A versatile single-plasmid system for tissue-specific and inducible control of gene expression in transgenic mice

Systems for Targeted Silencing of Gene Expression and Their Application in Plants and Animals

At present, there are a variety of different approaches to the targeted regulation of gene expression. However, most approaches are devoted to the activation of gene transcription, and the methods for gene silencing are much fewer in number. In this review, we describe the main systems used for the targeted suppression of gene expression (including RNA interference (RNAi), chimeric transcription factors, chimeric zinc finger proteins, transcription activator-like effectors (TALEs)-based repressors, optogenetic tools, and CRISPR/Cas-based repressors) and their application in eukaryotes-plants and animals. We consider the advantages and disadvantages of each approach, compare their effectiveness, and discuss the peculiarities of their usage in plant and animal organisms. This review will be useful for researchers in the field of gene transcription suppression and will allow them to choose the optimal method for suppressing the expression of the gene of interest depending on the research object.

Structure, Function, and Regulation of the Junctophilin Family

In both excitable and nonexcitable cells, diverse physiological processes are linked to different calcium microdomains within nanoscale junctions that form between the plasma membrane and endo-sarcoplasmic reticula. It is now appreciated that the junctophilin protein family is responsible for establishing, maintaining, and modulating the structure and function of these junctions. We review foundational findings from more than two decades of research that have uncovered how junctophilin-organized ultrastructural domains regulate evolutionarily conserved biological processes. We discuss what is known about the junctophilin family of proteins. Our goal is to summarize the current knowledge of junctophilin domain structure, function, and regulation and to highlight emerging avenues of research that help our understanding of the transcriptional, translational, and post-translational regulation of this gene family and its roles in health and during disease.

Activation of MG53 Enhances Cell Survival and Engraftment of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes in Injured Hearts

BACKGROUND AND OBJECTIVE

Our previous studies demonstrated that MG53 protein can protect the myocardium, but its use as a therapeutic is challenging due to its short half-life in blood circulation. This study aimed to investigate the cardioprotective role of MG53 on human induced pluripotent stem cell-derived cardiomyocytes (HiPSC-CMs) in the context of myocardial ischemia/reperfusion (I/R).

METHODS

In vitro: HiPSC-CMs were transfected with adenoviral MG53 (HiPSC-CMsMG53), in which the expression of MG53 can be controlled by doxycycline (Dox), and the cells were then exposed to H2O2 to mimic ischemia/reperfusion injury. In vivo: HiPSC-CMsMG53 were transplanted into the peri-infarct region in NSG™ mice after I/R. After surgery, mice were treated with Dox (+ Dox) to activate MG53 expression (sucrose as a control of -Dox) and then assessed by echocardiography and immunohistochemistry.

RESULTS

MG53 can be expressed in HiPSC-CMMG53 and released into the culture medium after adding Dox. The cell survival rate of HiPSC-CMMG53 was improved by Dox under the H2O2 condition. After 14 and 28 days of ischemia/reperfusion (I/R), transplanted HiPSC-CMsMG53 + Dox significantly improved heart function, including ejection fraction (EF) and fractional shortening (FS) in mice, compared to HiPSC-CMsMG53-Dox, and reduced the size of the infarction. Additionally, HiPSC-CMMG53 + Dox mice demonstrated significant engraftment in the myocardium as shown by staining human nuclei-positive cells. In addition, the cell survival-related AKT signaling was found to be more active in HiPSC-CMMG53 + Dox transplanted mice's myocardium compared to the HiPSC-CMMG53-Dox group. Notably, the Dox treatment did not cause harm to other organs.

CONCLUSION

Inducible MG53 expression is a promising approach to enhance cell survival and engraftment of HiPSC-CMs for cardiac repair.

