Use of a novel collagen matrix with oriented pore structure for muscle cell differentiation in cell culture and in grafts

Tissue engineering of skeletal muscle from cultured cells has been attempted using a variety of synthetic and natural macromolecular scaffolds. Our study describes the application of artificial scaffolds (collagen sponges, CS) consisting of collagen-I with parallel pores (width 20–50 μm) using the permanent myogenic cell line C₂C₁₂. CS were infiltrated with a high-density cell suspension, incubated in medium for proliferation of myoblasts prior to further culture in fusion medium to induce differentiation and formation of multinucleated myotubes. This resulted in a parallel arrangement of myotubes within the pore structures. CS with either proliferating cells or with myotubes were grafted into the beds of excised anterior tibial muscles of immunodeficient host mice. The recipient mice were transgenic for enhanced green fluorescent protein (eGFP) to determine a host contribution to the regenerated muscle tissue. Histological analysis 14–50 days after surgery showed that donor muscle fibres had formed in situ with host contributions in the outer portions of the regenerates. The function of the regenerates was assessed by direct electrical stimulation which resulted in the generation of mechanical force. Our study demonstrated that biodegradable CS with parallel pores support the formation of oriented muscle fibres and are compatible with force generation in regenerated muscle.

Maternal versus paternal expression of a LacZ transgene in preimplantation mouse embryos: effects of genetic background and 2-cell block

The expression of a transgene NI-ROSA LacZ (LacZtg) trapped into the genes for two presumably untranslated, ubiquitously expressed RNAs, was studied in preimplantation mouse embryos with respect to penetrance (fraction of expressing embryos) and to localisation of beta-galactosidase activity. With maternal origin in NMRI mice beta-galactosidase was first detected within one dot in the cytoplasm of zygotes at 30 h post-hCG. The staining pattern progressed to small clusters and to dense, homogeneous staining of the entire cytoplasm during further development. Within the NMRI background, penetrance in utero was delayed by at least 6 h when the transgene was of paternal as compared with maternal origin. Paternal transgene expression increased marginally during culture to 50 h after explantation of embryos at 30-48 h post-hCG and remained low or decreased in the '2-cell block'. Expression of a paternal transgene in preimplantation embryos developing in utero was further delayed in the maternal MF1 as compared with the NMRI background. In contrast to NMRI x NMRI embryos with paternally derived transgene, expression increased with time during the 2-cell block in MF1 x NMRI embryos. Thus, in the earliest phase of mammalian development expression of this LacZtg is influenced by parental origin, maternal genetic background and environment. The spatial distribution of the gene product is developmentally controlled.

Mutants of tobacco mosaic virus with temperature-sensitive coat proteins induce heat shock response in tobacco leaves

We analyzed, with respect to heat shock proteins (HSPs), systemically reacting tobacco leaves inoculated with Tobacco mosaic virus (TMV), wild-type vulgare, and temperature-sensitive coat protein (CP) mutants Ni 118 (P20L) and flavum (D19A), kept at 23 or 30 degrees C. HSP18 and HSP70 mRNAs and proteins were induced with temperature-sensitive CP mutants after 1 to 2 days at 30 degrees C. After 4 to 6 days, HSP70 was also induced at 23 degrees C. The induction of HSPs paralleled the amount of insoluble TMV CP in leaf extracts, indicating that denatured TMV CP by itself induces a heat-shock response.

Lack of dystrophin is associated with altered integration of the mitochondria and ATPases in slow-twitch muscle cells of MDX mice

The potential role of dystrophin-mediated control of systems integrating mitochondria with ATPases was assessed in muscle cells. Mitochondrial distribution and function in skinned cardiac and skeletal muscle fibers from dystrophin-deficient (MDX) and wild-type mice were compared. Laser confocal microscopy revealed disorganized mitochondrial arrays in m. gastrocnemius in MDX mice, whereas the other muscles appeared normal in this group. Irrespective of muscle type, the absence of dystrophin had no effect on the maximal capacity of oxidative phosphorylation, nor on coupling between oxidation and phosphorylation. However, in the myocardium and m. soleus, the coupling of mitochondrial creatine kinase to adenine nucleotide translocase was attenuated as evidenced by the decreased effect of creatine on the Km for ADP in the reactions of oxidative phosphorylation. In m. soleus, a low Km for ADP compared to the wild-type counterpart was found, which implies increased permeability for that nucleotide across the mitochondrial outer membrane. In normal cardiac fibers 35% of the ADP flux generated by ATPases was not accessible to the external pyruvate kinase-phosphoenolpyruvate system, which suggests the compartmentalized (direct) channeling of that fraction of ADP to mitochondria. Compared to control, the direct ADP transfer was increased in MDX ventricles. In conclusion, our data indicate that in slow-twitch muscle cells, the absence of dystrophin is associated with the rearrangement of the intracellular energy and feedback signal transfer systems between mitochondria and ATPases. As the mechanisms mediated by creatine kinases become ineffective, the role of diffusion of adenine nucleotides increases due to the higher permeability of the mitochondrial outer membrane for ADP and enhanced compartmentalization of ADP flux.

Growth and differentiation of permanent and secondary mouse myogenic cell lines on microcarriers

Myogenesis involves the determination of progenitor cells to myoblasts, their fusion to yield multinuclear myotubes, and the maturation of myotubes to muscle fibres. This development is reflected in a time pattern of gene expression, e.g. of genes coding for desmin, the myogenic factors myogenin and myoD, the acetylcholine receptor alpha-subunit and the muscular chloride channel CIC-1. We attempted to improve yields and myogenic differentiation in culture by using three-dimensional microcarrier systems. Out of a variety of carriers tested in stationary cultures, collagen-coated dextran Cytodex3 beads proved optimal for the proliferation and differentiation of the murine myogenic cell line C2C12. With C2C12 myoblasts in stationary and stirred systems (Spinner- and SuperSpinner flasks), surface adherence, differentiation into myotubes and expression of muscle-specific mRNAs on Cytodex3 beads were the same as in conventional cultures. Other carriers tested (DEAE cellulose, glass, plastic, cellulose, polyester) did not support growth and differentiation of C2C12 cells. The secondary mouse myogenic stem cells M12 and M2.7-MDX proliferated and differentiated well in stationary Cytodex3 cultures, but no differentiation occurred in Spinner flasks. As indicated by light and scanning electron microscopy, C2C12 myotubes formed not only on but also in between Cytodex beads. The secondary cell lines may succumb to shear forces under these conditions.

