Search PubMed⌕ Search

Biomedical subjects

M Kirschner

Publications and source records attributed to M Kirschner.

At least 37 records · Page 2Linked to original sources
PubMed1994

In vitro reconstitution of centrosome assembly and function: the central role of gamma-tubulin.

The centrosome nucleates microtubule polymerization, affecting microtubule number, polarity, and structure. We use an in vitro system based on extracts of Xenopus eggs to examine the role of gamma-tubulin in centrosome assembly and function. gamma-Tubulin is present in the cytoplasm of frog eggs and vertebrate somatic cells in a large approximately 25S complex. The egg extracts assemble centrosomes around sperm centrioles. Formation of a centrosome in the extract requires both the gamma-tubulin complex and ATP and can take place in the absence of microtubules. gamma-Tubulin is not present on the sperm prior to incubation in extract, but is recruited from the cytoplasm during centrosome assembly. The gamma-tubulin complex also binds to microtubules, likely the minus end, independent of the centrosome. These results suggest that gamma-tubulin is an essential component of the link between the centrosome and the microtubule, probably playing a direct role in microtubule nucleation.

Animals↗
PubMed1994

Pericentrin, a highly conserved centrosome protein involved in microtubule organization.

Antisera from scleroderma patients that react widely with centrosomes in plants and animals were used to isolate cDNAs encoding a novel centrosomal protein. The nucleotide sequence is consistent with a 7 kb mRNA and contains an open reading frame encoding a protein with a putative large coiled-coil domain flanked by noncoiled ends. Antisera recognize a 220 kd protein and stain centrosomes and acentriolar microtubule-organizing centers, where the protein is localized to the pericentriolar material (hence, the name pericentrin). Anti-pericentrin antibodies disrupt mitotic and meiotic divisions in vivo and block microtubule aster formation in Xenopus extracts, but do not block gamma-tubulin assembly or microtubule nucleation from mature centrosomes. These results suggest that pericentrin is a conserved integral component of the filamentous matrix of the centrosome involved in the initial establishment of organized microtubule arrays.

Amino Acid Sequence↗
PubMed1994

[Hypoxia-radiotherapy: the changes in selected physiological parameters of healthy tissue during the breathing of a hypoxic gas mixture].

BACKGROUND: Chronic hypoxia in tumors is generally accepted to be one of the most important factors concerning sensibility to irradiation. The principle of hypoxyradiotherapy is to achieve a limited (maximal 30 min) radioresistance of the healthy (actually euoxic) tissue by acute aerogen hypoxia, without protecting effects in the average chronic hypoxic tissue of the tumor. This study investigates changes in selected physiological parameters in vivo during acute aerogen hypoxia. PATIENTS AND METHODS: In 31 persons (group A) we documented changes in blood-gas-analysis, ECG-parameters, blood pressure and pulse during acute hypoxia lasting 5 minutes. During 30 minutes of acute hypoxia we determined in another 10 persons (group B) the relations between different phosphoryl-metabolites by means of 31-phosphorus magnetic resonance spectroscopy (31P-MRS). The 31P-MRS of the Musculus triceps surae was performed on our 1.5 T imaging system (Magnetom, Siemens) using a 8 cm diameter surface coil. To achieve the hypoxic condition in the persons tested, a gas-mixture with 8.2% O2 was used. RESULTS: In group A the acute hypoxia led to significant changes in O2-pressure, the blood O2-saturation and pulse frequency. The pO2-values drop in the 5th minute to about 50% of the initial value. Concerning group B the relations of the phosphoryl metabolites show no significant changes during acute hypoxia. CONCLUSIONS: The results show an excellent tolerability of breathing of the hypoxic gas-mixture, and confirm that this mixture containing 8.2% of O2 ensures a decrease in the pO2-levels of more than 50%. This is necessary to achieve a dose modifying factor (DMF) of at least 1.15 to 1.2. The changes in relations of phosphoryl metabolites seem to reflect the ability of healthy tissue to adapt to hypoxic conditions starting after 10 minutes and leading to the loss of the radioprotective effect after 30 minutes.

Acute Disease↗
PubMed1993

Weight loss and OSA and pulmonary function in obesity.

Obstructive sleep apnea (OSA), daytime hypoxemia, and hypercapnia complicate obesity and are alleviated by weight loss. The flow-volume curve is a sensitive screening tool for most patients; the curve can monitor therapeutic efficacy of weight reduction.

Adult↗
PubMed1992

The cell cycle then and now.

In the last few years a general model of cell cycle control has been established for all eukaryotic cells. Experiments from a variety of organisms and from a variety of experimental approaches have identified a protein kinase and its unstable regulatory subunit as the activator of mitosis; related molecules seem to be involved in the activation of chromosome replication. The identification of the biochemical components of these important regulatory pathways is providing several new insights into homeostatic and developmental control mechanisms in higher organisms.

