A novel postsynaptic density protein: the monocarboxylate transporter MCT2 is co-localized with delta-glutamate receptors in postsynaptic densities of parallel fiber-Purkinje cell synapses.

Confocal immunofluorescence microscopy showed strong monocarboxylate transporter 2 (MCT2) labeling of Purkinje cell bodies and punctate labeling in the molecular layer. By immunogold cytochemistry, it could be demonstrated that the MCT2 immunosignal was concentrated at postsynaptic densities of parallel fiber-Purkinje cell synapses. The distribution of MCT2 transporters within the individual postsynaptic densities mimicked that of the delta2 glutamate receptor, as shown by use of two different gold-particle sizes. The MCT2 distribution was also compared with the distributions of other monocarboxylate transporters (MCT1 and MCT4). The MCT1 immunolabeling was localized in the endothelial cells, while MCT4 immunogold particles were associated with glial profiles, including those abutting the synaptic cleft of the parallel fiber-spine synapses. The postsynaptic density (PSD) molecules identified so far can be divided into five classes: receptors, their anchoring molecules, molecules involved in signal transduction, ion channels, and attachment proteins. Here, we provide evidence that this list of molecules must now be extended to comprise an organic molecule transporter: the monocarboxylate transporter MCT2. The present data suggest that MCT2 has specific transport functions related to the synaptic cleft and that this transporter may allow an influx of lactate derived from perisynaptic glial processes. The expression of MCT2 in synaptic membranes may allow energy supply to be tuned to the excitatory drive.