Aberrant regulation of insulin receptor alternative splicing is associated with insulin resistance in myotonic dystrophy

Comparison of the Functional Insulin Binding Epitopes of the A and B Isoforms of the Insulin Receptor

The human insulin receptor is expressed as two isoforms that are generated by alternate splicing of its mRNA; the B isoform has 12 additional amino acids (718-729) encoded by exon 11 of the gene. The isoforms have been reported to have different ligand binding properties. To further characterize their insulin binding properties, we have performed structure-directed alanine-scanning mutagenesis of a major insulin binding site of the receptor, formed from the receptor L1 domain (amino acids 1-470) and amino acids 705-715 at the C terminus of the alpha subunit. Alanine mutants of each isoform were transiently expressed as recombinant secreted extracellular domain in 293 cells, and their insulin binding properties were evaluated by competitive binding assays. Mutation of Arg(86) and Phe(96) of each isoform resulted in receptors that were not secreted. The Kds of unmutated receptors were almost identical for both isoforms. Several new mutations compromising insulin binding were identified. In L1, mutation of Leu(37) decreased affinity 20- to 40-fold and mutations of Val(94), Glu(97), Glu(120), and Lys(121) 3 to 10-fold for each isoform. A number of mutations produced differential effects on the two isoforms. Mutation of Asn(15) in the L1 domain and Phe(714) at the C terminus of the alpha subunit inactivated the A isoform but only reduced the affinity of the B isoform 40- to 60-fold. At the C terminus of the alpha subunit, mutations of Asp(707), Val(713), and Val(715) produced 7- to 16-fold reductions in affinity of the A isoform but were without effect on the B isoform. In contrast, alanine mutations of Tyr(708) and Asn(711) inactivated the B isoform but only reduced the affinities of the A isoform 11- and 6-fold, respectively. In conclusion, alanine-scanning mutagenesis of the insulin receptor A and B isoforms has identified several new side chains contributing to insulin binding and indicates that the energetic contributions of certain side chains differ in each isoform, suggesting that different molecular mechanisms are used to obtain the same affinity.

Mutant DMPK 3'-UTR transcripts disrupt C2C12 myogenic differentiation by compromising MyoD

Myotonic dystrophy (DM) is caused by two similar noncoding repeat expansion mutations (DM1 and DM2). It is thought that both mutations produce pathogenic RNA molecules that accumulate in nuclear foci. The DM1 mutation is a CTG expansion in the 3' untranslated region (3'-UTR) of dystrophia myotonica protein kinase (DMPK). In a cell culture model, mutant transcripts containing a (CUG)200 DMPK 3'-UTR disrupt C2C12 myoblast differentiation; a phenotype similar to what is observed in myoblast cultures derived from DM1 patient muscle. Here, we have used our cell culture model to investigate how the mutant 3'-UTR RNA disrupts differentiation. We show that MyoD protein levels are compromised in cells that express mutant DMPK 3'-UTR transcripts. MyoD, a transcription factor required for the differentiation of myoblasts during muscle regeneration, activates differentiation-specific genes by binding E-boxes. MyoD levels are significantly reduced in myoblasts expressing the mutant 3'-UTR RNA within the first 6 h under differentiation conditions. This reduction correlates with blunted E-box-mediated gene expression at time points that are critical for initiating differentiation. Importantly, restoring MyoD levels rescues the differentiation defect. We conclude that mutant DMPK 3'-UTR transcripts disrupt myoblast differentiation by reducing MyoD levels below a threshold required to activate the differentiation program.

