Seeding competent TDP-43 persists in human patient and mouse muscle.

TAR DNA-binding protein 43 (TDP-43) is an RNA binding protein that accumulates as aggregates in the central nervous system of some neurodegenerative diseases. However, TDP-43 aggregation is also a sensitive and specific pathologic feature found in a family of degenerative muscle diseases termed inclusion body myopathy (IBM). TDP-43 aggregates from ALS and FTD brain lysates may serve as self-templating aggregate seeds in vitro and in vivo, supporting a prion-like spread from cell to cell. Whether a similar process occurs in IBM patient muscle is not clear. We developed a mouse model of inducible, muscle-specific cytoplasmic localized TDP-43. These mice develop muscle weakness with robust accumulation of insoluble and phosphorylated sarcoplasmic TDP-43, leading to eosinophilic inclusions, altered proteostasis and changes in TDP-43-related RNA processing that resolve with the removal of doxycycline. Skeletal muscle lysates from these mice also have seeding competent TDP-43, as determined by a FRET-based biosensor, that persists for weeks upon resolution of TDP-43 aggregate pathology. Human muscle biopsies with TDP-43 pathology also contain TDP-43 aggregate seeds. Using lysates from muscle biopsies of patients with IBM, IMNM and ALS we found that TDP-43 seeding capacity was specific to IBM. Surprisingly, TDP-43 seeding capacity anti-correlated with TDP-43 aggregate and vacuole abundance. These data support that TDP-43 aggregate seeds are present in IBM skeletal muscle and represent a unique TDP-43 pathogenic species not previously appreciated in human muscle disease.

DNAJB6 isoform specific knockdown: Therapeutic potential for limb girdle muscular dystrophy D1.

Dominant missense mutations in DNAJB6, a co-chaperone of HSP70, cause limb girdle muscular dystrophy (LGMD) D1. No treatments are currently available. Two isoforms exist, DNAJB6a and DNAJB6b, each with distinct localizations in muscle. Mutations reside in both isoforms, yet evidence suggests that DNAJB6b is primarily responsible for disease pathogenesis. Knockdown treatment strategies involving both isoforms carry risk, as DNAJB6 knockout is embryonic lethal. We therefore developed an isoform-specific knockdown approach using morpholinos. Selective reduction of each isoform was achieved in vitro in primary mouse myotubes and human LGMDD1 myoblasts, as well as in vivo in mouse skeletal muscle. To assess isoform specific knockdown in LGMDD1, we created primary myotube cultures from a knockin LGMDD1 mouse model. Using mass spectrometry, we identified an LGMDD1 protein signature related to protein homeostasis and myofibril structure. Selective reduction of DNAJB6b levels in LGMDD1 myotubes corrected much of the proteomic disease signature toward wild type levels. Additional in vivo functional data is required to determine if selective reduction of DNAJB6b is a viable therapeutic target for LGMDD1.

Genetic deletion of skeletal muscle iPLA2γ results in mitochondrial dysfunction, muscle atrophy and alterations in whole-body energy metabolism.

Skeletal muscle is the major site of glucose utilization in mammals integrating serum glucose clearance with mitochondrial respiration. To mechanistically elucidate the roles of iPLA2γ in skeletal muscle mitochondria, we generated a skeletal muscle-specific calcium-independent phospholipase A2γ knockout (SKMiPLA2γKO) mouse. Genetic ablation of skeletal muscle iPLA2γ resulted in pronounced muscle weakness, muscle atrophy, and increased blood lactate resulting from defects in mitochondrial function impairing metabolic processing of pyruvate and resultant bioenergetic inefficiency. Mitochondria from SKMiPLA2γKO mice were dysmorphic displaying marked changes in size, shape, and interfibrillar juxtaposition. Mitochondrial respirometry demonstrated a marked impairment in respiratory efficiency with decreases in the mass and function of oxidative phosphorylation complexes and cytochrome c. Further, a pronounced decrease in mitochondrial membrane potential and remodeling of cardiolipin molecular species were prominent. Collectively, these alterations prevented body weight gain during high-fat feeding through enhanced glucose disposal without efficient capture of chemical energy thereby altering whole-body bioenergetics.

