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

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.