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The glycolytic enzyme enolase (EC 4.2.1.11) exists as dimers formed from three structurally related subunits α, β, and γ, encoded by separate genes. The gene encoding the β-subunit is expressed only in striated muscles. We have previously shown that the β-enolase gene belongs to a small subset of muscle-specific genes showing transcriptional activity in cultured myoblasts, prior to withdrawal from the cell cycle. An increase in the level of β-enolase mRNA occurs during terminal differentiation of myoblasts. To investigate the mechanisms underlying this increase, we have simultaneously estimated, under steady state conditions, the rate of synthesis and the stability of β-enolase mRNA in proliferating C2.7 myoblasts as well as in differentiating myotubes. The method used is based on the isolation of newly synthesized RNA from the total RNA pool, following pulse-labeling of intact cells in the presence of 4-thiouridine. The results described here demonstrate a coordinate increase in newly synthesized and total β-enolase mRNA, while the mRNA half-life, about 4 hr, remains unchanged in the course of terminal differentiation. The expression of the gene for insulin-like growth factor-II (IGF-II), a major positive regulator of myogenesis, was analyzed using the same approach. It is concluded that the up-regulation of β-enolase as well as IGF-II gene expression in differentiating muscle cells reflects an increased rate of entry of newly synthesized mRNAs into the general pool of transcripts without changes in their respective half-lives. © 1995 Wiley-Liss, Inc.

Enolase (EC 4.2.1.11) is a glycolytic enzyme active as a dimer. In adult brain extracts, three forms, alpha alpha, alpha gamma and gamma gamma, have been described, with the alpha gamma 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 alpha subunit in astrocytes and of the gamma subunit in neurons at the adult stage, but failed to identify a cell type containing both the alpha and gamma subunits necessary for the formation of the alpha gamma 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 alpha and gamma mRNAs. In agreement with the immunocytological reports, we observe a specific accumulation of the gamma enolase transcripts in neurons and a high accumulation of alpha 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 alpha and gamma enolase genes occurs in many neurons of different brain regions. These results render highly probable the formation of the alpha gamma hybrid in mature neurons. Furthermore, we observe a differential expression of the genes encoding the alpha and gamma enolase subunits in various neuronal populations of the brain. The implications of these observations are discussed.

The cDNA for rabbit muscle-specific (betabeta) enolase was cloned, sequenced and expressed in Escherichia coli. This betabeta-enolase differs at eight positions from that sequenced by Chin (17). Site-directed mutagenesis was used to change residue 414 from glutamate to leucine, thereby abolishing a salt bridge involved in subunit contacts. Recombinant wild-type and mutant enolase were purified from E. coli and compared to enolase isolated from rabbit muscle. Molecular weights were determined by mass spectrometry. All three betabeta-enolases had similar kinetics, and UV and circular dichroism (CD) spectra. The mutant enolase was far more sensitive to inactivation by pressure, by KCl or EDTA, and by sodium perchlorate. We confirmed, by analytical ultracentrifugation, that the sodium perchlorate inactivation was due to dissociation. DeltaG(o) for dissociation of enolase was decreased from 49.7 kJ/mol for the wild-type enzyme to 42.3 kJ/mol for the mutant. In contrast to the wild-type enzyme, perchlorate inactivation of E414L was accompanied by a small loss of secondary structure.

During muscle development, an isozymic transition of the glycolytic enzyme enolase occurs from the embryonic and ubiquitous alphaalpha-isoform to the muscle-specific betabeta-isoform. Here, we demonstrate a stimulatory role of thyroid hormones on these two enolase genes during rat development in hindlimb muscles and an inhibitory effect on the muscle-specific enolase gene in cardiac muscle. In hindlimb muscles the ubiquitous alpha-transcript level is diminished by hypothyroidism, starting at birth. On the contrary, the more abundant muscle-specific beta-transcript is insensitive to hypothyroidism before establishment of the functional diversification of fibers and is greatly decreased thereafter. Our data support the hypothesis of a role of thyroid hormones in coordinating the expressions of contractile proteins and metabolic enzymes during muscle development. The subcellular localization of isoenolases, established here, is not modified by hypothyroidism. Our results underline the specificity of action of thyroid hormones, which modulate differentially two isozymes in the same muscle and regulate, in opposite directions, the expression of the same gene in two different muscles.