Comprehensive functional characterization of SGCB coding variants predicts pathogenicity in limb-girdle muscular dystrophy type R4/2E.

Genetic testing is essential for patients with a suspected hereditary myopathy. More than 50% of patients clinically diagnosed with a myopathy carry a variant of unknown significance in a myopathy gene, often leaving them without a genetic diagnosis. Limb-girdle muscular dystrophy (LGMD) type R4/2E is caused by mutations in ß-sarcoglycan (SGCB). Together, ß-, α-, γ-, and δ-sarcoglycan form a 4-protein transmembrane complex (SGC) that localizes to the sarcolemma. Biallelic loss-of-function mutations in any subunit can lead to LGMD. To provide functional evidence for the pathogenicity of missense variants, we performed deep mutational scanning of SGCB and assessed SGC cell surface localization for all 6,340 possible amino acid changes. Variant functional scores were bimodally distributed and perfectly predicted pathogenicity of known variants. Variants with less severe functional scores more often appeared in patients with slower disease progression, implying a relationship between variant function and disease severity. Amino acid positions intolerant to variation mapped to points of predicted SGC interactions, validated in silico structural models, and enabled accurate prediction of pathogenic variants in other SGC genes. These results will be useful for clinical interpretation of SGCB variants and improving diagnosis of LGMD; we hope they enable wider use of potentially life-saving gene therapy.

Connective tissue presentation in two families expands the phenotypic spectrum of PYROXD1 disorders.

Recessive variants in the oxidoreductase PYROXD1 are reported to cause a myopathy in 22 affected individuals from 15 families. Here, we describe two female probands from unrelated families presenting with features of a congenital connective tissue disorder including osteopenia, blue sclera, soft skin, joint hypermobility and neuromuscular junction dysfunction in addition to known features of PYROXD1 myopathy including respiratory difficulties, weakness, hypotonia and oromotor dysfunction. Proband AII:1 is compound heterozygous for the recurrent PYROXD1 variant Chr12(GRCh38):g.21452130A>G;NM_024854.5:c.464A>G;p.(N155S) and Chr12(GRCh38):g.21462019_21462022del;NM_024854.5:c.892_895del;p.(V298Mfs*4) and proband BII:1 is compound heterozygous for Chr12(GRCh38):g.21468739-21468741del;NM_024854.5:c.1488_1490del;p.(E496del) and Chr12(GRCh38):g.21467619del;NM_024854.5:c.1254+1del. RNA studies demonstrate c.892_895del;p.(V298Mfs*4) is targeted by nonsense mediated decay and c.1254+1delG elicits in-frame skipping of exon-11. Western blot from cultured fibroblasts shows reduced PYROXD1 protein levels in both probands. Testing urine from BII:1 and six individuals with PYROXD1 myopathy showed elevated levels of deoxypyridinoline, a mature collagen crosslink, correlating with PYROXD1-disorder severity. Urine and serum amino acid testing of the same individuals revealed no reportable changes. In contrast to PYROXD1 knock-out, we find no evidence for disrupted tRNA ligase activity, as measured via XBP1 splicing, in fibroblasts expressing PYROXD1 variants. In summary, we expand the clinical spectrum of PYROXD1-related disorders to include an overlapping connective tissue and myopathy presentation, identify three novel, pathogenic PYROXD1 variants, and provide preliminary evidence that elevated urine DPD crosslinks may provide a clinical biomarker for PYROXD1 disorders. Our results advocate consideration of PYROXD1 variants in the differential diagnosis for undiagnosed individuals presenting with a connective tissue disorder and myopathy.

Distinctive chaperonopathy in skeletal muscle associated with the dominant variant in DNAJB4.

