Prenatal onset of Axonopathy in Dystonia musculorum mice

Dystonia musculorum (dt) is a recessive hereditary neuropathy of the mouse. Affected animals display loss of limb coordination and twisting of the trunk. Sensory nerve fibers of these mice are severely reduced in number, and the remaining fibers present numerous axonal swellings. The gene defective in dt, dystonin (Dst), encodes a cytoskeletal linker protein that forms the bridge between F-actin and intermediate filaments. Dst is expressed during embryogenesis, whereas overt phenotype in dt mice only appears during the second week after birth. Here we show that axonal swellings are present in sensory nerve fibers of dt embryos as early as E15.5, before myelination and radial axonal growth have begun. Thus disease progression is gradual in dt mice, having begun during embryogenesis. In dt embryos, microtubule network disorganization and cytoplasmic organelle accumulation within axonal swellings were consistently observed. In addition, a few of the axonal swellings presented intermediate filament accumulation. These results demonstrate that dystonin is required for cytoskeleton organization during axonogenesis. They also suggest that axonal transport defects, through microtubule network perturbation, may be the primary mechanism of neurodegeneration in dt mice. Dev. Genet. 22:160–168, 1998. © 1998 Wiley-Liss, Inc.     

BPAG1 isoform-b: Complex distribution pattern in striated and heart muscle and association with plectin and α-actinin

BPAG1-b is the major muscle-specific isoform encoded by the dystonin gene, which expresses various protein isoforms belonging to the plakin protein family with complex, tissue-specific expression profiles. Recent observations in mice with either engineered or spontaneous mutations in the dystonin gene indicate that BPAG1-b serves as a cytolinker important for the establishment and maintenance of the cytoarchitecture and integrity of striated muscle. Here, we studied in detail its distribution in skeletal and cardiac muscles and assessed potential binding partners. BPAG1-b was detectable in vitro and in vivo as a high molecular mass protein in striated and heart muscle cells, co-localizing with the sarcomeric Z-disc protein α-actinin-2 and partially with the cytolinker plectin as well as with the intermediate filament protein desmin. Ultrastructurally, like α-actinin-2, BPAG1-b was predominantly localized at the Z-discs, adjacent to desmin-containing structures. BPAG1-b was able to form complexes with both plectin and α-actinin-2, and its NH₂-terminus, which contains an actin-binding domain, directly interacted with that of plectin and α-actinin. Moreover, the protein level of BPAG1-b was reduced in muscle tissues from plectin-null mutant mice versus wild-type mice. These studies provide new insights into the role of BPAG1-b in the cytoskeletal organization of striated muscle.     

The Intermediate Filament Protein Peripherin Is the Specific Interaction Partner of Mouse BPAG1-n (Dystonin) in Neurons

The dystonia musculorum (dt) mouse suffers from severe degeneration of primary sensory neurons. The mutated gene product is named dystonin and is identical to the neuronal isoform of bullous pemphigoid antigen 1 (BPAG1-n). BPAG1-n contains an actin-binding domain at its NH₂ terminus and a putative intermediate filament-binding domain at its COOH terminus. Because the degenerating sensory neurons of dt mice display abnormal accumulations of intermediate filaments in the axons, BPAG1-n has been postulated to organize the neuronal cytoskeleton by interacting with both the neurofilament triplet proteins (NFTPs) and microfilaments. In this paper we show by a variety of methods that the COOH-terminal tail domain of mouse BPAG1 interacts specifically with peripherin, but in contrast to a previous study (Yang, Y., J. Dowling, Q.C. Yu, P. Kouklis, D.W. Cleveland, and E. Fuchs. 1996. Cell. 86:655–665), mouse BPAG1 fails to associate with full-length NFTPs. The tail domains interfered with the association of the NFTPs with BPAG1. In dt mice, peripherin is present in axonal swellings of degenerating sensory neurons in the dorsal root ganglia and is downregulated even in other neural regions, which have no obvious signs of pathology. Since peripherin and BPAG1-n also display similar expression patterns in the nervous system, we suggest that peripherin is the specific interaction partner of BPAG1-n in vivo.     

