The nuclear localization signal (NLS) of PDX-1 is part of the homeodomain and represents a novel type of NLS

PDX-1: A Promising Therapeutic Target to Reverse Diabetes

The pancreatic duodenum homeobox-1 (PDX-1) is a transcription factor encoded by a Hox-like homeodomain gene that plays a crucial role in pancreatic development, β-cell differentiation, and the maintenance of mature β-cell functions. Research on the relationship between PDX-1 and diabetes has gained much attention because of the increasing prevalence of diabetes melitus (DM). Recent studies have shown that the overexpression of PDX-1 regulates pancreatic development and promotes β-cell differentiation and insulin secretion. It also plays a vital role in cell remodeling, gene editing, and drug development. Conversely, the absence of PDX-1 increases susceptibility to DM. Therefore, in this review, we summarized the role of PDX-1 in pancreatic development and the pathogenesis of DM. A better understanding of PDX-1 will deepen our knowledge of the pathophysiology of DM and provide a scientific basis for exploring PDX-1 as a potential target for treating diabetes.

DNA binding fluorescent proteins as single-molecule probes

DNA binding fluorescent proteins are useful probes for a broad range of biological applications.

PITX3 mutations associated with autosomal dominant congenital cataract in the Chinese population

The present study aimed to identify the disease‑causing gene of a four‑generation Chinese family affected with congenital posterior subcapsular cataracts (CPSC), to additionally investigate the frequency of paired like homeodomain 3 (PITX3) mutations in Chinese patients with autosomal dominant congenital cataract (ADCC) and to analyze the pathogenesis of the mutations identified in the present study. Whole exome sequencing (WES) was utilized to identify the genetic cause of CPSC in the four‑generation family. Sanger sequencing was performed to verify the WES results and to screen for mutations of the PITX3 gene in probands of an additional 194 Chinese ADCC families. Co‑segregation analysis was performed in the family members with available DNA. Subcellular localization analyses and transactivation assays were performed for the PITX3 mutations identified. From the WES data, the c.608delC (p.A203GfsX106) mutation of PITX3 was identified in the four‑generation family with CPSC. A second PITX3 mutation c.640_656del (p.A214RfsX42) was detected in two of the additional 194 ADCC families and one of these two families exhibited incomplete penetrance. Functional studies indicated that these 2 PITX3 mutant proteins retained a nuclear localization pattern, but resulted in decreased transactivation activity, similar to other previously identified PITX3 mutations. In the present study, 2 different mutations (p.A203GfsX106 and p.A214RfsX42) in PITX3 were identified as the causative defect in a four‑generation family with CPSC and two ADCC families, respectively. The prevalence of PITX3 gene‑associated cataract was 1.54% (3/195) in the Chinese congenital cataract (CC) family cohort. In vitro functional analyses of these 2 PITX3 mutations were performed, in order to enhance understanding of the pathogenesis of CC caused by PITX3 mutations.

The Phosphorylation of PDX-1 by Protein Kinase CK2 Is Crucial for Its Stability

The homeodomain protein PDX-1 is a critical regulator of pancreatic development and insulin production in pancreatic β-cells. We have recently shown that PDX-1 is a substrate of protein kinase CK2; a multifunctional protein kinase which is implicated in the regulation of various cellular aspects, such as differentiation, proliferation, and survival. The CK2 phosphorylation site of PDX-1 is located within the binding region of the E3 ubiquitin ligase adaptor protein PCIF1. To study the interaction between PDX-1 and PCIF1 we used immunofluorescence analysis, co-immunoprecipitation, GST-pull-down studies, and proximity ligation assay (PLA). For the analysis of the stability of PDX-1 we performed a cycloheximide chase. We used PDX-1 in its wild-type form as well as phosphomutants of the CK2 phosphorylation site. In pancreatic β-cells PDX-1 binds to PCIF1. The phosphorylation of PDX-1 by CK2 increases the ratio of PCIF1 bound to PDX-1. The stability of PDX-1 is extended in the absence of CK2 phosphorylation. Our results identified protein kinase CK2 as new important modulator of the stability of PDX-1.

Nuclear import of Nkx2-2 is mediated by multiple pathways

Nkx2-2 homeoprotein is essential for the development of the central nervous system and pancreas. Although the nuclear localization signals of Nkx2-2 have been identified, the responsible transport receptor is still unknown. Here, we demonstrate that imp α1 not only interacts with Nkx2-2 but also transports it into the nucleus in vitro by acting together with imp β1. However, the nuclear import of Nkx2-2 in cells was not inhibited in response to knockdown expression of endogenous imp β1 or over-expression of Bimax2. Furthermore, imp β1 and imp 13, but not imp 4, directly interact with Nkx2-2 and are capable of transporting Nkx2-2 in an in vitro import assay. By GST pull-down assay, we demonstrate that mutation of NLS1 or NLS2 has no effect on interaction with imp α1 or imp 13, but significantly reduced binding to imp β1. Thus, the nuclear import of Nkx2-2 is mediated not only by the classical import pathway but also directly by imp β1 or imp 13.

Low cost delivery of proteins bioencapsulated in plant cells to human non-immune or immune modulatory cells

Targeted oral delivery of GFP fused with a GM1 receptor binding protein (CTB) or human cell penetrating peptide (PTD) or dendritic cell peptide (DCpep) was investigated. Presence of GFP(+) intact plant cells between villi of ileum confirm their protection in the digestive system from acids/enzymes. Efficient delivery of GFP to gut-epithelial cells by PTD or CTB and to M cells by all these fusion tags confirm uptake of GFP in the small intestine. PTD fusion delivered GFP more efficiently to most tissues or organs than the other two tags. GFP was efficiently delivered to the liver by all fusion tags, likely through the gut-liver axis. In confocal imaging studies of human cell lines using purified GFP fused with different tags, GFP signal of DCpep-GFP was only detected within dendritic cells. PTD-GFP was only detected within kidney or pancreatic cells but not in immune modulatory cells (macrophages, dendritic, T, B, or mast cells). In contrast, CTB-GFP was detected in all tested cell types, confirming ubiquitous presence of GM1 receptors. Such low-cost oral delivery of protein drugs to sera, immune system or non-immune cells should dramatically lower their cost by elimination of prohibitively expensive fermentation, protein purification cold storage/transportation and increase patient compliance.

