PubMed Health⌕ Search

Biomedical subjects

G D Clark

Publications and source records attributed to G D Clark.

At least 19 recordsLinked to original sources
PubMed2000

Lissencephaly associated mutations suggest a requirement for the PAFAH1B heterotrimeric complex in brain development.

Human brain malformations, such as Miller-Dieker syndrome (MDS) or isolated lissencephaly sequence (ILS) may result from abnormal neuronal migration during brain development. MDS and ILS patients have a hemizygous deletion or mutation in the LIS1 gene (PAFAH1B1), therefore, the LIS1 encoded protein (Lis1) may play a role in neuronal migration. Lis1 is a subunit of a brain platelet-activating factor acetylhydrolase (PAFAH1B) where it forms a heterotrimeric complex with two hydrolase subunits, referred to as 29 kDa (PAFAH1B3) and 30 kDa (PAFAH1B2). In order to determine whether this heterotrimer is required for the developmental functions of PAFAH1B, we examined the binding properties of 29 and 30 kDa subunits to mutant Lis1 proteins. The results defined the critical regions of Lis1 for PAFAH1B complex formation and demonstrated that all human LIS1 mutations examined resulted in abolished or reduced capacity of Lis1 to interact with the 29 and 30 kDa subunits, suggesting that the PAFAH1B complex participates in the process of neuronal migration.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗
PubMed1999

Neuronal cytoskeletal alterations evoked by a platelet-activating factor (PAF) analogue.

Platelet-activating factor (PAF), a phospholipid signaling molecule found in brain, modulates several neural functions and is implicated in the human developmental brain disorder Miller-Dieker Lissencephaly (MDL). Exposure to PAF, and a non-hydrolyzable analogue, methyl carbamyl PAF (mc-PAF), produces the following rapid, reversible effects upon cultured hippocampal neurites: growth cone collapse, neurite retraction, and neurite varicosity formation. In this study, the cytoskeletal alterations that mediate these shape changes were investigated by comparing the effects of mc-PAF with other cytoskeletal-altering drugs, through the fluorescent labeling of cytoskeletal proteins and mitochondria, and by electron microscopy. Results indicate that rearrangements of microtubules (MTs), F-actin, and mitochondria underlie the neurite shape changes produced by mc-PAF. Evidence for MT alteration was obtained by comparing the effects of mc-PAF with nocodozole and taxol. Exposure to nocodazole, a MT-depolymerizing agent, produced growth cone collapse and neurite varicosity formation similar to mc-PAF, whereas pre-incubation of neurites in taxol, a MT-stabilizing drug, was effective in blocking mc-PAF-induced neurite effects. Immunofluorescent labeling and EM revealed MT splaying and unbundling within neurite varicosities following mc-PAF treatment. Immunofluorescent labeling also revealed that F-actin shifted from concentration in the growth cone to a diffuse distribution along the neurite shaft following mc-PAF exposure. Fluorescent labeling and EM also revealed retrograde movement and morphological alterations of mitochondria following mc-PAF exposure, resulting in mitochondrial aggregates within neurite varicosities. These cytoskeletal rearrangements may provide insights into the mechanisms by which PAF influences neuronal activity, and could have important implications for the impairment of neuronal motility observed in MDL.

Actins↗
PubMed1998

Platelet-activating factor receptor stimulation disrupts neuronal migration In vitro.

LIS-1 is a gene whose hemi-deletion causes the human neuronal migration disorder Miller-Dieker lissencephaly. It encodes a subunit of a brain platelet-activating factor (PAF) acetylhydrolase, an enzyme that inactivates PAF by hydrolyzing the acetyl moiety in the sn2 position of this phospholipid. Because PAF receptor activation has been shown to affect the developing neuronal cytoskeleton, we have hypothesized that a role for PAF in neurodevelopment is that of a modulator of neuroblast movement (a cytoskeletal function) and that an aberrant regulation of PAF could lead to an early arrest in migration. This report examines the effects of the nonhydrolyzable PAF receptor agonist methyl carbamyl PAF (mc-PAF) on the unidirectional in vitro migration of granule cells from cerebellar cell reaggregates on a laminin substrate. Bath treatment with mc-PAF yields a dose-dependent decrease in granule cell migration compared with controls. This effect can be blocked by the simultaneous bath application of BN 52021 and trans-BTD, PAF receptor-specific antagonists. Although mc-PAF minimally inhibited neurite growth, its primary effect was on somal movement along preextended neurites. These experiments suggest that the stimulation of neuronal PAF receptors could be one crucial step for the regulation of neuroblast migration and that disturbed PAF catabolism during neurodevelopment could contribute to the neuronal migration defects observed in Miller-Dieker lissencephaly.

