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Biomedical subjects

H Krämer

Publications and source records attributed to H Krämer.

At least 19 recordsLinked to original sources

Pioneers of movement disorders: Georges Gilles de la Tourette.

Georges Albert Edouard Brutus Gilles de la Tourette (1857-1904), a French neurologist and pupil of Jean Martin Charcot at the Salpêtrière hospital in Paris, has gained common recognition through his description of the 'Maladie des Tics'. This complex neuropsychiatric disorder, later known as the 'Tourette's syndrome', nowadays is accepted as a specific entity of movement disorders. Gilles had started working under Charcot (1825-1893), the first physician to occupy a designated chair of neurology of neuropsychiatric history, in 1884. Then the Salpêtrière hospital was a centre of intensive research with an emphasis on hysteria and hypnosis. Tourette took an interest in hysteria, but also dedicated himself to various other neuropsychiatric disorders and to neuropathology. He published scientific works on epilepsia, neurasthenia and syphilitic myelitis. Although he devoted much time to his neuropsychiatric research and the publication of articles in medical journals, his career did not make significant progress, despite Charcot's unrestricted support. One reason was, that he disregarded questions, answers and problems, which were outside his interest fields. Hence, he was accused for having acquired an extremely filtered and one-sided knowledge. Also, his alienated and critical behaviour, which had not helped him to find many friends over the years, prevented him from professional promotion. In 1893 an assassination attempt on Gilles de la Tourette raised considerable public interest: Gilles was shot in his appartement in the Rue de l'Université 39 by a young woman, who had been his patient in the Salpêtrière and who claimed that she had been hypnotized without her agreement and thereby had lost her mental health. However, the patient was diagnosed with a disease nowadays called paranoid schizophrenia and therefore hypnosis was not attributed to any part of the disease. Due to episodes of melancholia and phases of delusions of grandeur and megalomania Gilles de la Tourette was forced to leave his hospital appointment in 1901. These symptoms and the corresponding neurological signs were attributed to the paretic neurosyphilis. He was institutionalized to the psychiatric hospital Cery near Lausanne, Switzerland. In the course of the following three years he became increasingly psychotic and demented, suffered from epileptic seizures and finally died in hospital on 22nd May 1904.

France↗

[Brugada or not-Brugada: misdiagnosis of recorder-induced artifact].

The Brugada-Brugada syndrome is a life threatening cardiac arrhythmia that features syncopal events or aborted sudden death in combination with electrocardiographic characteristics (ST-segment elevation of V(1)-V(3) and right bundle branch block). Typical ECG alterations were recorded in a young man who was admitted to our hospital after syncope and a Brugada-Brugada syndrome was suspected. The following days similar pathological ECG recordings of other patients were noticed. The electrocardiographic artifact was diagnosed as being caused by incorrect handling of the ECG recorder.

Arrhythmias, Cardiac↗

Molecular characterization of mammalian homologues of class C Vps proteins that interact with syntaxin-7.

Vesicle-mediated protein sorting plays an important role in segregation of intracellular molecules into distinct organelles. Extensive genetic studies using yeast have identified more than 40 vacuolar protein sorting (VPS) genes involved in vesicle transport to vacuoles. However, their mammalian counterparts are not fully elucidated. In this study, we identified two human homologues of yeast Class C VPS genes, human VPS11 (hVPS11) and human VPS18 (hVPS18). We also characterized the subcellular localization and interactions of the protein products not only from these genes but also from the other mammalian Class C VPS homologue genes, hVPS16 and rVPS33a. The protein products of hVPS11 (hVps11) and hVPS18 (hVps18) were ubiquitously expressed in peripheral tissues, suggesting that they have a fundamental role in cellular function. Indirect immunofluorescence microscopy revealed that the mammalian Class C Vps proteins are predominantly associated with late endosomes/lysosomes. Immunoprecipitation and gel filtration studies showed that the mammalian Class C Vps proteins constitute a large hetero-oligomeric complex that interacts with syntaxin-7. These results indicate that like their yeast counterparts, mammalian Class C Vps proteins mediate vesicle trafficking steps in the endosome/lysosome pathway.

