Transcription factor hoxa-5 is taken up by cells in culture and conveyed to their nuclei

Engrailed: Pathological and physiological effects of a multifunctional developmental gene

Engrailed-1 (EN1) is a developmental gene that encodes En1, a highly conserved transcription factor involved in regionalization during early embryogenesis and in the later maintenance of normal neurons. After birth, EN1 still plays a role in the development and physiology of the body; for example, it exerts a protective effect on midbrain dopaminergic (mDA) neurons, and loss of EN1 causes mDA neurons in the ventral midbrain to gradually die approximately 6 weeks after birth, resulting in motor and nonmotor symptoms similar to those observed in Parkinson's disease. Notably, EN1 has been identified as a possible susceptibility gene for idiopathic Parkinson's disease in humans. EN1 is involved in the processes of wound-healing scar production and tissue and organ fibrosis. Additionally, EN1 can lead to tumorigenesis and thus provides a target for the treatment of some tumors. In this review, we summarize the effects of EN1 on embryonic organ development, describe the consequences of the deletion or overexpression of the EN1 gene, and discuss the pathways in which EN1 is involved. We hope to clarify the role of EN1 as a developmental gene and present potential therapeutic targets for diseases involving the EN1 gene.

Plasma membrane depolarization reveals endosomal escape incapacity of cell-penetrating peptides

Cell-penetrating peptides (CPPs) are short (<30 amino acids), generally cationic, peptides that deliver diverse cargos into cells. CPPs access the cytosol either by direct translocation through the plasma membrane or via endocytosis followed by endosomal escape. Both direct translocation and endosomal escape can occur simultaneously, making it non-trivial to specifically study endosomal escape alone. Here we depolarize the plasma membrane and showed that it inhibits the direct translocation of several CPPs but does not affect their uptake into endosomes. Despite good endocytic uptake many CPPs previously considered to access the cytosol via endosomal escape, failed to access the cytosol once direct translocation was abrogated. Even CPPs designed for enhanced endosomal escape actually showed negligible endosomal escape into the cytosol. Our data reveal that cytosolic localization of CPPs occurs mainly by direct translocation across the plasma membrane. Cell depolarization represents a simple manipulation to stringently test the endosomal escape capacity of CPPs.

HOX Protein Activity Regulation by Cellular Localization

While the functions of HOX genes have been and remain extensively studied in distinct model organisms from flies to mice, the molecular biology of HOX proteins remains poorly documented. In particular, the mechanisms involved in regulating the activity of HOX proteins have been poorly investigated. Nonetheless, based on data available from other well-characterized transcription factors, it can be assumed that HOX protein activity must be finely tuned in a cell-type-specific manner and in response to defined environmental cues. Indeed, records in protein–protein interaction databases or entries in post-translational modification registries clearly support that HOX proteins are the targets of multiple layers of regulation at the protein level. In this context, we review here what has been reported and what can be inferred about how the activities of HOX proteins are regulated by their intracellular distribution.

What are the roles of retinoids, other morphogens, and Hox genes in setting up the vertebrate body axis?

This article is concerned with the roles of retinoids and other known anterior-posterior morphogens in setting up the embryonic vertebrate anterior-posterior axis. The discussion is restricted to the very earliest events in setting up the anterior-posterior axis (from blastula to tailbud stages in Xenopus embryos). In these earliest developmental stages, morphogen concentration gradients are not relevant for setting up this axis. It emerges that at these stages, the core patterning mechanism is timing: BMP-anti BMP mediated time space translation that regulates Hox temporal and spatial collinearities and Hox-Hox auto- and cross- regulation. The known anterior-posterior morphogens and signaling pathways--retinoids, FGF's, Cdx, Wnts, Gdf11 and others--interact with this core mechanism at and after space-time defined "decision points," leading to the separation of distinct axial domains. There are also other roles for signaling pathways. Besides the Hox regulated hindbrain/trunk part of the axis, there is a rostral part (including the anterior part of the head and the extreme anterior domain [EAD]) that appears to be regulated by additional mechanisms. Key aspects of anterior-posterior axial patterning, including: the nature of different phases in early patterning and in the whole process; the specificities of Hox action and of intercellular signaling; and the mechanisms of Hox temporal and spatial collinearities, are discussed in relation to the facts and hypotheses proposed above.

EN2 in Prostate Cancer

Despite extensive efforts to identify a clinically useful diagnostic biomarker in prostate cancer, no new test has been approved by regulatory authorities. As a result, this unmet need has shifted to biomarkers that additionally indicate presence or absence of "significant" disease. EN2 is a homeodomain-containing transcription factor secreted by prostate cancer into the urine and can be detected by enzyme-linked immunoassay. EN2 may be an ideal biomarker because normal prostate tissue and benign prostatic hypertrophic cells do not secrete EN2. This review discusses the enormous potential of EN2 to address this unmet need and provide the urologist with a simple, inexpensive, and reliable prostate cancer biomarker.

Homeoprotein signaling in the developing and adult nervous system

Signaling classically involves the secretion of diverse molecules that bind specific cell-surface receptors and engage intracellular transduction cascades. Some exceptions-namely, lipophilic agents-can cross plasma membranes to bind intracellular receptors and be carried to the nucleus to regulate transcription. Homeoprotein transcription factors are among the few proteins with such a capacity. Here, we review the signaling activities of homeoproteins in the developing and adult nervous system, with particular emphasis on axon/cell migration and postnatal critical periods of cerebral cortex plasticity. We also describe homeoprotein non-cell-autonomous mechanisms and explore how this "novel" signaling pathway impacts emerging research in brain development and physiology. In this context, we explore hypotheses on the evolution of signaling, the role of homeoproteins as early morphogens, and their therapeutic potential for neurological and psychiatric diseases.

