,2018).This dynamic folding is closely related to many nuclear processes such as DNA replication and repair,chromosomal translo­cation,recombination,and segregation,as well as RNA transcription,splicing,and transport.In particular,dynamic long-distance chromatin looping interactions,which result in close spatial contacts between distal enhancers and target promoters,are thought to play a role in controlling precise spatiotemporal as well as cell-type specific gene expression during animal development(Rowley and Corces,2018).Mammalian genomes contain numerous noncoding regula­tory elements that regulate these dynamic long-distance chromatin looping interactions.

Ser68 phosphoregulation is essential for CENP-A deposition,centromere function and viability in mice

Centromere identity is defined by nucleosomes containing CENP-A,a histone H3 variant.The deposition of CENP-A at centromeres is tightly regulated in a cell-cycle-dependent manner.We previously reported that the spatiotemporal control of centromeric CENP-A incorporation is mediated by the phosphorylation of CENP-A Ser68.However,a recent report argued that Ser68 phosphoregulation is dispensable for accurate CENP-A loading.Here,we report that the substitution of Ser68 of endogenous CENP-A with either Gln68 or Glu68 severely impairs CENP-A deposition and cell viability.We also find that mice harboring the corresponding mutations are lethal.Together,these results indicate that the dynamic phosphorylation of Ser68 ensures cell-cycle-dependent CENP-A deposition and cell viability.

Role of farnesoid X receptor in establishment of ontogeny of phase-I drug metabolizing enzyme genes in mouse liver

The expression of phase-I drug metabolizing enzymes in liver changes dramatically during postnatal liver maturation.Farnesoid X receptor(FXR) is critical for bile acid and lipid homeostasis in liver.However,the role of FXR in regulating ontogeny of phase-I drug metabolizing genes is not clear.Hence,we applied RNA-sequencing to quantify the developmental expression of phase-I genes in both Fxr-null and control(C57BL/6) mouse livers during development.Liver samples of male C57BL/6 and Fxr-null mice at6 different ages from prenatal to adult were used.The Fxr-null showed an overall effect to diminish the "day-1 surge" of phase-I gene expression,including cytochrome P450 s at neonatal ages.Among the 185 phase-I genes from 12 different families,136 were expressed,and differential expression during development occurred in genes from all 12 phase-I families,including hydrolysis: carboxylesterase(Ces),paraoxonase(Pon),and epoxide hydrolase(Ephx); reduction: aldoketo reductase(Akr),quinone oxidoreductase(Nqo),and dihydropyrimidine dehydrogenase(Dpyd); and oxidation: alcohol dehydrogenase(Adh),aldehyde dehydrogenase(Aldh),flavin monooxygenases(Fmo),molybdenum hydroxylase(Aox and Xdh),cytochrome P450(P450),and cytochrome P450 oxidoreductase(Por).The data also suggested new phase-I genes potentially targeted by FXR.These results revealed an important role of FXR in regulation of ontogeny of phase-I genes.

Three-dimensional genome architectural CCCTC-binding factor makes choice in duplicated enhancers at Pcdhα locus

During development, gene expression is spatiotemporally regulated by long-distance chromatin interactions between distal enhancers and target promoters. However, how specificity of the interactions between enhancers and promoters is achieved remains largely unknown. As there are far more enhancers than promoters in mammalian genomes, the complexities of enhancer choice during gene regulation remain obscure. CTCF, the CCCTC-binding factor that directionally binds to a vast range of genomic sites known as CBSs(CTCF-binding sites), mediates oriented chromatin looping between a substantial set of distal enhancers and target promoters. To investigate mechanisms by which CTCF engages in enhancer choice, we used CRISPR/Cas9-based DNA-fragment editing to duplicate CBS-containing enhancers and promoters in the native genomic locus of the clustered Pcdhα genes. We found that the promoter is regulated by the proximal one among duplicated enhancers and that this choice is dependent on CTCF-mediated directional enhancer-promoter looping. In addition, gene expression is unaltered upon the switch of enhancers. Moreover, after promoter duplication, only the proximal promoter is chosen by CTCF-mediated directional chromatin looping to contact with the distal enhancer. Finally, we demonstrated that both enhancer activation and chromatin looping with the promoter are essential for gene expression. These findings have important implications regarding the role of CTCF in specific interactions between enhancers and promoters as well as developmental regulation of gene expression by enhancer switching.