DnaJ homolog, subfamily B, member 4, a member of the heat shock protein 40 chaperones encoded by DNAJB4, is highly expressed in myofibers. We identified a heterozygous c.270 T > A (p.F90L) variant in DNAJB4 in a family with a dominantly inherited distal myopathy, in which affected members have specific features on muscle pathology represented by the presence of cytoplasmic inclusions and the accumulation of desmin, p62, HSP70, and DNAJB4 predominantly in type 1 fibers. Both Dnajb4F90L knockin and knockout mice developed muscle weakness and recapitulated the patient muscle pathology in the soleus muscle, where DNAJB4 has the highest expression. These data indicate that the identified variant is causative, resulting in defective chaperone function and selective muscle degeneration in specific muscle fibers. This study demonstrates the importance of DNAJB4 in skeletal muscle proteostasis by identifying the associated chaperonopathy.

VCP suppresses proteopathic seeding in neurons.

BACKGROUND: Neuronal uptake and subsequent spread of proteopathic seeds, such as αS (alpha-synuclein), Tau, and TDP-43, contribute to neurodegeneration. The cellular machinery participating in this process is poorly understood. One proteinopathy called multisystem proteinopathy (MSP) is associated with dominant mutations in Valosin Containing Protein (VCP). MSP patients have muscle and neuronal degeneration characterized by aggregate pathology that can include αS, Tau and TDP-43. METHODS: We performed a fluorescent cell sorting based genome-wide CRISPR-Cas9 screen in αS biosensors. αS and TDP-43 seeding activity under varied conditions was assessed using FRET/Flow biosensor cells or immunofluorescence for phosphorylated αS or TDP-43 in primary cultured neurons. We analyzed in vivo seeding activity by immunostaining for phosphorylated αS following intrastriatal injection of αS seeds in control or VCP disease mutation carrying mice. RESULTS: One hundred fifty-four genes were identified as suppressors of αS seeding. One suppressor, VCP when chemically or genetically inhibited increased αS seeding in cells and neurons. This was not due to an increase in αS uptake or αS protein levels. MSP-VCP mutation expression increased αS seeding in cells and neurons. Intrastriatal injection of αS preformed fibrils (PFF) into VCP-MSP mutation carrying mice increased phospho αS expression as compared to control mice. Cells stably expressing fluorescently tagged TDP-43 C-terminal fragment FRET pairs (TDP-43 biosensors) generate FRET when seeded with TDP-43 PFF but not monomeric TDP-43. VCP inhibition or MSP-VCP mutant expression increases TDP-43 seeding in TDP-43 biosensors. Similarly, treatment of neurons with TDP-43 PFFs generates high molecular weight insoluble phosphorylated TDP-43 after 5 days. This TDP-43 seed dependent increase in phosphorlyated TDP-43 is further augmented in MSP-VCP mutant expressing neurons. CONCLUSION: Using an unbiased screen, we identified the multifunctional AAA ATPase VCP as a suppressor of αS and TDP-43 aggregate seeding in cells and neurons. VCP facilitates the clearance of damaged lysosomes via lysophagy. We propose that VCP's surveillance of permeabilized endosomes may protect against the proteopathic spread of pathogenic protein aggregates. The spread of distinct aggregate species may dictate the pleiotropic phenotypes and pathologies in VCP associated MSP.

Neuronal VCP loss of function recapitulates FTLD-TDP pathology.

The pathogenic mechanism by which dominant mutations in VCP cause multisystem proteinopathy (MSP), a rare neurodegenerative disease that presents as fronto-temporal lobar degeneration with TDP-43 inclusions (FTLD-TDP), remains unclear. To explore this, we inactivate VCP in murine postnatal forebrain neurons (VCP conditional knockout [cKO]). VCP cKO mice have cortical brain atrophy, neuronal loss, autophago-lysosomal dysfunction, and TDP-43 inclusions resembling FTLD-TDP pathology. Conditional expression of a single disease-associated mutation, VCP-R155C, in a VCP null background similarly recapitulates features of VCP inactivation and FTLD-TDP, suggesting that this MSP mutation is hypomorphic. Comparison of transcriptomic and proteomic datasets from genetically defined patients with FTLD-TDP reveal that progranulin deficiency and VCP insufficiency result in similar profiles. These data identify a loss of VCP-dependent functions as a mediator of FTLD-TDP and reveal an unexpected biochemical similarity with progranulin deficiency.

