Mbnl2 loss alters novel context processing and impairs object recognition memory.

Patients with myotonic dystrophy type I (DM1) demonstrate visuospatial dysfunction and impaired performance in tasks requiring recognition or memory of figures and objects. In DM1, CUG expansion RNAs inactivate the muscleblind-like (MBNL) proteins. We show that constitutive Mbnl2 inactivation in mice selectively impairs object recognition memory in the novel object recognition test.

Mbnl1 and Mbnl2 regulate brain structural integrity in mice.

Myotonic Dystrophy Type I (DM1) patients demonstrate widespread and variable brain structural alterations whose etiology is unclear. We demonstrate that inactivation of the Muscleblind-like proteins, Mbnl1 and Mbnl2, initiates brain structural defects. 2D FSE T2w MRIs on 4-month-old Mbnl1/Mbnl2 mice demonstrate whole-brain volume reductions, ventriculomegaly and regional gray and white matter volume reductions.

The Werner Syndrome Helicase Coordinates Sequential Strand Displacement and FEN1-Mediated Flap Cleavage during Polymerase δ Elongation.

The Werner syndrome protein (WRN) suppresses the loss of telomeres replicated by lagging-strand synthesis by a yet to be defined mechanism. Here, we show that whereas either WRN or the Bloom syndrome helicase (BLM) stimulates DNA polymerase δ progression across telomeric G-rich repeats, only WRN promotes sequential strand displacement synthesis and FEN1 cleavage, a critical step in Okazaki fragment maturation, at these sequences. Helicase activity, as well as the conserved winged-helix (WH) motif and the helicase and RNase D C-terminal (HRDC) domain play important but distinct roles in this process.

Recent advances in understanding the role of lamins in health and disease.

Lamins are major components of the nuclear lamina, a network of proteins that supports the nuclear envelope in metazoan cells. Over the past decade, biochemical studies have provided support for the view that lamins are not passive bystanders providing mechanical stability to the nucleus but play an active role in the organization of the genome and the function of fundamental nuclear processes. It has also become apparent that lamins are critical for human health, as a large number of mutations identified in the gene that encodes for A-type lamins are associated with tissue-specific and systemic genetic diseases, including the accelerated aging disorder known as Hutchinson-Gilford progeria syndrome.

Muscleblind-like 3 deficit results in a spectrum of age-associated pathologies observed in myotonic dystrophy.

Myotonic dystrophy type I (DM1) exhibits distinctive disease specific phenotypes and the accelerated onset of a spectrum of age-associated pathologies. In DM1, dominant effects of expanded CUG repeats result in part from the inactivation of the muscleblind-like (MBNL) proteins. To test the role of MBNL3, we deleted Mbnl3 exon 2 (Mbnl3(ΔE2)) in mice and examined the onset of age-associated diseases over 4 to 13 months of age.

Muscleblind-Like 1 and Muscleblind-Like 3 Depletion Synergistically Enhances Myotonia by Altering Clc-1 RNA Translation.

Unlabelled: Loss of Muscleblind-like 1 (Mbnl1) is known to alter Clc-1 splicing to result in myotonia. Mbnl1(ΔE3/ΔE3)/Mbnl3(ΔE2) mice, depleted of Mbnl1 and Mbnl3, demonstrate a profound enhancement of myotonia and an increase in the number of muscle fibers with very low Clc-1 currents, where gClmax values approach ~ 1 mS/cm(2), with the absence of a further enhancement in Clc-1 splice errors, alterations in polyA site selection or Clc-1 localization. Significantly, Mbnl1(ΔE3/ΔE3)/Mbnl3(ΔE2) muscles demonstrate an aberrant accumulation of Clc-1 RNA on monosomes and on the first polysomes.

Loss of muscleblind-like 1 results in cardiac pathology and persistence of embryonic splice isoforms.

Cardiac dysfunction is a prominent cause of mortality in myotonic dystrophy I (DM1), a disease where expanded CUG repeats bind and disable the muscleblind-like family of splice regulators. Deletion of muscleblind-like 1 (Mbnl1(ΔE2/ΔE2)) in 129 sv mice results in QRS, QTc widening, bundle block and STc narrowing at 2-4 months of age. With time, cardiac function deteriorates further and at 6 months, decreased R wave amplitudes, sinus node dysfunction, cardiac hypertrophy, interstitial fibrosis, multi-focal myocardial fiber death and calcification manifest.

