Sequence diversity in the 5' untranslated region of rabbit muscle phosphofructokinase mRNA

Novel testis- and embryo-specific isoforms of the phosphofructokinase-1 muscle type gene

We have identified novel transcriptional isoforms of the human and mouse genes encoding muscle type phosphofructokinase-1 (PFK-M). These isoforms are expressed specifically in the testis and in the mid-gestation embryo, and have been termed TE-PFK-M (testis- and embryo-specific PFK-M). The 5'UTR of TE-PFK-M is composed of three newly identified exons that lie much farther upstream of the PFK-M coding region than the previously characterized 5'UTR. In addition, this upstream region encodes a series of small polyadenylated transcripts, some of which share the same exons found in the 5'UTR of TE-PFK-M, and which may play some role in regulating TE-PFK-M expression. These findings indicate an even more complex level of control of PFK-M expression than previously thought.

Alteration of the levels of the M-type 6-phosphofructo-1-kinase mRNA isoforms during neonatal maturation of heart, brain and muscle

During muscle, heart, and brain neonatal maturation, the capacity to utilize glucose in energy metabolism is directly related to the extent of accumulation of the 6-phosphofructo-1-kinase (PFK) M-type subunit. Neonatal development of other organs, such as liver and kidney, which are not characterized by large increases in the capacity to use glucose do not exhibit large increases in the M-type subunit protein. The presence of the M-type subunit in a PFK isozyme pool fosters a higher affinity utilization of carbohydrate and increased responsiveness to the levels of regulatory metabolites. To better appreciate this phenomenon, which is vital for normal development, the different isoforms of the M-type subunit mRNA's and alteration of their levels during maturation have been examined. Further, the potential promoter regions, i.e., the regions upstream from the sites of initiation of transcription, which are involved in expression of the different M-type subunit mRNA isoforms have been isolated, sequenced, and examined for possible transcription factor interaction sites. Using cDNA libraries produced from adult rat brain or skeletal muscle RNA, two primary forms of rat M-type subunit cDNA's were detected. Although the translated regions of these mRNA's were essentially identical, the 5'-untranslated region (5'-UTR) exhibited different lengths (90 or 59 bp) and sequences. Each M-type subunit cDNA had 10 common nucleotides immediately upstream from the initiator ATG, and the remaining 5'-UTR's had insignificant identity. A genomic fragment which interacted with probes complimentary to the sequences of the 5'-UTR of each M-type subunit mRNA isoform was isolated and sequenced by primer walking. It was discovered that the 5'-UTR of one of the mRNA's (proximal mRNA) was located immediately upstream from exon I and was apparently transcribed without splicing. Subsequently, the initial bp in the sequence of the other mRNA isoform (distal mRNA) was located 4010 bp upstream from the ATG in exon 1. Employing Reverse Transcription-Polymerase Chain Reaction using total RNA and scanning densitometry, the relative levels of the proximal and distal mRNA's during neonatal maturation of brain, heart, and muscle were measured. In these tissues, both forms of M-type subunit mRNA's were present, and during maturation tissue-specific differences were noted.

Alteration of 6-phosphofructo-1-kinase subunit protein, synthesis rates, and mRNA during rat neonatal development

For the three 6-phosphofructo-1-kinase (PFK) subunits in heart, skeletal muscle, liver and kidney, developmentally-associated changes in protein, mRNA and apparent synthesis rates were observed. During neonatal maturation, all three phenomena for the M-type in heart and skeletal muscle exhibited large increases. Also, during neonatal development, the L-type and C-type subunits were unaffected in heart but disappeared from skeletal muscle. In the newborn liver and kidney, the amounts of each type of PFK subunit protein were nearly identical. During neonatal development, the levels of all three PFK subunit proteins in kidney increased more than twofold; and this was associated with a similar increase in apparent subunit synthesis rates and mRNA levels. During liver neonatal development, the L-type subunit protein, synthesis and mRNA levels also increased more than twofold. However, during hepatic maturation, M-type subunit protein, synthesis and mRNA levels were unchanged and apparently unaffected. The C-type subunit protein during neonatal liver development decreased approximately 80% as did its apparent synthesis rate. These data suggest that regulation of the alteration of the PFK subunit proteins during neonatal maturation can vary among these tissues and is not the same for each subunit type. Different mechanisms, such as transcription, translation, and mRNA stability could be involved.