Increased Cardiac Myocyte PDE5 Levels in Human and Murine Pressure Overload Hypertrophy Contribute to Adverse LV Remodeling

Background

The intracellular second messenger cGMP protects the heart under pathological conditions. We examined expression of phosphodiesterase 5 (PDE5), an enzyme that hydrolyzes cGMP, in human and mouse hearts subjected to sustained left ventricular (LV) pressure overload. We also determined the role of cardiac myocyte-specific PDE5 expression in adverse LV remodeling in mice after transverse aortic constriction (TAC).

Methodology/Principal Findings

In patients with severe aortic stenosis (AS) undergoing valve replacement, we detected greater myocardial PDE5 expression than in control hearts. We observed robust expression in scattered cardiac myocytes of those AS patients with higher LV filling pressures and BNP serum levels. Following TAC, we detected similar, focal PDE5 expression in cardiac myocytes of C57BL/6NTac mice exhibiting the most pronounced LV remodeling. To examine the effect of cell-specific PDE5 expression, we subjected transgenic mice with cardiac myocyte-specific PDE5 overexpression (PDE5-TG) to TAC. LV hypertrophy and fibrosis were similar as in WT, but PDE5-TG had increased cardiac dimensions, and decreased dP/dt_max and dP/dt_min with prolonged tau (P<0.05 for all). Greater cardiac dysfunction in PDE5-TG was associated with reduced myocardial cGMP and SERCA2 levels, and higher passive force in cardiac myocytes in vitro.

Conclusions/Significance

Myocardial PDE5 expression is increased in the hearts of humans and mice with chronic pressure overload. Increased cardiac myocyte-specific PDE5 expression is a molecular hallmark in hypertrophic hearts with contractile failure, and represents an important therapeutic target.     

Evaluation of Skeletal and Cardiac Muscle Function after Chronic Administration of Thymosin β-4 in the Dystrophin Deficient Mouse

Thymosin beta-4 (Tβ4) is a ubiquitous protein with many properties relating to cell proliferation and differentiation that promotes wound healing and modulates inflammatory mediators. We studied the effects of chronic administration of Tβ4 on the skeletal and cardiac muscle of dystrophin deficient mdx mice, the mouse model of Duchenne muscular dystrophy. Female wild type (C57BL10/ScSnJ) and mdx mice, 8–10 weeks old, were treated with 150 µg of Tβ4 twice a week for 6 months. To promote muscle pathology, mice were exercised for 30 minutes twice a week. Skeletal and cardiac muscle function were assessed via grip strength and high frequency echocardiography. Localization of Tβ4 and amount of fibrosis were quantified using immunohistochemistry and Gomori''s tri-chrome staining, respectively. Mdx mice treated with Tβ4 showed a significant increase in skeletal muscle regenerating fibers compared to untreated mdx mice. Tβ4 stained exclusively in the regenerating fibers of mdx mice. Although untreated mdx mice had significantly decreased skeletal muscle strength compared to untreated wild type, there were no significant improvements in mdx mice after treatment. Systolic cardiac function, measured as percent shortening fraction, was decreased in untreated mdx mice compared to untreated wild type and there was no significant difference after treatment in mdx mice. Skeletal and cardiac muscle fibrosis were also significantly increased in untreated mdx mice compared to wild type, but there was no significant improvement in treated mdx mice. In exercised dystrophin deficient mice, chronic administration of Tβ4 increased the number of regenerating fibers in skeletal muscle and could have a potential role in treatment of skeletal muscle disease in Duchenne muscular dystrophy.     

Functional Substitution by TAT-Utrophin in Dystrophin-Deficient Mice

Background

The loss of dystrophin compromises muscle cell membrane stability and causes Duchenne muscular dystrophy and/or various forms of cardiomyopathy. Increased expression of the dystrophin homolog utrophin by gene delivery or pharmacologic up-regulation has been demonstrated to restore membrane integrity and improve the phenotype in the dystrophin-deficient mdx mouse. However, the lack of a viable therapy in humans predicates the need to explore alternative methods to combat dystrophin deficiency. We investigated whether systemic administration of recombinant full-length utrophin (Utr) or ΔR4-21 “micro” utrophin (μUtr) protein modified with the cell-penetrating TAT protein transduction domain could attenuate the phenotype of mdx mice.