Animals↗
PubMed1998

Graded reduction of Pafah1b1 (Lis1) activity results in neuronal migration defects and early embryonic lethality.

Heterozygous mutation or deletion of the beta subunit of platelet-activating factor acetylhydrolase (PAFAH1B1, also known as LIS1) in humans is associated with type I lissencephaly, a severe developmental brain disorder thought to result from abnormal neuronal migration. To further understand the function of PAFAH1B1, we produced three different mutant alleles in mouse Pafah1b1. Homozygous null mice die early in embryogenesis soon after implantation. Mice with one inactive allele display cortical, hippocampal and olfactory bulb disorganization resulting from delayed neuronal migration by a cell-autonomous neuronal pathway. Mice with further reduction of Pafah1b1 activity display more severe brain disorganization as well as cerebellar defects. Our results demonstrate an essential, dosage-sensitive neuronal-specific role for Pafah1b1 in neuronal migration throughout the brain, and an essential role in early embryonic development. The phenotypes observed are distinct from those of other mouse mutants with neuronal migration defects, suggesting that Pafah1b1 participates in a novel pathway for neuronal migration.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗
PubMed1997

Elvax as a slow-release delivery agent for a platelet-activating factor receptor agonist and antagonist.

The ability of the slow-release polymer, Elvax, to deliver a large number of different molecules to distinct brain regions has been well-documented. However, there are currently no reports of Elvax-mediated delivery of ether phospholipids, whose lipid structure renders them difficult to solubilize and detect under experimental conditions. In order to potentially examine the role of platelet-activating factor (PAF) receptors in the brain in vivo, we tested the ability of Elvax to release the ether phospholipid. PAF receptor agonist, methyl-carbamyl platelet-activating factor (mc-PAF), and the ginkolide, PAF receptor antagonist, BN 52021. Mc-PAF and BN 52021 containing Elvax sections (200 microns) prepared in methylene chloride were assayed for release characteristics with a rabbit platelet aggregation bioassay. We measured a slow, sustained release of mc-PAF and BN 52021 from Elvax in vitro over a 5 day period. Similar in vivo mc-PAF and BN 52021 release kinetics were determined. Therefore, we believe that this novel method of slow-release delivery of PAF receptor agonists and antagonists as measured with platelet aggregation is viable and offers a simple, sensitive and inexpensive method for potential in vivo studies of the roles of PAF and its receptors in neural systems.

Animals↗
PubMed1997

Survey of sinonasal inverted papillomata for Epstein-Barr virus.

BACKGROUND: Several studies have indicated an etiologic role for viruses in the development of sinonasal inverted papillomata (IP). A recent report demonstrates a strong relationship (65%) between Epstein-Barr virus (EBV) and these lesions using polymerase chain reaction (PCR) analysis. METHODS: The present study analyzes a series of paraffin-embedded tissues, comprising 25 surgically resected IPs and four fungiform papillomata (FP) for the presence of EBV using a sensitive in situ hybridization (ISH) assay and PCR. RESULTS: None of the specimens examined showed evidence of EBV infection by ISH, and only two papillomata (one sinonasal IP and one FP) gave positive reactions for EBV using PCR. CONCLUSIONS: These data challenge the previous report and suggest that EBV is not a significant etiopathologic factor to be considered in the development of sinonasal IP.

Aged↗
PubMed1997

Predominant localization of the LIS family of gene products to Cajal-Retzius cells and ventricular neuroepithelium in the developing human cortex.

Mutations that perturb neuronal migration provide important biological clues that can lead to an understanding of the role of specific cells and molecules in the formation of the cortex. The human neuronal migration disorder, Miller-Dieker Lissencephaly, results from a hemideletion of LIS-1, which encodes a subunit of a brain platelet-activating factor acetylhydrolase. The cellular localization of the LIS-1 gene product in human fetal brain and its normal role in neuronal migration have yet to be determined. LIS-1 belongs to a family of genes that have identical coding sequences (LIS-1 [chromosome 17] and LIS-2 [chromosome 2]). In the brain, LIS-1 is the more abundant gene as determined by Northern blot analysis. Using antibodies raised against 2 epitopes of the LIS-1/LIS-2 protein sequence, we have localized the LIS family of gene products in the developing human brain to the Cajal-Retzius cells, some subplate neurons, thalamic neurons, the ventricular neuroepithelium, and at later gestational ages, to the ependyma. Therefore, LIS-1 bears some resemblance to reelin, the gene product involved in the cortical mouse mutant reeler, in that Cajal-Retzius cells demonstrate immunolocalization. However, unlike reelin, LIS proteins are expressed not only in the Cajal Retzius cells, but also in the ventricular neuroepithelium, suggesting a potential role for this structure in neuronal migration.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗
PubMed1995

Platelet-activating factor produces neuronal growth cone collapse.

