Clathrin assembly lymphoid myeloid leukemia (CALM) protein: localization in endocytic-coated pits, interactions with clathrin, and the impact of overexpression on clathrin-mediated traffic

Low-density lipoprotein receptor-related protein-1: a serial clearance homeostatic mechanism controlling Alzheimer's amyloid β-peptide elimination from the brain

Low-density lipoprotein receptor-related protein-1 (LRP1), a member of the low-density lipoprotein receptor family, has major roles in the cellular transport of cholesterol, endocytosis of 40 structurally diverse ligands, transcytosis of ligands across the blood-brain barrier, and transmembrane and nuclear signaling. Recent evidence indicates that LRP1 regulates brain and systemic clearance of Alzheimer's disease (AD) amyloid β-peptide (Aβ). According to the two-hit vascular hypothesis for AD, vascular damage precedes cerebrovascular and brain Aβ accumulation (hit 1) which then further amplifies neurovascular dysfunction (hit 2) preceding neurodegeneration. In this study, we discuss the roles of LRP1 during the hit 1 and hit 2 stage of AD pathogenesis and describe a three-level serial LRP1-dependent homeostatic control of Aβ clearance including (i) cell-surface LRP1 at the blood-brain barrier and cerebrovascular cells mediating brain-to-blood Aβ clearance (ii) circulating LRP1 providing a key endogenous peripheral 'sink' activity for plasma Aβ which prevents free Aβ access to the brain, and (iii) LRP1 in the liver mediating systemic Aβ clearance. Pitfalls in experimental Aβ brain clearance measurements with the concurrent use of peptides/proteins such as receptor-associated protein and aprotinin are also discussed. We suggest that LRP1 has a critical role in AD pathogenesis and is an important therapeutic target in AD.

The genetics of Alzheimer disease: back to the future

Three decades of genetic research in Alzheimer disease (AD) have substantially broadened our understanding of the pathogenetic mechanisms leading to neurodegeneration and dementia. Positional cloning led to the identification of rare, disease-causing mutations in APP, PSEN1, and PSEN2 causing early-onset familial AD, followed by the discovery of APOE as the single most important risk factor for late-onset AD. Recent genome-wide association approaches have delivered several additional AD susceptibility loci that are common in the general population, but exert only very small risk effects. As a result, a large proportion of the heritability of AD continues to remain unexplained by the currently known disease genes. It seems likely that much of this "missing heritability" may be accounted for by rare sequence variants, which, owing to recent advances in high-throughput sequencing technologies, can now be assessed in unprecedented detail.

Clathrin assembly proteins AP180 and CALM in the embryonic rat brain

Clathrin-coated vesicles are known to play diverse and pivotal roles in cells. The proper formation of clathrin-coated vesicles is dependent on, and highly regulated by, a large number of clathrin assembly proteins. These assembly proteins likely determine the functional specificity of clathrin-coated vesicles, and together they control a multitude of intracellular trafficking pathways, including those involved in embryonic development. In this study, we focus on two closely related clathrin assembly proteins, AP180 and CALM (clathrin assembly lymphoid myeloid leukemia protein), in the developing embryonic rat brain. We find that AP180 begins to be expressed at embryonic day 14 (E14), but only in postmitotic cells that have acquired a neuronal fate. CALM, on the other hand, is expressed as early as E12, by both neural stem cells and postmitotic neurons. In vitro loss-of-function studies using RNA interference (RNAi) indicate that AP180 and CALM are dispensable for some aspects of embryonic neurogenesis but are required for the growth of postmitotic neurons. These results identify the developmental stage of AP180 and CALM expression and suggest that each protein has distinct functions in neural development.

Neuronal activity and the expression of clathrin-assembly protein AP180

The clathrin-assembly protein AP180 is known to promote the assembly of clathrin-coated vesicles in the neuron. However, it is unknown whether the expression of AP180 is influenced by neuronal activity. In this study, we report that chronic depolarization results in a reduction of AP180 from hippocampal neurons, while acute depolarization causes a dispersed synaptic distribution of AP180. Activity-induced effects are observed only for AP180, but not for the structurally-related clathrin-assembly proteins CALM, epsin1, or HIP1. These findings suggest that AP180 levels and synaptic distribution are highly sensitive to neuronal activity.

Distribution and expression of picalm in Alzheimer disease

PICALM, the gene encoding phosphatidylinositol-binding clathrin assembly (picalm) protein, was recently shown to be associated with risk of Alzheimer disease (AD). Picalm is a key component of clathrin-mediated endocytosis. It recruits clathrin and adaptor protein 2 (AP-2) to the plasma membrane and, along with, AP-2 recognizes target proteins. The attached clathrin triskelions cause membrane deformation around the target proteins enclosing them within clathrin-coated vesicles to be processed in lysosomes or endosomes. We examined the distribution of picalm in control and AD brain tissue and measured levels of picalm messenger RNA (mRNA) by real-time polymerase chain reaction. Immunolabeling of brain tissue showed that picalm is predominately present in endothelial cells. This was further supported by the demonstration of picalm in human cerebral microvascular cells grown in culture. Picalm mRNA was elevated in relation to glyceraldehyde-3-phosphate dehydrogenase but not factor VIII-related antigen or CD31 mRNA in the frontal cortex in AD. No change was seen in the temporal cortex or thalamus. The transport of Aβ across vessel walls and into the bloodstream is a major pathway of Aβ removal from the brain and picalm is ideally situated within endothelial cells to participate in this process. Further research is needed to determine whether PICALM expression is influenced by Aβ levels and whether it affects Aβ uptake and transport by endothelial cells.

APP, APOE, complement receptor 1, clusterin and PICALM and their involvement in the herpes simplex life cycle

The major Alzheimer's disease susceptibility genes (APOE, clusterin, complement receptor 1 (CR1) and phosphatidylinositol binding clathrin assembly protein, PICALM) can be implicated directly (APOE, CR1) or indirectly (clusterin and PICALM) in the herpes simplex life cycle. The virus binds to proteoliposomes containing APOE or APOA1 and also to CR1, and both clusterin and PICALM are related to a mannose-6-phosphate receptor used by the virus for cellular entry and intracellular transport. PICALM also binds to a nuclear exportin used by the virus for nuclear egress. Clusterin and complement receptor 1 are both related to the complement pathways and play a general role in pathogen defence. In addition, the amyloid precursor protein APP is involved in herpes viral transport and gamma-secretase cleaves a number of receptors used by the virus for cellular entry. APOE, APOA1 and clusterin, or alpha 2-macroglobulin, insulysin and caspase 3, which also bind to the virus, are involved in beta-amyloid clearance or degradation, as are the viral binding complement components, C3 and CR1. There are multiple ways in which the products of key susceptibility genes might be able to modify the viral life cycle and in turn the virus interacts with key proteins involved in APP and beta-amyloid processing. These interactions support a role for the herpes simplex virus in Alzheimer's disease pathology and suggest that antiviral agents or vaccination might be considered as viable therapeutic strategies in Alzheimer's disease.

