PDGFRβ promotes oncogenic progression via STAT3/STAT5 hyperactivation in anaplastic large cell lymphoma

BACKGROUND

Anaplastic large cell lymphoma (ALCL) is an aggressive non-Hodgkin T cell lymphoma commonly driven by NPM-ALK. AP-1 transcription factors, cJUN and JUNb, act as downstream effectors of NPM-ALK and transcriptionally regulate PDGFRβ. Blocking PDGFRβ kinase activity with imatinib effectively reduces tumor burden and prolongs survival, although the downstream molecular mechanisms remain elusive.

METHODS AND RESULTS

In a transgenic mouse model that mimics PDGFRβ-driven human ALCL in vivo, we identify PDGFRβ as a driver of aggressive tumor growth. Mechanistically, PDGFRβ induces the pro-survival factor Bcl-xL and the growth-enhancing cytokine IL-10 via STAT5 activation. CRISPR/Cas9 deletion of both STAT5 gene products, STAT5A and STAT5B, results in the significant impairment of cell viability compared to deletion of STAT5A, STAT5B or STAT3 alone. Moreover, combined blockade of STAT3/5 activity with a selective SH2 domain inhibitor, AC-4-130, effectively obstructs tumor development in vivo.

CONCLUSIONS

We therefore propose PDGFRβ as a novel biomarker and introduce PDGFRβ-STAT3/5 signaling as an important axis in aggressive ALCL. Furthermore, we suggest that inhibition of PDGFRβ or STAT3/5 improve existing therapies for both previously untreated and relapsed/refractory ALK+ ALCL patients.

A Three-Month Follow-Up Study Evaluating Changes in Clinical Profile and Attitudes Towards Involuntary Admission

Introduction

Involuntary admission and treatment is often a traumatic experience for patients and there is a wide variation in attitudes towards care even when patients are recovered.

Objectives/aims

The purpose of this large prospective study was to identify clinical predictors of attitudes towards care during involuntary admission.

Methods

Three hundred and ninety-one consecutively admitted involuntarily patients to three psychiatric inpatient units over a 30-month period were invited to participate in the study. Comprehensive assessments at admission and 3 months after discharge were attained including measures of symptoms, insight, functioning, attitudes towards involuntary admission and coercive experiences. Multiple linear regression modelling was used to determine the optimal explanatory variables for attitudes towards care.

Results

Two hundred and sixty-three individuals participated at baseline and 156 (59%) successfully completed follow-up assessments. Individuals improved significantly over time clinically and in their attitudes towards their care. At baseline greater insight (P < 0.001) and less symptoms (P = 0.02) were associated with more positive attitudes towards care as was older age (P = 0.001). At follow-up, greater insight (P < 0.001), less symptoms (P = 0.02) and being older (P = 0.04) were associated with more positive attitudes towards care. More positive attitudes towards care at follow-up were associated with greater improvements in insight over time (P < 0.001) and having a diagnosis of an affective psychosis (P = 0.0009).

Conclusions

The best predictors of positive attitudes towards care during and after involuntary admission are illness related factors, such as levels of insight and improvement in insight, rather than service or legislation related factors, such as the use of coercive measures, seclusion and restraint.

Disclosure of interest

The authors have not supplied their declaration of competing interest.

'Venus trapped, Mars transits': Cu and Fe redox chemistry, cellular topography and in situ ligand binding in terrestrial isopod hepatopancreas

Woodlice efficiently sequester copper (Cu) in ‘cuprosomes' within hepatopancreatic ‘S' cells. Binuclear ‘B’ cells in the hepatopancreas form iron (Fe) deposits; these cells apparently undergo an apocrine secretory diurnal cycle linked to nocturnal feeding. Synchrotron-based µ-focus X-ray spectroscopy undertaken on thin sections was used to characterize the ligands binding Cu and Fe in S and B cells of Oniscus asellus (Isopoda). Main findings were: (i) morphometry confirmed a diurnal B-cell apocrine cycle; (ii) X-ray fluorescence (XRF) mapping indicated that Cu was co-distributed with sulfur (mainly in S cells), and Fe was co-distributed with phosphate (mainly in B cells); (iii) XRF mapping revealed an intimate morphological relationship between the basal regions of adjacent S and B cells; (iv) molecular modelling and Fourier transform analyses indicated that Cu in the reduced Cu⁺ state is mainly coordinated to thiol-rich ligands (Cu–S bond length 2.3 Å) in both cell types, while Fe in the oxidized Fe³⁺ state is predominantly oxygen coordinated (estimated Fe–O bond length of approx. 2 Å), with an outer shell of Fe scatterers at approximately 3.05 Å; and (v) no significant differences occur in Cu or Fe speciation at key nodes in the apocrine cycle. Findings imply that S and B cells form integrated unit-pairs; a functional role for secretions from these cellular units in the digestion of recalcitrant dietary components is hypothesized.

Erratum to: 36th International Symposium on Intensive Care and Emergency Medicine: Brussels, Belgium. 15-18 March 2016

[This corrects the article DOI: 10.1186/s13054-016-1208-6.].

Development and validation of an equine nerve block simulator to supplement practical skills training in undergraduate veterinary students

Lameness is the most common presenting complaint in equine practice. Performing diagnostic nerve blocks is an integral part of any lameness work-up, and is therefore an essential skill for equine practitioners. However, the opportunities for veterinary students to practice this skill are limited. The aim of this study was to design and validate an equine nerve block simulator. It was hypothesised that the simulator would improve students' ability and enhance their confidence in performing nerve blocks. A simulator was built using an equine forelimb skeleton and building foam. Wire wool targets were placed under the foam in the positions corresponding to the anatomical location of the most palmar digital, abaxial and low four-point nerve blocks and attached to an electrical circuit. The circuit became complete when the operator placed a needle in the correct position and immediate audible feedback with a buzzer was provided. To validate the simulator, it was compared with two established teaching methods: cadaver training and theoretical training with a hand-out. Cadaver-trained students achieved the best results (73 per cent correct blocks), compared with simulator-trained students (71 per cent correct blocks), and a hand-out trained group (58 per cent correct blocks). Feedback obtained with a questionnaire showed that students enjoyed simulator training more, and that they felt more confident in performing diagnostic nerve blocks than the other two groups. The equine nerve block simulator provides a safe, cost-effective method to supplement the teaching of diagnostic analgesia to undergraduate veterinary students.