Defects in pre-mRNA processing as causes of and predisposition to diseases

Humans possess a surprisingly low number of genes and intensively use pre-mRNA splicing to achieve the high molecular complexity needed to sustain normal body functions and facilitate responses to altered conditions. Because hundreds of thousands of proteins are generated by 25,000 to 40,000 genes, pre-mRNA processing events are highly important for the regulation of human gene expression. Both inherited and acquired defects in pre-mRNA processing are increasingly recognized as causes of human diseases, and almost all pre-mRNA processing events are controlled by a combination of protein factors. This makes defects in these processes likely candidates for causes of diseases with complicated inheritance patterns that affect seemingly unrelated functions. The elucidation of genetic mechanisms regulating pre-mRNA processing, combined with the development of drugs targeted at consensus RNA sequences and/or corresponding proteins, can lead to novel diagnostic and therapeutic approaches.

Myotonic syndromes

PURPOSE OF REVIEW

To highlight recent advances in understanding the clinical manifestations and molecular genetics of myotonic syndromes, with particular emphasis on the myotonic dystrophies.

RECENT FINDINGS

Myotonic syndromes include the non-dystrophic myotonias, caused by mutations in genes encoding the chloride or sodium channels that are specific to skeletal muscle, and the myotonic dystrophies. Previous studies have shown that myotonic dystrophy type 1 is caused by the expansion of a CTG repeat in the gene. Recently, it was discovered that myotonic dystrophy type 2 (proximal myotonic myopathy) is also caused by a DNA expansion mutation. In both types of myotonic dystrophy the expanded repeat is transcribed and the RNA produced from the mutant allele is retained in nuclear inclusions. Recent studies suggest that the mutant RNA has a toxic effect on muscle fibers by interfering with the essential functions of the myonucleus, such as RNA processing.

SUMMARY

It now appears likely that myotonic dystrophy is the first instance of a genetic disease in which the harmful effect of a mutation involves the production of a pathogenic RNA. However, the exact mechanism is not understood, and it is unclear whether this RNA-mediated disease process is also responsible for the manifestations of myotonic dystrophy in non-muscle tissues.

Myotonic dystrophy: clinical and molecular parallels between myotonic dystrophy type 1 and type 2

Myotonic dystrophy (DM) is a dominantly inherited disorder with a peculiar pattern of multisystemic clinical features affecting skeletal muscle, the heart, the eye, and the endocrine system. Two genetic loci have been associated with the DM phenotype: DM1 on chromosome 19, and DM2 on chromosome 3. In 1992, the mutation responsible for DM1 was identified as a CTG expansion located in the 3' untranslated region of the dystrophica myotonica-protein kinase gene (DMPK). How this untranslated CTG expansion causes DM1 has been a matter of controversy. The recent discovery that DM2 is caused by an untranslated CCTG expansion, along with other discoveries on DM1 pathogenesis, indicate that the clinical features common to both diseases are caused by a gain of function RNA mechanism in which the CUG and CCUG repeats alter cellular function, including alternative splicing of various genes.

IRS proteins and the common path to diabetes

Although a full understanding of insulin/insulin-like growth factor (IGF) action is evolving, the discovery of insulin receptor substrate (IRS) proteins and their role to link cell surface receptors to the intracellular signaling cascades provided an important step forward. Moreover, Insulin/IGF receptors use common signaling pathways to accomplish many tasks, the IRS proteins add a unique layer of specificity and control. Importantly, the IRS-2 branch of the insulin/IGF-signaling pathway is a common element in peripheral insulin response and pancreatic beta-cell growth and function. Failure of IRS-2 signaling might explain the eventual loss of compensatory hyperinsulinemia during prolonged periods of peripheral insulin resistance. Moreover, short-term inhibition of IRS protein functions by serine phosphorylation, or sustained inhibition by ubiquitin-targeted proteosome-mediated degradation suggests a common molecular mechanism for insulin resistance during acute injury or infection, or the sensitivity of beta-cells to autoimmune destruction. The broad role of IRS-1 and IRS-2 in cell growth and survival reveals a common regulatory pathway linking development, somatic growth, fertility, neuronal proliferation, and aging to the core mechanisms used by vertebrates for nutrient sensing.

