Peptide-based targeting of fluorophores to peroxisomes in living cells

IR-783 Labeling of a Peptide Receptor for 'Turn-On' Fluorescence Based Sensing

In this study, we examine a means for developing near-IR fluorescent sensors through streamlined, site-specific coupling with peptide-based receptors. As the penultimate step of solid-phase synthesis of a peptide-based receptor, we show a simple means of labeling the N' terminus with the near IR fluorophore IR-783 to afford a viable fluorescent sensor after cleavage from the resin. The proof-of-concept probe utilized a biotin mimetic peptide sequence as the receptive moiety. Here we revealed a "turn-on" fluorescence enhancement upon binding of the biotin mimetic probe to its intended streptavidin target. Not all peptide-receptive moieties tested were able to generate such an enhancement upon target binding, and as such, the rationale for the observed fluorescence response properties is discussed.

Protein-mimicking nanoparticles for the reproduction of transient protein-receptor interactions

One of the major concerns in target identification has been the need for new methods to detect target molecules in the native cellular environment. In conventional target identification, affinity-based pull down has been conducted using cell lysates. However, interactions in cell lysate do not reflect real endogenous interactions in living cells, and can produce false-positive or false-negative results. This study aimed to develop a new method of target protein identification in living cells. Targeting probes were conjugated onto magnetic nanoparticles (MNPs). After nanoparticle cellular uptake, identification and recruitment of target proteins were conducted in living cells, and the target protein was finally recovered under the magnetic field. As a proof-of-concept study, we developed a functionalized MNP (PTS1-MNP) to mimic a peroxisomal protein containing a synthetic peroxisomal targeting signal 1 (PTS1). The PTS1-MNPs were imported into human hepatoma HepG2 cells to recruit PTS1-receptor protein Pex5p. Successful peroxisomal translocation of PTS-MNPs was achieved via transient interaction with Pex5p. Pull-down of Pex5p in lysed or living HepG2 cells confirmed the selective recruiting functionality of synthetic PTS1. The specific detection of Pex5p before complete PTS1-MNPs translocation in living HepG2 cells further demonstrated the transient interaction between Pex5p and PTS1-MNPs. This is the first report showing the peroxisomal translocation of nanostructured materials in living cells. This approach can be applied as a new concept to study transient interactions and target identification or recruiting in living cells.

Exploring cells with targeted biosensors

Cellular signaling networks are composed of multiple pathways, often interconnected, that form complex networks with great potential for cross-talk. Signal decoding depends on the nature of the message as well as its amplitude, temporal pattern, and spatial distribution. In addition, the existence of membrane-bound organelles, which are both targets and generators of messages, add further complexity to the system. The availability of sensors that can localize to specific compartments in live cells and monitor their targets with high spatial and temporal resolution is thus crucial for a better understanding of cell pathophysiology. For this reason, over the last four decades, a variety of strategies have been developed, not only to generate novel and more sensitive probes for ions, metabolites, and enzymatic activity, but also to selectively deliver these sensors to specific intracellular compartments. In this review, we summarize the principles that have been used to target organic or protein sensors to different cellular compartments and their application to cellular signaling.

Golgi-dependent signaling: self-coordination of membrane trafficking

Multiple studies have shown that endomembranes can act as signaling platforms for plasma-membrane-originated signaling. In particular, the Golgi complex operates as a relay station for signaling, which is initiated by classical ligand-receptor systems at the plasma membrane, acting as a positive or negative regulator of these plasma-membrane signals. Thus, the Golgi complex has emerged as a hub for intracellular signaling. Furthermore, recent evidence has indicated that the Golgi complex can also trigger its own signaling cascades, which involve some of the molecular players that are classically engaged in signal transduction at the plasma membrane. This aspect of the Golgi complex, namely, the ability to generate autonomous signaling, has been experimentally addressed only in the last few years. These studies have revealed that the transport vesicles that leave the ER for the Golgi complex also carry signal molecules that can then be sensed by a receptor in the Golgi complex to coordinate secretory organelles. The receptor involved in the sensing of incoming traffic at the Golgi complex has been shown to be the KDEL receptor (KDELR), a proposed new G-protein-coupled receptor. Upon binding to a KDEL-containing ligand (a chaperone), the KDELR can activate a signaling cascade that regulates anterograde intra-Golgi trafficking. However, this Golgi-based signaling response is only partially understood to date. Here we report on several approaches that are suitable for the study of traffic-initiated and KDELR-dependent signaling.