Gamma tropomyosin gene products are required for embryonic development

The actin filament system is essential for many cellular functions, including shape, motility, cytokinesis, intracellular trafficking, and tissue organization. Tropomyosins (Tms) are rod-like components of most actin filaments that differentially affect their stability and flexibility. The Tm gene family consists of four genes, alphaTm, betaTm, gammaTm (Tm5 NM, where "NM" indicates "nonmuscle"), and deltaTm (Tm4). Multiple isoforms of the Tm family are generated by alternative splicing of three of these genes, and their expression is highly regulated. Extensive spatial and temporal sorting of Tm isoforms into different cellular compartments has been shown to occur in several cell types. We have addressed the function of the low-molecular-weight Tms encoded by the gammaTm gene by eliminating the corresponding amino-terminal coding sequences from this gene. Heterozygous mice were generated, and subsequent intercrossing of the F1 pups did not result in any viable homozygous knockouts. Genotype analysis of day 2.5 morulae also failed to detect any homozygous knockouts. We have failed in our attempts to delete the second allele and generate in vitro double-knockout cells, although 51 clones displayed homologous recombination back into the originally targeted locus. We therefore conclude that low-molecular-weight products from the gammaTm gene are essential for both embryonic development and cell survival.

Human cytoplasmic actin proteins are encoded by a multigene family

We characterized nine human actin genes that we isolated (Engel et al., Proc. Natl. Acad. Sci. U.S.A. 78:4674-4678, 1981) from a library of cloned human DNA. Measurements of the thermal stability of hybrids formed between each cloned actin gene and alpha-, beta-, and gamma-actin mRNA demonstrated that only one of the clones is most homologous to sarcomeric actin mRNA, whereas the remaining eight clones are most homologous to cytoplasmic actin mRNA. By the following criteria we show that these nine clones represent nine different actin gene loci rather than different alleles or different parts of a single gene: (i) the restriction enzyme maps of the coding regions are dissimilar; (ii) each clone contains sufficient coding region to encode all or most of an entire actin gene; and (iii) each clone contains sequences homologous to both the 5' and 3' ends of the coding region of a cloned chicken beta-actin cDNA. We conclude, therefore, that the human cytoplasmic actin proteins are encoded by a multigene family.

De novo missense mutation in a constitutively expressed exon of the slow alpha-tropomyosin gene TPM3 associated with an atypical, sporadic case of nemaline myopathy

We describe an atypical case of nemaline myopathy with an unusual distribution of muscle weakness who presented at 14 years of age with kyphoscoliosis. In this patient, we demonstrate heterozygosity for a de novo CGT-CAT (Arg167His) mutation in a constitutively expressed exon (exon 5) of slow alpha-tropomyosin (TPM3). This is the first mutation identified in a constitutively expressed exon of TPM3 in a nemaline myopathy patient, but is similar to recently described mutations in beta-tropomyosin (TPM2) associated with nemaline myopathy and mutations in fast alpha-tropomyosin (TPM1) which cause hypertrophic cardiomyopathy.

Sorting of tropomyosin isoforms in synchronised NIH 3T3 fibroblasts: evidence for distinct microfilament populations

The nonmuscle actin cytoskeleton consists of multiple networks of actin microfilaments. Many of these filament systems are bound by the actin-binding protein tropomyosin (Tm). We investigated whether Tm isoforms could be cell cycle regulated during G0 and G1 phases of the cell cycle in synchronised NIH 3T3 fibroblasts. Using Tm isoform-specific antibodies, we investigated protein expression levels of specific Tms in G0 and G1 phases and whether co-expressed isoforms could be sorted into different compartments. Protein levels of Tms 1, 2, 5a, 6, from the alpha Tm(fast) and beta-Tm genes increased approximately 2-fold during mid-late G1. Tm 3 levels did not change appreciably during G1 progression. In contrast, Tm 5NM gene isoform levels (Tm 5NM-1-11) increased 2-fold at 5 h into G1 and this increase was maintained for the following 3 h. However, Tm 5NM-1 and -2 levels decreased by a factor of three during this time. Comparison of the staining of the antibodies CG3 (detects all Tm 5NM gene products), WS5/9d (detects only two Tms from the Tm 5NM gene, Tm 5NM-1 and -2) and alpha(f)9d (detects specific Tms from the alpha Tm(fast) and beta-Tm genes) antibodies revealed 3 spatially distinct microfilament systems. Tm isoforms detected by alpha(f)9d were dramatically sorted from isoforms from the Tm 5NM gene detected by CG3. Tm 5NM-1 and Tm 5NM-2 were not incorporated into stress fibres, unlike other Tm 5NM isoforms, and marked a discrete, punctate, and highly polarised compartment in NIH 3T3 fibroblasts. All microfilament systems, excluding that detected by the WS5/9d antibody, were observed to coalign into parallel stress fibres at 8 h into G1. However, Tms detected by the CG3 and alpha(f)9d antibodies were incorporated into filaments at different times indicating distinct temporal control mechanisms. Microfilaments in NIH 3T3 cells containing Tm 5NM isoforms were more resistant to cytochalasin D-mediated actin depolymerisation than filaments containing isoforms from the alpha Tm(fast) and beta-Tm genes. This suggests that Tm 5NM isoforms may be in different microfilaments to alpha Tm(fast) and beta-Tm isoforms even when present in the same stress fibre. Staining of primary mouse fibroblasts showed identical Tm sorting patterns to those seen in cultured NIH 3T3 cells. Furthermore, we demonstrate that sorting of Tms is not restricted to cultured cells and can be observed in human columnar epithelial cells in vivo. We conclude that the expression and localisation of Tm isoforms are differentially regulated in G0 and G1 phase of the cell cycle. Tms mark multiple microfilament compartments with restricted tropomyosin composition. The creation of distinct microfilament compartments by differential sorting of Tm isoforms is observable in primary fibroblasts, cultured 3T3 cells and epithelial cells in vivo.

Identification and in situ hybridization mapping of a mouse Tpd52l1 (D53) orthologue to chromosome 10A4-B2

We report the identification of a mouse cDNA Tpd52l1 (tumor protein D52-like 1), which represents the first demonstrated orthologue of the human TPD52L1 (alias D53) gene, a member of the breast carcinoma-associated TPD52 (alias D52) gene family. In situ hybridization mapping located the Tpd52l1 gene to chromosome 10A4-10B2. Since the TPD52L1 gene is found at human chromosome 6q22-->q23, the mouse and human TPD52L1 loci are syntenically conserved.

