MYRF: A unique transmembrane transcription factor- from proteolytic self-processing to its multifaceted roles in animal development.

The Myelin Regulator Factor (MYRF) is a master regulator governing myelin formation and maintenance in the central nervous system. The conservation of MYRF across metazoans and its broad tissue expression suggest it has functions extending beyond the well-established role in myelination. Loss of MYRF results in developmental lethality in both invertebrates and vertebrates, and MYRF haploinsufficiency in humans causes MYRF-related Cardiac Urogenital Syndrome, underscoring its importance in animal development; however, these mechanisms are largely unexplored. MYRF, an unconventional transcription factor, begins embedded in the membrane and undergoes intramolecular chaperone mediated trimerization, which triggers self-cleavage, allowing its N-terminal segment with an Ig-fold DNA-binding domain to enter the nucleus for transcriptional regulation. Recent research suggests developmental regulation of cleavage, yet the mechanisms remain enigmatic. While some parts of MYRF's structure have been elucidated, others remain obscure, leaving questions about how these motifs are linked to its intricate processing and function.

Context-dependent modulation of Pol II CTD phosphatase SSUP-72 regulates alternative polyadenylation in neuronal development.

Alternative polyadenylation (APA) is widespread in neuronal development and activity-mediated neural plasticity. However, the underlying molecular mechanisms are largely unknown. We used systematic genetic studies and genome-wide surveys of the transcriptional landscape to identify a context-dependent regulatory pathway controlling APA in the Caenorhabditis elegans nervous system. Loss of function in ssup-72, a Ser5 phosphatase for the RNA polymerase II (Pol II) C-terminal domain (CTD), dampens transcription termination at a strong intronic polyadenylation site (PAS) in unc-44/ankyrin yet promotes termination at the weak intronic PAS of the MAP kinase dlk-1. A nuclear protein, SYDN-1, which regulates neuronal development, antagonizes the function of SSUP-72 and several nuclear polyadenylation factors. This regulatory pathway allows the production of a neuron-specific isoform of unc-44 and an inhibitory isoform of dlk-1. Dysregulation of the unc-44 and dlk-1 mRNA isoforms in sydn-1 mutants impairs neuronal development. Deleting the intronic PAS of unc-44 results in increased pre-mRNA processing of neuronal ankyrin and suppresses sydn-1 mutants. These results reveal a mechanism by which regulation of CTD phosphorylation controls coding region APA in the nervous system.

Interactive Repression of MYRF Self-Cleavage and Activity in Oligodendrocyte Differentiation by TMEM98 Protein.

Myelin sheath formed by oligodendrocytes (OLs) is essential for the rapid propagation of action potentials in the vertebrate CNS. Myelin regulatory factor (MYRF) is one of the critical factors that control OL differentiation and myelin maintenance. Previous studies showed that MYRF is a membrane-bound transcription factor associated with the endoplasmic reticulum (ER). After self-cleavage, the N-fragment of MYRF is released from the ER and translocated into the nucleus where it functions as a transcription factor to activate myelin gene expression. At present, it remains unknown whether MYRF self-cleavage and functional activation can be regulated during OL differentiation. Here, we report that TMEM98, an ER-associated transmembrane protein, is capable of binding to the C-terminal of MYRF and inhibiting its self-cleavage and N-fragment nuclear translocation. In the developing CNS, TMEM98 is selectively expressed in early maturing OLs in mouse pups of either sex. Forced expression of TMEM98 in embryonic chicken spinal cord of either sex suppresses endogenous OL differentiation and MYRF-induced ectopic expression of myelin genes. These results suggest that TMEM98, through inhibiting the self-cleavage of MYRF, functions as a negative feedback regulator of MYRF in oligodendrocyte differentiation and myelination.SIGNIFICANCE STATEMENT MYRF protein is initially synthesized as an ER-associated membrane protein that undergoes autoproteolytic cleavage to release the N-fragment, which is then transported into the nucleus and activates the transcription of myelin genes. To date, the molecular mechanisms that regulate the self-cleavage and function of MYRF in regulating oligodendrocyte differentiation have remained unknown. In this study, we present the molecular and functional evidence that TMEM98 membrane protein physically interacts with MYRF in the ER and subsequently blocks its self-cleavage, N-terminal nuclear translocation, and functional activation of myelin gene expression. To our knowledge, this is the first report on the regulation of MYRF self-proteolytic activity and function by an interacting protein, providing new insights into the molecular regulation of OL differentiation and myelinogenesis.