BODIPY probes to study peroxisome dynamics in vivo

There is a continuing need for bioprobes that are target-specific and combine speed of delivery with maintenance of normal cell behaviour. Towards this end, we are developing small pro-fluorescent molecules that provide such specificity through chemical activation by biomolecules. We have generated a set of BODIPY (boron dipyrromethane) fluorophores, including one that is intrinsically non-fluorescent but on incubation with cells becomes fluorescent at its target site. Addition of these BODIPY probes to plant cells identifies peroxisomes, as verified by co-localization with an SKL-FP construct. Interestingly, in mammalian cells, co-localization with the mammalian peroxisomal marker SelectFX(TM) was not observed. These data suggest fundamental differences in peroxisome composition, development or function between plant and animal cells.

The c-MYC NHE III(1): function and regulation

c-MYC is an important regulator of a wide array of cellular processes necessary for normal cell growth and differentiation, and its dysregulation is one of the hallmarks of many cancers. Consequently, understanding c-MYC transcriptional activation is critical for understanding developmental and cancer biology, as well as for the development of new anticancer drugs. The nuclease hypersensitive element (NHE) III(1) region of the c-MYC promoter has been shown to be particularly important in regulating c-MYC expression. Specifically, the formation of a G-quadruplex structure appears to promote repression of c-MYC transcription. This review focuses on what is known about the formation of a G-quadruplex in the NHE III(1) region of the c-MYC promoter, as well as on those factors that are known to modulate its formation. Last, we discuss the development of small molecules that stabilize or induce the formation of G-quadruplex structures and could potentially be used as anticancer agents.

Gene Ontology-driven transcriptional analysis of CD34+ cell-initiated megakaryocytic cultures identifies new transcriptional regulators of megakaryopoiesis

Differentiation of hematopoietic stem and progenitor cells is an intricate process controlled in large part at the level of transcription. While some key megakaryocytic transcription factors have been identified, the complete network of megakaryocytic transcriptional control is poorly understood. Using global gene expression microarray analysis, Gene Ontology-based functional annotations, and a novel interlineage comparison with parallel, isogenic granulocytic cultures as a negative control, we closely examined the mRNA level of transcriptional regulators in megakaryocytes derived from human mobilized peripheral blood CD34(+) hematopoietic cells. This approach identified 199 differentially expressed transcription factors or transcriptional regulators. We identified and detailed the transcriptional kinetics of most known megakaryocytic transcription factors including GATA1, FLI1, and MAFG. Furthermore, many genes with transcription factor activity or transcription factor binding activity were identified in megakaryocytes that had not previously been associated with that lineage, including BTEB1, NR4A2, FOXO1A, MEF2C, HDAC5, VDR, and several genes associated with the tumor suppressor p53 (HIPK2, FHL2, and TADA3L). Protein expression and nuclear localization were confirmed in megakaryocytic cells for four of the novel candidate megakaryocytic transcription factors: FHL2, MXD1, E2F3, and RFX5. In light of the hypothesis that transcription factors expressed in a particular differentiation program are important contributors to such a program, these data substantially expand our understanding of transcriptional regulation in megakaryocytic differentiation of stem and progenitor cells.

Induction of a novel histone deacetylase 1/c-Myc/Mnt/Max complex formation is implicated in parity-induced refractoriness to mammary carcinogenesis

Refractoriness to carcinogen-induced increases in epithelial cell proliferation is a very important characteristic of parous mammary glands. We found that N-methyl-N-nitrosourea (MNU)-induced proliferative burst in the mammary ductal epithelium was blocked in parous glands but not in age-matched virgin (AMV) glands. The inhibition of the proliferative burst in MNU-treated parous mammary glands coincided with the upregulation of Mnt, a Myc-suppressor, and the formation of histone deacetylase 1/Mnt/Max complexes that unexpectedly contained c-Myc. These complexes formed on the promoters of Myc targets, such as ornithine decarboxylase, cyclin D2, and transforming growth factor beta1 genes, in quiescent fibroblasts, and were disassembled in serum-stimulated cells. These results suggest that the complexes also function as transcription repressors of the growth-related Myc targets in MNU-treated parous mammary glands. Using the chemical mammary carcinogenesis model of human c-Ha-ras transgenic (Tg) rats, we confirmed that parity protected the mammary glands at the postinitiation phase of tumorigenesis. Although the incidence of 7,12-dimethylbenz[alpha]anthracene-induced palpable tumors was reduced from 61.5% in the AMV Tg rats to 28.5% in the parous animals, the incidence of early neoplastic lesions in the parous rats was the same as that in the AMV rats. Restriction fragment length polymorphism analysis detected mutations in the human c-Ha-ras gene in most of the normal-appearing parous Tg glands, as well as in the virgin glands. We propose that accelerated formation of HDAC1/c-Myc/Mnt/Max complexes in response to carcinogen exposure results in down-regulation of growth-related genes, leading to the refractoriness of parous mammary glands at the postinitiation phase of carcinogenesis.