Shiga toxin-producing Escherichia coli can impair T84 cell structure and function without inducing attaching/effacing lesions

Enteropathogenic Escherichia coli (EPEC) intimately adhere to epithelial cells producing cytoskeletal rearrangement with typical attaching and effacing lesions and altered epithelial barrier and transport function. Since EPEC and Shiga toxin-producing E. coli (STEC) share similar genes in the "locus for enterocyte effacement" (LEE) thought to cause these changes, it has been assumed that STEC shares similar pathogenic mechanisms with EPEC. The aims of this study were to compare the effects of EPEC and STEC on bacterial-epithelial interactions and to examine changes in epithelial function. T84 monolayers were infected with STEC O157:H7 (wild strain EDL 933 or non-toxin-producing strain 85/170), EPEC (strain E2348/69), or HB101 (nonpathogenic) and studied at various times after infection. EPEC bound more avidly than EDL 933, but both strains exhibited greater binding than HB101. Attaching and effacing lesions and severe disruption to the actin cytoskeleton were observed in EPEC by 3 h postinfection but not in EDL 933 or HB101 at any time point. EPEC and EDL 933 increased monolayer permeability to [(3)H]mannitol 5- to 10-fold. In contrast to EPEC, EDL 933 completely abolished secretagogue-stimulated anion secretion as assessed under voltage clamp conditions in Ussing chambers. Several other STEC strains induced changes similar to those of EDL 933. In conclusion, STEC impairs epithelial barrier function and ion transport without causing major disruption to the actin cytoskeleton. Pathogenic factors other than products of LEE may be operant in STEC.

Specific isoforms of actin-binding proteins on distinct populations of Golgi-derived vesicles

Golgi membranes and Golgi-derived vesicles are associated with multiple cytoskeletal proteins and motors, the diversity and distribution of which have not yet been defined. Carrier vesicles were separated from Golgi membranes, using an in vitro budding assay, and different populations of vesicles were separated using sucrose density gradients. Three main populations of vesicles labeled with beta-COP, gamma-adaptin, or p200/myosin II were separated and analyzed for the presence of actin/actin-binding proteins. beta-Actin was bound to Golgi cisternae and to all populations of newly budded vesicles. Centractin was selectively associated with vesicles co-distributing with beta-COP-vesicles, while p200/myosin II (non-muscle myosin IIA) and non-muscle myosin IIB were found on different vesicle populations. Isoforms of the Tm5 tropomyosins were found on selected Golgi-derived vesicles, while other Tm isoforms did not colocalize with Tm5 indicating the association of specialized actin filaments with Golgi-derived vesicles. Golgi-derived vesicles were shown to bind to F-actin polymerized from cytosol with Jasplakinolide. Thus, newly budded, coated vesicles derived from Golgi membranes can bind to actin and are customized for differential interactions with microfilaments by the presence of selective arrays of actin-binding proteins.

Isoform sorting and the creation of intracellular compartments

The generation of isoforms via gene duplication and alternative splicing has been a valuable evolutionary tool for the creation of biological diversity. In addition to the formation of molecules with related but different functional characteristics, it is now apparent that isoforms can be segregated into different intracellular sites within the same cell. Sorting has been observed in a wide range of genes, including those encoding structural molecules, receptors, channels, enzymes, and signaling molecules. This results in the creation of intracellular compartments that (a) can be independently controlled and (b) have different functional properties. The sorting mechanisms are likely to operate at the level of both proteins and mRNAs. Isoform sorting may be an important consequence of the evolution of isoforms and is likely to have contributed to the diversity of functional properties within groups of isoforms.

Cloning of a third member of the D52 gene family indicates alternative coding sequence usage in D52-like transcripts

D52 proteins are emerging as signalling molecules which may be regulators of cell proliferation. Having previously reported the existence of the human D52 gene family, comprising the hD52 and hD53 genes expressed in human breast carcinoma, we report the identification of a novel human gene hD54 (TPD52L2), which represents a third D52 gene family member. In situ mapping placed the hD54 gene on human chromosome 20q13.2-q13.3, a localization distinct from those of both hD52 and hD53 genes. The identified hD54 cDNAs predicted three hD54 isoforms, suggesting that alternatively-spliced transcripts may be produced from D52-like genes. This was confirmed by directly sequencing reverse transcriptase-polymerase chain reaction (RT-PCR) products amplified from D52-like gene transcripts expressed in developing and adult rat tissues, and by performing sequence analyses of the expressed sequence tag divisions of nucleotide databases. Alternative splicing of sequences encoding two regions, termed ins2 and ins3, was identified in one or more D52-like genes, with these alternative splicing events being differentially regulated. The functional consequences of alternative splicing were examined by characterizing the protein-protein interactions mediated by a truncated hD53 isoform within the yeast two-hybrid system. This hD53 isoform displayed altered interaction capabilities with respect to those of full-length hD53, suggesting that alternative splicing within the D52 gene family functions in part to alter the protein-protein interaction capabilities of encoded isoforms.

Creating intracellular structural domains: spatial segregation of actin and tropomyosin isoforms in neurons

Actin microfilaments play a direct role in a variety of cell processes. Distinct populations of microfilaments are associated with different cellular compartments, such as growth cones, filipodia, stress fibers, and lamellipodia. It is becoming clear that these different populations are often composed of different isoforms of the two core microfilament components, actin and tropomyosin. This is particularly true in neurons, where actin and tropomyosin isoforms are segregated into different intracellular compartments which correspond to functionally distinct regions of the neuron. Developmental regulation of this isoform sorting suggests a specific role for some isoforms in growth and for others in stabilization of neuronal structure. This provides a mechanism by which a neuron can create and independently regulate intracellular domains composed of microfilaments with different functional properties.

Tropomyosin isoform diversity and neuronal morphogenesis

Tropomyosins (Tm) are a large family of isoforms obtained from multiple genes and by extensive alternative splicing. They bind in the alpha-helical groove of the actin filament and are therefore core components of this extensive cytoskeletal system. In non-muscle cells the Tm isoforms have been implicated in a diversity of processes including cytokinesis, vesicle transport, motility, morphogenesis and cell transformation. Using immunohistochemical localization in cultured primary cortical neurons with an antibody that potentially identifies all non-muscle TM5 gene isoforms compared with one that specifically identifies a subset of isoforms, the possibility was raised that there were considerably more isoforms derived from this gene than the four previously described. Using polymerase chain reaction (PCR) analysis we have now shown that the rat brain generates at least 10 mRNA isoforms using multiple combinations of terminal exons and two internal exons. There is extensive developmental regulation of these isoforms in the brain and there appears to be a switch in the preferential use of the two internal exons 6a to 6b from the embryonic to the adult isoforms. Specific isoforms using alternate carboxyl-terminal exons are differentially localized within the adult rat cerebellum. It is suggested that the tightly regulated spatial and temporal expression of Tm isoforms plays an important role in the development and maintenance of specific neuronal compartments. This may be achieved by isoforms providing unique structural properties to actin-based filaments within functionally distinct neuronal domains.