Phosphofructokinase deficiency: recent advances in molecular biology

Phosphofructokinase (PFK) plays a major role in glycolysis. Deficiency of PFK-M is characterized by muscle weakness due to fuel crisis in exercising muscles. To elucidate the gene defect of PFK-deficient patients, we have cloned and determined the complete structure and transcription mechanism of human PFK-M mRNA and gene. Molecular defects were investigated in three unrelated Japanese family cases. The first case was characterized by a point mutation at the donor site of intron 15 of the PFK-M gene. Cryptic splicing resulted in a 25 amino acid truncation in the patient's PFK-M. The second case possessed a point mutation at the donor site of intron 19, resulting in the skipping of exon 19 and the truncation of 55 amino acids. In the third case, a missense mutation was identified in the coding region. The review of an updated mutation repertoire indicates the heterogeneity of the molecular mechanism of the disease.

Mutations in muscle phosphofructokinase gene

Mutations in the muscle phosphofructokinase gene (PFK-M) result in a metabolic myopathy characterized by exercise intolerance and compensated hemolysis. PFK deficiency, glycogenosis type VII (Tarui disease) is a rare, autosomal, recessively inherited disorder. Multiple mutations, including splicing defects, frameshifts, and missense mutations, have recently been identified in patients from six different ethnic backgrounds establishing genetic heterogeneity of the disease. There is no obvious correlation between the genotype and phenotypic expression of the disease. PFK-M deficiency appears to be prevalent among people of Ashkenazi Jewish descent. Molecular diagnosis is now feasible for Ashkenazi patients who share two common mutations in the gene; the more frequent is an exon 5 splicing defect, which accounts for approximately 68% of mutant alleles in this population.

Expression of mouse phosphofructokinase-M gene alternative transcripts: evidence for the conserved two-promoter system

Molecular cloning of the 5' part of mouse phosphofructokinase-M cDNA was performed. In the 46 cDNA clones isolated, there were two classes of 5' untranslated sequences. One had an EcoRI site within its 5' untranslated sequence. This showed 83.0% similarity with human type B mRNA for phosphofructokinase-M. The other lacked an EcoRI site, showing 92.9% similarity with human type C mRNA. Using the reverse-transcription PCR technique, we found that the transcript with an EcoRI site was exclusively expressed in cardiac and skeletal muscles, while that without an EcoRI site was expressed in all the mouse tissues examined. The results suggested that the mouse phosphofructokinase-M gene was transcribed through alternative splicing by the multiple promoters. This transcription mechanism was considered to be evolutionarily conserved. The level of phosphofructokinase-M gene expression in mouse cardiac and skeletal muscles decreased in the ketotic diabetic state. Although the regulatory mechanism and the physiological significance are not fully known, this would indicate that phosphofructokinase-M gene transcripts are affected during the diabetic state.

Control of the murine phosphofructokinase-A gene during muscle differentiation

The muscle-specific isoform of phosphofructokinase (PFK-A) is induced during muscle development. To understand expression of PFK at the molecular level, transcription of the mouse PFK-A gene was examined during C2 myoblast differentiation to myotubes. PFK-A gene transcription increased 5-7-fold during differentiation in vitro. To identify cis-acting elements which direct muscle-specific transcription of the PFK-A gene, its 5'-flanking region and first exon were cloned and characterized. S1 nuclease protection and primer extension assays showed four sites of transcription initiation at 106, 105, 88, and 87 bp upstream of the translation initiation codon. Stable transfection of fusion genes linking -1900 to +99 of PFK-A 5'-flanking sequence to chloramphenicol acetyltransferase coding sequences into myogenic C2 cells did not confer muscle-specific expression. However, larger fragments of PFK-A 5'-flanking region (-5800 to +99) showed muscle-specific expression by transient transfection assay. The sequences directing muscle-specific transcription were further defined by linking various PFK-A upstream fragments to the luciferase gene under the control of the PFK-A proximal promoter, -335 to +99 bp. We found DNA sequence responsible for muscle-specific expression of the PFK-A gene between -4800 and -3900 bp.