The glycolytic enzyme enolase (EC 4.2.1.11) is active as dimers formed from three subunits encoded by different genes. The embryonic alphaalpha isoform remains distributed in many adult cell types, whereas a transition towards betabeta and gammagamma isoforms occurs in striated muscle cells and neurons respectively. It is not understood why enolase exhibits tissue-specific isoforms with very close functional properties. We approached this problem by the purification of native betabeta-enolase from mouse hindlimb muscles and by raising specific antibodies of high titre against this protein. These reagents have been useful in revealing a heterogeneity of the beta-enolase subunit that changes with in vivo and in vitro maturation. A basic carboxypeptidase appears to be involved in generating an acidic beta-enolase variant, and may regulate plasminogen binding by this subunit. We show for the first time that pure betabeta-enolase binds with high affinity the adjacent enzymes in the glycolytic pathway (pyruvate kinase and phosphoglycerate mutase), favouring the hypothesis that these three enzymes form a functional glycolytic segment. betabeta-Enolase binds with high affinity sarcomeric troponin but not actin and tropomyosin. Some of these binding properties are shared by the alphaalpha-isoenolase, which is also expressed in striated muscle, but not by the neuron-specific gammagamma-enolase. These results support the idea that specific interactions with macromolecules will address muscle enolase isoforms at the subcellular site where ATP, produced through glycolysis, is most needed for contraction. Such a specific targeting could be modulated by post-translational modifications.

During striated muscle development, the glycolytic enzyme enolase (EC 4.2.1.11) undergoes an isozymic transition, from the embryonic αα form towards the muscle-specific forms αβ and ββ. The regulation of this transition was analyzed in mouse hindlimb muscles from embryonic day 15 (E15) to the adult stage. The quantitative modulations of the levels of the transcripts and subunits of α and β enolase genes were determined. The absolute amounts of α and β enolase mRNAs were estimated using in vitro synthesized transcripts as calibration standards, thus allowing an evaluation of their relative contribution at each stage examined. The muscle-specific β enolase mRNA is already present at E15. Its level then increases and, from E17, this transcript becomes predominant. This accumulation is biphasic: a steep prenatal rise, corresponding to a net increase per fiber, accompanies the formation of secondary myofibers and the development of innervation; a second rise, beginning at postnatal day 5, is temporally correlated with the definitive specialization of the myofibers. Most of the decrease in α mRNA level occurs postnatally. No temporal or quantitative correlation between the up-regulation of β mRNA and the down-regulation of α mRNA levels is observed throughout hindlimb muscle development. Quantitative immunoblotting analyses carried out in parallel show that the enolase isozymic transition is mainly controlled at the mRNA level. The developmental profile of α-skeletal actin mRNA matches the biphasic pattern of β enolase mRNA. Interestingly, the second phase of transcript increases coincides with that of the mRNA of an adult-specific myosin heavy chain, the accumulation of which depends on increasing thyroid hormone levels. It has been proposed that thyroid hormones could coordinate the expression of contractile proteins and enzymes of energy metabolism during the postnatal muscle development. Our results support this notion and suggest a role of thyroid hormones in the regulation of the β enolase gene.

We define the spatial and temporal patterns of expression of the gene encoding the glycolytic enzyme, β-enolase, during mouse ontogenesis. Transcripts were detected by in situ hybridization using 35 S labelled cRNA probes. The β-enolase gene is expressed only in striated muscles. It is first detected in the embryo, in the cardiac tube and in newly formed myotomes. In the muscle masses of the limb, β gene expression occurs at a low level in primary fibers, and subsequently greatly increases at a time which corresponds to the onset of innervation and secondary fiber formation. Later in development, it becomes undetectable in slow-twitch fibers. Our results demonstrate the multistep regulation of the β-enolase gene. The regulation of this muscle-specific gene in somites is discussed in terms of the myogenic sequences of the MyoD family shown to be present when it is activated.