Amino Acid Sequence↗

The Golgi-associated hook3 protein is a member of a novel family of microtubule-binding proteins.

Microtubules are central to the spatial organization of diverse membrane-trafficking systems. Here, we report that Hook proteins constitute a novel family of cytosolic coiled coil proteins that bind to organelles and to microtubules. The conserved NH(2)-terminal domains of Hook proteins mediate attachment to microtubules, whereas the more divergent COOH-terminal domains mediate the binding to organelles. Human Hook3 bound to Golgi membranes in vitro and was enriched in the cis-Golgi in vivo. Unlike other cis-Golgi-associated proteins, however, a large fraction of Hook3 maintained its juxtanuclear localization after Brefeldin A treatment, indicating a Golgi-independent mechanism for Hook3 localization. Because overexpression of Hook3 caused fragmentation of the Golgi complex, we propose that Hook3 participates in defining the architecture and localization of the mammalian Golgi complex.

Animals↗

Molecular cloning and characterization of human VPS18, VPS 11, VPS16, and VPS33.

In multicellular organisms, the delivery of proteins to lysosomes is essential. Many of the genes necessary for this process have first been identified by their requirement for vacuolar delivery in yeast. A subset of these genes, the four class C vps genes, is necessary for the delivery of endocytic and biosynthetic cargo in yeast, and also in Drosophila. Here, we describe the sequence and expression pattern of four human homologs of these genes. This initial molecular description of these four genes is an important step towards their evaluation as candidate genes that may be involved in the pathogenesis of Hermansky-Pudlak syndrome-related diseases.

Amino Acid Sequence↗

Neuralized: regulating notch by putting away delta.

Neuralized acts in a subset of Notch-dependent cell fate decisions including lateral inhibition in Drosophila neurogenesis. Three recent papers reveal that Neuralized acts as a ubiquitin ligase and triggers endocytosis of the ligand Delta.

Animals↗

Drosophila endosomal proteins hook and deep orange regulate synapse size but not synaptic vesicle recycling.

To study the function of endosomes at synapses we analyzed the localization and function of two Drosophila endosomal proteins, Hook and Deep orange (Dor), at the larval neuromuscular junction. Hook, a negative regulator of endocytic trafficking, and Dor, a positive regulator of endocytic trafficking, are highly enriched at synapses, especially close to postsynaptic membranes. Mutations in hook (hk) and dor do not affect synaptic vesicle recycling, as assessed by electrophysiological analysis of synaptic transmission and behavioral studies of double mutants with shi(ts) mutations that alter vesicle recycling. However, hk and dor mutations alter the number of presynaptic varicosities (synapse size) in opposing ways. Synapse size is increased in hk(11) mutants and is decreased in dor(4) mutants. Double mutants for dor and hk show a dor-like phenotype. These effects on synapse size parallel known functions of Hook and Dor in endocytosis and strongly indicate a role for endocytic trafficking in the regulation of synapse size in vivo. Our observations suggest a model in which Hook and Dor function in later stages of endocytosis is essential for regulating synaptic plasma membrane composition but not synaptic vesicle recycling.

Animals↗

RIPping notch apart: a new role for endocytosis in signal transduction?

Notch proteins are receptors that are important in mediating several developmental processes. Notch receptors are activated upon binding transmembrane ligands, the DSL proteins. Notch is cleaved at several sites and activation of Notch leads to the cleavage of the intracellular domain, which then is translocated to the nucleus and regulates the transcription of target genes. Krämer discusses how binding of Notch to the DSL ligand, Delta, leads to cleavage and trans-endocytosis of the Notch extracellular domain into the Delta-expressing cell. This trans-endocytosis event contributes to the cleavage and release of the active Notch intracellular domain. The Perspective is accompanied by a movie illustrating the trans-endocytosis of Notch.

Animals↗

A role for the deep orange and carnation eye color genes in lysosomal delivery in Drosophila.