Vertical signalling involves transmission of Hox information from gastrula mesoderm to neurectoderm

Development and patterning of neural tissue in the vertebrate embryo involves a set of molecules and processes whose relationships are not fully understood. Classical embryology revealed a remarkable phenomenon known as vertical signalling, a gastrulation stage mechanism that copies anterior-posterior positional information from mesoderm to prospective neural tissue. Vertical signalling mediates unambiguous copying of complex information from one tissue layer to another. In this study, we report an investigation of this process in recombinates of mesoderm and ectoderm from gastrulae of Xenopus laevis. Our results show that copying of positional information involves non cell autonomous autoregulation of particular Hox genes whose expression is copied from mesoderm to neurectoderm in the gastrula. Furthermore, this information sharing mechanism involves unconventional translocation of the homeoproteins themselves. This conserved primitive mechanism has been known for three decades but has only recently been put into any developmental context. It provides a simple, robust way to pattern the neurectoderm using the Hox pattern already present in the mesoderm during gastrulation. We suggest that this mechanism was selected during evolution to enable unambiguous copying of rather complex information from cell to cell and that it is a key part of the original ancestral mechanism mediating axial patterning by the highly conserved Hox genes.

Investigation of homeodomain membrane translocation properties: insights from the structure determination of engrailed-2 homeodomain in aqueous and membrane-mimetic environments

In addition to their well-known DNA-binding properties, homeodomains have the ability to efficiently translocate across biological membranes through still poorly-characterized mechanisms. To date, most biophysical studies addressing the mechanisms of internalization have focused on small synthetic peptides rather than full-length globular homeodomains. In this work, we characterized the conformational properties of chicken Engrailed 2 homeodomain (En2HD) in aqueous solution and in membrane mimetic environments using circular dichroism, Trp fluorescence, and NMR spectroscopy. En2HD adopts a well-defined three-helical bundle fold in aqueous solution. The Trp-48 residue, which is critical for internalization, is fully buried in the hydrophobic core. Circular dichroism and fluorescence reveal that a conformational transition occurs in anionic lipid vesicles and in micelles. En2HD loses its native three-dimensional structure in micellar environments but, remarkably, near-native helical secondary structures are maintained. Long-range interactions could be detected using site-directed spin labels, indicating that the three helices do not adopt extended orientations. Noncovalent paramagnetic probes yielded information about helix positioning and unveiled the burial of critical aromatic and basic residues within the micelles. Our results suggest that electrostatic interactions with membranes may be determinant in inducing a conformational change enabling Trp-48 to insert into membranes.

Characterization of a novel cell penetrating peptide derived from human Oct4

Background

Oct4 is a transcription factor that plays a major role for the preservation of the pluripotent state in embryonic stem cells as well as for efficient reprogramming of somatic cells to induced pluripotent stem cells (iPSC) or other progenitors. Protein-based reprogramming methods mainly rely on the addition of a fused cell penetrating peptide. This study describes that Oct4 inherently carries a protein transduction domain, which can translocate into human and mouse cells.

Results

A 16 amino acid peptide representing the third helix of the human Oct4 homeodomain, referred to as Oct4 protein transduction domain (Oct4-PTD), can internalize in mammalian cells upon conjugation to a fluorescence moiety thereby acting as a cell penetrating peptide (CPP). The cellular distribution of Oct4-PTD shows diffuse cytosolic and nuclear staining, whereas penetratin is strictly localized to a punctuate pattern in the cytoplasm. By using a Cre/loxP-based reporter system, we show that this peptide also drives translocation of a functionally active Oct4-PTD-Cre-fusion protein. We further provide evidence for translocation of full length Oct4 into human and mouse cell lines without the addition of any kind of cationic fusion tag. Finally, physico-chemical properties of the novel CPP are characterized, showing that in contrast to penetratin a helical structure of Oct4-PTD is only observed if the FITC label is present on the N-terminus of the peptide.

Conclusions

Oct4 is a key transcription factor in stem cell research and cellular reprogramming. Since it has been shown that recombinant Oct4 fused to a cationic fusion tag can drive generation of iPSCs, our finding might contribute to further development of protein-based methods to generate iPSCs.

Moreover, our data support the idea that transcription factors might be part of an alternative paracrine signalling pathway, where the proteins are transferred to neighbouring cells thereby actively changing the behaviour of the recipient cell.

Electronic supplementary material

The online version of this article (doi: 10.1186/2045-9769-3-2) contains supplementary material, which is available to authorized users.

Signaling with homeoprotein transcription factors in development and throughout adulthood

The concept of homeoprotein transduction as a novel signaling pathway has dramatically evolved since it was first proposed in 1991. It is now well established in several biological systems from plants to mammals. In this review, the different steps that have led to this unexpected observation are recalled and the developmental and physiological models that have allowed us (and a few others) to consolidate the original hypothesis are described. Because homeoprotein signaling is active in plants and animals it is proposed that it has predated the separation between animals and plants and is thus very ancient. This may explain why the basic phenomenon of homeoprotein transduction is so minimalist, requiring no specific receptors or transduction pathways beside those offered by mitochondria, organelles present in all eukaryotic cells. Indeed complexity has been added in the course of evolution and the conservation of homeoprotein transduction is discussed in the context of its synergy with bona fide signaling mechanism that may have added robustness to this primitive cell communication device. The same synergy possibly explains why homeoprotein signaling is important both in embryonic development and in adult functions fulfilled by signaling entities (e.g. growth factors) themselves active throughout development and in the adult. The cell biological mechanism of homeoprotein transfer is also discussed. Although it is clear that many questions are still in want of precise answers, it appears that the sequences responsible both for secretion and internalization are in the DNA-binding domain and very highly conserved among most homeoproteins. On this basis, it is proposed that this signaling pathway is likely to imply as many as 200 proteins that participate in a myriad of developmental and physiological pathways.

