Covalent labeling of the cocaine-sensitive catecholamine transporter

Effects of Antidepressants on DSP4/CPT-Induced DNA Damage Response in Neuroblastoma SH-SY5Y Cells

DNA damage is a form of cell stress and injury. Increased systemic DNA damage is related to the pathogenic development of neurodegenerative diseases. Depression occurs in a relatively high percentage of patients suffering from degenerative diseases, for whom antidepressants are often used to relieve depressive symptoms. However, few studies have attempted to elucidate why different groups of antidepressants have similar effects on relieving symptoms of depression. Previously, we demonstrated that neurotoxins N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4)- and camptothecin (CPT) induced the DNA damage response in SH-SY5Y cells, and DSP4 caused cell cycle arrest which was predominately in the S-phase. The present study shows that CPT treatment also resulted in similar cell cycle arrest. Some classic antidepressants could reduce the DNA damage response induced by DSP4 or CPT in SH-SY5Y cells. Cell viability examination demonstrated that both DSP4 and CPT caused cell death, which was prevented by spontaneous administration of some tested antidepressants. Flow cytometric analysis demonstrated that a majority of the tested antidepressants protect cells from being arrested in S-phase. These results suggest that blocking the DNA damage response may be an important pharmacologic characteristic of antidepressants. Exploring the underlying mechanisms may allow for advances in the effort to improve therapeutic strategies for depression appearing in degenerative and psychiatric diseases.

Neurotoxin-induced DNA damage is persistent in SH-SY5Y cells and LC neurons

Degeneration of the noradrenergic neurons has been reported in the brain of patients suffering from neurodegenerative diseases. However, their pathological characteristics during the neurodegenerative course and underlying mechanisms remain to be elucidated. In the present study, we used the neurotoxin camptothecin (CPT) to induce the DNA damage response in neuroblastoma SH-SY5Y cells, normal fibroblast cells, and primarily cultured locus coeruleus (LC) and raphe neurons to examine cellular responses and repair capabilities after neurotoxin exposure. To our knowledge, the present study is the first to show that noradrenergic SH-SY5Y cells are more sensitive to CPT-induced DNA damage and deficient in DNA repair, as compared to fibroblast cells. Furthermore, similar to SH-SY5Y cells, primarily cultured LC neurons are more sensitive to CPT-induced DNA damage and show a deficiency in repairing this damage. Moreover, while N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) exposure also results in DNA damage in cultured LC neurons, neither CPT nor DSP4 induce DNA damage in neuronal cultures from the raphe nuclei. Taken together, noradrenergic SH-SY5Y cells and LC neurons are sensitive to CPT-induced DNA damage and exhibit a repair deficiency, providing a mechanistic explanation for the pathological characteristics of LC degeneration when facing endogenous and environmental DNA-damaging insults in vivo.

Effects of DSP4 on the noradrenergic phenotypes and its potential molecular mechanisms in SH-SY5Y cells

Dopamine β-hydroxylase (DBH) and norepinephrine (NE) transporter (NET) are the noradrenergic phenotypes for their functional importance to noradrenergic neurons. It is known that in vivo N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) treatment induces degeneration of noradrenergic terminals by interacting with NET and depleting intracellular NE. However, DSP4's precise mechanism of action remains unclear. In this study various biochemical approaches were employed to test the hypothesis that DSP4 down-regulates the expression of DBH and NET, and to determine molecular mechanisms that may be involved. The results showed that treatment of SH-SY5Y neuroblastoma cells with DSP4 significantly decreased mRNA and protein levels of DBH and NET. DSP4-induced reduction of DBH mRNA and proteins, as well as NET proteins showed a time- and concentration-dependent manner. Flow cytometric analysis demonstrated that DSP4-treated cells were arrested predominantly in the S-phase, which was reversible. The arrest was confirmed by several DNA damage response markers (phosphorylation of H2AX and p53), suggesting that DSP4 causes replication stress which triggers cell cycle arrest via the S-phase checkpoints. Moreover, the comet assay verified that DSP4 induced single-strand DNA breaks. In summary, the present study demonstrated that DSP4 down-regulates the noradrenergic phenotypes, which may be mediated by its actions on DNA replication, leading to replication stress and cell cycle arrest. These action mechanisms of DSP4 may account for its degenerative consequence after systematic administration for animal models.