The possible influence of noradrenaline (NA) upon cell differentiation has been studied by comparing NA-supplemented cultures of foetal pallial cells with control cultures grown in normal medium. Two days after plating, the cultures were processed for immunocytochemical detection of either an adhesion molecule and marker of early stages of neuronal differentiation (L1) or a marker expressed at relatively late stages (γ-enolase). In both cases, the NA supplement reduced the expression of the antigen. The effects were more clear-cut for the late than for the early marker. In conclusion, the NA supplement to the culture medium, in our model, seemed to have a ‘differentiation regulating’ rather than a ‘neurotrophic’ function sensu stricto. It remains to be clarified, however, to which extent this finding can be generalized to in vivo situations.

Using the rabbit reticulocyte cell-free translation system we have estimated during ontogenesis the proportions of in vitro translatable alpha and gamma brain enolase mRNAs, which are two minor mRNA species. No polypeptide precursor to these enzyme subunits appears to be synthesized during translation in vitro. During brain development, the changes in translatable alpha and gamma mRNA content seem to parallel those of the corresponding antigens. The proportion of each of the enolase mRNAs is highest in adult mouse brain. Mechanisms controlling alpha and gamma antigen expression are discussed. In order to prepare the specific cDNA probes, purification of alpha and gamma mRNAs was undertaken.

Resume Les polysomes prepares a partie de poudre de cerveau de rat adulte prealablement congeles ont ete separes sur gradient de sucrose. La traduction in vitro dirigee par les polysomes ainsi fractionnes a ete realisee dans le systeme acellulaire de lysat de reticulocyte. Parmi les produits synthetises in vitro l'enolase specifique du cerveau (proteine 14.3.2) et la proteine S100 ont ete identifiees par immunoprecipitation; la tubuline a ete analysee par electrophorese bidimensionnelle. La synthese relative de ces proteines par l'ensemble des polysomes libres du cerveau de rat represente respectivement 0,1 p. cent, 0,05 p. cent et 0,7 p. cent des produits formes in vitro. Les polysomes responsables de la synthese de la proteine S100 sont separes de ceux dirigeant la formation des deux autres marqueurs par centrifugation sur gradient de sucrose. Ce fractionnement permet d'enrichir trois fois l'activite RNA messager specifique de chacune des proteines etudiees.

An analysis of enolase expression during differentiation of neuroblastoma clones in homogeneous culture is presented. The enolases expressed in these neuroblast-like cells are identical to those of mouse brain with respect to the examined properties. Our biochemical investigation has premitted us to demonstrate formally that neuroblastoma cells undergo a transition from the embryonic αα form to the neuronal γγ form and contain both enolases as well as the αγ hybrid form during maturation. These results suggest that the same phenomenon must exist in vivo for neuroblasts. In neuroblastoma cells, an increase in both αγ and γγ neuron specific enolases is related to cell maturation and expression of the αγ form precedes that of the γγ form during differentiation. Modulation of neuronal enolase activities is similar in the various conditions of differentiation studied and appears not to be necessarily related with morphological differentiation, although concomitant with an arrest of cell division. The evolution of specific neuronal enolases in neuroblastoma cells parallels that observed in vivo , in brain from embryonic day 15 to post-natal day 7. Moreover, at least one treatment (dimethylsulfoxide) causes an important decrease in the high specific αα activity of these cells as occurs in vivo . This enolase can therefore also be considered as a biochemical marker for neuroblastoma maturation. As observed with other markers and other cell types, neuroblastoma cells in culture express an immature biochemical differentiation of the enolase isozymes.

