Biochemical characterization and functional analysis of the POU transcription factor POU-M2 of Bombyx mori

Clinical and Molecular Aspects Associated with Defects in the Transcription Factor POU3F4: A Review

X-linked deafness (DFNX) is estimated to account for up to 2% of cases of hereditary hearing loss and occurs in both syndromic and non-syndromic forms. POU3F4 is the gene most commonly associated with X-linked deafness (DFNX2, DFN3) and accounts for about 50% of the cases of X-linked non-syndromic hearing loss. This gene codes for a transcription factor of the POU family that plays a major role in the development of the middle and inner ear. The clinical features of POU3F4-related hearing loss include a pathognomonic malformation of the inner ear defined as incomplete partition of the cochlea type 3 (IP-III). Often, a perilymphatic gusher is observed upon stapedectomy during surgery, possibly as a consequence of an incomplete separation of the cochlea from the internal auditory canal. Here we present an overview of the pathogenic gene variants of POU3F4 reported in the literature and discuss the associated clinical features, including hearing loss combined with additional phenotypes such as cognitive and motor developmental delays. Research on the transcriptional targets of POU3F4 in the ear and brain is in its early stages and is expected to greatly advance our understanding of the pathophysiology of POU3F4-linked hearing loss.

Dynamic chromatin conformation and accessibility changes mediate the spatial-specific gene regulatory network in Bombyx mori

Silk gland genes of Bombyx mori can have strict spatial expression patterns, which impact their functions and silk quality; however, our understanding of their regulation mechanisms is currently insufficient. To address this, the middle silk gland (MSG) and posterior silk gland (PSG) of the silkworm were investigated. Gene ontology annotation showed that spatially specific expressed genes were involved in the formation of H3k9me and chromatin topology. Chromatin conformation data generated by Hi-C showed that the topologically associated domain boundaries around FibL and Sericin1 genes were significantly different between MSG and PSG. Changes in chromatin conformation led to changes in chromatin activity, which significantly affected the expression of nearby genes in silkworm. Chromatin accessibility regions of MSG and PSG were analyzed using FAIRE-seq, and 1006 transcription factor motifs were identified in open chromatin regions. Furthermore, the spatial-specific expression patterns of silk gland genes were mainly associated with homeobox-contained transcription factors, such as POU-M2, which was specifically bound and relatively highly expressed in the MSG. The regulatory network mediated by POU-M2 regulated most of the spatial-specific expressed genes in MSG, such as ADH1. These results can aid in improving silk performance, optimizing silkworm breeding, and improving the gene spatial regulatory model research for insects.

Analysis of histomorphometric and proteome dynamics inside the silk gland lumen of Bombyx mori revealed the dynamic change of silk protein during the molt stage

Silkworms spin different silks at different growth stages for specific purposes. The silk spun before the end of each instar is stronger than that at the beginning of each instar and cocoon silk. However, the compositional changes in silk proteins during this process are unknown. Consequently, we performed histomorphological and proteomic analyses of the silk gland to characterize changes from the instar end to the next instar beginning. The silk glands were collected on day 3 of third- and fourth-instar larvae (III-3 and IV-3) and the beginning of fourth-instar larvae (IV-0). Proteomic analysis identified 2961 proteins from all silk glands. Silk proteins P25 and Ser5 were significantly more abundant in III-3 and IV-3 than in IV-0, and many cuticular proteins and protease inhibitors increased significantly in IV-0 compared with III-3 and IV-3. This shift may cause mechanical property differences between the instar end and beginning silk. Using section staining, qPCR, and western blotting, we found for the first time that silk proteins were degraded first and then resynthesized during the molting stage. Furthermore, we revealed that fibroinase mediated the changes of silk proteins during molting. Our results provide insights into the molecular mechanisms of silk proteins dynamic regulation during molting.