The homeodomain transcription factor Hoxa2 interacts with and promotes the proteasomal degradation of the E3 ubiquitin protein ligase RCHY1

Hox proteins are conserved homeodomain transcription factors known to be crucial regulators of animal development. As transcription factors, the functions and modes of action (co-factors, target genes) of Hox proteins have been very well studied in a multitude of animal models. However, a handful of reports established that Hox proteins may display molecular activities distinct from gene transcription regulation. Here, we reveal that Hoxa2 interacts with 20S proteasome subunits and RCHY1 (also known as PIRH2), an E3 ubiquitin ligase that targets p53 for degradation. We further show that Hoxa2 promotes proteasome-dependent degradation of RCHY1 in an ubiquitin-independent manner. Correlatively, Hoxa2 alters the RCHY1-mediated ubiquitination of p53 and promotes p53 stabilization. Together, our data establish that Hoxa2 can regulate the proteasomal degradation of RCHY1 and stabilization of p53.

Homeoprotein signaling in development, health, and disease: a shaking of dogmas offers challenges and promises from bench to bed

Homeoproteins constitute a major class of transcription factors active throughout development and in adulthood. Their membrane transduction properties were discovered over 20 years ago, opening an original field of research in the domain of vector peptides and signal transduction. In early development, homeoprotein transfer participates in tissue patterning, cell/axon guidance, and migration. In the axon guidance model, homeoproteins exert their non-cell autonomous activity through the regulation of translation, in particular, that of nuclear-transcribed mitochondrial mRNAs. An important aspect of these studies on patterning and migration is that homeoproteins sensitize the cells to the action of other growth factors, thus cooperating with established signaling pathways. The role of homeoprotein signaling at later developmental stages is also of interest. In particular, the transfer of homeoprotein Otx2 into parvalbumin-expressing inhibitory neurons (PV-cells) in the visual cortex regulates cortical plasticity. The molecular deciphering of the interaction of Otx2 with binding sites at the surface of PV-cells has allowed the development of a specific Otx2 antagonist that reopens plasticity in the adult cortex and cures mice from experimental amblyopia, a neurodevelopmental disease. Finally, the use of homeoproteins as therapeutic proteins in mouse models of glaucoma and Parkinson disease is reviewed. In the latter case, engrailed homeoproteins protect mesencephalic dopaminergic neurons by increasing the local translation of complex I mitochondrial mRNAs. In conclusion, this review synthesizes 20 years of work on the fundamental and potentially translational aspects of homeoprotein signaling.

Engrailed homeoprotein acts as a signaling molecule in the developing fly

Homeodomain transcription factors classically exert their morphogenetic activities through the cell-autonomous regulation of developmental programs. In vertebrates, several homeoproteins have also been shown to have direct non-cell-autonomous activities in the developing nervous system. We present the first in vivo evidence for homeoprotein signaling in Drosophila. Focusing on wing development as a model, we first demonstrate that the homeoprotein Engrailed (En) is secreted. Using single-chain anti-En antibodies expressed under the control of a variety of promoters, we delineate the wing territories in which secreted En acts. We show that En is a short-range signaling molecule that participates in anterior crossvein development, interacting with the Dpp signaling pathway. This report thus suggests that direct signaling with homeoproteins is an evolutionarily conserved phenomenon that is not restricted to neural tissues and involves interactions with bona fide signal transduction pathways.

Human HOXA5 homeodomain enhances protein transduction and its application to vascular inflammation

Cellular protein delivery is an emerging technique by which exogenous recombinant proteins are delivered into mammalian cells across the membrane. We have developed an Escherichia coli expression vector including human specific gene sequences for protein cellular delivery. The plasmid was generated by ligation the nucleotides 770-817 of the homeobox A5 mRNA sequence which was matched with protein transduction domain (PTD) of homeodomain protein A5 (HOXA5) into pET expression vector. The cellular uptake of HOXA5-PTD-EGFP was detected in 1min and its transduction reached a maximum at 1h within cell lysates. The cellular uptake of HOXA5-EGFP at 37°C was greater than in 4°C. For study for the functional role of human HOXA5-PTD, we purified HOXA5-APE1/Ref-1 and applied it on monocyte adhesion. Pretreatment with HOXA5-APE1/Ref-1 (100nM) inhibited TNF-α-induced monocyte adhesion to endothelial cells, compared with HOXA5-EGFP. Taken together, our data suggested that human HOXA5-PTD vector provides a powerful research tools for uncovering cellular functions of proteins or for the generation of human PTD-containing proteins.

Control of DNA replication: a new facet of Hox proteins?

Hox proteins are well-known as developmental transcription factors controlling cell and tissue identity, but recent findings suggest that they are also part of the cell replication machinery. Hox-mediated control of transcription and replication may ensure coordinated control of cell growth and differentiation, two processes that need to be tightly and precisely coordinated to allow proper organ formation and patterning. In this review we summarize the available data linking Hox proteins to the replication machinery and discuss the developmental and pathological implications of this new facet of Hox protein function.

Recent Advances in Protein Transduction Technology

During the past 15 years, a variety of peptides, known as protein transduction domains (PTDs), or cell-penetrating peptides (CPPs), have been characterized for their ability to translocate into live cells. There are now numerous examples of biologically active full-length proteins and peptides that have been successfully delivered to cells and tissues, both in vitro and in vivo. One of the principal mechanisms of protein transduction is via electrostatic interactions with the plasma membrane, subsequent penetration into the cells by macropinocytosis, and release into the cytoplasm and nuclei by retrograde transport. Recent reports have also now shown that some of the limitations of protein transduction technology have been overcome. In particular, the use of ubiquitination-resistant proteins has been demonstrated to be a more effective strategy for transduction because the half-life of these molecules is significantly increased. Moreover, the use of the NH2-terminal domain of the influenza virus hemagglutinin-2 subunit (HA2) or photosensitive PTDs has been shown to specifically enhance macropinosome escape. Hence, these and other recent advances in protein transduction technologies have created a number of possibilities for the development of new peptide-based drugs.