Ablation of Calsequestrin-1, Ca2+ unbalance, and susceptibility to heat stroke

Introduction: Ca2+ levels in adult skeletal muscle fibers are mainly controlled by excitation-contraction (EC) coupling, a mechanism that translates action potentials in release of Ca2+ from the sarcoplasmic reticulum (SR) release channels, i.e. the ryanodine receptors type-1 (RyR1). Calsequestrin (Casq) is a protein that binds large amounts of Ca2+ in the lumen of the SR terminal cisternae, near sites of Ca2+ release. There is general agreement that Casq is not only important for the SR ability to store Ca2+, but also for modulating the opening probability of the RyR Ca2+ release channels. The initial studies: About 20 years ago we generated a mouse model lacking Casq1 (Casq1-null mice), the isoform predominantly expressed in adult fast twitch skeletal muscle. While the knockout was not lethal as expected, lack of Casq1 caused a striking remodeling of membranes of SR and of transverse tubules (TTs), and mitochondrial damage. Functionally, CASQ1-knockout resulted in reduced SR Ca2+ content, smaller Ca2+ transients, and severe SR depletion during repetitive stimulation. The myopathic phenotype of Casq1-null mice: After the initial studies, we discovered that Casq1-null mice were prone to sudden death when exposed to halogenated anaesthetics, heat and even strenuous exercise. These syndromes are similar to human malignant hyperthermia susceptibility (MHS) and environmental-exertional heat stroke (HS). We learned that mechanisms underlying these syndromes involved excessive SR Ca2+ leak and excessive production of oxidative species: indeed, mortality and mitochondrial damage were significantly prevented by administration of antioxidants and reduction of oxidative stress. Though, how Casq1-null mice could survive without the most important SR Ca2+ binding protein was a puzzling issue that was not solved. Unravelling the mystery: The mystery was finally solved in 2020, when we discovered that in Casq1-null mice the SR undergoes adaptations that result in constitutively active store-operated Ca2+ entry (SOCE). SOCE is a mechanism that allows skeletal fibers to use external Ca2+ when SR stores are depleted. The post-natal compensatory mechanism that allows Casq1-null mice to survive involves the assembly of new SR-TT junctions (named Ca2+ entry units) containing Stim1 and Orai1, the two proteins that mediate SOCE.

MG53 suppresses tumor progression and stress granule formation by modulating G3BP2 activity in non-small cell lung cancer

Background

Cancer cells develop resistance to chemotherapeutic intervention by excessive formation of stress granules (SGs), which are modulated by an oncogenic protein G3BP2. Selective control of G3BP2/SG signaling is a potential means to treat non-small cell lung cancer (NSCLC).

Methods

Co-immunoprecipitation was conducted to identify the interaction of MG53 and G3BP2. Immunohistochemistry and live cell imaging were performed to visualize the subcellular expression or co-localization. We used shRNA to knock-down the expression MG53 or G3BP2 to test the cell migration and colony formation. The expression level of MG53 and G3BP2 in human NSCLC tissues was tested by western blot analysis. The ATO-induced oxidative stress model was used to examine the effect of rhMG53 on SG formation. Moue NSCLC allograft experiments were performed on wild type and transgenic mice with either knockout of MG53, or overexpression of MG53. Human NSCLC xenograft model in mice was used to evaluate the effect of MG53 overexpression on tumorigenesis.

Results

We show that MG53, a member of the TRIM protein family (TRIM72), modulates G3BP2 activity to control lung cancer progression. Loss of MG53 results in the progressive development of lung cancer in mg53^-/- mice. Transgenic mice with sustained elevation of MG53 in the bloodstream demonstrate reduced tumor growth following allograft transplantation of mouse NSCLC cells. Biochemical assay reveals physical interaction between G3BP2 and MG53 through the TRIM domain of MG53. Knockdown of MG53 enhances proliferation and migration of NSCLC cells, whereas reduced tumorigenicity is seen in NSCLC cells with knockdown of G3BP2 expression. The recombinant human MG53 (rhMG53) protein can enter the NSCLC cells to induce nuclear translation of G3BP2 and block arsenic trioxide-induced SG formation. The anti-proliferative effect of rhMG53 on NSCLC cells was abolished with knockout of G3BP2. rhMG53 can enhance sensitivity of NSCLC cells to undergo cell death upon treatment with cisplatin. Tailored induction of MG53 expression in NSCLC cells suppresses lung cancer growth via reduced SG formation in a xenograft model.

Conclusion

Overall, these findings support the notion that MG53 functions as a tumor suppressor by targeting G3BP2/SG activity in NSCLCs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12943-021-01418-3.