Methods and Findings

Recombinant TAT-Utr and TAT-μUtr proteins were expressed using the baculovirus system and purified using FLAG-affinity chromatography. Age-matched mdx mice received six twice-weekly intraperitoneal injections of either recombinant protein or PBS. Three days after the final injection, mice were analyzed for several phenotypic parameters of dystrophin deficiency. Injected TAT-μUtr transduced all tissues examined, integrated with members of the dystrophin complex, reduced serum levels of creatine kinase (11,290±920 U versus 5,950±1,120 U; PBS versus TAT), the prevalence of muscle degeneration/regeneration (54%±5% versus 37%±4% of centrally nucleated fibers; PBS versus TAT), the susceptibility to eccentric contraction-induced force drop (72%±5% versus 40%±8% drop; PBS versus TAT), and increased specific force production (9.7±1.1 N/cm² versus 12.8±0.9 N/cm²; PBS versus TAT).

Conclusions

These results are, to our knowledge, the first to establish the efficacy and feasibility of TAT-utrophin-based constructs as a novel direct protein-replacement therapy for the treatment of skeletal and cardiac muscle diseases caused by loss of dystrophin.     

The Superfast Human Extraocular Myosin Is Kinetically Distinct from the Fast Skeletal IIa,IIb, and IId Isoforms

Humans express five distinct myosin isoforms in the sarcomeres of adult striated muscle (fast IIa, IId, the slow/cardiac isoform I/β, the cardiac specific isoform α, and the specialized extraocular muscle isoform). An additional isoform, IIb, is present in the genome but is not normally expressed in healthy human muscles. Muscle fibers expressing each isoform have distinct characteristics including shortening velocity. Defining the properties of the isoforms in detail has been limited by the availability of pure samples of the individual proteins. Here we study purified recombinant human myosin motor domains expressed in mouse C₂C₁₂ muscle cells. The results of kinetic analysis show that among the closely related adult skeletal isoforms, the affinity of ADP for actin·myosin (K_AD) is the characteristic that most readily distinguishes the isoforms. The three fast muscle myosins have K_AD values of 118, 80, and 55 μm for IId, IIa, and IIb, respectively, which follows the speed in motility assays from fastest to slowest. Extraocular muscle is unusually fast with a far weaker K_AD = 352 μm. Sequence comparisons and homology modeling of the structures identify a few key areas of sequence that may define the differences between the isoforms, including a region of the upper 50-kDa domain important in signaling between the nucleotide pocket and the actin-binding site.     

The structure of isolated cardiac Myosin thick filaments from cardiac Myosin binding protein-C knockout mice

Mutations in the thick filament associated protein cardiac myosin binding protein-C (cMyBP-C) are a major cause of familial hypertrophic cardiomyopathy. Although cMyBP-C is thought to play both a structural and a regulatory role in the contraction of cardiac muscle, detailed information about the role of this protein in stability of the thick filament and maintenance of the ordered helical arrangement of the myosin cross-bridges is limited. To address these questions, the structure of myosin thick filaments isolated from the hearts of wild-type mice containing cMyBP-C (cMyBP-C^+/+) were compared to those of cMyBP-C knockout mice lacking this protein (cMyBp-C^−/−). The filaments from the knockout mice hearts lacking cMyBP-C are stable and similar in length and appearance to filaments from the wild-type mice hearts containing cMyBP-C. Both wild-type and many of the cMyBP-C^−/− filaments display a distinct 43 nm periodicity. Fourier transforms of electron microscope images typically show helical layer lines to the sixth layer line, confirming the well-ordered arrangement of the cross-bridges in both sets of filaments. However, the “forbidden” meridional reflections, thought to derive from a perturbation from helical symmetry in the wild-type filament, are weaker or absent in the transforms of the cMyBP-C^−/− myocardial thick filaments. In addition, the cross-bridge array in the absence of cMyBP-C appears more easily disordered.     