It is generally believed that neuronal growth cone collapsing factors are proteins that interact with membrane-bound receptors. Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine; PAF) - a phospholipid autocoid, also interacts with a membrane-bound neuronal receptor which is similar in nature to collapsing factor receptors. We report that PAF and the nonhydrolyzable PAF agonist, methyl carbamyl PAF (1-O-hexadecyl-2N-methylcarbamyl-sn-glycero-3-phosphocholine, mc-PAF), evoke a dose-dependent neuronal growth cone collapse. This collapse is specifically attenuated by the PAF receptor antagonist BN-52021. These data point to a PAF receptor-mediated growth cone collapse. Therefore, PAF must be added to the list of collapsing factors which potentially guide axons to their proper targets in the developing nervous system.

Animals↗
PubMed1994

Platelet-activating factor as a potential retrograde messenger in CA1 hippocampal long-term potentiation.

Long-term potentiation (LTP) refers to a persisting enhancement of neurotransmission that follows high-frequency activation of certain synapses. Although both pre- and postsynaptic mechanisms contribute to LTP, it is believed that the enhanced release of neurotransmitter that accompanies this process results from the production of a diffusible messenger in postsynaptic neurons which traverses the synaptic cleft and alters the function of presynaptic terminals. One candidate for such a messenger is arachidonic acid, a metabolite produced by phospholipase A2 which augments synaptic transmission when coupled with presynaptic stimulation. However, the effects of arachidonic acid require activation of the postsynaptic receptor for N-methyl-D-aspartate. Previously we found that platelet-activating factor (1 O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), another phospholipase A2-derived messenger, selectively enhances excitatory postsynaptic currents in hippocampal neurons by a presynaptic mechanism. We now present evidence that platelet-activating factor, acting at a receptor localized to synaptic regions, participates in LTP in the CA1 region of rat hippocampal slices and may serve as part of a retrograde signalling cascade.

Action Potentials↗
PubMed1994

Ethical dilemmas and decisions concerning the do-not-resuscitate patient undergoing anesthesia.

The growing geriatric population in this country makes it increasingly difficult to deal with the number of do-not-resuscitate (DNR) orders. In part, this is due to an increase in the number of elderly undergoing anesthesia and surgery. It can also be attributed to a rise in complex legal, ethical, and moral issues these orders pose for the healthcare professional caring for the DNR patients, including anesthetists. The term "DNR" is confusing to many, including healthcare professionals. As patients progress through the perioperative period, this confusion is compounded by the fact that administration of anesthesia encompasses interventions that include intubation, ventilation, and fluid replacement. These interventions may be regarded as resuscitative efforts outside the operating room. The anesthetist must identify and sort through a maze of conflicting courses of action, which must match the patient's desires and personal rights. The topic of DNR orders is addressed as well as some of the moral and ethical dilemmas they pose for the Certified Registered Nurse Anesthetist (CRNA). Some solutions are offered to help the nurse anesthetist make those decisions that are most "right" for the patient.

Decision Making↗
PubMed1993

Recurrent respiratory papillomatosis.

Recurrent respiratory papillomatosis lesions of the respiratory tract affect both children and adults. The etiology currently accepted by researchers is the human papillomavirus. Though relatively rare in children, these lesions can be serious and potentially fatal if the airway should become compromised or obstructed. Of the numerous treatments tested since recurrent respiratory papillomatosis was first described, the surgical laser is the most effective tool for controlling these lesions. A 5-year-old female patient who suffered from recurrent respiratory papillomatosis since the age of 2 and had over 80 laser treatments in a 3-year period is discussed. Laser and interferon administered concurrently lessened the frequency of laser therapy in this patient. Variations in number and size of recurrent respiratory papillomatosis lesions present a challenge to the anesthetist. Delivery of anesthesia requires precise preanesthetic planning and coordination. Since these lesions are most prevalent in the laryngeal airway, the surgical field must be shared by the anesthetist and the surgeon. The etiology, medical and physical therapeutic management, anesthetic management, and complications of patients with RRP are presented.