A novel role of the Mad family member Mad3 in cerebellar granule neuron precursor proliferation

During development, Sonic hedgehog (Shh) regulates the proliferation of cerebellar granule neuron precursors (GNPs) in part via expression of Nmyc. We present evidence supporting a novel role for the Mad family member Mad3 in the Shh pathway to regulate Nmyc expression and GNP proliferation. Mad3 mRNA is transiently expressed in GNPs during proliferation. Cultured GNPs express Mad3 in response to Shh stimulation in a cyclopamine-dependent manner. Mad3 is necessary for Shh-dependent GNP proliferation as measured by bromodeoxyuridine incorporation and Nmyc expression. Furthermore, Mad3 overexpression, but not that of other Mad proteins, is sufficient to induce GNP proliferation in the absence of Shh. Structure-function analysis revealed that Max dimerization and recruitment of the mSin3 corepressor are required for Mad3-mediated GNP proliferation. Surprisingly, basic-domain-dependent DNA binding of Mad3 is not required, suggesting that Mad3 interacts with other DNA binding proteins to repress transcription. Interestingly, cerebellar tumors and pretumor cells derived from patched heterozygous mice express high levels of Mad3 compared with adjacent normal cerebellar tissue. Our studies support a novel role for Mad3 in cerebellar GNP proliferation and possibly tumorigenesis, and they challenge the current paradigm that Mad3 should antagonize Nmyc by competition for direct DNA binding via Max dimerization.

Peroxisomes and oxidative stress

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.

The ins and outs of peroxisomes: Co-ordination of membrane transport and peroxisomal metabolism

Peroxisomes perform a range of metabolic functions which require the movement of substrates, co-substrates, cofactors and metabolites across the peroxisomal membrane. In this review, we discuss the evidence for and against specific transport systems involved in peroxisomal metabolism and how these operate to co-ordinate biochemical reactions within the peroxisome with those in other compartments of the cell.

Genomic binding and transcriptional regulation by the Drosophila Myc and Mnt transcription factors

Deregulated expression of members of the myc oncogene family has been linked to the genesis of a wide range of cancers, whereas their normal expression is associated with growth, proliferation, differentiation, and apoptosis. Myc proteins are transcription factors that function within a network of transcriptional activators (Myc) and repressors (Mxd/Mad and Mnt), all of which heterodimerize with the bHLHZ protein Mad and bind E-box sequences in DNA. These transcription factors recruit coactivator or corepressor complexes that in turn modify histones. Myc, Mxd/Max, and Mnt proteins have been thought to act on a specific subset of genes. However, expression array studies and, most recently, genomic binding studies suggest that these proteins exhibit widespread binding across the genome. Here we demonstrate by immunostaining of Drosophila polytene chromosome that Drosophila Myc (dMyc) is associated with multiple euchromatic chromosomal regions. Furthermore, many dMyc-binding regions overlap with regions containing active RNA polymerase II, although dMyc can also be found in regions lacking active polymerase. We also demonstrate that the pattern of dMyc expression in nuclei overlaps with histone markers of active chromatin but not pericentric heterochromatin. dMyc binding is not detected on the X chromosome rDNA cluster (bobbed locus). This is consistent with recent evidence that in Drosophila cells dMyc regulates rRNA transcription indirectly, in contrast to mammalian cells where direct binding of c-Myc to rDNA has been observed. We further show that the dMyc antagonist dMnt inhibits rRNA transcription in the wing disc. Our results support the view that the Myc/Max/Mad network influences transcription on a global scale.

