Biomedical subjects

Houbo Jiang

Publications and source records attributed to Houbo Jiang.

5 recordsLinked to original sources

PubMed2006

Parkin suppresses the expression of monoamine oxidases.

Mutations of parkin are linked to early onset Parkinson disease. Here we show that stable transfection of parkin in the human dopaminergic neuroblastoma cell line SH-SY5Y markedly reduced the activities of both monoamine oxidase (MAO) A and B. The amount of 3,4-dihydroxyphenylacetic acid, which is produced during dopamine oxidation by MAO, was greatly reduced by parkin overexpression. Radioligand binding assays showed that MAO binding sites were decreased accordingly. Consistent with these, MAO-B protein level was much lower, whereas the amount of MAO-A protein was not determined due to the lack of a suitable antibody. Co-transfection of either MAO with parkin in HEK293 cells did not significantly alter ubiquitination and degradation of each MAO. When we measured MAO expression by real-time quantitative reverse transcription-PCR, marked reductions were seen in SH-SY5Y cells stably expressing parkin compared with the parental cells or a control line stably transfected with luciferase. In addition, parkin mutants defective in E3 ligase activity exhibited different effects on MAO expression. We found that parkin also significantly decreased mRNA levels of both MAOs in the mouse fibroblast cell line NIH3T3. Furthermore, MAO expression was significantly increased in human B lymphocyte cell lines derived from Parkinson disease patients with homozygous but not heterozygous deletion of exon 4 of parkin. Together these results suggest that parkin suppresses MAO expression. This function may limit the production of reactive oxygen species generated by MAO in dopamine oxidation and would, thus, be beneficial to the survival of dopaminergic neurons.

3,4-Dihydroxyphenylacetic Acid↗

PubMed2005

Selective vulnerability of dopaminergic neurons to microtubule depolymerization.

Parkinson disease (PD) is characterized by the specific degeneration of dopaminergic (DA) neurons in substantia nigra and has been linked to a variety of environmental and genetic factors. Rotenone, an environmental PD toxin, exhibited much greater toxicity to DA neurons in midbrain neuronal cultures than to non-DA neurons. The effect was significantly decreased by the microtubule-stabilizing drug taxol and mimicked by microtubule-depolymerizing agents such as colchicine or nocodazole. Microtubule depolymerization disrupted vesicular transport along microtubules and caused the accumulation of dopamine vesicles in the soma. This led to increased oxidative stress due to oxidation of cytosolic dopamine leaked from vesicles. Inhibition of dopamine metabolism significantly reduced rotenone toxicity. Thus, our results suggest that microtubule depolymerization induced by PD toxins such as rotenone plays a key role in the selective death of dopaminergic neurons.

Cell Death↗

PubMed2004

Parkin increases dopamine uptake by enhancing the cell surface expression of dopamine transporter.

Mutations of parkin, a protein-ubiquitin E3 ligase, are linked to Parkinson's disease (PD). Although a variety of parkin substrates have been identified, none of these is selectively expressed in dopaminergic neurons, whose degeneration plays a critical role in PD. Here we show that parkin significantly increased dopamine uptake in the human dopaminergic neuroblastoma cell line SH-SY5Y. This effect was accompanied by increased V(max) of dopamine uptake and unchanged K(m). Consistent with this, increased binding sites for dopamine transporter (DAT) ligand were observed in SH-SY5Y cells overexpressing parkin. The results were confirmed when parkin was transfected in HEK293 cells stably expressing DAT. In these cells, parkin enhanced the ubiquitination and degradation of DAT, increased its cell surface expression, and augmented dopamine uptake. The effects of parkin were significantly abrogated by its PD-causing mutations. Because the cell surface expression of functional DAT requires its oligomerization, misfolded DAT, induced either by the protein glycosylation inhibitor tunicamycin or by its C-terminal truncation, significantly attenuated cell surface expression of native DAT and reduced dopamine uptake. Expression of parkin, but not its T240R mutant, significantly alleviated these detrimental effects of misfolded DAT. Thus, our studies suggest that parkin increases dopamine uptake by enhancing the ubiquitination and degradation of misfolded DAT, so as to prevent it from interfering with the oligomerization and cell surface expression of native DAT. This function of parkin would enhance the precision of dopaminergic transmission, increase the efficiency of dopamine utilization, and reduce dopamine toxicity on neighboring cells.