Molecular evolution of the HD-ZIP I gene family in legume genomes

Homeodomain leucine zipper I (HD-ZIP I) genes were used to increase the plasticity of plants by mediating external signals and regulating growth in response to environmental conditions. The way genomic histories drove the evolution of the HD-ZIP I family in legume species was described; HD-ZIP I genes were searched in Lotus japonicus, Medicago truncatula, Cajanus cajan and Phaseolus vulgaris, and then divided into five clades through phylogenetic analysis. Microsynteny analysis was made based on genomic segments containing the HD-ZIP I genes. Some pairs turned out to conform with syntenic genome regions, while others corresponded to those that were inverted, expanded, or contracted after the divergence of legumes. Besides, we dated their duplications by Ks analysis and demonstrated that all the blocks were formed after the monocot-dicot split; we observed Ka/Ks ratios representing strong purifying selections in the four legume species which might have been followed by gene loss and rearrangement.

Nuclear localization of Obox4 is dependent on its homeobox domain

Objective

Oocyte-specific homeobox 4 (Obox4) is preferentially expressed in oocytes and plays an important role in the completion of meiosis of oocytes. However, the Obox4 expression pattern has not been reported yet. In this study, we investigated the subcellular localization of Obox4 using a green fluorescent protein (GFP) fusion expression system.

Methods

Three regions of Obox4 were divided and fused to the GFP expression vector. The partly deleted homeodomain (HD) regions of Obox4 were also fused to the GFP expression vector. The recombinant vectors were transfected into HEK-293T cells plated onto coated glass coverslips. The transfected cells were stained with 4',6-diamidino-2-phenylindol and photographed using a fluorescence microscope.

Results

Mutants containing the HD region as well as full-length Obox4 were clearly localized to the nucleus. In contrast, the other mutants of either the N-terminal or C-terminal region without HD had impaired nuclear localization. We also found that the N-terminal and C-terminal of the Obox HD contributed to nuclear localization and the entire HD was necessary for nuclear localization of Obox4.

Conclusion

Based on the results of the present study, we demonstrated that the intact HD region of Obox4 is responsible for the nuclear localization of Obox4 protein in cells.

Karyopherins in nuclear transport of homeodomain proteins during development

Homeodomain proteins are crucial transcription factors for cell differentiation, cell proliferation and organ development. Interestingly, their homeodomain signature structure is important for both their DNA-binding and their nucleocytoplasmic trafficking. The accurate nucleocytoplasmic distribution of these proteins is essential for their functions. We summarize information on (a) the roles of karyopherins for import and export of homeoproteins, (b) the regulation of their nuclear transport during development, and (c) the corresponding complexity of homeoprotein nucleocytoplasmic transport signals. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

The activation of the rat insulin gene II by BETA2 and PDX-1 in rat insulinoma cells is repressed by Pax6

The transcriptional transactivator Pax6 binds the pancreatic islet cell-specific enhancer sequence (PISCES) of the rat insulin I gene. However the human, mouse, and rat insulin gene II promoters do not contain a PISCES element. To analyze the role of Pax6 in those PISCES-less promoters, we investigated its influence on rat insulin gene II expression and included in our studies the main activators: pancreatic and duodenal homeobox protein-1 (PDX-1) and BETA2/E47. Luciferase assays, Northern blots, and RIA were used to study effects of Pax6 overexpression, gel shift and chromatin precipitation assays to study its binding to the DNA, and yeast two-hybrid assays and glutathione S transferase capture assays to investigate its interactions with PDX-1 and BETA2. Finally, glucose-dependent intracellular transport of Pax6 was demonstrated by fluorescence microscopy. Overexpression of Pax6 prevents activation of the rat insulin II gene by BETA2 and PDX-1 and hence suppresses insulin synthesis and secretion. In vitro, Pax6 binds to the A-boxes, thereby blocking binding of PDX-1, and at the same time, its paired domain interacts with BETA2. Fluorescence microscopy demonstrated that the nuclear-cytoplasmic localization of Pax6 and PDX-1 are oppositely regulated by glucose. From the results, it is suggested that at low concentrations of glucose, Pax6 is localized in the nucleus and prevents the activation of the insulin gene by occupying the PDX-1 binding site and by interacting with BETA2.

CK2 phosphorylation of Pdx-1 regulates its transcription factor activity

The duodenal homeobox-1 protein Pdx-1 is one of the regulators for the transcription of the insulin gene. Pdx-1 is a phosphoprotein, and there is increasing evidence for the regulation of some of its functions by phosphorylation. Here, we asked whether protein kinase CK2 might phosphorylate Pdx-1 and how this phosphorylation could be implicated in the functional regulation of Pdx-1. We used fragments of Pdx-1 as well as phosphorylation mutants for experiments with protein kinase CK2. Transactivation was measured by reporter assays using the insulin promoter. Our data showed that Pdx-1 is phosphorylated by protein kinase CK2 at amino acids thr(231) and ser(232), and this phosphorylation was implicated in the regulation of the transcription factor activity of Pdx-1. Furthermore, inhibition of protein kinase CK2 by specific inhibitors led to an elevated release of insulin from pancreatic beta-cells. Thus, these findings identify CK2 as a novel mediator of the insulin metabolism.

Glucose regulation of insulin gene expression in pancreatic β-cells

Production and secretion of insulin from the β-cells of the pancreas is very crucial in maintaining normoglycaemia. This is achieved by tight regulation of insulin synthesis and exocytosis from the β-cells in response to changes in blood glucose levels. The synthesis of insulin is regulated by blood glucose levels at the transcriptional and post-transcriptional levels. Although many transcription factors have been implicated in the regulation of insulin gene transcription, three β-cell-specific transcriptional regulators, Pdx-1 (pancreatic and duodenal homeobox-1), NeuroD1 (neurogenic differentiation 1) and MafA (V-maf musculoaponeurotic fibrosarcoma oncogene homologue A), have been demonstrated to play a crucial role in glucose induction of insulin gene transcription and pancreatic β-cell function. These three transcription factors activate insulin gene expression in a co-ordinated and synergistic manner in response to increasing glucose levels. It has been shown that changes in glucose concentrations modulate the function of these β-cell transcription factors at multiple levels. These include changes in expression levels, subcellular localization, DNA-binding activity, transactivation capability and interaction with other proteins. Furthermore, all three transcription factors are able to induce insulin gene expression when expressed in non-β-cells, including liver and intestinal cells. The present review summarizes the recent findings on how glucose modulates the function of the β-cell transcription factors Pdx-1, NeuroD1 and MafA, and thereby tightly regulates insulin synthesis in accordance with blood glucose levels.