Cloning and characterization of the human muscle phosphofructokinase gene

A 35-kbp region of genomic DNA encoding the human muscle phosphofructokinase (HPFK-M) gene including all of the coding exons (1-22) plus 2.2-kbp of 5'-flanking sequence has been cloned. The exon boundaries are the same as has been observed for the rabbit muscle phosphofructokinase (RPFK-M), the human liver phosphofructokinase (HPFK-L), and the mouse liver phosphofructokinase (MPFK-L) genes. Characterization of the structure of the HPFK-M gene and its transcript in Epstein-Barr virus transformed B-cell lines derived from patients with glycogen storage disease type VII (GSDVII or Tarui's disease) demonstrated that this single-copy gene encodes a normal sized 3.0-kb transcript in the four cases examined. This suggests the lesion in these cases represents either a point mutation or possibly a small insertion or deletion resulting in the synthesis of a defective HPFK-M protein. Analysis of the 5'-flanking region demonstrated the presence of a functional promoter located within 114 nucleotides of a proposed transcription initiation site. This promoter was active in the human cervical carcinoma cell line, HeLa S3, the dedifferentiated human hepatoma cell line, HepG2.1, and the mouse myoblast cell line, C2C12, suggesting this promoter has a broad cell-type specificity. In addition, from the known HPFK-M cDNA sequences, this observation indicates that the HPFK-M gene has a second promoter located upstream from the genomic region isolated in this study.

Structure of the entire human muscle phosphofructokinase-encoding gene: a two-promoter system

We have recently shown that three types (A,B, and C) of mRNA species are transcribed from a single gene encoding human muscle phosphofructokinase (hPFK-M) through alternative splicing [Nakajima et al., Biochem. Biophys. Res. Commun. 166 (1990) 637-641]. To determine its complete structure and elucidate the mechanism of alternative RNA splicing, we isolated the hPFK-M gene, which spans about 30 kb, and contains 24 exons. Transcription start points were observed for both exon 1 and exon 2 by S1 nuclease protection assay and primer extension. Motifs of an Sp1-binding site were observed in the upstream region of exon 1 (promoter 1). A TATA-box-like sequence and a CAAT-box-like sequence were identified in the upstream region of exon 2 (promoter 2). Reporter assay revealed that the promoter 1 region was functional both in HeLa cells and myoblastic clonal cells, and that the promoter 2 region was active only in myoblastic cells. Motifs of M-CAT known as a muscle-specific enhancer, were observed in the promoter 2 region. These results indicated that the hPFK-M gene contains at least two promoter regions, facilitating the expression of the heterogeneous gene transcripts in a cell-type-specific manner.

Molecular cloning and expression of a human brain inositol 1,4,5-trisphosphate 3-kinase

A human hippocampus cDNA library was screened by hybridization with a rat brain inositol 1,4,5-trisphosphate (InsP3) 3-kinase cDNA. Sequencing of three overlapping clones identified a 1383 bp open reading frame encoding a 461 amino acid protein with a calculated molecular weight of 50988. The coding amino acid sequence showed an overall 93% similarity with the sequence of rat brain InsP3 3-kinase. The cDNA insert of one isolated partial clone (i.e. hh 26) was in frame with the beta galactosidase fragment fused to it as a Bluescript plasmid; it displayed InsP3 3-kinase activity when expressed in Escherichia coli (E. coli). Biochemical characterization of human brain InsP3 3-kinase by SDS polyacrylamide gel electrophoresis and regeneration of enzyme activity reveals three active fractions with apparent Mr of 58,000-64,000, 45,000-50,000 and 37,000-39,000.

Tissue specificity in expression and alternative RNA splicing of human phosphofructokinase-M and -L genes

Mode of the expression of phosphofructokinase (PFK) -M and -L genes was examined in various human tissues including muscle, placenta, liver, kidney, pancreas, stomach and reticulocytes. The gross level of mRNA expression of PFK-M and -L genes was estimated by Northern analysis. Polymerase chain reaction was used to detect mRNA expressed at low levels in these tissues. Tissue-specific expression of alternatively spliced PFK-M gene transcripts was also determined by polymerase chain reaction. The results indicated that alternative splicing of PFK-M gene transcripts was controlled in a tissue-specific manner.