Animal microRNAs (miRNAs) are gene regulatory factors that prevent the expression of specific messenger RNA targets by binding to their 3' untranslated region. The Caenorhabditis elegans lsy-6 miRNA (for lateral symmetry defective) is required for the left/right asymmetric expression of guanyl cyclase (gcy) genes in two chemosensory neurons termed ASE left (ASEL) and ASE right (ASER). The asymmetric expression of these putative chemoreceptors in turn correlates with the functional lateralization of the ASE neurons. Here we find that a mutation in the die-1 zinc-finger transcription factor disrupts both the chemosensory laterality and left/right asymmetric expression of chemoreceptor genes in the ASE neurons. die-1 controls chemosensory laterality by activating the expression of lsy-6 specifically in ASEL, but not in ASER, where die-1 expression is downregulated through two sites in its 3' untranslated region. These two sites are complementary to mir-273, a previously uncharacterized miRNA, whose expression is strongly biased towards ASER. Forced bilateral expression of mir-273 in ASEL and ASER causes a loss of asymmetric die-1 expression and ASE laterality. Thus, an inverse distribution of two sequentially acting miRNAs in two bilaterally symmetric neurons controls laterality of the nematode chemosensory system.

[Keywords: C. elegans; left/right asymmetry; transcription factor; homeobox] Supplemental material is available at http://www.genesdev.org.

The elucidation of the architecture of gene regulatory networks that control cell-type specific gene expression programs represents a major challenge in developmental biology. We describe here a cell fate decision between two alternative neuronal fates and the architecture of a gene regulatory network that controls this cell fate decision. The two Caenorhabditis elegans taste receptor neurons ''ASE left'' (ASEL) and ''ASE right'' (ASER) share many bilaterally symmetric features, but each cell expresses a distinct set of chemoreceptors that endow the gustatory system with the capacity to sense and discriminate specific environmental inputs. We show that these left͞right asymmetric fates develop from a precursor state in which both ASE neurons express equivalent features. This hybrid precursor state is unstable and transitions into the stable ASEL or ASER terminal end state. Although this transition is spatially stereotyped in wild-type animals, mutant analysis reveals that each cell has the potential to transition into either the ASEL or ASER stable end state. The stability and irreversibility of the terminal differentiated state is ensured by the interactions of two microRNAs (miRNAs) and their transcription factor targets in a double-negative feedback loop. Simple feedback loops are found as common motifs in many gene regulatory networks, but the loop described here is unusually complex and involves miRNAs. The interaction of miRNAs in double-negative feedback loops may not only be a means for miRNAs to regulate their own expression but may also represent a general paradigm for how terminal cell fates are selected and stabilized.left͞right asymmetry ͉ bistable ͉ network motif ͉ regulatory RNA ͉ cellular diversification N ervous systems are characterized by a striking degree of cellular diversity. The molecular correlates to morphological and functional diversity of nervous systems are neuron-type specific gene expression programs. The experimental accessibility of the nematode Caenorhabditis elegans offers the opportunity to (i) determine the nature of neuron-type specific gene expression programs on a single-cell level and (ii) to genetically dissect the mechanisms that establish and maintain these singlecell specific programs. The two main gustatory neurons of C. elegans, ASE left (ASEL) and ASE right (ASER), display a particularly intricate level of neuronal diversity. Although bilaterally symmetric in many different regards (cell position, axodendritic morphology, synaptic connectivity, and molecular features), each neuron expresses a distinct spectrum of putative chemoreceptors, a feature that the worm requires to navigate through complex sensory environments (1, 2). The ASE neurons therefore not only provide a model to study sensory neuron fate diversification but also to study neuronal laterality, a common but poorly understood feature of many nervous systems.To elucidate the nature of the gene regulatory program that diversifies ASEL and ASER, we have isolated mutants in which ASE asymmetry is disrupted...