Interference of the noradrenergic neurotoxin DSP4 with neuronal and nonneuronal monoamine transporters

The haloalkylamine DSP4 (N[-2-chloroethyl]-N-ethyl-2-bromobenzylamine) is a noradrenergic neurotoxin, which is used for the chemical denervation of noradrenergic neurons, and it has been proposed to be a selective substrate for the neuronal, Na(+)- and Cl(-)-dependent noradrenaline transporter (NAT). In the present study, we investigated whether DSP4 not only interacts with the human NAT (hNAT) but also with other neuronal monoamine transporters such as the transporters for dopamine (hDAT) and serotonin (hSERT) or with nonneuronal (Na(+)-independent) monoamine transporters also known as organic cation transporters (OCTs), such as hOCT(1), hOCT(2), and hOCT(3). Using human embryonic kidney HEK293 cells heterologously expressing the corresponding transporter, we show that DSP4 irreversibly inhibits the hNAT, hDAT, hSERT, and hOCT(3). However, this inhibition includes a reversible component at the hDAT, hSERT, and hOCT(3) but not at the hNAT. The inhibitory potency of DSP4 at the neuronal transporters was highest at the hNAT (IC(50) about 5 microM), and it was about five and 40 times lower at the hSERT and hDAT, respectively. DSP4 inhibited all three hOCTs with high potency (IC(50) about 1 microM) but in a completely reversible manner at hOCT(1) and hOCT(2). Cytotoxicity by 24-h exposure of hNAT- or hOCT-expressing cells to low DSP4 concentrations (<10 microM) could be observed only in hNAT-expressing cells. Thus, DSP4's high-affinity uptake through the NAT together with its completely irreversible mode of interaction with the NAT may contribute to its selectivity as noradrenergic neurotoxin.

Antipeptide antibodies confirm the topology of the human norepinephrine transporter

We have raised polyclonal antibodies (N6-28, L211-226, L371-384, and C590-607) against peptides corresponding to hydrophilic sequences of the human norepinephrine transporter (hNET). The antisera immunoprecipitated the [35S]Met-labeled hNET. Antiserum L211-226, directed against a sequence of the putative second (large) extracellular loop of hNET, also immunoprecipitated the human dopamine transporter. Antisera N6-28 and C590-607, raised against a hNET peptide region of the N and the C termini, respectively, recognized a 58-kDa protein from transfected COS-7 cells expressing the hNET. This 58-kDa species represents a functional, glycosylated form of the hNET and not a degradation product. Tunicamycin treatment of transfected COS-7 cells as well as peptide-N-glycosidase F digestion of the transporter converted the 58-kDa species to a 50-kDa form, indicating that the latter represents the hNET core protein. In indirect immunofluorescence studies, our antisera confirmed the originally proposed topology of hNET. Antisera N6-28 and C590-607 detected hNET only in permeabilized cells. In contrast, antisera L211-226 and L371-384 directed against peptide sequences of the second and fourth putative extracellular loop displayed fluorescence signals with the intact cells.

PC12 variants deficient in norepinephrine transporter mRNA have wild type activities of several other related transporters

Wild type PC12 pheochromocytoma cells express a Na(+)-dependent norepinephrine transporter that operates in the uptake of catecholamines. In addition to the previously described Na(+)-dependent system A for the uptake of alpha-amino-isobutyric acid and system Gly for glycine, we have identified two other Na(+)-dependent transporter systems for amino acid uptake in these cells: 1) system beta for beta-alanine and taurine; and 2) a system for creatine. Uptake of alpha-amino-isobutyric acid, glycine, beta-alanine, and creatine is not affected in some PC12 variants that were previously shown to be deficient in catecholamine uptake and to have decreased levels of norepinephrine transporter mRNA. We have isolated two PC12 cDNA clones that are essentially identical in sequence to recently reported cDNAs for rat brain taurine and creatine transporters, respectively, and a third cDNA that appears to code for a novel transporter. mRNAs for these three transporters are present at wild type levels in those variants that express no or little norepinephrine transporter mRNA. These results support the notion that the expression of catecholamine reuptake transporters may be particularly susceptible to down-regulation.

Molecular aspects of the neuronal noradrenaline transporter

The neuronal noradrenaline transporter was partially purified by means of low and high pressure liquid chromatography using anion exchange, gel filtration and lectin affinity columns. A protein characterized by a molecular weight of 50-53 kilodalton was enriched; it may represent the transporter or a component of it. In addition, a RNA fraction characterized by a mean size of 2 kilobases was isolated from PC12 rat phaeochromocytoma cells and from bovine adrenal medulla; this RNA fraction caused expression of the noradrenaline transporter after microinjection into Xenopus laevis oocytes.