Differential expression of the Ca2+-binding protein parvalbumin during myogenesis in Xenopus laevis

Oncomodulin: The Enigmatic Parvalbumin Protein

EF-hand Ca²⁺-binding protein family members, α- and β-parvalbumins have been studied for decades. Yet, considerable information is lacking distinguishing functional differences between mammalian α-parvalbumin (PVALB) and oncomodulin (OCM), a branded β-parvalbumin. Herein, we provide an overview detailing the current body of work centered around OCM as an EF-Hand Ca²⁺-binding protein and describe potential mechanisms of OCM function within the inner ear and immune cells. Additionally, we posit that OCM is evolutionarily distinct from PVALB and most other β-parvalbumins. This review summarizes recent studies pertaining to the function of OCM and emphasizes OCM as a parvalbumin possessing a unique cell and tissue distribution, Ca²⁺ buffering capacity and phylogenetic origin.

Muscle differentiation induced up-regulation of calcium-related gene expression in quail myoblasts

Objective

In the poultry industry, the most important economic traits are meat quality and carcass yield. Thus, many studies were conducted to investigate the regulatory pathways during muscle differentiation. To gain insight of muscle differentiation mechanism during growth period, we identified and validated calcium-related genes which were highly expressed during muscle differentiation through mRNA sequencing analysis.

Methods

We conducted next-generation-sequencing (NGS) analysis of mRNA from undifferentiated QM7 cells and differentiated QM7 cells (day 1 to day 3 of differentiation periods). Subsequently, we obtained calcium related genes related to muscle differentiation process and examined the expression patterns by quantitative reverse-transcription polymerase chain reaction (qRT-PCR).

Results

Through RNA sequencing analysis, we found that the transcription levels of six genes (troponin C1, slow skeletal and cardiac type [TNNC1], myosin light chain 1 [MYL1], MYL3, phospholamban [PLN], caveolin 3 [CAV3], and calsequestrin 2 [CASQ2]) particularly related to calcium regulation were gradually increased according to days of myotube differentiation. Subsequently, we validated the expression patterns of calcium-related genes in quail myoblasts. These results indicated that TNNC1, MYL1, MYL3, PLN, CAV3, CASQ2 responded to differentiation and growth performance in quail muscle.

Conclusion

These results indicated that calcium regulation might play a critical role in muscle differentiation. Thus, these findings suggest that further studies would be warranted to investigate the role of calcium ion in muscle differentiation and could provide a useful biomarker for muscle differentiation and growth.

Preparation of developing Xenopus muscle for sarcomeric protein localization by high-resolution imaging

Mutations in several sarcomeric proteins have been linked to various human myopathies. Therefore, having an in vivo developmental model available that develops quickly and efficiently is key for investigators to elucidate the critical steps, components and signaling pathways involved in building a myofibril; this is the pivotal foundation for deciphering disease mechanisms as well as the development of myopathy-related therapeutics. Although striated muscle cell culture studies have been extremely informative in providing clues to both the distribution and functions of sarcomeric proteins, myocytes in vivo develop in an irreproducible 3D environment. Xenopus laevis (frog) embryos are cost effective, compliant to protein level manipulations and develop relatively quickly (⩽ a week) in a petri dish, thus providing a powerful system for de novo myofibrillogenesis studies. Although fluorophore-conjugated phalloidin labeling is the gold standard approach for investigating actin-thin filament architecture, it is well documented that phalloidin-labeling can be challenging and inconsistent within Xenopus embryos. Therefore we highlight several techniques that can be utilized to preserve both antibody and fluorophore-conjugated phalloidin labeling within Xenopus embryos for high-resolution fluorescence microscopy.

Hedgehog regulation of superficial slow muscle fibres inXenopusand the evolution of tetrapod trunk myogenesis