Calcium entry units (CEUs): perspectives in skeletal muscle function and disease

In the last decades the term Store-operated Ca²⁺ entry (SOCE) has been used in the scientific literature to describe an ubiquitous cellular mechanism that allows recovery of calcium (Ca²⁺) from the extracellular space. SOCE is triggered by a reduction of Ca²⁺ content (i.e. depletion) in intracellular stores, i.e. endoplasmic or sarcoplasmic reticulum (ER and SR). In skeletal muscle the mechanism is primarily mediated by a physical interaction between stromal interaction molecule-1 (STIM1), a Ca²⁺ sensor located in the SR membrane, and ORAI1, a Ca²⁺-permeable channel of external membranes, located in transverse tubules (TTs), the invaginations of the plasma membrane (PM) deputed to propagation of action potentials. It is generally accepted that in skeletal muscle SOCE is important to limit muscle fatigue during repetitive stimulation. We recently discovered that exercise promotes the assembly of new intracellular junctions that contains colocalized STIM1 and ORAI1, and that the presence of these new junctions increases Ca²⁺ entry via ORAI1, while improving fatigue resistance during repetitive stimulation. Based on these findings we named these new junctions Ca²⁺ Entry Units (CEUs). CEUs are dynamic organelles that assemble during muscle activity and disassemble during recovery thanks to the plasticity of the SR (containing STIM1) and the elongation/retraction of TTs (bearing ORAI1). Interestingly, similar structures described as SR stacks were previously reported in different mouse models carrying mutations in proteins involved in Ca²⁺ handling (calsequestrin-null mice; triadin and junctin null mice, etc.) or associated to microtubules (MAP6 knockout mice). Mutations in Stim1 and Orai1 (and calsequestrin-1) genes have been associated to tubular aggregate myopathy (TAM), a muscular disease characterized by: (a) muscle pain, cramping, or weakness that begins in childhood and worsens over time, and (b) the presence of large accumulations of ordered SR tubes (tubular aggregates, TAs) that do not contain myofibrils, mitochondria, nor TTs. Interestingly, TAs are also present in fast twitch muscle fibers of ageing mice. Several important issues remain un-answered: (a) the molecular mechanisms and signals that trigger the remodeling of membranes and the functional activation of SOCE during exercise are unclear; and (b) how dysfunctional SOCE and/or mutations in Stim1, Orai1 and calsequestrin (Casq1) genes lead to the formation of tubular aggregates (TAs) in aging and disease deserve investigation.

Transverse tubule remodeling enhances Orai1-dependent Ca2+ entry in skeletal muscle

Exercise promotes the formation of intracellular junctions in skeletal muscle between stacks of sarcoplasmic reticulum (SR) cisternae and extensions of transverse-tubules (TT) that increase co-localization of proteins required for store-operated Ca²⁺ entry (SOCE). Here, we report that SOCE, peak Ca²⁺ transient amplitude and muscle force production during repetitive stimulation are increased after exercise in parallel with the time course of TT association with SR-stacks. Unexpectedly, exercise also activated constitutive Ca²⁺ entry coincident with a modest decrease in total releasable Ca²⁺ store content. Importantly, this decrease in releasable Ca²⁺ store content observed after exercise was reversed by repetitive high-frequency stimulation, consistent with enhanced SOCE. The functional benefits of exercise on SOCE, constitutive Ca²⁺ entry and muscle force production were lost in mice with muscle-specific loss of Orai1 function. These results indicate that TT association with SR-stacks enhances Orai1-dependent SOCE to optimize Ca²⁺ dynamics and muscle contractile function during acute exercise.

PGC-1a integrates a metabolism and growth network linked to caloric restriction

Deleterious changes in energy metabolism have been linked to aging and disease vulnerability, while activation of mitochondrial pathways has been linked to delayed aging by caloric restriction (CR). The basis for these associations is poorly understood, and the scope of impact of mitochondrial activation on cellular function has yet to be defined. Here, we show that mitochondrial regulator PGC-1a is induced by CR in multiple tissues, and at the cellular level, CR-like activation of PGC-1a impacts a network that integrates mitochondrial status with metabolism and growth parameters. Transcriptional profiling reveals that diverse functions, including immune pathways, growth, structure, and macromolecule homeostasis, are responsive to PGC-1a. Mechanistically, these changes in gene expression were linked to chromatin remodeling and RNA processing. Metabolic changes implicated in the transcriptional data were confirmed functionally including shifts in NAD metabolism, lipid metabolism, and membrane lipid composition. Delayed cellular proliferation, altered cytoskeleton, and attenuated growth signaling through post-transcriptional and post-translational mechanisms were also identified as outcomes of PGC-1a-directed mitochondrial activation. Furthermore, in vivo in tissues from a genetically heterogeneous mouse population, endogenous PGC-1a expression was correlated with this same metabolism and growth network. These data show that small changes in metabolism have broad consequences that arguably would profoundly alter cell function. We suggest that this PGC-1a sensitive network may be the basis for the association between mitochondrial function and aging where small deficiencies precipitate loss of function across a spectrum of cellular activities.