Downregulation of the Werner syndrome protein induces a metabolic shift that compromises redox homeostasis and limits proliferation of cancer cells.

The Werner syndrome protein (WRN) is a nuclear protein required for cell growth and proliferation. Loss-of-function mutations in the Werner syndrome gene are associated with the premature onset of age-related diseases. How loss of WRN limits cell proliferation and induces replicative senescence is poorly understood.

RNA splicing is responsive to MBNL1 dose.

Myotonic dystrophy (DM1) is a highly variable, multi-system disorder resulting from the expansion of an untranslated CTG tract in DMPK. In DM1 expanded CUG repeat RNAs form hairpin secondary structures that bind and aberrantly sequester the RNA splice regulator, MBNL1. RNA splice defects resulting as a consequence of MBNL1 depletion have been shown to play a key role in the development of DM1 pathology.

A filtering strategy identifies FOXQ1 as a potential effector of lamin A dysfunction.

Small increases in the expression of wild-type prelamin A are sufficient to recapitulate the reduced cell proliferation and altered nuclear membrane morphology observed in cells expressing progerin, the mutant lamin A associated with progeria. We hypothesized that the manifestation of these phenotypes in cells expressing elevated levels of wild-type prelamin A or progerin is caused by the same molecular effectors, which play a central role in the onset of the progeroid phenotype. To experimentally test this hypothesis, we compared the transcriptomes of isogenic diploid fibroblasts expressing progerin or elevated levels of wild-type prelamin A with that of wild-type fibroblasts.

Transcriptome-wide regulation of pre-mRNA splicing and mRNA localization by muscleblind proteins.

The muscleblind-like (Mbnl) family of RNA-binding proteins plays important roles in muscle and eye development and in myotonic dystrophy (DM), in which expanded CUG or CCUG repeats functionally deplete Mbnl proteins. We identified transcriptome-wide functional and biophysical targets of Mbnl proteins in brain, heart, muscle, and myoblasts by using RNA-seq and CLIP-seq approaches. This analysis identified several hundred splicing events whose regulation depended on Mbnl function in a pattern indicating functional interchangeability between Mbnl1 and Mbnl2.

Expanded CUG repeats Dysregulate RNA splicing by altering the stoichiometry of the muscleblind 1 complex.

To understand the role of the splice regulator muscleblind 1 (MBNL1) in the development of RNA splice defects in myotonic dystrophy I (DM1), we purified RNA-independent MBNL1 complexes from normal human myoblasts and examined the behavior of these complexes in DM1 myoblasts. Antibodies recognizing MBNL1 variants (MBNL1(CUG)), which can sequester in the toxic CUG RNA foci that develop in DM1 nuclei, were used to purify MBNL1(CUG) complexes from normal myoblasts. In normal myoblasts, MBNL1(CUG) bind 10 proteins involved in remodeling ribonucleoprotein complexes including hnRNP H, H2, H3, F, A2/B1, K, L, DDX5, DDX17, and DHX9.

Lamin A, farnesylation and aging.

Lamin A is a component of the nuclear envelope that is synthesized as a precursor prelamin A molecule and then processed into mature lamin A through sequential steps of posttranslational modifications and proteolytic cleavages. Remarkably, over 400 distinct point mutations have been so far identified throughout the LMNA gene, which result in the development of at least ten distinct human disorders, collectively known as laminopathies, among which is the premature aging disease Hutchinson-Gilford progeria syndrome (HGPS). The majority of HGPS cases are associated with a single point mutation in the LMNA gene that causes the production of a permanently farnesylated mutant lamin A protein termed progerin.

RNA steady-state defects in myotonic dystrophy are linked to nuclear exclusion of SHARP.

We describe a new mechanism by which CTG tract expansion affects myotonic dystrophy (DM1). Changes to the levels of a panel of RNAs involved in muscle development and function that are downregulated in DM1 are due to aberrant localization of the transcription factor SHARP (SMART/HDAC1-associated repressor protein). Mislocalization of SHARP in DM1 is consistent with increased CRM1-mediated export of SHARP to the cytoplasm.

Depletion of Ku70/80 reduces the levels of extrachromosomal telomeric circles and inhibits proliferation of ALT cells.