The genomic basis of K(V)3.4 potassium channel mRNA diversity in mice

K(V)3.4 belongs to the shaw subfamily of shaker-type potassium channels. It conducts fast inactivating, high threshold currents in the central nervous system and in fast-twitch skeletal muscle fibers. The corresponding mouse gene, Kcnc4, consists of five exons spanning a region of 20 kb. Approximately 700 bp of regulatory sequence were delineated. It is GC-rich and lacks typical TATA and CAAT motifs. Instead, seven Sp-1 and three E-box elements define putative regulatory sequences. The mouse K(V)3.4 mRNA has a size of 3639 bp, 1120 bp of which are 3' untranslated region. A transcript initiated from an alternative 5'-exon was identified by RACE and verified by genomic analysis. This isoform, designated K(V)3.4d, is predominantly expressed in skeletal muscle and probably results from alternative promoter usage. It encodes a channel protein with a novel N-terminal cytoplasmic domain. It lacks the conserved sequence motifs encoding the shaw-type tetramerization domain and the 'ball' peptide, which confers fast inactivation properties. Another splice variant, K(V)3.4c, is derived by exon skipping in the C-terminal region and is expressed at similar levels in brain and muscle. These data demonstrate that differential splicing and alternative transcription start sites are utilised to generate a set of K(V)3.4 variants in skeletal muscle and brain, presumably involved in the regulation of excitability.

Generation of tension by skinned fibers and intact skeletal muscles from desmin-deficient mice

We have investigated the physiological role of desmin in skeletal muscle by measuring isometric tension generated in skinned fibres and intact skeletal muscles from desmin knock-out (DES-KO) mice. About 80% of skinned single extensor digitorum longus (EDL) fibres from adult DES-KO mice generated tensions close to that of wild-type (WT) controls. Weights and maximum tensions of intact EDL but not of soleus (SOL) muscles were lowered in DES-KO mice. Repeated contractions with stretch did not affect subsequent isometric tension in EDL muscles of DES-KO mice. Tension during high frequency fatigue (HFF) declined faster and this deficiency was compensated in DES-KO EDL muscles by 5 mM caffeine which had no influence on HFF in WT EDL. Furthermore, caffeine evoked twitch potentiation was higher in DES-KO than in WT muscles. We conclude that desmin is not essential for acute tensile strength but rather for optimal activation of intact myofibres during E-C coupling.

Tumor necrosis factor alpha induces a metalloprotease-disintegrin, ADAM8 (CD 156): implications for neuron-glia interactions during neurodegeneration

ADAM proteases, defined by extracellular disintegrin and metalloprotease domains, are involved in protein processing and cell-cell interactions. Using wobbler (WR) mutant mice, we investigated the role of ADAMs in neurodegeneration and reactive glia activation in the CNS. We found that ADAM8 (CD 156), a suspected leukocyte adhesion molecule, is expressed in the CNS and highly induced in affected CNS areas of WR mice, in brainstem and spinal cord. ADAM8 mRNA and protein are found at low levels throughout the normal mouse CNS, in neurons and oligodendrocytes. In the WR CNS regions in which neurodegeneration occurs, ADAM8 is induced in neurons, reactive astrocytes, and activated microglia. Similarly, the proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha) is upregulated and shows the same cellular distribution. In primary astrocytes from wild-type and WR mice, in primary cerebellar neurons, and in mouse motoneuron-like NSC19 cells, ADAM8 expression was induced up to 15-fold by mouse TNF-alpha, in a dose-dependent manner. In both cell types, ADAM8 was also induced by human TNF-alpha, indicating that TNF receptor type I (p55) is involved. Induction of ADAM8 mRNA was suppressed by treatment with an interferon-regulating factor 1 (IRF-1) antisense oligonucleotide. We conclude that IRF-1-mediated induction of ADAM8 by TNF-alpha is a signaling pathway relevant for neurodegenerative disorders with glia activation, proposing a role for ADAM8 in cell adhesion during neurodegeneration.

Elevated expression of membrane type 1 metalloproteinase (MT1-MMP) in reactive astrocytes following neurodegeneration in mouse central nervous system

Reactive astrocytes occurring in response to neurodegeneration are thought to play an important role in neuronal regeneration by upregulating the expression of extracellular matrix (ECM) components as well as the ECM degrading metalloproteinases (MMPs). We examined the mRNA levels and cellular distribution of membrane type matrix metalloproteinase 1 (MT1-MMP) and tissue inhibitors 1-4 of MMPs (TIMPs) in brain stem and spinal cord of wobbler (WR) mutant mice affected by progressive neurodegeneration and astrogliosis. MT1-MMP, TIMP-1 and TIMP-3 mRNA levels were elevated, whereas TIMP-2 and TIMP-4 expression was not affected. MT1-MMP was expressed in reactive astrocytes of WR. In primary astrocyte cultures, MT1-MMP mRNA was upregulated by exogeneous tumor necrosis factor alpha. Increased plasma membrane and secreted MMP activities were found in primary WR astrocytes.

Isolation and characterization of the novel popeye gene family expressed in skeletal muscle and heart

We identified a novel gene family in vertebrates which is preferentially expressed in developing and adult striated muscle. Three genes of the Popeye (POP) family were detected in human and mouse and two in chicken. Chromosomal mapping indicates that Pop1 and Pop3 genes are clustered on mouse chromosome 10, whereas Pop2 maps to mouse chromosome 16. We found evidence that POP1 and POP3 in chicken may also be linked and multiple transcript isoforms are generated from this locus. The POP genes encode proteins with three potential transmembrane domains that are conserved in all family members. Individual POP genes exhibit specific expression patterns during development and postnatally. Chicken POP3 and mouse Pop1 are first preferentially expressed in atrium and later also in the subepicardial compact layer of the ventricles. Chicken POP1 and mouse Pop2 are expressed in the entire heart except the outflow tract. All three Pop genes are expressed in heart and skeletal muscle of the adult mouse and lower in lung. Pop1 and Pop2 expression is upregulated in uterus of pregnant mice. Like the mouse genes, human POP genes are predominantly expressed in skeletal and cardiac muscle. The strong conservation of POP genes during evolution and their preferential expression in heart and skeletal muscle suggest that these novel proteins may have an important function in these tissues in vertebrates.