Photo-inducible cell ablation in Caenorhabditis elegans using the genetically encoded singlet oxygen generating protein miniSOG.

We describe a method for light-inducible and tissue-selective cell ablation using a genetically encoded photosensitizer, miniSOG (mini singlet oxygen generator). miniSOG is a newly engineered fluorescent protein of 106 amino acids that generates singlet oxygen in quantum yield upon blue-light illumination. We transgenically expressed mitochondrially targeted miniSOG (mito-miniSOG) in Caenorhabditis elegans neurons. Upon blue-light illumination, mito-miniSOG causes rapid and effective death of neurons in a cell-autonomous manner without detectable damages to surrounding tissues. Neuronal death induced by mito-miniSOG appears to be independent of the caspase CED-3, but the clearance of the damaged cells partially depends on the phagocytic receptor CED-1, a homolog of human CD91. We show that neurons can be killed at different developmental stages. We further use this method to investigate the role of the premotor interneurons in regulating the convulsive behavior caused by a gain-of-function mutation in the neuronal acetylcholine receptor acr-2. Our findings support an instructive role for the interneuron AVB in controlling motor neuron activity and reveal an inhibitory effect of the backward premotor interneurons on the forward interneurons. In summary, the simple inducible cell ablation method reported here allows temporal and spatial control and will prove to be a useful tool in studying the function of specific cells within complex cellular contexts.

Complementary annealing mediated by exonuclease: a method for seamless cloning and conditioning site-directed mutagenesis.

Traditional cut-paste DNA cloning is often limited by the availability of restriction enzyme sites. Here, we described the complementary annealing mediated by exonuclease (CAME), in which the insert or vector fragment is amplified to carry sequences complementary to the other, and both fragments are modified by exonuleases to create directional single-stranded overhangs. The two recessed DNA fragments are joined through complementary strand annealing. The CAME is highly efficient for cloning the DNA of at least 12 kb and single DNA fragment out of a complex DNA sample. Moreover, the application of CAME greatly improved the efficiency of site-directed mutagenesis.

Hyperactivation of B-type motor neurons results in aberrant synchrony of the Caenorhabditis elegans motor circuit.

Excitatory acetylcholine motor neurons drive Caenorhabditis elegans locomotion. Coordinating the activation states of the backward-driving A and forward-driving B class motor neurons is critical for generating sinusoidal and directional locomotion. Here, we show by in vivo calcium imaging that expression of a hyperactive, somatodendritic ionotropic acetylcholine receptor ACR-2(gf) in A and B class motor neurons induces aberrant synchronous activity in both ventral- and dorsal-innervating B and A class motor neurons. Expression of ACR-2(gf) in either ventral- or dorsal-innervating B neurons is sufficient for triggering the aberrant synchrony that results in arrhythmic convulsions. Silencing of AVB, the premotor interneurons that innervate B motor neurons suppresses ACR-2(gf)-dependent convulsion; activating AVB by channelrhodopsin induces the onset of convulsion. These results support that the activity state of B motor neurons plays an instructive role for the coordination of motor circuit.

A genetically encoded tag for correlated light and electron microscopy of intact cells, tissues, and organisms.