Inhibition of DNAJ-HSP70 interaction improves strength in muscular dystrophy.

Dominant mutations in the HSP70 cochaperone DNAJB6 cause a late-onset muscle disease termed limb-girdle muscular dystrophy type D1 (LGMDD1), which is characterized by protein aggregation and vacuolar myopathology. Disease mutations reside within the G/F domain of DNAJB6, but the molecular mechanisms underlying dysfunction are not well understood. Using yeast, cell culture, and mouse models of LGMDD1, we found that the toxicity associated with disease-associated DNAJB6 required its interaction with HSP70 and that abrogating this interaction genetically or with small molecules was protective. In skeletal muscle, DNAJB6 localizes to the Z-disc with HSP70. Whereas HSP70 normally diffused rapidly between the Z-disc and sarcoplasm, the rate of diffusion of HSP70 in LGMDD1 mouse muscle was diminished, probably because it had an unusual affinity for the Z-disc and mutant DNAJB6. Treating LGMDD1 mice with a small-molecule inhibitor of the DNAJ-HSP70 complex remobilized HSP70, improved strength, and corrected myopathology. These data support a model in which LGMDD1 mutations in DNAJB6 are a gain-of-function disease that is, counterintuitively, mediated via HSP70 binding. Thus, therapeutic approaches targeting HSP70-DNAJB6 may be effective in treating this inherited muscular dystrophy.

Desmin forms toxic, seeding-competent amyloid aggregates that persist in muscle fibers.

Desmin-associated myofibrillar myopathy (MFM) has pathologic similarities to neurodegeneration-associated protein aggregate diseases. Desmin is an abundant muscle-specific intermediate filament, and disease mutations lead to its aggregation in cells, animals, and patients. We reasoned that similar to neurodegeneration-associated proteins, desmin itself may form amyloid. Desmin peptides corresponding to putative amyloidogenic regions formed seeding-competent amyloid fibrils. Amyloid formation was increased when disease-associated mutations were made within the peptide, and this conversion was inhibited by the anti-amyloid compound epigallocatechin-gallate. Moreover, a purified desmin fragment (aa 117 to 348) containing both amyloidogenic regions formed amyloid fibrils under physiologic conditions. Desmin fragment-derived amyloid coaggregated with full-length desmin and was able to template its conversion into fibrils in vitro. Desmin amyloids were cytotoxic to myotubes and disrupted their myofibril organization compared with desmin monomer or other nondesmin amyloids. Finally, desmin fragment amyloid persisted when introduced into mouse skeletal muscle. These data suggest that desmin forms seeding-competent amyloid that is toxic to myofibers. Moreover, small molecules known to interfere with amyloid formation and propagation may have therapeutic potential in MFM.

Novel mutation in TNPO3 causes congenital limb-girdle myopathy with slow progression.