Exercise-dependent formation of new junctions that promote STIM1-Orai1 assembly in skeletal muscle

Store-operated Ca²⁺ entry (SOCE), a ubiquitous mechanism that allows recovery of Ca²⁺ ions from the extracellular space, has been proposed to limit fatigue during repetitive skeletal muscle activity. However, the subcellular location for SOCE in muscle fibers has not been unequivocally identified. Here we show that exercise drives a significant remodeling of the sarcotubular system to form previously unidentified junctions between the sarcoplasmic reticulum (SR) and transverse-tubules (TTs). We also demonstrate that these new SR-TT junctions contain the molecular machinery that mediate SOCE: stromal interaction molecule-1 (STIM1), which functions as the SR Ca²⁺ sensor, and Orai1, the Ca²⁺-permeable channel in the TT. In addition, EDL muscles isolated from exercised mice exhibit an increased capability of maintaining contractile force during repetitive stimulation in the presence of 2.5 mM extracellular Ca²⁺, compared to muscles from control mice. This functional difference is significantly reduced by either replacement of extracellular Ca²⁺ with Mg²⁺ or the addition of SOCE inhibitors (BTP-2 and 2-APB). We propose that the new SR-TT junctions formed during exercise, and that contain STIM1 and Orai1, function as Ca²⁺Entry Units (CEUs), structures that provide a pathway to rapidly recover Ca²⁺ ions from the extracellular space during repetitive muscle activity.

Selective inhibitory effects of zinc on cell proliferation in esophageal squamous cell carcinoma through Orai1

Zinc, an essential micronutrient, has a cancer preventive role. Zinc deficiency has been shown to contribute to the progression of esophageal cancer. Orai1, a store-operated Ca²⁺ entry (SOCE) channel, was previously reported to be highly expressed in tumor tissues removed from patients with esophageal squamous cell carcinoma (ESCC) with poor prognosis, and elevation of its expression contributes to both hyperactive intracellular Ca²⁺ oscillations and fast cell proliferation in human ESCC cells. However, the molecular basis of cancer preventive functions of zinc and its association with Orai1-mediated cell proliferation remains unknown. The present study shows that zinc supplementation significantly inhibits proliferation of ESCC cell lines and that the effect of zinc is reversible with N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine, a specific Zn²⁺ chelator, whereas nontumorigenic esophageal epithelial cells are significantly less sensitive to zinc treatment. Fluorescence live cell imaging revealed that extracellular Zn²⁺ exerted rapid inhibitory effects on Orai1-mediated SOCE and on intracellular Ca²⁺ oscillations in the ESCC cells. Knockdown of Orai1 or expression of Orai1 mutants with compromised zinc binding significantly diminished sensitivity of the cancer cells to zinc treatment in both SOCE and cell proliferation analyses. These data suggest that zinc may inhibit cell proliferation of esophageal cancer cells through Orai1-mediated intracellular Ca²⁺ oscillations and reveal a possible molecular basis for zinc-induced cancer prevention and Orai1-SOCE signaling pathway in cancer cells.-Choi, S., Cui, C., Luo, Y., Kim, S.-H., Ko, J.-K., Huo, X., Ma, J., Fu, L.-W., Souza, R. F., Korichneva, I., Pan, Z. Selective inhibitory effects of zinc on cell proliferation in esophageal squamous cell carcinoma through Orai1.

Characterization of fast-twitch and slow-twitch skeletal muscles of calsequestrin 2 (CASQ2)-knock out mice: unexpected adaptive changes of fast-twitch muscles only

This study investigates the functional role of calsequestrin 2 (CASQ2) in both fast-twitch and slow-twitch skeletal muscles by using CASQ2-/- mice; CASQ2 is expressed throughout life in slow-twitch muscles, but only in the developmental and neonatal stages in fast-twitch muscles. CASQ2-/- causes increase in calsequestrin 1 (CASQ1) expression, but without functional changes in both muscle types. CASQ2-/- mice have ultrastructural changes in fast-twitch muscles only, i.e., formation of pentads and stacks in the sarcoplasmic reticulum.