Animals↗
PubMed1991

Gamma-tubulin is a highly conserved component of the centrosome.

We have cloned and characterized gamma-tubulin genes from both X. laevis and S. pombe, and partial genes from maize, diatom, and a budding yeast. The proteins encoded by these genes are very similar to each other and to the original Aspergillus protein, indicating that gamma-tubulins are an ubiquitous and highly conserved subfamily of the tubulin family. A null mutation of the S. pombe gene is lethal. gamma-tubulin is a minor protein, present at less than 1% the level of alpha- and beta-tubulin, and is limited to the centrosome. In particular, gamma-tubulin is associated with the pericentriolar material, the microtubule-nucleating material of the centrosome. gamma-Tubulin remains associated with the centrosome when microtubules are depolymerized, suggesting that it is an integral component that might play a role in microtubule organization.

Amino Acid Sequence↗
PubMed1991

mik1 and wee1 cooperate in the inhibitory tyrosine phosphorylation of cdc2.

wee1 acts antagonistically to cdc25 in the tyrosine dephosphorylation and activation of cdc2, yet biochemical evidence suggests that wee1 is not required for tyrosine phosphorylation and its role is obscure. We show here that a related 66 kd kinase, called mik1, acts redundantly with wee1 in the negative regulation of cdc2 in S. pombe. A null allele of mik1 has no discernible phenotype, but a mik1 wee1 double mutant is hypermitotically lethal: all normal M phase checkpoints are bypassed, including the requirement for initiation of cell cycle "start," completion of S phase, and function of the cdc25+ mitotic activator. In the absence of mik1 and wee1 activity, cdc2 rapidly loses phosphate on tyrosine, both in strains undergoing mitotic lethality and in those that are viable owing to a compensating mutation within cdc2. The data suggest that mik1 and wee1 act cooperatively on cdc2, either directly as the inhibitory tyrosine kinase or as essential activators of that kinase.

Amino Acid Sequence↗
PubMed1991

Microtubule polymer assembly and transport during axonal elongation.

As axons elongate, tubulin, which is synthesized in the cell body, must be transported and assembled into new structures in the axon. The mechanism of transport and the location of assembly are presently unknown. We report here on the use of tubulin tagged with a photoactivatable fluorescent group to investigate these issues. Photoactivatable tubulin, microinjected into frog embryos at the two-cell stage, is incorporated into microtubules in neurons obtained from explants of the neural tube. When activated by light, a fluorescent mark is made on the microtubules in the axon, and transport and turnover can be visualized directly. We find that microtubules are generated in or near the cell body and continually transported distally as a coherent phase of polymer during axon elongation. This vectorial polymer movement was observed at all levels on the axon, even in the absence of axonal elongation. Measurements of the rate of polymer translocation at various places in the axon suggest that new polymer is formed by intercalary assembly along the axon and assembly at the growth cone in addition to transport of polymer from the cell body. Finally, polymer movement near the growth cone appeared to respond in a characteristic manner to growth cone behavior, while polymer proximally in the axon moved more consistently. These results suggest that microtubule translocation is the principal means of tubulin transport and that translocation plays an important role in generating new axon structure at the growth cone.

Animals↗
PubMed1991

Microtubule behavior during guidance of pioneer neuron growth cones in situ.

The growth of an axon toward its target results from the reorganization of the cytoskeleton in response to environmental guidance cues. Recently developed imaging technology makes it possible to address the effect of such cues on the neural cytoskeleton directly. Although high resolution studies can be carried out on neurons in vitro, these circumstances do not recreate the complexity of the natural environment. We report here on the arrangement and dynamics of microtubules in live neurons pathfinding in response to natural guidance cues in situ using the embryonic grasshopper limb fillet preparation. A rich microtubule network was present within the body of the growth cone and normally extended into the distal growth cone margin. Complex microtubule loops often formed transiently within the growth cone. Branches both with and without microtubules were regularly observed. Microtubules did not extend into filopodia. During growth cone steering events in response to identified guidance cues, microtubule behaviour could be monitored. In turns towards guidepost cells, microtubules selectively invaded branches derived from filopodia that had contacted the guidepost cell. At limb segment boundaries, microtubules displayed a variety of behaviors, including selective branch invasion, and also invasion of multiple branches followed by selective retention in branches oriented in the correct direction. Microtubule invasion of multiple branches also was seen in growth cones migrating on intrasegmental epithelium. Both selective invasion and selective retention generate asymmetrical microtubule arrangements within the growth cone, and may play a key role in growth cone steering events.

Animals↗
PubMed1991

The need for unity in the biomedical research community.