OBJECTIVE: We report a second family with autosomal dominant transportinopathy presenting with congenital or early-onset myopathy and slow progression, causing proximal and less pronounced distal muscle weakness. METHODS: Patients had clinical examinations, muscle MRI, EMG, and muscle biopsy studies. The MYOcap gene panel was used to identify the gene defect in the family. Muscle biopsies were used for histopathologic and protein expression studies, and TNPO3 constructs were used to study the effect of the mutations in transfected cells. RESULTS: We identified a novel heterozygous mutation, c.2757delC, in the last part of the transportin-3 (TNPO3) gene in the affected family members. The mutation causes an almost identical frameshift affecting the stop codon and elongating the C-term protein product of the TNPO3 transcript, as was previously reported in the first large Spanish-Italian LGMD1F kindred. TNPO3 protein was increased in the patient muscle and accumulated in the subsarcolemmal and perinuclear areas. At least one of the cargo proteins, the splicing factor SRRM2 was normally located in the nucleus. Transiently transfected mutant TNPO3 constructs failed to localize to cytoplasmic annulate lamellae pore complexes in cells. CONCLUSIONS: We report the clinical, molecular genetic, and histopathologic features of the second transportinopathy family. The variability of the clinical phenotype together with histopathologic findings suggests that several molecular pathways may be involved in the disease pathomechanism, such as nucleocytoplasmic shuttling, protein aggregation, and defective protein turnover.

Lithium chloride corrects weakness and myopathology in a preclinical model of LGMD1D.

OBJECTIVE: To understand DNAJB6's function in skeletal muscle and identify therapeutic targets for limb-girdle muscular dystrophy 1D (LGMD1D). METHODS: DNAJB6 knockout (KO) myoblasts were generated with Crispr/cas9 technology, and differentially accumulated proteins were identified using stable isotope labeling, followed by quantitative mass spectrometry. Cultured KO myotubes and mouse muscle from DNAJB6b-WT or DNAJB6b-F93L mice were analyzed using histochemistry, immunohistochemistry, and immunoblot. Mouse functional strength measures included forelimb grip strength and inverted wire hang. RESULTS: DNAJB6 inactivation leads to the accumulation of sarcomeric proteins and hypertrophic myotubes with an enhanced fusion index. The increased fusion in DNAJB6 KO myotubes correlates with diminished glycogen synthase kinase-ß (GSK3ß) activity. In contrast, LGMD1D mutations in DNAJB6 enhance GSK3ß activation and suppress ß-catenin and NFAT3c signaling. GSK3ß inhibition with lithium chloride improves muscle size and strength in an LGMD1D preclinical mouse model. CONCLUSIONS: Our results suggest that DNAJB6 facilitates protein quality control and negatively regulates myogenic signaling. In addition, LGMD1D-associated DNAJB6 mutations inhibit myogenic signaling through augmented GSK3ß activity. GSK3ß inhibition with lithium chloride may be a therapeutic option in LGMD1D.

VCP maintains lysosomal homeostasis and TFEB activity in differentiated skeletal muscle.