Regulation of Hox activity: insights from protein motifs

Deciphering the molecular bases of animal body plan construction is a central question in developmental and evolutionary biology. Genome analyses of a number of metazoans indicate that widely conserved regulatory molecules underlie the amazing diversity of animal body plans, suggesting that these molecules are reiteratively used for multiple purposes. Hox proteins constitute a good example of such molecules and provide the framework to address the mechanisms underlying transcriptional specificity and diversity in development and evolution. Here we examine the current knowledge of the molecular bases of Hox-mediated transcriptional control, focusing on how this control is encoded within protein sequences and structures. The survey suggests that the homeodomain is part of an extended multifunctional unit coordinating DNA binding and activity regulation and highlights the need for further advances in our understanding of Hox protein activity.

The third helix of the Hoxc8 homeodomain peptide enhances the efficiency of gene transfer in combination with lipofectamine

Protein transduction domains (PTDs) have been shown to cross the biological cell membranes efficiently through a receptor and energy independent mechanism. Because of its ease in membrane transducing ability, PTDs could be used as a gene delivery vector. Since we already have shown that purified Hoxc8 homeoprotein has the ability to cross the cellular membrane, we analyzed the possibility of the third helix of the Hoxc8 homeodomain as a useful gene delivery vector. For that purpose, a 16-aa long synthetic oligopeptide Hoxc8 Protein Transduction Domain (HPTD) was chemically synthesized and then tested to see whether the HPTD could form a complex with DNA or not. Gel retardation analysis revealed that the HPTD interacts with plasmid DNA efficiently but failed to transfer the DNA into the cells. However, HPTD can enhance the efficiency of gene transfer in combination with Lipofectamine which doubled the gene transfer rate into COS-7 cells compared with the DNA/Lipofectamine control. An MTT assay indicated that the amount of HPTD used in the complex for the transfection did not show any cytotoxicty in COS-7 cells. The TEM studies showed compact particle formation in the presence of HPTD. These results indicate that the HPTD could be a good candidate adjuvant molecule to enhance the gene transfer efficiency of Lipofectamine in eukaryotic cells.

Homeoproteins as natural Penetratin cargoes with signaling properties

Most of this volume is dedicated to a very important and pragmatic issue which is to design ways of internalizing active pharmacological compounds into cells. In fact, many vectors have now been developed and the improvement in the technology can be seen on two main fronts. A first one is the identification of extremely efficient cargoes, for example siRNAs, which can enter the cells once attached to the vectors. A second one is the development of chemical vectors designed after the properties of the peptides and of tags allowing in vivo addressing to specific organs, for example the brain, cell types or sub-cellular compartments. This chapter is of a different nature, as it is devoted to the physiological significance of protein transduction and to the comparative analysis of the Penetratin PTD with its parental proteins, the homeoproteins. Although very academic, these two issues are of practical interest for the rational design of new vectors and the identification of unforeseen pathological mechanisms and pharmacological targets.

Efficient cellular uptake of recombinant murine Hoxc8 homeoprotein in COS-7 cells

In order to analyze the self-delivery activity of Hoxc8, recombinant Hoxc8 protein (rHoxc8) was designed to be expressed and purified in E. coli as a glutathione S-transferase and green fluorescent protein-fused form (GST-GFP-Hoxc8). After purification using glutathione sepharose beads, the 82 kDa fusion protein was separated on the SDS-PAGE gel and confirmed by detecting the fluorescence through luminescent image analyzer. When rHoxc8 was added to culture media for 30 h, most of the COS-7 cells contained the fusion proteins, showing green fluorescence under the fluorescent microscope. When the efficiency of cellular uptake was examined after Hoechst staining, almost 100% of the cells exhibited the GFP signal, revealing that rHoxc8 can traverse the cellular membrane of COS-7 cells efficiently, suggesting that the rHoxc8 could be applied in the development of efficient and useful delivery vectors for therapeutic molecules.

HOXBES2: a novel epididymal HOXB2 homeoprotein and its domain-specific association with spermatozoa

The sperm from the testis acquires complete fertilizing ability and forward progressive motility following its transit through the epididymis. Acquisition of these characteristics results from the modification of the sperm proteome following interactions with epididymal secretions. In our attempts to identify epididymis-specific sperm plasma membrane proteins, a partial 2.83-kb clone was identified by immunoscreening a monkey epididymal cDNA library with an agglutinating monoclonal antibody raised against washed human spermatozoa. The sequence of the 2.83-kb clone exhibited homology to the region between 1 and 1097 bp of the homeobox gene, Hoxb2. This sequence was found to be species conserved, as revealed by RT-PCR analysis. To obtain a full-length clone of the sequence, 5' RACE-PCR (rapid amplification of cDNA ends PCR) was carried out using rat epididymal RNA as the template. It resulted in a full-length 1.657-kb cDNA encoding a 32.9-kDa putative protein. The protein designated HOXBES2 exhibited homology to the conserved 61-amino acid homeodomain region of the HOXB2 homeoprotein. However, characteristic differences were noted in its amino and carboxyl termini compared with HOXB2. A putative 30-kDa protein was detected in the tissue extracts from adult rat epididymis and caudal spermatozoa, and a 37-kDa protein was detected in the rat embryo when probed with a polyclonal antibody against HOXB2 protein. Multiple tissue Western blot and immunohistochemical analysis further indicated its expression in the cytoplasm of the principal and basal epithelial cells, with maximal expression in the distal epididymal segments. Northern blot analysis detected a single approximately 2.5-kb transcript from the adult epididymis. Indirect immunofluorescence localized the protein to the acrosome, midpiece, and equatorial segments of rat caudal and ejaculated human and monkey spermatozoa, respectively. In conclusion, we have identified and characterized a novel epididymal homeoprotein different from HOXB2 protein and hereafter referred to as HOXBES2, (HOXB2 homeodomain containing epididymis-specific sperm protein) with a probable role in fertilization.

Engrailed: complexity and economy of a multi-functional transcription factor

Engrailed is a homeodomain-containing transcription factor with numerous, overlapping roles in neural development. Its multifunctional nature depends upon an extremely diverse group of molecular functions including the ability to regulate both transcription and translation, and to be released from and internalized by cells. Recent findings have shown how some of these functions relate to specific roles in development and disease.

