Cell-Specific Expression Pattern of Monocarboxylate Transporters in Astrocytes and Neurons Observed in Different Mouse Brain Cortical Cell Cultures Ruth Debernardi, Karin Pierre, Sylvain Lengacher, Pierre J. Magistretti, and Luc Pellerin * Institut de Physiologie, Lausanne, Switzerland Evidence suggests that lactate could be a preferential energy substrate transferred from astrocytes to neurons. Such a process implies the presence of specific mono- carboxylate transporters on both cell types. Expression of MCT1 and MCT2, two isoforms of the monocarboxy- late transporter (MCT) family, was studied in enriched cultures of mouse cortical astrocytes or neurons. It was observed that, at both the mRNA and the protein levels, astrocytes strongly expressed MCT1 but had very little if any MCT2. By contrast, neurons had high amounts of MCT2 mRNA, although MCT1 mRNA was also detected. Double immunofluorescent labelings with appropriate markers confirmed the cell-specific preference in the expression of MCT1 and MCT2, but they revealed that a subset of neurons expresses low to moderate levels of MCT1. Parallel immunocytochemical stainings of cul- tured neurons with the presynaptic marker synaptophy- sin showed that MCT2 expression is correlated with syn- aptic development. Although MCT2 and synaptophysin were not colocalized, their distribution was similar, and they were often closely apposed, suggesting that MCT2 could be associated with postsynaptic terminals. Inter- action between astrocytes and neurons, as occurring in layered cultures, did not modify the levels of MCT1 and MCT2 expression or their distribution and cell-specific preference under the conditions used. However, a close apposition between neurites and MCT1-expressing as- trocytic processes was apparent and developed as cul- tures evolved. In addition to providing an extensive de- scription of MCT distribution in cultured cells, our data underscore the potential of such preparations for future studies on the regulation of MCT expression. © 2003 Wiley-Liss, Inc. Key words: MCT1; MCT2; lactate; energy metabolism Monocarboxylate transporters (MCTs) form a family of H + -dependent membrane carriers involved in the transport of lactate and pyruvate as well as the ketone bodies acetoacetate and -hydroxybutyrate (for review see Halestrap and Price, 1999). Nine members of this group, MCT1–MCT9, have been identified so far by sequence homologies, but only the first four MCTs (MCT1– MCT4) have been functionally characterized and proven to transport monocarboxylates (Garcia et al., 1994, 1995; Yoon et al., 1997; Manning Fox et al., 2000). Evidence has been found that some of these proteins are present in the central nervous system, in addition to their expression in many peripheral tissues. This is the case for MCT1 and MCT2, for which a distribution at the mRNA and protein levels has been provided in both rat and mouse brain (Gerhart et al., 1997, 1998; Koehler-Stec et al., 1998; Pellerin et al., 1998a; Pierre et al., 2000, 2002). Moreover, some indications that MCT4 could be expressed at least in the cerebellum have been obtained recently (Bergersen et al., 2001, 2002), whereas MCT3 appears to be restricted to the retinal pigment epithelium (Philp et al., 1998). It has been known for some time that, at the cellular level, both astrocytes and neurons in cultures exhibit sat- urable monocarboxylate transport (Dringen et al., 1993, 1995; Nedergaard and Goldman, 1993; Tildon et al., 1993). Moreover, it was suggested based on kinetic studies that each cell type might contain distinct monocarboxylate transporter isoforms and maybe even more than one iso- form (Tildon et al., 1993; McKenna et al., 1998). Little information is available on the identity of these MCT isoforms present on astrocytes and neurons in vitro. A first study reported that mouse cortical astrocytes contained large amounts of MCT1 mRNA, whereas mouse cortical neurons were enriched in MCT2 mRNA (Bro ¨er et al., 1997). In another study performed on rat cortical astro- cytes, it was shown that both MCT1 and MCT2 were present, as revealed by immunocytochemistry (Hanu et al., 2000). To provide a more comprehensive picture of the expression and distribution of MCTs on both brain cell *Correspondence to: Dr. Luc Pellerin, Institut de Physiologie, 7 rue du Bugnon, 1005 Lausanne, Switzerland. E-mail: luc.pellerin@iphysiol.unil.ch Received 27 January 2003; Revised 18 March 2003; Accepted 21 March 2003 Journal of Neuroscience Research 73:141–155 (2003) © 2003 Wiley-Liss, Inc.