Coexpression of alpha and gamma enolase genes in neurons of adult rat brain

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Abstract
Enolase (EC 4.2.1.11) is a glycolytic enzyme active as a dimer. In adult brain extracts, three forms, αα, αγ and γγ, have been described, with the αγ hybrid accounting for 30% of total enolase activity (Fletcher et al., Dev Biol 65:462–475, 1978; Lucas et al., Dev Neurosci 10:91–98, 1988). Previous biochemical studies strongly suggest that this hybrid is not generated artefactually during the extraction procedures (Keller et al., J Neurochem 36:1389–1397, 1981; Shimizu et al., BBA 748:278–284, 1983). Immunocytological observations have demonstrated the cell specific localization of the α subunit in astrocytes and of the γ subunit in neurons at the adult stage, but failed to identify a cell type containing both the α and γ subunits necessary for the formation of the αγ hybrid isoform (Ghandour et al., Exp Brain Res 41:271–279, 1981; Vinores et al., J Histochem Cytochem 32:1295–1302, 1984; Iwanaga et al., Arch Histol Cytol [Suppl] 52:13–24, 1989). We sought to approach this question by performing in situ hybridization studies in order to visualize the α and γ mRNAs. In agreement with the immunocytological reports, we observe a specific accumulation of the γ enolase transcripts in neurons and a high accumulation of α enolase transcripts in some glial cells such as the ependymocytes lining the ventricles. Our observations, following hybridization with 35S labeled oligonucleotide specific probes on adjacent thin sections, demonstrate for the first time that transcription of both α and γ enolase genes occurs in many neurons of different brain regions. These results render highly probable the formation of the αγ hybrid in mature neurons. Furthermore, we observe a differential expression of the genes encoding the α and γ enolase subunits in various neuronal populations of the brain. The implications of these observations are discussed.