Conservation of the 3'-untranslated region of the Rab1a gene in amniote vertebrates: exceptional structure in marsupials and possible role for posttranscriptional regulation

The YPT1/RAB1 protein, a key regulator of the intracellular vesicle transport in eukaryotes, is highly conserved in function and amino acid sequence. Here we report that the most highly conserved nucleotide sequence of the Rab1a gene of amniote vertebrates corresponds to the 3'-untranslated region (3'-UTR) of the mRNA. Sequences of 27 species ranging from mammals to sauropsida are >91% identical in this region. Secondary structure prediction procedures applied to the 3'-UTR sequences between positions 750 and 984 and 1428 (mouse cDNA: Y00094), respectively, of the RAB1a mRNAs revealed families of alternative structures around nucleotide position 800 as recurrent features. The two hairpin loops are also predicted for marsupials, despite of their exceptional extension of the A-rich sequence in between. Yet, sequence conservation is much higher than required to conserve secondary structure. Implications for posttranscriptional regulation and protein binding are discussed.

A functional knock-out of titin results in defective myofibril assembly

Titin, also called connectin, is a giant muscle protein that spans the distance from the sarcomeric Z-disc to the M-band. Titin is thought to direct the assembly of sarcomeres and to maintain sarcomeric integrity by interacting with numerous sarcomeric proteins and providing a mechanical linkage. Since severe defects of such an important molecule are likely to result in embryonic lethality, a cell culture model should offer the best practicable tool to probe the cellular functions of titin. The myofibroblast cell line BHK-21/C13 was described to assemble myofibrils in culture. We have now characterized the sub-line BHK-21-Bi, which bears a small deletion within the titin gene. RNA analysis revealed that in this mutant cell line only a small internal portion of the titin mRNA is deleted. However, western blots, immunofluorescence microscopy and immunoprecipitation experiments showed that only the N-terminal, approx. 100 kDa central Z-disc portion of the 3 MDa titin protein is expressed, due to the homozygous deletion in the gene. Most importantly, in BHK-21-Bi cells the formation of thick myosin filaments and the assembly of myofibrils are impaired, although sarcomeric proteins are expressed. Lack of thick filament formation and of ordered actin-myosin arrays was confirmed by electron microscopy. Myogenisation induced by transfection with MyoD yielded myofibrils only in myotubes formed from wild type and not from mutant cells, ruling out that a principal failure in myogenic commitment of the BHK-21-Bi cells might cause the observed effects. These experiments provide the first direct evidence for the crucial role of titin in both thick filament formation as a molecular ruler and in the coordination of myofibrillogenesis.

Steroid RU 486 inducible myogenesis by 10T1/2 fibroblastic mouse cells

For reconstruction or repair of damaged tissues, an artificially regulated switch from proliferation to differentiation would be of great advantage. To achieve conditional myogenesis, we expressed MyoD in mouse C3H 10T1/2 fibroblastic cells, using a gene regulation system based on the synthetic steroid RU 486. By stable co-transfection of a plasmid construct with the RU 486 dependent activator and an integrating inducible MyoD construct, a cell clone, designated 10T-RM, was obtained in which MyoD expression was stringently controlled by RU 486. 12 h after addition of 10 nM RU 486 to 10T-RM cells, saturation levels of MyoD mRNA were observed and >/=2 days later, mRNA for embryonal myosin heavy chain (MyHC(emb)) was abundant and mononucleated cells fused into myotubes.

A role for polyproline motifs in the spinal muscular atrophy protein SMN. Profilins bind to and colocalize with smn in nuclear gems

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by the loss of alpha-motoneurons in the spinal cord followed by atrophy of skeletal muscles. SMA-determining candidate genes, SMN1 and SMN2, have been identified on human chromosome 5q. The corresponding SMN protein is expressed ubiquitously. It is coded by seven exons and contains conspicuous proline-rich motifs in its COOH-terminal third (exons 4, 5, and 6). Such motifs are known to bind to profilins (PFNs), small proteins engaged in the control of actin dynamics. We tested whether profilins interact with SMN via its polyproline stretches. Using the yeast two-hybrid system we show that profilins bind to SMN and that this binding depends on its proline-rich motifs. These results were confirmed by coimmunoprecipitation and by in vitro binding studies. Two PFN isoforms, I and II, are known, of which II is characteristic for central nervous system tissue. We show by in situ hybridization that both PFNs are highly expressed in mouse spinal cord and that PFN II is expressed predominantly in neurons. In motoneurons, the primary target of neurodegeneration in SMA, profilins are highly concentrated and colocalize with SMN in the cytoplasm of the cell body and in nuclear gems. Likewise, SMN and PFN I colocalize in gems of HeLa cells. Although SMN interacts with both profilin isoforms, binding of PFN II was stronger than of PFN I in all assays employed. Because the SMN genes are expressed ubiquitously, our findings suggest that the interaction of PFN II with SMN may be involved in neuron-specific effects of SMN mutations.

Spatiotemporal progression of neurodegeneration and glia activation in the wobbler neuropathy of the mouse

The wobbler mouse (phenotype WR; genotype wr/wr) has been investigated as a model for neurodegenerative diseases like SMA and ALS. A new diagnostic marker based on a polymorphism in the closely linked chaperonine gene Cct4 enabled us to diagnose the allelic status at the wr locus within the original background strain C57BL/6. Using this marker, we investigated the spatiotemporal progression of neuropathology in WR mice from postnatal day (d.p.n.) 10 to 60. Neurodegeneration starts at 13 d.p.n. in the thalamus (N. ventralis), in deep cerebellar nuclei, brain stem (N. vestibularis) and spinal cord interneurons. The motor nuclei of spinal nerves and motoneurons degenerate from 15 d.p.n. onward. Reactive astrocytes are observed around 17 d.p.n. in the white and grey matter of the spinal cord. Microgliosis occurs only from 23 d.p.n. onward. Our data demonstrate that in the WR disease, neurodegeneration in thalamus, cerebellum, and brain stem precedes motoneuron degeneration, astrogliosis and microgliosis.

