Protein interacting with C kinase 1 (PICK1) and GluR2 are associated with presynaptic plasma membrane and vesicles in hippocampal excitatory synapses

AMPA receptors have been identified in different populations of presynaptic terminals and found to be involved in the modulation of neurotransmitter release. The mechanisms that govern the expression of presynaptic AMPA receptors are not known. One possibility is that pre- and postsynaptic AMPA receptors are regulated according to the same principles. To address this hypothesis we investigated whether protein interacting with C kinase 1 (PICK1), known to interact with AMPA receptors postsynaptically, also is expressed presynaptically, together with AMPA receptors. Subfractionation and high-resolution immunogold analyses of the rat hippocampus revealed that GluR2 and PICK1 are enriched postsynaptically, but also in presynaptic membrane compartments, including the active zone and vesicular membranes. PICK1 and GluR2 are associated with the same vesicles, which are immunopositive also for synaptophysin and vesicle-associated membrane protein 2. Based on what is known about the function of PICK1 postsynaptically, the present data suggest that PICK1 is involved in the regulation of presynaptic AMPA receptor trafficking and in determining the size of the AMPA receptor pool that modulates presynaptic glutamate release.

Specificity of antibodies: unexpected cross-reactivity of antibodies directed against the excitatory amino acid transporter 3 (EAAT3)

Specific antibodies are essential tools for identifying individual proteins in biological samples. While generation of antibodies is often straightforward, determination of the antibody specificity is not. Here we illustrate this by describing the production and characterization of antibodies to excitatory amino acid transporter 3 (EAAT3). We synthesized 13 peptides corresponding to parts of the EAAT3 sequence and immunized 6 sheep and 30 rabbits. All sera were affinity purified against the relevant immobilized peptide. Antibodies to the peptides were obtained in almost all cases. Immunoblotting with tissue extracts from wild type and EAAT3 knockout animals revealed that most of the antibodies did not recognize the native EAAT3 protein, and that some recognized other proteins. Several immunization protocols were tried, but strong reactions with EAAT3 were only seen with antibodies to the C-terminal peptides. In contrast, good antibodies were obtained to several parts of EAAT2. EAAT3 was only detected in neurons. However, rabbits immunized with an EAAT3-peptide corresponding to residues 479-498 produced antibodies that labeled axoplasm and microtubules therein particularly strongly. On blots, these antibodies recognized both EAAT3 and a slightly smaller, but far more abundant protein that turned out to be tubulin. The antibodies were fractionated on columns with immobilized tubulin. One fraction contained antibodies apparently specific for EAAT3 while another fraction contained antibodies recognizing both EAAT3 and tubulin despite the lack of primary sequence identity between the two proteins. Addition of free peptide to the incubation solution blocked immunostaining of both EAAT3 and tubulin.

CONCLUSIONS

Not all antibodies to synthetic peptides recognize the native protein. The peptide sequence is more important than immunization protocol. The specificity of an antibody is hard to predict because cross-reactivity can be specific and to unrelated molecules. The antigen preabsorption test is of little value in testing the specificity of affinity purified antibodies.

Highly differential expression of the monocarboxylate transporters MCT2 and MCT4 in the developing rat brain

Monocarboxylate transporters (MCTs) play an important role in the metabolism of all cells. They mediate the transport of lactate and pyruvate but also some other substrates such as ketone bodies. It has been proposed that glial cells release monocarboxylates to fuel neighbouring neurons. A key element in this hypothesis is the existence of neuronal MCTs. Amongst the three MCTs known to be expressed in the brain (MCT1, 2 and 4) only MCT2 has been found in neurons. Here we have studied the expression pattern of MCT2 during postnatal development. By use of immunoperoxidase and double immunofluorescence microscopy we report that neuronal MCT2 occurs in most brain areas, including the hippocampus and cerebellum, from birth to adult. MCT2 is also expressed in specific subpopulations of astrocytes. Neuronal MCT2 is most abundant in the first 3 postnatal weeks and thereafter decreases toward adulthood. In contrast to MCT2, MCT4 is exclusively present in astroglia during all stages of development. Furthermore, MCT4 expression is very low at birth and reaches adult level by P14. Our results are consistent with previous data suggesting that in the immature brain much of the energy demand is met by monocarboxylates and ketone bodies.