Cell Type-Specific and Inducible PTEN Gene Silencing by a Tetracycline Transcriptional Activator-Regulated Short Hairpin RNA

Inducible and reversible gene silencing in desired types of cells is instrumental for deciphering gene functions using cultured cells or in vivo models. However, efficient conditional gene knockdown systems remain to be established. Here, we report the generation of an inducible expression system for short hairpin RNA (shRNA) targeted to PTEN, a well-documented dual-specificity phosphatase involved in tumor suppression and ontogenesis. Upon induction by doxycycline (DOX), the reverse tetracycline transcriptional activator (rtTA) switched on the concomitant expression of GFP and a miR-30 precursor, the subsequent processing of which released the embedded PTEN-targeted shRNA. The efficacy and reversibility of PTEN knockdown by this construct was validated in normal and neoplastic cells, in which PTEN deficiency resulted in accelerated cell proliferation, suppressed apoptosis, and increased invasiveness. Transgenic mice harboring the conditional shRNA-expression cassette were obtained; GFP expression and concurrent PTEN silencing were observed upon ectopic expression of rtTA and induction with Dox. Therefore, this study provides novel tools for the precise dissection of PTEN functions and the generation of PTEN loss of function models in specific subsets of cells during carcinogenesis and ontogenesis.

Junctophilin-1 is a modifier gene of GDAP1-related Charcot-Marie-Tooth disease

Mutations in the GDAP1 gene cause different forms of Charcot-Marie-Tooth (CMT) disease, and the primary clinical expression of this disease is markedly variable in the dominant inheritance form (CMT type 2K; CMT2K), in which carriers of the GDAP1 p.R120W mutation can display a wide range of clinical severity. We investigated the JPH1 gene as a genetic modifier of clinical expression variability because junctophilin-1 (JPH1) is a good positional and functional candidate. We demonstrated that the JPH1-GDAP1 cluster forms a paralogon and is conserved in vertebrates. Moreover, both proteins play a role in Ca(2+) homeostasis, and we demonstrated that JPH1 is able to restore the store-operated Ca(2+) entry (SOCE) activity in GDAP1-silenced cells. After the mutational screening of JPH1 in a series of 24 CMT2K subjects who harbour the GDAP1 p.R120W mutation, we characterized the JPH1 p.R213P mutation in one patient with a more severe clinical picture. JPH1(p.R213P) cannot rescue the SOCE response in GDAP1-silenced cells. We observed that JPH1 colocalizes with STIM1, which is the activator of SOCE, in endoplasmic reticulum-plasma membrane puncta structures during Ca(2+) release in a GDAP1-dependent manner. However, when GDAP1(p.R120W) is expressed, JPH1 seems to be retained in mitochondria. We also established that the combination of GDAP1(p.R120W) and JPH1(p.R213P) dramatically reduces SOCE activity, mimicking the effect observed in GDAP1 knock-down cells. In summary, we conclude that JPH1 and GDAP1 share a common pathway and depend on each other; therefore, JPH1 can contribute to the phenotypical consequences of GDAP1 mutations.

TRIM72 is required for effective repair of alveolar epithelial cell wounding

The molecular mechanisms for lung cell repair are largely unknown. Previous studies identified tripartite motif protein 72 (TRIM72) from striated muscle and linked its function to tissue repair. In this study, we characterized TRIM72 expression in lung tissues and investigated the role of TRIM72 in repair of alveolar epithelial cells. In vivo injury of lung cells was introduced by high tidal volume ventilation, and repair-defective cells were labeled with postinjury administration of propidium iodide. Primary alveolar epithelial cells were isolated and membrane wounding and repair were labeled separately. Our results show that absence of TRIM72 increases susceptibility to deformation-induced lung injury whereas TRIM72 overexpression is protective. In vitro cell wounding assay revealed that TRIM72 protects alveolar epithelial cells through promoting repair rather than increasing resistance to injury. The repair function of TRIM72 in lung cells is further linked to caveolin 1. These data suggest an essential role for TRIM72 in repair of alveolar epithelial cells under plasma membrane stress failure.