Deep orange and carnation are two of the classic eye color genes in Drosophila. Here, we demonstrate that Deep orange is part of a protein complex that localizes to endosomal compartments. A second component of this complex is Carnation, a homolog of Sec1p-like regulators of membrane fusion. Because complete loss of deep orange function is lethal, the role of this complex in intracellular trafficking was analyzed in deep orange mutant clones. Retinal cells devoid of deep orange function completely lacked pigmentation and exhibited exaggerated multivesicular structures. Furthermore, a defect in endocytic trafficking was visualized in developing photoreceptor cells. These results provide direct evidence that eye color mutations of the granule group also disrupt vesicular trafficking to lysosomes.

Amino Acid Sequence↗

Genetic dissection of endocytic trafficking in Drosophila using a horseradish peroxidase-bride of sevenless chimera: hook is required for normal maturation of multivesicular endosomes.

Mutations in the hook gene alter intracellular trafficking of internalized ligands in Drosophila. To dissect this defect in more detail, we developed a new approach to visualize the pathway taken by the Bride of Sevenless (Boss) ligand after its internalization into R7 cells. A chimeric protein consisting of HRP fused to Boss (HRP-Boss) was expressed in R8 cells. This chimera was fully functional: it rescued the boss mutant phenotype, and its trafficking was indistinguishable from that of the wild-type Boss protein. The HRP activity of the chimera was used to follow HRP-Boss trafficking on the ultrastructural level through early and late endosomes in R7 cells. In both wild-type and hook mutant eye disks, HRP-Boss was internalized into R7 cells. In wild-type tissue, Boss accumulated in mature multivesicular bodies (MVBs) within R7 cells; such accumulation was not observed in hook eye disks, however. Quantitative electron microscopy revealed a loss of mature MVBs in hook mutant tissue compared with wild type, whereas more than twice as many multilammelar late endosomes were detected. Our genetic analysis indicates that Hook is required late in endocytic trafficking to negatively regulate delivery from mature MVBs to multilammelar late endosomes and lysosomes.

Amino Acid Sequence↗

Genetic analysis of hook, a gene required for endocytic trafficking in drosophila.

The Drosophila hook gene encodes a novel component of the endocytic compartment. Previously identified hook alleles, which still expressed truncated Hook proteins, affected the accumulation of internalized transmembrane ligands into multivesicular bodies (MVBs). To determine the hook null phenotype, we isolated nine new hook alleles on the basis of their characteristic hooked-bristle phenotype. At least one of these alleles, hk11, is a complete loss-of-function allele. Flies carrying the hk11 allele are viable and fertile but neither transmembrane ligands nor soluble ligands accumulate in MVBs. This effect on endocytosed ligands can be mimicked by the expression of Hook proteins truncated for the N- and C-terminal domains flanking the central coiled-coil region. The importance of all three domains for Hook function was confirmed by their conservation between two Drosophila and two human Hook proteins.

Alleles↗

Not just pretty eyes: Drosophila eye-colour mutations and lysosomal delivery.

Analysis of Drosophila eye-colour mutations has made seminal contributions to the fields of genetics and biochemistry. Recent findings suggest that a subset of eye-colour genes is crucial for vesicular transport of proteins to pigment granules, specialized lysosomes of eye-pigment cells. Thus, classical work describing more than 85 eye-colour mutations and their genetic interactions offers a remarkable, untapped resource for the genetic analysis of protein delivery to lysosomes.

Animals↗

Oligomerization of the extracellular domain of Boss enhances its binding to the Sevenless receptor and its antagonistic effect on R7 induction.

In the developing compound eye of Drosophila, neuronal differentiation of the R7 photoreceptor cell is induced by the interaction of the receptor tyrosine kinase Sevenless with its ligand Bride of sevenless (Boss), which is expressed on the neighboring R8 cell. Boss is an unusual ligand of a receptor tyrosine kinase: it is composed of a large extracellular domain, a transmembrane domain with seven membrane-spanning segments and a cytoplasmic tail. Expression of a monomeric, secreted form of the extracellular domain of Boss is not sufficient for Sevenless activation, and instead acts as a weak antagonist. Because oligomerization appears to be a critical step in the activation of receptor tyrosine kinases, we used oligomerized forms of the Boss extracellular domain to test their ability to bind to Sevenless in vivo and restore R7 induction in vivo. Oligomerization was achieved by fusion to the leucine zipper of the yeast transcription factor GCN4 or to the tetramerization helix of Lac repressor. Binding of these multivalent proteins to Sevenless could be detected in vitro by immunoprecipitation of cross-linked ligand/receptor complexes and in vivo by receptor-dependent ligand localization. However, neither R8-specific or ubiquitous expression of multivalent Exboss ligands rescued the boss phenotype. Instead, these ligands acted as competitive inhibitors for wild-type Boss protein and thereby suppressed R7 induction. Therefore the role of the transmembrane or cytoplasmic domains of Boss in the activation of the Sev receptor cannot be replaced by oligomerization.