In tetrapod phylogeny, the dramatic modifications of the trunk have received less attention than the more obvious evolution of limbs. In somites,several waves of muscle precursors are induced by signals from nearby tissues. In both amniotes and fish, the earliest myogenesis requires secreted signals from the ventral midline carried by Hedgehog (Hh) proteins. To determine if this similarity represents evolutionary homology, we have examined myogenesis in Xenopus laevis, the major species from which insight into vertebrate mesoderm patterning has been derived. Xenopus embryos form two distinct kinds of muscle cells analogous to the superficial slow and medial fast muscle fibres of zebrafish. As in zebrafish, Hh signalling is required for XMyf5 expression and generation of a first wave of early superficial slow muscle fibres in tail somites. Thus, Hh-dependent adaxial myogenesis is the likely ancestral condition of teleosts, amphibia and amniotes. Our evidence suggests that midline-derived cells migrate to the lateral somite surface and generate superficial slow muscle. This cell re-orientation contributes to the apparent rotation of Xenopussomites. Xenopus myogenesis in the trunk differs from that in the tail. In the trunk, the first wave of superficial slow fibres is missing,suggesting that significant adaptation of the ancestral myogenic programme occurred during tetrapod trunk evolution. Although notochord is required for early medial XMyf5 expression, Hh signalling fails to drive these cells to slow myogenesis. Later, both trunk and tail somites develop a second wave of Hh-independent slow fibres. These fibres probably derive from an outer cell layer expressing the myogenic determination genes XMyf5, XMyoD and Pax3 in a pattern reminiscent of amniote dermomyotome. Thus, Xenopus somites have characteristics in common with both fish and amniotes that shed light on the evolution of somite differentiation. We propose a model for the evolutionary adaptation of myogenesis in the transition from fish to tetrapod trunk.

Postmetamorphic changes in parvalbumin expression in the hindlimb skeletal muscle of the bullfrog, Rana catesbeiana

Anuran amphibians, animals that spend a terrestrial life after metamorphosis, exhibit a marked development of hindlimbs during and after metamorphosis. In order to see whether changes occur in the muscle protein components in the course of postmetamorphic development, we subjected gastrocnemius muscle extracts from growing froglets to two-dimensional electrophoresis (2DE). As a result, we found two proteins to undergo a change in level. One spot, indicating a molecular mass of approximately 12 kDa and an isoelectric point (pI) of 5.0 first became detectable at 45 days after metamorphosis. Another spot, corresponding to a protein of 11 kDa and pI 4.8, was prominent until the former spot appeared. N-terminal amino acid sequence analysis and comparison of the spots with those of parvalbumin (PA) revealed that these two proteins were PA alpha and PA beta. Northern blot analysis using PA alpha and PA beta cDNAs as probes revealed that the PA beta mRNA level declined whereas that of PA alpha mRNA rose as the frogs grew.

Xenopus muscle development: from primary to secondary myogenesis

Xenopus myogenesis is characterized by specific features, different from those of mammalian and avian systems both at the cellular level and in gene expression patterns. During early embryogenesis, after the initial molecular signals inducing mesoderm, the myogenic determination factors XMyoD and XMyf-5 are activated in presomitic mesoderm in response to mesoderm-inducing factors. After these first inductions of the myogenic program, forming muscles in Xenopus can have different destinies, some of these resulting in cell death before adulthood. In particular, it is quite characteristic of this species that, during metamorphosis, the primary myotomal myofibers completely die and are progressively replaced by secondary "adult" multinucleated myofibers. This feature offers the unique opportunity to totally separate the molecular analysis of these two distinct types of myogenesis. The aim of this review is to summarize our knowledge on the cellular and molecular events as well as the epigenetic regulations involved in the construction of Xenopus muscles during development.

Xenopus bagpipe-related gene, koza, may play a role in regulation of cell proliferation

The homeobox gene koza is a new member of the vertebrate bagpipe-related gene family. Embryonic expression of koza is observed at highest levels in the muscle layer of the somites and, during later development, is restricted to the lateral somitic cells, which correspond to slow twitch muscle tissue. Expression of koza is also observed in the myocardial layer of the heart and in the cement gland. In each of these tissues, koza transcription commences only after the expression of terminal differentiation markers. By injection of synthetic mRNA, we show that overexpression of koza leads to an apparent decrease in the number of cells in the somites. No reduction in cell number is observed when koza is present in neural tissues, suggesting that koza exhibits some tissue specificity in regulation of cell proliferation. Embryonic manipulations show that restriction of koza expression to the slow twitch muscle layer is independent of axial structures but is, at least partly, regulated by signals arising in ectodermal tissue. Finally, in Drosophila, bagpipe expression is regulated by the hedgehog signaling pathway. By using ectopic expression, we show that koza transcription is positively regulated by banded hedgehog. This result indicates that regulation of bagpipe expression by hedgehog signaling is evolutionarily conserved.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Calcium signaling in the developing Xenopus myotome