Electron microscopy (EM) achieves the highest spatial resolution in protein localization, but specific protein EM labeling has lacked generally applicable genetically encoded tags for in situ visualization in cells and tissues. Here we introduce "miniSOG" (for mini Singlet Oxygen Generator), a fluorescent flavoprotein engineered from Arabidopsis phototropin 2. MiniSOG contains 106 amino acids, less than half the size of Green Fluorescent Protein. Illumination of miniSOG generates sufficient singlet oxygen to locally catalyze the polymerization of diaminobenzidine into an osmiophilic reaction product resolvable by EM. MiniSOG fusions to many well-characterized proteins localize correctly in mammalian cells, intact nematodes, and rodents, enabling correlated fluorescence and EM from large volumes of tissue after strong aldehyde fixation, without the need for exogenous ligands, probes, or destructive permeabilizing detergents. MiniSOG permits high quality ultrastructural preservation and 3-dimensional protein localization via electron tomography or serial section block face scanning electron microscopy. EM shows that miniSOG-tagged SynCAM1 is presynaptic in cultured cortical neurons, whereas miniSOG-tagged SynCAM2 is postsynaptic in culture and in intact mice. Thus SynCAM1 and SynCAM2 could be heterophilic partners. MiniSOG may do for EM what Green Fluorescent Protein did for fluorescence microscopy.

Essential function of transmembrane transcription factor MYRF in promoting transcription of miRNA lin-4 during C. elegans development.

Precise developmental timing control is essential for organism formation and function, but its mechanisms are unclear. In C. elegans, the microRNA lin-4 critically regulates developmental timing by post-transcriptionally downregulating the larval-stage-fate controller LIN-14. However, the mechanisms triggering the activation of lin-4 expression toward the end of the first larval stage remain unknown. We demonstrate that the transmembrane transcription factor MYRF-1 is necessary for lin-4 activation. MYRF-1 is initially localized on the cell membrane, and its increased cleavage and nuclear accumulation coincide with lin-4 expression timing. MYRF-1 regulates lin-4 expression cell-autonomously and hyperactive MYRF-1 can prematurely drive lin-4 expression in embryos and young first-stage larvae. The tandem lin-4 promoter DNA recruits MYRF-1GFP to form visible loci in the nucleus, suggesting that MYRF-1 directly binds to the lin-4 promoter. Our findings identify a crucial link in understanding developmental timing regulation and establish MYRF-1 as a key regulator of lin-4 expression.

Nuclear pre-mRNA 3'-end processing regulates synapse and axon development in C. elegans.

Nuclear pre-mRNA 3'-end processing is vital for the production of mature mRNA and the generation of the 3' untranslated region (UTR). However, the roles and regulation of this event in cellular development remain poorly understood. Here, we report the function of a nuclear pre-mRNA 3'-end processing pathway in synapse and axon formation in C. elegans. In a genetic enhancer screen for synaptogenesis mutants, we identified a novel polyproline-rich protein, Synaptic defective enhancer-1 (SYDN-1). Loss of function of sydn-1 causes abnormal synapse and axon development, and displays striking synergistic interactions with several genes that regulate specific aspects of synapses. SYDN-1 is required in neurons and localizes to distinct regions of the nucleus. Through a genetic suppressor screen, we found that the neuronal defects of sydn-1 mutants are suppressed by loss of function in Polyadenylation factor subunit-2 (PFS-2), a conserved WD40-repeat protein that interacts with multiple subcomplexes of the pre-mRNA 3'-end processing machinery. PFS-2 partially colocalizes with SYDN-1, and SYDN-1 influences the nuclear abundance of PFS-2. Inactivation of several members of the nuclear 3'-end processing complex suppresses sydn-1 mutants. Furthermore, lack of sydn-1 can increase the activity of 3'-end processing. Our studies provide in vivo evidence for pre-mRNA 3'-end processing in synapse and axon development and identify SYDN-1 as a negative regulator of this cellular event in neurons.

Allosterically inhibited PFKL via prostaglandin E2 withholds glucose metabolism and ovarian cancer invasiveness.