In vertebrates, the glycolytic enzyme enolase (EC 4.2.1.11) is present as homodimers and heterodimers formed from three distinct subunits of identical molecular weight, alpha, beta, and gamma. We report the cloning and sequencing of a cDNA encoding the beta subunit of murine muscle-specific enolase. The corresponding amino acid sequence shows greater than 80% homology with the beta subunit from chicken obtained by protein sequencing and with alpha and gamma subunits from rat and mouse deduced from cloned cDNAs. In contrast, there is no homology between the 3' untranslated regions of mouse alpha, beta, and gamma enolase mRNAs, which also differ greatly in length. The short 3' untranslated region of beta enolase mRNA accounts for its distinct length, 1600 bases. It is known that a progressive transition from alpha alpha to beta beta enolase occurs in developing skeletal muscle. We show that this transition mainly results from a differential regulation of alpha and beta mRNA levels. Analysis of myogenic cell lines shows that beta enolase gene is expressed at the myoblast stage. Moreover, transfection of premyogenic C3H10T1/2 cells with MyoD1 cDNA shows that the initial expression of beta transcripts occurs during the very first steps of the myogenic pathway, suggesting that it could be a marker event of myogenic lineage determination.

A specific event which accompanies the terminal differentiation of most neurons is the isozymic enolase transition from the αα form to the αγ and γγ adult neuronal forms. It is partly expressed during maturation of a mouse neuroblastoma clonal cell line (NIE-115). We demonstrate, in these cell cultures, that the significant increase in the concentration of γ gene product, expressed as αγ enolase during differentiation, is due to a parallel and similar increase in its synthesis. The capacities of poly (A)+ RNA from undifferentiated and differentiated cultures to direct the synthesis of γ gene product were compared in a reticulocyte cell-free protein-synthesizing system. This translating capacity is stimulated in the same proportion as the rate of synthesis of γ antigen in differentiating cell cultures. Therefore the increase in the level of γ protein (expressed mostly as αγ enolase) during differentiation results from the increased translating activity or relative amount of γmRNA.

Expression of the neuronal marker 14-3-2 or NSE (neuron-specific enolase) has been studied during in vitro differentiation of cells in culture. The 14-3-2 protein of neuroblastoma cells is immunologically identical with that found in mouse brain extract. The lack of detectable 14-3-2 in cultures of non-neuronal lines shows that this protein, as has been already shown in vivo, is also a specific marker of neurons in vitro. The presence of 14-3-2 in a differentiated hypothalamic clone—but not in its presumptive precursor—indicates selective initial derepression of 14-3-2. Moreover, modulation of the amount of 14-3-2 already present in dividing neuroblastoma cells is related to the confluent phase of growth or morphological differentiation of neuroblasts. Both mechanisms may be related to the mechanisms underlying initial differentiation and subsequent maturation of neurons in vivo. In dividing neuroblastoma cells modulation of the basal level of 14-3-2 is not necessarily associated with expression of the morphological differentiation, but seems generally concomitant with an arrest of cell division.

We have compared the rodent developmental pattern of the 14-3-2 antigen estimated by a microcomplement fixation technique with that of the cerebral enolases. Chromatographic separation of enolase isozymes on microcolumns demonstrates that the embryonic neuron specific enolase is firstly and mostly represented by the αγ isozyme. The most important increase in 14-3-2 antigen and γγ enolase occurs between post-natal days 7th and 15th. By post-natal day 30, adult levels have been reached. An interesting observation is—during embryonic development—the decrease in the specific activity of the cerebral enolase isozyme αα. This could be explained by the replacement—in neuroblasts—of αα enolase by neuron specific enolase. A comparison between 14-3-2 antigen and neuron specific enolase (γγ) purified by completely different methods is presented. The 14-3-2 antigen exhibits an enolase specific activity comparable to that of purified enzyme and has the same electrophoretic mobility. Antibodies raised against either antigen have an identical specificity. Pre and post-natal developmental pattern in rodent brains are similar for both proteins. Thus neuron specific 14-3-2 antigen is identical to neuron specific enolase. Thus we have precisely described the ontogenic transition between the three cerebral enolase isozymes at the tissue level. This study is completed by the analysis of these transitions at the neuronal cell level, using homogenous cell lines (Part II of this paper).