Structural compartments within neurons: developmentally regulated organization of microfilament isoform mRNA and protein

The microfilament system is thought to be a crucial cytoskeletal component regulating development and mature function of neurons. The intracellular distribution of the microfilament isoform components, actin and tropomyosin (Tm), in neurons primarily in vivo, has been investigated at both the mRNA and the protein level using isoform specific riboprobes and antibodies. Our in vivo and in vitro studies have identified at least six neuronal compartments based on microfilament isoform mRNA localization: the developing soma, the mature soma, growth cone, developing axon hillock/proximal axon, mature somatodendritic and mature axonal pole soma. Protein localization patterns revealed that the isoforms were frequently distributed over a wider area than their respective mRNAs, suggesting that isoform specific patterns of mRNA targeting may influence, but do not absolutely determine, microfilament isoform location. Tm4 and Tm5 showed identical mRNA targeting in the developing neuron but distinct protein localization patterns. We suggest that in this instance mRNA location may best be viewed as a regulated site of synthesis and assembly, rather than a regulator of protein localization per se. In addition, Tm5 and beta-actin mRNA and protein locations were developmentally regulated, suggesting the possibility that environmental signals modulate targeting of specific mRNAs and their proteins. Thus, developmentally regulated mRNA localization and positional translation may act in concert with protein transport to regulate neuronal microfilament composition and consequently neuronal structure.

Splicing of two internal and four carboxyl-terminal alternative exons in nonmuscle tropomyosin 5 pre-mRNA is independently regulated during development

Four nonmuscle tropomyosin isoforms have been reported to be produced from the rat Tm5 gene by alternative splicing (Beisel, K. W., and Kennedy, J. E. (1994) Gene (Amst.) 145, 251-256). In order to detect additional isoforms that might be expressed from that gene, we used reverse transcriptase-polymerase chain reaction assays and evaluated the presence of all product combinations of two alternative internal exons (6a and 6b) and four carboxyl-terminal exons (9a, 9b, 9c, and 9d) in developing and adult rat brain. We identified five different combinations for exon 9 (9a + 9b, 9a + 9c, 9a + 9d, 9c, and 9d), and the exon combinations 9a + 9c and 9a + 9d were previously unreported. Each of these combinations existed with both exon 6a and exon 6b. Thus, the rat brain generates at least 10 different isoforms from the Tm5 gene. Northern blot hybridization with alternative exon-specific probes revealed that these isoforms were also expressed in a number of different adult rat tissues, although some exons are preferentially expressed in particular tissues. Studies of regulation of the 10 different Tm5 isoform mRNAs during rat brain development indicated that no two isoforms are coordinately accumulated. Furthermore, there is a developmental switch in the use of exon 6a to exon 6b from embryonic to adult isoforms. TM5 protein isoforms show a differential localization in the adult cerebellum.

Identification of homo- and heteromeric interactions between members of the breast carcinoma-associated D52 protein family using the yeast two-hybrid system

The hD52 gene was originally identified through its elevated expression level in human breast carcinoma. Cloning of D52 homologues from other species has indicated that D52 may play roles in calcium-mediated signal transduction and cell proliferation. Two human homologues of hD52, hD53 and hD54, have also been identified, demonstrating the existence of a novel gene/protein family. Since D52-like protein sequences are all predicted to contain a coiled-coil domain, we used the yeast two-hybrid system and glutathione S-transferase pull-down assays to investigate whether homo- and/or heteromeric interactions occur between D52-like proteins. Analyses of yeast strains co-transfected with paired D52-like constructs indicated that D52-like fusion proteins interact in homo- and heteromeric fashions through their predicted coiled-coil domains. Similarly, extensive two-hybrid screenings of a human breast carcinoma expression library identified hD53 and hD52 as potential interactors for both hD52 and hD53 baits. Thus, D52-like proteins appear to exert and/or regulate their activities through specific interactions with other D52-like proteins, which in turn may be intrinsic to potential roles of these molecules in controlling cell proliferation.

The 3'-end of the human beta-actin gene enhances activity of the beta-actin expression vector system: construction of improved vectors

The human beta-actin promoter has been widely used to drive expression of genes of interest in mammalian cell lines and transgenic mice. The original form of the human beta-actin expression vector contains upstream sequences, 5'UTR (untranslated region) and intron 1 from the beta-actin gene linked to a three restriction site polylinker and SV40 (Simian Virus 40) 3'UTR. We have modified this vector now to contain the highly conserved beta-actin 3'UTR plus flanking region which replaces the SV40 sequences. An additional modification has removed the mRNA peripheral localization sequences present in the beta-actin 3'UTR. The new vectors also contain an improved polylinker. The activity of these two new vectors has been compared with that of the original vector and that of a vector using the popular cytomegalovirus (CMV) promoter. Mouse C2 myoblasts were transfected with each vector driving expression of enhanced green fluorescent protein (EGFP) and analyzed for EGFP mRNA levels. We find that both new vectors drive twice the level of mRNA accumulation of the original vector and over 30-times that of the CMV promoter. This suggests that these new vectors will provide a substantial elevation in levels of expression by virtue of inclusion of the beta-actin 3'UTR plus flanking region.

Actin and tropomyosin isoforms in morphogenesis

The major components of the actin microfilament system, actin and tropomyosin (Tm), are encoded by multigene families. There are at least 6 actin and over 20 Tm isoforms in mammals. The observation that isoforms are expressed in a tissue-specific manner has encouraged the hypothesis that they contribute to the formation of cell type-specific structures. Recent studies have indicated that certain specific isoforms do play unique structural roles. One nonmuscle actin isoform, beta, is implicated in the regulation of cell spreading and membrane organisation. The intracellular location of beta-actin mRNA has been shown to be regulated by growth factor stimulation of signal transduction pathways. Actin isoforms have also been shown to differ in their contractile properties in both muscle and non-muscle cells. Tropomyosins have been found to show isoform specific regulation in response to cell transformation. This has correlated with the view that some isoforms of tropomyosin promote filament stability whereas others are associated with more dynamic structures. Neuronal development and maturation are accompanied by dynamic spatial sorting of tropomyosin isoforms into different cellular compartments. It is now apparent that isoforms of these proteins perform different structural tasks. The challenge is now to link the significance of spatial sorting to the different physicochemical properties of these isoforms.