Embryonic Xenopus myocytes generate spontaneous calcium (Ca(2+)) transients during differentiation in culture. Suppression of these transients disrupts myofibril organization and the formation of sarcomeres through an identified signal transduction cascade. Since transients often occur during myocyte polarization and migration in culture, we hypothesized they might play additional roles in vivo during tissue formation. We have tested this hypothesis by examining Ca(2+) dynamics in the intact Xenopus paraxial mesoderm as it differentiates into the mature myotome. We find that Ca(2+) transients occur in cells of the developing myotome with characteristics remarkably similar to those in cultured myocytes. Transients produced within the myotome are correlated with somitogenesis as well as myocyte maturation. Since transients arise from intracellular stores in cultured myocytes, we examined the functional distribution of both IP(3) and ryanodine receptors in the intact myotome by eliciting Ca(2+) elevations in response to photorelease of caged IP(3) and superfusion of caffeine, respectively. As in culture, transients in vivo depend on Ca(2+) release from ryanodine receptor (RyR) stores, and blocking RyR during development interferes with somite maturation.

Expression of myogenic regulatory factors during muscle development of Xenopus: myogenin mRNA accumulation is limited strictly to secondary myogenesis

To clarify the acquisition of the adult muscle pattern in Xenopus laevis, in situ hybridization and reverse transcriptase-polymerase chain reaction were used to correlate the time course of gene expression for myogenic regulatory factors (Myf-5, MyoD, and myogenin) with the expression of contractile protein (myosin heavy chain; MHC) genes during hindlimb formation compared with their expression in dorsal body muscles. After the precocious expression of Myf-5 and MyoD mRNA in limb bud (stage 50), myogenin mRNA strongly accumulated later at paddle stages (stages 52/53) concomitantly with the accumulation of both the larval and the adult MHC mRNAs. In dorsal body muscles, as early as stage 52, myogenin transcripts accumulated in a few small, secondary myofibers expressing the adult MHC mRNA that were located along the dorsomedial edge, but they were never detected in the large, primary myofibers of the body expressing the larval MHC mRNA. During metamorphosis, the areas expressing both the adult MHC and the myogenin transcripts gradually expanded from the dorsomedial edge to the ventral side of the dorsal body muscles, accounting for the progression of the secondary "adult" myogenesis described previously (Nishikawa and Hayashi [1994] Dev. Biol. 165:86-94). This work shows that, in Xenopus, the accumulation of myogenin mRNA is restricted to secondary myogenesis, including the formation of new muscles in developing limbs as well as in dorsal muscles during body remodeling. This shows that myogenin is not required for primary myogenesis, and it suggests a crucial role for myogenin in the terminal differentiation program, including myoblast fusion and the activation of adult-type muscle genes.

Oncomodulin is abundant in the organ of Corti

A small, acidic Ca(2+)-binding protein (CBP-15) was recently detected in extracts of the mammalian auditory receptor organ, the organ of Corti [Senarita et al. (1995) Hear. Res. 90, 169-175]. N-terminal sequence data for CBP-15 [Thalmann et al. (1995) Biochem. Biophys. Res. Commun. 215, 142-147] implied membership in the parvalbumin family and possible identity with the mammalian beta-parvalbumin oncomodulin. As shown herein, the latter conclusion is supported by strong cross-reactivity between CBP-15 and isoform-specific antibodies to oncomodulin. Moreover, we have succeeded in amplifying the guinea pig CBP-15 coding sequence from organ of Corti cDNA using degenerate oligonucleotide primers based on the rat oncomodulin sequence. The deduced amino acid sequence of guinea pig CBP-15 displays 90%, 92%, and 98% identity with mouse, rat, and human oncomodulin isoforms. Demonstration of the presence of oncomodulin in the organ of Corti is the first documentation of this substance in a postnatal mammalian tissue.