Differentiated tissue is particularly vulnerable to alterations in protein and organelle homeostasis. The essential protein VCP, mutated in hereditary inclusion body myopathy, amyotrophic lateral sclerosis and frontotemporal dementia, is critical for efficient clearance of misfolded proteins and damaged organelles in dividing cells, but its role in terminally differentiated tissue affected by disease mutations is less clear. To understand the relevance of VCP in differentiated tissue, we inactivated it in skeletal muscle of adult mice. Surprisingly, knockout muscle demonstrated a necrotic myopathy with increased macroautophagic/autophagic proteins and damaged lysosomes. This was not solely due to a defect in autophagic degradation because age-matched mice with muscle inactivation of the autophagy essential protein, ATG5, did not demonstrate a myopathy. Notably, myofiber necrosis was preceded by upregulation of LGALS3/Galectin-3, a marker of damaged lysosomes, and TFEB activation, suggesting early defects in the lysosomal system. Consistent with that, myofiber necrosis was recapitulated by chemical induction of lysosomal membrane permeabilization (LMP) in skeletal muscle. Moreover, TFEB was activated after LMP in cells, but activation and nuclear localization of TFEB persisted upon VCP inactivation or disease mutant expression. Our data identifies VCP as central mediator of both lysosomal clearance and biogenesis in skeletal muscle. Abbreviations: AAA: ATPases Associated with diverse cellular Activities; TUBA1A/α-tubulin: tubulin alpha 1a; ATG5: autophagy related 5; ATG7: autophagy related 7; ACTA1: actin alpha 1, skeletal muscle; CLEAR: coordinated lysosomal expression and regulation; CTSB/D: cathepsin B/D; Ctrl: control; DAPI: diamidino-2-phenylindole; EBSS: Earle's balanced salt solution; ELDR: endolysosomal damage response; ESCRT: endosomal sorting complexes required for transport; Gastroc/G: gastrocnemius; H&E: hematoxylin and eosin; HSPA5/GRP78: heat shock protein family A (Hsp70) member 5; IBMPFD/ALS: inclusion body myopathy associated with Paget disease of the bone, frontotemporal dementia and amyotrophic lateral sclerosis; i.p.: intraperitoneal; LAMP1/2: lysosomal-associated membrane protein 1/2; LLOMe: Leu-Leu methyl ester hydrobromide; LGALS3/Gal3: galectin 3; LMP: lysosomal membrane permeabilization; MTOR: mechanistic target of rapamycin kinase; MYL1: myosin light chain 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MSP: multisystem proteinopathy; PBS: phosphate-buffered saline; PCR: polymerase chain reaction; Quad/Q: quadriceps; RHEB: Ras homolog, mTORC1 binding; SQSTM1: sequestosome 1; TFEB: transcription factor EB; TA: tibialis anterior; siRNA: small interfering RNA; SQSTM1/p62, sequestosome 1; TARDBP/TDP-43: TAR DNA binding protein; TBS: Tris-buffered saline; TXFN, tamoxifen; UBXN6/UBXD1: UBX domain protein 6; VCP: valosin containing protein; WT: wild-type.

Loss of lipin 1-mediated phosphatidic acid phosphohydrolase activity in muscle leads to skeletal myopathy in mice.

Lipin 1 regulates glycerolipid homeostasis by acting as a phosphatidic acid phosphohydrolase (PAP) enzyme in the triglyceride-synthesis pathway and by regulating transcription factor activity. Mutations in human lipin 1 are a common cause of recurrent rhabdomyolysis in children. Mice with constitutive whole-body lipin 1 deficiency have been used to examine mechanisms connecting lipin 1 deficiency to myocyte injury. However, that mouse model is confounded by lipodystrophy not phenocopied in people. Herein, 2 muscle-specific mouse models were studied: 1) Lpin1 exon 3 and 4 deletion, resulting in a hypomorphic protein without PAP activity, but which preserved transcriptional coregulatory function; and 2) Lpin1 exon 7 deletion, resulting in total protein loss. In both models, skeletal muscles exhibited a chronic myopathy with ongoing muscle fiber necrosis and regeneration and accumulation of phosphatidic acid and, paradoxically, diacylglycerol. Additionally, lipin 1-deficient mice had abundant, but abnormal, mitochondria likely because of impaired autophagy. Finally, these mice exhibited increased plasma creatine kinase following exhaustive exercise when unfed. These data suggest that mice lacking lipin 1-mediated PAP activity in skeletal muscle may serve as a model for determining the mechanisms by which lipin 1 deficiency leads to myocyte injury and for testing potential therapeutic approaches.-Schweitzer, G. G., Collier, S. L., Chen, Z., McCommis, K. S., Pittman, S. K., Yoshino, J., Matkovich, S. J., Hsu, F.-F., Chrast, R., Eaton, J. M., Harris, T. E., Weihl, C. C., Finck, B. N. Loss of lipin 1-mediated phosphatidic acid phosphohydrolase activity in muscle leads to skeletal myopathy in mice.