Transduction peptides within naturally occurring proteins

First identified as peptides derived from the human immunodeficiency virus (HIV) transcriptional regulator Tat and the Drosophila transcription factor Antennapedia, transduction (or cell-penetrating) peptide sequences enable soluble proteins to cross biological membranes and interact with cytosolic and nuclear targets. Proteins containing such sequences have been found to function as transcription factors, to inhibit apoptosis, to play roles in axon guidance, or to transport viral mRNA between cells. The recent demonstration that dynorphins are able to act as transduction peptides suggests that these neuropeptides may have roles independent of opiate receptor activation.

Peptide-enhanced cellular internalization of proteins in neuroscience

Over the last 15 years, many publications described the use of peptide sequences that have been dubbed cell penetrating peptides (CPP), Trojan Horse peptides, protein transduction domains, or membrane-translocating sequences. These mostly positively charged domains bring attached cargo across biological membranes. One of the reasons for the interest in CPP is their potential as delivery tools to enhance the pharmacodynamics of drugs otherwise poorly bioavailable. In particular, the neuroscientist aiming to deliver a protein or other compound into the brain for analytical or therapeutic reasons is faced with the challenge that few drugs cross the blood-brain barrier. CPP are valuable tools to overcome the plasma membrane or the blood-brain barrier in basic research, and in relevant models of neural disease, and will hopefully help to increase the precious few treatments or even cures for people with diseases of the brain and nervous system. Here, we review applications in neuroscience and recent insights into the mechanism of CPP-mediated trafficking.

Knockdown of the complete Hox paralogous group 1 leads to dramatic hindbrain and neural crest defects

The Hox paralogous group 1 (PG1) genes are the first and initially most anterior Hox genes expressed in the embryo. In Xenopus, the three PG1 genes, Hoxa1, Hoxb1 and Hoxd1, are expressed in a widely overlapping domain, which includes the region of the future hindbrain and its associated neural crest. We used morpholinos to achieve a complete knockdown of PG1 function. When Hoxa1, Hoxb1 and Hoxd1 are knocked down in combination, the hindbrain patterning phenotype is more severe than in the single or double knockdowns, indicating a degree of redundancy for these genes. In the triple PG1 knockdown embryos the hindbrain is reduced and lacks segmentation. The patterning of rhombomeres 2 to 7 is lost, with a concurrent posterior expansion of the rhombomere 1 marker, Gbx2. This effect could be via the downregulation of other Hox genes, as we show that PG1 function is necessary for the hindbrain expression of Hox genes from paralogous groups 2 to 4. Furthermore, in the absence of PG1 function, the cranial neural crest is correctly specified but does not migrate into the pharyngeal arches. Embryos with no active PG1 genes have defects in derivatives of the pharyngeal arches and, most strikingly, the gill cartilages are completely missing. These results show that the complete abrogation of PG1 function in Xenopus has a much wider scope of effect than would be predicted from the single and double PG1 knockouts in other organisms.

Delivery of bioactive molecules into the cell: the Trojan horse approach

In recent years, vast amounts of data on the mechanisms of neural de- and regeneration have accumulated. However, only in disproportionally few cases has this led to efficient therapies for human patients. Part of the problem is to deliver cell death-averting genes or gene products across the blood-brain barrier (BBB) and cellular membranes. The discovery of Antennapedia (Antp)-mediated transduction of heterologous proteins into cells in 1992 and other "Trojan horse peptides" raised hopes that often-frustrating attempts to deliver proteins would now be history. The demonstration that proteins fused to the Tat protein transduction domain (PTD) are capable of crossing the BBB may revolutionize molecular research and neurobiological therapy. However, it was only recently that PTD-mediated delivery of proteins with therapeutic potential has been achieved in models of neural degeneration in nerve trauma and ischemia. Several groups have published the first positive results using protein transduction domains for the delivery of therapeutic proteins in relevant animal models of human neurological disorders. Here, we give an extensive review of peptide-mediated protein transduction from its early beginnings to new advances, discuss their application, with particular focus on a critical evaluation of the limitations of the method, as well as alternative approaches. Besides applications in neurobiology, a large number of reports using PTD in other systems are included as well. Because each protein requires an individual purification scheme that yields sufficient quantities of soluble, transducible material, the neurobiologist will benefit from the experiences of other researchers in the growing field of protein transduction.

VP22-mediated intercellular transport correlates with enhanced biological activity of MybEngrailed but not (HSV-I) thymidine kinase fusion proteins in primary vascular cells following non-viral transfection

BACKGROUND

The intercellular transport properties of the herpes simplex virus (HSV) protein VP22 have been harnessed to enhance the effectiveness of viral gene transfer. We investigated the intercellular transport and biological effects of VP22 fused with the dominant negative c-Myb chimera, MybEngrailed (MybEn) and HSV-I thymidine kinase (TK), in primary vascular smooth muscle cells (VSMC) following non-viral transfection.

MATERIALS AND METHODS

Porcine VSMC transfected with plasmids encoding MybEn, TK and their respective N- and C-terminal VP22 fusion proteins were assayed for the extent and distribution of transgene expression (by immunohistochemistry), culture growth and apoptosis.

RESULTS

The N-terminal MybEn fusion with VP22 (MybEnVP22) and both TK fusions, but not VP22MybEn, exhibited intercellular spread from primary transfected to up to 200 surrounding cells. pMybEnVP22-transfected cultures exhibited growth inhibition and apoptosis rates that were 10.6 +/- 3.6 and 3.2 +/- 1.0 fold higher than in pMybEn-transfected cultures; pVP22MybEn-transfected cultures showed no difference in these parameters. pTK-transfected cultures underwent 60-70% cell death in the presence of ganciclovir despite <2% primary transfection, which was not increased in cultures transfected with plasmids encoding VP22-TK fusions.

CONCLUSIONS

The close correlation between immunocytochemical and biological assays suggests that intercellular transport is crucial to the enhanced biological activity of the MybEnVP22 fusion. The "intrinsic" bystander activity of TK was 4-fold greater than was "engineered" by VP22 fusion, probably reflecting the abundance of gap junctions between VSMC. VP22 fusion may enhance the efficiency of non-viral gene delivery when combined with the appropriate therapeutic transgene, target tissue and transfection method.