Secreted PLA2 induces proliferation in astrocytoma through the EGF receptor: another inflammation-cancer link

We have investigated mechanisms that contribute to reinforce the relationship between inflammation and cancer. Secreted phospholipase A(2) group IIA (sPLA(2)-IIA) is a molecule relevant in inflammatory events and has been proposed as a marker for some of these. Previously, we reported the mitogenic properties of this sPLA(2) in the human astrocytoma cell line 1321N1. Here, we go deeper into the mechanisms that link this inflammatory protein with proliferation in one of the most aggressive types of tumors. We found that phosphorylation of the extracellular regulated kinase (ERK) was preceded by the activation of the small GTPase Ras, and both failed to be activated by inhibiting protein kinase C (PKC). Fractionation and immunofluorescence studies revealed translocation of PKC alpha, delta, and epsilon to the membrane fraction upon stimulation with sPLA(2)-IIA. Immunoprecipitation analysis showed that sPLA(2)-IIA induces phosphorylation of the epidermal growth factor receptor (EGFR) through a PKC-dependent pathway. We found that phosphorylation of this receptor contributed to Ras and ERK activation and that inhibition of ERK, PKC, and EGFR blocked the mitogenic response induced by sPLA(2)-IIA. This study showed that sPLA(2)-IIA is able to bring into play EGFR to trigger its signaling and that PKC leads the distribution of resources. Interestingly, we found that this is not a cell-specific response, because sPLA(2)-IIA was also able to transactivate EGFR in MCF7 human breast cancer cells. Therefore, this mechanism could contribute to worsen the prognosis of a tumor in an inflammatory microenvironment. We also present more links of the tumor chain possibly susceptible to targeting.

Effect of high intratesticular estrogen on global gene expression and testicular cell number in rats

BACKGROUND

The identification of estrogen receptors alpha and beta and aromatase in the testis has highlighted the important role of estrogens in regulating spermatogenesis. There is a wealth of information on the deleterious effects of fetal and neonatal exposure of estrogens and xenoestrogens in the testis, including spermiation failure and germ cell apoptosis. However, very little is known about gene transcripts affected by exogenous estradiol exposure in the testis. The objective of the present study was to unveil global gene expression profiles and testicular cell number changes in rats after estradiol treatment.

METHODS

17beta-estradiol was administered to adult male rats at a dose of 100 micrograms/kg body weight in saline daily for 10 days; male rats receiving only saline were used as controls. Microarray analysis was performed to examine global gene expression profiles with or without estradiol treatment. Real time RT-PCR was conducted to verify the microarray data. In silico promoter and estrogen responsive elements (EREs) analysis was carried out for the differentially expressed genes in response to estradiol. Quantitation of testicular cell number based on ploidy was also performed using flow cytometry in rats with or without estradiol treatment.

RESULTS

We found that 221 genes and expressed sequence tags (ESTs) were differentially expressed in rat testes treated with estradiol compared to the control; the microarray data were confirmed by real time RT-PCR. Gene Ontology analysis revealed that a number of the differentially expressed genes are involved in androgen and xenobiotic metabolism, maintenance of cell cytoskeleton, endocytosis, and germ cell apoptosis. A total of 33 up-regulated genes and 67 down-regulated genes showed the presence of EREs. Flow cytometry showed that estradiol induced a significant decrease in 2n cells (somatic and germ cells) and 4n cells (pachytene spermatocytes) and a marked increase in the number of elongated and elongating spermatids.

CONCLUSIONS

This study provides a novel insight into the molecular basis for spermiation failure and apoptosis caused by 17beta-estradiol and it also offers new mechanisms by which adult exposure to environmental estrogens can affect spermatogenesis and fertility.

Replication of CLU, CR1, and PICALM associations with alzheimer disease

OBJECTIVE

To test for replication of the association between variants in the CLU, CR1, and PICALM genes with Alzheimer disease.

DESIGN

Follow-up case-control association study.

SETTING

The Mayo Clinics at Jacksonville, Florida, and Rochester, Minnesota.

PARTICIPANTS

Community-based patients of European descent with late-onset Alzheimer disease (LOAD) and controls without dementia who were seen at the Mayo clinics, and autopsy-confirmed cases and controls whose pathology was evaluated at the Mayo Clinic in Jacksonville. Additional samples were obtained from the National Cell Repository for Alzheimer Disease (NCRAD). A total of 1829 LOAD cases and 2576 controls were analyzed.

INTERVENTIONS

The most significant single-nucleotide polymorphisms in CLU (rs11136000), CR1 (rs3818361), and PICALM (rs3851179) were tested for allelic association with LOAD. Main Outcome Measure Clinical or pathology-confirmed diagnosis of LOAD.

RESULTS

Odds ratios for CLU, CR1, and PICALM were 0.82, 1.15, and 0.80, respectively, comparable in direction and magnitude with those originally reported. P values were 8.6 x 10(-5), .014, and 1.3 x 10(-5), respectively; they remain significant even after Bonferroni correction for the 3 single-nucleotide polymorphisms tested.

CONCLUSION

These results show near-perfect replication and provide the first additional evidence that CLU, CR1, and PICALM are associated with the risk of LOAD.

Recruitment of Uev1B to Hrs-containing endosomes and its effect on endosomal trafficking

Endocytosis of signaling receptors, such as epidermal growth factor receptor (EGFR), tightly controls the signal transduction process triggered by ligand activation of these receptors. To identify new regulators of the endocytic trafficking of EGFR an RNA interference screen was performed for genes involved in ubiquitin conjugation and down-regulation of EGFR. The screen revealed that small interfering RNAs (siRNAs) that target the conserved ubiquitin-binding domain Uev1 increased down-regulation of EGFR. Since these siRNAs simultaneously targeted multiple genes containing a Uev1 domain, we analyzed the role of these gene products by overexpressing individual Uev1-related proteins. This analysis revealed that overexpression of Uev1A (UBE2V1) has no effect on the degradation of EGFR:EGF complexes. In contrast, overexpression of Uev1B (TMEM189-UBE2V1 isoform 2) slowed the degradation of EGF:receptor complexes. The Uev1B protein was found to strongly colocalize and associate with ubiquitin and Hrs in endosomes. Moreover, overexpression of Uev1B abrogated the ability of Hrs to colocalize with EGFR. The B-domain of Uev1B, and not the UEV-domain, was mainly responsible for the observed phenotypes suggesting the presence of a novel endosomal targeting sequence within the B-domain. Together, the data show that elevated levels of Uev1B protein in cells lead to decreased efficiency of endosomal sorting by associating with ubiquitinated proteins and Hrs.