Differential regulation of actin depolymerizing factor and cofilin in response to alterations in the actin monomer pool

Myoblasts, transfected with a human gene encoding a beta-actin point mutation, down-regulate expression of actin depolymerizing factor (ADF) and its mRNA. Regulation is posttranscriptional. Expression of cofilin, a structurally similar protein, and profilin, CapG, and tropomodulin is not altered with increasing mutant beta-actin expression. Myoblasts expressing either human gamma-actin or the mutant beta-actin down-regulate the endogenous mouse actin genes to keep a constant level of actin mRNA, whereas the gamma-actin transfectants do not down-regulate ADF. Thus, ADF expression is regulated differently from actin expression. The mutant beta-actin binds to ADF with about the same affinity as normal actin; however, it does not assemble into normal actin filaments. The decrease in ADF expression correlates with an increase in the unassembled actin pool. When the actin monomer pool in untransfected myoblasts is increased 70% by treatment with latrunculin A, synthesis of ADF and actin are down-regulated compared with cofilin and 19 other proteins selected at random. Increasing the actin monomer pool also results in nearly complete phosphorylation of both ADF and cofilin. Thus, ADF and cofilin are coordinately regulated by posttranslational modification, but their expression is differentially regulated. Furthermore, expression of ADF is responsive to the utilization of actin by the cell.

Variations in the relative mRNA levels of actins and myosin heavy chains do not produce corresponding differences in their proteins in the adult human heart

This paper examines the quantitative relationship between the expression of myosin heavy chain (MHC) and actin at both the levels of their mRNAs and their proteins. Explanted human left ventricle tissues were obtained from non-diseased (ND) individuals and from dilated cardiomyopathy (DCM) patients with terminally failing hearts who underwent heart transplantation. We found: (1) there are substantial differences in the stoichiometry of sarcomeric MHC and actin transcripts in hearts of DCM patients as well as in ND individuals; (2) there are substantial differences between levels of total sarcomeric actin transcripts from different individual patients; (3) by and large variations in transcript levels between samples from the same heart are much less than between samples from different hearts; and (4) the ratio of MHC to sarcomeric actin proteins expressed by different ND and DCM hearts remains essentially constant. We conclude that the human ventricle can accommodate a substantial imbalance between sarcomeric MHC and actin mRNA levels while maintaining a constant ratio of their corresponding proteins.

Impact of alpha-skeletal actin but not alpha-cardiac actin on myoblast morphology

Muscle differentiation involves a profound change in cell cytoarchitecture. This is accompanied by extensive isoform replacement in which the major non-muscle isoforms of the actin filament system are replaced by their muscle isoform counterparts. We have tested whether the sequential expression of the actin isoforms is functionally significant by precociously expressing the two striated muscle actins (alpha-skeletal and alpha-cardiac) in mouse myoblasts. The human alpha-skeletal and alpha-cardiac actin genes were transfected into mouse C2 myoblasts and clones expressing the human genes at the highest level were identified. Expression of the human alpha-skeletal actin gene was low with the highest mRNA level found to be 4% of that in adult human skeletal muscle. Clones expressing alpha-cardiac actin accumulated the mRNA up to 13% of the level of alpha-skeletal actin in adult human skeletal muscle. Despite the low level of alpha-skeletal actin expression, myoblasts transfected with this gene displayed a profound decrease in cell spreading. In contrast, alpha-cardiac actin had no impact on cell spreading. Neither alpha-skeletal actin nor alpha-cardiac actin had any impact on the total actin protein pool nor on the levels of the high molecular weight tropomyosins. The organisation of actin and tropomyosin into stress fibres was similar between transfected and control cells. We conclude that precocious expression of alpha-skeletal actin, but not alpha-cardiac actin, compromises myoblast morphology but not the ability of the cell to assemble stress-fibre-like structures.

Tropomyosin localization reveals distinct populations of microfilaments in neurites and growth cones

The functional and structural differences between neurites and growth cones suggests the possibility that distinct microfilament populations may exist in each domain. Tropomyosins are integral components of the actin-based microfilament system. Using antibodies which detect three different sets of tropomyosin isoforms, we found that the vast majority of tropomyosin was found in a microfilament-enriched fraction of cultured cortical neurons, therefore enabling us to use the antisera to evaluate compositional differences in neuritic and growth cone microfilaments. An antibody which reacts with all known nonmuscle isoforms of the alpha Tms gene (Tm5NM1-4) stains both neurites and growth cones, whereas a second antibody against the isoform subset, Tm5NM1-2, reacts only with the neurite. A third antibody which reacts with the Tm5a/5b isoforms encoded by a separate gene from alpha Tms was strongly reactive with both neurites and growth cones in 16-h cultures but only with the neurite shaft in 40-h cultures. Treatment of neurons with cytochalasin B allowed neuritic Tm5NM1-2 to spread into growth cones. Removal of the drug resulted in the disappearance of Tm5NM1-2 from the growth cone, indicating that isoform segregation is an active process dependent on intact microfilaments. Treatment of 40-h cultures with nocodazole resulted in the removal of Tm5NM1-2 from the neurite whereas Tm5a/5b now spread back into the growth cone. We conclude that the organization of Tm5NM1-2 and Tm5a/5b in the neurite is at least partially dependent on microtubule integrity. These results indicate that tropomyosin isoforms Tm5NM1-2, Tm5NM3-4, and Tm5a/5b mark three distinct populations of actin filaments in neurites and growth cones. Further, the composition of microfilaments differs between neurites and growth cones and is subject to temporal regulation.