Cardiac myosin heavy chain expression during heart development in Xenopus laevis

Muscle-specific gene expression in the heart during Xenopus development was investigated using reverse transcription-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization to detect transcripts of the gene for the cardiac myosin heavy chain (CMHC). RT-PCR analysis determined that CMHC transcripts are present in the cardiac mesoderm at state 13, demonstrating that muscle-specific gene expression in the primitive myocardium has begun by the early neurula stage, approximately 30 h before the heart beat begins. Xenopus, therefore, is similar to amniotes and mammals in that cardiac precursor cells begin to express muscle-specific gene transcripts soon after commitment to the cardiac myocyte lineage. The earliest CMHC gene transcripts can be detected in the heart using whole-mount in situ hybridization is early tailbud stage 28, which coincides with the onset of heart tube morphogenesis. CMHC gene expression was also detected in skeletal muscle: RT-PCR analysis determined that CMHC transcripts are transiently expressed in the somite during the initial phases of skeletal muscle differentiation. Furthermore, CMHC mRNAs are expressed in a subset of head muscles of the feeding tadpole. CMHC gene expression is induced in ectodermal cells of the animal cap in blastula-stage embryos injected with synthetic MyoD or Myf5 RNA, suggesting that the CMHC gene contains regulatory elements that are responsive to the activity of those skeletal-muscle-specific transcription factors.

Oligomerization of an avian thymic parvalbumin. Chemical evidence for a Ca(2+)-specific conformation

CPV3, the third parvalbumin isoform to be identified in the chicken, is produced exclusively in the thymus gland. Although parvalbumins are typically cysteine-deficient, CPV3 contains two cysteine residues, at positions 18 and 72. The reported three-dimensional parvalbumin structures suggest that the side chain of cysteine-72 should be solvent-accessible. Accordingly, we find that CPV3 readily forms disulfide-linked oligomers in the absence of reducing agents. The reaction, employing either O2 or ferricyanide ion as the oxidant, is apparently restricted to the Ca(2+)-bound form of the protein. The differing reactivity of the Ca2+, Mg2+, and apo-forms has significant structural implications.

Parvalbumin-immunoreactive material in the kidney of Xenopus laevis

Parvalbumin-immunoreactive material has been localized in various parts of the Xenopus kidney. Ciliated epithelia in the neck segment, the intermediate segment and in the peritoneal funnel, epithelial cells intermingled among flask cells in the connecting tubule, and cells in the proximal and distal tubules cells were labelled. Renal corpuscle and collecting duct cells did not exhibit parvalbumin immunoreactivities. Western-blot analysis of kidney homogenates indicated various isoforms of parvalbumin-immunoreactive proteins.

Myosin, parvalbumin and myofibril expression in barbel (Barbus barbus L.) lateral white muscle during development

Histo- and immunohistochemical techniques have recently been used to study the fibre type and myosin expression in fish muscle during development. In the present work, embryonic, larval and adult myosin isozymes (heavy and light chains) and parvalbumin isotypes were analyzed, from fertization to the adult stage, by polyacrylamide gel electrophoresis of barbel (Barbus barbus L.) trunk muscle extracts. The examined myosins display the sequential transitions from embryonic to larval and adult forms characteristic of higher vertebrates. They are characterized by specific heavy chains but their light chains differ only by the LC1/LC3 stoichiometry with LC3 exceeding LC1 after 10 days. Sarcoplasmic parvalbumins show considerable and unforeseen developmental transitions in their isotype distribution: the PA II isotype first appears after hatching and becomes the predominant form until the length reaches about 6 cm. One month after hatching, the amount of PA II then decreases and the synthesis of PA III and IV further increases to reach the typical adult pattern at a size of 18 cm. These observations show that the distribution of parvalbumin isotypes reflects the stage of development. It suggests a specific role for each isotype in relation to muscle activity. Microscopy illustrates the progressive development of somites, muscles cells, and myofibrils, which accelerates at hatching when movements increase.

The structure of the mouse parvalbumin gene

Parvalbumin (PV) is a calcium-binding protein of the EF-hand family, expressed mainly in fast contracting/relaxing muscles of vertebrates. We have isolated five overlapping genomic PV clones which overall span 28 kilobase pairs (kb) around the Pva locus on mouse Chromosome (Chr) 15. The positions of four introns were determined by DNA sequencing. They interrupt the coding sequences at positions corresponding to those in rat and human PV genes. The transcription start site, 25 bp downstream from the TATA-box, was mapped by oligonucleotide primer extension on poly(A)(+)-RNA. The analysis of 0.4 kb promoter sequence of the mouse PV gene revealed CCAAT- and TATA-box sequences and a 59 bp GC-rich stretch between positions -59 and -118. Similar motifs have been found in the parvalbumin genes of rat and human. A perfect 11-bp repeat upstream to positions -149 and -163 respectively is homologous only to the rat promoter. These results will be related to tissue and species differences in PV expression.