The pursuit of susceptibility genes for Alzheimer's disease: progress and prospects

The recent discoveries in genome-wide association studies (GWAS) of novel susceptibility loci (CLU, CR1 and PICALM) for Alzheimer's disease (AD) have elicited considerable interest in the AD community. But what are the implications of these purely epidemiological findings for our understanding of disease etiology and patient care? In this review, we attempt to place these findings in the context of current and future AD genetics research. CLU, CR1 and PICALM support existing hypotheses about the amyloid, lipid, chaperone and chronic inflammatory pathways in AD pathogenesis. We discuss how these and future findings can be translated into efforts to ameliorate patient care by genetic profiling for risk prediction and pharmacogenetics and by guiding drug development.

Genome-wide association study identifies variants at CLU and PICALM associated with Alzheimer's disease

We undertook a two-stage genome-wide association study of Alzheimer's disease involving over 16,000 individuals. In stage 1 (3,941 cases and 7,848 controls), we replicated the established association with the APOE locus (most significant SNP: rs2075650, p= 1.8×10⁻¹⁵⁷) and observed genome-wide significant association with SNPs at two novel loci: rs11136000 in the CLU or APOJ gene (p= 1.4×10⁻⁹) and rs3851179, a SNP 5′ to the PICALM gene (p= 1.9×10⁻⁸). Both novel associations were supported in stage 2 (2,023 cases and 2,340 controls), producing compelling evidence for association with AD in the combined dataset (rs11136000: p= 8.5×10⁻¹⁰, odds ratio= 0.86; rs3851179: p= 1.3×10⁻⁹, odds ratio= 0.86). We also observed more variants associated at p< 1×10⁻⁵ than expected by chance (p=7.5×10⁻⁶), including polymorphisms at the BIN1, DAB1 and CR1 loci.

The clathrin assembly protein AP180 regulates the generation of amyloid-beta peptide

The overproduction and extracellular buildup of amyloid-beta peptide (Abeta) is a critical step in the etiology of Alzheimer's disease. Recent data suggest that intracellular trafficking is of central importance in the production of Abeta. Here we use a neuronal cell line to examine two structurally similar clathrin assembly proteins, AP180 and CALM. We show that RNA interference-mediated knockdown of AP180 reduces the generation of Abeta1-40 and Abeta1-42, whereas CALM knockdown has no effect on Abeta generation. Thus AP180 is among the traffic controllers that oversee and regulate amyloid precursor protein processing pathways. Our results also suggest that AP180 and CALM, while similar in their domain structures and biochemical properties, are in fact dedicated to separate trafficking pathways in neurons.

Global reduction of the epigenetic H3K79 methylation mark and increased chromosomal instability in CALM-AF10-positive leukemias

Chromosomal translocations generating fusion proteins are frequently found in human leukemias. The fusion proteins play an important role in leukemogenesis by subverting the function of one or both partner proteins. The leukemogenic CALM-AF10 fusion protein is capable of interacting with the histone H3 lysine 79 (H3K79)-specific methyltransferase hDOT1L through the fused AF10 moiety. This interaction leads to local H3K79 hypermethylation on Hoxa5 loci, which up-regulates the expression of Hoxa5 and contributes to leukemogenesis. However, the long latency of leukemogenesis of CALM-AF10 transgenic mice suggests that the direct effects of fusion oncogene are not sufficient for the induction of leukemia. In this study, we show that the CALM-AF10 fusion protein can also greatly reduce global H3K79 methylation in both human and murine leukemic cells by disrupting the AF10-mediated association of hDOT1L with chromatin. Cells with reduced H3K79 methylation are more sensitive to gamma-irradiation and display increased chromosomal instability. Consistently, leukemia patients harboring CALM-AF10 fusion have more secondary chromosomal aberrations. These findings suggest that chromosomal instability associated with global epigenetic alteration contributes to malignant transformation in certain leukemias, and that leukemias with this type of epigenetic alteration might benefit from treatment regimens containing DNA-damaging agents. This study is registered with www.clinicaltrials.gov as NCT00266136.

Clathrin assembly protein AP180 and CALM differentially control axogenesis and dendrite outgrowth in embryonic hippocampal neurons

Emerging data suggest that, much like epithelial cells, the polarized growth of neurons requires both the secretory and endocytic pathways. The clathrin assembly proteins AP180 and CALM (clathrin assembly lymphoid myeloid protein) are known to be involved in clathrin-mediated endocytosis, but their roles in mammalian neurons and, in particular, in developmental processes before synaptogenesis are unknown. Here we provide evidence that AP180 and CALM play critical roles in establishing the polarity and controlling the growth of axons and dendrites in embryonic hippocampal neurons. Knockdown of AP180 primarily impairs axonal development, whereas reducing CALM levels results in dendritic dystrophy. Conversely, neurons that overexpress AP180 or CALM generate multiple axons. Ultrastructural analysis shows that CALM affiliates with a wider range of intracellular trafficking organelles than does AP180. Functional analysis shows that endocytosis is reduced in both AP180-deficient and CALM-deficient neurons. Additionally, CALM-deficient neurons show disrupted secretory transport. Our data demonstrate previously unknown functions for AP180 and CALM in intracellular trafficking that are essential in the growth of neurons.

The CALM and CALM/AF10 interactor CATS is a marker for proliferation

The CATS protein was recently identified as a novel CALM interacting protein. CATS increases the nuclear and specifically the nucleolar localization of the leukemogenic CALM/AF10 fusion protein. We cloned and characterized the murine Cats gene. Detailed analysis of murine Cats expression during mouse embryogenesis showed an association with rapidly proliferating tissues. Interestingly, the Cats transcript is highly expressed in murine hematopoietic cells transformed by CALM/AF10. The CATS protein is highly expressed in leukemia, lymphoma and tumor cell lines but not in non-proliferating T-cells or human peripheral blood lymphocytes. CATS protein levels are cell cycle dependent and it is induced by mitogens, suggesting a role of CATS in the control of cell proliferation and possibly CALM/AF10-mediated leukemogenesis.

Interaction between Epsin/Yap180 adaptors and the scaffolds Ede1/Pan1 is required for endocytosis

The spatial and temporal regulation of the interactions among the approximately 60 proteins required for endocytosis is under active investigation in many laboratories. We have identified the interaction between monomeric clathrin adaptors and endocytic scaffold proteins as a critical prerequisite for the recruitment and/or spatiotemporal dynamics of endocytic proteins at early and late stages of internalization. Quadruple deletion yeast cells (DeltaDeltaDeltaDelta) lacking four putative adaptors, Ent1/2 and Yap1801/2 (homologues of epsin and AP180/CALM proteins), with a plasmid encoding Ent1 or Yap1802 mutants, have defects in endocytosis and growth at 37 degrees C. Live-cell imaging revealed that the dynamics of the early- and late-acting scaffold proteins Ede1 and Pan1, respectively, depend upon adaptor interactions mediated by adaptor asparagine-proline-phenylalanine motifs binding to scaffold Eps15 homology domains. These results suggest that adaptor/scaffold interactions regulate transitions from early to late events and that clathrin adaptor/scaffold protein interaction is essential for clathrin-mediated endocytosis.