DUX4c, an FSHD candidate gene, interferes with myogenic regulators and abolishes myoblast differentiation

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant neuromuscular disease. It maps to the D4Z4 repeat array at 4q35, and correlates with a repeat contraction which derepresses transcription of local genes. Which, if any, of these genes is pathogenic to muscle, and through what molecular mechanism is unknown. The present study investigates the function of one candidate gene, DUX4c, encoded by a truncated inverted D4Z4 element located 42 kb proximal to the D4Z4 repeats. Using a gain of function approach we tested DUX4c for toxicity and effects on differentiation in C2C12 myoblasts. DUX4c-expressing myoblasts appear morphologically normal but have reduced expression of myogenic regulators, including MyoD and Myf5. Consistent with this, DUX4c-expressing myoblasts are unable to differentiate into myotubes. Furthermore, overexpression of Myf5 or MyoD rescued myoblast differentiation, suggesting that reductions in expression of these regulators are the relevant DUX4c-induced physiological changes that inhibit differentiation. Our results suggest that upregulation of DUX4c can have a deleterious effect on muscle homeostasis and regeneration, and point to a possible role for DUX4c in the pathology of FSHD.

Cell-specific nuclear import of plasmid DNA in smooth muscle requires tissue-specific transcription factors and DNA sequences

Two shortcomings of nonviral gene therapy are a lack of tissue-specific targeting of vectors and low levels of gene transfer. Our laboratory has begun to address these limitations by designing plasmids that enter the nucleus of specific cell types in the absence of cell division, thereby enhancing expression in a controlled manner. We have shown that a 176 bp portion of the smooth muscle gamma-actin (SMGA) promoter can mediate plasmid nuclear import specifically in smooth muscle cells (SMCs). Here, we demonstrate that the binding sites for serum response factor (SRF) and NKX3-1/3-2 within this DNA nuclear targeting sequence (DTS) are required for plasmid nuclear import. Knockdown of these factors with siRNA abrogates plasmid nuclear import, indicating that they are necessary cofactors. In addition, coinjection of recombinant SRF and Nkx3.2 with the vector in TC7 epithelial cells rescues import. Finally, we show that the SRF nuclear localization sequence (NLS) is required for vector nuclear import. We propose that SRF and NKX3-1/3-2 bind the SMGA DTS in the cytoplasm, thus coating the plasmid with NLSs that mediate translocation across the nuclear pore complex. This discovery could aid in the development of more efficient nonviral vectors for gene transfer to SMCs.

Functional analysis of human mutations in homeodomain transcription factor PITX3

BACKGROUND

The homeodomain-containing transcription factor PITX3 was shown to be essential for normal eye development in vertebrates. Human patients with point mutations in PITX3 demonstrate congenital cataracts along with anterior segment defects in some cases when one allele is affected and microphthalmia with brain malformations when both copies are mutated. The functional consequences of these human mutations remain unknown.

RESULTS

We studied the PITX3 mutant proteins S13N and G219fs to determine the type and severity of functional defects. Our results demonstrate alterations in DNA-binding profiles and/or transactivation activities and suggest a partial loss-of-function in both mutants with the G219fs form being more severely affected. No anomalies in cellular distribution and no dominant-negative effects were discovered for these mutants. Interestingly, the impairment of the G219fs activity varied between different ocular cell lines.

CONCLUSION

The G219fs mutation was found in multiple families affected with congenital cataracts along with anterior segment malformations in many members. Our data suggest that the presence/severity of anterior segment defects in families affected with G219fs may be determined by secondary factors that are expressed in the developing anterior segment structures and may modify the effect(s) of this mutation. The S13N mutant showed only minor alteration of transactivation ability and DNA binding pattern and may represent a rare polymorphism in the PITX3 gene. A possible contribution of this mutation to human disease needs to be further investigated.

An intact homeobox domain is required for complete nuclear localization of human Nanog

Nanog is a homeobox-containing transcriptional factor required for maintaining the pluripotent state of stem cells. We investigated the nuclear localization signal (NLS) motif required for human Nanog (hNanog) nuclear import. Mutation analysis revealed that a mutant containing only the homeodomain (HD) was exclusively localized to the nucleus, while other mutants containing either the N- or C-terminal region (NR or CR) had impaired nuclear localization. In addition, NR and CR were exclusively localized to the nucleus when they were fused to the HD, indicating that complete nuclear localization is only driven by functional NLS motif(s) within the HD. Furthermore, partial loss of HD led to the incomplete localization of hNanog, suggesting that the intact HD is required for hNanog nuclear import. A series of deletion and site-directed mutagenesis within the HD revealed that two basic NLS motifs are located at the N-terminus and C-terminus of the HD and that both motifs are required for complete hNanog nuclear localization.

Amino acid 1-209 is essential for PDX-1-mediated repression of human CMV IE promoter activity

AIM

To explore the different roles of pancreatic duodenal homeobox factors-1 (PDX-1) domains in PDX-1 mediated repression of human cytomegalovirus immediately early (CMV IE) promoter.

METHODS

A series of truncated PDX-1 mutants were constructed. The binding of PDX-1 and CMV IE promoter was identified by electrophoretic mobility shift assay (EMSA). The dual-reporter assay was applied to examine the repression activities of PDX-1 mutants on CMV IE promoter. In addition, RNAi technology was used to specifically knock down the endogenous PDX-1 expression.

RESULTS

The reporter assay indicated that compared to the mock controls (pEGFP-N2), overexpression of PDX-1 resulted in a 41% decrease of CMV IE promoter activity in the 293 cells (P< 0.05) and 43% decrease in HeLa cells (P< 0.05), and the repression levels of various truncated mutants played on CMV IE promoter were different. Specific knock down of the endogenous PDX-1 expression significantly restored the activity of CMV IE promoter. EMSA demonstrated that domain 3 is necessary for nuclear localization and DNA binding activity of PDX-1. However, binding of PDX-1 alone to CMV IE promoter was not sufficient to inhibit its transcriptional activity, and other domains of PDX-1 presented were also required.

CONCLUSION

Our data suggested that the DNA binding activity of PDX-1 domain 3 and the cooperative binding of PDX-1 domain 1/2 with other proteins were required for PDX-1 mediated repression of CMV IE promoter.

