Emanuele Loro

Autophagy Is Required to Maintain Muscle Mass

The ubiquitin-proteasome and autophagy-lysosome pathways are the two major routes for protein and organelle clearance. In skeletal muscle, both systems are under FoxO regulation and their excessive activation induces severe muscle loss. Although altered autophagy has been observed in various myopathies, the specific role of autophagy in skeletal muscle has not been determined by loss-of-function approaches. Here, we report that muscle-specific deletion of a crucial autophagy gene, Atg7, resulted in profound muscle atrophy and age-dependent decrease in force. Atg7 null muscles showed accumulation of abnormal mitochondria, sarcoplasmic reticulum distension, disorganization of sarcomere, and formation of aberrant concentric membranous structures. Autophagy inhibition exacerbated muscle loss during denervation and fasting. Thus, autophagy flux is important to preserve muscle mass and to maintain myofiber integrity. Our results suggest that inhibition/alteration of autophagy can contribute to myofiber degeneration and weakness in muscle disorders characterized by accumulation of abnormal mitochondria and inclusions.

The nuclear receptor Rev-erbα controls circadian thermogenic plasticity

Circadian oscillation of body temperature is a basic, evolutionarily conserved feature of mammalian biology. In addition, homeostatic pathways allow organisms to protect their core temperatures in response to cold exposure. However, the mechanism responsible for coordinating daily body temperature rhythm and adaptability to environmental challenges is unknown. Here we show that the nuclear receptor Rev-erbα (also known as Nr1d1), a powerful transcriptional repressor, links circadian and thermogenic networks through the regulation of brown adipose tissue (BAT) function. Mice exposed to cold fare considerably better at 05:00 (Zeitgeber time 22) when Rev-erbα is barely expressed than at 17:00 (Zeitgeber time 10) when Rev-erbα is abundant. Deletion of Rev-erbα markedly improves cold tolerance at 17:00, indicating that overcoming Rev-erbα-dependent repression is a fundamental feature of the thermogenic response to cold. Physiological induction of uncoupling protein 1 (Ucp1) by cold temperatures is preceded by rapid downregulation of Rev-erbα in BAT. Rev-erbα represses Ucp1 in a brown-adipose-cell-autonomous manner and BAT Ucp1 levels are high in Rev-erbα-null mice, even at thermoneutrality. Genetic loss of Rev-erbα also abolishes normal rhythms of body temperature and BAT activity. Thus, Rev-erbα acts as a thermogenic focal point required for establishing and maintaining body temperature rhythm in a manner that is adaptable to environmental demands.

Overexpression of microRNA-206 in the skeletal muscle from myotonic dystrophy type 1 patients.

BackgroundMicroRNAs are highly conserved, noncoding RNAs involved in post-transcriptional gene silencing. They have been shown to participate in a wide range of biological processes, including myogenesis and muscle regeneration. The goal of this study is to test the hypothesis that myo-miRs (myo = muscle + miR = miRNA) expression is altered in muscle from patients affected by myotonic dystrophy type 1 (DM1), the most frequently inherited neuromuscular disease in adults. In order to gain better insights about the role of miRNAs in the DM1 pathogenesis, we have also analyzed the muscular expression of miR-103 and miR-107, which have been identified in silico as attractive candidates for binding to the DMPK mRNA.MethodsTo this aim, we have profiled the expression of miR-133 (miR-133a, miR-133b), miR-1, miR-181 (miR-181a, miR-181b, miR-181c) and miR-206, that are specifically induced during myogenesis in cardiac and skeletal muscle tissues. miR-103 and miR-107, highly expressed in brain, heart and muscle have also been included in this study. QRT-PCR experiments have been performed on RNA from vastus lateralis biopsies of DM1 patients (n = 7) and control subjects (n = 4). Results of miRNAs expression have been confirmed by Northern blot, whereas in situ hybridization technique have been performed to localize misexpressed miRNAs on muscle sections from DM1 and control individuals.ResultsOnly miR-206 showed an over-expression in 5 of 7 DM1 patients (threshold = 2, fold change between 1.20 and 13.22, average = 5.37) compared to the control group. This result has been further confirmed by Northern blot analysis (3.37-fold overexpression, R2 = 0.89). In situ hybridization localized miR-206 to nuclear site both in normal and DM1 tissues. Cellular distribution in DM1 tissues includes also the nuclear regions of centralized nuclei, with a strong signal corresponding to nuclear clumps.ConclusionsThis work provides, for the first time, evidences about miRNAs misexpression in DM1 muscle tissues, adding a new element in the pathogenesis of this complex genetic disease.

