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

Missense mutation in PFKM associated with muscle-type phosphofructokinase deficiency in the Wachtelhund dog

Hereditary muscle-type phosphofructokinase (PFK) deficiency causing intermittent hemolytic anemia and exertional myopathy due to a single nonsense mutation in PFKM has been previously described in English Springer and American Cocker Spaniels, Whippets, and mixed breed dogs. We report here on a new missense mutation associated with PFK deficiency in Wachtelhunds. Coding regions of the PFKM gene were amplified from genomic DNA and/or cDNA reverse-transcribed from RNA of EDTA blood of PFK-deficient and clinically healthy Wachtelhunds and control dogs. The amplicons were sequenced and compared to the published canine PFKM sequence. A point mutation (c.550C>T, in the coding sequence of PFKM expressed in blood) was found in all 4 affected Wachtelhunds. This missense mutation results in an amino acid substitution of arginine (Arg) to tryptophan (Trp) at position 184 of the protein expressed in blood (p.Arg184Trp). The mutation is located within an alpha-helix, and based on the SIFT analysis, this amino acid substitution is not tolerated. Amplifying the region around this mutation and digesting the PCR fragment with the restriction enzyme MspI, produces fragments that readily differentiate between PFK-deficient, carrier, and normal animals. Furthermore, we document 2 additional upstream PFKM exons expressed in canine testis but not in blood. Despite their similar phenotypic appearance and use for hunting, Wachtelhunds and English Springer Spaniels are not thought to have common ancestors. Thus, it is not surprising that different mutations are responsible for PFK deficiency in these breeds. Knowledge of the molecular basis of PFK deficiency in Wachtelhunds provides an opportunity to screen and control the spread of this deleterious trait.

Posttranslational modification of 6-phosphofructo-1-kinase as an important feature of cancer metabolism

Background

Human cancers consume larger amounts of glucose compared to normal tissues with most being converted and excreted as lactate despite abundant oxygen availability (Warburg effect). The underlying higher rate of glycolysis is therefore at the root of tumor formation and growth. Normal control of glycolytic allosteric enzymes appears impaired in tumors; however, the phenomenon has not been fully resolved.

Methodology/Principal Findings

In the present paper, we show evidence that the native 85-kDa 6-phosphofructo-1-kinase (PFK1), a key regulatory enzyme of glycolysis that is normally under the control of feedback inhibition, undergoes posttranslational modification. After proteolytic cleavage of the C-terminal portion of the enzyme, an active, shorter 47-kDa fragment was formed that was insensitive to citrate and ATP inhibition. In tumorigenic cell lines, only the short fragments but not the native 85-kDa PFK1 were detected by immunoblotting. Similar fragments were detected also in a tumor tissue that developed in mice after the subcutaneous infection with tumorigenic B16-F10 cells. Based on limited proteolytic digestion of the rabbit muscle PFK-M, an active citrate inhibition-resistant shorter form was obtained, indicating that a single posttranslational modification step was possible. The exact molecular masses of the active shorter PFK1 fragments were determined by inserting the truncated genes constructed from human muscle PFK1 cDNA into a pfk null E. coli strain. Two E. coli transformants encoding for the modified PFK1s of 45,551 Da and 47,835 Da grew in glucose medium. The insertion of modified truncated human pfkM genes also stimulated glucose consumption and lactate excretion in stable transfectants of non-tumorigenic human HEK cell, suggesting the important role of shorter PFK1 fragments in enhancing glycolytic flux.

Conclusions/Significance

Posttranslational modification of PFK1 enzyme might be the pivotal factor of deregulated glycolytic flux in tumors that in combination with altered signaling mechanisms essentially supports fast proliferation of cancer cells.

Tissue-dependent loss of phosphofructokinase-M in mice with interrupted activity of the distal promoter: impairment in insulin secretion

Phosphofructokinase is a key enzyme of glycolysis that exists as homo- and heterotetramers of three subunit isoforms: muscle, liver, and C type. Mice with a disrupting tag inserted near the distal promoter of the phosphofructokinase-M gene showed tissue-dependent differences in loss of that isoform: 99% in brain and 95-98% in islets, but only 50-75% in skeletal muscle and little if any loss in heart. This correlated with the continued presence of proximal transcripts specifically in muscle tissues. These data strongly support the proposed two-promoter system of the gene, with ubiquitous use of the distal promoter and additional use of the proximal promoter selectively in muscle. Interestingly, the mice were glucose intolerant and had somewhat elevated fasting and fed blood glucose levels; however, they did not have an abnormal insulin tolerance test, consistent with the less pronounced loss of phosphofructokinase-M in muscle. Isolated perifused islets showed about 50% decreased glucose-stimulated insulin secretion and reduced amplitude and regularity of secretory oscillations. Oscillations in cytoplasmic free Ca(2+) and the rise in the ATP/ADP ratio appeared normal. Secretory oscillations still occurred in the presence of diazoxide and high KCl, indicating an oscillation mechanism not requiring dynamic Ca(2+) changes. The results suggest the importance of phosphofructokinase-M for insulin secretion, although glucokinase is the overall rate-limiting glucose sensor. Whether the Ca(2+) oscillations and residual insulin oscillations in this mouse model are due to the residual 2-5% phosphofructokinase-M or to other phosphofructokinase isoforms present in islets or involve another metabolic oscillator remains to be determined.