No abstract

We describe here the results of genetic screens for Caenorhabditis elegans mutants in which a single neuronal fate decision is inappropriately executed. In wild-type animals, the two morphologically bilaterally symmetric gustatory neurons ASE left (ASEL) and ASE right (ASER) undergo a left/right asymmetric diversification in cell fate, manifested by the differential expression of a class of putative chemoreceptors and neuropeptides. Using single cell-specific gfp reporters and screening through a total of almost 120,000 haploid genomes, we isolated 161 mutants that define at least six different classes of mutant phenotypes in which ASEL/R fate is disrupted. Each mutant phenotypic class encompasses one to nine different complementation groups. Besides many alleles of 10 previously described genes, we have identified at least 16 novel ''lsy'' genes (''laterally symmetric''). Among mutations in known genes, we retrieved four alleles of the miRNA lsy-6 and a gain-of-function mutation in the 39-UTR of a target of lsy-6, the cog-1 homeobox gene. Using newly found temperature-sensitive alleles of cog-1, we determined that a bistable feedback loop controlling ASEL vs. ASER fate, of which cog-1 is a component, is only transiently required to initiate but not to maintain ASEL and ASER fate. Taken together, our mutant screens identified a broad catalog of genes whose molecular characterization is expected to provide more insight into the complex genetic architecture of a left/right asymmetric neuronal cell fate decision.A PART from expressing a core set of features that distinguish neuronal from nonneuronal cell types, individual cell types in the nervous system express distinct batteries of genes that generate the structural and functional diversity of neuronal cell types. The diversification of cell fate in the developing nervous system presumably relies on the interplay of a host of regulatory factors. Screens for mutant animals defective in neuronal fate specification provide a powerful and unbiased approach to identify these regulatory factors. Such screens have been successfully conducted in various model systems, most prominently flies and worms, yielding valuable insights into the molecular mechanisms that control neuronal fate specification. We describe here genetic screens for neuronal fate mutants in the nematode Caenorhabditis elegans that focus on a cell fate decision executed by a single neuron class, the ASE gustatory neurons. This neuron class is composed of two morphologically bilaterally symmetric neurons, ASE We have previously reported the identification of mutants that affect the development of the left/right asymmetry of the ASE neurons (Chang et al. 2003; Hobert 2003, 2005;Johnston et al. 2006). These mutants, which we termed lsy mutants (for laterally symmetric) to indicate their role in controlling the fate of two left/right asymmetric neurons, fell into four distinct classes ( Figure 1A). In class I mutants, both ASE neurons adopt the fate of the ASEL neuron, with a concomitant loss of ASE...

We investigated the electroluminescence and relatively external quantum efficiency (EQE) of m-plane InGaN/GaN light emitting diodes (LEDs) emitting at 480 nm to elucidate the droop behaviors in nitride-based LEDs. With increasing the injection current density to 100 A/cm2, the m-plane LEDs exhibit only 13% efficiency droop, whereas conventional c-plane LEDs suffer from efficiency droop at very low injection current density and the EQE of c-plane LEDs decrease to as little as 50% of its maximum value. Our simulation models show that in m-plane LEDs the absence of polarization fields manifest not only the hole distribution more uniform among the wells but also the reduction in electron overflow out of electron blocking layer. These results suggest that the nonuniform distribution of holes and electron leakage current due to strong polarization fields are responsible for the relatively significant efficiency droop of conventional c-plane LEDs.

A graded-composition electron blocking layer (GEBL) with aluminum composition increasing along the [0001] direction was designed for c-plane InGaN/GaN light-emitting diodes (LEDs) by employing the band-engineering. The simulation results demonstrated that such GEBL can effectively enhance the capability of hole transportation across the EBL as well as the electron confinement. Consequently, the LED with GEBL grown by metal-organic chemical vapor deposition exhibited lower forward voltage and series resistance and much higher output power at high current density as compared to conventional LED. Meanwhile, the efficiency droop was reduced from 34% in conventional LED to only 4% from the maximum value at low injection current to 200 A/cm2.

No abstract