A splicing silencer that regulates smooth muscle specific alternative splicing is active in multiple cell types

Alternative splicing of alpha-tropomyosin (alpha-TM) involves mutually exclusive selection of exons 2 and 3. Selection of exon 2 in smooth muscle (SM) cells is due to inhibition of exon 3, which requires both binding sites for polypyrimidine tract-binding protein as well as UGC (or CUG) repeat elements on both sides of exon 3. Point mutations or substitutions of the UGC-containing upstream regulatory element (URE) with other UGC elements disrupted the alpha-TM splicing pattern in transfected cells. Multimerisation of the URE caused enhanced exon skipping in SM and various non-SM cells. In the presence of multiple UREs the degree of splicing regulation was decreased due to the high levels of exon skipping in non-SM cell lines. These results suggest that the URE is not an intrinsically SM- specific element, but that its functional strength is fine tuned to exploit differences in the activities of regulatory factors between SM and other cell types. Co-transfection of tropomyosin reporters with members of the CUG-binding protein family, which are candidate URE-binding proteins, indicated that these factors do not mediate repression of tropomyosin exon 3.

Genetic approaches to the molecular understanding of type 2 diabetes

The appreciation that individual susceptibility to type 2 diabetes (T2D) and related components of the dysmetabolic syndrome has a strong inherited component provides a coherent framework within which to develop a molecular understanding of the pathogenesis of T2D. This review focuses on the main approaches currently adopted by researchers seeking to identify the inherited basis of T2D and the present state of our knowledge. One central theme that emerges is that progress in defining the genetic basis of the common, multifactorial forms of T2D is hindered by etiological heterogeneity: T2D is likely to represent the final common pathway of diverse interacting primary disturbances. Such heterogeneity equally compromises efforts to understand the basis for T2D by use of other approaches, such as cellular biochemistry and classical physiology. Analyses that seek to ally sophisticated physiological characterization with measures of genomic variation are likely to provide powerful tools for redressing the loss of power associated with such heterogeneity.

Expanded CTG repeats inhibit neuronal differentiation of the PC12 cell line

Myotonic dystrophy (DM) is a dominant neuromuscular disorder caused by the expansion of trinucleotide CTG repeats in the 3-untranslated region (3'-UTR) of the MtPK gene. Although DM-associated mental retardation suggests that neuronal functions are disturbed by the expansion mutation, the effect of this alteration in neuronal cells has not been approached. In this study we established stable transfectans of PC12 neuronal cell line expressing the reporter gene CAT alone (empty-vector clone) or fused to the MtPK 3'-UTR with 5, 60, or 90 CTG repeats (CTG5, CTG60, and CTG90 clones, respectively). CTG90 cells exhibited a suppression of NGF-induced neuronal differentiation while empty-vector, CTG5 and CTG60 clones differentiated normally. CTG90 cells displayed normal activation of early differentiation markers, ERK1/2, but the up-regulation of the late marker MAP2 was dramatically reduced. Our neuronal cell system provides the first information of how the mutant MtPK 3'-UTR mRNA affects neuronal functions.

Loss of the muscle-specific chloride channel in type 1 myotonic dystrophy due to misregulated alternative splicing

Myotonic dystrophy type 1 (DM1) is a dominant multisystemic disorder caused by a CTG expansion in the 3' untranslated region of the DMPK gene. A predominant characteristic of DM1 is myotonia resulting from skeletal muscle membrane hyperexcitability. Here we demonstrate loss of the muscle-specific chloride channel (ClC-1) mRNA and protein in DM1 skeletal muscle tissue due to aberrant splicing of the ClC-1 pre-mRNA. The splicing regulator, CUG binding protein (CUG-BP), which is elevated in DM1 striated muscle, binds to the ClC-1 pre-mRNA, and overexpression of CUG-BP in normal cells reproduces the aberrant pattern of ClC-1 splicing observed in DM1 skeletal muscle. We propose that disruption of alternative splicing regulation causes a predominant pathological feature of DM1.