POUM2 homeostasis regulates intimal remodeling and cells fate in the anterior silk gland of the silkworm

Tissue/organ remodeling and cells fate determination play key roles in the life cycle of animals. However, they are still poorly understood in insects, especially in the silkworm. The anterior silk gland (ASG) of the silkworm is essential for the formation and performance of silk fibers, but the regulatory mechanism of ASG remodeling and cells fate determination is less known. Here we found that silencing of POUM2 caused shorter ASG length, intimal structural defects, silkworm spinning failure, and the resultant naked pupae death, but cells number was not affected. Cells staining showed that DNA endoreduplication was not affected in the ASG. Transmission electron microscopy and chitin staining showed cuticle proteins and chitin were greatly reduced in the ASG during the molting period. Transcriptional analysis showed the expression profiles of cuticle proteins and chitin synthase were similar to that of POUM2 during the molting period, and POUM2 down-regulation reduced the expression of cuticle proteins, chitin synthase, autophagy and apoptosis-related genes. While the phenotype resulting from POUM2 over-expression was similar to that of POUM2 down-regulation. Cells staining revealed marked cells apoptosis with cells number reduction and inhibition of DNA endoreduplication in the ASG. Transcriptional analysis showed the expression of autophagy and apoptosis-related genes, and some cuticle proteins and chitin synthase were significantly up-regulated. The results suggest that POUM2 homeostasis regulates ASG intimal remodeling and cells fate, thus affecting ASG development, silkworm spinning and metamorphosis. Our studies not only offer potential molecular targets for genetic improvement of silk performance and molecular breeding of the silkworm, but also provide new insights into POU factor-mediated tissue remodeling and cells fate determination in insects.

POU-M2 promotes juvenile hormone biosynthesis by directly activating the transcription of juvenile hormone synthetic enzyme genes in Bombyx mori

Juvenile hormone (JH) plays a key role in preventing larval precocious metamorphosis, maintaining larval state, controlling adult sexual development and promoting insect egg maturation. Genetic studies have shown that POU factor ventral veins lacking regulates JH synthesis to control the timing of insect metamorphosis. However, how POU factor regulates JH synthesis is largely unknown. Here, we found POU-M2 was highly expressed in corpora allata (CA) and specifically localized in the nucleus of CA. The overexpression of POU-M2 promoted the expression of JH synthase genes and kr-h1 and enhanced the activity of JH synthase genes promoter. Further, POU-M2 promoted the transcription of JH acid O-methyltransferase (JHAMT) by directly binding to the key cis-regulatory elements -207, -249 and -453 within the proximal regions of JHAMT promoter. Both the POU domain and homeodomain were vital for the activation of POU-M2 on JHAMT transcription. Our study reveals the mechanism by which POU-M2 regulates JHAMT transcription.

MAP4K4 controlled transcription factor POUM1 regulates PxABCG1 expression influencing Cry1Ac resistance in Plutella xylostella (L.)

Deciphering the molecular mechanisms of insect resistance to Bacillus thuringiensis (Bt) based biotechnology products including Bt sprays and Bt crops is critical for the long-term application of Bt technology. Previously, we established that down-regulation of the ABC transporter gene PxABCG1, trans-regulated by the MAPK signaling pathway, contributed to high-level resistance to Bt Cry1Ac toxin in diamondback moth, Plutella xylostella (L.). However, the underlying transcriptional regulatory mechanism was unknown. Herein, we identified putative binding sites (PBSs) of the transcription factor (TF) POUM1 in the PxABCG1 promoter and used a dual-luciferase reporter assay (DLRA) and yeast one-hybrid (Y1H) assay to reveal that POUM1 activates PxABCG1 via interaction with one of these sites. The expression of POUM1 was significantly decreased in the midgut tissue of Cry1Ac-resistant P. xylostella strains compared to a Cry1Ac-susceptible P. xylostella strain. Silencing of POUM1 expression resulted in reduced expression of the PxABCG1 gene and an increase in larval tolerance to Bt Cry1Ac toxin in the Cry1Ac-susceptible P. xylostella strain. Furthermore, silencing of PxMAP4K4 expression increased the expression of both POUM1 and PxABCG1 genes in the Cry1Ac-resistant P. xylostella strain. These results indicate that the POUM1 induces PxABCG1 expression, while the activated MAPK cascade represses PxABCG1 expression thus reducing Cry1Ac susceptibility in P. xylostella. This result deepens our understanding of the transcriptional regulatory mechanism of midgut Cry receptor genes and the molecular basis of the evolution of Bt resistance in insects.