In normal cells, telomeres shorten each time a cell divides ultimately resulting in cell senescence. In contrast, cancer cells counteract the loss of telomeric DNA either by inducing the expression of telomerase or by activating the alternative lengthening of telomeres (ALT) pathway. ALT cells are characterized by heterogeneous telomeres and the presence of extrachromosomal circular double-stranded DNA molecules containing telomeric repeat sequences.

Accumulation of distinct prelamin A variants in human diploid fibroblasts differentially affects cell homeostasis.

Lamin A is a component of the nuclear lamina that plays a major role in the structural organization and function of the nucleus. Lamin A is synthesized as a prelamin A precursor which undergoes four sequential post-translational modifications to generate mature lamin A. Significantly, a large number of point mutations in the LMNA gene cause a range of distinct human disorders collectively known as laminopathies.

Muscleblind-like 1 (Mbnl1) promotes insulin receptor exon 11 inclusion via binding to a downstream evolutionarily conserved intronic enhancer.

The insulin receptor exists as two isoforms, IR-A and IR-B, which result from alternative splicing of exon 11 in the primary transcript. These two isoforms show a cell-specific distribution, and their relative proportions also vary during development, aging, and in different disease states. We have previously demonstrated that both intron 10 and the alternatively spliced exon 11 contain regulatory sequences that affect insulin receptor splicing both positively and negatively and that these sequences bind the serine/arginine-rich (SR) proteins SRp20 and SF2/ASF and the CELF protein CUG-BP1.

Muscleblind1, but not Dmpk or Six5, contributes to a complex phenotype of muscular and motivational deficits in mouse models of myotonic dystrophy.

Assessment of molecular defects that underlie cognitive deficits observed in mendelian disorders provides a unique opportunity to identify key regulators of human cognition. Congenital Myotonic Dystrophy 1 (cDM1), a multi-system disorder is characterized by both cognitive deficits and a spectrum of behavioral abnormalities, which include visuo-spatial memory deficits, anxiety and apathy. Decreased levels of DMPK (Dystrophia Myotonica-protein kinase), SIX5, a transcription factor or MBNL1 (Muscleblind-like 1), an RNA splice regulator have been demonstrated to contribute to distinct features of cDM1.

Sequence-specific processing of telomeric 3' overhangs by the Werner syndrome protein exonuclease activity.

Werner syndrome is a premature aging disease caused by loss of function mutations in the Werner syndrome protein (WRN) gene. WRN is a RecQ helicase that in contrast to every other member of this family of proteins possesses an exonuclease activity. The findings that cells lacking WRN activity display accelerated telomere shortening and WRN can be detected at chromosome ends suggest that this protein participates in some aspects of telomere metabolism.

Altered nuclear functions in progeroid syndromes: a paradigm for aging research.

Syndromes of accelerated aging could provide an entry point for identifying and dissecting the cellular pathways that are involved in the development of age-related pathologies in the general population. However, their usefulness for aging research has been controversial, as it has been argued that these diseases do not faithfully reflect the process of natural aging. Here we review recent findings on the molecular basis of two progeroid diseases, Werner syndrome (WS) and Hutchinson-Gilford progeria syndrome (HGPS), and highlight functional connections to cellular processes that may contribute to normal aging.

Cytoplasmic CUG RNA foci are insufficient to elicit key DM1 features.

The genetic basis of myotonic dystrophy type I (DM1) is the expansion of a CTG tract located in the 3' untranslated region of DMPK. Expression of mutant RNAs encoding expanded CUG repeats plays a central role in the development of cardiac disease in DM1. Expanded CUG tracts form both nuclear and cytoplasmic aggregates, yet the relative significance of such aggregates in eliciting DM1 pathology is unclear.

WRN controls formation of extrachromosomal telomeric circles and is required for TRF2DeltaB-mediated telomere shortening.

Telomere dysfunction has been proposed to contribute to the pathogenesis of Werner syndrome (WS), a premature-aging disorder. The WS protein WRN binds TRF2, a telomere-specific factor that protects chromosome ends. TRF2 possesses an amino-terminal domain that plays an essential role in preventing telomere shortening, as expression of TRF2(DeltaB), which lacks this domain, leads to the formation of telomeric circles, telomere shortening, and cell senescence.