Metastasis is the leading cause of high ovarian-cancer-related mortality worldwide. Three major processes constitute the whole metastatic cascade: invasion, intravasation, and extravasation. Tumor cells often reprogram their metabolism to gain advantages in proliferation and survival. However, whether and how those metabolic alterations contribute to the invasiveness of tumor cells has yet to be fully understood. Here we performed a genome-wide CRISPR-Cas9 screening to identify genes participating in tumor cell dissemination and revealed that PTGES3 acts as an invasion suppressor in ovarian cancer. Mechanistically, PTGES3 binds to phosphofructokinase, liver type (PFKL) and generates a local source of prostaglandin E2 (PGE2) to allosterically inhibit the enzymatic activity of PFKL. Repressed PFKL leads to downgraded glycolysis and the subsequent TCA cycle for glucose metabolism. However, ovarian cancer suppresses the expression of PTGES3 and disrupts the PTGES3-PGE2-PFKL inhibitory axis, leading to hyperactivation of glucose oxidation, eventually facilitating ovarian cancer cell motility and invasiveness.

A neuronal acetylcholine receptor regulates the balance of muscle excitation and inhibition in Caenorhabditis elegans.

In the nematode Caenorhabditis elegans, cholinergic motor neurons stimulate muscle contraction as well as activate GABAergic motor neurons that inhibit contraction of the contralateral muscles. Here, we describe the composition of an ionotropic acetylcholine receptor that is required to maintain excitation of the cholinergic motor neurons. We identified a gain-of-function mutation that leads to spontaneous muscle convulsions. The mutation is in the pore domain of the ACR-2 acetylcholine receptor subunit and is identical to a hyperactivating mutation in the muscle receptor of patients with myasthenia gravis. Screens for suppressors of the convulsion phenotype led to the identification of other receptor subunits. Cell-specific rescue experiments indicate that these subunits function in the cholinergic motor neurons. Expression of these subunits in Xenopus oocytes demonstrates that the functional receptor is comprised of three alpha-subunits, UNC-38, UNC-63 and ACR-12, and two non-alpha-subunits, ACR-2 and ACR-3. Although this receptor exhibits a partially overlapping subunit composition with the C. elegans muscle acetylcholine receptor, it shows distinct pharmacology. Recordings from intact animals demonstrate that loss-of-function mutations in acr-2 reduce the excitability of the cholinergic motor neurons. By contrast, the acr-2(gf) mutation leads to a hyperactivation of cholinergic motor neurons and an inactivation of downstream GABAergic motor neurons in a calcium dependent manner. Presumably, this imbalance between excitatory and inhibitory input into muscles leads to convulsions. These data indicate that the ACR-2 receptor is important for the coordinated excitation and inhibition of body muscles underlying sinusoidal movement.

Atbf1 is required for the Pit1 gene early activation.

Enhancers have been functionally described for >35 years, but the molecular principles underlying the integration of regulatory inputs to alternate gene enhancers used during mammalian organogenesis remain incompletely understood. Using a combination of in vivo enhancer mapping and proteomics approaches, we have established that two distant and distinct early enhancers, each requiring different transcription complexes, are required for full activation of the gene encoding the pituitary lineage determining factor, Pit1. A transcription factor belonging to the "giant, multiple-homeodomain and zinc finger family," Atbf1, serves as a novel pituitary regulator for one of the two required enhancers as shown by genetic and in vitro analysis.

The LRR-TM protein PAN-1 interacts with MYRF to promote its nuclear translocation in synaptic remodeling.