The N-Terminal Extension of Cardiac Troponin T Stabilizes the Blocked State of Cardiac Thin Filament

Cardiac troponin T (cTnT) is a key component of contractile regulatory proteins. cTnT is characterized by a ～32 amino acid N-terminal extension (NTE), the function of which remains unknown. To understand its function, we generated a transgenic (TG) mouse line that expressed a recombinant chimeric cTnT in which the NTE of mouse cTnT was removed by replacing its 1–73 residues with the corresponding 1–41 residues of mouse fast skeletal TnT. Detergent-skinned papillary muscle fibers from non-TG (NTG) and TG mouse hearts were used to measure tension, ATPase activity, Ca²⁺ sensitivity (pCa₅₀) of tension, rate of tension redevelopment, dynamic muscle fiber stiffness, and maximal fiber shortening velocity at sarcomere lengths (SLs) of 1.9 and 2.3 μm. Ca²⁺ sensitivity increased significantly in TG fibers at both short SL (pCa₅₀ of 5.96 vs. 5.62 in NTG fibers) and long SL (pCa₅₀ of 6.10 vs. 5.76 in NTG fibers). Maximal cross-bridge turnover and detachment kinetics were unaltered in TG fibers. Our data suggest that the NTE constrains cardiac thin filament activation such that the transition of the thin filament from the blocked to the closed state becomes less responsive to Ca²⁺. Our finding has implications regarding the effect of tissue- and disease-related changes in cTnT isoforms on cardiac muscle function.     

A Role for BK Channels in Heart Rate Regulation in Rodents

The heart generates and propagates action potentials through synchronized activation of ion channels allowing inward Na⁺ and Ca²⁺ and outward K⁺ currents. There are a number of K⁺ channel types expressed in the heart that play key roles in regulating the cardiac cycle. Large conductance calcium-activated potassium (BK) ion channels are not thought to be directly involved in heart function. Here we present evidence that heart rate can be significantly reduced by inhibiting the activity of BK channels. Agents that specifically inhibit BK channel activity, including paxilline and lolitrem B, slowed heart rate in conscious wild-type mice by 30% and 42%, respectively. Heart rate of BK channel knock-out mice (Kcnma1^−/−) was not affected by these BK channel inhibitors, suggesting that the changes to heart rate were specifically mediated through BK channels. The possibility that these effects were mediated through BK channels peripheral to the heart was ruled out with experiments using isolated, perfused rat hearts, which showed a significant reduction in heart rate when treated with the BK channel inhibitors paxilline (1 µM), lolitrem B (1 µM), and iberiotoxin (0.23 µM), of 34%, 60%, and 42%, respectively. Furthermore, paxilline was shown to decrease heart rate in a dose-dependent manner. These results implicate BK channels located in the heart to be directly involved in the regulation of heart rate.     

A Highlights from MBoC Selection: Calpain-mediated proteolysis of tropomodulin isoforms leads to thin filament elongation in dystrophic skeletal muscle

Duchenne muscular dystrophy (DMD) induces sarcolemmal mechanical instability and rupture, hyperactivity of intracellular calpains, and proteolytic breakdown of muscle structural proteins. Here we identify the two sarcomeric tropomodulin (Tmod) isoforms, Tmod1 and Tmod4, as novel proteolytic targets of m-calpain, with Tmod1 exhibiting ∼10-fold greater sensitivity to calpain-mediated cleavage than Tmod4 in situ. In mdx mice, increased m-calpain levels in dystrophic soleus muscle are associated with loss of Tmod1 from the thin filament pointed ends, resulting in ∼11% increase in thin filament lengths. In mdx/mTR mice, a more severe model of DMD, Tmod1 disappears from the thin filament pointed ends in both tibialis anterior (TA) and soleus muscles, whereas Tmod4 additionally disappears from soleus muscle, resulting in thin filament length increases of ∼10 and ∼12% in TA and soleus muscles, respectively. In both mdx and mdx/mTR mice, both TA and soleus muscles exhibit normal localization of α-actinin, the nebulin M1M2M3 domain, Tmod3, and cytoplasmic γ-actin, indicating that m-calpain does not cause wholesale proteolysis of other sarcomeric and actin cytoskeletal proteins in dystrophic skeletal muscle. These results implicate Tmod proteolysis and resultant thin filament length misspecification as novel mechanisms that may contribute to DMD pathology, affecting muscles in a use- and disease severity–dependent manner.     

