DNA-binding properties of Mblk-1, a putative transcription factor from the honeybee

Mblk-1/E93, an ecdysone related-transcription factor, targets synaptic plasticity-related genes in the honey bee mushroom bodies

Among hymenopteran insects, aculeate species such as bees, ants, and wasps have enlarged and morphologically elaborate mushroom bodies (MBs), a higher-order brain center in the insect, implying their relationship with the advanced behavioral traits of aculeate species. The molecular bases leading to the acquisition of complicated MB functions, however, remains unclear. We previously reported the constitutive and MB-preferential expression of an ecdysone-signaling related transcription factor, Mblk-1/E93, in the honey bee brain. Here, we searched for target genes of Mblk-1 in the worker honey bee MBs using chromatin immunoprecipitation sequence analyses and found that Mblk-1 targets several genes involved in synaptic plasticity, learning, and memory abilities. We also demonstrated that Mblk-1 expression is self-regulated via Mblk-1-binding sites, which are located upstream of Mblk-1. Furthermore, we showed that the number of the Mblk-1-binding motif located upstream of Mblk-1 homologs increased associated with evolution of hymenopteran insects. Our findings suggest that Mblk-1, which has been focused on as a developmental gene transiently induced by ecdysone, has acquired a novel expression pattern to play a role in synaptic plasticity in honey bee MBs, raising a possibility that molecular evolution of Mblk-1 may have partly contributed to the elaboration of MB function in insects.

E93 confers steroid hormone responsiveness of digestive enzymes to promote blood meal digestion in the midgut of the mosquito Aedes aegypti

Anautogenous female mosquitoes obtain the nutrients needed for egg development from vertebrate blood, and consequently they transmit numerous pathogens of devastating human diseases. Digestion of blood proteins into amino acids that are used for energy production, egg maturation and replenishment of maternal reserves is an essential part of the female mosquito reproductive cycle. However, the regulatory mechanisms underlying this process remain largely unknown. Here, we report that the transcription factor E93 is a critical factor promoting blood meal digestion in adult females of the major arboviral vector Aedes aegypti in response to the steroid hormone 20-hydroxyecdysone (20E). E93 was upregulated in the female mosquito midgut after a blood meal, and RNA interference (RNAi)-mediated knockdown of E93 inhibited midgut blood digestion. E93 RNAi depletion repressed late trypsin (LT), serine protease I (SPI), SPVI and SPVII, and activated early trypsin (ET) expression in the female mosquito midgut after a blood meal. Injection of 20E activated E93, LT, SPI, SPVI and SPVII, and repressed ET expression, whereas RNAi knockdown of the ecdysone receptor (EcR) repressed E93, LT, SPI, SPVI and SPVII, and activated ET expression in the midgut. Furthermore, E93 depletion resulted in a complete loss of 20E responsiveness of LT, SPVI and SPVII. Our findings reveal important mechanisms regulating blood meal digestion in disease-transmitting mosquitoes.

Reproductive potential does not cause loss of heat shock response performance in honey bees

In other species characterized to date, aging, as a function of reproductive potential, results in the breakdown of proteaostasis and a decreased capacity to mount responses by the heat shock response (HSR) and other proteostatic network pathways. Our understanding of the maintenance of stress pathways, such as the HSR, in honey bees, and in the reproductive queen in particular, is incomplete. Based on the findings in other species showing an inverse relationship between reproductive potential and HSR function, one might predict that that HSR function would be lost in the reproductive queens. However, as queens possess an atypical uncoupling of the reproduction-maintenance trade-off typically found in solitary organisms, HSR maintenance might also be expected. Here we demonstrate that reproductive potential does not cause loss of HSR performance in honey bees as queens induce target gene expression to levels comparable to those induced in attendant worker bees. Maintenance of HSR function with advent of reproductive potential is unique among invertebrates studied to date and provides a potential model for examining the molecular mechanisms regulating the uncoupling of the reproduction-maintenance trade-off in queen bees, with important consequences for understanding how stresses impact different types of individuals in honey bee colonies.

