ClC-1 mutations in myotonia congenita patients: insights into molecular gating mechanisms and genotype-phenotype correlation.

KEY POINTS: Loss-of-function mutations of the skeletal muscle ClC-1 channel cause myotonia congenita with variable phenotypes. Using patch clamp we show that F484L, located in the conducting pore, probably induces mild dominant myotonia by right-shifting the slow gating of ClC-1 channel, without exerting a dominant-negative effect on the wild-type (WT) subunit. Molecular dynamics simulations suggest that F484L affects the slow gate by increasing the frequency and the stability of H-bond formation between E232 in helix F and Y578 in helix R. Three other myotonic ClC-1 mutations are shown to produce distinct effects on channel function: L198P shifts the slow gate to positive potentials, V640G reduces channel activity, while L628P displays a WT-like behaviour (electrophysiology data only). Our results provide novel insight into the molecular mechanisms underlying normal and altered ClC-1 function. ABSTRACT: Myotonia congenita is an inherited disease caused by loss-of-function mutations of the skeletal muscle ClC-1 chloride channel, characterized by impaired muscle relaxation after contraction and stiffness. In the present study, we provided an in-depth characterization of F484L, a mutation previously identified in dominant myotonia, in order to define the genotype-phenotype correlation, and to elucidate the contribution of this pore residue to the mechanisms of ClC-1 gating. Patch-clamp recordings showed that F484L reduced chloride currents at every tested potential and dramatically right-shifted the voltage dependence of slow gating, thus contributing to the mild clinical phenotype of affected heterozygote carriers. Unlike dominant mutations located at the dimer interface, no dominant-negative effect was observed when F484L mutant subunits were co-expressed with wild type. Molecular dynamics simulations further revealed that F484L affected the slow gate by increasing the frequency and stability of the H-bond formation between the pore residue E232 and the R helix residue Y578. In addition, using patch-clamp electrophysiology, we characterized three other myotonic ClC-1 mutations. We proved that the dominant L198P mutation in the channel pore also right-shifted the voltage dependence of slow gating, recapitulating mild myotonia. The recessive V640G mutant drastically reduced channel function, which probably accounts for myotonia. In contrast, the recessive L628P mutant produced currents very similar to wild type, suggesting that the occurrence of the compound truncating mutation (Q812X) or other muscle-specific mechanisms accounted for the severe symptoms observed in this family. Our results provide novel insight into the molecular mechanisms underlying normal and altered ClC-1 function.

Slug transcription factor and nuclear Lamin B1 are upregulated in osteoarthritic chondrocytes.

OBJECTIVE: To contribute to clarify molecular mechanisms supporting senescence and de-differentiation of chondrocytes in chondrocyte pathologies such as osteoarthritis (OA). Specifically, we investigated the relationship between the nuclear lamina protein Lamin B1 and the negative regulator of chondrogenesis Slug transcription factor in osteoarthritic chondrocytes. METHODS: Lamin B1 and Slug proteins were analyzed in cartilage explants from normal subjects and OA patients by immunohistochemical technique. Their expression was confirmed on isolated chondrocytes both at passage 0 and passage 2 (de-differentiated chondrocytes) by immunofluorescence and western blot. Subsequently, we explored the "in vivo" binding of Slug on LMNB1 promoter by chromatin immunoprecipitation assay (ChIP). RESULTS: In this study we demonstrated that nuclear lamina protein Lamin B1 and anti-chondrogenic Slug transcription factor are upregulated in cartilage and OA chondrocytes. Furthermore, we found that Slug is "in vivo" recruited by LMNB1 gene promoter mostly when chondrocytes undergo de-differentiation or OA degeneration. CONCLUSIONS: We described for the first time a potential regulatory role of Slug on the LMNB1 gene expression in OA chondrocytes. These findings may have important implications for the study of premature senescence, and degeneration of cartilage, and may contribute to develop effective therapeutic strategies against signals supporting cartilage damage in different subsets of patients.

Reduced adipogenic gene expression in fibroblasts from a patient with type 2 congenital generalized lipodystrophy.