Neural regulation of calmodulin in adult Xenopus leg muscle

Two Ca(2+)-binding proteins important in regulating muscle responses to Ca2+ flux are differentially expressed following denervation of Xenopus laevis gastrocnemius. Levels of parvalbumin (PV) RNA transcripts and proteins decrease in abundance, while calmodulin (CaM) transcript and protein levels increase. Our studies on PV kinetics in Xenopus follow a pattern observed in other species, however, our observation of a concomitant increase in CaM has not been documented in any system. Molecular analyses of the Xenopus CaM gene indicate that its structure and upstream sequences are highly conserved across several vertebrate species and implicate several transcription factors in the regulation of its expression.

Expression of Ca2(+)-ATPase isoforms in denervated, regenerating, and dystrophic chicken skeletal muscle

The expression of fast and slow isoforms of the sarcoplasmic reticulum Ca2(+)-ATPase was studied in denervated, regenerating, and dystrophic fast and slow avian skeletal muscles. We found that both fast and slow Ca2(+)-ATPase isoforms were expressed in most myofibers following denervation of adult fast-twitch muscle, but only the slow Ca2(+)-ATPase isoform was found in slow-tonic muscle which had been denervated. Regenerating myotubes in normally innervated and previously denervated adult fast-twitch or slow-tonic muscle expressed both Ca2(+)-ATPase isoforms. Expression of the slow Ca2(+)-ATPase isoform was found to persist in dystrophic fast-twitch muscle, long after it had disappeared from normal fast-twitch muscle. However, the fast Ca2(+)-ATPase isoform disappeared from slow-tonic muscle similarly in normal and dystrophic birds. These results demonstrate that the appearance of myosin heavy chain isoforms characteristic of developing muscle is correlated with similar changes in the expression of sarcoplasmic reticulum Ca2(+)-ATPases.

Junctional acetylcholine receptor channel open time is not presynaptically regulated in developing muscle

The role of motor innervation in controlling the development of acetylcholine receptor (AChR) channel open time was tested by examining synaptic current durations in transplanted muscles of Xenopus tadpoles. The presumptive lower jaw region, which gives rise to the interhyoideus muscle, was transplanted to the tail, overlying the myotomal muscle cells. The transplanted muscles became innervated, presumably by spinal nerves which normally innervate myotomal muscle. Despite development in the presence of foreign innervation, synaptic currents in the transplanted interhyoideus were predominantly long in duration and resembled those in the normally innervated interhyoideus. They did not resemble those in the myotomal muscle, where synaptic currents are brief. The apparent lack of neural influence on development of AChR function in muscle contrasts with the evidence for presynaptic control of AChR open time in frog sympathetic ganglia. This may reflect a fundamental difference between nerve and muscle in the regulation of postsynaptic function.

Parvalbumin expression in normal and mutant Xenopus embryos

The levels and cellular distribution of PV were analyzed at the molecular level in normal and mutant Xenopus laevis embryos. In developing tadpoles, PV protein expression was restricted to fast-twitch myotomal cells, as shown by immunofluorescence experiments with myosin isoform-specific monoclonal antibodies. Both PV expressing and non-expressing myotomal cells can be cultured in vitro without neurons, and non-expressing cells have been shown to lack hybridizable levels of PV mRNA. In a paralyzed mutant, unresponsive, the level of PV protein was normal, but its cellular distribution was not. Thus, while neural input may not be necessary for PV expression in embryonic cells, one gene sequence appears needed for proper cellular distribution of PV in tadpole tail muscles.

Parvalbumin genes from human and rat are identical in intron/exon organization and contain highly homologous regulatory elements and coding sequences