A novel deletion in the GTPase domain of OPA1 causes defects in mitochondrial morphology and distribution, but not in function

Autosomal dominant optic atrophy (ADOA), the commonest cause of inherited optic atrophy, is caused by mutations in the ubiquitously expressed gene optic atrophy 1 (OPA1), involved in fusion and biogenesis of the inner membrane of mitochondria. Bioenergetic failure, mitochondrial network abnormalities and increased apoptosis have all been proposed as possible causal factors. However, their relative contribution to pathogenesis as well as the prominent susceptibility of the retinal ganglion cell (RGC) in this disease remains uncertain. Here we identify a novel deletion of OPA1 gene in the GTPase domain in three patients affected by ADOA. Muscle biopsy of the patients showed neurogenic atrophy and abnormal morphology and distribution of mitochondria. Confocal microscopy revealed increased mitochondrial fragmentation in fibroblasts as well as in myotubes, where mitochondria were also unevenly distributed, with clustered organelles alternating with areas where mitochondria were sparse. These abnormalities were not associated with altered bioenergetics or increased susceptibility to pro-apoptotic stimuli. Therefore, changes in mitochondrial shape and distribution can be independent of other reported effects of OPA1 mutations, and therefore may be the primary cause of the disease. The arrangement of mitochondria in RGCs, which degenerate in ADOA, may be exquisitely sensitive to disturbance, and this may lead to bioenergetic crisis and/or induction of apoptosis. Our results highlight the importance of mitochondrial dynamics in the disease per se, and point to the loss of the fine positioning of mitochondria in the axons of RGCs as a possible explanation for their predominant degeneration in ADOA.

ChIP sequencing of cyclin D1 reveals a transcriptional role in chromosomal instability in mice

Chromosomal instability (CIN) in tumors is characterized by chromosomal abnormalities and an altered gene expression signature; however, the mechanism of CIN is poorly understood. CCND1 (which encodes cyclin D1) is overexpressed in human malignancies and has been shown to play a direct role in transcriptional regulation. Here, we used genome-wide ChIP sequencing and found that the DNA-bound form of cyclin D1 occupied the regulatory region of genes governing chromosomal integrity and mitochondrial biogenesis. Adding cyclin D1 back to Ccnd1(-/-) mouse embryonic fibroblasts resulted in CIN gene regulatory region occupancy by the DNA-bound form of cyclin D1 and induction of CIN gene expression. Furthermore, increased chromosomal aberrations, aneuploidy, and centrosome abnormalities were observed in the cyclin D1-rescued cells by spectral karyotyping and immunofluorescence. To assess cyclin D1 effects in vivo, we generated transgenic mice with acute and continuous mammary gland-targeted cyclin D1 expression. These transgenic mice presented with increased tumor prevalence and signature CIN gene profiles. Additionally, interrogation of gene expression from 2,254 human breast tumors revealed that cyclin D1 expression correlated with CIN in luminal B breast cancer. These data suggest that cyclin D1 contributes to CIN and tumorigenesis by directly regulating a transcriptional program that governs chromosomal stability.

Inhibition of Mitochondrial Fission Favours Mutant Over Wild-type Mitochondrial DNA

Biased segregation of mitochondrial DNA variants has been widely documented, but little was known about its molecular basis. We set out to test the hypothesis that altering the balance between mitochondrial fusion and fission could influence the segregation of mutant and wild-type mtDNA variants, because it would modify the number of organelles per cell. Therefore human cells heteroplasmic for the pathological A3243G mitochondrial DNA mutation were transfected with constructs designed to silence Drp1 or hFis1, whose gene products are required for mitochondrial fission. Drp1 and hFis1 gene silencing were both associated with increased levels of mutant mitochondrial DNA. Thus, the extent of the mitochondrial reticular network appears to be an important factor in determining mutant load. The fact that the level of mutant and wild-type mitochondrial DNA can be manipulated by altering the expression of nuclear encoded factors involved in mitochondrial fission suggests new interventions for mitochondrial DNA disorders.