Two-pore Channels (TPC2s) and Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) at Lysosomal-Sarcoplasmic Reticular Junctions Contribute to Acute and Chronic β-Adrenoceptor Signaling in the Heart

Ca²⁺-permeable type 2 two-pore channels (TPC2) are lysosomal proteins required for nicotinic acid adenine dinucleotide phosphate (NAADP)-evoked Ca²⁺ release in many diverse cell types. Here, we investigate the importance of TPC2 proteins for the physiology and pathophysiology of the heart. NAADP-AM failed to enhance Ca²⁺ responses in cardiac myocytes from Tpcn2^−/− mice, unlike myocytes from wild-type (WT) mice. Ca²⁺/calmodulin-dependent protein kinase II inhibitors suppressed actions of NAADP in myocytes. Ca²⁺ transients and contractions accompanying action potentials were increased by isoproterenol in myocytes from WT mice, but these effects of β-adrenoreceptor stimulation were reduced in myocytes from Tpcn2^−/− mice. Increases in amplitude of L-type Ca²⁺ currents evoked by isoproterenol remained unchanged in myocytes from Tpcn2^−/− mice showing no loss of β-adrenoceptors or coupling mechanisms. Whole hearts from Tpcn2^−/− mice also showed reduced inotropic effects of isoproterenol and a reduced tendency for arrhythmias following acute β-adrenoreceptor stimulation. Hearts from Tpcn2^−/− mice chronically exposed to isoproterenol showed less cardiac hypertrophy and increased threshold for arrhythmogenesis compared with WT controls. Electron microscopy showed that lysosomes form close contacts with the sarcoplasmic reticulum (separation ∼25 nm). We propose that Ca²⁺-signaling nanodomains between lysosomes and sarcoplasmic reticulum dependent on NAADP and TPC2 comprise an important element in β-adrenoreceptor signal transduction in cardiac myocytes. In summary, our observations define a role for NAADP and TPC2 at lysosomal/sarcoplasmic reticulum junctions as unexpected but major contributors in the acute actions of β-adrenergic signaling in the heart and also in stress pathways linking chronic stimulation of β-adrenoceptors to hypertrophy and associated arrhythmias.     

The ALP-Enigma Protein ALP-1 Functions in Actin Filament Organization to Promote Muscle Structural Integrity in Caenorhabditis elegans

Mutations that affect the Z-disk–associated ALP-Enigma proteins have been linked to human muscular and cardiac diseases. Despite their clear physiological significance for human health, the mechanism of action of ALP-Enigma proteins is largely unknown. In Caenorhabditis elegans, the ALP-Enigma protein family is encoded by a single gene, alp-1; thus C. elegans provides an excellent model to study ALP-Enigma function. Here we present a molecular and genetic analysis of ALP-Enigma function in C. elegans. We show that ALP-1 and α-actinin colocalize at dense bodies where actin filaments are anchored and that the proper localization of ALP-1 at dense bodies is dependent on α-actinin. Our analysis of alp-1 mutants demonstrates that ALP-1 functions to maintain actin filament organization and participates in muscle stabilization during contraction. Reducing α-actinin activity enhances the actin filament phenotype of the alp-1 mutants, suggesting that ALP-1 and α-actinin function in the same cellular process. Like α-actinin, alp-1 also interacts genetically with a connectin/titin family member, ketn-1, to provide mechanical stability for supporting body wall muscle contraction. Taken together, our data demonstrate that ALP-1 and α-actinin function together to stabilize actin filaments and promote muscle structural integrity.     

Myocardial Creatine Levels Do Not Influence Response to Acute Oxidative Stress in Isolated Perfused Heart

Background

Multiple studies suggest creatine mediates anti-oxidant activity in addition to its established role in cellular energy metabolism. The functional significance for the heart has yet to be established, but antioxidant activity could contribute to the cardioprotective effect of creatine in ischaemia/reperfusion injury.