Expanded CUG repeats trigger aberrant splicing of ClC-1 chloride channel pre-mRNA and hyperexcitability of skeletal muscle in myotonic dystrophy

In myotonic dystrophy (dystrophia myotonica, DM), expression of RNAs that contain expanded CUG or CCUG repeats is associated with degeneration and repetitive action potentials (myotonia) in skeletal muscle. Using skeletal muscle from a transgenic mouse model of DM, we show that expression of expanded CUG repeats reduces the transmembrane chloride conductance to levels well below those expected to cause myotonia. The expanded CUG repeats trigger aberrant splicing of pre-mRNA for ClC-1, the main chloride channel in muscle, resulting in loss of ClC-1 protein from the surface membrane. We also have identified a similar defect in ClC-1 splicing and expression in two types of human DM. We propose that a transdominant effect of mutant RNA on RNA processing leads to chloride channelopathy and membrane hyperexcitability in DM.

Dominantly inherited, non-coding microsatellite expansion disorders

Dominantly inherited diseases are generally caused by mutations resulting in gain of function protein alterations. However, a CTG expansion located in the 3' untranslated portion of a kinase gene was found to cause myotonic dystrophy type 1, a multisystemic dominantly inherited disorder. The recent discovery that an untranslated CCTG expansion causes the same constellation of clinical features in myotonic dystrophy type 2 (DM2), along with other recent discoveries on DM1 pathogenesis, have led to the understanding that both DM1 and DM2 mutations are pathogenic at the RNA level. These findings indicate the existence of a new category of disease wherein repeat expansions in RNA alter cellular function. Pathogenic repeat expansions in RNA may also be involved in spinocerebellar ataxia types 8, 10 and 12, and Huntington's disease-like type 2.

SRp30c is a repressor of 3' splice site utilization

Several intron elements influence exon 7B skipping in the mammalian hnRNP A1 pre-mRNA. We have shown previously that the 38-nucleotide CE9 element located in the intron separating alternative exon 7B from exon 8 can repress the use of a downstream 3' splice site. The ability of CE9 to act on heterologous substrates, combined with the results of competition and gel shift assays, indicates that the activity of CE9 is mediated by a trans-acting factor. UV cross-linking analysis revealed the specific association of a 25-kDa nuclear protein with CE9. Using RNA affinity chromatography, we isolated a 25-kDa protein that binds to CE9 RNA. This protein corresponds to SRp30c. Consistent with a role for SRp30c in the activity of CE9, recombinant SRp30c interacts specifically with CE9 and can promote splicing repression in vitro in a CE9-dependent manner. The closest homologue of SRp30c, ASF/SF2, does not bind to CE9 and does not repress splicing even when the intronic SRp30c binding sites are replaced with high-affinity ASF/SF2 binding sites. Only the first 7 nucleotides of CE9 are sufficient for binding to SRp30c, and mutations that abolish binding also prevent repression. Our results indicate that SRp30c can function as a repressor of 3' splice site utilization and suggest that the SRp30c-CE9 interaction may contribute to the control of hnRNP A1 alternative splicing.

Advances in understanding of fragile X pathogenesis and FMRP function, and in identification of X linked mental retardation genes

The fragile X mental retardation syndrome is caused by large methylated expansions of a CGG repeat in the FMR1 gene that lead to the loss of expression of FMRP, an RNA-binding protein. FMRP is proposed to act as a regulator of mRNA transport or translation that plays a role in synaptic maturation and function. The recent observations of unexpected phenotypes in some carriers of fragile X premutations suggest a pathological role, in these individuals, of an abnormal FMR1 mRNA. FMRP was recently shown to interact preferentially with mRNAs containing a G quartet structure. Mouse and Drosophila models are used to decipher the function of FMRP, which was found to inhibit translation of some mRNA targets, but may be stimulatory in other cases. Proteins interacting with FMRP have been identified, and suggest a link with the Rac1 GTPase pathway that is important in neuronal maturation. Recent advances also include identification of other genes implicated in X-linked mental retardation.