Keap1/Cullin3 Modulates p62/SQSTM1 Activity via UBA Domain Ubiquitination.

p62/SQSTM1 (p62) is a scaffolding protein that facilitates the formation and degradation of ubiquitinated aggregates via its self-interaction and ubiquitin binding domains. The regulation of this process is unclear but may relate to the post-translational modification of p62. In the present study, we find that Keap1/Cullin3 ubiquitinates p62 at lysine 420 within its UBA domain. Substitution of lysine 420 with an arginine diminishes p62 sequestration and degradation activity similar what is seen when the UBA domain is deleted. Overexpression of Keap1/Cullin3 in p62-WT-expressing cells increases ubiquitinated inclusion formation and p62's association with LC3 and rescues proteotoxicity. This effect is not seen in cells expressing a mutant p62 that fails to interact with Keap1. Interestingly, p62 disease mutants have diminished or absent UBA domain ubiquitination. These data suggest that the ubiquitination of p62's UBA domain at lysine 420 may regulate p62's function and be disrupted in p62-associated disease.

TMEM184b Promotes Axon Degeneration and Neuromuscular Junction Maintenance.

UNLABELLED: Complex nervous systems achieve proper connectivity during development and must maintain these connections throughout life. The processes of axon and synaptic maintenance and axon degeneration after injury are jointly controlled by a number of proteins within neurons, including ubiquitin ligases and mitogen activated protein kinases. However, our understanding of these molecular cascades is incomplete. Here we describe the phenotype resulting from mutation of TMEM184b, a protein identified in a screen for axon degeneration mediators. TMEM184b is highly expressed in the mouse nervous system and is found in recycling endosomes in neuronal cell bodies and axons. Disruption of TMEM184b expression results in prolonged maintenance of peripheral axons following nerve injury, demonstrating a role for TMEM184b in axon degeneration. In contrast to this protective phenotype in axons, uninjured mutant mice have anatomical and functional impairments in the peripheral nervous system. Loss of TMEM184b causes swellings at neuromuscular junctions that become more numerous with age, demonstrating that TMEM184b is critical for the maintenance of synaptic architecture. These swellings contain abnormal multivesicular structures similar to those seen in patients with neurodegenerative disorders. Mutant animals also show abnormal sensory terminal morphology. TMEM184b mutant animals are deficient on the inverted screen test, illustrating a role for TMEM184b in sensory-motor function. Overall, we have identified an important function for TMEM184b in peripheral nerve terminal structure, function, and the axon degeneration pathway. SIGNIFICANCE STATEMENT: Our work has identified both neuroprotective and neurodegenerative roles for a previously undescribed protein, TMEM184b. TMEM184b mutation causes delayed axon degeneration following peripheral nerve injury, indicating that it participates in the degeneration process. Simultaneously, TMEM184b mutation causes progressive structural abnormalities at neuromuscular synapses and swellings within sensory terminals, and animals with this mutation display profound weakness. Thus, TMEM184b is necessary for normal peripheral nerve terminal morphology and maintenance. Loss of TMEM184b results in accumulation of autophagosomal structures in vivo, fitting with emerging studies that have linked autophagy disruption and neurological disease. Our work recognizes TMEM184b as a new player in the maintenance of the nervous system.

Myofibrillar disruption and RNA-binding protein aggregation in a mouse model of limb-girdle muscular dystrophy 1D.

Limb-girdle muscular dystrophy type 1D (LGMD1D) is caused by dominantly inherited missense mutations in DNAJB6, an Hsp40 co-chaperone. LGMD1D muscle has rimmed vacuoles and inclusion bodies containing DNAJB6, Z-disc proteins and TDP-43. DNAJB6 is expressed as two isoforms; DNAJB6a and DNAJB6b. Both isoforms contain LGMD1D mutant residues and are expressed in human muscle. To identify which mutant isoform confers disease pathogenesis and generate a mouse model of LGMD1D, we evaluated DNAJB6 expression and localization in skeletal muscle as well as generating DNAJB6 isoform specific expressing transgenic mice. DNAJB6a localized to myonuclei while DNAJB6b was sarcoplasmic. LGMD1D mutations in DNAJB6a or DNAJB6b did not alter this localization in mouse muscle. Transgenic mice expressing the LGMD1D mutant, F93L, in DNAJB6b under a muscle-specific promoter became weak, had early lethality and developed muscle pathology consistent with myopathy after 2 months; whereas mice expressing the same F93L mutation in DNAJB6a or overexpressing DNAJB6a or DNAJB6b wild-type transgenes remained unaffected after 1 year. DNAJB6b localized to the Z-disc and DNAJB6b-F93L expressing mouse muscle had myofibrillar disorganization and desmin inclusions. Consistent with DNAJB6 dysfunction, keratin 8/18, a DNAJB6 client also accumulated in DNAJB6b-F93L expressing mouse muscle. The RNA-binding proteins hnRNPA1 and hnRNPA2/B1 accumulated and co-localized with DNAJB6 at sarcoplasmic stress granules suggesting that these proteins maybe novel DNAJB6b clients. Similarly, hnRNPA1 and hnRNPA2/B1 formed sarcoplasmic aggregates in patients with LGMD1D. Our data support that LGMD1D mutations in DNAJB6 disrupt its sarcoplasmic function suggesting a role for DNAJB6b in Z-disc organization and stress granule kinetics.

