In 1959, while analysing the bacterial flagellar proteins, Ambler and Rees observed an unknown species of amino acid that they eventually identified as methylated lysine. Over half a century later, ...protein methylation is known to have a regulatory role in many essential cellular processes that range from gene transcription to signal transduction. However, the road to this now burgeoning research field was obstacle-ridden, not least because of the inconspicuous nature of the methyl mark itself. Here, we chronicle the milestone achievements and discuss the future of protein methylation research.
Although high-throughput RNA sequencing (RNA-seq) has greatly advanced small non-coding RNA (sncRNA) discovery, the currently widely used complementary DNA library construction protocol generates ...biased sequencing results. This is partially due to RNA modifications that interfere with adapter ligation and reverse transcription processes, which prevent the detection of sncRNAs bearing these modifications. Here, we present PANDORA-seq (panoramic RNA display by overcoming RNA modification aborted sequencing), employing a combinatorial enzymatic treatment to remove key RNA modifications that block adapter ligation and reverse transcription. PANDORA-seq identified abundant modified sncRNAs-mostly transfer RNA-derived small RNAs (tsRNAs) and ribosomal RNA-derived small RNAs (rsRNAs)-that were previously undetected, exhibiting tissue-specific expression across mouse brain, liver, spleen and sperm, as well as cell-specific expression across embryonic stem cells (ESCs) and HeLa cells. Using PANDORA-seq, we revealed unprecedented landscapes of microRNA, tsRNA and rsRNA dynamics during the generation of induced pluripotent stem cells. Importantly, tsRNAs and rsRNAs that are downregulated during somatic cell reprogramming impact cellular translation in ESCs, suggesting a role in lineage differentiation.
Lysine-specific demethylase 1 (LSD1) has been reported to repress and activate transcription by mediating histone H3K4me1/2 and H3K9me1/2 demethylation, respectively. The molecular mechanism that ...underlies this dual substrate specificity has remained unknown. Here we report that an isoform of LSD1, LSD1+8a, does not have the intrinsic capability to demethylate H3K4me2. Instead, LSD1+8a mediates H3K9me2 demethylation in collaboration with supervillin (SVIL), a new LSD1+8a interacting protein. LSD1+8a knockdown increases H3K9me2, but not H3K4me2, levels at its target promoters and compromises neuronal differentiation. Importantly, SVIL co-localizes to LSD1+8a-bound promoters, and its knockdown mimics the impact of LSD1+8a loss, supporting SVIL as a cofactor for LSD1+8a in neuronal cells. These findings provide insight into mechanisms by which LSD1 mediates H3K9me demethylation and highlight alternative splicing as a means by which LSD1 acquires selective substrate specificities (H3K9 versus H3K4) to differentially control specific gene expression programs in neurons.
Display omitted
•LSD1+8a isoform does not have the intrinsic property to demethylate H3K4•LSD1+8a functions as a co-activator by demethylating the repressive H3K9me2 mark•LSD1+8a interacts with SVIL; LSD1+8a/SVIL-containing complex demethylates H3K9me2•SVIL regulates neuronal maturation by controlling LSD1+8a mediated H3K9 demethylation
Benoit Laurent et al. find that the neuro-enriched LSD1 isoform, LSD1+8a, mediates H3K9me2 demethylation in collaboration with the supervillin protein (SVIL), a new LSD1+8a partner. These findings highlight alternative splicing as a means by which LSD1 acquires selective substrate specificities (H3K9 versus H3K4) to control specific transcriptional programs in neurons.