Inflammatory disease and lymphomagenesis caused by deletion of the Myc antagonist Mnt in T cells

Mnt is a Max-interacting protein that can antagonize the activities of Myc oncoproteins in cultured cells. Mnt null mice die soon after birth, but conditional deletion of Mnt in breast epithelium leads to tumor formation. These and related data suggest that Mnt functions as a tumor suppressor. Here we show that conditional deletion of Mnt in T cells leads to tumor formation but also causes inflammatory disease. Deletion of Mnt caused increased apoptosis of thymic T cells and interfered with T-cell development yet led to spleen, liver, and lymph node enlargement. The proportion of T cells in the spleen and lymph nodes was reduced, and the numbers of cells in non-T-cell immune cell populations were elevated. The disruption of immune homeostasis is linked to a strong skewing toward production of T-helper 1 (Th1) cytokines and enhanced proliferation of activated Mnt-deficient CD4+ T cells. Consistent with Th1 polarization in vivo, extensive intestinal inflammation and liver necrosis developed. Finally, most mice lacking Mnt in T cells ultimately succumbed to T-cell lymphoma. These results strengthen the argument that Mnt functions as a tumor suppressor and reveal a critical and surprising role for Mnt in the regulation of T-cell development and in T-cell-dependent immune homeostasis.

alphavbeta3 and macrophage colony-stimulating factor: partners in osteoclast biology

Osteoclasts, the sole bone-resorbing cells, arise by fusion and differentiation of monocyte/macrophage precursors. Matrix degradation requires adhesion of the osteoclast to bone, an integrin alphavbeta3-mediated event that also stimulates signals which polarize the cell and secrete resorptive molecules such as hydrochloric acid and acidic proteases. Two cytokines are necessary and sufficient for osteoclastogenesis, receptor activator of nuclear factor kappaB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF), both produced by mesenchymal cells in the bone marrow environment. M-CSF promotes survival and proliferation of osteoclast precursors. It also contributes to their differentiation and regulates the cytoskeletal changes that accompany bone resorption. Binding of M-CSF to c-Fms, its receptor, recruits adapter proteins and cytosolic kinases, thereby activating a variety of intracellular signals. We herein review how alphavbeta3 and M-CSF, alone and in concert, impact production, survival, and function of the osteoclast, thereby controlling skeletal mass. Signals from alphavbeta3 and/or c-Fms activate Syk and Vav3, originally defined by their function in lymphoid cells. Genetic depletion of either protein generates a strong bone phenotype, underscoring the promise of osteoimmunobiology.

The transcriptional repressor dMnt is a regulator of growth in Drosophila melanogaster

The Myc-Max-Mad/Mnt network of transcription factors has been implicated in oncogenesis and the regulation of proliferation in vertebrate cells. The identification of Myc and Max homologs in Drosophila melanogaster has demonstrated a critical role for dMyc in cell growth control. In this report, we identify and characterize the third member of this network, dMnt, the sole fly homolog of the mammalian Mnt and Mad family of transcriptional repressors. dMnt possesses two regions characteristic of Mad and Mnt proteins: a basic helix-loop-helix-zipper domain, through which it dimerizes with dMax to form a sequence-specific DNA binding complex, and a Sin-interacting domain, which mediates interaction with the dSin3 corepressor. Using the upstream activation sequence/GAL4 system, we show that expression of dMnt results in an inhibition of cellular growth and proliferation. Furthermore, we have generated a dMnt null allele, which results in flies with larger cells, increased weight, and decreased life span compared to wild-type flies. Our results demonstrate that dMnt is a transcriptional repressor that regulates D. melanogaster body size.

Whole-genome analysis reveals a strong positional bias of conserved dMyc-dependent E-boxes

Myc is a transcription factor with diverse biological effects ranging from the control of cellular proliferation and growth to the induction of apoptosis. Here we present a comprehensive analysis of the transcriptional targets of the sole Myc ortholog in Drosophila melanogaster, dMyc. We show that the genes that are down-regulated in response to dmyc inhibition are largely identical to those that are up-regulated after dMyc overexpression and that many of them play a role in growth control. The promoter regions of these targets are characterized by the presence of the E-box sequence CACGTG, a known dMyc binding site. Surprisingly, a large subgroup of (functionally related) dMyc targets contains a single E-box located within the first 100 nucleotides after the transcription start site. The relevance of this E-box and its position was confirmed by a mutational analysis of a selected dMyc target and by the observation of its evolutionary conservation in a different Drosophila species, Drosophila pseudoobscura. These observations raise the possibility that a subset of Myc targets share a distinct regulatory mechanism.