The author provides a perspective on how basic scientists view contemporary issues of biomedical research funding by discussing five topics of concern to the biomedical research community and the need for that community to form a new consensus as the basis for new approaches to legislative efforts in Congress and communication with the public. The topics are (1) the reasons scientific societies became active in legislative efforts in Congress, (2) the reasons there was initial friction between these societies and other institutions in the biomedical research community, (3) the history of the founding of a biomedical research community, (3) the history of the founding of a biomedical research caucus in the House of Representatives, (4) the effects of the Institute of Medicine's recent report on funding health sciences research, and (5) the weakness of the present institutional coalition. The author proposes specific steps to develop a more unified approach to achieve a common biomedical research agenda.

Humans↗
PubMed1990

[3 new antibody detection tests. A comparative study].

Erythrocyte antibodies are of clinical importance since they may cause decreased red cell survival as the result of hemolytic transfusion reactions, hemolytic disease of the newborn or autoimmune hemolytic anemia. A variety of in vitro antibody screening tests are employed to detect their presence in donor and patient sera. Three new methods now available for pretransfusion testing--two solid phase antiglobulin tests and a gel centrifugation test--are compared in parallel with a conventional tube test.

Blood Group Incompatibility↗
PubMed1989

Contractures in patients with Williams syndrome.

Williams syndrome is a multisystem disorder, including a characteristic facies and habitus. Intellectual, neurologic, and cardiovascular dysfunction occur; vascular disease may be progressive. Joint contractures affected 10 of a group of 20 children and young adults. In 3 of 10 patients, the contractures were severe enough to interfere with daily activities; in 3 the effect was only moderate. The contractures developed in early childhood and usually did not improve or worsen with time. Large and small joints were affected and usually were symmetrical. Their occurrence did not correlate with vascular disease. The contractures could not be attributed to a neurologic cause.

Adolescent↗
PubMed1988

New features of microtubule behaviour observed in vivo.

The microtubule cytoskeleton is thought to be intimately involved in generating and maintaining cell polarity and can generate many different morphological structures from a few structural elements. The mechanism by which these structures are generated has been partially elucidated from studies of microtubule polymerization both in vitro and in vivo. Microtubules in vitro exist in growing (polymerizing) and shrinking (depolymerizing) populations that interconvert infrequently. This behaviour, termed dynamic instability, permits microtubules in the cell rapidly to explore different arrangements and allows selective stabilization of specific morphologies. To investigate the regulation of these processes, we have implemented techniques for direct observation of fluorescently labelled microtubules and developed them to observe the dynamic behaviour of individual microtubules in single living cells. Sammak and Borisy recently used this technique to show that the dynamics of microtubules in fibroblasts is explained by dynamic instability. Although we also conclude here that dynamic instability explains much of microtubule behaviour in vivo, we find significant deviations from the properties of tubulin in vitro. These results suggest that local cytoplasmic factors strongly influence microtubule dynamics; such control has important implications for cellular morphogenesis.

Animals↗
PubMed1988

The primary structure and heterogeneity of tau protein from mouse brain.

Tau protein is a family of microtubule binding proteins, heterogeneous in molecular weight, that are induced during neurite outgrowth and are found prominently in neurofibrillary tangles in Alzheimer's disease. The predicted amino acid sequences of two forms of tau protein from mouse brain were determined from complementary DNA clones. These forms are identical in their amino-terminal sequences but differ in their carboxyl-terminal domains. Both proteins contain repeated sequences that may be tubulin binding sites. The sequence suggests that tau is an elongated molecule with no extensive alpha-helical or beta-sheet domains. These complementary DNAs should enable the study of various functional domains of tau and the study of tau expression in normal and pathological states.

Amino Acid Sequence↗
PubMed1988

Regulation of microtubule protein levels during cellular morphogenesis in nerve growth factor-treated PC12 cells.

Nerve growth factor induces neurite process formation in pheochromacytoma (PC12) cells and causes the parallel increase in levels of the microtubule-associated proteins, tau and MAP1, as well as increases in tubulin levels. Mechanisms to insure balanced accumulation of microtubule proteins and make their levels highly responsive to nerve growth factor were investigated. The effects on tau, MAP1, and tubulin are due to changes in protein synthesis rates, which for tau and tubulin we could show are due in part to changes in the mRNA levels. Whereas tubulin shows feedback regulation to modulate synthesis up or down, tau protein synthesis is not affected in a straightforward way by microtubule polymerization and depolymerization. The degradation of tau, MAP1, and both tubulin polypeptides, however, are stimulated by microtubule depolymerization caused by colchicine, or nerve growth factor removal. Combined feedback on synthesis and stability make tubulin levels highly responsive to assembly states. In addition, the linkage of tau and MAP1 turnover with the state of microtubule polymerization amplifies any change in their rate of synthesis, since tau and MAP1 promote microtubule polymerization. This linkage lends itself to rapid changes in the state of the system in response to nerve growth factor.

Animals↗