Functional dissection of the HNF-1alpha transcription factor: a study on nuclear localization and transcriptional activation

Hepatocyte nuclear factor-1alpha (HNF-1alpha) is a homeodomain-containing transcription factor regulating the expression of liver and pancreas-specific genes. Mutations in the HNF-1alpha-encoding gene TCF1 cause maturity-onset diabetes of the young, type 3 (MODY3). These mutations may affect nuclear import or reduce the ability of HNF-1alpha to stimulate transcription. We performed a functional dissection of HNF-1alpha, attempting both to define its nuclear localization signals (NLSs) and to identify important elements of the Cterminal transactivation domain. Three HNF-1alpha regions, A (amino acids 158-171), B (197-205), and C (271-282), highly similar to consensus NLSs, were studied by immunolocalization in HeLa cells. Region B could be identified as the most critical for correct nuclear localization. Deletion of two subregions (amino acids 398-470 and 544-631, respectively) in the HNF-1alpha C-terminal transactivation domain, resulted in the greatest reduction in stimulation of transcription compared to wild-type protein. However, this domain probably consists of many elements that work in concert to give the full transactivation potential of the protein.

The Forkhead Transcription Factor Foxo1 Bridges the JNK Pathway and the Transcription Factor PDX-1 through Its Intracellular Translocation

It has been shown that oxidative stress and activation of the c-Jun N-terminal kinase (JNK) pathway induce the nucleocytoplasmic translocation of the pancreatic transcription factor PDX-1, which leads to pancreatic beta-cell dysfunction. In this study, we have shown that the forkhead transcription factor Foxo1/FKHR plays a role as a mediator between the JNK pathway and PDX-1. Under oxidative stress conditions, Foxo1 changed its intracellular localization from the cytoplasm to the nucleus in the pancreatic beta-cell line HIT-T15. The overexpression of JNK also induced the nuclear localization of Foxo1, but in contrast, suppression of JNK reduced the oxidative stress-induced nuclear localization of Foxo1, suggesting the involvement of the JNK pathway in Foxo1 translocation. In addition, oxidative stress or activation of the JNK pathway decreased the activity of Akt in HIT cells, leading to the decreased phosphorylation of Foxo1 following nuclear localization. Furthermore, adenovirus-mediated Foxo1 overexpression reduced the nuclear expression of PDX-1, whereas repression of Foxo1 by Foxo1-specific small interfering RNA retained the nuclear expression of PDX-1 under oxidative stress conditions. Taken together, Foxo1 is involved in the nucleocytoplasmic translocation of PDX-1 by oxidative stress and the JNK pathway.

Structure, alternative splicing, and expression of the human and mouse KCNIP gene family

Potassium channel-interacting proteins (KCNIPs, also named KChIPs) modulate A-type potassium channels and favor their surface expression. In addition, KCNIPs have been shown to interact with presenilins and also to function as transcriptional repressors. Here we describe the structures and alternative splicing of the human and mouse KCNIP genes, including novel splice variants for KCNIP1, KCNIP3, and KCNIP4, and show the expression of different KCNIP mRNAs in various mouse and human tissues and brain regions by RT-PCR. Furthermore, we describe the expression of KCNIP1, KCNIP2, KCNIP3, and KCNIP4 mRNAs in the adult mouse brain with in situ hybridization and show that all KCNIP mRNAs were expressed in the neurons of the mouse brain with specific patterns for each KCNIP. Our results show that alternatively spliced KCNIP mRNAs are expressed differentially and could contribute to the diversity of functions of the KCNIP proteins.

Cloning and characterization of an HDZip I gene GmHZ1 from soybean

By using cDNA-AFLP, we analyzed a recombinant inbred line population of soybean that was derived from a soybean mosaic virus (SMV) resistant cultivar Kefeng No.1 and a susceptible cultivar Nannong 1138-2. One hundred and eight fragments showing polymorphism between SMV resistant and susceptible pools were identified. One fragment w27 was 96 bp in length and showed homology to homeobox ggth with a coding region of 738 bp, encoding a protein of 245 amino acids. The genomic sequence analysis defined an intron of 521 bp in the coding region. GmHZ1 was characterized by the presence of a homeodomain (HD) with a closely linked leucine zipper motif (Zip). Southern blot analysis indicated that there was a single copy of GmHZ1 in the soybean genome. When inoculated with SMV strain N3, resistant and susceptible varieties showed reduced and increased expression of the GmHZ1, respectively. The fusion protein of GmHZ1 with GFP was targeted only in nucleus. Yeast two hybrid studies revealed that the GmHZ1 had transcriptional activation activity and can form homodimer. GmHZ1 can bind two 9-bp pseudopalindromic elements (CAAT(A/T)ATTG and CAAT(C/G)ATTG) with different affinity. Using GUS as a reporter gene, GmHZ1 was proved to be a transcriptional activator and enhanced GUS expression by binding with the two elements in plant cells. These results indicate that the GmHZ1 may have a transcriptional activator function in plant response to SMV infection.

Intracellular trafficking and dynamics of double homeodomain proteins

Double homeodomain (DUX) proteins are encoded by a family of 3.3-kilobase repeated elements dispersed in the human genome. One of these elements named D4Z4 is found in a tandem repeat array on chromosome 4 that is partially deleted in facioscapulohumeral muscular dystrophy. We have evaluated the trafficking and mobility of two DUX proteins, DUX1 and DUX4. We transfected C2C12 myoblasts with cDNA encoding these proteins fused to the green fluorescent protein and studied their intracellular localization and diffusional mobilities using fluorescence recovery after photobleaching and fluorescence loss in photobleaching. We also studied truncated forms of the proteins, containing one or both homeodomains or a region outside the homeodomains. We show that both full-length proteins are actively transported into the nucleus, and that the homeodomains contain the signals required for this localization. DUX1 is more mobile than DUX4 within the nucleus (t(1/2) = 4.8 s for DUX1 and 13.4 s for DUX4), suggesting differences in the way the two proteins interact with nuclear components.

Transcription factors, pancreatic development, and β-cell maintenance

Transcription factors play an important role during pancreatic development ensuring normal differentiation of the islet endocrine cells. In mature beta-cells, expression of specific transcription factors is essential in maintaining normal beta-cell function.