Lymphocytic choriomeningitis virus uses a novel endocytic pathway for infectious entry via late endosomes

The endocytic entry of lymphocytic choriomeningitis virus (LCMV) into host cells was compared to the entry of viruses known to exploit clathrin or caveolae/raft-dependent pathways. Pharmacological inhibitors, expression of pathway-specific dominant-negative constructs, and siRNA silencing of clathrin together with electron and light microscopy provided evidence that although a minority population followed a classical clathrin-mediated mechanism of entry, the majority of these enveloped RNA viruses used a novel endocytic route to late endosomes. The pathway was clathrin, dynamin-2, actin, Arf6, flotillin-1, caveolae, and lipid raft independent but required membrane cholesterol. Unaffected by perturbation of Rab5 or Rab7 and apparently without passing through Rab5/EEA1-positive early endosomes, the viruses reached late endosomes and underwent acid-induced penetration. This membrane trafficking route between the plasma membrane and late endosomes may function in the turnover of a select group of surface glycoproteins such as the dystroglycan complex, which serves as the receptor of LCMV.

A functional GFP fusion for imaging clathrin-mediated endocytosis

The ability to localize proteins of interest in live cells through imaging inherently fluorescent protein tags has provided an unprecedented level of information on cellular organization. However, there are numerous cases where fluorescent tags alter the localization and/or function of the proteins to which they are appended. Clathrin-mediated endocytosis from the plasma membrane is a physiologically important process evolutionarily conserved from yeast to humans. Some proteins that are associated with the machinery of clathrin-mediated endocytosis have been tagged with fluorescent proteins. However, it has not yet been possible to study this process through a protein marker that is specific to this step and still fully functional when linked to a fluorescent protein. In this study, we present the first demonstration that one of these proteins, in this case a green fluorescent protein (GFP) fusion to alpha-adaptin, a marker of the adaptor protein-2 complex, functionally complements knockdown of endogenous protein through small interfering RNA silencing. GFP-alpha-adaptin, as well as the techniques used to test the fusion protein, represents an important contribution to the cell biologist's toolbox, which will permit a greater understanding of vesicle trafficking in live cells.

The role of CALM-AF10 gene fusion in acute leukemia

Chromosomal translocations are important genetic perturbations frequently associated with hematologic malignancies; characterization of these events has been a rich source of insights into the mechanisms that lead to malignant transformation. The t(10;11)(p13;q14-21) results in a recently identified rare but recurring chromosomal translocation seen in patients with ALL as well as AML, and results in the production of a CALM-AF10 fusion gene. Although the details by which the CALM-AF10 fusion protein exerts its leukemogenic effect remain unclear, emerging data suggests that the CALM-AF10 fusion impairs differentiation of hematopoietic cells, at least in part via an upregulation of HOXA cluster genes. This review discusses the normal structure and function of CALM and AF10, describes the spectrum of clinical findings seen in patients with CALM-AF10 fusions, summarizes recently published CALM-AF10 mouse models and highlights the role of HOXA cluster gene activation in CALM-AF10 leukemia.

The leukemogenic CALM/AF10 fusion protein alters the subcellular localization of the lymphoid regulator Ikaros

The t(10;11)(p13;q14) translocation leads to the fusion of the CALM and AF10 genes. This translocation can be found as the sole cytogenetic abnormality in acute lymphoblastic leukemia, acute myeloid leukemia and in malignant lymphomas. The expression of CALM/AF10 in primary murine bone marrow cells results in the development of an aggressive leukemia in a murine bone marrow transplantation model. Using a yeast two-hybrid screen, we identified the lymphoid regulator Ikaros as an AF10 interacting protein. Interestingly, Ikaros is required for normal development of lymphocytes, and aberrant expression of Ikaros has been found in leukemia. In a murine model, the expression of a dominant negative isoform of Ikaros causes leukemias and lymphomas. The Ikaros interaction domain of AF10 was mapped to the leucine zipper domain of AF10, which is required for malignant transformation both by the CALM/AF10 and the MLL/AF10 fusion proteins. The interaction between AF10 and Ikaros was confirmed by GST pull down and co-immunoprecipitation. Coexpression of CALM/AF10 but not of AF10 alters the subcellular localization of Ikaros in murine fibroblasts. The transcriptional repressor activity of Ikaros is reduced by AF10. These results suggest that CALM/AF10 might interfere with normal Ikaros function, and thereby block lymphoid differentiation in CALM/AF10 positive leukemias.

Intoxication of epithelial cells by plasmid-encoded toxin requires clathrin-mediated endocytosis

It has been shown that the autotransporter plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli (EAEC) produces cytotoxic and enterotoxic effects. Both effects can be explained by the proteolytic activity of Pet on its intracellular target alpha-fodrin (alphaII spectrin). In addition, Pet cytotoxicity and enterotoxicity depend on Pet serine protease activity, and on its internalization into epithelial cells. However, the mechanisms of Pet uptake by epithelial cells are unknown. Here, we show that Pet interacts with the plasma membrane of epithelial cells, and afterwards is detected inside the cells. Furthermore, Pet was internalized via clathrin-mediated endocytosis, since its internalization was inhibited by monodansylcadaverine and sucrose, but not by filipin or methyl-beta-cyclodextrin, which are drugs that interfere with protein entry via a clathrin-independent pathway. Additionally, Pet was immunoprecipitated by anti-clathrin antibodies, but not by anti-caveolin antibodies. Moreover, small interfering RNA (siRNA), designed to knock out clathrin gene expression in HEp-2 cells, prevented Pet internalization, and thereby the Pet-induced cytotoxic effect. However, the use of siRNA to knock out caveolin expression had no effect on Pet internalization, and the cytotoxic effect was clearly observed. Together, these data indicate that Pet secreted by EAEC binds to the cell surface via an unknown receptor, to be taken up by clathrin-mediated endocytosis, and exert its toxic effect in the cytoplasm.

AP180 and CALM in the developing hippocampus: expression at the nascent synapse and localization to trafficking organelles

Genetic and biochemical evidence has established that clathrin assembly protein AP180 is required for the proper assembly of synaptic vesicles via clathrin-mediated endocytosis. The assembly protein CALM, the ubiquitously expressed homolog of AP180, also regulates the formation of clathrin-coated vesicles. In this study we found high expression levels of AP180 and CALM in hippocampal tissues as early as embryonic day 18, before the expression of synaptophysin. We also used immunoelectron microscopy to establish the distribution of AP180 and CALM in the developing hippocampal synapses. We found AP180 and CALM in synapses at all developmental stages and in nonsynaptic growing processes. In addition to localization on the plasma membrane and clathrin-coated vesicles that originated from the plasma membrane, we also report the presence of AP180 and CALM on other types of membrane structures. Our observations link AP180 and CALM to multiple vesicular organelles and raise the possibility that these proteins may play additional roles in developing neurons.

