Successful identification of glycine transporter inhibitors using an adaptation of a functional cell-based assay.

Glycine transporter (GlyT1) function is typically measured by radiolabeled glycine uptake using lysis methods or scintillation proximity assays (SPAs), which have limited throughput. This study shows the adaptation of the standard cell lysis method to a screening assay with improved throughput and assay characteristics. The assay takes advantage of the 384-well format, standard laboratory automation, and cryopreserved CHO-K1 cells stably overexpressing human GlyT1a transporter (CHO-K1/hGlyT1a) that were validated and banked in advance of screening. The assay was evaluated for the time course of glycine uptake, K(m), V(max), Z' factor analysis, and IC(50) value determination with reference GlyT1 inhibitors. Screening of 118,000 compounds at 10 microM identified 4556 compounds (3.9%) as inhibitors. Positive compounds (>50% inhibition) were retested in the assay at 4 inhibitor concentrations. Compounds demonstrating greater than 40% inhibition at 10 microM were considered as confirmed positives, yielding a 68% confirmation rate from the original screen. To eliminate compounds that nonspecifically inhibited glycine uptake, IC(50) values were determined in both GlyT1 and GlyT2 assays, and those compounds that inhibited GlyT2 were removed from consideration. The screening campaign identified 300 small molecules as selective GlyT1 inhibitors for lead optimization, demonstrating the utility of this cost-effective method.

Inhibition of mixed lineage kinase 3 attenuates MPP+-induced neurotoxicity in SH-SY5Y cells.

The neuropathology of Parkinson's Disease has been modeled in experimental animals following MPTP treatment and in dopaminergic cells in culture treated with the MPTP neurotoxic metabolite, MPP(+). MPTP through MPP(+) activates the stress-activated c-Jun N-terminal kinase (JNK) pathway in mice and SH-SY5Y neuroblastoma cells. Recently, it was demonstrated that CEP-1347/KT7515 attenuated MPTP-induced nigrostriatal dopaminergic neuron degeneration in mice, as well as MPTP-induced JNK phosphorylation. Presumably, CEP-1347 acts through inhibition of at least one upstream kinase within the mixed lineage kinase (MLK) family since it has been shown to inhibit MLK 1, 2 and 3 in vitro. Activation of the MLK family leads to JNK activation. In this study, the potential role of MLK and the JNK pathway was examined in MPP(+)-induced cell death of differentiated SH-SY5Y cells using CEP-1347 as a pharmacological probe and dominant negative adenoviral constructs to MLKs. CEP-1347 inhibited MPP(+)-induced cell death and the morphological features of apoptosis. CEP-1347 also prevented MPP(+)-induced JNK activation in SH-SY5Y cells. Endogenous MLK 3 expression was demonstrated in SH-SY5Y cells through protein levels and RT-PCR. Adenoviral infection of SH-SY5Y cells with a dominant negative MLK 3 construct attenuated the MPP(+)-mediated increase in activated JNK levels and inhibited neuronal death following MPP(+) addition compared to cultures infected with a control construct. Adenoviral dominant negative constructs of two other MLK family members (MLK 2 and DLK) did not protect against MPP(+)-induced cell death. These studies show that inhibition of the MLK 3/JNK pathway attenuates MPP(+)-mediated SH-SY5Y cell death in culture and supports the mechanism of action of CEP-1347 as an MLK family inhibitor.