Epigenetic regulation of drug metabolism and transport

The drug metabolism is a biochemical process on modification of pharmaceutical substances through specialized enzymatic systems. Changes in the expression of drug-metabolizing enzyme genes can affect drug metabolism. Recently, epigenetic regulation of drug-metabolizing enzyme genes has emerged as an important mechanism. Epigenetic regulation refers to heritable factors of genomic modifications that do not involve changes in DNA sequence. Examples of such modifications include DNA methylation, histone modifications, and non-coding RNAs. This review examines the widespread effect of epigenetic regulations on genes involved in drug metabolism, and also suggests a network perspective of epigenetic regulation. The epigenetic mechanisms have important clinical implications and may provide insights into effective drug development and improve safety of drug therapy.

Wiring the Brain by Clustered Protocadherin Neural Codes

There are more than a thousand trillion specific synaptic connections in the human brain and over a million new specific connections are formed every second during the early years of life. The assembly of these staggeringly complex neuronal circuits requires specific cell-surface molecular tags to endow each neuron with a unique identity code to discriminate self from non-self. The clustered protocadherin(Pcdh) genes, which encode a tremendous diversity of cell-surface assemblies, are candidates for neuronal identity tags. We describe the adaptive evolution,genomic structure, and regulation of expression of the clustered Pcdhs. We specifically focus on the emerging3-D architectural and biophysical mechanisms that generate an enormous number of diverse cell-surface Pcdhs as neural codes in the brain.

Efficient inversions and duplications of mammalian regulatory DNA elements and gene clusters by CRISPR/Cas9

The human genome contains millions of DNA regulatory elements and a large number of gene clusters,most of which have not been tested experimentally.The clustered regularly interspaced short palindromic repeats(CRISPR)/CRISPR-associated nuclease 9(Cas9)programed with a synthetic single-guide RNA(sgRNA)emerges as a method for genome editing in virtually any organisms.Here we report that targeted DNA fragment inversions and duplications could easily be achieved in human and mouse genomes by CRISPR with two sgRNAs.Specifically,we found that,in cultured human cells and mice,efficient precise inversions of DNA fragments ranging in size froma few tens of bp to hundreds of kb could be generated.In addition,DNA fragment duplications and deletions could also be generated by CRISPR through trans-allelic recombination between the Cas9-induced double-strand breaks(DSBs)on two homologous chromosomes(chromatids).Moreover,junctions of combinatorial inversions and duplications of the protocadherin(Pcdh)gene clusters induced by Cas9 with four sgRNAs could be detected.In mice,we obtained founders with alleles of precise inversions,duplications,and deletions of DNA fragments of variable sizes by CRISPR.Interestingly,we found that very efficient inversions were mediated by microhomology-mediated end joining(MMEJ)through short inverted repeats.We showed for the first time that DNA fragment inversions could be transmitted through germlines in mice.Finally,we applied this CRISPR method to a regulatory element of the Pcdha cluster and found a new role in the regulation of members of the Pcdhg cluster.This simple and efficient method should be useful in manipulating mammalian genomes to study millions of regulatory DNA elements as well as vast numbers of gene clusters.