The cerebellar transcriptome during postnatal development of the Ts1Cje mouse, a segmental trisomy model for Down syndrome

The central nervous system of persons with Down syndrome presents cytoarchitectural abnormalities that likely result from gene-dosage effects affecting the expression of key developmental genes. To test this hypothesis, we have investigated the transcriptome of the cerebellum of the Ts1Cje mouse model of Down syndrome during postnatal development using microarrays and quantitative PCR (qPCR). Genes present in three copies were consistently overexpressed, with a mean ratio relative to euploid of 1.52 as determined by qPCR. Out of 63 three-copy genes tested, only five, nine and seven genes had ratios >2 or <1.2 at postnatal days 0 (P0), P15 and P30, respectively. This gene-dosage effect was associated with a dysregulation of the expression of some two-copy genes. Out of 8258 genes examined, the Ts1Cje/euploid ratios differed significantly from 1.0 for 406 (80 and 154 with ratios above 1.5 and below 0.7, respectively), 333 (11 above 1.5 and 55 below 0.7) and 246 genes (59 above 1.5 and 69 below 0.7) at P0, P15 and P30, respectively. Among the two-copy genes differentially expressed in the trisomic cerebellum, six homeobox genes, two belonging to the Notch pathway, were severely repressed. Overall, at P0, transcripts involved in cell differentiation and development were over-represented among the dysregulated genes, suggesting that cell differentiation and migration might be more altered than cell proliferation. Finally, global gene profiling revealed that transcription in Ts1Cje mice is more affected by the developmental changes than by the trisomic state, and that there is no apparent detectable delay in the postnatal development of the cerebellum of Ts1Cje mice.

Intracellular Delivery of Functional Proteins via Decoration with Transporter Peptides

Despite numerous attractive intracellular targets, protein therapeutics have been principally confined to the extracellular space due to the lack of a straightforward way to deliver functional polypeptides to the cell interior. Peptide sequences facilitating intracellular protein delivery have been identified; however, current strategies to apply them require problematic steps, such as generation of new in-frame fusion proteins, covalent chemical conjugation, and denaturation. We have developed a new approach to protein transfer into cells and tissues that relies on single-step decoration by cysteine-flanked, arginine-rich transporter peptides. This approach facilitated cell and tissue delivery of a variety of functional proteins, including antibodies and enzymes. Decoration with transporter peptides thus provides an attractive general means of intracellular delivery of functional proteins in vitro and in tissue.

Transduction peptides: from technology to physiology

During the past fifteen years, a variety of peptides have been characterized for their ability to translocate into live cells. Most are efficient vectors that can internalize hydrophilic cargoes, and so provide a valuable biological (and potentially therapeutic) tool for targeting proteins into cells. Furthermore, translocation of cell-permeable peptides across the plasma membrane and their subsequent access to the cytosol, even when fused to large hydrophilic proteins, is challenging the perception of the plasma membrane as an impermeable barrier.

Can transcription factors function as cell–cell signalling molecules?

Recent data support the view that transcription factors - in particular, homeoproteins - can be transferred from cell to cell and have direct non-cell-autonomous (and therefore paracrine) activities. This intercellular transfer, based on atypical internalization and secretion, has important biotechnological consequences. But the real excitement stems from the physiological and developmental implications of this mode of signal transduction.

Nonclassical transport proteins and peptides: an alternative to classical macromolecule delivery systems

The number of peptides and proteins known to exhibit nonclassical transport activity has increased significantly in recent years. In most cases, these entities have been studied in relation to their ability to deliver high molecular weight compounds, including proteins and DNA, for the ultimate purpose of developing new drug delivery strategies. In this review, an overview of the various types of vectors is presented. The in vitro and in vivo delivery successes of this technology, as well as preliminary therapeutic efforts, are described. Although a comprehensive mechanism of nonclassical transport has not yet been clearly established, we propose a straightforward model based on the cationic nature of the vectors and the need for lack of highly organized structure. In this hypothesis we suggest that the movement of polycations is mediated by a network of extra- and intracellular polyanions while transport across the bilayer is facilitated by cation-pi interactions between the vectors' basic groups and aromatic amino acid side chains in the bilayer spanning helices of membrane proteins.

"Translocatory proteins" and "protein transduction domains": a critical analysis of their biological effects and the underlying mechanisms

It has been suggested that several proteins, termed "translocatory" or "messenger" proteins, can move between living cells-exiting the cell of synthesis via an uncharacterized secretory pathway and entering adjacent cells by a nonendocytic mechanism that is active even at 4 degrees C. These activities, which have been mapped to short, highly basic regions termed "protein transduction domains" (PTDs), have engendered considerable interest in the gene therapy and vaccine research communities. If these proteins, and PTDs, are to be used in human or veterinary medicine, it is vital that the mechanisms underlying their effects be understood. This article presents a critical evaluation of the current literature and describes recent findings that indicate that the effects of these sequences might be explained by well-established biological principles.

Joint regulation of the MAP1B promoter by HNF3beta/Foxa2 and Engrailed is the result of a highly conserved mechanism for direct interaction of homeoproteins and Fox transcription factors

The MAP1B (Mtap1b) promoter presents two evolutionary conserved overlapping homeoproteins and Hepatocyte nuclear factor 3beta (HNF3beta/Foxa2) cognate binding sites (defining putative homeoprotein/Fox sites, HF1 and HF2). Accordingly, the promoter domain containing HF1 and HF2 is recognized by cerebellum nuclear extracts containing Engrailed and Foxa2 and has regulatory functions in primary cultures of embryonic mesmetencephalic nerve cells. Transfection experiments further demonstrate that Engrailed and Foxa2 interact physiologically in a dose-dependent manner: Foxa2 antagonizes the Engrailed-driven regulation of the MAP1B promoter, and vice versa. This led us to investigate if Engrailed and Foxa2 interact directly. Direct interaction was confirmed by pull-down experiments, and the regions participating in this interaction were identified. In Foxa2 the interacting domain is the Forkhead box DNA-binding domain. In Engrailed, two independent interacting domains exist: the homeodomain and a region that includes the Pbx-binding domain. Finally, Foxa2 not only binds Engrailed but also Lim1, Gsc and Hoxa5 homeoproteins and in the four cases Foxa2 binds at least the homeodomain. Based on the involvement of conserved domains in both classes of proteins, it is proposed that the interaction between Forkhead box transcription factors and homeoproteins is a general phenomenon.