Functional characterization of SIX3 homeodomain mutations in holoprosencephaly: interaction with the nuclear receptor NR4A3/NOR1

Holoprosencephaly (HPE) is a relatively common brain malformation resulting in an incomplete separation of the two cerebral hemispheres. A number of mutations in different genes have been linked to this malformation, including three missense mutations in the homeodomain of the transcription factor SIX3. In this study, we investigated the functional consequences of these SIX3 mutations with respect to the ability of the protein to interact with and stimulate the transcriptional activity of the nuclear receptor NOR1 (NR4A3). Using glutathione S-transferase fusion protein pull-down assays and transient cotransfections of Neuro-2a cells with expression and reporter vectors, we found that one mutation, c.676C>G (p.L226V), does not alter the properties of SIX3 toward NOR1. Another mutation, c.749T>C (p.V250A), results in the production of a highly unstable protein in Neuro-2a cells. The third mutation, c.770G>C (p.R257P), results in a mutant SIX3 protein that no longer interacts with NOR1 in vivo. These observations suggest that different SIX3 mutations in HPE2 may affect different signaling pathways, and that one of these pathways may involve the nuclear receptor NOR1.

Thrombopoietin induces HOXA9 nuclear transport in immature hematopoietic cells: potential mechanism by which the hormone favorably affects hematopoietic stem cells

Members of the homeobox family of transcription factors are major regulators of hematopoiesis. Overexpression of either HOXB4 or HOXA9 in primitive marrow cells enhances the expansion of hematopoietic stem cells (HSCs). However, little is known of how expression or function of these proteins is regulated during hematopoiesis under physiological conditions. In our previous studies we demonstrated that thrombopoietin (TPO) enhances levels of HOXB4 mRNA in primitive hematopoietic cells (K. Kirito, N. Fox, and K. Kaushansky, Blood 102:3172-3178, 2003). To extend our studies, we investigated the effects of TPO on HOXA9 in this same cell population. Although overall levels of the transcription factor were not affected, we found that TPO induced the nuclear import of HOXA9 both in UT-7/TPO cells and in primitive Sca-1(+)/c-kit(+)/Gr-1(-) hematopoietic cells in a mitogen-activated protein kinase-dependent fashion. TPO also controlled MEIS1 expression at mRNA levels, at least in part due to phosphatidylinositol 3-kinase activation. Collectively, TPO modulates the function of HOXA9 by leading to its nuclear translocation, likely mediated by effects on its partner protein MEIS1, and potentially due to two newly identified nuclear localization signals. Our data suggest that TPO controls HSC development through the regulation of multiple members of the Hox family of transcription factors through multiple mechanisms.

Impairment of SHOX nuclear localization as a cause for Léri-Weill syndrome

We report the characterization of the nuclear localization signal (NLS) of the short stature homeobox gene SHOX. Mutations within the SHOX gene cause Léri-Weill dyschondrosteosis (LWD) and Langer mesomelic dysplasia (LD) as well as idiopathic short stature (ISS). Furthermore, haploinsufficiency of SHOX has also been implicated in Turner syndrome. SHOX has been shown to be a cell-type-specific transcriptional activator that localizes to the nucleus. The SHOX protein contains a central homeodomain that together with its transactivation domain regulates the transcription of its target sequences within the nucleus. The sequences for its nuclear localization have not been identified yet. Experimental characterization of SHOX-NLS by deletion mapping identified a non-classic type basic signal, AKCRK, in the recognition helix of the homeodomain. Fusion of this stretch of five amino acids to a cytoplasmic reporter protein resulted in its nuclear translocation. Functional analysis of a missense mutation R173C (C517T) affecting the identified SHOX-NLS in two families with LWS and LD showed that the mutated SHOX protein is unable to enter the nucleus. Conversely, we can demonstrate that insertion of the identified signal adjacent to the mutant site can restore its nuclear translocation. These results establish impairment of nuclear localization as a mechanistic basis for SHOX-related diseases.

Intracellular calcium modulates the nuclear translocation of calsenilin

Calsenilin, which was originally identified as a presenilin interacting protein, has since been shown to be involved in the processing of presenilin(s), the modulation of amyloid beta-peptide (Abeta) levels and apoptosis. Subsequent to its original identification, calsenilin was shown to act as a downstream regulatory element antagonist modulator (and termed DREAM), as well as to interact with and modulate A-type potassium channels (and termed KChIP3). Calsenilin is primarily a cytoplasmic protein that must translocate to the nucleus to perform its function as a transcriptional repressor. This study was designed to determine the cellular events that modulate the translocation of calsenilin from the cytoplasm to the nucleus. The nuclear translocation of calsenilin was found to be enhanced following serum deprivation. A similar effect was observed when cells were treated with pharmacological agents that directly manipulate the levels of intracellular calcium (caffeine and the calcium ionophore A23187), suggesting that the increased levels of calsenilin in the nucleus are mediated by changes in intracellular calcium. A calsenilin mutant that was incapable of binding calcium retained the ability to translocate to the nucleus. Taken together, these findings indicate that the level of intracellular calcium can modulate the nuclear translocation of calsenilin and that this process does not involve the direct binding of calcium to calsenilin.

Importin beta1 mediates the glucose-stimulated nuclear import of pancreatic and duodenal homeobox-1 in pancreatic islet beta-cells (MIN6)

The transcription factor PDX-1 (pancreatic and duodenal homeobox-1) is essential for pancreatic development and the maintainence of expression of islet beta-cell-specific genes. In an previous study [Rafiq, Kennedy and Rutter (1998) J. Biol. Chem. 273, 23241-23247] we demonstrated that PDX-1 may be activated at elevated glucose concentrations by translocation from undefined binding sites in the cytosol and nuclear membrane into the nucleoplasm. In the present study, we show that PDX-1 interacts directly and specifically in vitro with the nuclear import receptor family member, importin beta1, and that this interaction is mediated by the PDX-1 homeodomain (amino acids 146-206). Demonstrating the functional importance of the PDX-1-importin beta1 interaction, microinjection of MIN6 beta-cells with anti-(importin beta1) antibodies blocked both the nuclear translocation of PDX-1, and the activation by glucose (30 mM versus 3 mM) of the pre-proinsulin promoter. However, treatment with extracts from pancreatic islets incubated at either low or high glucose concentrations had no impact on the ability of PDX-1 to interact with importin beta1 in vitro. Furthermore, importin beta1 also interacted with SREBP1c (sterol-regulatory-element-binding protein 1c) in vitro, and microinjection of importin beta1 antibodies blocked the activation by glucose of SREBP1c target genes. Since the subcellular distribution of SREBP1c is unaffected by glucose, these findings suggest that a redistribution of importin beta1 is unlikely to explain the glucose-stimulated nuclear uptake of PDX-1. Instead, we conclude that the uptake of PDX-1 into the nucleoplasm, as glucose concentrations increase, may be mediated by release of the factor both from sites of retention in the cytosol and from non-productive complexes with importin beta1 at the nuclear membrane.