AIMS: Beradinelli-Seip congenital generalized lipodystrophy is a rare autosomal recessive disorder characterized by near-complete absence of adipose tissue, Herculean appearance, insulin resistance, hypoleptinaemia and diabetes mellitus. The aim of this study was to investigate the in vitro effects of pioglitazone on the expression of genes involved in adipogenesis in fibroblasts from a patient with this condition due to a seipin mutation. METHODS: Primary cultures of fibroblasts from the skin of the patient were obtained. Fibroblasts were treated with classic adipose differentiation medium, with and without pioglitazone. Several adipogenes were evaluated by real-time reverse transcriptase-polymerase chain reaction and western blotting. Intracellular localization of prelamin A was studied by immunofluorescence microscopy. RESULTS: The expression of the adipogenic genes PPARG, LPL, LEP and SLC2A4 was reduced in lipodystrophic fibroblasts, while treatment with pioglitazone increased the expression of these genes. Moreover, and unexpectedly, we found an accumulation of farnesylated prelamin A in lipodystrophic fibroblasts. CONCLUSIONS: The process of adipocyte differentiation is compromised in patients with Beradinelli-Seip congenital lipodystrophy owing to diminished expression of the regulatory genes involved, which pioglitazone treatment partially rescues. Prelamin A accumulation establishes a link with other types of familial lipodystrophies, as familial partial lipodystrophy.

Site-dependent differences in both prelamin A and adipogenic genes in subcutaneous adipose tissue of patients with type 2 familial partial lipodystrophy.

BACKGROUND: Type 2 familial partial lipodystrophy (FPLD2) is characterised by loss of fat in the limbs and buttocks and results from mutations in the LMNA gene. AIM: To evaluate the role of several genes involved in adipogenesis in order to better understand the underlying mechanisms of regional loss of subcutaneous adipose tissue (scAT) in patients with FPLD2. METHODS: In total, 7 patients with FPLD2 and 10 healthy control participants were studied. A minimal model was used to calculate the insulin sensitivity (IS). scAT was obtained from abdomen and thigh by biopsy. Relative gene expression was quantified by real-time reverse transcription PCR in a thermal cycler. Prelamin A western blot analysis was carried out on scAT and prelamin A nuclear localisation was determined using immunofluorescence. Adipocyte nuclei were examined by electron microscopy. RESULTS: Patients with FPLD2 were found to have significantly lower IS. The expression of LMNA was similar in both groups. The expression of PPARG2, RB1, CCND3 and LPL in thigh but not in abdomen scAT was significantly reduced (67%, 25%, 38% and 66% respectively) in patients with FPLD2. Significantly higher levels of prelamin A were found in peripheral scAT of patients with FPLD2. Defects in the peripheral heterochromatin and a nuclear fibrous dense lamina were present in the adipocytes of patients with FPLD2. CONCLUSIONS: In FPLD2 participants, prelamin A accumulation in peripheral scAT is associated with a reduced expression of several genes involved in adipogenesis, which could perturb the balance between proliferation and differentiation in adipocytes, leading to less efficient tissue regeneration.

Different prelamin A forms accumulate in human fibroblasts: a study in experimental models and progeria.

Lamin A is a component of the nuclear lamina mutated in a group of human inherited disorders known as laminopathies. Among laminopathies, progeroid syndromes and lipodystrophies feature accumulation of prelamin A, the precursor protein which, in normal cells, undergoes a multi-step processing to yield mature lamin A. It is of utmost importance to characterize the prelamin A form accumulated in each laminopathy, since existing evidence shows that drugs acting on protein processing can improve some pathological aspects.We report that two antibodies raised against differently modified prelamin A peptides show a clear specificity to full-length prelamin A or carboxymethylated farnesylated prelamin A, respectively. Using these antibodies, we demonstrated that inhibition of the prelamin A endoprotease ZMPSTE24 mostly elicits accumulation of full-length prelamin A in its farnesylated form, while loss of the prelamin A cleavage site causes accumulation of carboxymethylated prelamin A in progeria cells. These results suggest a major role of ZMPSTE24 in the first prelamin A cleavage step.

Involvement of prelamin A in laminopathies.

The precursor protein of the nuclear lamina constituent lamin A is a 74-kDa protein called prelamin A which undergoes subsequent steps of posttranslational modification at its C-terminal CaaX residue. The unexpected finding that accumulation of unprocessable prelamin A is the molecular basis of the most severe laminopathies so far identified, including Hutchinson-Gilford progeria and restrictive dermopathy, has opened new perspectives in the study of the pathogenic mechanisms causing all lamin A/C-linked disorders, as well as new interest in the analysis of molecular mechanisms regulating prelamin A processing. However, complete knowledge of the cellular pathways affected downstream of prelamin A accumulation is still lacking, but it could give new insights both in normal and pathogenic mechanisms regulated by lamins. In this article, we review the involvement of defects of prelamin A processing in the pathogenesis of a group of laminopathies. In particular, we discuss the possibility that mutations leading to accumulation of particular forms of prelamin A result in specific nuclear abnormalities and impairment of nuclear functions leading to cell senescence or altered metabolism.