Maturation and myotonia influence the abundance of cation channels KDR, KIR and CIR differently: a patch-clamp study on mouse interosseus muscle fibres

To detect cation channels, the expression of which is dependent on the physiological state of muscle, single-channel activities of dissociated fibres of the mouse interosseus muscle were recorded using the patch-clamp technique in the cell-attached mode. Fibres were prepared from juvenile and adult wild-type (WT), from chloride channel-deficient myotonic and from denervated adult WT muscles. In all cases delayed-rectifier K+ channels (KDR) with a unitary conductance of 11 pS were recorded in more than 95% of sarcolemmal patches, but with a low, steady-state open probability. Inwards-rectifying K+ channels (KIR) with a conductance of 31 pS in 140 mM [K+]o were active in about 50% of the membrane patches from WT and in more than 90% of those from myotonic fibres. A hitherto undescribed, inwards-rectifying, cation channel, provisionally termed CIR, with fast kinetics and a unitary conductance of 36 pS, was active in nearly every membrane patch from juvenile mice, both WT and myotonic. The abundance of CIR decreased during development, but was not changed 7 days after denervation of adult WT muscle. Ca(2+)-dependent K+ channels were seen sporadically. Channels with the characteristics of adenosine 5'-triphosphate (ATP)-sensitive K+ channels were recorded frequently upon excision of membrane patches, but remained inactive in most cell-attached recordings. In conclusion, of the investigated ion channels, only KIR was responsive to the activity pattern of adult muscle, whereas CIR was down-regulated during muscle maturation.

Role of phosphorylation and physiological state in the regulation of the muscular chloride channel ClC-1: a voltage-clamp study on isolated M. interosseus fibers

Chloride currents (I(Cl)) were investigated with the two-electrode voltage-clamp technique in enzymatically isolated fibers from interosseus muscles of wild-type (WT), denervated WT, and myotonic (ADR, ClC-1-deficient) mice. Characteristics of I(Cl) were consistent with previous observations on rat muscle fibers and cultured nonmuscle cells transfected with hClC-1 cDNA. In the presence of 0.1 mM anthracene-9-carboxylic acid and in ADR fibers, I(Cl) was reduced by >90%. WT interosseus fibers denervated 6-7 days prior to isolation showed approximately 50% I(Cl) compared to control fibers. Addition of 3.3 microM staurosporine, a nonspecific inhibitor of protein kinases, increased I(Cl) in WT interosseus fibers by a factor of approximately two and altered its kinetic characteristics. We conclude that in dissociated fibers cultured for 1-2 days, in contrast to freshly isolated muscles, chloride conductance is downregulated by a mechanism involving protein phosphorylation. In situ, this short-term regulation may complement transcriptional long-term regulation of ClC-1.

A multicopy c-Myc transgene as a nuclear label: overgrowth of Myctg50 cells in allophenic mice

To trace cell lineages and the origin and fate of cells in transplantation and embryo chimeras, a DNA/DNA in situ hybridization cell labelling system was developed, based on a 50-copy murine c-myc transgene on mouse chromosome 8. Elevated levels of cMyc mRNA were found in Myc*tg50 (Myctg50/0 and Myctg50/Myctg50) transgenic tissues, but adult transgenic NMRI mice were anatomically and histologically indistinguishable from control NMRI mice and did not develop tumours on a wild-type or nude (nu/nu) background. The hybridization label detected transgenic nuclei with an efficiency of approximately 80%. In muscle grafts, this transgene label was successfully applied to trace donor cells in a labelled host and to study the invasion of a graft by host cells. When the cMyc hybridization was used in allophenic mice of the control<-->NMRI-Myctg50/? (nu/+ or +/+) type, an up to a three-fold excess of MYC*tg50 positive over control nuclei was found in all organs examined (ventricle, skeletal muscle, liver, small intestine). This overgrowth of MYC*tg50 cells is probably due to transgene expression. Four out of seven (C57BL/6xBALB/c) or (C57BL/6xNMRI)<-->MYC*tg50 allophenic mice displayed anatomical abnormalities, e.g. an enlarged thymus and a tumour in the groin region. As these abnormalities were only observed in allophenic mice, they might be due to the imbalance of growth potential between MYC*tg50 transgenic and normal cells in the same individual.

Mutual interference of myotonia and muscular dystrophy in the mouse: a study on ADR-MDX double mutants

For Duchenne muscular dystrophy (DMD, dystrophin deficiency) and Thomsen/Becker myotonia (muscular chloride channel deficiency) genetically homologous mouse models are available, the dystrophin-deficient MDX mouse and the myotonic ADR mouse. Whereas the latter shows more severe symptoms than human myotonia patients, the MDX mouse, in contrast to DMD patients, is only mildly affected. We have introduced, by appropriate breeding, the defect leading to myotonia (Clc1 null mutation, adr allele) into MDX mice, thus creating ADR-MDX double mutants. The expectation was that, due to mechanical stress during myotonic cramps, the ADR status should symptomatically aggravate the muscle fibre necrosis caused by the dystrophin deficiency. The overall symptoms of the double mutants were dominated by myotonia. Weight reduction and premature death rate were higher in ADR-MDX than in ADR mice. Sarcolemmal ruptures as indicated by influx into muscle fibres of serum globulins and injected Evans blue were found with great inter-individual variation in MDX and in ADR-MDX muscles. Affected fibres were found mainly in large groups in MDX but single or in small clusters in ADR-MDX leg muscles. The symptoms of myotonia (aftercontractions, shift towards oxidative fibres) were less pronounced in ADR-MDX than in ADR muscles. Conversely, numbers of damaged fibres as well as the percentage of central nuclei (an indicator of fibre regeneration) were significantly lower in ADR-MDX than in MDX skeletal muscles. Thus it appears that, at the level of the muscle fibre, myotonia and muscular dystrophy attenuate each other.

Homology between human chromosome 2p13.3 and the wobbler critical region on mouse chromosome 11: comparative high-resolution mapping of STS and EST loci on YAC/BAC contigs

Human Chr 2p13-14 and homologous regions on mouse Chrs 6 and 11 have been subjects of previous studies because they comprise the loci for several neuromuscular diseases. Here we report on high-resolution mapping of 55 STS and EST loci on human Chr 2p13.3 and of 47 markers on the corresponding region on proximal mouse Chr. 11. The maps comprise several known genes, MEIS1/Meis1, RAB1a/Rab1a, MDH1/Mor2, OTX1/Otx1, and REL on human 2p13.3 and mouse Chr 11, respectively, as well as the wobbler (wr) critical region of the mouse. Whereas a perfect correspondence was found in most of the 4-Mb region, a small rearrangement was discovered around the OTX1/Otx1 locus. The detailed STS and EST transcript maps of these regions and a further narrowing down of the mouse wr critical region to the interval between D11Mit79 and D11Mit19 allow for the selection of positional candidate genes for wr, and the exclusion of others.