Expression of a CALM-AF10 fusion gene leads to Hoxa cluster overexpression and acute leukemia in transgenic mice

To assess the role of the CALM-AF10 fusion gene in leukemic transformation in vivo, we generated transgenic mice that expressed a CALM-AF10 fusion gene. Depending on the transgenic line, at least 40% to 50% of the F(1) generation mice developed acute leukemia at a median age of 12 months. Leukemic mice typically had enlarged spleens, invasion of parenchymal organs with malignant cells, and tumors with myeloid markers such as myeloperoxidase, Mac1, and Gr1. Although most leukemias were acute myeloid leukemia, many showed lymphoid features, such as CD3 staining, or clonal Tcrb or Igh gene rearrangements. Mice were clinically healthy for the first 9 months of life and had normal peripheral blood hemograms but showed impaired thymocyte differentiation, manifested by decreased CD4(+)/CD8(+) cells and increased immature CD4(-)/CD8(-) cells in the thymus. Hematopoietic tissues from both clinically healthy and leukemic CALM-AF10 mice showed up-regulation of Hoxa cluster genes, suggesting a potential mechanism for the impaired differentiation. The long latency period and incomplete penetrance suggest that additional genetic events are needed to complement the CALM-AF10 transgene and complete the process of leukemic transformation.

A novel AAK1 splice variant functions at multiple steps of the endocytic pathway

Phosphorylation is a critical step in regulating receptor transport through the endocytic pathway. AAK1 is a serine/threonine kinase that is thought to coordinate the recruitment of AP-2 to receptors containing tyrosine-based internalization motifs by phosphorylating the micro2 subunit. Here we have identified a long form of AAK1 (AAK1L) that contains an extended C-terminus that encodes an additional clathrin-binding domain (CBD2) consisting of multiple low-affinity interaction motifs. Protein interaction studies demonstrate that AAK1L CBD2 directly binds clathrin. However, in vitro kinase assays reveal little difference between AAK1 isoforms in their basal or clathrin-stimulated kinase activity toward the AP-2 micro2 subunit. However, overexpression of AAK1L CBD2 impairs transferrin endocytosis, confirming an endocytic role for AAK1. Surprisingly, CBD2 overexpression or AAK1 depletion by RNA interference significantly impairs transferrin recycling from the early/sorting endosome. These observations suggest that AAK1 functions at multiple steps of the endosomal pathway by regulating transferrin internalization and its rapid recycling back to the plasma membrane from early/sorting endosome.

Clathrin Adaptor GGA1 Polymerizes Clathrin into Tubules

GGAs, a class of monomeric clathrin adaptors, are involved in the sorting of cargo at the trans-Golgi network of eukaryotic cells. They are modular structures consisting of the VHS, the GAT, hinge, and GAE domains, which have been shown to interact directly with cargo, ARF, clathrin, and accessory proteins, respectively. Previous studies have shown that GGAs interact with clathrin both in solution and in the cell, but it has yet been shown whether they assemble clathrin. We find that GGA1 promoted assembly of clathrin with complete assembly achieved when one GGA1 molecule is bound per heavy chain. In the presence of excess GGA1, we obtained the unusual stoichiometry of five GGA1s per heavy chain, and even at this stoichiometry the binding was not saturated. The assembled structures were mostly baskets, but approximately 10% of the structures were tubular with an average length of 180 +/- 40 nm and width of approximately 50 nm. The truncated GGA1 fragment consisting of the hinge+GAE domains bound to clathrin with similar affinity as the full-length molecule and polymerized clathrin into baskets. Unlike the full-length molecule, this fragment saturated the lattices at one molecule per heavy chain and assembled clathrin only into baskets. The separated hinge and GAE domains bound much weaker to clathrin than the intact molecule, and these domains do not significantly polymerize clathrin into baskets. We conclude that clathrin adaptor GGA1 is a clathrin assembly protein, but it is unique in its ability to polymerize clathrin into tubules.

Clathrin-Coated Pits: Vive La Différence?

Because of the discovery of coated pits and vesicles more than 40 years ago and the identification of clathrin as a major component of the coat, it has been assumed that clathrin-coated pits (CCPs) are responsible for the uptake of most plasma membrane receptors undergoing internalization. The recent molecular characterization of clathrin-independent routes of endocytosis confirms that several alternative endocytic pathways operate at the plasma membrane of mammalian cells. This heterogeneous view of endocytosis has been expanded still further by recent studies, suggesting that different subpopulations of CCPs responsible for the internalization of specific sets of cargo may coexist. In the present review, we have discussed the experimental evidence in favor or against the existence of distinct parallel clathrin-dependent pathways at the plasma membrane.

Cell biology of membrane trafficking in human disease

Understanding the molecular and cellular mechanisms underlying membrane traffic pathways is crucial to the treatment and cure of human disease. Various human diseases caused by changes in cellular homeostasis arise through a single gene mutation(s) resulting in compromised membrane trafficking. Many pathogenic agents such as viruses, bacteria, or parasites have evolved mechanisms to subvert the host cell response to infection, or have hijacked cellular mechanisms to proliferate and ensure pathogen survival. Understanding the consequence of genetic mutations or pathogenic infection on membrane traffic has also enabled greater understanding of the interactions between organisms and the surrounding environment. This review focuses on human genetic defects and molecular mechanisms that underlie eukaryote exocytosis and endocytosis and current and future prospects for alleviation of a variety of human diseases.

Ptr ToxA interacts with a chloroplast-localized protein

Pyrenophora tritici-repentis, causal agent of tan spot of wheat, produces host-selective toxins that are determinants of pathogenicity or virulence. Ptr ToxA (ToxA), a proteinaceous toxin produced by P. tritici-repentis, is a necrotizing toxin produced by the most common races isolated from infected wheat. Recent studies have shown that ToxA is internalized into the mesophyll cells and localizes to chloroplasts of sensitive wheat cultivars only. We employed a yeast two-hybrid screen in an effort to determine plant proteins that interact with ToxA and found that ToxA interacts with a chloroplast protein, designated ToxA binding protein 1 (ToxABP1). ToxABP1 contains a lysine-rich region within a coiled-coil domain that is similar to phosphotidyl-inositol binding sites present in animal proteins involved in endocytosis. In both ToxA-sensitive and -insensitive cultivars, ToxABP1 is expressed at similar levels and encodes an identical protein. ToxABP1 protein is present in both chloroplast membranes and chloroplast stroma. ToxA appears to interact primarily with a multimeric complex of ToxABP1 protein associated with the chloroplast membrane.

Selective blocking of clathrin-mediated endocytosis by RNA interference: epsin as target protein

Epsin is an essential accessory protein exclusively implicated in clathrin-mediated endocytosis and therefore an ideal target to study involvement of this entry route in the uptake of bioligands. The technique of RNA interference (RNAi) was exploited to generate a cell line constitutively silencing epsin expression in a sequence-specific manner In these Caco-2(eps-) cells, quantitative reverse transcription PCR (RT-PCR) revealed a severe depletion of the epsin messenger RNA (mRNA) level in cells, reaching a factor > 10(6). The reduction at the mRNA level in the Caco-2(eps-) cells was paralleled by a decrease of 75% at the protein level. In order to evaluate transfection effects at the functional level, uptake of transferrin and epidermal growth factor (EGF) in transfected Caco-2(eps-) and control cells was evaluated. In control cells, respectively, approximately 72% of transferrin and approximately 66% of EGF were internalized, whereas in Caco-2(eps-) cells only approximately 25% of transferrin and approximately 34% of EGF was taken up, confirming that in the transfected cells, endocytosis via coated pits was prominently compromised. The reduced uptake was not the result of an inhibition of transferrin recycling. The effects of direct treatment with chlorpromazine on Caco-2 cells, also monitored from the degree of transferrin internalization, were compared with those elicited by RNAi.