The molecular composition of neuronal microfilaments is spatially and temporally regulated

The actin-based microfilament system is thought to play a critical role in neuronal development. We have determined specific changes in the composition of microfilaments accompanying neuronal morphogenesis. By using specific antibodies against the isoforms for tropomyosin (Tm) (Tm-5 and TmBr-1/-3) and actin (beta- and gamma-actin), we found that during early morphogenesis in vivo immature growing axons contain beta- and gamma-actin and Tm-5. In particular, Tm-5 is exclusively located in the immature axonal processes relative to the neuronal cell body. In contrast, beta-actin and Tm-5 are absent in mature, quiescent axons. This developmental loss from axons is associated with an approximately twofold downregulation of beta-actin and Tm-5 levels in the brain; gamma-actin levels do not change, and this molecule is widely distributed throughout neurons during development. The loss of beta-actin and Tm-5 from axons is accompanied by a progressive appearance of TmBr-1/-3. This apparent replacement of Tm-5 with TmBr-1/-3 occurs over a 2 d time period during rat embryonic hindbrain development and is conserved in evolution between birds and mammals. The loss of Tm-5 from axons involves a redistribution of this molecule to the cell soma and dendrites. These findings suggest that specialized microfilament domains are associated with the development and maintenance of neuronal polarity. We conclude that these Tm isoforms and beta-actin are subject to specific patterns of segregation associated with axonal development and neuronal differentiation. This provides a potential molecular basis for the temporal and spatial specificity of microfilament function during neuronal differentiation.

Intracellular localization of tropomyosin mRNA and protein is associated with development of neuronal polarity

Neuronal differentiation involves extensive rearrangement of the cytoskeleton, including the actin-based microfilament system, and establishment of molecular compartments within the neuron. The intracellular distribution of tropomyosin (Tm) mRNA in vivo and in vitro has been examined and correlated with protein targetting. The mRNAs encoding two Tm isoforms were found to be differentially localized in developing neurons. Tm-5 mRNA is localized to the axonal pole of differentiating embryonic rat neurons, in contrast to TmBr-2 mRNA distribution throughout the cell body. Tm-5 mRNA is transported into the axon of differentiating primary cultured neurons. This mRNA localization is developmentally regulated and correlates with the targeting of Tm-5 protein to growing axons. Tm-5 colocalizes with a subset of neuronal microfilaments associated with the initiation and maintenance of outgrowth. The segregation of Tm-5 is the earliest known marker of neuronal polarity and may play a role in the establishment of polarity.

Impact of altered actin gene expression on vinculin, talin, cell spreading, and motility

Previous studies have demonstrated a strong correlation between the expression of vinculin and the shape and motility of a cell (Rodriguez Fernandez et al., 1992a, b, 1993). This hypothesis was tested by comparing the expression of vinculin and talin with the motility of morphologically altered myoblasts. These mouse C2 myoblasts were previously generated by directly perturbing the cell cytoskeleton via the stable transfection of a mutant-form of the beta-actin gene (beta sm) and three different forms of the gamma-actin gene; gamma, gamma minus 3'UTR (gamma delta'UTR), and gamma minus intron III (gamma delta IVSIII) (Schevzov et al., 1992; Lloyd and Gunning, 1993). In the case of the beta sm and gamma-actin transfectants, a two-fold decrease in the cell surface area was coupled, as predicted, with a decrease in vinculin and talin expression. In contrast, the gamma delta IVSIII transfectants with a seven-fold decrease in the cell surface area showed an unpredicted slight increase in vinculin and talin expression and the gamma delta 3'-UTR transfectants with a slight increase in the cell surface area showed no changes in talin expression and a decrease in vinculin expression. We conclude that changes in actin gene expression alone can impact on the expression of vinculin and talin. Furthermore, we observed that these actin transfectants failed to show a consistent relationship between cell shape, motility, and the expression of vinculin. However, a relationship between talin and cell motility was found to exist, suggesting a role for talin in the establishment of focal contacts necessary for motility.

Serum-induced signal transduction determines the peripheral location of beta-actin mRNA within the cell

Cell motility is dependent upon the reorganization of the cellular cytoskeleton. Actin filaments form the major component of the cytoskeleton and respond rapidly to serum growth factors. We have previously shown that myoblasts sort the two cytoskeletal beta- and gamma-actin isoform mRNAs to different intracellular regions and that only beta-actin mRNA was associated with peripheral regions of cell motility (Hill, M.A. and P. Gunning. 1993. J. Cell Biol. 122: 825-832). We now show by in situ hybridization that 3T3 fibroblasts similarly sort actin isoform mRNAs and that peripheral beta-actin mRNA is regulated by serum. In the absence of serum, we could not detect beta-actin mRNA at the periphery. Addition of serum rapidly redistributed beta-actin mRNA to the periphery. gamma-actin mRNA distribution was not altered by serum addition at any time. Both proteins, as identified by immunochemistry with isoform-specific antibodies, were found in similar cellular structures. Serum-stimulated cell motility is mediated through the GTPase signal transduction pathway. We find that an RNA-binding protein, p62, that is part of this pathway, displays a localization pattern similar to beta-actin mRNA. Our results suggest a new biological mechanism which integrates signal transduction with the supply of an architectural component required for membrane remodeling. We propose that active transport of beta-actin mRNA to regions of cell motility is one possible objective of these signal transduction pathways.

Nerve-dependent and -independent patterns of mRNA expression in regenerating skeletal muscle

Muscle regeneration was used to assess the roles of innervation and intrinsic genetic information in the acquisition of predominant slow and fast contractile protein mRNA profiles in adult skeletal muscle. Rat soleus (slow) and EDL (fast) muscles were allowed to regenerate in the presence and absence of their respective slow and fast nerves. Levels of mRNAs encoding fast, slow, and cardiac contractile protein isoforms were quantitatively determined at specific times during regeneration. All muscles initially expressed a heterogeneous pattern of fast, slow, and cardiac isoform mRNAs during myotube formation. Subsequently, the EDL muscle made a transition to a predominantly fast profile of mRNAs which was independent of innervation. The mRNA profile for the noninnervated regenerating soleus muscle was similar to both the innervated and the noninnervated EDL muscle profile. Thus, the decision to express predominantly fast isoform mRNAs is intrinsic to in vivo muscle regeneration with the fast nerve not appearing to be informative. In contrast, acquisition of a slow mRNA profile is dependent on the presence of a slow nerve. The mRNAs encoding slow isoforms from all of the contractile protein gene families are upregulated during the period of reestablishment of neuromuscular transmission. Additionally, there is no concomitant down-regulation of the fast isoform mRNAs upon reinnervation in the soleus regenerate. We propose that information both intrinsic to the muscle and supplied by the in vivo environment acts to provide potential isoform mRNA options to the regenerating muscle.