PDX-1 and the pancreas

Many transcription factors are critical for ensuring proper embryonic development of the endocrine pancreas and normal islet function. The transcription factor pancreatic duodenal homeobox 1 (PDX-1) is uniformly expressed in early pancreatic buds of embryos as well as the beta and delta cells of the islets of Langerhans. PDX-1 has also been found in dispersed endocrine cells of the duodenum in adults and plays a key role in pancreas formation. It has been reported that null mutation of PDX-1 in mice results in a failure of the pancreatic bud to expand; thus, the mice die 2-3 days after birth from hyperglycemia and dehydration. Heterozygous PDX-1 mice developed a pancreas but were diabetic. It has been shown that PDX-1 is required for maintaining the pancreatic islet functions by activating gene transcriptions including insulin, somatostatin (SST), islet amyloid polypeptide, glucose transporter type 2, and glucokinase. PDX-1 serves a dual role in pancreatic development. It initially contributes to pancreatic formation during embryogenesis and subsequently regulates the pancreatic islet cell physiology in mature islet cells. Understanding the underlying molecular mechanisms of pancreas formation, especially the function of PDX-1, may contribute to the enhanced treatment and prevention of debilitating diseases such as diabetes, insulinomas, and pancreatic carcinomas.

Oxidative stress induces nucleo-cytoplasmic translocation of pancreatic transcription factor PDX-1 through activation of c-Jun NH(2)-terminal kinase

Oxidative stress is induced in pancreatic beta-cells under diabetic conditions and causes beta-cell dysfunction. Antioxidant treatment of diabetic animals leads to recovery of insulin biosynthesis and increases the expression of its controlling transcription factor, pancreatic duodenal homeobox-1 (PDX-1), in pancreatic beta-cells. Here, we show that PDX-1 is translocated from the nuclei to the cytoplasm of pancreatic beta-cells in response to oxidative stress. When oxidative stress was charged upon beta-cell-derived HIT-T15 cells, both endogenous PDX-1 and exogenously introduced green fluorescent protein-tagged PDX-1 moved from the nuclei to the cytoplasm. The addition of a dominant negative form of c-Jun NH(2)-terminal kinase (JNK) inhibited oxidative stress-induced PDX-1 translocation, suggesting an essential role of JNK in mediating this phenomenon. Whereas the nuclear localization signal (NLS) in PDX-1 was not affected by oxidative stress, leptomycin B, a specific inhibitor of the classical leucine-rich nuclear export signal (NES), inhibited nucleo-cytoplasmic translocation of PDX-1 induced by oxidative stress. Moreover, we identified an NES at position 82-94 of the mouse PDX-1 protein. Thus, our present results revealed a novel mechanism that negatively regulates PDX-1 function. The identification of the NES, which overrides the function of the NLS in an oxidative stress-responsive, JNK-dependent manner, supports the complicated regulation of PDX-1 function in vivo and may further the understanding of beta-cell pathophysiology in diabetes.

Sumoylation of Pdx1 is associated with its nuclear localization and insulin gene activation

Pancreatic duodenal homeobox-1 (Pdx1) is a transcription factor, and its phosphorylation is thought to be essential for activation of insulin gene expression. This phosphorylation is related to a concomitant shift in molecular mass from 31 to 46 kDa. However, we found that Pdx1 was modified by SUMO-1 (small ubiquitin-related modifier 1) in beta-TC-6 and COS-7 cells, which were transfected with Pdx1 cDNA. This modification contributed to the increase in molecular mass of Pdx1 from 31 to 46 kDa. Additionally, sumoylated Pdx1 localized in the nucleus. The reduction of SUMO-1 protein by use of RNA interference (SUMO-iRNAs) resulted in a significant decrease in Pdx1 protein in the nucleus. A 34-kDa form of Pdx1 was detected by the cells exposed to SUMO-iRNAs in the presence of lactacystin, a proteasome inhibitor. Furthermore, the reduced nuclear sumoylated Pdx1 content was associated with significant lower transcriptional activity of the insulin gene. These findings indicate that SUMO-1 modification is associated with both the localization and stability of Pdx1 as well as its effect on insulin gene activation.

Human agmatinase is diminished in the clear cell type of renal cell carcinoma

The proteome of RCC was analyzed by 2D PAGE to search for tumor-associated proteins. Agmatinase, which hydrolyzes agmatine to putrescine and urea, was identified by mass spectrometry and database searches and shown to be downregulated in tumor cells. Additionally, RT-PCR and Northern blot analyses demonstrated a clearly decreased amount of agmatinase mRNA in tumor cells. The differential expression of agmatinase mRNA was confirmed at the protein level. Western blot analysis showed almost no detectable agmatinase protein in tumor cells compared to corresponding normal renal tissue. Agmatinase mRNA is most abundant in human liver and kidney but expressed to a lesser extent in several other tissues, including skeletal muscle and small intestine. The human agmatinase gene encodes a 352-residue protein with a putative mitochondrial targeting sequence at the N-terminus. Using transfection and immunohistochemical studies, we show that agmatinase is localized in the mitochondria. Immunohistochemical studies revealed that agmatinase in the normal kidney is restricted to tubulus epithelial cells, while in tumors staining was low and heterogeneous. Thus, expression of human agmatinase is altered in RCC. We discuss the consequences of these findings in terms of polyamine, NO metabolism and macrophage function.