Melanocytes and the microphthalmia transcription factor network

The first mouse microphthalmia transcription factor (Mitf ) mutation was discovered over 60 years ago, and since then over 24 spontaneous and induced mutations have been identified at the locus. Mitf encodes a member of the Myc supergene family of basic helix-loop-helix zipper (bHLH-Zip) transcription factors. Like Myc, Mitf regulates gene expression by binding to DNA as a homodimer or as a heterodimer with another related family member, in the case of Mitf the Tfe3, Tfeb, and Tfec proteins. The study of Mitf has provided many insights into the biology of melanocytes and helped to explain how melanocyte-specific gene expression and signaling is regulated. The human homologue of MITF is mutated in patients with the pigmentary and deafness disorder Waardenburg Syndrome Type 2A (WS2A). The mouse Mitf mutations therefore serve as a model for the study of this human disease. Mutations and/or aberrant expression of several MITF family member genes have also been reported in human cancer, including melanoma (MITF), papillary renal cell carcinoma (TFE3, TFEB), and alveolar soft part sarcoma (TFE3). Genes in the MITF/TFE pathway may therefore also represent valuable therapeutic targets for the treatment of human cancer. Here we review recent developments in the analysis of Mitf function in vivo and in vitro and show how traditional genetics, modern forward genetics and in vitro biochemical analyses have combined to produce an intriguing story on the role and actions of a gene family in a living organism.

Mammalian peroxisomes and reactive oxygen species

The central role of peroxisomes in the generation and scavenging of hydrogen peroxide has been well known ever since their discovery almost four decades ago. Recent studies have revealed their involvement in metabolism of oxygen free radicals and nitric oxide that have important functions in intra- and intercellular signaling. The analysis of the role of mammalian peroxisomes in a variety of physiological and pathological processes involving reactive oxygen species (ROS) is the subject of this review. The general characteristics of peroxisomes and their enzymes involved in the metabolism of ROS are briefly reviewed. An expansion of the peroxisomal compartment with proliferation of tubular peroxisomes is observed in cells exposed to UV irradiation and various oxidants and is apparently accompanied by upregulation of PEX genes. Significant reduction of peroxisomes and their enzymes is observed in inflammatory processes including infections, ischemia-reperfusion injury, and allograft rejection and seems to be related to the suppressive effect of tumor necrosis factor-alpha on peroxisome function and peroxisome proliferator activated receptor-alpha. Xenobiotic-induced proliferation of peroxisomes in rodents is accompanied by the formation of hepatic tumors, and evidently the imbalance in generation and decomposition of ROS plays an important role in this process. In PEX5-/- knockout mice lacking functional peroxisomes severe alterations of mitochondria in various organs are observed which seem to be due to a generalized increase in oxidative stress confirming the important role of peroxisomes in homeostasis of ROS and the implications of its disturbances for cell pathology.

Functional analysis of the Mad1-mSin3A repressor-corepressor interaction reveals determinants of specificity, affinity, and transcriptional response

The recruitment of corepressors by DNA-bound repressors is likely to be a critical rate-limiting step in the transcriptional regulation of many genes. An excellent paradigm for such an interaction is the association of the basic helix-loop-helix zipper protein Mad1 with the corepressor mSin3A. When bound together, the Sin3 interaction domain (SID) of Mad1 forms extensive hydrophobic contacts with the four-helix bundle formed by the paired amphipathic helix 2 (PAH2) domain of mSin3A. Using the costructure to predict the principle residues required for binding, we have carried out an extensive mutational analysis to examine the Mad1 SID-mSin3A PAH2 interaction in vitro and in vivo. Bulky hydrophobic residues in the alpha1 (I308 and V311) and alpha2 (L329 and L332) helices of the PAH2 domain are necessary to accommodate the precise arrangement of bulky (L12) and short (A15 and A16) hydrophobic residues in the amphipathic Mad1 SID. We have also used phage display to derive an optimal SID, which shows an essentially identical arrangement of key residues. By manipulating these key residues, we have generated altered-specificity Mad1 SID mutants that bind only to a PAH2 domain with a reciprocal mutation, permitting us to demonstrate for the first time that these domains interact directly in vivo. We have also found that the integrity of the PAH1 domain affects the Mad1 SID-PAH2 interaction. It is conceivable that cross talk between different PAH domains and their binding partners helps to determine the subunit composition and order of assembly of mSin3A complexes.