Lamin A N-terminal phosphorylation is associated with myoblast activation: impairment in Emery-Dreifuss muscular dystrophy.

BACKGROUND: Skeletal muscle disorders associated with mutations of lamin A/C gene include autosomal Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy 1B. The pathogenic mechanism underlying these diseases is unknown. Recent data suggest an impairment of signalling mechanisms as a possible cause of muscle malfunction. A molecular complex in muscle cells formed by lamin A/C, emerin, and nuclear actin has been identified. The stability of this protein complex appears to be related to phosphorylation mechanisms. OBJECTIVE: To analyse lamin A/C phosphorylation in control and laminopathic muscle cells. METHODS: Lamin A/C N-terminal phosphorylation was determined in cultured mouse myoblasts using a specific antibody. Insulin treatment of serum starved myoblast cultures was carried out to evaluate involvement of insulin signalling in the phosphorylation pathway. Screening of four Emery-Dreifuss and one limb girdle muscular dystrophy 1B cases was undertaken to investigate lamin A/C phosphorylation in both cultured myoblasts and mature muscle fibres. RESULTS: Phosphorylation of lamin A was observed during myoblast differentiation or proliferation, along with reduced lamin A/C phosphorylation in quiescent myoblasts. Lamin A N-terminus phosphorylation was induced by an insulin stimulus, which conversely did not affect lamin C phosphorylation. Lamin A/C was also hyperphosphorylated in mature muscle, mostly in regenerating fibres. Lamin A/C phosphorylation was strikingly reduced in laminopathic myoblasts and muscle fibres, while it was preserved in interstitial fibroblasts. CONCLUSIONS: Altered lamin A/C interplay with a muscle specific phosphorylation partner might be involved in the pathogenic mechanism of Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy 1B.

Nuclear envelope proteins and chromatin arrangement: a pathogenic mechanism for laminopathies.

The involvement of the nuclear envelope in the modulation of chromatin organization is strongly suggested by the increasing number of human diseases due to mutations of nuclear envelope proteins. A common feature of these diseases, named laminopathies, is the occurrence of major chromatin defects. We previously reported that cells from laminopathic patients show an altered nuclear profile, and loss or detachment of heterochromatin from the nuclear envelope. Recent evidence indicates that processing of the lamin A precursor is altered in laminopathies featuring pre-mature aging and/or lipodystrophy phenotype. In these cases, pre-lamin A is accumulated in the nucleus and heterochromatin is severely disorganized. Here we report evidence indicating that pre-lamin A is mis-localized in the nuclei of Emery-Dreifuss muscular dystrophy fibroblasts, either bearing lamin A/C or emerin mutations. Abnormal pre-lamin A-containing structures are formed following treatment with a farnesyl-transferase inhibitor, a drug that causes accumulation of pre-lamin A. Pre-lamin A-labeled structures co-localize with heterochromatin clumps. These data indicate that in almost all laminopathies the expression of the mutant lamin A precursor disrupts the organization of heterochromatin domains. Our results further show that the absence of emerin expression alters the distribution of pre-lamin A and of heterochromatin areas, suggesting a major involvement of emerin in pre-lamin A-mediated mechanisms of chromatin remodeling.

Changes in polyphosphoinositide levels in rat liver nuclei in response to prolactin, a known hepatic mitogen.

The effect of prolactin action on nuclear polyphosphoinositide synthesis was investigated in isolated rat liver nuclei. An increased uptake of phosphate from [gamma 32P] adenosinetriphosphate was observed in both phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate with a maximum response at 10(-12) M concentration of hormone. Pulse-chase experiments in isolated nuclei following prolactin treatment indicate that the observed increase in accumulation of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate is mainly due to a decrease in their rate of turnover possibly induced by a change in activity of polyphosphoinositide-specific monoesterases. In vitro prolactin also reduces the activity of nuclear phospholipase C specific for phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. Moreover, this feature is strongly supported by the concomitant decrease in nuclear diacylglycerol mass. Thus these data suggest that once prolactin reaches the nucleus an intranuclear signalling is evoked through inositol lipid metabolism.