Objectives

To determine whether intracellular creatine levels influence responses to acute reactive oxygen species (ROS) exposure in the intact beating heart. We hypothesised that mice with elevated creatine due to over-expression of the creatine transporter (CrT-OE) would be relatively protected, while mice with creatine-deficiency (GAMT KO) would fare worse.

Methods and Results

CrT-OE mice were pre-selected for creatine levels 20–100% above wild-type using in vivo ¹H^–MRS. Hearts were perfused in isovolumic Langendorff mode and cardiac function monitored throughout. After 20 min equilibration, hearts were perfused with either H₂O₂ 0.5 µM (30 min), or the anti-neoplastic drug doxorubicin 15 µM (100 min). Protein carbonylation, creatine kinase isoenzyme activities and phospho-PKCδ expression were quantified in perfused hearts as markers of oxidative damage and apoptotic signalling. Wild-type hearts responded to ROS challenge with a profound decline in contractile function that was ameliorated by co-administration of catalase or dexrazoxane as positive controls. In contrast, the functional deterioration in CrT-OE and GAMT KO hearts was indistinguishable from wild-type controls, as was the extent of oxidative damage and apoptosis. Exogenous creatine supplementation also failed to protect hearts from doxorubicin-induced dysfunction.

Conclusions

Intracellular creatine levels do not influence the response to acute ROS challenge in the intact beating heart, arguing against creatine exerting (patho-)physiologically relevant anti-oxidant activity.     

The Early-Onset Myocardial Infarction Associated PHACTR1 Gene Regulates Skeletal and Cardiac Alpha-Actin Gene Expression

The phosphatase and actin regulator 1 (PHACTR1) locus is a very commonly identified hit in genome-wide association studies investigating coronary artery disease and myocardial infarction (MI). However, the function of PHACTR1 in the heart is still unknown. We characterized the mechanisms regulating Phactr1 expression in the heart, used adenoviral gene delivery to investigate the effects of Phactr1 on cardiac function, and analyzed the relationship between MI associated PHACTR1 allele and cardiac function in human subjects. Phactr1 mRNA and protein levels were markedly reduced (60%, P<0.01 and 90%, P<0.001, respectively) at 1 day after MI in rats. When the direct myocardial effects of Phactr1 were studied, the skeletal α-actin to cardiac α-actin isoform ratio was significantly higher (1.5-fold, P<0.05) at 3 days but 40% lower (P<0.05) at 2 weeks after adenovirus-mediated Phactr1 gene delivery into the anterior wall of the left ventricle. Similarly, the skeletal α-actin to cardiac α-actin ratio was lower at 2 weeks in infarcted hearts overexpressing Phactr1. In cultured neonatal cardiac myocytes, adenovirus-mediated Phactr1 overexpression for 48 hours markedly increased the skeletal α-actin to cardiac α-actin ratio, this being associated with an enhanced DNA binding activity of serum response factor. Phactr1 overexpression exerted no major effects on the expression of other cardiac genes or LV structure and function in normal and infarcted hearts during 2 weeks’ follow-up period. In human subjects, MI associated PHACTR1 allele was not associated significantly with cardiac function (n = 1550). Phactr1 seems to regulate the skeletal to cardiac α-actin isoform ratio.     