The IRE1 pathway regulates honey bee Unfolded Protein Response gene expression

Our molecular understanding of honey bee cellular stress responses is incomplete. Previously, we sought to identify and began functional characterization of the components of the Unfolded Protein Response (UPR) in honey bees. We observed that UPR stimulation resulted in induction of target genes upon IRE1 pathway activation, as assessed by splicing of Xbp1 mRNA. However, we were not able to determine the relative role of the various UPR pathways in gene activation. Our understanding of honey bee signal transduction and transcriptional regulation has been hampered by a lack of tools. After using RNA-seq to expand the known UPR targets in the honey bee, we used the Drosophila melanogaster S2 cell line and honey bee trans and cis elements to investigate the role of the IRE1 pathway in the transcriptional activation of one of these targets, the honey bee Hsc70-3 gene. Using a luciferase reporter, we show that honey bee Hsc70 promoter activity is inducible by UPR activation. In addition, we show that this activation is IRE1-dependent and relies on specific cis regulatory elements. Experiments using exogenous honey bee or fruit fly XBP1S proteins demonstrate that both factors can activate the Hsc70-3 promoter and further support a role for the IRE1 pathway in control of Hsc70-3 expression in the honey bee. By providing foundational knowledge about the UPR in the honey bee and demonstrating the usefulness of a heterologous cell line for molecular characterization of honey bee pathways, this work stands to improve our understanding of this critical species.

Developmental stage-specific distribution and phosphorylation of Mblk-1, a transcription factor involved in ecdysteroid-signaling in the honey bee brain

In the honey bee, the mushroom bodies (MBs), a higher-order center in insect brain, comprise interneurons termed Kenyon cells (KCs). We previously reported that Mblk-1, which encodes a transcription factor involved in ecdysteroid-signaling, is expressed preferentially in the large-type KCs (lKCs) in the pupal and adult worker brain and that phosphorylation by the Ras/MAPK pathway enhances the transcriptional activity of Mblk-1 in vitro. In the present study, we performed immunoblotting and immunofluorescence studies using affinity-purified anti-Mblk-1 and anti-phosphorylated Mblk-1 antibodies to analyze the distribution and phosphorylation of Mblk-1 in the brains of pupal and adult workers. Mblk-1 was preferentially expressed in the lKCs in both pupal and adult worker brains. In contrast, some Mblk-1 was phosphorylated almost exclusively in the pupal stages, and phosphorylated Mblk-1 was preferentially expressed in the MB neuroblasts and lKCs in pupal brains. Immunofluorescence studies revealed that both Mblk-1 and phosphorylated Mblk-1 are located in both the cytoplasm and nuclei of the lKC somata in the pupal and adult worker brains. These findings suggest that Mblk-1 plays a role in the lKCs in both pupal and adult stages and that phosphorylated Mblk-1 has pupal stage-specific functions in the MB neuroblasts and lKCs in the honey bee brain.

Mblk-1 Transcription Factor Family: Its Roles in Various Animals and Regulation by NOL4 Splice Variants in Mammals

Transcription factors play critical roles in regulation of neural development and functions. A transcription factor Mblk-1 was previously reported from a screen for factors possibly important for the higher brain functions of the honeybee. This review first summarizes how Mblk-1 was identified, and then provides an overview of the studies of Mblk-1 and their homologs. Mblk-1 family proteins are found broadly in animals and are shown to affect transcription activities. Studies have revealed that the mammalian homologs can interact with several cofactors and together regulate transcription. Interestingly, a recent study using the mouse homologs, Mlr1 and Mlr2, showed that one of their cofactor proteins, NOL4, have several splice variants with different effects on the transactivation activities of Mlr proteins. These findings suggest that there is an additional layer of the regulation of Mblk-1 family proteins by cofactor splice variants and provide novel insights into our current understanding of the roles of the conserved transcription factor family.

Analysis of the Differentiation of Kenyon Cell Subtypes Using Three Mushroom Body-Preferential Genes during Metamorphosis in the Honeybee (Apis mellifera L.)

The adult honeybee (Apis mellifera L.) mushroom bodies (MBs, a higher center in the insect brain) comprise four subtypes of intrinsic neurons: the class-I large-, middle-, and small-type Kenyon cells (lKCs, mKCs, and sKCs, respectively), and class-II KCs. Analysis of the differentiation of KC subtypes during metamorphosis is important for the better understanding of the roles of KC subtypes related to the honeybee behaviors. In the present study, aiming at identifying marker genes for KC subtypes, we used a cDNA microarray to comprehensively search for genes expressed in an MB-preferential manner in the honeybee brain. Among the 18 genes identified, we further analyzed three genes whose expression was enriched in the MBs: phospholipase C epsilon (PLCe), synaptotagmin 14 (Syt14), and discs large homolog 5 (dlg5). Quantitative reverse transcription-polymerase chain reaction analysis revealed that expression of PLCe, Syt14, and dlg5 was more enriched in the MBs than in the other brain regions by approximately 31-, 6.8-, and 5.6-fold, respectively. In situ hybridization revealed that expression of both Syt14 and dlg5 was enriched in the lKCs but not in the mKCs and sKCs, whereas expression of PLCe was similar in all KC subtypes (the entire MBs) in the honeybee brain, suggesting that Syt14 and dlg5, and PLCe are available as marker genes for the lKCs, and all KC subtypes, respectively. In situ hybridization revealed that expression of PLCe is already detectable in the class-II KCs at the larval fifth instar feeding stage, indicating that PLCe expression is a characteristic common to the larval and adult MBs. In contrast, expression of both Syt14 and dlg5 became detectable at the day three pupa, indicating that Syt14 and dlg5 expressions are characteristic to the late pupal and adult MBs and the lKC specific molecular characteristics are established during the late pupal stages.