SQSTM1 splice site mutation in distal myopathy with rimmed vacuoles.

OBJECTIVE: To identify the genetic etiology and characterize the clinicopathologic features of a novel distal myopathy. METHODS: We performed whole-exome sequencing on a family with an autosomal dominant distal myopathy and targeted exome sequencing in 1 patient with sporadic distal myopathy, both with rimmed vacuolar pathology. We also evaluated the pathogenicity of identified mutations using immunohistochemistry, Western blot analysis, and expression studies. RESULTS: Sequencing identified a likely pathogenic c.1165+1 G>A splice donor variant in SQSTM1 in the affected members of 1 family and in an unrelated patient with sporadic distal myopathy. Affected patients had late-onset distal lower extremity weakness, myopathic features on EMG, and muscle pathology demonstrating rimmed vacuoles with both TAR DNA-binding protein 43 and SQSTM1 inclusions. The c.1165+1 G>A SQSTM1 variant results in the expression of 2 alternatively spliced SQSTM1 proteins: 1 lacking the C-terminal PEST2 domain and another lacking the C-terminal ubiquitin-associated (UBA) domain, both of which have distinct patterns of cellular and skeletal muscle localization. CONCLUSIONS: SQSTM1 is an autophagic adaptor that shuttles aggregated and ubiquitinated proteins to the autophagosome for degradation via its C-terminal UBA domain. Similar to mutations in VCP, dominantly inherited mutations in SQSTM1 are now associated with rimmed vacuolar myopathy, Paget disease of bone, amyotrophic lateral sclerosis, and frontotemporal dementia. Our data further suggest a pathogenic connection between the disparate phenotypes.

Autophagic vacuolar pathology in desminopathies.

Autophagic vacuolar myopathies are an emerging group of muscle diseases with common pathologic features. These include autophagic vacuoles containing both lysosomal and autophagosomal proteins sometimes lined with sarcolemmal proteins such as dystrophin. These features have been most clearly described in patients with Danon's disease due to LAMP2 deficiency and X-linked myopathy with excessive autophagy (XMEA) due to mutations in VMA21. Disruptions of these proteins lead to lysosomal dysfunction and subsequent autophagic vacuolar pathology. We performed whole exome sequencing on two families with autosomal dominantly inherited myopathies with autophagic vacuolar pathology and surprisingly identified a p.R454W tail domain mutation and a novel p.S6W head domain mutation in desmin, DES. In addition, re-evaluation of muscle tissue from another family with a novel p.I402N missense DES mutation also identified autophagic vacuoles. We suggest that autophagic vacuoles may be an underappreciated pathology present in desminopathy patient muscle. Moreover, autophagic vacuolar pathology can be due to genetic etiologies unrelated to primary defects in the lysosomes or autophagic machinery. Specifically, cytoskeletal derangement and the accumulation of aggregated proteins such as desmin may activate the autophagic system leading to the pathologic features of an autophagic vacuolar myopathy.