Beta-adaptin: key molecule for microglial scavenger receptor function under oxidative stress

Scavenger receptors internalize chemically modified low density lipoprotein particles (ac-LDL) and other ligands through the process of receptor-mediated endocytosis. During this investigation using amyloid-beta as a natural ligand for the SR, we studied under a ligand-induced oxidative stress condition, changes in protein expression of several adaptor proteins important in the organization of the endocytic machinery in microglia and macrophages. Differential expression experiments of beta-adaptin, alpha-adaptin, SR-AI, and SR-BI in RAW (macrophages) and EOC (microglia) cells were performed according to dosage and exposure time to amyloid-beta. Our results show that according to dosage, amyloid-beta produces an oxidative stress state that importantly affects the availability of beta-adaptin. Under these conditions, RT-PCR assays show that beta-adaptin mRNA is normally synthesized, reason why protein translation or protein structure of beta-adaptin might be altered. These observations might have impact in the understanding of the mechanisms microglia employ to process amyloid-beta in the brain.

The monomeric clathrin assembly protein, AP180, regulates contractile vacuole size in Dictyostelium discoideum

AP180, one of many assembly proteins and adaptors for clathrin, stimulates the assembly of clathrin lattices on membranes, but its unique contribution to clathrin function remains elusive. In this study we identified the Dictyostelium discoideum ortholog of the adaptor protein AP180 and characterized a mutant strain carrying a deletion in this gene. Imaging GFP-labeled AP180 showed that it localized to punctae at the plasma membrane, the contractile vacuole, and the cytoplasm and associated with clathrin. AP180 null cells did not display defects characteristic of clathrin mutants and continued to localize clathrin punctae on their plasma membrane and within the cytoplasm. However, like clathrin mutants, AP180 mutants, were osmosensitive. When immersed in water, AP180 null cells formed abnormally large contractile vacuoles. Furthermore, the cycle of expansion and contraction for contractile vacuoles in AP80 null cells was twice as long as that of wild-type cells. Taken together, our results suggest that AP180 plays a unique role as a regulator of contractile vacuole morphology and activity in Dictyostelium.

Leukaemic transformation by CALM-AF10 involves upregulation of Hoxa5 by hDOT1L

Chromosomal translocation is a common cause of leukaemia and the most common chromosome translocations found in leukaemia patients involve the mixed lineage leukaemia (MLL) gene. AF10 is one of more than 30 MLL fusion partners in leukaemia. We have recently demonstrated that the H3K79 methyltransferase hDOT1L contributes to MLL-AF10-mediated leukaemogenesis through its interaction with AF10 (ref. 5). In addition to MLL, AF10 has also been reported to fuse to CALM (clathrin-assembly protein-like lymphoid-myeloid) in patients with T-cell acute lymphoblastic leukaemia (T-ALL) and acute myeloid leukaemia (AML). Here, we analysed the molecular mechanism of leukaemogenesis by CALM-AF10. We demonstrate that CALM-AF10 fusion is both necessary and sufficient for leukaemic transformation. Additionally, we provide evidence that hDOT1L has an important role in the transformation process. hDOT1L contributes to CALM-AF10-mediated leukaemic transformation by preventing nuclear export of CALM-AF10 and by upregulating the Hoxa5 gene through H3K79 methylation. Thus, our study establishes CALM-AF10 fusion as a cause of leukaemia and reveals that mistargeting of hDOT1L and upregulation of Hoxa5 through H3K79 methylation is the underlying mechanism behind leukaemia caused by CALM-AF10 fusion.

Acute monocytic leukemia with coexpression of minor BCR-ABL1 and PICALM-MLLT10 fusion genes along with overexpression of HOXA9

The t(9;22)(q34;q11) translocation occurs in chronic myeloid leukemia (CML) and adult B-cell acute lymphoblastic leukemia (ALL), leading to fusion of BCR to ABL1 and constitutive activation of ABL1 tyrosine kinase activity. The main BCR-ABL1 breakpoints result in P190 BCR-ABL1 or P210 BCR-ABL1 fusion proteins. The latter is found in almost all cases of CML and in one third of the cases of t(9;22)-positive adult B-ALL. P190 BCR-ABL1 is found in the remaining two thirds of t(9;22)-positive adult B-ALL cases but only exceptionally in CML. We describe here the first case of t(9;22)(q34;q11) associated with t(10;11)(p13;q14) in acute monocytic leukemia. The recurrent t(10;11)(p13;q14) translocation, usually found in acute myeloid leukemia (AML) and T-ALL, merges PICALM to MLLT10. RT-PCR enabled identification of PICALM-MLLT10 and BCR-ABL1 e1-a2 fusion transcripts; in the context of chronic and acute myeloid leukemia, the latter usually has a monocytic presentation. We also identified overexpression of HOXA9, a gene essential to myeloid differentiation that is expressed in PICALM-MLLT10 and MLL-rearranged acute leukemias. This case fits with and extends a recently proposed multistage AML model in which constitutive activation of tyrosine kinases by mutations (BCR-ABL1) are associated with deregulation of transcription factors central to myeloid differentiation (HOXA9 secondary to PICALM-MLLT10).

Dynamic regulation of mammalian numb by G protein-coupled receptors and protein kinase C activation: Structural determinants of numb association with the cortical membrane

The cell fate determinant Numb is a membrane-associated adaptor protein involved in both development and intracellular vesicular trafficking. It has a phosphotyrosine-binding (PTB) domain and COOH-terminal endocytic-binding motifs for alpha-adaptin and Eps15 homology domain-containing proteins. Four isoforms of Numb are expressed in vertebrates, two of which selectively associate with the cortical membrane. In this study, we have characterized a cortical pool of Numb that colocalizes with AP2 and Eps15 at substratum plasma membrane punctae and cortical membrane-associated vesicles. Green fluorescent protein (GFP)-tagged mutants of Numb were used to identify the structural determinants required for localization. In addition to the previously described association of the PTB domain with the plasma membrane, we show that the AP2-binding motifs facilitate the association of Numb with cortical membrane punctae and vesicles. We also show that agonist stimulation of G protein-coupled receptors (GPCRs) that are linked to phospholipase Cbeta and protein kinase C (PKC) activation causes redistribution of Numb from the cortical membrane to the cytosol. This effect is correlated with Numb phosphorylation and an increase in its Triton X-100 solubility. Live-imaging analysis of mutants identified two regions within Numb that are independently responsive to GPCR-mediated lipid hydrolysis and PKC activation: the PTB domain and a region encompassing at least three putative PKC phosphorylation sites. Our data indicate that membrane localization of Numb is dynamically regulated by GPCR-activated phospholipid hydrolysis and PKC-dependent phosphorylation events.