Splicing variants of NOL4 differentially regulate the transcription activity of Mlr1 and Mlr2 in cultured cells

Mlr1 (Mblk-1-related protein-1) and Mlr2 are mouse homologs of transcription factor Mblk-1 (Mushroom body large-type Kenyon cell-specific protein-1), which we originally identified from the honeybee brain. In the present study, aiming at identifying coregulator(s) of Mlr1 and Mlr2 from the mouse brain, we used yeast two-hybrid screening of mouse brain cDNA library to search for interaction partners of Mlr 1 and Mlr2, respectively. We identified nucleolar protein 4 (NOL4) splicing variants as major interaction partners for both Mlr1 and Mlr2. Among the three murine NOL4 splicing variants, we further characterized NOL4-S, which lacks an N-terminal part of NOL4-L, and NOL4-SΔ, which lacks nuclear localization signal (NLS)-containing domain of NOL4-S. A GST pull-down assay revealed that Mlr1 interacts with both NOL4-S and NOL4-SΔ, whereas Mlr2 interacts with NOL4-S, but not with NOL4-SΔ. These results indicate that the NLS-containing domain of NO4-S Is necessary for in vitro binding with Mlr2, but not for that with Mlr1. Furthermore, a luciferase assay using Schneider's Line 2 cells revealed that transactivation activity of Mlr1 was significantly suppressed by both NOL4-S and NOL4-SΔ, with almost complete suppression by NOL4-SΔ. In contrast, transactivation activity of Mlr2 was significantly suppressed by NOL4-S but rather activated by NOL4-SΔ. Our findings suggest that transactivation activities of Mlr1 and Mlr2 are differentially regulated by splicing variants of NOL4, which are expressed in a tissue-selective manner.

Control of target gene specificity during metamorphosis by the steroid response gene E93

Hormonal control of sexual maturation is a common feature in animal development. A particularly dramatic example is the metamorphosis of insects, in which pulses of the steroid hormone ecdysone drive the wholesale transformation of the larva into an adult. The mechanisms responsible for this transformation are not well understood. Work in Drosophila indicates that the larval and adult forms are patterned by the same underlying sets of developmental regulators, but it is not understood how the same regulators pattern two distinct forms. Recent studies indicate that this ability is facilitated by a global change in the responsiveness of target genes during metamorphosis. Here we show that this shift is controlled in part by the ecdysone-induced transcription factor E93. Although long considered a dedicated regulator of larval cell death, we find that E93 is expressed widely in adult cells at the pupal stage and is required for many patterning processes at this time. To understand the role of E93 in adult patterning, we focused on a simple E93-dependent process, the induction of the Dll gene within bract cells of the pupal leg by EGF receptor signaling. In this system, we show that E93 functions to cause Dll to become responsive to EGF receptor signaling. We demonstrate that E93 is both necessary and sufficient for directing this switch. E93 likely controls the responsiveness of many other target genes because it is required broadly for patterning during metamorphosis. The wide conservation of E93 orthologs suggests that similar mechanisms control life-cycle transitions in other organisms, including vertebrates.

EcR-A expression in the brain and ovary of the honeybee (Apis mellifera L.)

We previously demonstrated that six genes involved in ecdysteroid signaling are expressed preferentially in Kenyon-cell subtypes in the mushroom bodies of the honeybee (Apis mellifera L.). To further examine the possible involvement of ecdysteroid signaling in honeybee brain function, we isolated a cDNA for the A isoform of the ecdysone receptor gene homolog AmEcR-A and analyzed its expression in the brain. In situ hybridization revealed that AmEcR-A is expressed selectively in the small-type Kenyon cells of the mushroom bodies in the worker and queen brain, like AmE74 and AmHR38, suggesting a possible association of these gene products. Analysis of AmEcR-A expression in queen and worker abdomens demonstrated that AmEcR-A is strongly expressed in nurse cells of the queen ovary, suggesting that ecdysteroid and ecdysteroid signaling have roles in oogenesis. Our present results further support the possible involvement of ecdysteroid signaling in brain function, as well as in regulating queen reproductive physiology in the adult honeybee.

