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

An induction gene trap for identifying a homeoprotein-regulated locus

An important issue in developmental biology is the identification of homeoprotein target genes. We have developed a strategy based on the internalization and nuclear addressing of exogenous homeodomains, using an engrailed homeodomain (EnHD) to screen an embryonic stem (ES) cell gene trap library. Eight integrated gene trap loci responded to EnHD. One is within the bullous pemphigoid antigen 1 (BPAG1) locus, in a region that interrupts two neural isoforms. By combining in vivo electroporation with organotypic cultures, we show that an already identified BPAG1 enhancer/promoter is differentially regulated by homeoproteins Hoxc-8 and Engrailed in the embryonic spinal cord and mesencephalon. This strategy can therefore be used for identifying and mutating homeoprotein targets. Because homeodomain third helices can internalize proteins, peptides, phosphopeptides, and antisense oligonucleotides, this strategy should be applicable to other intracellular targets for characterizing genetic networks involved in a large number of physiopathological states.

Homeodomain-derived peptides. In and out of the cells

The internalization of homeodomains and of homeopeptides derived from the third helix of the homeodomain of Antennapedia, a Drosophila transcription factor, is used by some investigators to target exogenous hydrophilic compounds into live cells. In addition to this very practical aspect of drug delivery, translocation across biologic membranes of peptides subsequently addressed to the cell cytoplasm and nucleus raises several questions. A first series of questions pertains to the mechanism of translocation. Thanks to the synthesis of several peptides derived from the third helix of the Antennapedia homeodomain, we began to investigate the mechanism of translocation and we have shown that it is not dependent upon the presence of a chiral receptor and probably involves the formation of inverted micelles. A second series of questions is related to the physiologic significance of the phenomenon. In a first approach, we demonstrated that some full-length homeoproteins are internalized and secreted in vitro. The mechanism of internalization is probably similar to that of the homeodomain or of its third helix, but secretion involves a different mechanism which requires an association with specialized intracellular membranous structures. The existence of specific mechanisms for homeoprotein internalization and secretion suggests that this class of transcription factors may have important signaling properties.

Cell-penetrating peptides

The established view in cellular biology dictates that the cellular internalization of hydrophilic macromolecules can only be achieved through the classical endocytosis pathway. However, in the past five years several peptides have been demonstrated to translocate across the plasma membrane of eukaryotic cells by a seemingly energy-independent pathway. These peptides have been used successfully for the intracellular delivery of macromolecules with molecular weights several times greater than their own. Cellular delivery using these cell-penetrating peptides offers several advantages over conventional techniques because it is efficient for a range of cell types, can be applied to cells en masse and has a potential therapeutic application.

The amyloid precursor protein interacts with Go heterotrimeric protein within a cell compartment specialized in signal transduction

The function of the beta-amyloid protein precursor (betaAPP), a transmembrane molecule involved in Alzheimer pathologies, is poorly understood. We recently reported the presence of a fraction of betaAPP in cholesterol and sphingoglycolipid-enriched microdomains (CSEM), a caveolae-like compartment specialized in signal transduction. To investigate whether betaAPP actually interferes with cell signaling, we reexamined the interaction between betaAPP and Go GTPase. In strong contrast with results obtained with reconstituted phospholipid vesicles (Okamoto et al., 1995), we find that incubating total neuronal membranes with 22C11, an antibody that recognizes an N-terminal betaAPP epitope, reduces high-affinity Go GTPase activity. This inhibition is specific of Galphao and is reproduced, in the absence of 22C11, by the addition of the betaAPP C-terminal domain but not by two distinct mutated betaAPP C-terminal domains that do not bind Galphao. This inhibition of Galphao GTPase activity by either 22C11 or wild-type betaAPP cytoplasmic domain suggests that intracellular interactions between betaAPP and Galphao could be regulated by extracellular signals. To verify whether this interaction is preserved in CSEM, we first used biochemical, immunocytochemical, and ultrastructural techniques to unambiguously confirm the colocalization of Galphao and betaAPP in CSEM. We show that inhibition of basal Galphao GTPase activity also occurs within CSEM and correlates with the coimmunoprecipitation of Galphao and betaAPP. The regulation of Galphao GTPase activity by betaAPP in a compartment specialized in signaling may have important consequences for our understanding of the physiopathological functions of betaAPP.

The neuronal zootype. An hypothesis

We present an hypothesis, derived from the zootype concept of Slack, Holland and Graham. The main point of this hypothesis is to postulate that the primordial function of the zootype genes is to design an appropriate neuronal network in bilaterian animals, by controlling the genes involved in the specificity of the axon pathways. This would be the primary function of the zootype genes in development and their primitive function in evolution. The hypothesis is discussed in view of the current knowledge on the Hox genes, their evolution, their genomic organization, their expression and their targets.