A survey of the primary structure and the interspecies conservation of I-band titin's elastic elements in vertebrates

Titin is a >3000-kDa large filamentous protein of vertebrate-striated muscle, and single titin molecules extend from the Z disc to the M line. In its I-band section, titin behaves extensible and is responsible for myofibrillar passive tension during stretch. However, details of the molecular basis of titin's elasticity are not known. We have compared the motif sequences of titin elastic elements from different vertebrate species and from different regions of the molecule. The I-band titin Ig repeats that are expressed in the stiff cardiac muscle and those that are tissue-specifically expressed in more elastic skeletal muscles represent distinct subgroups. Within the tissue-specifically expressed Ig repeats, a super-repeat structure is found. For the PEVK titin sequences, we surveyed interspecies conservation by hybridization experiments. The sequences of the titin gene which code for the C-terminal region of the PEVK domain are conserved in the genomes of a larger variety of vertebrates, whereas the N-terminal PEVK sequences are more divergent. Future comparisons of titin gene sequences from different vertebrates may improve our understanding of how titin contributes to species diversity of myofibrillar elasticity. Within one species, different classes of Ig repeat families may contribute to elastic diversity of the titin spring in different segments.

LacZ transgene expression as a cell marker to analyse rescue from the 2-cell block in mouse aggregation chimeras

Embryos from certain mouse strains are arrested at the 2-cell stage in cell culture ('2-cell block'), whereas those from other strains develop to the blastocyst stage under the same conditions. It was previously shown that blocking embryos can be rescued in culture by aggregation with an excess of 2-cell stages of a non-blocking strain such as CBA x C57BL/6 F2. Here we have employed a LacZ transgene in a blocking strain (NMRI) to follow the fate of rescued blastomeres up to the blastocyst stage. We found that rescued blastomeres can participate in both inner cell mass and trophoblast formation, thus completely overcoming the 2-cell block.

The protein kinase N (PKN) gene PRKCL1/Prkcl1 maps to human chromosome 19p12-p13.1 and mouse chromosome 8 with close linkage to the myodystrophy (myd) mutation

Protein kinase N (PKN) is a fatty acid- and Rho-activated serine/threonine protein kinase involved in the regulation of cell motility by association with cytoskeletal components such as neurofilament and alpha-actinin. We determined the chromosomal location of the human PKN gene PRKCL1 by fluorescence in situ hybridization and by radiation hybrid mapping. The corresponding mouse gene Prkcl1 was mapped by segregation analysis. We found by FISH that PRKCL1 is localized to chromosome 19p12-p13.1 and, more precisely, by radiation hybrid mapping, about 11 cR from EST WI-6344 in subband 19p12. Prkcl1 maps to mouse chromosome 8 between D8Mit6 and junb. This region of mouse Chr 8 shows a scrambled syntenic conservation to human chromosomes 4q, 8p, and 19p. As the mouse mutation myodystrophy myd has been mapped to the same region, Prkcl1 is a candidate gene for myd.

Expression of the potassium channel KV3.4 in mouse skeletal muscle parallels fiber type maturation and depends on excitation pattern

We report the detailed expression pattern of the voltage-dependent potassium channel KV3.4 (rat homologue, Raw3) in mouse skeletal muscle. Using semi-quantitative RT-PCR, we show that its expression is detectable at embryonic day 17 and rises to adult levels within 2 weeks after birth. Expression is fiber type-dependent, with mRNA levels being 5-6-fold lower in the mixed slow/fast soleus muscle than in the fast tibialis anterior and extensor digitorum longus muscles. Fast muscles from myotonic mice exhibit low KV3.4 mRNA levels similar to those of wild-type soleus. In denervated extensor digitorum longus, KV3.4 expression declines to perinatal levels. We conclude that KV3.4 expression in mouse skeletal muscle is regulated by the pattern of excitation.

Chloride conductance in mouse muscle is subject to post-transcriptional compensation of the functional Cl- channel 1 gene dosage

1. In mature mammalian muscle, the muscular chloride channel ClC-1 contributes about 75% of the sarcolemmal resting conductance (Gm). In mice carrying two defective alleles of the corresponding Clc1 gene, chloride conductance (GCl) is reduced to less than 10% of that of wild-type, and this causes hyperexcitability, the salient feature of the disease myotonia. Potassium conductance (GK) values in myotonic mouse muscle fibres are lowered by about 60% compared with wild-type. 2. The defective Clcadr allele causes loss of the 4.5 kb ClC-1 mRNA. Mice heterozygous for the defective Clc1adr allele contain about 50% functional mRNA in their muscles compared with homozygous wild-type mice. 3. Despite a halved functional gene dosage, heterozygous muscles display an average GCl which is not significantly different from that of homozygous wild-type animals. The GK values in heterozygotes are also indistinguishable from homozygous wild-type animals. 4. These results indicate that a regulatory mechanism acting at the post-transcriptional level limits the density of ClC-1 channels. GK is probably indirectly regulated by muscle activity.

The mouse Clc1/myotonia gene: ETn insertion, a variable AATC repeat, and PCR diagnosis of alleles

Myotonias are muscle diseases in which the function of the muscular chloride channel ClC-1 is impaired. Null alleles of the corresponding Clc1 gene on mouse chromosome (Chr) 6 provide animal models for human myotonias. It was shown that the allele adr (Clc1adr) is due to an insertion of an ETn type transposon that is transcribed and leads to multiple splicing events; the allele mto (Clc1adr-mto) involves a stop codon near the N-terminus. We have determined the genomic organization of the mouse Clc1 gene and the sequence requirements for the transposon insertion in the Clc1adr allele. The mouse Clc1 gene is composed of 23 exons, ranging from 39 to 372 bp, and spans approximately 23 kb of genomic DNA. The exon/intron organization is highly homologous to that of the human CLCN1 gene; the homology of the coding sequence is 97% to rat and 89% to human. In the adr allele the ETn transposon is inserted into intron 12, the largest intron. Whereas the 5' and 3' LTR sequences of the ETn transposon are homologous to those reported for other insertional mutations of the mouse, no consensus motif for an insertion target site could be defined. On the basis of flanking sequences, we provide duplex PCR diagnoses for the adr, adr-mto, and wild-type alleles of Clc1. Close to the 3' end of intron 12, a tetranucleotide repeat (AATC)n was found that is polymorphic between mouse species Mus musculus, M. molossinus, M. castaneus, and M. spretus, and can thus be used for chromosomal mapping studies.

