Search

Although precise tuning of gene expression levels is critical for most developmental pathways, the mechanisms by which the transcriptional output of dosage-sensitive molecules is established or modulated by the environment remain poorly understood. Here, we provide a mechanistic framework for how the conserved transcription factor BLMP-1/Blimp1 operates as a pioneer factor to decompact chromatin near its target loci during embryogenesis (hours prior to major transcriptional activation) and, by doing so, regulates both the duration and amplitude of subsequent target gene transcription during post-embryonic development. This priming mechanism is genetically separable from the mechanisms that establish the timing of transcriptional induction and functions to canalize aspects of cell-fate specification, animal size regulation, and molting. A key feature of the BLMP-1-dependent transcriptional priming mechanism is that chromatin decompaction is initially established during embryogenesis and maintained throughout larval development by nutrient sensing. This anticipatory mechanism integrates transcriptional output with environmental conditions and is essential for resuming normal temporal patterning after animals exit nutrient-mediated developmental arrests., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Glutamate is a major excitatory neurotransmitter, and impaired glutamate clearance following synaptic release promotes spillover, inducing extra-synaptic signaling. The effects of glutamate spillover on animal behavior and its neural correlates are poorly understood. We developed a glutamate spillover model in Caenorhabditis elegans by inactivating the conserved glial glutamate transporter GLT-1. GLT-1 loss drives aberrant repetitive locomotory reversal behavior through uncontrolled oscillatory release of glutamate onto AVA, a major interneuron governing reversals. Repetitive glutamate release and reversal behavior require the glutamate receptor MGL-2/mGluR5, expressed in RIM and other interneurons presynaptic to AVA. mgl-2 loss blocks oscillations and repetitive behavior; while RIM activation is sufficient to induce repetitive reversals in glt-1 mutants. Repetitive AVA firing and reversals require EGL-30/Gαq, an mGluR5 effector. Our studies reveal that cyclic autocrine presynaptic activation drives repetitive reversals following glutamate spillover. That mammalian GLT1 and mGluR5 are implicated in pathological motor repetition suggests a common mechanism controlling repetitive behaviors.

Long-term studies of Caenorhabditis elegans larval development traditionally require tedious manual observations because larvae must move to develop, and existing immobilization techniques either perturb development or are unsuited for young larvae. Here, we present a simple microfluidic device to simultaneously follow development of ten C. elegans larvae at high spatiotemporal resolution from hatching to adulthood (∼3 days). Animals grown in microchambers are periodically immobilized by compression to allow high-quality imaging of even weak fluorescence signals. Using the device, we obtain cell-cycle statistics for C. elegans vulval development, a paradigm for organogenesis. We combine Nomarski and multichannel fluorescence microscopy to study processes such as cell-fate specification, cell death, and transdifferentiation throughout post-embryonic development. Finally, we generate time-lapse movies of complex neural arborization through automated image registration. Our technique opens the door to quantitative analysis of time-dependent phenomena governing cellular behavior during C. elegans larval development., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Stochastic mechanisms diversify cell fate in organisms ranging from bacteria to humans [ 1–4 ]. In the anchor cell/ventral uterine precursor cell (AC/VU) fate decision during C. elegans gonadogenesis, two "α cells," each with equal potential to be an AC or a VU, interact via LIN-12/Notch and its ligand LAG-2/DSL [ 5, 6 ]. This LIN-12/Notch-mediated interaction engages feedback mechanisms that amplify a stochastic initial difference between the two α cells, ensuring that the cell with higher lin-12 activity becomes the VU while the other becomes the AC [ 7–9 ]. The initial difference between the α cells was originally envisaged as a random imbalance from "noise" in lin-12 expression/activity [ 6 ]. However, subsequent evidence that the relative birth order of the α cells biases their fates suggested other factors may be operating [ 7 ]. Here, we investigate the nature of the initial difference using high-throughput lineage analysis [ 10 ]; GFP-tagged endogenous LIN-12, LAG-2, and HLH-2, a conserved transcription factor that orchestrates AC/VU development [ 7, 11 ]; and tissue-specific hlh-2 null alleles. We identify two stochastic elements: relative birth order, which largely originates at the beginning of the somatic gonad lineage three generations earlier, and onset of HLH-2 expression, such that the α cell whose parent expressed HLH-2 first is biased toward the VU fate. We find that these elements are interrelated, because initiation of HLH-2 expression is linked to the birth of the parent cell. Finally, we provide a potential deterministic mechanism for the HLH-2 expression bias by showing that hlh-2 is required for LIN-12 expression in the α cells. • LIN-12/Notch resolves a stochastic cell fate decision of two equipotential α cells • Two stochastic events bias their fates: relative birth order and HLH-2 expression • Onset of HLH-2 expression is linked to the birth of the parents of the α cells • LIN-12 expression requires HLH-2: a potential deterministic decision mechanism Two bipotential α cells interact via LIN-12/Notch and its ligand LAG-2/DSL, engaging feedback mechanisms that amplify a stochastic initial difference between them. Attner et al. identify two interrelated stochastic elements and provide a potential deterministic mechanism by showing that hlh-2 is required for LIN-12 expression in the α cells. [ABSTRACT FROM AUTHOR]

Development relies on the exquisite control of both the timing and the levels of gene expression to achieve robust developmental transitions. How cis - and trans -acting factors control both aspects simultaneously is unclear. We show that transcriptional pulses of the temporal patterning microRNA (miRNA) lin-4 are generated by two nuclear hormone receptors (NHRs) in C. elegans , NHR-85 and NHR-23, whose mammalian orthologs, Rev-Erb and ROR, function in the circadian clock. Although Rev-Erb and ROR antagonize each other to control once-daily transcription in mammals, NHR-85/NHR-23 heterodimers bind cooperatively to lin-4 regulatory elements to induce a single pulse of expression during each larval stage. Each pulse's timing, amplitude, and duration are dictated by the phased expression of these NHRs and the C. elegans Period ortholog, LIN-42, that binds to and represses NHR-85. Therefore, during nematode temporal patterning, an evolutionary rewiring of circadian clock components couples the timing of gene expression to the control of transcriptional dosage. [Display omitted] • lin-4 miRNA transcription is highly dynamic, with ∼100 min pulses per larval stage • NHR-85/NHR-23 heterodimers bind cooperatively to the upstream lin-4 enhancers • The duration of NHR-85/NHR-23 dimerization controls lin-4 transcriptional dynamics • LIN-42 binds directly to NHR-85 to dynamically modulate lin-4 transcriptional dosage Kinney, Sahu, et al. report that genes implicated in mammalian circadian transcription are rewired in C. elegans to generate the oscillatory transcriptional patterns of miRNAs that program temporal patterning during post-embryonic development. This gene regulatory network directly controls lin-4 gene dosage in the heterochronic pathway, maintaining developmental robustness. [ABSTRACT FROM AUTHOR]