Expression of Two Ecdysteroid-Regulated Genes,Broad-ComplexandE75, in the Brain and Ovary of the Honeybee (Apis mellifera L.)

We previously demonstrated that two ecdysteroid-regulated genes, Mblk-1/E93 and E74, are expressed selectively in Kenyon cell subtypes in the mushroom bodies of the honeybee (Apis mellifera L.) brain. To further examine the possible involvement of ecdysteroid-regulated genes in brain function as well as in oogenesis in the honeybee, we isolated cDNAs for two other ecdysteroid-regulated genes, Broad-Complex (BR-C) and E75, and analyzed their expression in the worker brain as well as in the queen abdomen. In situ hybridization revealed that BR-C, like Mblk-1/ E93, is expressed selectively in the large-type Kenyon cells of the mushroom bodies in the worker brain, whereas E75 is expressed in all mushroom body neuron subtypes, suggesting a difference in the mode of response to ecdysteroid among Kenyon cell subtypes. In the queen ovary, both BR-C and E75 are expressed preferentially in the follicle cells that surround egg cells at the late stage, suggesting their role in oogenesis. These results suggest that BR-C and E75 are involved in the regulation of brain function as well as in reproductive physiology in the adult honeybee.

MBR-1, a novel helix-turn-helix transcription factor, is required for pruning excessive neurites in Caenorhabditis elegans

In the developing brain, excessive neurites are actively pruned in the construction and remodeling of neural circuits. We demonstrate for the first time that the pruning of neurites occurs in the simple neural circuit of Caenorhabditis elegans and that a novel transcription factor, MBR-1, is involved in this process. We identified MBR-1 as a C. elegans ortholog of Mblk-1, a transcription factor that is expressed preferentially in the mushroom bodies of the honeybee brain. Although Mblk-1 homologs are conserved among animal species, their roles in the nervous system have never been analyzed. We used C. elegans as an ideal model animal for analysis of neuronal development. mbr-1 is expressed in various neurons in the head and tail ganglia. A comparison of the morphology of mbr-1-expressing neurons revealed that excessive neurites connecting the left and right AIM interneurons are eliminated during larval stages in wild-type but are sustained through the adult stage in the mbr-1 mutant. In addition, mbr-1 expression is regulated by UNC-86, a POU domain transcription factor, and the pruning of the excessive AIM connection is impaired in the unc-86 mutant. These findings provide an important clue for further genetic dissection of neurite pruning.

Further exploration into the adaptive design of the arthropod "microbrain": I. Sensory and memory-processing systems

Arthropods have small but sophisticated brains that have enabled them to adapt their behavior to a diverse range of environments. In this review, we first discuss some of general characteristics of the arthropod "microbrain" in comparison with the mammalian "megalobrain". Then we discuss about recent progress in the study of sensory and memory-processing systems of the arthropod "microbrain". Results of recent studies have shown that (1) insects have excellent capability for elemental and context-dependent forms of olfactory learning, (2) mushroom bodies, higher olfactory and associative centers of arthropods, have much more elaborated internal structures than previously thought, (3) many genes involved in the formation of basic brain structures are common among arthropods and vertebrates, suggesting that common ancestors of arthropods and vertebrates already had organized head ganglia, and (4) the basic organization of sensori-motor pathways of the insect brain has features common to that of the mammalian brain. These findings provide a starting point for the study of brain mechanisms of elaborated behaviors of arthropods, many of which remain unexplored.

Gene expression of ecdysteroid-regulated gene E74 of the honeybee in ovary and brain

To facilitate studies of hormonal control in the honeybee (Apis mellifera L.), a cDNA for a honeybee homologue of the ecdysteroid-regulated gene E74 (AmE74) was isolated and its expression was analysed. Northern blot analysis indicated strong expression in the adult queen abdomen, and no significant expression in the adult drone and worker abdomens. In situ hybridization demonstrated that this gene was expressed selectively in the ovary and gut in the queen abdomen. Furthermore, this gene was also expressed selectively in subsets of mushroom body interneurones in the brain of the adult worker bees. These findings suggest that AmE74 is involved in neural function as well as in reproduction in adult honeybees.