The structural organization of the chromosomal gene for human parvalbumin was determined mostly by sequencing exons and intron exon junctions of a 7500 base-pair (bp) long genomic clone derived from a chromosome 22-specific gene library. Four exons coding for 100 from a total of 109 amino acids were detected in this clone and 472 bp of the 5'-flanking region were sequenced. The region corresponding to the C-terminal amino acids 101 to 109 of human parvalbumin was determined by sequencing a cDNA fragment derived from human brain mRNA after amplification by the polymerase chain reaction. The first intron is placed 7 bp upstream from the ATG translation start signal, whereas all other splice sites divide putative Ca2+-binding domains. All intron positions coincide exactly with those reported for the rat parvalbumin gene. The 5' mRNA leader sequence has a similarity of 57%, the coding region of 91% and the 3' non-coding region of 83% to the corresponding rat sequences. Only nine conservative amino acid replacements were observed between human and rat parvalbumins. The predicted secondary structures for human, rat, mouse and rabbit parvalbumins are very similar, indicating a strong structural relationship among mammalian parvalbumins. Several elements with potential transcription regulatory activities were found in the region immediately 5' to the transcription start site including a TATA box (TATATA) and a CAAT box (CCAAAAT). Several regions in the putative promoter are strongly conserved between the human and rat parvalbumin genes. One of these with a length of 32 bp is identical with the rat counterpart and has a high degree of homology to a promoter region in the myosin light chain 3F gene, which is expressed in fast contracting/relaxing muscle fibers (anaerobic/type IIb), the cell type that also exhibits highest levels of parvalbumin expression. The human parvalbumin mRNA contains the putative polyadenylation signal AATAAA 13 nucleotides upstream from the polyadenylation site. A 700-nucleotide long parvalbumin mRNA is synthesized at low levels in the human cerebellum as well as in the neuroblastoma cell line SK-N-BE.

Identification and quantification in single muscle fibers of four isoforms of parvalbumin in the iliofibularis muscle of Xenopus laevis

The major parvalbumins present in the iliofibularis muscle of Xenopus laevis were identified and the total parvalbumin content of different types of single fibers of this muscle was determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate (SDS). The criteria used in the identification of proteins as parvalbumins were: a relative molecular mass (Mr) between 10,000 and 14,000, an isoelectric point (pI) between 4.0 and 5.0, and a Ca2+-dependent mobility when run on a polyacrylamide gel in the absence of SDS. Four proteins were thus identified as parvalbumins: PA1, Mr 14,000, pI 4.90; PA2, Mr 11,000, pI 4.90; PA3, Mr 11,000, pI 4.95; and PA4, Mr 11,000, pI 4.25. An ultraviolet absorbance spectrum characteristic of parvalbumins was recorded for a purified preparation of these four proteins. Because the apparent Mr of rabbit parvalbumin in the gel system used was 14,000, whereas the true value is 12,100, it is not excluded that the Mr of component PA1 of 14,000 is an overestimation. The total parvalbumin content of muscles and single muscle fibers was determined using the supernatant obtained after centrifugation of tissue homogenates. Analysis of the protein pattern after electrophoresis in the presence of SDS of this fraction indicated that the Mr 14,000 and 11,000 protein bands contained virtually only parvalbumin. Quantification of the total parvalbumin content of relatively fast (type 1) and slow (type 2) contracting and relaxing single muscle fibers, using laser densitometric analysis of minigels, yielded mean values (mg protein/g wet wt., +/- S.D.) of 5.2 +/- 0.8 for nine type 1 fibers, and 1.9 +/- 1.0 for five type 2 fibers. Both fiber types contained about 2.5-times as much of the Mr 14,000 isoform relative to the combined Mr 11,000 isoforms.

Expression of myosin heavy chain transcripts during Xenopus laevis development

In amphibians, the composition and pattern of expression of the myosin heavy chain (myHC) gene family are mostly unknown. To understand better the regulation of this complex family, we screened cDNA libraries from swimming tadpole and adult leg muscle RNA and have isolated and partially sequenced clones for several myHCs. Two of these, E3 and E19, first appear in mesoderm at late gastrula stage and are coexpressed with muscle actin. They increase in abundance in trunk and tail myotome until metamorphosis, when they begin to decline. E3 and E19 are also both expressed in hind leg muscle at the beginning of metamorphosis, but decline to low levels in adult leg muscle. A new transcript, A7, first appears during early metamorphosis in both the tail and hind leg skeletal muscle. A7 transcripts then decline in degenerating tail but persist in hind leg through metamorphosis and in adults. Two other embryonic myHCs, E14 and E15, code for isoforms closely related to E19 and may be alleles or duplications. Although sequence comparisons with myHCs from other vertebrates could not correlate the transcripts with specific protein isoforms, from the pattern of expression E3 and E19 apparently code for embryonic fast skeletal myHC isoforms, whereas A7 codes for an adult skeletal isoform.