Neural circuits develop through a plastic phase orchestrated by genetic programs and environmental signals. We have identified a leucine-rich-repeat domain transmembrane protein PAN-1 as a factor required for synaptic rewiring in C. elegans. PAN-1 localizes on cell membrane and binds with MYRF, a membrane-bound transcription factor indispensable for promoting synaptic rewiring. Full-length MYRF was known to undergo self-cleavage on ER membrane and release its transcriptional N-terminal fragment in cultured cells. We surprisingly find that MYRF trafficking to cell membrane before cleavage is pivotal for C. elegans development and the timing of N-MYRF release coincides with the onset of synaptic rewiring. On cell membrane PAN-1 and MYRF interact with each other via their extracellular regions. Loss of PAN-1 abolishes MYRF cell membrane localization, consequently blocking myrf-dependent neuronal rewiring process. Thus, through interactions with a cooperating factor on the cell membrane, MYRF may link cell surface activities to transcriptional cascades required for development.

Co-localization of Nkx6.2 and Nkx2.2 homeodomain proteins in differentiated myelinating oligodendrocytes.

Recent studies have suggested that Nkx6.2/Gtx and Nkx2.2 homeodomain transcription factors are involved in the regulation of oligodendrocyte maturation and/or myelination which occur predominantly in postnatal stages. However, their cellular specificity in postnatal central nervous system has not been characterized and their dynamic expressional relationship during oligodendrocyte lineage progression has not been determined. Here we report that both Nkx2.2 and Nkx6.2 are selectively expressed in Olig2+ cells of oligodendrocyte lineage in postnatal spinal cords. Although Nkx6.2 is specifically expressed in the APC+ mature oligodendrocytes, Nkx2.2 is initially expressed in differentiating oligodendrocyte precursor cells (OPCs) but quickly down-regulated as OPCs undergo terminal differentiation. Intriguingly, Nkx2.2 expression is up-regulated in mature myelinating oligodendrocytes at later stages. The co-expression of Nkx2.2 and Nkx6.2 transcription factors in myelinating oligodendrocytes suggests their functional interactions in the regulation of myelin sheath formation and/or maintenance.

Generation of oligodendrocyte precursor cells from mouse dorsal spinal cord independent of Nkx6 regulation and Shh signaling.

In the developing spinal cord, early progenitor cells of the oligodendrocyte lineage are induced in the motor neuron progenitor (pMN) domain of the ventral neuroepithelium by the ventral midline signal Sonic hedgehog (Shh). The ventral generation of oligodendrocytes requires Nkx6-regulated expression of the bHLH gene Olig2 in this domain. In the absence of Nkx6 genes or Shh signaling, the initial expression of Olig2 in the pMN domain is completely abolished. In this study, we provide the in vivo evidence for a late phase of Olig gene expression independent of Nkx6 and Shh gene activities and reveal a brief second wave of oligodendrogenesis in the dorsal spinal cord. In addition, we provide genetic evidence that oligodendrogenesis can occur in the absence of hedgehog receptor Smoothened, which is essential for all hedgehog signaling.

A Tet/Q Hybrid System for Robust and Versatile Control of Transgene Expression in C. elegans.

Binary gene regulatory tools such as the Tetracycline (Tet)-controlled transcription system have revolutionized genetic research in multiple organisms, but their applications to the worm remain very limited. Here we report that the canonical Tet system is largely inactive in the worm but can be adapted for the worm by introducing multiple modifications, a crucial one being the use of the transcription activation domain from the fungal Q binary system. The resultant Tet/Q hybrid system proves more robust and flexible than either of its precursors, enabling elaborate modes of transgene manipulation previously hard to achieve in the worm, including inducible intersectional regulation and, in combination with the Q system, independent control of distinct transgenes within the same cells. Furthermore, we demonstrated, as an example of its applications, that the hybrid system can tightly and efficiently control Cre expression. This study establishes Tet/Q as a premier binary system for worm genetic research.

Building stereotypic connectivity: mechanistic insights into structural plasticity from C. elegans.

The ability of neurons to modify or remodel their synaptic connectivity is critical for the function of neural circuitry throughout the life of an animal. Understanding the mechanisms underlying neuronal structural changes is central to our knowledge of how the nervous system is shaped for complex behaviors and how it further adapts to developmental and environmental demands. Caenorhabditis elegans provides a powerful model for examining developmental processes and for discovering mechanisms controlling neural plasticity. Recent findings have identified conserved themes underlying neural plasticity in development and under environmental stress.