Loss of NAD Homeostasis Leads to Progressive and Reversible Degeneration of Skeletal Muscle

NAD is an obligate co-factor for the catabolism of metabolic fuels in all cell types. However, the availability of NAD in several tissues can become limited during genotoxic stress and the course of natural aging. The point at which NAD restriction imposes functional limitations on tissue physiology remains unknown. We examined this question in murine skeletal muscle by specifically depleting Nampt, an essential enzyme in the NAD salvage pathway. Knockout mice exhibited a dramatic 85% decline in intramuscular NAD content, accompanied by fiber degeneration and progressive loss of both muscle strength and treadmill endurance. Administration of the NAD precursor nicotinamide riboside rapidly ameliorated functional deficits and restored muscle mass despite having only a modest effect on the intramuscular NAD pool. Additionally, lifelong overexpression of Nampt preserved muscle NAD levels and exercise capacity in aged mice, supporting a critical role for tissue-autonomous NAD homeostasis in maintaining muscle mass and function.

Cyclins and Cell Cycle Control in Cancer and Disease

Cyclin D1 overexpression is found in more than 50% of human breast cancers and causes mammary cancer in transgenic mice. Dysregulation of cyclin D1 gene expression or function contributes to the loss of normal cell cycle control during tumorigenesis. Recent studies have demonstrated that cyclin D1 conducts additional specific functions to regulate gene expression in the context of local chromatin, promote cellular migration, and promote chromosomal instability. It is anticipated that these additional functions contribute to the pathology associated with dysregulated cyclin D1 abundance. This article discusses evidence that examines the functional roles that cyclin D1 may play in cancer with an emphasis on other cyclin family members that also may contribute to cancer and disease in a similar fashion.

Normal myogenesis and increased apoptosis in myotonic dystrophy type-1 muscle cells

Myotonic dystrophy (DM) is caused by a (CTG)n expansion in the 3′-untranslated region of DMPK gene. Mutant transcripts are retained in nuclear RNA foci, which sequester RNA binding proteins thereby misregulating the alternative splicing. Controversy still surrounds the pathogenesis of the DM1 muscle distress, characterized by myotonia, weakness and wasting with distal muscle atrophy. Eight primary human cell lines from adult-onset (DM1) and congenital (cDM1) patients, (CTG)n range 90–1800, were successfully differentiated into aneural-immature and contracting-innervated-mature myotubes. Morphological, immunohistochemical, RT-PCR and western blotting analyses of several markers of myogenesis indicated that in vitro differentiation–maturation of DM1 myotubes was comparable to age-matched controls. In all pathological muscle cells, (CTG)n expansions were confirmed by long PCR and RNA fluorescence in situ hybridization. Moreover, the DM1 myotubes showed the splicing alteration of insulin receptor and muscleblind-like 1 (MBNL1) genes associated with the DM1 phenotype. Considerable myotube loss and atrophy of 15-day-differentiated DM1 myotubes indicated activated catabolic pathways, as confirmed by the presence of apoptotic (caspase-3 activation, cytochrome c release, chromatin fragmentation) and autophagic (P62/LC3) markers. Z-VAD treatment significantly reduced the decrease in myonuclei number and in average width in 15-day-differentiated DM1 myotubes. We thus propose that the muscle wasting typical in DM1 is due to impairment of muscle mass maintenance–regeneration, through premature apoptotic–autophagic activation, rather than altered myogenesis.

The CTG repeat expansion size correlates with the splicing defects observed in muscles from myotonic dystrophy type 1 patients.

Background: Myotonic dystrophy type 1 is caused by an unstable (CTG)n repetition located in the 3′UTR of the DM protein kinase gene (DMPK). Untranslated expanded DMPK transcripts are retained in ribonuclear foci which sequester CUG-binding proteins essential for the maturation of pre-mRNAs. Aim: To investigate the effects of CTG expansion length on three molecular parameters associated with the DM1 muscle pathology: (1) the expression level of the DMPK gene; (2) the degree of splicing misregulation; and (3) the number of ribonuclear foci. Methods: Splicing analysis of the IR, MBNL1, c-TNT and CLCN1 genes, RNA-FISH experiments and determination of the DMPK expression on muscle samples from DM1 patients with an expansion below 500 repetitions (n = 6), DM1 patients carrying a mutation above 1000 CTGs (n = 6), and from controls (n = 6). Results: The level of aberrant splicing of the IR, MBNL1, c-TNT and CLCN1 genes is different between the two groups of DM1 muscle samples and correlates with the CTG repeat length. RNA-FISH analysis revealed that the number of ribonuclear foci in DM1 muscle sections increases in patients with a higher (CTG)n number. No relationships were found between the expression level of the DMPK gene transcript and average expansion sizes. Conclusion: The CTG repeat length plays a key role in the extent of splicing misregulation and foci formation, thus providing a useful link between the genotype and the molecular cellular phenotype in DM1.