| Summary: | 博士 === 國立中山大學 === 生物科學系研究所 === 93 === In eukaryotic cells, most proteins in the cytosol and nucleus are degraded via the ubiquitin-proteasome pathway. Ubiquitin is best known for its role in targeting proteins for degradation by the proteasome. Ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) is found specifically in central and peripheral neurons, and is responsible for the removal of small peptide fragments from the ubiquitin chain and for co-translational processing of ubiquitin gene products to generate free monomeric ubiquitin. In response to extreme conditions, cells exhibit an up-regulation of heat shock protein (HSP) expression, which contributes to repair and protective mechanisms. Within the HSP family, HSP70 is the major inducible member that protects against cell death. Based on the pharmacologic property of organophosphates as an inhibitor of cholinesterase, it is generally contended that manifestations of organophosphate poisoning, including secretion and muscle fasciculation, stupor, cardiopulmonary collapse, respiratory failure, coma or death, result from accumulation of, and over-stimulation by acetylcholine at peripheral of central synapses. One approach in furthering our understanding on organophosphate poisoning is delineation of its potential protective mechanisms. In this regard, the information on the cellular and molecular mechanisms that underlie organophosphate poisoning has received attention.
Our laboratory demonstrated previously that a crucial brain site via which mevinphos (Mev), an organophosphate insecticide of the P=O type, acts is the rostral ventrolateral medulla (RVLM), the medullary origin of premotor sympathetic neurons that are responsible for the maintenance of vasomotor tone. The phasic changes in cardiovascular events over the course of acute Mev intoxication also parallel fluctuations of the “life-and-death” signals that emanate form the RVLM. Based on a rat model of organophosphate poisoning that provides continuous information on cellular and molecular mechanisms in the RVLM, the present study was undertaken to evaluate whether changes in protein level of UCH-L1 or HSP70 are associated with death arising from Mev intoxication. We also evaluated the efficacy of both of them in the neuroprotection against fatality during Mev intoxication.
The first part of this study investigated whether UCH-L1 plays a neuroprotective role at the RVLM, where Mev acts to elicit cardiovascular toxicity. In Sprague-Dawley rats maintained under propofol anesthesia, Mev (960 µg/kg, i.v.) induced a parallel and progressive augmentation in UCH-L1 or ubiquitin expression at the ventrolateral medulla during the course of Mev intoxication. The increase in UCH-L1 level was significantly blunted on pretreatment with microinjection bilaterally into the RVLM of a transcription inhibitor, actinomycin D (5 nmol) or a translation inhibitor, cycloheximide (20 nmol). Compared to artificial cerebrospinal fluid (aCSF) or sense uch-L1 oligonucleotide (100 pmol) pretreatment, microinjection of an antisense uch-L1 oligonucleotide (100 pmol) bilaterally into the RVLM significantly increased mortality, reduced the duration of the phase I (“pro- life” phase), blunted the increase in ubiquitin expression in ventrolateral medulla, and augmented the induced hypotension in rats that received Mev.
The second part of this study investigated whether HSP70 plays a neuroprotective role at the RVLM. Intravenous administration of Mev (960
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