Cellular differentiation involves profound remodelling of chromatic landscapes, yet the mechanisms by which somatic cell identity is subsequently maintained remain incompletely understood. To further ...elucidate regulatory pathways that safeguard the somatic state, we performed two comprehensive RNA interference (RNAi) screens targeting chromatin factors during transcription-factor-mediated reprogramming of mouse fibroblasts to induced pluripotent stem cells (iPS cells). Subunits of the chromatin assembly factor-1 (CAF-1) complex, including Chaf1a and Chaf1b, emerged as the most prominent hits from both screens, followed by modulators of lysine sumoylation and heterochromatin maintenance. Optimal modulation of both CAF-1 and transcription factor levels increased reprogramming efficiency by several orders of magnitude and facilitated iPS cell formation in as little as 4 days. Mechanistically, CAF-1 suppression led to a more accessible chromatin structure at enhancer elements early during reprogramming. These changes were accompanied by a decrease in somatic heterochromatin domains, increased binding of Sox2 to pluripotency-specific targets and activation of associated genes. Notably, suppression of CAF-1 also enhanced the direct conversion of B cells into macrophages and fibroblasts into neurons. Together, our findings reveal the histone chaperone CAF-1 to be a novel regulator of somatic cell identity during transcription-factor-induced cell-fate transitions and provide a potential strategy to modulate cellular plasticity in a regenerative setting.
Cell fate commitment is driven by dynamic changes in chromatin architecture and activity of lineage-specific transcription factors (TFs). The chromatin assembly factor-1 (CAF-1) is a histone ...chaperone that regulates chromatin architecture by facilitating nucleosome assembly during DNA replication. Accumulating evidence supports a substantial role of CAF-1 in cell fate maintenance, but the mechanisms by which CAF-1 restricts lineage choice remain poorly understood. Here, we investigate how CAF-1 influences chromatin dynamics and TF activity during lineage differentiation. We show that CAF-1 suppression triggers rapid differentiation of myeloid stem and progenitor cells into a mixed lineage state. We find that CAF-1 sustains lineage fidelity by controlling chromatin accessibility at specific loci, and limiting the binding of ELF1 TF at newly-accessible diverging regulatory elements. Together, our findings decipher key traits of chromatin accessibility that sustain lineage integrity and point to a powerful strategy for dissecting transcriptional circuits central to cell fate commitment.
Correct orchestration of nervous system development is a profound challenge that involves coordination of complex molecular and cellular processes. Mechanistic target of rapamycin (mTOR) signaling is ...a key regulator of nervous system development and synaptic function. The mTOR kinase is a hub for sensing inputs including growth factor signaling, nutrients and energy levels. Activation of mTOR signaling causes diseases with severe neurological manifestations, such as tuberous sclerosis complex and focal cortical dysplasia. However, the molecular mechanisms by which mTOR signaling regulates nervous system development and function are poorly understood. Unkempt is a conserved zinc finger/RING domain protein that regulates neurogenesis downstream of mTOR signaling in Drosophila. Unkempt also directly interacts with the mTOR complex I component Raptor. Here we describe the generation and characterisation of mice with a conditional knockout of Unkempt (Unk
) in the nervous system. Loss of Unkempt reduces Raptor protein levels in the embryonic nervous system but does not affect downstream mTORC1 targets. We also show that nervous system development occurs normally in Unk
mice. However, we find that Unkempt is expressed in the adult cerebellum and hippocampus and behavioural analyses show that Unk
mice have improved memory formation and cognitive flexibility to re-learn. Further understanding of the role of Unkempt in the nervous system will provide novel mechanistic insight into the role of mTOR signaling in learning and memory.
Advances in cellular reprogramming and stem cell differentiation now enable ex vivo studies of human neuronal differentiation. However, it remains challenging to elucidate the underlying regulatory ...programs because differentiation protocols are laborious and often result in low neuron yields. Here, we overexpressed two Neurogenin transcription factors in human‐induced pluripotent stem cells and obtained neurons with bipolar morphology in 4 days, at greater than 90% purity. The high purity enabled mRNA and microRNA expression profiling during neurogenesis, thus revealing the genetic programs involved in the rapid transition from stem cell to neuron. The resulting cells exhibited transcriptional, morphological and functional signatures of differentiated neurons, with greatest transcriptional similarity to prenatal human brain samples. Our analysis revealed a network of key transcription factors and microRNAs that promoted loss of pluripotency and rapid neurogenesis via progenitor states. Perturbations of key transcription factors affected homogeneity and phenotypic properties of the resulting neurons, suggesting that a systems‐level view of the molecular biology of differentiation may guide subsequent manipulation of human stem cells to rapidly obtain diverse neuronal types.