Amino Acid Sequence↗

Mutations in the Drosophila hook gene inhibit endocytosis of the boss transmembrane ligand into multivesicular bodies.

Transmembrane ligands can be internalized across cell boundaries into receptor-expressing cells. In the developing Drosophila eye imaginal disc, the bride of sevenless transmembrane protein (boss) is expressed on the surface of R8 cells. After internalization into neighboring R7 cells, the boss protein accumulates in multivesicular bodies. In a search for genes that affect this cell-type-specific pattern of boss endocytosis, we found that mutations in the hook gene inhibit the accumulation of boss in multivesicular bodies of R7 cells. In addition, hook flies exhibit pleiotropic phenotypes including abnormal bristle morphology and eye degeneration. The wild-type-pattern of boss endocytosis was restored in hook mutants by a genomic rescue fragment containing the hook gene or by a hook cDNA expressed in R7 cells under control of a sevenless (sev) enhancer. The hook gene encodes a novel cytoplasmic protein of 679 amino acids with a central coiled-coil domain of some 200 amino acids. Truncated, epitope-tagged hook proteins coimmunoprecipitated the full-length protein, indicating dimerization mediated by the coiled-coil domain. The hook protein localizes to vesicular structures that are part of the endocytic compartment. The requirement of the hook protein in R7 cells for the accumulation of boss protein in multivesicular bodies, and the localization of the hook protein to endocytic vesicles indicate that the hook gene encodes a novel component of the endocytic compartment that plays an important role in the endocytosis of transmembrane ligands or their transport to multivesicular bodies.

Amino Acid Sequence↗

Determination of photoreceptor cell fate in the Drosophila retina.

Cell-cell communication directs the development of photoreceptor cells in the Drosophila retina. A recent set of studies has provided a genetic dissection of one of these interactions, the induction of the R7 photoreceptor cell by the neighboring R8 photoreceptor cell. The results from these experiments shape our understanding of both the molecular basis of signal transduction mediated through receptor tyrosine kinases, as well as the developmental strategies used to generate different cell types in neuronal systems.

Animals↗

Visualization of neonatal anatomy and pathology with a new computerized three-dimensional model as a basis for teaching, diagnosis and therapy.

A new computerized three-dimensional (3D) volume model derived from a post-mortem MRI series of the chest and abdomen of a human newborn allows interactive dissection by removing, adding organs or cutting in unlimited directions. The advanced technique of real volume visualization instead of using contours allows one to study the normal and pathological anatomy of the neonate. Anatomical details of the pleural, pericardial and peritoneal cavities and of abdominal veins are demonstrated. Compared to conventional methods, the advantages of this model for teaching and as a basis for diagnostic imaging and therapeutic procedures are evident.

Abdomen↗

Extracellular domain of the boss transmembrane ligand acts as an antagonist of the sev receptor.

The fate of the R7 photoreceptor cell in the Drosophila compound eye is established by a specific inductive interaction between the R8 photoreceptor neuron and the R7 precursor cell. This induction is mediated by two cell-surface proteins: the ligand, bride of sevenless (boss), and sevenless (sev), a tyrosine-kinase receptor. The structure of boss is unique for a ligand of a tyrosine-kinase receptor. It contains a large extracellular domain, seven transmembrane segments, and a carboxy-terminal cytoplasmic tail. Here we report that: (1) boss activates tyrosine phosphorylation of the sev receptor; (2) the seven transmembrane domain of boss is necessary for its function; and (3) a soluble form of boss acts as an antagonist of the sev receptor both in vivo and in vitro.

Animals↗