Emerin increase in regenerating muscle fibers.

The fate of emerin during skeletal muscle regeneration was investigated in an animal model by means of crush injury. Immunofluorescence, immunoblotting and mRNA analysis demonstrated that emerin level is increased in regenerating rat muscle fibers with respect to normal mature myofibers. This finding suggests an involvement of emerin during the muscle fiber regeneration process, in analogy with its reported involvement in muscle cell differentiation in vitro. The impairment of skeletal muscle physiological regeneration or reorganization could be a possible pathogenetic mechanism for Emery Dreifuss muscular dystrophy.

Collagen VI deficiency affects the organization of fibronectin in the extracellular matrix of cultured fibroblasts.

Fibronectin is one of the main components of the extracellular matrix and associates with a variety of other matrix molecules including collagens. We demonstrate that the absence of secreted type VI collagen in cultured primary fibroblasts affects the arrangement of fibronectin in the extracellular matrix. We observed a fine network of collagen VI filaments and fibronectin fibrils in the extracellular matrix of normal murine and human fibroblasts. The two microfibrillar systems did not colocalize, but were interconnected at some discrete sites which could be revealed by immunoelectron microscopy. Direct interaction between collagen VI and fibronectin was also demonstrated by far western assay. When primary fibroblasts from Col6a1 null mutant mice were cultured, collagen VI was not detected in the extracellular matrix and a different pattern of fibronectin organization was observed, with fibrils running parallel to the long axis of the cells. Similarly, an abnormal fibronectin deposition was observed in fibroblasts from a patient affected by Bethlem myopathy, where collagen VI secretion was drastically reduced. The same pattern was also observed in normal fibroblasts after in vivo perturbation of collagen VI-fibronectin interaction with the 3C4 anti-collagen VI monoclonal antibody. Competition experiments with soluble peptides indicated that the organization of fibronectin in the extracellular matrix was impaired by added soluble collagen VI, but not by its triple helical (pepsin-resistant) fragments. These results indicate that collagen VI mediates the three-dimensional organization of fibronectin in the extracellular matrix of cultured fibroblasts.

Staurosporine treatment and serum starvation promote the cleavage of emerin in cultured mouse myoblasts: involvement of a caspase-dependent mechanism.

Emerin is a nuclear membrane-anchored protein which is absent or mutated in patients affected by Emery-Dreifuss muscular dystrophy. In this study, we induced apoptosis in cultured mouse myoblasts to evaluate emerin fate during the nuclear destabilization involved in programmed cell death. Emerin proteolysis was observed in myocytes during the apoptotic process. Myoblast apoptosis and emerin degradation were associated with chromatin compaction and detachment from the nuclear lamina, as detected by electron microscopy. In vivo specific inhibition of caspase 3 or caspase 6 activity completely abolished emerin proteolysis. These results show that the process of programmed cell death in muscle cells leads to emerin proteolysis, which appears to be related to caspase 6 activation and to cleavage of other nuclear envelope proteins, that share sequence homologies or functional features with emerin.

Functional domains of the nucleus: implications for Emery-Dreifuss muscular dystrophy.

Elucidation of the pathophysiology of Emery-Dreifuss muscular dystrophy, caused by mutations in emerin or lamin A/C, will require deciphering the role of these proteins in the functional organization of the nuclear envelope. This review focuses on nuclear envelope related mechanisms that modulate chromatin arrangement and control of gene transcription, both potential targets of the disease process in Emery-Dreifuss muscular dystrophy. Interactions of these proteins with chromatin- and nuclear matrix-associated proteins are now of particular interest, since chromatin alterations occur in cells in Emery-Dreifuss muscular dystrophy. Both emerin and lamin A/C interact with nuclear actin, a component of the chromatin remodeling complex associated with the nuclear matrix, suggesting that either chromatin arrangement, or gene transcription, or both, might be impaired in the disease.

Ultrastructural aspects of the DNA polymerase alpha distribution during the cell cycle.