Alternative splicing: combinatorial output from the genome

Alternative splicing has emerged as a mechanism that can account for a large proportion of the disparity between the modest number of genes in the human genome and the much higher complexity of the expressed proteome. At least a third, and probably the majority, of human genes are alternatively spliced, and some genes can generate thousands of protein isoforms by complex alternative splicing events. Analysis of the transcriptome will therefore require the development of massively parallel technologies that are able to encompass the complexity arising from alternative splicing.

Alternative splicing: multiple control mechanisms and involvement in human disease

Alternative splicing is an important mechanism for controlling gene expression. It allows large proteomic complexity from a limited number of genes. An interplay of cis-acting sequences and trans-acting factors modulates the splicing of regulated exons. Here, we discuss the roles of the SR and hnRNP families of proteins in this process. We also focus on the role of the transcriptional machinery in the regulation of alternative splicing, and on those alterations of alternative splicing that lead to human disease.

Dynamic antagonism between ETR-3 and PTB regulates cell type-specific alternative splicing

Inclusion of cardiac troponin T (cTNT) exon 5 in embryonic muscle requires conserved flanking intronic elements (MSEs). ETR-3, a member of the CELF family, binds U/G motifs in two MSEs and directly activates exon inclusion in vitro. Binding and activation by ETR-3 are directly antagonized by polypyrimidine tract binding protein (PTB). We use dominant-negative mutants to demonstrate that endogenous CELF and PTB activities are required for MSE-dependent activation and repression in muscle and nonmuscle cells, respectively. Combined use of CELF and PTB dominant-negative mutants provides an in vivo demonstration that antagonistic splicing activities exist within the same cells. We conclude that cell-specific regulation results from the dominance of one among actively competing regulatory states rather than modulation of a nonregulated default state.

c-Jun ARE targets mRNA deadenylation by an EDEN-BP (embryo deadenylation element-binding protein)-dependent pathway

In mammalian cells, certain mRNAs encoding cytokines or proto-oncogenes are especially unstable, because of the presence of a particular sequence element in their 3'-untranslated region named ARE (A/U-rich element). AREs cause this instability by provoking the rapid shortening of the poly(A) tail of the mRNA. The deadenylation of mRNAs mediated by AREs containing repeats of the AUUUA motif (class I/II AREs) is conserved in Xenopus embryos. Here, we first extend these observations by showing that c-Jun ARE, a representative of class III (non-AUUUA) AREs, also provokes the deadenylation of a reporter RNA in Xenopus embryos. Next, by immunodepletion and immunoneutralization experiments, we show that, in Xenopus, the rapid deadenylation of RNAs that contain the c-Jun ARE, but not an AUUUA ARE, requires EDEN-BP. This RNA-binding protein was previously shown to provoke the rapid deadenylation of certain Xenopus maternal RNAs. Finally, we show that CUG-BP, the human homologue of EDEN-BP, specifically binds to c-Jun ARE. Together, these results identify CUG-BP as a plausible deadenylation factor responsible for the post-transcriptional control of c-Jun proto-oncogene mRNA in mammalian cells.

Highlights of alternative splicing regulation session: yes, no, maybe--a history of paradigm shifts

Highlights from the Sixth Annual Meeting of the RNA Society, Banff, Alberta, Canada, 29 May to 3 June 2001. Cooper summarizes the discussions and presentations from the session entitled "Control of Splice Site Selection" held at the Sixth Annual Meeting of the RNA Society. Paradigms are shifting as experiments show that some of the proteins involved in regulating splicing can act as splicing enhancers or repressors, depending on the cellular context. The complex interactions among the ribonucleoproteins (RNPs) and proteins, and the role of cis elements, in controlling cell-specific splicing are highlighted. The importance of properly regulated splicing is emphasized by examples of disease pathologies in which alternative splicing is aberrant.