Homeodomain proteins POU-M2, antennapedia and abdominal-B are involved in regulation of the segment-specific expression of the clip-domain serine protease gene CLIP13 in the silkworm, Bombyx mori

Clip-domain serine proteases (CLIPs) play important roles in insect innate immunity and development. Our previous studies indicated that CLIP13, an epidermis-specific gene, was involved in cuticle remodeling during molting and metamorphosis in the silkworm, Bombyx mori. However, the transcriptional regulatory mechanism and regulatory pathways of CLIP13 remained unclear. In the present study, we investigated CLIP13 expression and the regulation pathway controlled by 20-hydroxyecdysone (20E) in the silkworm. At the transcriptional level, expression of CLIP13 exhibited pronounced spatial and temporal specificity in different regions of the epidermis; homeodomain transcription factors POU-M2, antennapedia (Antp), and abdominal-B (Abd-B) showed similar expression change trends as CLIP13 in the head capsule, thorax, and abdomen, respectively. Furthermore, results of cell transfection assays, electrophoretic mobility shift assays, and chromatin immunoprecipitation demonstrated that POU-M2, Antp, and Abd-B were involved in the transcriptional regulation of CLIP13 by directly binding to their cis-response elements in CLIP13 promoter. RNA interference-mediated silencing of POU-M2, Antp, and Abd-B led to a decrease of CLIP13 expression in the head capsule, the epidermis of the 1st to 3rd thoracic segments and the 7th to 10th abdominal segments, respectively. Consistent with CLIP13, 20E treatment significantly upregulated expression of POU-M2, Antp, and Abd-B in the silkworm epidermis. Taken together, these data suggest that 20E positively regulates transcription of CLIP13 via homeodomain proteins POU-M2, Antp, and Abd-B in different regions of the silkworm epidermis during metamorphosis, thus affecting the molting process. Our findings provide new insight into the functions of homeodomain transcription factors in insect molting.

Genome-wide survey and characterization of transcription factors in the silk gland of the silkworm, Bombyx mori

Transcription factors (TFs) are key proteins that modulate gene transcription and thereby lead to changes in the gene expression profile and the subsequent alteration of cellular functions. In the silk gland (SG) of silkworm Bombyx mori, an important silk-producing insect, TFs are of vital importance in the regulation of silk protein synthesis in this organ. However, which TFs exist and express in the SG remains largely unknown. Here, we report the large-scale identification of TFs in the SG based on available full-length transcript sequences and the most recent version of silkworm genome data. In total, 348 candidate TFs were identified by strict filtration and were classified into 56 TF families. Chromosomal distribution, motif composition, and phylogenetic relationship analyses revealed the typical characteristics of these TFs. In addition, the expression patterns of 348 TFs in various tissues of B. mori, especially the SG of fourth-molt (4LM) and day-3 and day-4 fifth-instar (5L3D and 5L4D) larvae, were investigated based on public RNA-seq data and gene microarray data, followed by spatiotemporal verification of TF expression levels by quantitative real-time PCR (qRT-PCR). This report describes the first comprehensive analysis of TFs in the B. mori SG. The results can serve as a baseline for further studies of the roles of TFs in the B. mori SG.

New insights into the proteins interacting with the promoters of silkworm fibroin genes

The silkworm, Bombyx mori, is a silk-producing insect that has contributed greatly to human society. The silk gland of B. mori is a specialized organ responsible for synthesizing silk fibroin and sericin proteins under control of numerous factors. However, which factors are involved in direct silk protein synthesis regulation remains largely unknown. We report the identification of promoter-interacting proteins (PIPs) necessary for the regulation of genes encoding fibroin proteins, including the fibroin heavy chain (fibH), fibroin light chain (fibL), and a 25-kD polypeptide protein (P25). In the fourth larval molting stage (M4) or day 5 fifth-instar larvae (L5D5), a total of 198, 292, and 247 or 330, 305, and 460 proteins interacting with the promoter region of fibH, fibL and P25, respectively, were identified from the posterior silk gland by DNA pull-down combined with mass spectrometry. Many PIPs were particularly involved in ribosome- and metabolism-related pathways. Additionally, 135 and 212 proteins were identified as common PIPs of fibH, fibL and P25 in M4 and L5D5, respectively. Among all PIPs, we identified 31 potential transcription factors, such as Y-box and poly A-binding proteins, which play roles in nucleotide binding, ATP binding, or protein folding. This study provides the first in-depth profile of proteins interacting with fibroin gene promoters and contributes to a better understanding of silk protein synthesis regulation.

Research progress of the transcription factor Brn4 (Review)

Brain 4 (Brn4) is a transcription factor belonging to the POU3 family, and it is important for the embryonic development of the neural tube, inner ear and pancreas. In addition, it serves a crucial role in neural stem cell differentiation and reprogramming. The present review aimed to summarize the chromosomal location, species homology, protein molecular structure and tissue distribution of Brn4, in addition to its biological processes, with the aim of providing a reference of its structure and function for further studies, and its potential use as a gene therapy target.