Microarray gene expression profiles in dilated and hypertrophic cardiomyopathic end-stage heart failure

Despite similar clinical endpoints, heart failure resulting from dilated cardiomyopathy (DCM) or hypertrophic cardiomyopathy (HCM) appears to develop through different remodeling and molecular pathways. Current understanding of heart failure has been facilitated by microarray technology. We constructed an in-house spotted cDNA microarray using 10,272 unique clones from various cardiovascular cDNA libraries sequenced and annotated in our laboratory. RNA samples were obtained from left ventricular tissues of precardiac transplantation DCM and HCM patients and were hybridized against normal adult heart reference RNA. After filtering, differentially expressed genes were determined using novel analyzing software. We demonstrated that normalization for cDNA microarray data is slide-dependent and nonlinear. The feasibility of this model was validated by quantitative real-time reverse transcription-PCR, and the accuracy rate depended on the fold change and statistical significance level. Our results showed that 192 genes were highly expressed in both DCM and HCM (e.g., atrial natriuretic peptide, CD59, decorin, elongation factor 2, and heat shock protein 90), and 51 genes were downregulated in both conditions (e.g., elastin, sarcoplasmic/endoplasmic reticulum Ca2+-ATPase). We also identified several genes differentially expressed between DCM and HCM (e.g., alphaB-crystallin, antagonizer of myc transcriptional activity, beta-dystrobrevin, calsequestrin, lipocortin, and lumican). Microarray technology provides us with a genomic approach to explore the genetic markers and molecular mechanisms leading to heart failure.

Peptide-based targeting of fluorophores to organelles in living cells

Peptides carrying organelle-specific import or retention sequences can target the fluorophore BODIPY(581/591) to the nucleus, peroxisomes, endoplasmic reticulum (ER), or the trans-Golgi network (TGN). The peroxisomal peptide contains the PTS1 sequence AKL. For targeting to the ER or TGN, the peptides carry the retention sequences KDEL and SDYQRL, respectively. A peptide carrying the nuclear leader sequence of the simian virus SV40 large tumor antigen, KKKRK, was used to direct the fluorophore to the nucleus. The fluorescent peptides for peroxisomes, ER, and the TGN spontaneously incorporate into living fibroblasts at 37 degrees C and accumulate in their target organelles within minutes. The uptake is still significant at 4 degrees C, indicating that endocytosis is not required for internalization. The highly charged nuclear peptide (net charge +4) does not spontaneously internalize. However, by transient permeabilization of the plasma membrane, this fluorescent peptide was found to rapidly accumulate in the nucleus. These fluorescent peptides open new opportunities to follow various aspects of specific organelles such as their morphology, biogenesis, dynamics, degradation, and their internal parameters (pH, redox).

Targeted fluorescent probes in peroxisome function

Fluorescent peptides form a new generation of analytical tools for visualizing intracellular processes and molecular interactions at the level of single cells. The peptide-based reporters combine the sensitivity of fluorescence detection with the information specificity of amino acid sequences. Recently we have succeeded in targeting a fluorescent heptapeptide (acetyl-CKGGAKL) carrying a peroxisomal targeting signal (PTS1) to peroxisomes in intact cells. The fluorophores conjugated to the PTS1-peptide were fluorescein, BODIPY and the pH-sensitive SNAFL-2. When added to cells, these fluorescent peptides were internalized at 37 degrees C and typically visible in the cell after 15 min or less. Cells lacking an active peroxisomal protein import system, as in the case of Zellweger syndrome, were stained diffusely throughout the cell. Uptake of the peptide probes was not inhibited at 4 degrees C or when the cells were depleted of ATP. Under these conditions translocation to peroxisomes was blocked. This indicates that the uptake by cells is diffusion-driven and not an active process. Using the SNAFL-2-PTS1 peptide, we established by ratio-imaging that peroxisomes of human fibroblasts have an internal pH of 8.2. The concurrent pH gradient over the peroxisomal membrane was dissipated when an ionophore (CCCP) was added. In fibroblasts of chondrodysplasia punctata patients with defects in the peroxisomal import of proteins carrying a PTS2 sequence, import of the PTS1-peptide probe into peroxisomes appeared normal, but these peroxisomes have a pH of 6.8 equal to that of the cytosol. Coupling different fluorophores to the PTS1-peptide offers the possibility of determining in time and space as to how peroxisomes function in living cells.