Neuronal differentiation of PC12 cells induced by engrailed homeodomain is DNA-binding specific and independent of MAP kinases

Neuronal differentiation may be induced by different mechanisms. In PC12 cells, differentiation can be achieved after stimulation by nerve growth factor through the sustained activation and nuclear translocation of MAPKs. A peptide covering the homeodomain of Drosophila Antennapedia translocates through the cell membrane in primary neurons in culture and reaches their nuclei. This process accelerates neurite elongation. We have examined whether the capacity for neuronal induction is a general characteristic of homeodomains, and whether differentiation proceeds through the same pathway as that induced by growth factors or represents a distinct cellular response. We show here that Engrailed homeodomain is internalized by UR61 cells, a PC12 cell derivative, and that it promotes and sustains neurite outgrowth. This event appears to proceed independently of MAPKs activation, suggesting that either parallel signal transduction pathways are under the control of homeoproteins or that they act downstream of MAPKs. The Fushi tarazu homeodomain also causes neurite outgrowth in UR61 cells and the neurotrophic activities of Engrailed and Fushi tarazu homeodomains correlate with their DNA binding specificities. However, neurite outgrowth is not promoted by Bicoid homeodomain, which recognizes a different DNA sequence. Therefore, the neurotrophic activity of the homeodomains depends not only on DNA-binding ability but also on the specificity of this binding.

Identification of a signal sequence necessary for the unconventional secretion of Engrailed homeoprotein

BACKGROUND

Engrailed-1 and Engrailed-2 are homeoproteins--transcription factors implicated in the morphogenesis of discrete structures. Engrailed proteins have a role in patterning the midbrain-hindbrain region and are expressed in the nuclei of rat embryo midbrain-hindbrain cells. We have previously found that both endogenous and exogenously expressed Engrailed proteins also associate with membrane regions implicated in signal transduction and secretion. Within total membrane fractions, a small proportion of Engrailed--about 5%--is protected against proteinase K proteolysis, suggesting that Engrailed has access to a luminal compartment. Together with our finding that homeodomains and homeoproteins can be internalized by live cells, these observations suggest that Engrailed might act as a polypeptidic messenger. In order to investigate this possibility, we looked to see if Engrailed could be secreted.

RESULTS

Engrailed expressed in COS cells can be recovered in abutting primary neurons and this is dependent on a short sequence in its homeodomain distinct from 'classical' secretion signals. This sequence, which overlaps with the sequence necessary for Engrailed internalization and which is highly conserved among homeoproteins, is the first example of an 'unconventional' sequence necessary for secretion. Less than 50% of total intracellular Engrailed is secreted and there is a correlation between secretion and access to the membrane compartment where the protein is protected against proteinase K.

CONCLUSIONS

Our results lend weight to the proposal that Engrailed, and possibly other homeoproteins, might act as intercellular polypeptidic messengers.

The efficient and rapid import of a peptide into primary B and T lymphocytes and a lymphoblastoid cell line

Lymphocytes are notoriously difficult to transfect. The favoured technique, electroporation, has three major disadvantages: it is highly disruptive, causing large scale cell death; it is inefficient; quiescent primary lymphocytes are refractory to electroporation unless they have been partially activated. We have investigated the cellular import of the third helix of the Antennapedia homeodomain protein (pAntp) as an alternative method for introducing peptides into primary lymphocytes and lymphoid cell lines. The pAntp peptide is taken up rapidly into the cytoplasm and nucleus of the cells where it is retained for at least 48 h. The system displays none of the disadvantages of electroporation; no toxicity of the pAntp peptide was detected at the concentrations tested and the process was efficient with up to 95% of lymphocytes importing the pAntp peptide. Finally, quiescent primary lymphocytes were as efficient as activated primary lymphocytes and a lymphoid cell line at importing the pAntp peptide. This demonstrates that the pAntp peptide delivery system has major advantages over electroporation as a method of delivering molecules to lymphocytes and a lymphoid cell line.

Homeobox genes and disease

To date, not many disorders have been associated with homeobox genes, especially with those belonging to the HOX family. This is particularly surprising, considering the body of evidence accumulated for a role of these genes in the control of mammalian development. Recently, this situation has changed and some congenital or somatic defects have been demonstrated to involve mutations in homeobox genes of the HOX, EMX, PAX, and MSX families, as well as in other novel genes containing either a paired- or bicoid-type homeobox.