The intracrine hypothesis and intracellular peptide hormone action

There is evidence that many peptide growth factors and hormones act in the intracellular space after either internalization or retention in their cells of synthesis. These factors, commonly called intracrines, are structurally diverse while sharing some common functional features. Reports of intracellular peptide hormone binding and action are reviewed here. Also, this laboratory has made proposals regarding the origin and actions of intracrines and these areas are further explored. Intracrine interactions and the relationship of intracrines to transcription factors are discussed. The intracellular/intracrine renin-angiotensin system (iRAS) is reviewed to illustrate the intracrine analogue of a well-established physiological system. The role of intracrine action in metazoan development is also considered.

Cellular uptake of Antennapedia Penetratin peptides is a two-step process in which phase transfer precedes a tryptophan-dependent translocation

Several homeodomains and homeodomain-containing proteins enter live cells through a receptor- and energy-independent mechanism. Translocation through biological membranes is conferred by the third alpha-helix of the homeodomain, also known as Penetratin. Biophysical studies demonstrate that entry of Penetratin into cells requires its binding to surface lipids but that binding and translocation are differentially affected by modifications of some physico-chemical properties of the peptide, like helical amphipathicity or net charge. This suggests that the plasma membrane lipid composition affects the internalization of Penetratin and that internalization requires both lipid binding and other specific properties. Using a phase transfer assay, it is shown that negatively charged lipids promote the transfer of Penetratin from a hydrophilic into a hydrophobic environment, probably through charge neutralization. Accordingly, transfer into a hydrophobic milieu can also be obtained in the absence of negatively charged lipids, by the addition of DNA oligonucleotides. Strikingly, phase transfer by charge neutralization was also observed with a variant peptide of same charge and hydrophobicity in which the tryptophan at position 6 was replaced by a phenylalanine. However, Penetratin, but not its mutant version, is internalized by live cells. This underscores that charge neutralization and phase transfer represent only a first step in the internalization process and that further crossing of a biological membrane necessitates the critical tryptophan residue at position 6.

Taking the cell by stealth or storm? Protein transduction domains (PTDs) as versatile vectors for delivery

A cell delivery system is increasing in use in many areas of cell and molecular biology and bio-medicine. This system is based on a number of naturally occurring protein motifs and/or sequences which show the remarkable ability to rapidly cross the mammalian cell membrane without compromising its structure or function. These so-called Protein Transduction Domains (PTDs) offer unprecedented advantages for intracellular delivery. These advantages include, but are not limited to, their applicability to all cell types (no cell type has yet been described which is not transduced by these PTDs), and the range of cargoes that can be transduced (including peptides, small proteins, full-length enzymes, DNA oligomers, peptide-nucleic acid oligomers, liposomes, and magnetic nanoparticles). Furthermore, the PTDs have been demonstrated to be suitable for in vivo delivery including delivery across the blood brain barrier, and have been shown to cross the plasma membrane rapidly and enter the cytoplasm and nuclear regions of the cell. In this review, the general properties of the most commonly used PTDs are described. The strategies currently being undertaken also highlight that improvements in membrane transduction are possible despite our lack of understanding of the exact biochemical and/or physical mechanisms of transduction. Recent examples of the range of potential applications are also discussed.

Hoxa5 overexpression correlates with IGFBP1 upregulation and postnatal dwarfism: evidence for an interaction between Hoxa5 and Forkhead box transcription factors

Transgenic mice expressing the homeobox gene Hoxa5 under the control of Hoxb2 regulatory elements present a growth arrest during weeks two and three of postnatal development, resulting in proportionate dwarfism. These mice present a liver phenotype illustrated by a 12-fold increase in liver insulin-like growth factor binding protein 1 (IGFBP1) mRNA and a 50% decrease in liver insulin-like growth factor 1 (IGF1) mRNA correlated with a 50% decrease in circulating IGF1. We show that the Hoxa5 transgene is expressed in the liver of these mice, leading to an overexpression of total (endogenous plus transgene) Hoxa5 mRNA in this tissue. We have used several cell lines to investigate a possible physiological interaction of Hoxa5 with the main regulator of IGFBP1 promoter activity, the Forkhead box transcription factor FKHR. In HepG2 cells, Hoxa5 has little effect by itself but inhibits the FKHR-dependent activation of the IGFBP1 promoter. In HuF cells, Hoxa5 cooperates with FKHR to dramatically enhance IGFBP1 promoter activity. This context-dependent physiological interaction probably corresponds to the existence of a direct interaction between Hoxa5 and FKHR and FoxA2/HNF3β, as demonstrated by pull-down experiments achieved either in vitro or after cellular co-expression. In conclusion, we propose that the impaired growth observed in this transgenic line relates to a liver phenotype best explained by a direct interaction between Hoxa5 and liver-specific Forkhead box transcription factors, in particular FKHR but also Foxa2/HNF3β. Because Hoxa5 and homeogenes of the same paralog group are normally expressed in the liver, the present results raise the possibility that homeoproteins, in addition to their established role during early development, regulate systemic physiological functions.