The effects of low levels of dystrophin on mouse muscle function and pathology

Duchenne muscular dystrophy (DMD) is a severe progressive muscular disorder caused by reading frame disrupting mutations in the DMD gene, preventing the synthesis of functional dystrophin. As dystrophin provides muscle fiber stability during contractions, dystrophin negative fibers are prone to exercise-induced damage. Upon exhaustion of the regenerative capacity, fibers will be replaced by fibrotic and fat tissue resulting in a progressive loss of function eventually leading to death in the early thirties. With several promising approaches for the treatment of DMD aiming at dystrophin restoration in clinical trials, there is an increasing need to determine more precisely which dystrophin levels are sufficient to restore muscle fiber integrity, protect against muscle damage and improve muscle function.To address this we generated a new mouse model (mdx-Xist ^Δhs) with varying, low dystrophin levels (3–47%, mean 22.7%, stdev 12.1, n = 24) due to skewed X-inactivation. Longitudinal sections revealed that within individual fibers, some nuclei did and some did not express dystrophin, resulting in a random, mosaic pattern of dystrophin expression within fibers. Mdx-Xist ^Δhs, mdx and wild type females underwent a 12 week functional test regime consisting of different tests to assess muscle function at base line, or after chronic treadmill running exercise. Overall, mdx-Xist ^Δhs mice with 3–14% dystrophin outperformed mdx mice in the functional tests. Improved histopathology was observed in mice with 15–29% dystrophin and these levels also resulted in normalized expression of pro-inflammatory biomarker genes, while for other parameters >30% of dystrophin was needed. Chronic exercise clearly worsened pathology, which needed dystrophin levels >20% for protection. Based on these findings, we conclude that while even dystrophin levels below 15% can improve pathology and performance, levels of >20% are needed to fully protect muscle fibers from exercise-induced damage.     

Phosphofructo-1-Kinase Deficiency Leads to a Severe Cardiac and Hematological Disorder in Addition to Skeletal Muscle Glycogenosis

Mutations in the gene for muscle phosphofructo-1-kinase (PFKM), a key regulatory enzyme of glycolysis, cause Type VII glycogen storage disease (GSDVII). Clinical manifestations of the disease span from the severe infantile form, leading to death during childhood, to the classical form, which presents mainly with exercise intolerance. PFKM deficiency is considered as a skeletal muscle glycogenosis, but the relative contribution of altered glucose metabolism in other tissues to the pathogenesis of the disease is not fully understood. To elucidate this issue, we have generated mice deficient for PFKM (Pfkm^−/−). Here, we show that Pfkm^−/− mice had high lethality around weaning and reduced lifespan, because of the metabolic alterations. In skeletal muscle, including respiratory muscles, the lack of PFK activity blocked glycolysis and resulted in considerable glycogen storage and low ATP content. Although erythrocytes of Pfkm^−/− mice preserved 50% of PFK activity, they showed strong reduction of 2,3-biphosphoglycerate concentrations and hemolysis, which was associated with compensatory reticulocytosis and splenomegaly. As a consequence of these haematological alterations, and of reduced PFK activity in the heart, Pfkm^−/− mice developed cardiac hypertrophy with age. Taken together, these alterations resulted in muscle hypoxia and hypervascularization, impaired oxidative metabolism, fiber necrosis, and exercise intolerance. These results indicate that, in GSDVII, marked alterations in muscle bioenergetics and erythrocyte metabolism interact to produce a complex systemic disorder. Therefore, GSDVII is not simply a muscle glycogenosis, and Pfkm^−/− mice constitute a unique model of GSDVII which may be useful for the design and assessment of new therapies.     

Computational analyses reveal spatial relationships between nuclear pore complexes and specific lamins

Nuclear lamin isoforms form fibrous meshworks associated with nuclear pore complexes (NPCs). Using datasets prepared from subpixel and segmentation analyses of 3D–structured illumination microscopy images of WT and lamin isoform knockout mouse embryo fibroblasts, we determined with high precision the spatial association of NPCs with specific lamin isoform fibers. These relationships are retained in the enlarged lamin meshworks of Lmna^−/− and Lmnb1^−/− fibroblast nuclei. Cryo-ET observations reveal that the lamin filaments composing the fibers contact the nucleoplasmic ring of NPCs. Knockdown of the ring-associated nucleoporin ELYS induces NPC clusters that exclude lamin A/C fibers but include LB1 and LB2 fibers. Knockdown of the nucleoporin TPR or NUP153 alters the arrangement of lamin fibers and NPCs. Evidence that the number of NPCs is regulated by specific lamin isoforms is presented. Overall the results demonstrate that lamin isoforms and nucleoporins act together to maintain the normal organization of lamin meshworks and NPCs within the nuclear envelope.