In vivo gene transfer into the adult honeybee brain by using electroporation

The honeybee, Apis mellifera L., is a social insect and they show wide variety of exquisite social behaviors to maintain colony activity. To enable the elucidation of those social behaviors at a molecular level and gene function in the nervous system, we developed an in vivo method to perform gene transfer in the adult brain of living honeybee by electroporation. When green fluorescent protein-expressing plasmid was transferred to the brain with this system, green fluorescence was observed near the anode location. The expression of transfected genes was confirmed at both transcriptional and translational levels by reverse transcription-polymerase chain reaction and immunoblot analyses. This system will facilitate the analysis of gene function and the regulatory mechanisms of gene networks in the nervous system and provide clues to clarify the relation between those genes and the complex behaviors of the honeybee.

Novel insect picorna-like virus identified in the brains of aggressive worker honeybees

To identify candidate genes involved in the aggressive behavior of worker honeybees, we used the differential display method to search for RNAs exclusively detected in the brains of aggressive workers that had attacked a hornet. We identified a novel, 10,152-nucleotide RNA, termed Kakugo RNA. Kakugo RNA encodes a protein of 2,893 amino acid residues that shares structural features and sequence similarities with various picorna-like virus polyproteins, especially those from sacbrood virus, which infects honeybees. The Kakugo protein contains several domains that correspond to the virion protein, helicase, protease, and RNA-dependent RNA polymerase domains of various picorna-like virus polyproteins. When the worker bee tissue lysate was subjected to sucrose density gradient centrifugation, Kakugo RNA, except for the material at the bottom, was separated into two major peaks. One of the peaks corresponded to the position of Kakugo mRNA, and the other corresponded to the position of the poliovirus virion. These results suggest that the Kakugo RNA exists as an mRNA-like free RNA and virion RNA in the honeybee. Furthermore, injection of the lysate supernatant from the attacker heads into the heads of noninfected bees resulted in a marked increase in Kakugo RNA. These results demonstrate that Kakugo RNA is a plus-strand RNA of a novel picorna-like virus and that the brains of aggressive workers are infected by this novel virus. Kakugo RNA was detected in aggressive workers but not in nurse bees or foragers. In aggressive workers, Kakugo RNA was detected in the brain but not in the thorax or abdomen, indicating a close relation between viral infection in the brain and aggressive worker behaviors.

The activity of Mblk-1, a mushroom body-selective transcription factor from the honeybee, is modulated by the ras/MAPK pathway

We previously identified a gene, termed Mblk-1, that encodes a putative transcription factor with two DNA-binding motifs expressed preferentially in the mushroom body of the honeybee brain, and its preferred binding sequence, termed Mblk-1-binding element (MBE) (Takeuchi, H., Kage, E., Sawata, M., Kamikouchi, A., Ohashi, K., Ohara, M., Fujiyuki, T., Kunieda, T., Sekimizu, K., Natori, S., and Kubo, T. (2001) Insect Mol Biol 10, 487-494; Park, J.-M., Kunieda. T., Takeuchi, H., and Kubo, T. (2002) Biochem. Biophys. Res. Commun. 291, 23-28). In the present study, the effect of Mblk-1 on transcription of genes containing MBE in Drosophila Schneider's Line 2 cells was examined using a luciferase assay. Mblk-1 expression transactivated promoters containing MBEs approximately 2-7-fold. Deletion experiments revealed that RHF2, the second DNA-binding domain of Mblk-1, was necessary for the transcriptional activity. Furthermore, mitogen-activated protein kinase (MAPK) phosphorylated Mblk-1 at Ser-444 in vitro, and the Mblk-1-induced transactivation was stimulated by phosphorylation of Ser-444 by the Ras/MAPK pathway in the luciferase assay. These results suggest that Mblk-1 is a transcription factor that might function in the mushroom body neuronal circuits downstream of the Ras/MAPK pathway in the honeybee brain.

Identification and characterization of Mlr1,2: two mouse homologues of Mblk-1, a transcription factor from the honeybee brain(1)

We previously identified the Mblk-1 gene in the honeybee brain, which encodes a transcription factor containing two DNA binding motifs, termed RHF1 and 2 (Takeuchi et al. (2001) Insect Mol. Biol. 121, 134-140). Here, we identified two mouse Mblk1 homologues, Mlr1 and Mlr2. Both encode proteins containing a single DNA-binding motif highly conserved with RHF2 and activate transcription mediated by a DNA element recognized by honeybee Mblk-1. Mlr1 was expressed predominantly in the spermatocytes of the testis, while Mlr2 was expressed in various tissues other than testis. Mlr1 transcripts were lost in the testis of W/W(v) mutant mice, suggesting a role in spermatogenesis.