Beta and gamma actin mRNAs are differentially located within myoblasts

Actin is of fundamental importance to all eukaryotic cells. Of the six mammalian actins, beta (beta) and gamma (gamma) cytoplasmic are the isoforms found in all nonmuscle cells and differ by only four amino acids at the amino-terminal region. Both genes are regulated temporally and spatially, though no differences in protein function have been described. Using fluorescent double in situ hybridization we describe the simultaneous intracellular localization of both beta and gamma actin mRNA. This study shows that myoblasts differentially segregate the beta and gamma actin mRNAs. The distribution of gamma actin mRNA, only to perinuclear and nearby cytoplasm, suggests a distribution based on diffusion or restriction to nearby cytoplasm. The distribution of beta actin mRNA, perinuclear and at the cell periphery, implicates a peripheral localizing signal which is unique to the beta isoform. The peripheral beta actin mRNA corresponded to cellular morphologies, extending processes, and ruffling edges that reflect cell movement. Total actin and gamma actin protein steady-state distributions were identified by specific antibodies. gamma actin protein was found in both stress fibers and at the cell plasma membrane and does not correspond to its mRNA distribution. We suggest that localized protein synthesis rather than steady-state distribution functionally differentiates between the actin isoforms.

Differential regulation of tropomyosin isoform organization and gene expression in response to altered actin gene expression

Phenotypically altered C2 myoblast cells, generated by the stable transfection of human nonmuscle actin genes (Schevzov, G., C. Lloyd, and P. Gunning. 1992. J. Cell Biol. 117:775-786), exhibit a differential pattern of tropomyosin cellular organization and isoform gene expression. The beta-actin transfectants displaying a threefold increase in the cell surface area, showed no significant changes in the pattern of organization of the high M(r) tropomyosin isoform, Tm 2, or the low M(r) tropomyosin isoform, Tm 5. In contrast, the gamma- and beta sm-actin gene transfectants, exhibiting a twofold decrease in the cell surface area, had an altered organization of Tm 2 but not Tm 5. In these actin transfectants, Tm 2 did not preferentially segregate into stress fiber-like structures and the intensity of staining was greatly diminished. Conversely, a well-defined stress fiber-like organization of Tm 5 was observed. The pattern of organization of these tropomyosin isoforms correlated with their expression such that a profound decrease in Tm 2 expression was observed both at the transcript and protein levels, whereas Tm 5 remained relatively unchanged. These results suggest that relative changes in nonmuscle actin gene expression can affect the organization and expression of tropomyosin in an isoform specific manner. Furthermore, this apparent direct link observed between actin and tropomyosin expression suggests that nonpharmacological signals originating in the cytoskeleton can regulate cytoarchitectural gene expression.

Myosin light chain gene expression associated with disease states of the human heart

During development of the human heart, the atrial isoform of alkali myosin light chain (MLC1A) is expressed in the ventricle. With maturation of the heart, MLC1A expression is completely replaced by that of the adult ventricular myosin light chain, MLC1V. We have evaluated the re-expression of MLC1A as a marker of different disease states of the human ventricle. RNA was isolated from the ventricles of patients with idiopathic dilated cardiomyopathy (CM) and severe congenital cardiac defects (CCD). Northern blot analysis was used to measure the mRNA levels of MLC1A and MLC1V in these samples. As a control, the level of regulatory MLC2V mRNA was also measured. We find that the level of MLC2V mRNA per microgram total ventricular RNA is very similar in CM, CCD and normal human samples. In contrast, we find that MLC1V mRNA levels tend to be reduced in both CM and CCD samples. In the case of the CCD samples, this apparent drop in MLC1V is compensated by expression of the developmental MLC1A isoform. However, in CM patients in end-stage failure, expression of MLC1A is barely detectable. The expression of MLC1A in CCD samples may reflect an adaptive mechanism in response to cardiac overload. The failure to detect substantial MLC1A expression in the CM samples may reflect the failure of such an adaptive mechanism.

Noncoding regions of the gamma-actin gene influence the impact of the gene on myoblast morphology

We have addressed the question of whether two highly conserved noncoding regions of the gamma-actin gene are of functional importance. Human gamma-actin gene constructs deleted for either the entire 3' untranslated region (UTR) and 3' flank or intron III sequences were transfected into mouse myoblasts and the resulting clones were analyzed for cell morphology and actin protein expression. Transfectants carrying the gamma-actin gene deleted for the 3' end (gamma 22) exhibited numerous long pseudopods and increased surface area. In contrast, transfectants expressing the gamma-actin gene deleted for intron III (gamma 156) were rounded with blebs over the cell surface and showed decreased surface area. The relative expression of beta- to gamma-actin protein decreased for both transfectant types. The total actin protein levels remained constant for the gamma 22 cells but decreased for the gamma 156 cells. The results indicate that alterations to transfectant cell morphology can be influenced by the presence or absence of different noncoding regions in the transfected gamma-actin gene. The mechanisms by which noncoding regions of the gamma-actin gene influence the impact of the gene are unknown. Nevertheless, these noncoding regions are isoform specific and highly conserved in evolution. We propose that the functional significance of the different actin isoforms may involve the properties of these noncoding regions in addition to the differences in protein sequence.

Induction of neuron-specific tropomyosin mRNAs by nerve growth factor is dependent on morphological differentiation

We have examined the expression of brain-specific tropomyosins during neuronal differentiation. Both TmBr-1 and TmBr-3 were shown to be neuron specific. TmBr-1 and TmBr-3 mRNA levels increased during the most active phase of neurite outgrowth in the developing rat cerebellum. In PC12 cells stimulated by nerve growth factor (NGF) to differentiate to the neuronal phenotype, TmBr-1 and TmBr-3 levels increased with an increasing degree of morphological differentiation. Induction of TmBr-1 and TmBr-3 expression only occurred under conditions where PC12 cells were permitted to extend neurites. NGF was unable to maintain levels of TmBr-1 and TmBr-3 with the loss of neuronal phenotype by resuspension of differentiated PC12 cells. The unique cellular expression and regulation in vivo and in vitro of TmBr-1 and TmBr-3 strongly suggests a critical role of these tropomyosins in neuronal microfilament function. The findings reveal that the induction and maintenance of the neuronal tropomyosins is dependent on morphological differentiation and the maintenance of the neuronal phenotype.