The POU Transcription Factor POU-M2 Regulates Vitellogenin Receptor Gene Expression in the Silkworm, Bombyx mori

Vitellogenin receptors (VgRs) play critical roles in egg formation by transporting vitellogenin (Vg) into oocytes in insects. Although the function of VgR in insects is well studied, the transcriptional regulation of this gene is still unclear. Here, we cloned the promoter of the VgR gene from Bombyx mori (BmVgR), and predicted many POU cis-response elements (CREs) in its promoter. Electrophoretic mobility shift and chromatin immunoprecipitation assays showed that the POU transcription factor POU-M2 bound directly to the CREs of the promoter. Overexpression of POU-M2 in an ovarian cell line (BmNs) enhanced BmVgR transcription and promoter activity detected by quantitative reverse transcription PCR and luciferase reporter assays. Analyses of expression patterns indicated that POU-M2 was expressed in ovary at day two of wandering stage initially, followed by BmVgR. RNA interference of POU-M2 significantly reduced the transcription of BmVgR in ovary and egg-laying rate. Our results suggest a novel function for the POU factor in silkworm oogenesis by its involvement in BmVgR regulation and expands the understanding of POU factors in insect VgR expression.

A Novel Adenosine Kinase from Bombyx mori: Enzymatic Activity, Structure, and Biological Function

Adenosine kinase (ADK) is the first enzyme in the adenosine remediation pathway that catalyzes adenosine phosphorylation into adenosine monophosphate, thus regulating adenosine homeostasis in cells. To obtain new insights into ADK from Bombyx mori (BmADK), we obtained recombinant BmADK, and analyzed its activity, structure, and function. Gel-filtration showed BmADK was a monomer with molecular weight of approximately 38 kDa. Circular dichroism spectra indicated BmADK had 36.8% α-helix and 29.9% β-strand structures, respectively. The structure of BmADK was stable in pH 5.0–11.0, and not affected under 30 °C. The melting temperature and the enthalpy and entropy changes in the thermal transition of BmADK were 46.51 ± 0.50 °C, 253.43 ± 0.20 KJ/mol, and 0.79 ± 0.01 KJ/(mol·K), respectively. Site-directed mutagenesis demonstrated G68, S201, E229, and D303 were key amino acids for BmADK structure and activity. In particular, S201A mutation significantly increased the α-helix content of BmADK and its activity. BmADK was located in the cytoplasm and highly expressed in the silk gland during the pre-pupal stage. RNA interference revealed the downregulation of BmADK decreased ATG-8, Caspase-9, Ec-R, E74A, and Br-C expression, indicating it was likely involved in 20E signaling, apoptosis, and autophagy to regulate silk gland degeneration and silkworm metamorphosis. Our study greatly expanded the knowledge on the activity, structure, and role of ADK.

A Convenient, Rapid, Sensitive, and Reliable Spectrophotometric Assay for Adenylate Kinase Activity

Enzymatic activity assays are essential and critical for the study of enzyme kinetics. Adenylate kinase (Adk) plays a fundamental role in cellular energy and nucleotide homeostasis. To date, assays based on different principles have been used for the determination of Adk activity. Here, we show a spectrophotometric analysis technique to determine Adk activity with bromothymol blue as a pH indicator. We analyzed the effects of substrates and the pH indicator on the assay using orthogonal design and then established the most optimal assay for Adk activity. Subsequently, we evaluated the thermostability of Adk and the inhibitory effect of KCl on Adk activity with this assay. Our results show that this assay is simple, rapid, and precise. It shows great potential as an alternative to the conventional Adk activity assay. Our results also suggest that orthogonal design is an effective approach, which is very suitable for the optimization of complex enzyme reaction conditions.

Cross-talk between juvenile hormone and ecdysone regulates transcription of fibroin modulator binding protein-1 in Bombyx mori