Child, Preschool↗
PubMed1992

Enhancement of hippocampal excitatory synaptic transmission by platelet-activating factor.

The biologically active lipid platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine; PAF) is a mediator of inflammatory and immune responses, and it accumulates in the brain during convulsions or ischemia. We have examined whether PAF may play a second messenger role in the central nervous system by studying effects on synaptic transmission in cultured hippocampal neurons. Carbamyl-PAF, a nonhydrolyzable PAF analog with a similar pharmacologic profile, augmented glutamate-mediated, evoked excitatory synaptic transmission and increased the frequency of spontaneous miniature excitatory synaptic events without increasing their amplitude or altering their time course. This compound had no significant effect on gamma-aminobutyric acid-mediated inhibitory synaptic responses. Lyso-PAF, the biologically inactive metabolic intermediate, had no effect on synaptic transmission. Moreover, the enhancement of excitatory synaptic transmission by carbamyl-PAF was blocked by a PAF receptor antagonist. These results indicate a specific presynaptic effect of PAF in enhancing excitatory synaptic transmission in cultured rat hippocampal neurons.

Animals↗
PubMed1992

Wheat germ agglutinin enhances EPSCs in cultured postnatal rat hippocampal neurons by blocking ionotropic quisqualate receptor desensitization.

1. The effect of the lectin wheat germ agglutinin (WGA), an inhibitor of ionotropic quisqualate receptor desensitization, on both evoked and spontaneous fast excitatory postsynaptic events was examined in cultured postnatal rat hippocampal neurons with the use of whole cell recordings. 2. WGA, at 580 nM, potentiated evoked fast excitatory postsynaptic currents (EPSCs) by increasing the amplitudes by 100 +/- 27% (mean +/- SE) and the time constant of decay from 5.8 +/- 0.6 to 7.9 +/- 0.5 ms. The increases in these parameters were not accompanied by changes in the current-voltage (I-V) relationship or pharmacological profile of the fast EPSCs. 3. WGA did not alter the amplitude or time course of decay of inhibitory postsynaptic currents (IPSCs), and it did not alter neuronal input resistance or action potentials. 4. WGA increased the amplitude of spontaneous fast miniature EPSCs (MEPSCs), defined as spontaneous EPSCs recorded in the presence of tetrodotoxin, by 53 +/- 11% and increased the time required to decay to 50% of the peak amplitude by 48 +/- 23%. These changes were not associated with a change in the rate of MEPSC occurrence. 5. These results suggest that WGA augments hippocampal excitatory postsynaptic events via a postsynaptic mechanism. The results further imply that ionotropic quisqualate receptor desensitization can modulate the amplitude and time course of decay of fast excitatory synaptic events. Thus desensitization may be one factor that regulates fast excitatory synaptic transmission.

Animals↗
PubMed1992

Measurement of free desipramine in serum by ultrafiltration with immunoassay.

We developed an ultrafiltration method for assaying free desipramine (DMI) in serum. An ultrafiltrate of DMI-containing serum was prepared by centrifugation through an Amicon Centrifree micropartition filter. Syva DMI solid-phase extraction (SPE) columns were used to extract the DMI from the serum and ultrafiltrate. The Syva monoclonal EMIT assay was used to quantify the DMI in the extract. In some experiments, the percent free DMI was quantified with radioactivity. Nonspecific losses of DMI in serum to the ultrafilter system were low (recoveries > 91%). Extraction of [3H]DMI from phosphate-buffered saline (to mimic serum ultrafiltrate) with the Syva SPE system was quantitative (recoveries of 98.4% +/- 4.6%). Free DMI concentrations, derived from serum containing 2.5-2500 micrograms/L DMI, were determined by ultrafiltration; results agreed well with values determined by equilibrium dialysis, the average percent of free DMI being 18.4% +/- 0.25% and 15.9% +/- 0.51%, respectively. To increase the sensitivity of the free DMI assay in the therapeutic range (total DMI 125-300 micrograms/L), we increased fourfold the ultrafiltrate volume applied to the SPE column. For free DMI at 11-130 micrograms/L, the within-run and between-run CVs for the ultrafiltration method were < 9% and < 15%, respectively. Binding of DMI to serum proteins decreased over the pH range 6.0-8.0, although temperatures between 20 and 28 degrees C did not affect binding. The ultrafiltration assay is fast, accurate, simple, and adaptable to standard laboratory instrumentation.

Desipramine↗