New insights in the transcriptional activity and coregulator molecules in the arterial wall

A number of vascular diseases are associated with abnormal expression of genes that contribute to their pathophysiological and clinical manifestations, but at the same time offer potential therapeutic targets. One of the promising therapeutic approaches targets the pathophysiological pathways leading to aberrant gene activation, namely transcriptional activity and its molecular modulators (agonists, antagonists, coregulators, and nuclear receptors). The transcription factors can be divided into four classes (I-IV) classified by structural elements, like basic leucine zipper (bZIP) or basic helix-loop-helix (bHLH), which mediate their DNA binding activity but also determine the classes of drugs that can affect their activity. For example, statins modulate activation of the class-I transcription factor sterol responsive element-binding protein (SREBP), whose target genes including hydroxyl-methyl-glutaryl acetyl Coenzyme-A (HMG-CoA) reductase, HMG-CoA synthase, and the low-density lipoprotein receptor, all of which are involved in cholesterol and fatty acid metabolism. Similarly, insulin-like drugs target the nuclear receptor peroxisome-proliferator-activator-receptor (PPAR)-gamma (class-II), several anti-inflammatory drugs inhibit activation of nuclear factor kappa B (NFkappaB) (class-IV), while others (e.g. flavopiridol, rapamycin, and paclitaxel) target regulation of cell-cycle proteins. Increased understanding of the genetic and molecular basis of disease (e.g. transcriptional activity and its coregulation) will potentially enhance future diagnosis, treatment, and prevention of vascular diseases.

LEDGF, a survival factor, activates stress-related genes

LEDGF is a survival factor and it enhances survival of various cell types against stress. LEDGF is also a transcriptional activator and it binds to promoter elements of heat shock and stress-related genes to activate expression of these genes. The elevated levels of the stress-related family of proteins, such as heat shock proteins, antioxidant proteins, and detoxication enzymes might suppress apoptosis induced by stress. The protective mechanisms against stress in mammalian cells and in yeast are surprisingly similar.

Missense mutations of human homeoboxes: A review

The homeodomain (encoded by the homeobox) is the DNA-binding domain of a large variety of transcriptional regulators involved in controlling cell fate decisions and development. Mutations of homeobox-containing genes cause several diseases in humans. A variety of missense mutations giving rise to human diseases have been described. These mutations are an excellent model to better understand homeodomain molecular functions. To this end, homeobox missense mutations giving rise to human diseases are reviewed. Seventy-four independent homeobox mutations have been observed in 17 different genes. In the same genes, 30 missense mutations outside the homeobox have been observed, indicating that the homeodomain is more easily affected by single amino acids changes than the rest of the protein. Most missense mutations have dominant effects. Several data indicate that dominance is mostly due to haploinsufficiency. Among proteins having the homeodomain as the only DNA-binding domain, three "hot spot" regions can be delineated: 1) at codon encoding for Arg5; 2) at codon encoding for Arg31; and 3) at codons encoding for amino acids of recognition helix. In the latter, mutations at codons encoding for Arg residues at positions 52 and 53 are prevalent. In the recognition helix, Arg residues at positions 52 and 53 establish contacts with phosphates in the DNA backbone. Missense mutations of amino acids that contribute to sequence discrimination (such as those at positions 50 and 54) are present only in a minority of cases. Similar data have been obtained when missense mutations of proteins possessing an additional DNA-binding domain have been analyzed. The only exception is observed in the POU1F1 (PIT1) homeodomain, in which Arg58 is a "hot spot" for mutations, but is not involved in DNA recognition.

Phosphorylation-dependent nucleocytoplasmic shuttling of pancreatic duodenal homeobox-1

Pancreatic duodenal homeobox-1 (PDX-1) is a homeodomain protein that plays an important role in the development of the pancreas and in maintaining the identity and function of the islets of Langerhans. It also regulates the expression of the insulin gene in response to changes in glucose and insulin concentrations. Glucose and insulin regulate PDX-1 by way of a signaling pathway involving phosphatidylinositol 3-kinase (PI 3-kinase) and SAPK2/p38. Activation of this pathway leads to phosphorylation of PDX-1 and its movement into the nucleus. To investigate the intracellular trafficking of PDX-1, immunocytochemistry was used to localize PDX-1 in the human beta-cell line NesPDX-1, in which PDX-1 is overexpressed, and in MIN6 beta-cells. In low-glucose conditions, PDX-1 localized predominantly to the nuclear periphery, with some staining in the cytoplasm. After stimulation with glucose, PDX-1 was present in the nucleoplasm. The translocation of PDX-1 to the nucleoplasm was complete within 15 min and occurred in 5-10 mmol/l glucose. Insulin and sodium arsenite, an activator of the stress-activated pathway, also stimulated PDX-1 movement from the nuclear periphery to the nucleoplasm. When cells were transferred between high glucose- and low glucose-containing medium, PDX-1 rapidly shuttled between the nuclear periphery and the nucleoplasm. Glucose- and insulin-stimulated translocation of PDX-1 to the nucleoplasm was inhibited by wortmannin and SB 203580, indicating that a pathway involving PI 3-kinase and SAPK2/p38 was involved; translocation was unaffected by PD 098959 and rapamycin, suggesting that neither mitogen-activated protein kinase nor p70(s6k) were involved. Arsenite-stimulated import of PDX-1 into the nucleus was inhibited by SB 203580 but not by wortmannin. Export from the nucleoplasm to the nuclear periphery was inhibited by calyculin A and okadaic acid, suggesting that dephosphorylation of PDX-1 was involved. These results demonstrated that PDX-1 shuttles between the nuclear periphery and nucleoplasm in response to changes in glucose and insulin concentrations and that these events are dependent on PI 3-kinase, SAPK2/p38, and a nuclear phosphatase(s).

Spectrum of mutations and genotype-phenotype analysis in Currarino syndrome

The triad of a presacral tumour, sacral agenesis and anorectal malformation constitutes the Currarino syndrome which is caused by dorsal-ventral patterning defects during embryonic development. The syndrome occurs in the majority of patients as an autosomal dominant trait associated with mutations in the homeobox gene HLXB9 which encodes the nuclear protein HB9. However, genotype-phenotype analyses have been performed only in a few families and there are no reports about the specific impact of HLXB9 mutations on HB9 function. We performed a mutational analysis in 72 individuals from nine families with Currarino syndrome. We identified a total of five HLXB9 mutations, four novel and one known mutation, in four out of four families and one out of five sporadic cases. Highly variable phenotypes and a low penetrance with half of all carriers being clinically asymptomatic were found in three families, whereas affected members of one family showed almost identical phenotypes. However, an obvious genotype-phenotype correlation was not found. While HLXB9 mutations were diagnosed in 23 patients, no mutation or microdeletion was detected in four sporadic patients with Currarino syndrome. The distribution pattern of here and previously reported HLXB9 mutations indicates mutational predilection sites within exon 1 and the homeobox. Furthermore, sequence homology to Drosophila homeobox genes suggest that some of these mutations located within the homeobox may alter the DNA-binding specificity of HB9 while those in sequences homologous to a recently identified NLS motif of the human homeobox gene PDX-1 may impair nuclear translocation of the mutated protein.