Integrated radiation hybrid map of human chromosome 2p13: possible involvement of dynactin in neuromuscular diseases

The genes for the human neuromuscular diseases limb-girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy are located on chromosome 2p13-p14, and two neuromuscular mutations of the mouse have been mapped to regions homologous to human chromosome 2p13 by conserved synteny, wobbler (wr) on proximal Chr 11 and motor neuron degeneration 2 (mnd2) on Chr 6. Neither one is a mouse homologue of LGMD2B. Recently the gene DCTN1, coding for the large subunit of the cytoskeletal protein dynactin, was shown by FISH to be located in this region and therefore should be considered a candidate for all these disease genes. Here we present mapping data based on radiation hybrid and physical mapping that more precisely define the location of nine genetic markers in the critical region and the homology relationship of human chromosome 2p with mouse proximal Chr 11 and Chr 6. The human dynactin gene was mapped between markers TGFA and D2S1394, implying that the mouse dynactin gene Dctn1 is located on Chr 6, distal to mnd2. Thus DCTN1/Dctn1 is a candidate for LGMD2B but not for mnd2 or wr.

The human gene ZFP161 on 18p11.21-pter encodes a putative c-myc repressor and is homologous to murine Zfp161 (Chr 17) and Zfp161-rs1 (X Chr)

A clone from a lambda gt11 cDNA expression library of HeLa cells was isolated, sequenced, and shown to encode a new human zinc finger protein. The cDNA of the gene termed ZFP161 has an open reading frame of 1347 bp. The predicted protein comprises 449 amino acid residues and contains five zinc finger motifs of the Krüppel type near the C-terminus and a BTB/POZ domain in the N-terminal region. The protein is 98% homologous to a murine zinc finger protein, ZF5 (M. Numoto et al., 1993, Nucleic Acids Res. 21: 3767-3775), which is a putative transcriptional repressor of c-myc and exhibits growth-suppressive activity in mouse cell lines. Through the use of a panel of somatic cell hybrids for chromosomal assignment and DNAs of somatic cell hybrids containing a deleted chromosome 18 for fine mapping, the human gene ZFP161 was localized to 18p11.21-pter. Therefore, ZFP161 is a candidate gene by position for the holoprosencephaly type 4 gene, HPE4, which is involved in congenital malformations. With DNAs from an interspecific backcross, two homologous mouse genes, Zfp161 and Zfp161-rs1, were mapped to chromosome 17 and the X chromosome, respectively. Mapping of Zfp161 confirms and extends a region of homology between distal mouse chromosome 17 and human 18p.

Evaluation of the antimyotonic activity of mexiletine and some new analogs on sodium currents of single muscle fibers and on the abnormal excitability of the myotonic ADR mouse

To search for use-dependent sodium channel blockers to selectively solve skeletal muscle hyperexcitability in hereditary myotonias, mexiletine (MEX; compound I) and its newly synthetized analogs, 2-(4-chloro-2-methylphenoxy)-benzenethanamine (compound II) and (-)-S-3-(2,6-dimethylphenoxy)-2-methylpropanamine (compound III), were tested on intercostal muscle fibers from the myotonic ADR mouse through use of the standard current-clamp microelectrode technique. In parallel, the effects of these compounds on the sodium channels were measured on frog muscle fibers under voltage-clamp conditions. The tonic and use-dependent blocks of peak sodium currents (I(Namax)) produced by each compound were evaluated by using a single depolarizing pulse and a pulse train at 10 Hz frequency, respectively. At 10 and 50 microM, MEX decreased the occurrence of spontaneous excitability in myotonic muscle fibers; 100 microM was required to decrease the amplitude of the action potential and the stimulus-induced firing of the membrane as well as to increase the threshold for generation of action potential. At 300 microM, MEX decreased the latency of the action potential and increased the threshold current to elicit a single action potential. MEX produced a tonic block of I(Namax) with an half-maximal concentration (IC50) of 83 microM, but the IC50 value for use-dependent block was 3-fold lower. Compound III, which differs from MEX in that it has a longer alkyl chain, similarly blocked first the spontaneous and then the stimulus-evoked excitability of myotonic muscle fibers but at 2-fold lower concentrations than MEX. Compound III was less potent than MEX in producing a tonic block of I(Namax) (IC50 = 108 microM) but was a strong use-dependent blocker with an IC50 close to 15 microM. The more lipophylic compound II irreversibly blocked both spontaneous and stimulus-evoked membrane excitability at concentrations as low as 10 microM and shortened the latency of the action potential in a concentration-dependent fashion. Compound II produced a potent tonic block of I(Namax) (IC50 = 30 microM), and its potency increased 2-fold during high-frequency stimulation. Both of the new analogs (compound II in particular), but not MEX, were less effective on the excitability parameters of striated fibers of healthy vs. ADR mice, a characteristic that increases their interest as potential antimyotonic agents.

Spinal muscular atrophy gene wobbler of the mouse: evidence from chimeric spinal cord and testis for cell-autonomous function

Human hereditary neurodegenerative diseases are genetically and mechanistically very heterogeneous and so are spinal muscular atrophies and cerebellar ataxias in the mouse, despite the common phenomenon of neuronal death. In this species, a number of mutations impair spermiogenesis in addition to neuron survival. Among these, the wobbler mutation on proximal chromosome 11 of the mouse leads to motoneuron degeneration in brain stem and spinal cord and to a defect of spermiogenesis. Chimeric mice of the type wr?/wr? <--> +/+ were produced, and their allelic status at the wr locus was determined by PCR diagnosis of a closely linked marker. Two overt chimeras, one female (XX <--> XX) and one male (XY <--> XY) were identified as wr/wr <--> +/+ and analyzed with respect to their pathological phenotype. Although there was patchy astrogliosis in the spinal cords of both chimeras, their motor performances were overtly normal and muscles were without signs of denervation. The male's testes revealed a mosaic pattern of normal and pathological spermatids. As no progeny was derived from wr spermatids, the spermatocytes appear as a primary target of the wr mutation in testis. Our results argue against a humoral mechanism of the wobbler disease and indicate a cell-autonomous action of the wr gene both in testis and in spinal cord.