Association of Engrailed homeoproteins with vesicles presenting caveolae-like properties

We report here that the homeoproteins Engrailed-1 and Engrailed-2 are present in specific non-nuclear subcellular compartments. Using electron microscopy, we observed that chick-Engrailed-2 expressed in COS-7 cells associates with membrane fractions that are characterized as caveolae. This characterization is based on morphological, biochemical and immunological criteria such as, in particular, the absence of clathrin coat and the presence of caveolin and cholera toxin-binding sites. These data are fully confirmed by subcellular fractionation experiments, which demonstrate that transfected chick-Engrailed-2 is present in low density membrane fractions that are resistant to Triton X-100, enriched in caveolin and solubilized by the addition of a cholesterol-binding detergent, a set of properties highly characteristic of caveolae. The association of Engrailed-2 with specific membrane fractions observed after transfection in COS-7 cells is also observed for endogenous Engrailed-1 and Engrailed-2 expressed at late embryonic stages in the cerebellum and posterior mesencephalon of the rodent. Indeed, the two proteins are present in membrane fractions that bear all the characteristics of microdomains or caveolae-like domains, i.e. Triton X-100 resistance, saponin solubilization, low density on sucrose gradients, enrichment in glycosphingolipid GM1, absence of transmembrane Neural Cell Adhesion Molecule, presence of the glypiated (GPI-anchored) glycoprotein F3/F11 and of the acylated growth-associated protein GAP-43. Finally we demonstrate that part of the membrane-associated Engrailed, either expressed in COS-7 cells or endogenously present in neural tissues, is not accessible to proteolytic enzymes unless the membranes have been permeabilized with detergent. This study suggests that, in addition to their well-known presence in the nucleus, Engrailed proteins are also associated with caveolae-like vesicles that are primarily transported anterogradely into the axon, and that they can get access to a compartment compatible with secretion.

Dlx-2 homeobox gene controls neuronal differentiation in primary cultures of developing basal ganglia

Homeodomain-containing genes of the Dlx family are expressed in the developing basal ganglia. To investigate the role of Dlx genes during development, we studied their cellular localization in primary cultures of embryonic basal telencephalon, and examined the changes in cellular phenotypes resulting from blockade of Dlx-2 expression. Cells containing Dlx-1, Dlx-2, and Dlx-5 mRNAs are immature cells of the neuronal lineage expressing the microtubule-associated proteins (MAPs) MAP1B and MAP2, but not glial fibrillary acidic protein (GFAP). Treatment of these cells with antisense oligonucleotides targeted to Dlx-2 caused a specific decrease of Dlx-2 mRNA and protein. This decrease in the Dlx-2 gene product was associated with a decrease in the expression of MAP2, a protein localized in neuronal dendrites, along with a smaller decrease in the 200-kDa neurofilament subunit (NF-H). Proteins expressed preferentially in axons were unchanged. This reduction in MAP2 expression was associated with a decrease in dendrite outgrowth and an increased level of cell proliferation. None of these changes were elicited by antisense oligonucleotides targeted to Dlx-1. We suggest that the Dlx-2 gene product regulates two interrelated aspects of neuronal differentiation: the exit from the mitotic cycle and the capability to grow MAP2-positive dendrites. As such, this gene product may be important for the establishment of neuronal polarity, setting the stage for afferent synaptic connectivity.

Getting hydrophilic compounds into cells: lessons from homeopeptides

The homeodomain of Antennapedia, a Drosophila transcription factor, translocates across biological membranes. Within this 60-amino-acid polypeptide, a shorter sequence of 16 amino acids was identified that can be used as an internalization vector for several types of cargo (i.e. other peptides and oligonucleotides) into the cytoplasm and nucleus of all cell types. This article describes our present understanding of this phenomenon and discusses its potential applications in cell biology.

Cell internalization of the third helix of the Antennapedia homeodomain is receptor-independent

We have recently reported that a 16-amino acid long polypeptide corresponding to the third helix of the DNA binding domain (homeodomain) of Antennapedia, a Drosophila transcription factor, is internalized by cells in culture (Derossi, D., Joliot, A. H., Chassaing, G., and Prochiantz, A.(1994) J. Biol. Chem. 269, 10444-10450). The capture of the homeodomain and of its third helix at temperatures below 10 degrees C raised the problem of the mechanism of internalization. The present demonstration, that a reverse helix and a helix composed of D-enantiomers still translocate across biological membranes at 4 and 37 degrees C strongly suggests that the third helix of the homeodomain is internalized by a receptor-independent mechanism. The finding that introducing 1 or 3 prolines in the structure does not hamper internalization also demonstrates that the alpha-helical structure is not necessary. The data presented are compatible with a translocation process based on the establishment of direct interactions with the membrane phospholipids. The third helix of the homeodomain has been used successfully to address biologically active substances to the cytoplasm and nucleus of cells in culture (Théodore, L., Derossi, D., Chassaing, G., Llirbat, B., Kubes, M., Jordan, P., Chneiweiss, H., Godement, P., and Prochiantz, A.(1995) J. Neurosci. 15, 7158-7167). Therefore, in addition to their physiological implications (Prochiantz, A., and Théodore, L.(1995) BioEssays 17, 39-45), the present results open the way to the molecular design of cellular vectors.