Animals↗

PubMed2004

Parkin protects human dopaminergic neuroblastoma cells against dopamine-induced apoptosis.

Parkinson's disease (PD) is characterized by the selective degeneration of dopaminergic (DA) neurons in substantia nigra pars compacta (SNpc). A combination of genetic and environmental factors contributes to such a specific loss. Among the five PD-linked genes identified so far, parkin, a protein-ubiquitin E3 ligase, appears to be the most prevalent genetic factor in PD. Although a variety of substrates have been identified for parkin, none of them is selectively expressed in nigral DA neurons. It remains unclear how accumulation of these substrates in the absence of functional parkin may cause the selective death of DA neurons in SNpc. Here, we show that overexpression of parkin protected human DA neuroblastoma cell line (SH-SY5Y) against apoptosis induced by DA or 6-OHDA, but not by H(2)O(2) or rotenone. Parkin significantly attenuated dopamine-induced activation of c-Jun N-terminal kinase (JNK) and caspase-3. It also decreased the level of reactive oxygen species (ROS) and protein carbonyls in the cell. Inhibiting DA uptake through dopamine transporter or treating the cell with antioxidants significantly reduced oxidative stress and dopamine toxicity. Furthermore, PD-linked mutations of parkin significantly abrogated the protective effect of wild-type parkin, as well as its ability to suppress ROS and protein carbonylation. These results suggest that parkin protects against dopamine toxicity by decreasing oxidative stress and ensuing activation of apoptotic programs such as the JNK/caspase pathway. This protective function of parkin, which is greatly attenuated by its PD-linked mutations, may be uniquely important for the survival of DA neurons, as they are constantly threatened by oxyradicals produced during dopamine oxidation.

Apoptosis↗

PubMed2003

Impaired modulation of GABAergic transmission by muscarinic receptors in a mouse transgenic model of Alzheimer's disease.

It has long been recognized that muscarinic acetylcholine receptors (mAChRs) are crucial for the control of cognitive processes, and drugs that activate mAChRs are helpful in ameliorating cognitive deficits of Alzheimer's disease (AD). On the other hand, GABAergic transmission in prefrontal cortex (PFC) plays a key role in "working memory" via controlling the timing of neuronal activity during cognitive operations. To test whether the muscarinic and gamma-aminobutyric acid (GABA) system are interconnected in normal cognition and dementia, we examined the muscarinic regulation of GABAergic transmission in PFC of an animal model of AD. Transgenic mice overexpressing a mutant gene for beta-amyloid precursor protein (APP) show behavioral and histopathological abnormalities resembling AD and, therefore, were used as an AD model. Application of the mAChR agonist carbachol significantly increased the spontaneous inhibitory postsynaptic current (sIPSC) frequency and amplitude in PFC pyramidal neurons from wild-type animals. In contrast, carbachol failed to increase the sIPSC amplitude in APP transgenic mice, whereas the carbachol-induced increase of the sIPSC frequency was not significantly changed in these mutants. Similar results were obtained in rat PFC slices pretreated with the beta-amyloid peptide (Abeta). Inhibiting protein kinase C (PKC) blocked the carbachol enhancement of sIPSC amplitudes, implicating the PKC dependence of this mAChR effect. In APP transgenic mice, carbachol failed to activate PKC despite the apparently normal expression of mAChRs. These results show that the muscarinic regulation of GABA transmission is impaired in the AD model, probably due to the Abeta-mediated interference of mAChR activation of PKC.

Action Potentials↗