Deletion in the Z-line region of the titin gene in a baby hamster kidney cell line, BHK-21-Bi

The gene for titin, a 4MDa myofibrillar protein, was analysed in golden hamster DNAs from different sources, using human cDNA probes and PCR. In the DNA from the BHK-21-Bi subline of baby hamster kidney cells, extended sequences coding for Z-line associated domains were missing, indicating a deletion that renders titin non-functional. These sequences were present in the original BHK-21 line and in hamster DNAs. Our finding shows that, due to the absence of selective pressure on a gene's function, genomic deterioration can occur in a permanent cell line and can lead to a loss of overlapping DNA stretches in both autosomes.

Dynamin genes Dnm1 and Dnm2 are located on proximal mouse chromosomes 2 and 9, respectively

Dynamins, microtubule-binding GTPases, are encoded by at least three genes in mammals. Two distinct gene-specific cDNAs were used to analyze the segregation of dynamin genes Dnm1 and Dnm2 in a mouse interspecies backcross. The nervous system-expressed gene Dnm1 was localized to Chr 2 between the genes for vimentin and nebulin, within a chromosomal region of conserved synteny to human chromosome 9q, consistent with the localization of the human dynamin-1 gene by FISH (see accompanying paper by Newman-Smith et al., 1997, Genomics 41:286-289). The ubiquitously expressed Dnm2 gene was found to be closely linked to the intercellular adhesion molecule-1 gene, Icam1, in a region with homologies to human chromosomes 19p, 8q, and 11q. Potential relations of both loci to disease genes are discussed.

Expression of nerve-regulated genes in muscles of mouse mutants affected by spinal muscular atrophies and muscular dystrophies

The expression of the genes for the alpha-subunit of AChR (AChR alpha), for the myogenic factors myogenin and MyoD, for the calcium-binding protein parvalbumin (PV), and for the muscular chloride channel CIC-1 was studied in the three mouse spinal muscular atrophies (SMAs). These were the mutants "wobbler" (WR), "muscle deficient" (MDF) and "progressive motor neuronopathy" (PMN). Murine myopathies "muscular dystrophy with myositis" (MDM) and "X-linked muscular dystrophy" (MDX) were used as controls. AChR alpha and myogenin mRNA levels were strongly elevated in muscles affected by SMAs (reflecting denervation), whereas only myogenin mRNA was moderately elevated in MDX and MDM muscles, probably due to fiber regeneration. As in denervated muscle, CIC-1 and PV mRNA levels were lowered in SMAs. No changes were seen in muscles of up to 222-day-old symptomless ciliary neurotrophic factor (CNTF) knockout mice. The patterns of gene expression were characteristic for the type of muscle disease, indicating their possible usefulness for clinical diagnosis.

Expression of chloride channel 1 mRNA in cultured myogenic cells: a marker of myotube maturation

The chloride channel CIC-1 is required to maintain a normal excitability of mature muscle fibers; its blockade leads to hyperexcitability, the hallmark of the disease myotonia. In mouse and rat myotubes, representing the embryonic stage of muscle, CIC-1 mRNA is not detectable by Northern blotting. From neonatal to adult, CIC-1 expression increases at least fourfold. Using RT-PCR and hybridization on cultured myotubes we found CIC-1 mRNA at a level of 0.4-1.1% of that in mature mouse muscle, and < or = 0.01% in myoblasts, at stages when desmin mRNA levels are already high. The level of CIC-1 mRNA is thus a sensitive and specific indicator of the maturation of skeletal muscle cells.

YAC contigs of the Rab1 and wobbler (wr) spinal muscular atrophy gene region on proximal mouse chromosome 11 and of the homologous region on human chromosome 2p

Despite rapid progress in the physical characterization of murine and human genomes, little molecular information is available on certain regions, e.g., proximal mouse chromosome 11 (Chr 11) and human chromosome 2p (Chr 2p). We have localized the wobbler spinal atrophy gene wr to proximal mouse Chr 11, tightly linked to Rab1, a gene coding for a small GTP-binding protein, and Glnsps1, an intronless pseudogene of the glutamine synthetase gene. We have now used these markers to construct a 1.3-Mb yeast artificial chromosome (YAC) contig of the Rab1 region on mouse Chr 11. Four YAC clones isolated from two independent YAC libraries were characterized by rare-cutting analysis, fluorescence in situ hybridization (FISH), and sequence-tagged site (STS) isolation and mapping. Rab1 and Glns-ps1 were found to be only 200 kb apart. A potential CpG island near a methylated NarI site and a trapped exon, ETG1.1, were found between these loci, and a new STS, AHY1.1, was found over 250 kb from Rab1. Two overlapping YACs were identified that contained a 150-kb region of human Chr 2p, comprising the RAB1 locus, AHY1.1, and the human homologue of ETG1.1, indicating a high degree of conservation of this region in the two species. We mapped AHY1.1 and thus human RAB1 on Chr 2p13.4-p14 using somatic cell hybrids and a radiation hybrid panel, thus extending a known region of conserved synteny between mouse Chr 11 and human Chr 2p. Recently, the gene LMGMD2B for a human recessive neuromuscular disease, limb girdle muscular dystrophy type 2B, has been mapped to 2p13-p16. The conservation between the mouse Rab1 and human RAB1 regions will be helpful in identifying candidate genes for the wobbler spinal muscular atrophy and in clarifying a possible relationship between wr and LMGMD2B.

Overshooting production of satellite cells in murine skeletal muscle affected by the mutation "muscular dystrophy with myositis" (mdm, Chr 2)

The hereditary degenerative muscle disease "muscular dystrophy with myositis" of the mouse (MDM, genotype mdm/mdm) is caused by a spontaneous recessive mutation on chromosome 2. Skeletal muscles of MDM mice degenerate during the first few postnatal weeks and subsequently regenerate. The cellular and molecular mechanisms of this muscle disease are not yet understood. An inflammatory component as implied by its name seems unlikely. By applying quantitative electron microscopy, we demonstrate a transient increase of up to 4-fold in the satellite cell frequency in both fast and slow muscles of affected animals. This difference from wildtype controls (+/+ or +/mdm) is also reflected in the yield of mononucleate myogenic cells upon dissociation of skeletal muscle and subsequent cell culture. Unlike the increase in satellite cell density in the myotonic ADR mouse, this phenomenon is not accompanied with a shift to more oxidative fibre types. It may however be related to the more profuse micro-vascularization of MDM muscle.