Synopsis
Rapid and homogeneous neuronal differentiation is attained in human stem cells upon overexpression of two Neurogenin transcription factors. mRNA and miRNA expression profiling during differentiation reveals a regulatory network mediating neurogenesis from stem cells.
Neurogenin‐1 and ‐2 drive homogeneous differentiation of human stem cells into bipolar neurons in 4 days in defined media.
The population homogeneity allowed mRNA and miRNA expression profiling over time during neurogenesis.
A network of key transcription factors and miRNAs that promote rapid neurogenesis and loss of pluripotency is identified.
Perturbations of key transcription factors affect the homogeneity and phenotypic properties of the resulting neurons.
Rapid and homogeneous neuronal differentiation is attained in human stem cells upon overexpression of two Neurogenin transcription factors. mRNA and miRNA expression profiling during differentiation reveals a regulatory network mediating neurogenesis from stem cells.
How RNA-binding proteins (RBPs) convey regulatory instructions to the core effectors of RNA processing is unclear. Here, we document the existence and functions of a multivalent RBP-effector ...interface. We show that the effector interface of a conserved RBP with an essential role in metazoan development, Unkempt, is mediated by a novel type of 'dual-purpose' peptide motifs that can contact two different surfaces of interacting proteins. Unexpectedly, we find that the multivalent contacts do not merely serve effector recruitment but are required for the accuracy of RNA recognition by Unkempt. Systems analyses reveal that multivalent RBP-effector contacts can repurpose the principal activity of an effector for a different function, as we demonstrate for the reuse of the central eukaryotic mRNA decay factor CCR4-NOT in translational control. Our study establishes the molecular assembly and functional principles of an RBP-effector interface.
Fragile X syndrome, a common form of inherited intellectual disability, is caused by loss of the fragile X mental retardation protein FMRP. FMRP is present predominantly in the cytoplasm, where it ...regulates translation of proteins that are important for synaptic function. We identify FMRP as a chromatin-binding protein that functions in the DNA damage response (DDR). Specifically, we show that FMRP binds chromatin through its tandem Tudor (Agenet) domain in vitro and associates with chromatin in vivo. We also demonstrate that FMRP participates in the DDR in a chromatin-binding-dependent manner. The DDR machinery is known to play important roles in developmental processes such as gametogenesis. We show that FMRP occupies meiotic chromosomes and regulates the dynamics of the DDR machinery during mouse spermatogenesis. These findings suggest that nuclear FMRP regulates genomic stability at the chromatin interface and may impact gametogenesis and some developmental aspects of fragile X syndrome.
Display omitted
•Fragile X mental retardation protein FMRP binds chromatin via its Agenet domain•FMRP participates in the DNA damage response in a chromatin-dependent manner•FMRP occupies chromosomes and regulates DNA damage machinery in male mouse meiosis•Lack of FMRP results in meiotic defects such as incomplete chromosome pairing
In addition to its classical role as a translational regulator at neuronal synapses, the fragile X mental retardation protein has a role in the nucleus, where it targets chromatin and regulates the DNA damage response and mammalian gametogenesis.
Unkempt is an evolutionarily conserved RNA-binding protein that regulates translation of its target genes and is required for the establishment of the early bipolar neuronal morphology. Here we ...determined the X-ray crystal structure of mouse Unkempt and show that its six CCCH zinc fingers (ZnFs) form two compact clusters, ZnF1-3 and ZnF4-6, that recognize distinct trinucleotide RNA substrates. Both ZnF clusters adopt a similar overall topology and use distinct recognition principles to target specific RNA sequences. Structure-guided point mutations reduce the RNA binding affinity of Unkempt both in vitro and in vivo, ablate Unkempt's translational control and impair the ability of Unkempt to induce a bipolar cellular morphology. Our study unravels a new mode of RNA sequence recognition by clusters of CCCH ZnFs that is critical for post-transcriptional control of neuronal morphology.
PDF