Components of the antigen processing and presentation pathway revealed by gene expression microarray analysis following B cell antigen receptor (BCR) stimulation

Background

Activation of naïve B lymphocytes by extracellular ligands, e.g. antigen, lipopolysaccharide (LPS) and CD40 ligand, induces a combination of common and ligand-specific phenotypic changes through complex signal transduction pathways. For example, although all three of these ligands induce proliferation, only stimulation through the B cell antigen receptor (BCR) induces apoptosis in resting splenic B cells. In order to define the common and unique biological responses to ligand stimulation, we compared the gene expression changes induced in normal primary B cells by a panel of ligands using cDNA microarrays and a statistical approach, CLASSIFI (Cluster Assignment for Biological Inference), which identifies significant co-clustering of genes with similar Gene Ontology™ annotation.

Results

CLASSIFI analysis revealed an overrepresentation of genes involved in ion and vesicle transport, including multiple components of the proton pump, in the BCR-specific gene cluster, suggesting that activation of antigen processing and presentation pathways is a major biological response to antigen receptor stimulation. Proton pump components that were not included in the initial microarray data set were also upregulated in response to BCR stimulation in follow up experiments. MHC Class II expression was found to be maintained specifically in response to BCR stimulation. Furthermore, ligand-specific internalization of the BCR, a first step in B cell antigen processing and presentation, was demonstrated.

Conclusion

These observations provide experimental validation of the computational approach implemented in CLASSIFI, demonstrating that CLASSIFI-based gene expression cluster analysis is an effective data mining tool to identify biological processes that correlate with the experimental conditional variables. Furthermore, this analysis has identified at least thirty-eight candidate components of the B cell antigen processing and presentation pathway and sets the stage for future studies focused on a better understanding of the components involved in and unique to B cell antigen processing and presentation.

SMAP2, a novel ARF GTPase-activating protein, interacts with clathrin and clathrin assembly protein and functions on the AP-1-positive early endosome/trans-Golgi network

We recently reported that SMAP1, a GTPase-activating protein (GAP) for Arf6, directly interacts with clathrin and regulates the clathrin-dependent endocytosis of transferrin receptors from the plasma membrane. Here, we identified a SMAP1 homologue that we named SMAP2. Like SMAP1, SMAP2 exhibits GAP activity and interacts with clathrin heavy chain (CHC). Furthermore, we show that SMAP2 interacts with the clathrin assembly protein CALM. Unlike SMAP1, however, SMAP2 appears to be a regulator of Arf1 in vivo, because cells transfected with a GAP-negative SMAP2 mutant were resistant to brefeldin A. SMAP2 colocalized with the adaptor proteins for clathrin AP-1 and EpsinR on the early endosomes/trans-Golgi-network (TGN). Moreover, overexpression of SMAP2 delayed the accumulation of TGN38/46 molecule on the TGN. This suggests that SMAP2 functions in the retrograde, early endosome-to-TGN pathway in a clathrin- and AP-1-dependent manner. Thus, the SMAP gene family constitutes an important ArfGAP subfamily, with each SMAP member exerting both common and distinct functions in vesicle trafficking.

The novel CALM interactor CATS influences the subcellular localization of the leukemogenic fusion protein CALM/AF10

The Clathrin Assembly Lymphoid Myeloid leukemia gene (CALM or PICALM) was first identified as the fusion partner of AF10 in the t(10;11)(p13;q14) translocation, which is observed in acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL) and malignant lymphoma. The CALM/AF10 fusion protein plays a crucial role in t(10;11)(p13;q14) associated leukemogenesis. Using the N-terminal half of CALM as a bait in a yeast two-hybrid screen, a novel protein named CATS (CALM interacting protein expressed in thymus and spleen) was identified. Multiple tissue Northern blot analysis showed predominant expression of CATS in thymus, spleen and colon. CATS codes for two protein isoforms of 238 and 248 amino acids (aa). The interaction between CALM and CATS could be confirmed using pull down assays, co-immunoprecipitation and colocalization experiments. The CATS interaction domain of CALM was mapped to aa 221-335 of CALM. This domain is contained in the CALM/AF10 fusion protein. CATS localizes to the nucleus and shows a preference for nucleoli. Expression of CATS was able to markedly increase the nuclear localization of CALM and of the leukemogenic fusion protein CALM/AF10. The possible implications of these findings for CALM/AF10-mediated leukemogenesis are discussed.

Partially overlapping distribution of epsin1 and HIP1 at the synapse: analysis by immunoelectron microscopy

Synapses of neurons use clathrin-mediated endocytic pathways for recycling of synaptic vesicles and trafficking of neurotransmitter receptors. Epsin 1 and huntingtin-interacting protein 1 (HIP1) are endocytic accessory proteins. Both proteins interact with clathrin and the AP2 adaptor complex and also bind to the phosphoinositide-containing plasma membrane via an epsin/AP180 N-terminal homology (ENTH/ANTH) domain. Epsin1 and HIP1 are found in neurons; however, their precise roles in synapses remain largely unknown. Using immunogold electron microscopy, we examine and compare the synaptic distribution of epsin1 and HIP1 in rat CA1 hippocampal synapse. We find that epsin1 is located across both sides of the synapse, whereas HIP1 displays a preference for the postsynaptic compartment. Within the synaptic compartments, espin1 is distributed similarly throughout, whereas postsynaptic HIP1 is concentrated near the plasma membrane. Our results suggest a dual role for epsin1 and HIP1 in the synapse: as broadly required factors for promoting clathrin assembly and as adaptors for specific endocytic pathways.

Effect of clathrin assembly lymphoid myeloid leukemia protein depletion on clathrin coat formation

The endocytic accessory clathrin assembly lymphoid myeloid leukemia protein (CALM) is the ubiquitously expressed homolog of the neuron-specific protein AP180 that has been implicated in the retrieval of synaptic vesicle. Here, we show that CALM associates with the alpha-appendage domain of the AP2 adaptor via the three peptide motifs 420DPF, 375DIF and 489FESVF and to a lesser extent with the amino-terminal domain of the clathrin heavy chain. Reducing clathrin levels by RNA interference did not significantly affect CALM localization, but depletion of AP2 weakens its association with the plasma membrane. In cells, where CALM levels were reduced by RNA interference, AP2 and clathrin remained organized in somewhat enlarged bright fluorescent puncta. Electron microscopy showed that the depletion of CALM drastically affected the clathrin lattice structure. Round-coated buds, which are the predominant features in control cells, were replaced by irregularly shaped buds and long clathrin-coated tubules. Moreover, we noted an increase in the number of very small cages that formed on flat lattices. Furthermore, we noticed a redistribution of endosomal markers and AP1 in cells that were CALM depleted. Taken together, our findings indicate a critical role for CALM in the regulation and orderly progression of coated bud formation at the plasma membrane.