Annexin VII relocalization as a result of dystrophin deficiency

Annexin VII (synexin) is a member of the annexin family of proteins, which are characterized by Ca(2+)-dependent binding to phospholipids. In normal skeletal muscle annexin VII is located preferentially at the plasma membrane and the t-tubule system [Selbert et al. (1995) J. Cell. Sci. 108, 85-95]. Here we have analyzed the distribution of annexin VII in muscle disorders in which the Ca2+ regulation is affected. A remarkable difference was observed in muscle specimens from patients suffering from Duchenne muscular dystrophy and also in muscle from the MDX mouse where annexin VII was gradually released from the sarcolemmal membrane into the cytosol and into the extracellular space during progression of the disease. Hypercontracted muscle fibers positive in Ca2+ staining were devoid of cytosolic annexin VII. Annexins IV and VI were similarly released into the extracellular space. Whereas normal skeletal muscle showed specifically the 51-kDa annexin VII isoform, in dystrophic muscle different ratios of the 51-kDa and the muscle-atypic 47-kDa isoforms were observed. The potential of annexin VII to serve as a tool with which cellular Ca2+ levels can be studied and different muscular disorders classified is discussed.

Chloride channel 2 gene (Clc2) maps to chromosome 16 of the mouse, extending a region of conserved synteny with human chromosome 3q

The Clc2 gene of the mouse codes for the ubiquitously expressed chloride channel ClC-2, a member of a family of at least seven voltage gated chloride channels, some of which are implicated in hereditary diseases. Using a mouse interspecies back-cross panel, we have mapped Clc2 to Chr 16, proximal to the somatostatin gene Smst, extending a region of documented conserved synteny to human Chr 3q.

Sarcolemmal chloride and potassium channels from normal and myotonic mouse muscle studied in lipid supplemented vesicles

By patch-clamp analysis of lipid supplemented vesicles prepared from the sarcolemma of mouse wildtype skeletal muscle, we could identify two known types of potassium channels, the inward rectifier and Ca2+ activated BK channels, and five types of chloride channels designated CIC-a, CIC-b, CIC-c, CIC-d, and CIC-e. CIC-b corresponds to a known chloride channel, whereas CICa, -c, -d and -e have not been described previously. The diversity may be due to a heterooligomeric composition of different subunits. None of these chloride channels nor the potassium channels were found in the sarcoplasmic reticulum fraction. In vesicles from myotonic mice, Clc-d and -e were not found.

Extent of shock-induced membrane leakage in human and mouse myotubes depends on dystrophin

A lack of the cytoskeletal protein dystrophin causes muscle fiber necrosis in Duchenne/Becker muscular dystrophies (DMD/BMD) and in murine X-linked muscular dystrophy (MDX). However, no overt disease symptoms are observed in dystrophin-less cultured myotubes, and the biological function of dystrophin in normal muscle cells is still unknown. In this work, we have extended our studies on a model system, using hypoosmotic shock to determine stress resistance of muscle cells. In frozen sections of control human and mouse myotubes, dystrophin was shown to be localized at the cell periphery as in mature muscle fibers. Dystrophin-less DMD and MDX myotubes were more susceptible to hypoosmotic shock than controls, as monitored by the uptake of external horseradish peroxidase and release of the soluble enzymes creatinine kinase or pyruvate kinase and of radiolabelled proteins. Control experiments indicated that this difference is not due to differences in metabolism or ion fluxes. Treatment with cytochalasin D drastically increased the shock sensitivity of myotubes and abolished the difference between dystrophin-less and control cells. These results lend further support to the suggested stabilizing role of dystrophin in the context of the membrane-cytoskeletal complex.

Role of innervation, excitability, and myogenic factors in the expression of the muscular chloride channel ClC-1. A study on normal and myotonic muscle

The muscular chloride channel (ClC-1) is essential for a normal excitability of mature mammalian muscle fibers; inactivation of the corresponding gene by mutations leads to hyperexcitability of muscle, the hallmark of the disease myotonia. In the mouse, there is very little ClC-1 mRNA in myotubes, and its concentration increases steeply during postnatal development, suggesting a role of the motor nerve in ClC-1 expression. We investigated the response of the expression of the corresponding gene Clc-1 to different patterns of muscle activity as controlled by sarcolemmal excitability and by innervation. In rat and mouse, the level of ClC-1 mRNA was higher in fast (extensor digitorum longus) than in slow (soleus) muscle. Myotonia in the ADR mouse is caused by an insertional mutation leading to the adr allele of the Clc-1 gene and to grossly abnormal ClC-1 mRNAs. Nevertheless, in +/adr heterozygous, phenotypically wild type (WT) animals, the expression levels of both alleles correspond to the gene dosage. However, in the myotonic ADR mouse in which both Clc-1 genes are defective, ClC-1 mRNA levels in slow muscle were nearly as high as in WT fast muscle. In WT muscle, denervation within 2 days caused a drastic reduction of the ClC-1 mRNA level and at the same time an increase of myogenin and MyoD mRNAs. Neither effect of denervation was observed in myotonic mice (homozygous for the alleles adr or adrK), suggesting that spontaneous electrical activity of the hyperexcitable sarcolemma may substitute for nerve activity. Furthermore, potential MyoD/myogenin-binding sequence motifs were identified in the 5' regulatory region of the Clc-1 gene. These findings suggest that the activity-dependent regulation of the muscular chloride channel 1 gene is mediated by myogenic factors.

Subtractive cDNA cloning as a tool to analyse secondary effects of a muscle disease. Characterization of affected genes in the myotonic ADR mouse

In myotonic ADR mice that are homozygous for a defect in the muscular chloride channel gene adr/Clc-1, the hyperexcitability of fast muscles is associated with secondary changes in gene expression and fibre type composition. cDNA clones derived from a set of genes down regulated in fast muscles of the myotonic ADR mouse were isolated by a subtractive cloning procedure. A total of 1200 clones were analysed for high expression in fast muscle of wild type and low expression in mutant mouse. Differential transcript levels were verified by northern blot hybridizations. The identities of the corresponding transcripts were determined by sequencing as myosin heavy chain IIB, alpha-tropomyosin, troponin C, a Ca2+ ATPase and parvalbumin mRNAs. Of these, mRNAs for parvalbumin and myosin heavy chain IIB were drastically downregulated in myotonic muscle (to < 10% of control). A full length cDNA clone for skeletal muscle alpha-tropomyosin was homologous to the mouse fibroblast tropomyosin isoform 2, except for the portion encoding the alpha-tropomyosin specific amino acids 258-284. A cDNA derived from the 1100 nucleotide parvalbumin transcript was cloned and the sequence for the as yet unknown 3' extended trailer, generated by alternative polyadenylation, was determined.