We studied the nuclear topography of the replicating enzyme DNA polymerase alpha in HeLa cells by transmission electron microscopy and field emission in lens scanning electron microscopy. Cells were synchronized at the G1/S-phase boundary and samples of the different phases of the cell cycle were labeled with an anti-DNA polymerase alpha antibody detected by an immunogold reaction. DNA synthesis was detected by immunogold labeling after bromodeoxyuridine administration. The typical labeling pattern of DNA polymerase alpha observed in G1- and S-phase cells was represented by circular structures 80-100 nm in diameter surrounding an electron-dense area. In double labeled samples these circular structures were associated with bromodeoxyuridine-containing DNA replication sites, forming rosette-like structures. Field emission scanning electron microscopy performed on ultrathin cryosections revealed the chromatin fibers underlying DNA polymerase alpha complexes and showed that the size of the rosette-like structures corresponded to the diameter of chromatin foldings. G2- and M-phase cells showed a spread distribution of DNA polymerase alpha. The evidence of DNA polymerase alpha circular arrangement exclusively in G1- and S-phase cells, obtained by such different approaches, allowed us to consider the three-dimensional structures as DNA replication areas.

High-resolution FEISEM detection of DNA centromeric probes in HeLa metaphase chromosomes.

HeLa metaphase chromosome spreads were hybridized with centromeric biotinylated DNA probes and detected with gold-conjugated anti-biotin antibodies. Chromosomes were observed by an in-lens field emission scanning electron microscope (FEISEM), which permits detection of biological samples without any coating. DNA probes were well localized in the centromeric region of chromosomes and there was clear discrimination between 10 nm fibers that hybridized to DNA probes and those that did not hybridize. This approach shows that in situ hybridization can be directly visualized at the FEISEM level by evaluating only secondary electron emission, which allows physical localization of the hybridized probe with high resolution so that backscatter detection represents only a control. Because chromosomes maintain the 10-nm fiber organization after in situ hybridization procedures, our data suggest that this fiber represents the lowest order of chromatin arrangement that permits transitory DNA denaturation.

Benzene adducts with rat nucleic acids and proteins: dose-response relationship after treatment in vivo.

The dose-response relationship of the benzene covalent interaction with biological macromolecules from rat organs was studied. The administered dose range was 3.6 x 10(7) starting from the highest dosage employed, 486 mg/kg, which is oncogenic for rodents, and included low and very low dosages. The present study was initially performed with tritium-labeled benzene, administered by IP injection. In order to exclude the possibility that part of the detected radioactivity was due to tritium incorporated into DNA from metabolic processes, 14C-benzene was then also used following a similar experimental design. By HPLC analysis, a single adduct from benzene-treated DNA was detected; adduct identification will be attempted in the near future. Linear dose-response relationship was observed within most of the range of explored doses. Linearity was particularly evident within low and very low dosages. Saturation of benzene metabolism did occur at the highest dosages for most of the assayed macromolecules and organs, especially in rat liver. This finding could be considered as indicative of the dose-response relationship of tumor induction and could be used in risk assessment.

Master equation approach to molecular motors.

A master equation approach to molecular motors allows us to describe a mechanochemical cyclic system where chemical and translational degrees of freedom are treated on an equal footing. A generalized detailed balance condition in the out-of-equilibrium regime is shown to be compatible with the Fokker-Planck equation in the continuum limit. The Onsager reciprocity relations hold for stationary states close to equilibrium, provided the generalized detailed balance condition is satisfied. Semiphenomenological considerations in the case of motor proteins lead to a discrete kinetics model, for which interesting observable quantities may be directly calculated and compared with experimental data.

Transverse fluctuations of grafted polymers.

We study the statistical mechanics of grafted polymers of arbitrary stiffness in a two-dimensional embedding space with Monte Carlo simulations. The probability distribution function of the free end is found to be highly anisotropic and non-Gaussian for typical semiflexible polymers. The reduced distribution in the transverse direction, a Gaussian in the stiff and flexible limits, shows a double-peak structure at intermediate stiffnesses. We also explore the response to a transverse force applied at the polymer free end. We identify F-Actin as an ideal benchmark for the effects discussed.

Phase shifts of synchronized oscillators and the systolic-diastolic blood pressure relation.

We study the phase-synchronization properties of systolic and diastolic arterial pressure in healthy subjects. We find that delays in the oscillatory components of the time series depend on the frequency bands that are considered, in particular we find a change of sign in the phase shift going from the very low frequency band to the high frequency band. This behavior should reflect a collective behavior of a system of nonlinear interacting elementary oscillators. We prove that some models describing such systems, e.g., the Winfree and the Kuramoto models, offer a clue to this phenomenon. For these theoretical models there is a linear relationship between phase shifts and the difference of natural frequencies of oscillators and a change of sign in the phase shift naturally emerges.