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.

Coregulation of fast contractile protein transgene and glycolytic enzyme expression in mouse skeletal muscle

Little is known of the gene regulatory mechanisms that coordinate the contractile and metabolic specializations of skeletal muscle fibers. Here we report a novel connection between fast isoform contractile protein transgene and glycolytic enzyme expression. In quantitative histochemical studies of transgenic mouse muscle fibers, we found extensive coregulation of the glycolytic enzyme glycerol-3-phosphate dehydrogenase (GPDH) and transgene constructs based on the fast skeletal muscle troponin I (TnIfast) gene. In addition to a common IIB > IIX > IIA fiber type pattern, TnIfast transgenes and GPDH showed correlated fiber-to-fiber variation within each fast fiber type, concerted emergence of high-level expression during early postnatal muscle maturation, and parallel responses to muscle under- or overloading. Regulatory information for GPDH-coregulated expression is carried by the TnIfast first-intron enhancer (IRE). These results identify an unexpected contractile/metabolic gene regulatory link that is amenable to further molecular characterization. They also raise the possibility that the equal expression in all fast fiber types observed for the endogenous TnIfast gene may be driven by different metabolically coordinated mechanisms in glycolytic (IIB) vs. oxidative (IIA) fast fibers.

Genomic organization, 5'flanking region and tissue-specific expression of mouse phosphofructokinase C gene

Using a combination of mouse bacterial artificial chromosome (BAC) genomic library screening, long-range polymerase chain reaction (PCR) amplification, genomic walking and DNA sequencing, we have characterized the intron/exon boundaries, the sizes of each intron and 5' flanking region of the mouse PFK-C gene. The gene spans approximately 55 kb and comprises 22 exons separated by 21 introns. All intron/exon splice junctions conform to the GT/AG rule. The mouse PFK-C gene organization is similar to that of the human and rabbit PFK-A and human and mouse PFK-B genes. However, PFK-C has much larger intronic sequences throughout the gene. Anchored PCR was performed to amplify about 1.0 kb of genomic DNA upstream of the translational start site. Sequence analysis of the PFK-C 5' flanking region revealed that it is devoid of TATA and CAAT boxes at the usual positions, but it contained several putative binding sites for transcription factors AP1, GATA1, NKX2.5 and STAT. The 5' flanking region was not enriched in GC dinucleotides and lacked CpG islands and putative binding sites for SP1. Four transcription initiation sites have been identified by full-length RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE) between -61 and -32 bp from the translation initiation codon. Reverse transcription-PCR analysis revealed that PFK-A, PFK-B and PFK-C genes were expressed, in all mouse tissues tested, at varying levels. PFK-A mRNA was more abundantly expressed in all tissues than were the PFK-B and PFK-C genes. Based on the mouse PFK-C signal normalized to 18S rRNA, the PFK-C mRNA was expressed at the highest levels in the brain, heart, thymus and testicles, whereas low levels were observed in the kidney, liver, muscle, and lung.

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.

Quantitative characterization of homo- and heteroassociations of muscle phosphofructokinase with aldolase

Dissociation of purified phosphofructokinase accompanied with inactivation was analyzed in the absence and presence of aldolase and the data were compared with those obtained with muscle extract. The kinetics of the decrease in enzymatic activity was highly dependent on the dilution factor in both cases, but the inactivation appeared to be biphasic only with extract. The inactivation of the phosphofructokinase was impeded by addition of excess of aldolase. Time courses of kinase inactivation were fitted by alternative kinetic models to characterize the multiple equilibria of several homo- and hetero-oligomers of phosphofructokinase. The combination of modeling data obtained with purified and extract systems suggests that aldolase binds to an intermediate dimer of phosphofructokinase and within this heterocomplex the kinase is completely active. The intermediate dimer is stabilized by association with microtubules and the kinase activity decreased due to dilution can be recovered by addition of excess aldolase. In extract, the phosphofructokinase is of sigmoidal character (Hill coefficient of 2.3); the addition of excess exogenous aldolase to phosphofructokinase resulted in heterocomplex formation displaying Michaelian kinetics. The possible physiological relevance of heterocomplex formation of phosphofructokinase in muscle extract is discussed.

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.