Engrailed homeoprotein secretion is a regulated process

Chicken Engrailed 2 homeoprotein is transported between cells in culture. This intercellular transfer is based on unconventional secretion and internalisation mechanisms: Engrailed 2 has access to vesicles but lacks a signal sequence for secretion and is internalised by a non-endocytic process. We show that phosphorylation of a serine-rich domain within Engrailed 2 by the protein kinase CK2 specifically inhibits Engrailed 2 secretion. The availability of the serine-rich domain to CK2 is highly increased when it is displaced from its normal position to the C terminus of Engrailed 2, leading to a constitutive blockage of Engrailed 2 intercellular transfer. This indicates that intercellular transfer of Engrailed 2 is a highly regulated process.

The identity and distribution of neural cells expressing the mesodermal determinant spadetail

BACKGROUND

The spadetail (spt) gene of zebrafish is expressed in presomitic mesoderm and in neural cells previously suggested to be Rohon-Beard neurons. The mechanism(s) generating the apparently irregular rostrocaudal distribution of spt-expressing cells in the developing CNS is unknown.

RESULTS

spt-expressing neural cells co-express huC, a marker of neurons. These cells also co-express the genes islet-1, -2 and -3 but not valentino. The islet-1 gene expression, irregular distribution and dorsolateral position of spt-expressing cells in the developing CNS are characteristic of dorsal longitudinal ascending (DoLA) interneurons. Shortly after their birth, these neurons extend processes rostrally into which spt mRNA is transported. At 24 hours post fertilisation(hpf), spt-expressing neurons occur most frequently at rostral levels caudal of the 5th-formed somite pair. There is no apparent bias in the number of spt-expressing cells on the left or right sides of embryos. Extended staining for spt-transcription reveals expression in the dorsocaudal cells of somites at the same dorsoventral level as the spt-expressing neurons. There is frequent juxtaposition of spt-expression in newly formed somites and in neurons. This suggests that both types of spt-expressing cell respond to a common positional cue or that neurons expressing spt are patterned irregularly by flanking somitic mesoderm.

CONCLUSIONS

spt-expressing cells in the developing CNS appear to be DoLA interneurons. The irregular distribution of these cells along the rostrocaudal axis of the spinal cord may be due to "inefficient" patterning of neural spt expression by a signal(s) from flanking, regularly distributed somites also expressing spt.

The origins of intracrine hormone action

A growing number of peptide hormones and growth factors have been shown to operate in the intracellular space after either internalization or retention in their cells of synthesis. These factors, called intracrines, in many cases are expressed as multiple isoforms, traffic to nucleus or nucleolus, and regulate gene transcription. Some intracrines are angiogenic. It is here argued that intracrine action is the modern analogue of a biologically ancient mechanism for regulating message translation and ribosome synthesis. The implications of this view for research and therapeutics are discussed.

The neuronal microtubule-associated protein 1B is under homeoprotein transcriptional control

To identify genes regulated by homeoprotein transcription factors in postnatal neurons, the DNA-binding domain (homeodomain) of Engrailed homeoprotein was internalized into rat cerebellum neurons. The internalized homeodomain (EnHD) acts as a competitive inhibitor of Engrailed and of several homeoproteins (Mainguy et al., 2000). Analysis by differential display revealed that microtubule-associated protein 1B (MAP1B) mRNA is upregulated by EnHD. This upregulation does not require protein synthesis, suggesting a direct effect of the homeodomain on MAP1B transcription. The promoter region of MAP1B was cut into several subdomains, and each subdomain was tested for its ability to bind Engrailed and EnHD and to associate with Engrailed-containing cerebellum nuclear extracts. In addition, the activity, and regulation by Engrailed, of each subdomain and of the entire promoter were evaluated in vivo by electroporation in the chick embryo neural tube. These experiments demonstrate that MAP1B promoter is regulated by Engrailed in vivo. Moreover, they show that one promoter domain that contains all ATTA homeoprotein cognate binding sites common to the rat and human genes is an essential element of this regulation. It is thus proposed that MAP1B, a cytoskeleton protein involved in neuronal growth and regeneration, is under homeoprotein transcriptional regulation.

Protein transduction: an alternative to genetic intervention?

Protein transduction, an emerging technology with potential applications in gene therapy, can best be described as the internalisation of proteins into the cell, from the external environment. This process relies on the inherent property of a small number of proteins and peptides of being able to penetrate the cell membrane. The transducing property of these molecules can be conferred upon proteins which are expressed as fusions with them and thus offers an alternative to gene therapy for the delivery of therapeutic proteins into target cells. This review describes the three most commonly used protein transduction vehicles; the antennapedia peptide, the herpes simplex virus VP22 protein and HIV TAT protein transduction domain. The future prospects for the application of this technology in gene therapy are also discussed.

"Alien" wasps and evolution of development

A comparative analysis of early developmental programs in a group of parasitic wasps reveals that closely related species can undergo dramatic evolutionary shifts in their patterns of embryogenesis. Developmental changes detected include alterations in early cleavage divisions, the establishment of embryonic anteroposterior polarity and modifications of the segmentation gene hierarchy described from Drosophila. These changes appear to be adaptations to parasitic development, taking place within the body of the host. Wasps illustrate a surprising plasticity in their early development and embryogenesis. The alterations associated with different parasitic strategies suggest that ecological adaptations may have profound influences on developmental processes in animals.

Potential use of non-classical pathways for the transport of macromolecular drugs

Since an increasing number of drug delivery strategies utilising proteins and peptides exhibiting 'non-classical' transport activities have been proposed, studies have begun to establish underlying functional relationships between different vectors. These attempts to find common factors have been hampered by a lack of biophysical data for the various potential protein and peptide transporters, as well as by the structural and functional diversity of the group as a whole. We describe the various types of vectors being considered for use and the preliminary therapeutic successes that have been achieved. Additionally, the various models that have been proposed for non-classical import and export are outlined and discussed in relation to therapeutic delivery. Possible future developments are also discussed.

Messenger proteins: homeoproteins, TAT and others

Some proteins are internalized by live cells by a process that does not involve classical endocytosis and thus gain direct access to the cytoplasm and nucleus. These same proteins are often secreted, despite the absence of a signal peptide. Recent studies of this unexpected mode of intercellular signaling have opened the way for biotechnological developments.