CRISPR transcriptional repression devices and layered circuits in mammalian cells

A key obstacle to creating sophisticated genetic circuits has been the lack of scalable device libraries. Here we present a modular transcriptional repression architecture based on clustered regularly interspaced palindromic repeats (CRISPR) system and examine approaches for regulated expression of guide RNAs in human cells. Subsequently we demonstrate that CRISPR regulatory devices can be layered to create functional cascaded circuits, which provide a valuable toolbox for engineering purposes.

Inducible, dose-adjustable and time-restricted reconstitution of STAT1 deficiency in vivo

Signal transducer and activator of transcription (STAT) 1 is a key player in interferon (IFN) signaling, essential in mediating host defense against viruses and other pathogens. STAT1 levels are tightly regulated and loss- or gain-of-function mutations in mice and men lead to severe diseases. We have generated a doxycycline (dox) -inducible, FLAG-tagged Stat1 expression system in mice lacking endogenous STAT1 (i.e. Stat1^ind mice). We show that STAT1 expression depends on the time and dose of dox treatment in primary cells and a variety of organs isolated from Stat1^ind mice. In bone marrow-derived macrophages, a fraction of the amount of STAT1 present in WT cells is sufficient for full expression of IFN-induced genes. Dox-induced STAT1 established protection against virus infections in primary cells and mice. The availability of the Stat1^ind mouse model will enable an examination of the consequences of variable amounts of STAT1. The model will also permit the study of STAT1 dose-dependent and reversible functions as well as of STAT1's contributions to the development, progression and resolution of disease.

A novel piggyBac transposon inducible expression system identifies a role for AKT signalling in primordial germ cell migration

In this work, we describe a single piggyBac transposon system containing both a tet-activator and a doxycycline-inducible expression cassette. We demonstrate that a gene product can be conditionally expressed from the integrated transposon and a second gene can be simultaneously targeted by a short hairpin RNA contained within the transposon, both in vivo and in mammalian and avian cell lines. We applied this system to stably modify chicken primordial germ cell (PGC) lines in vitro and induce a reporter gene at specific developmental stages after injection of the transposon-modified germ cells into chicken embryos. We used this vector to express a constitutively-active AKT molecule during PGC migration to the forming gonad. We found that PGC migration was retarded and cells could not colonise the forming gonad. Correct levels of AKT activation are thus essential for germ cell migration during early embryonic development.

Inducible and reversible regulation of endogenous gene in mouse

Methods for generating loss-of-function mutations, such as conventional or conditional gene knockout, are widely used in deciphering gene function in vivo. By contrast, inducible and reversible regulation of endogenous gene expression has not been well established. Using a mouse model, we demonstrate that a chimeric transcriptional repressor molecule (tTS) can reversibly inhibit the expression of an endogenous gene, Nmyc. In this system, a tetracycline response element (TRE) artificially inserted near the target gene’s promoter region turns the gene on and off in a tetracycline-inducible manner. Nmyc^TRE mice were generated by inserting a TRE into the first intron of Nmyc by the knockin technique. Nmyc^TRE mice were crossed to tTS transgenic mice to produce Nmyc^TRE/TRE: tTS embryos. In these embryos, tTS blocked Nmyc expression, and embryonic lethality was observed at E11.5d. When the dam was exposed to drinking water containing doxycycline (dox), normal endogenous Nmyc expression was rescued, and the embryo survived to birth. This novel genetic modification strategy based on the tTS–dox system for inducible and reversible regulation of endogenous mouse genes will be a powerful tool to investigate target genes that cause embryonic lethality or other defects where reversible regulation or temporary shutdown of the target gene is needed.

Tubular aggregates in skeletal muscle: just a special type of protein aggregates?

Tubular aggregates are inclusions, usually found in type II muscle fibers and in males, consisting of regular arrays of tubules derived from the sarcoplasmic reticulum. Tubular aggregates are associated with a wide variety of muscle disorders, including poorly defined "tubular aggregate myopathies" characterized by weakness and/or myalgia and/or cramps, and are also present in different mouse models, including normal aging muscles. The mechanism(s) responsible for inducing the formation of these structures have not been identified, because of the slow time course of their development in vivo, several months in mice. However, identical structures are formed in a few hours in rat muscles kept in vitro in hypoxic medium. Here I suggest that tubular aggregates result from reshaping of sarcoplasmic reticulum caused by misfolding and aggregation of membrane proteins and thus represent a special type of "protein aggregates" due to altered proteostasis.