Differential regulation of the atrial isoforms of the myosin light chains during striated muscle development

We have isolated a cDNA that encodes the human regulatory myosin light chain isoform predominant in adult atrial muscle. The cDNA contains an open reading frame of 175 amino acids and encodes a hydrophilic protein of a largely helical structure with two potential phosphorylation sites. The protein is different from any other regulatory myosin light chain so far described and is the product of a previously uncharacterized single copy gene. An isoform-specific probe was used to analyze the expression of this isoform in adult muscle and in cardiac and skeletal muscle development in vivo and in vitro. Parallel analysis of the corresponding human alkali myosin light chain (predominant in adult atrium) showed that both isoforms are expressed in early heart development, in both atrium and ventricle. Although the atrial alkali light chain is expressed throughout embryonic striated muscle development, the regulatory myosin light chain was not detected in skeletal myogenesis in vivo or in vitro. Thus the atrial isoforms are not universally or exclusively "paired" and can be independently regulated. We propose that the manner in which these particular isoforms fulfill the functional requirements of the muscle at different developmental times may have direct impact on their regulation.

Molecular cloning and primary structure of the avian Thy-1 glycoprotein

We have previously characterised, both biochemically and immunohistochemically, a 23 kDa putative avian Thy-1 protein homologue. In this report we have examined the carbohydrates present on the protein and determined the partial protein sequence of enzymatically and CNBr-produced peptides. The protein sequences enabled us to clone an essentially full-length (1854 bp) cDNA using PCR and colony screening of an embryonic day (ED) 18 forebrain pUEX-1 cDNA library. Analysis of deduced amino acid sequence shows the 23 kDa protein to be 110 amino acids in length compared to mouse (112) and human and rat (111) while still retaining the conserved cysteine residues. The N-glycosylation site at position 61 is the same as that in the human protein, but is different from that in the rodent (position 75). Northern blot analysis of Thy-1 mRNA expression in the forebrain, cerebellum and tectum show that all three tissues have low levels at ED4 (forebrain and tectum) and ED8 (cerebellum), rising most rapidly between ED16 and the first few days post-hatch. Analysis of various tissues at hatch and adult show expression to be predominantly neuronal with very low levels in some other organs, mainly at hatch, indicating the possibility of subsets of cells, which we have also seen in histological sections, in these tissues expressing Thy-1 mRNA. Bone marrow and blood cells were also negative for Thy-1 protein.

High level expression of transfected beta- and gamma-actin genes differentially impacts on myoblast cytoarchitecture

The impact of the human beta- and gamma-actin genes on myoblast cytoarchitecture was examined by their stable transfection into mouse C2 myoblasts. Transfectant C2 clones expressing high levels of human beta-actin displayed increases in cell surface area. In contrast, C2 clones with high levels of human gamma-actin expression showed decreases in cell surface area. The changes in cell morphology were accompanied by changes in actin stress-fiber organization. The beta-actin transfectants displayed well-defined filamentous organization of actin; whereas the gamma-actin transfectants displayed a more diffuse organization of the actin cables. The role of the beta-actin protein in generating the enlarged cell phenotype was examined by transfecting a mutant form of the human beta-actin gene. Transfectant cells were shown to incorporate the aberrant actin protein into stress-fiber-like structures. High level expression of the mutant beta-actin produced decreases in cell surface area and disruption of the actin microfilament network similar to that seen with transfection of the gamma-actin gene. In contrast, transfection of another mutant form of the beta-actin gene which encodes an unstable protein had no impact on cell morphology or cytoarchitecture. These results strongly suggest that it is the nature of the encoded protein that determines the morphological response of the cell. We conclude that the relative gene expression of beta- and gamma-actin is of relevance to the control of myoblast cytoarchitecture. In particular, we conclude that the beta- and gamma-actin genes encode functionally distinct cytoarchitectural information.

Transfection of nonmuscle beta- and gamma-actin genes into myoblasts elicits different feedback regulatory responses from endogenous actin genes

We have examined the role of feedback-regulation in the expression of the nonmuscle actin genes. C2 mouse myoblasts were transfected with the human beta- and gamma-actin genes. In gamma-actin transfectants we found that the total actin mRNA and protein pools remained unchanged. Increasing levels of human gamma-actin expression resulted in a progressive down-regulation of mouse beta- and gamma-actin mRNAs. Transfection of the beta-actin gene resulted in an increase in the total actin mRNA and protein pools and induced an increase in the levels of mouse beta-actin mRNA. In contrast, transfection of a beta-actin gene carrying a single-point mutation (beta sm) produced a feedback-regulatory response similar to that of the gamma-actin gene. Expression of a beta-actin gene encoding an unstable actin protein had no impact on the endogenous mouse actin genes. This suggests that the nature of the encoded actin protein determines the feedback-regulatory response of the mouse genes. The role of the actin cytoskeleton in mediating this feedback-regulation was evaluated by disruption of the actin network with Cytochalasin D. We found that treatment with Cytochalasin D abolished the down-regulation of mouse gamma-actin in both the gamma- and beta sm-actin transfectants. In contrast, a similar level of increase was observed for the mouse beta-actin mRNA in both control and transfected cells. These experiments suggest that the down-regulation of mouse gamma-actin mRNA is dependent on the organization of the actin cytoskeleton. In addition, the mechanism responsible for the down-regulation of beta-actin may be distinct from that governing gamma-actin. We conclude that actin feedback-regulation provides a biochemical assay for differences between the two nonmuscle actin genes.

Multiple mechanisms regulate muscle fiber diversity

Adult skeletal muscles are composed of clusters of multinucleated muscle cells called myofibers. At least three different types of myofibers can be detected within mammals based on their physiological properties and their expression of different contractile protein isoforms. Different skeletal muscles display a wide range of combinations of myofibers. Recent work has demonstrated that multiple mechanisms are responsible for the generation of these myofiber types during development. Muscle progenitor cells have been dissected into two categories on the basis of which isoforms of myosin heavy chain (MHC) they express when they differentiate. Neural and other environmental influences act to modify decisions concerning the type of contractile protein a myofiber may express, and this is most apparent for MHC. The other contractile protein gene families are initially regulated independent of the MHC gene family. One or more events late in development are responsible for coordinating isoform expression between the gene families to generate the adult phenotype. Studies of muscle gene expression have revealed that regulation can occur at the levels of transcription, alternative splicing of primary transcripts, mRNA stability, and translation. The current challenge is to decipher how environmental and functional information is interpreted in terms of the activity of the regulators of muscle gene expression.