CALM-AF10+ T-ALL expression profiles are characterized by overexpression of HOXA and BMI1 oncogenes

The t(10;11)(p13;q14-21) is found in T-ALL and acute myeloid leukemia and fuses CALM (Clathrin-Assembly protein-like Lymphoid-Myeloid leukaemia gene) to AF10. In order to gain insight into the transcriptional consequences of this fusion, microarray-based comparison of CALM-AF10+ vs CALM-AF10- T-ALL was performed. This analysis showed upregulation of HOXA5, HOXA9, HOXA10 and BMI1 in the CALM-AF10+ cases. Microarray results were validated by quantitative RT-PCR on an independent group of T-ALL and compared to mixed lineage leukemia-translocated acute leukemias (MLL-t AL). The overexpression of HOXA genes was associated with overexpression of its cofactor MEIS1 in CALM-AF10+ T-ALL, reaching levels of expression similar to those observed in MLL-t AL. Consequently, CALM-AF10+ T-ALL and MLL-t AL share a specific HOXA overexpression, indicating they activate common oncogenic pathways. In addition, BMI1, located close to AF10 breakpoint, was overexpressed only in CALM-AF10+ T-ALL and not in MLL-t AL. BMI1 controls cellular proliferation through suppression of the tumor suppressors encoded by the CDKN2A locus. This locus, often deleted in T-ALL, was conserved in CALM-AF10+ T-ALL. This suggests that decreased CDKN2A activity, as a result of BMI1 overexpression, contributes to leukemogenesis in CALM-AF10+ T-ALL. We propose to define a HOXA+ leukemia group composed of at least MLL-t, CALM-AF10 and HOXA-t AL, which may benefit from adapted management.

Sequence analysis of Arabidopsis thaliana E/ANTH-domain-containing proteins: membrane tethers of the clathrin-dependent vesicle budding machinery

The epsin N-terminal homology (ENTH) domain is a conserved protein module present in cytosolic proteins which are required in clathrin-mediated vesicle budding processes. A highly similar, yet unique module is the AP180 N-terminal homology (ANTH) domain, which is present in a set of proteins that also support clathrin-dependent endocytosis. Both ENTH and ANTH (E/ANTH) domains bind to phospholipids and proteins, in order to support the nucleation of clathrin coats on the plasma membrane or the trans-Golgi-network membrane. Therefore, E/ANTH proteins might be considered as universal tethering components of the clathrin-mediated vesicle budding machinery. Since the E/ANTH protein family appears to be crucial in the first steps of clathrin-coated vesicle budding, we performed data base searches of the Arabidopsis thaliana genome. Sequence analysis revealed three proteins containing the ENTH signature motif and eight proteins containing the ANTH signature motif. Another six proteins were found that do not contain either motif but seem to have the same domain structure and might therefore be seen as VHS-domain-containing plant proteins. Functional analysis of plant E/ANTH proteins are rather scarce, since only one ANTH homolog from A. thaliana, At-AP180, has been characterized so far. At-AP180 displays conserved functions as a clathrin assembly protein and as an alpha-adaptin binding partner, and in addition shows features at the molecular level that seem to be plant-specific.

Control of scavenger receptor-mediated endocytosis by novel ligands of different length

The scavenger receptor recognized as a multiligand family of receptors falls in the group that is internalised through endocytosis. In this report we used several recombinant fragments of the tapeworm protein paramyosin, known to form filamentous dimers that bind collagenous structures as ligands of different length for the class A type I scavenger receptor (SR-AI). While native CHO cells are unresponsive to any of the recombinant fragments, it is shown that CHO cells transfected with this receptor efficiently internalise recombinant fragments that correspond to two thirds of the full-length paramyosin. In contrast, recombinant products corresponding to one-third of the full-length paramyiosin are not internalised. It is also shown that important molecules in the organization of the coated pit, are enriched when the two-thirds long paramyosin fragments were bound and internalised through the SR-AI. Moreover, internalisation of these fragments trigger a classical apoptotic pathway shown by the presence of TUNEL positive cells and the appearance of apoptotic bodies. We report paramyosin as a new ligand for the scavenger receptor and provide evidence supporting the notion that these receptors upon the formation of arrays with length-specific molecules, not only trigger endocytosis but also seem to regulate the synthesis of molecules involved in the organization of coated pits.

Synaptic distribution of the endocytic accessory proteins AP180 and CALM

Clathrin-coated vesicles mediate a variety of endocytosis pathways in cells, including endocytic events at synapses. AP180 and clathrin assembly lymphoid myeloid leukemia protein (CALM) are clathrin accessory proteins that promote the formation of clathrin-coated vesicles. Both proteins bind to membrane lipids through their epsin N-terminal homology domains and interact with clathrin and related protein components through their carboxyl-terminal peptide motifs. We examine their neuronal expression and synaptic distribution. We show that both proteins are detected in synapses but demonstrate different distribution patterns. AP180 is located predominantly in presynaptic profiles, whereas CALM is found nonselectively in pre- and postsynaptic profiles and also in perisynaptic processes. These observations reveal an unexpected relationship between AP180 and the presumed non-neuronal homologue CALM. We propose that both AP180 and CALM function as endocytic accessory proteins at synapses, but each may regulate distinct clathrin pathways.

Met receptor dynamics and signalling

The receptor for hepatocyte growth factor (HGF), Met, controls a programme of invasive growth that combines proliferation with various moto- and morphogenetic processes. This process is important for development and organ regeneration, but dysregulation in transformed tissues can contribute to cancer progression and metastasis. Acute stimulation of tissue culture cells with HGF leads to Met downregulation via degradation through an endocytic mechanism that also requires proteasome activity. Perturbation of Met trafficking on the endocytic pathway, either at the level of the internalisation step or during sorting at the early endosome, leads to altered signalling outputs. Ubiquitination of Met through the E3-ligase Cbl is required for receptor downregulation, and a mutant receptor defective in Cbl binding is able to transform cells. We discuss the hypothesis that some naturally occurring Met mutants implicated in cancer may transform cells owing to defects in their trafficking along the endosomal degradation pathway.

Long-term follow-up of a patient with idiopathic myelofibrosis associated with chromosome 11 and 13 abnormalities

A case of a leukemic transformation following a 27-year history of idiopathic myelofibrosis (IMF) is presented. The patient had two chromosomal abnormalities: a deletion of chromosome 13, del 13(q12q14), and a deletion of chromosome 11, del 11(q14q23). This patient's final diagnosis was acute micromegakaryocytic leukemia, and she died 1 month after leukemic transformation with an additional chromosomal abnormality, trisomy 8. IMF with myeloid metaplasia associated with deletion of the long arms of chromosomes 11 and 13 has not been previously reported. We speculate that the leukemic transformation in this patient was associated with chromosomal abnormalities del 11 and trisomy 8.