Juvenile hormone (JH) and 20-hydroxyecdysone (20E) are the most important hormones in silkworm and play vital roles in silkworm development, metamorphosis, and silk protein synthesis. Fibroin modulator binding protein-1 (FMBP-1) is a novel transcription factor regulating fibroin heavy chain (fib-H) transcription in Bombyx mori. The roles of JH and 20E on FMBP-1 transcription are less known. Here, we show FMBP-1 transcription is repressed by juvenile hormone analog (JHA) and activated by 20E. We identify two Krüppel homolog 1 (Kr-h1) binding sites (KBS1 and KBS2) and an E74A binding site (EBS) in the promoter of FMBP-1. We demonstrate Kr-h1 directly binds to KBS1 and KBS2 to repress FMBP-1 transcription, and 20E promotes FMBP-1 transcription through E74A. In the presence of JH and 20E, E74A abolishes the repression of Kr-h1 and activates FMBP-1 transcription through direct binding to EBS between KBS1 and KBS2 in FMBP-1 promoter. Further, JHA and 20E treatment and RNA interference confirm the effects of JH and 20E on FMBP-1 transcription in vivo, thus affecting fib-H transcription. Our results reveal the molecular mechanism of FMBP-1 transcription regulated by the cross-talk between JH and 20E in Bombyx mori, and provide novel insights into FMBP-1 transcriptional regulation and silk protein synthesis.

A proteotranscriptomic study of silk-producing glands from the orb-weaving spiders

A proteotranscriptomic approach provides a biochemical basis for understanding the intricate spinning process and complex structural features of spider silk proteins.

Insights into the repression of fibroin modulator binding protein-1 on the transcription of fibroin H-chain during molting in Bombyx mori

Fibroin modulator binding protein-1 (FMBP-1) is a novel DNA-binding protein containing a conserved score and three amino acid peptide repeat (STPR) domain. The roles of factors containing STPR domain are less known. Although multiple transcription factors are involved in the transcriptional regulation of silk protein genes during the development of silkworm, the mechanism of transcriptional repression of silk protein genes during molting remains unclear. Here, we found that FMBP-1 expression was contrary to that of fibroin heavy chain (fib-H) during the fourth molting period of Bombyx mori. FMBP-1 repressed fib-H promoter activity by directly binding to the -130 element in the fib-H promoter region. We also identified two proteins, Bmsage and Bmdimm, that interacted with FMBP-1 in the posterior silk gland of silkworm larvae, and further verified these interactions by far western blotting and microscale thermophoresis in vitro, as well as co-immunoprecipitation and bimolecular fluorescence complementation at the cellular level. The luciferase reporter assay showed that the interaction between FMBP-1 and Bmdimm antagonized the activation of Bmdimm on fib-H transcription, but did not affect FMBP-1-mediated transcriptional repression on fib-H gene. Therefore, we proposed the following mechanism of fib-H transcriptional repression by FMBP-1 during the molting of silkworm larvae: 1) FMBP-1 directly binds to the -130 element in the fib-H promoter to repress fib-H transcription; 2) FMBP-1 interacts with Bmdimm to antagonize the activation of Bmdimm on fib-H transcription. Our findings promote a better understanding of fib-H transcriptional regulation and provide novel insights into the transcriptional repression of fib-H by FMBP-1 and basic helix-loop-helix factors Bmdimm during the molting of silkworm larvae. Our study also provides valuable information regarding the biological function of factors containing STPR domain.

LBD1 of Vitellogenin Receptor Specifically Binds to the Female-Specific Storage Protein SP1 via LBR1 and LBR3

Storage proteins are the major protein synthesized in the fat body, released into hemolymph and re-sequestered into the fat body before pupation in most insect species. Storage proteins are important amino acid and nutrition resources during the non-feeding pupal period and play essential roles for the metamorphosis and oogenesis of insects. The sequestration of storage protein is a selective, specific receptor-mediated process. However, to date, the potential receptor mediating the sequestration of storage protein has not been determined in Bombyx mori. In this study, we expressed and purified the first ligand binding domain of Bombyx mori vitellogenin receptor (BmVgR), LBD1, and found LBD1 could bind with an unknown protein from the hemolymph of the ultimate silkworm larval instar via pull-down assay. This unknown protein was subsequently identified to be the female-specific storage protein SP1 by mass spectrometry. Furthermore, far western blotting assay, immunoprecipitation and isothermal titration calorimetry analysis demonstrated LBD1 specifically bound with the female-specific SP1, rather than another unisex storage protein SP2. The specific binding of LBD1 with SP1 was dependent on the presence of Ca2+ as it was essential for the proper conformation of LBD1. Deletion mutagenesis and ITC analysis revealed the first and third ligand binding repeats LBR1 and LBR3 were indispensable for the binding of LBD1 with SP1, and LBR2 and LBR4 also had a certain contribution to the specific binding. Our results implied BmVgR may mediate the sequestration of SP1 from hemolymph into the fat body during the larval-pupal transformation of Bombyx mori.