Toward precise CRISPR DNA fragment editing and predictable 3D genome engineering

Ever since gene targeting or specific modification of genome sequences in mice was achieved in the early 1980s,the reverse genetic approach of precise editing of any genomic locus has greatly accelerated biomedical research and biotechnology development.In particular,the recent development of the CRISPR/Cas9 system has greatly expedited genetic dissection of 3D genomes.CRISPR gene-editing outcomes result from targeted genome cleavage by ectopic bacterial Cas9 nuclease followed by presumed random ligations via the host double-strand break repair machineries.Recent studies revealed,however,that the CRISPR genomeediting system is precise and predictable because of cohesive Cas9 cleavage of targeting DNA.Here,we synthesize the current understanding of CRISPR DNA fragment-editing mechanisms and recent progress in predictable outcomes from precise genetic engineering of 3D genomes.Specifically,we first briefly describe historical genetic studies leading to CRISPR and 3D genome engineering.We then summarize different types of chromosomal rearrangements by DNA fragment editing.Finally,we review significant progress from precise ID gene editing toward predictable 3D genome engineering and synthetic biology.The exciting and rapid advances in this emerging field provide new opportunities and challenges to understand or digest 3D genomes.

CRISPR Double Cutting through the Labyrinthine Architecture of 3D Genomes

The genomes are organized into ordered and hierarchical topological structures in interphase nuclei.Within discrete territories of each chromosome,topologically associated domains（TADs） play important roles in various nuclear processes such as gene regulation.Inside TADs separated by relatively constitutive boundaries,distal elements regulate their gene targets through specific chromatin-looping contacts such as long-distance enhancer-promoter interactions.High-throughput sequencing studies have revealed millions of potential regulatory DNA elements,which are much more abundant than the mere ~ 20,000 genes they control.The recently emerged CRISPRCas9 genome editing technologies have enabled efficient and precise genetic and epigenetic manipulations of genomes.The multiplexed and high-throughput CRISPR capabilities facilitate the discovery and dissection of gene regulatory elements.Here,we describe the applications of CRISPR for genome,epigenome,and 3D genome editing,focusing on CRISPR DNA-fragment editing with Cas9 and a pair of sgRNAs to investigate topological folding of chromatin TADs and developmental gene regulation.

Questions about NgAgo

Dear Editor： Gao et al. published data in Nature Biotechnology （Nat Biotechnol. 2016 May 2） showing that DNA-guided genome editing using the Natronobacterium gregoryi Argonaute （NgAgo） protein targeted 47 mammalian genomic loci with a 100% success rate and an efficiency of 21.3%-41.3% at various targets. This report led us to test NgAgo＇s utility in various cells and organisms such as mouse and zebrafish for gene editing.

Pyk2 suppresses contextual fear memory in an autophosphorylation-independent manner

Clustered protocadherins(Pcdhs)are a large family of cadherin-like cell adhesion proteins that are central for neurite selfavoidance and neuronal connectivity in the brain.Their downstream nonreceptor tyrosine kinase Pyk2(proline-rich tyrosine kinase 2,also known as Ptk2b,Cakb,Raftk,Fak2,and Cadtk)is predominantly expressed in the hippocampus.We constructed Pyk2-null mouse lines and found that these mutant mice showed enhancement in contextual fear memory,without significant change in auditory-cued and spatial-referenced learning and memory.In addition,by preparing Y402F mutant mice,we observed that Pyk2 suppressed contextual fear memory in an autophosphorylation-independent manner.Moreover,using high-throughput RNA sequencing,we found that immediate early genes,such as Npas4,cFos,Zif268/Egr1,Arc,and Nr4a1,were enhanced in Pyk2-null mice.We further showed that Pyk2 disruption affected pyramidal neuronal complexity and spine dynamics.Thus,we demonstrated that Pyk2 is a novel fear memory suppressor molecule and Pyk2-null mice provide a model for understanding fearrelated disorders.These findings have interesting implications regarding dysregulation of the Pcdh–Pyk2 axis in neuropsychiatric disorders.

Erratum to： Questions about NgAgo

In the original publication of this article the Figure 1E legend has been incorrectly published.