Excitatory motor neurons are local oscillators for backward locomotion.

Cell- or network-driven oscillators underlie motor rhythmicity. The identity of C. elegans oscillators remains unknown. Through cell ablation, electrophysiology, and calcium imaging, we show: (1) forward and backward locomotion is driven by different oscillators; (2) the cholinergic and excitatory A-class motor neurons exhibit intrinsic and oscillatory activity that is sufficient to drive backward locomotion in the absence of premotor interneurons; (3) the UNC-2 P/Q/N high-voltage-activated calcium current underlies A motor neuron's oscillation; (4) descending premotor interneurons AVA, via an evolutionarily conserved, mixed gap junction and chemical synapse configuration, exert state-dependent inhibition and potentiation of A motor neuron's intrinsic activity to regulate backward locomotion. Thus, motor neurons themselves derive rhythms, which are dually regulated by the descending interneurons to control the reversal motor state. These and previous findings exemplify compression: essential circuit properties are conserved but executed by fewer numbers and layers of neurons in a small locomotor network.

Novel Mutations in Synaptic Transmission Genes Suppress Neuronal Hyperexcitation in Caenorhabditis elegans.

Acetylcholine (ACh) receptors (AChR) regulate neural circuit activity in multiple contexts. In humans, mutations in ionotropic acetylcholine receptor (iAChR) genes can cause neurological disorders, including myasthenia gravis and epilepsy. In Caenorhabditis elegans, iAChRs play multiple roles in the locomotor circuit. The cholinergic motor neurons express an ACR-2-containing pentameric AChR (ACR-2R) comprised of ACR-2, ACR-3, ACR-12, UNC-38, and UNC-63 subunits. A gain-of-function mutation in the non-α subunit gene acr-2 [acr-2(gf)] causes defective locomotion as well as spontaneous convulsions. Previous studies of genetic suppressors of acr-2(gf) have provided insights into ACR-2R composition and assembly. Here, to further understand how the ACR-2R regulates neuronal activity, we expanded the suppressor screen for acr-2(gf)-induced convulsions. The majority of these suppressor mutations affect genes that play critical roles in synaptic transmission, including two novel mutations in the vesicular ACh transporter unc-17 In addition, we identified a role for a conserved major facilitator superfamily domain (MFSD) protein, mfsd-6, in regulating neural circuit activity. We further defined a role for the sphingosine (SPH) kinase (Sphk) sphk-1 in cholinergic neuron activity, independent of previously known signaling pathways. Overall, the genes identified in our study suggest that optimal modulation of synaptic activity is balanced by the differential activities of multiple pathways, and the novel alleles provide valuable reagents to further dissect neuronal mechanisms regulating the locomotor circuit.

Myrf ER-Bound Transcription Factors Drive C. elegans Synaptic Plasticity via Cleavage-Dependent Nuclear Translocation.

Synaptic refinement is a critical step in nervous system maturation, requiring a carefully timed reorganization and refinement of neuronal connections. We have identified myrf-1 and myrf-2, two C. elegans homologs of Myrf family transcription factors, as key regulators of synaptic rewiring. MYRF-1 and its paralog MYRF-2 are functionally redundant specifically in synaptic rewiring. They co-exist in the same protein complex and act cooperatively to regulate synaptic rewiring. We find that the MYRF proteins localize to the ER membrane and that they are cleaved into active N-terminal fragments, which then translocate into the nucleus to drive synaptic rewiring. Overexpression of active forms of MYRF is sufficient to accelerate synaptic rewiring. MYRF-1 and MYRF-2 are the first genes identified to be indispensable for promoting synaptic rewiring in C. elegans. These findings reveal a molecular mechanism underlying synaptic rewiring and developmental circuit plasticity.