Pdx1 level defines pancreatic gene expression pattern and cell lineage differentiation

The absence of Pdx1 and the expression of brain-4 distinguish alpha-cells from other pancreatic endocrine cell lineages. To define the transcription factor responsible for pancreatic cell differentiation, we employed the reverse tetracycline-dependent transactivator system in INS-I cell-derived subclones INSralphabeta and INSrbeta to achieve tightly controlled and conditional expression of wild type Pdx1 or its dominant-negative mutant, as well as brain-4. INSralphabeta cells express not only insulin but also glucagon and brain-4, while INSrbeta cells express only insulin. Overexpression of Pdx1 eliminated glucagon mRNA and protein in INSralphabeta cells and promoted the expression of beta-cell-specific genes in INSrbeta cells. Induction of dominant-negative Pdx1 in INSralphabeta cells resulted in differentiation of insulin-producing beta-cells into glucagon-containing alpha-cells without altering brain4 expression. Loss of Pdx1 function alone in INSrbeta cells, which do not express endogenous brain-4 and glucagon, was also sufficient to abolish the expression of genes restricted to beta-cells and to cause alpha-cell differentiation. In contrast, induction of brain-4 in INSrbeta cells initiated detectable expression of glucagon but did not affect beta-cell-specific gene expression. In conclusion, Pdx1 confers the expression of pancreatic beta-cell-specific genes, such as genes encoding insulin, islet amyloid polypeptide, Glut2, and Nkx6.1. Pdx1 defines pancreatic cell lineage differentiation. Loss of Pdx1 function rather than expression of brain4 is a prerequisite for alpha-cell differentiation.

Therapeutic modulation of transcription factor activity

Aberrant gene expression is a fundamental cause of many disease-associated pathophysiologies. The pharmacological modulation of transcription factor activity therefore represents an attractive therapeutic approach to such disorders. With the exception of nuclear receptors, which are the direct targets of pharmaceuticals, other known classes of transcription factors are largely regulated indirectly by drugs that impact upon those signal transduction cascades that alter transcription factor phosphorylation and dephosphorylation and/or nuclear import. However, recent advances in drug discovery technologies now enable high-throughput screens that can identify molecules that act directly at the level of transcription factor complexes.

Functional domains of the cone-rod homeobox (CRX) transcription factor

The paired-like homeodomain transcription factor CRX (cone-rod homeobox) is involved in regulating photoreceptor gene expression and rod outer segment development. Mutations in CRX have been associated with several retinal degenerative diseases. These conditions range from Leber congenital amaurosis (a severe cone and rod degeneration of childhood onset) to adult onset cone-rod dystrophy and retinitis pigmentosa (an adult onset condition that primarily affects rods). The goal of this study is to better understand the molecular basis of CRX function and to provide insight into how mutations in CRX cause such a variety of clinical phenotypes. We performed deletion analysis in conjunction with DNA binding and transient transfection-based transactivation studies to identify the functional domains within CRX. DNA binding requires a complete homeodomain. Furthermore, truncated proteins that did not contain an intact homeodomain failed to demonstrate detectable expression in tissue culture upon transfection. Transactivation analysis indicated that both the OTX tail and the WSP domain are important for controlling positive regulatory activity of CRX. Interestingly, the mapped CRX transactivation domains were also critical when coexpressed with NRL. Specifically, the synergy between CRX and NRL was constant regardless of which CRX variant was used.

Two nuclear localization signals are required for nuclear translocation of nuclear factor 1-A

Nuclear factor 1 (NF1) proteins are encoded by at least four genes (NF1-A, B, C, X). Although DNA-binding and the transcription regulation domains of these proteins are well characterized, the nuclear localization signals (NLSs) are still unknown in all NF1s. We have identified two NLSs in NF1-A, and both are required for full translocation to the nucleus, although one of them itself has a partial translocation ability. These two NLSs are conserved in all four NF1s. Interestingly, three isoforms of NF1-A (NF1-A1, A2, A4) have two NLSs and translocate completely to the nucleus. In contrast, NF1-A3 lacks the second NLS and partially stays in the cytoplasm. Since NF1s construct homodimer and heterodimer, these findings indicate the differential regulations of the NF1 translocation.

The homeodomain of Nkx2.2 carries two cooperatively acting nuclear localization signals

NK-2 family members of homeodomain proteins have been identified as important regulators of growth and development in the ventral forebrain, heart, lung, and thyroid. In addition, Nk2.2 expression has been detected in the pancreas, where it is vital for the final differentiation of beta-cells. In our present paper, we have analyzed the domains necessary for nuclear transport of Nkx2.2. With the help of deletion mutants we identified two separate nuclear localization signals (NLS). Interestingly, both NLSs are situated in the homeodomain. They belong to the monopartite class of NLS; the proximal NLS has the sequence KKRKRR and lies at the very N-terminus of the homeodomain, while the more distal NLS RYKMKRAR is at the homeodomain C-terminus. Each NLS per se is sufficient for nuclear transport of Nkx2.2 into the nucleus, although inefficiently. Both identified NLSs act cooperatively in mediating complete nuclear transport of Nkx2.2.

Specific induction of Z-DNA conformation by a nuclear localization signal peptide of lupin glutaminyl tRNA synthetase

Recently we have sequenced cDNA of plant glutaminyl-tRNA synthetase (GlnRS) from Lupinus luteus. At the N terminal part the protein contains a lysine rich polypeptide (KPKKKKEK), which is identical to a nuclear localization signal (NLS). In this paper we showed that two synthetic peptides (20 and 8 amino acids long), which were derived from lupin GlnRS containing the NLS sequence interact with DNA, but one of them (8aa long) changing its conformation from the B to the Z form. This observation clearly suggests that the presence of the NLS polypeptide in a leader sequence of GlnRS is required not only for protein transport into nucleus but also for regulation of a gene expression. This is the first report suggesting a role of the NLS signal peptide in structural changes of DNA.