Molecular Mechanisms of Noncanonical Autophagy.

Macroautophagy is a lysosomal catabolic process that maintains the homeostasis of eukaryotic cells, tissues, and organisms. Macroautophagy plays important physiological roles during development and aging processes and also contributes to immune responses. The process of macroautophagy is compromised in diseases, such as cancer, neurodegenerative disorders, and diabetes. The autophagosome, the double-membrane-bound organelle that sequesters cytoplasmic material to initiate macroautophagy, is formed by the hierarchical recruitment of about 15 autophagy-related (ATG) proteins and associated proteins, such as DFCP1, AMBRA1, the class III phosphatidyl-inositol 3-kinase VPS34, and p150/VPS15. Evidence suggests that in addition to the canonical pathway, noncanonical pathways that do not require the entire repertoire of ATGs can also result in formation of autophagosomes. Here we will discuss recent discoveries concerning the molecular regulation of these noncanonical forms of macroautophagy and their potential roles in cellular responses to stressful situations.

Muscle atrophy in Limb Girdle Muscular Dystrophy 2A: a morphometric and molecular study.

AIMS: The peculiar clinical features and the pathogenic mechanism related to calpain-3 deficiency (impaired sarcomere remodelling) suggest that the ubiquitin-proteasome degradation pathway may have a crucial role in Limb Girdle Muscular Dystrophy 2A (LGMD2A). We therefore investigated muscle atrophy and the role of the ubiquitin-proteasome and lysosomal-autophagic degradation pathways. METHODS: We selected 25 adult male LGMD2A patients (and seven controls), classified them using clinical severity score, analysed muscle fibre size by morphometry and protein and/or transcriptional expression levels of the most important atrophy- and autophagy-related genes (MuRF1, atrogin1, LC3, p62, Bnip3). RESULTS: Muscle fibre size was significantly lower in LGMD2A than in controls and it was significantly correlated with patients' clinical disability score recorded at the time of biopsy, suggesting that functional and structural muscle impairment are dependent. The large majority of atrophic fibres originate from a mechanism different from regeneration, as assessed by neonatal myosin immunolabelling. As compared with controls, LGMD2A muscles have higher MuRF1 (but not atrogin1) protein and MuRF1 gene expression levels, and MuRF1 protein levels significantly correlated with both muscle fibre size and clinical disability score. LGMD2A muscles have slightly increased levels of LC3-II and p62 proteins and a significant up-regulation of p62 and Bnip3 gene expression. CONCLUSIONS: In LGMD2A muscles the activation of the atrophy programme appeared to depend mainly upon induction of the ubiquitin-proteasome system and, to a lesser extent, the autophagic-lysosomal degradation pathway.

The role of autophagy in the pathogenesis of glycogen storage disease type II (GSDII).

Regulated removal of proteins and organelles by autophagy-lysosome system is critical for muscle homeostasis. Excessive activation of autophagy-dependent degradation contributes to muscle atrophy and cachexia. Conversely, inhibition of autophagy causes accumulation of protein aggregates and abnormal organelles, leading to myofiber degeneration and myopathy. Defects in lysosomal function result in severe muscle disorders such as Pompe (glycogen storage disease type II (GSDII)) disease, characterized by an accumulation of autophagosomes. However, whether autophagy is detrimental or not in muscle function of Pompe patients is unclear. We studied infantile and late-onset GSDII patients and correlated impairment of autophagy with muscle wasting. We also monitored autophagy in patients who received recombinant α-glucosidase. Our data show that infantile and late-onset patients have different levels of autophagic flux, accumulation of p62-positive protein aggregates and expression of atrophy-related genes. Although the infantile patients show impaired autophagic function, the late-onset patients display an interesting correlation among autophagy impairment, atrophy and disease progression. Moreover, reactivation of autophagy in vitro contributes to acid α-glucosidase maturation in both healthy and diseased myotubes. Together, our data suggest that autophagy protects myofibers from disease progression and atrophy in late-onset patients.

The clinical course of calpainopathy (LGMD2A) and dysferlinopathy (LGMD2B).

OBJECTIVE: Autosomal recessive limb girdle muscular dystrophies (LGMD type 2) are a clinically and genetically heterogeneous group of disorders, characterized by progressive involvement and wasting of limb girdle muscles. In order to describe the peculiar clinical features of LGMD2A (calpainopathy) and LGMD2B (dysferlinopathy), the most frequent forms of LGMD in European countries, we analysed and compared the phenotype and the clinical course in two relatively large groups of these patients. METHODS: We selected 22 patients with a molecular diagnosis of LGMD2A and 21 patients with LGMD2B and reported their clinical data collected from both clinical history and during periodical neuromuscular examinations: age and distribution of muscle involvement at onset, clinical functional score by the use of ten-point modified scale of Gardner-Medwin and Walton at onset and at last clinical examination, and the rate of disease progression. RESULTS: LGMD2A group included patients with different ages at onset (early-onset or late-onset), different phenotypes (upper girdle in Erb LGMD or lower girdle in Leyden-Moebius LGMD) and different disease progressions (rapid or slow course). LGMD2B patients differed for pattern of muscle involvement at onset (distal in Miyoshi dystrophy or proximal in Leyden-Moebius LGMD) but they had a rather homogeneous age at onset (in the second/third decade) and rate of disease progression. DISCUSSION: Our data show that besides the clinical differences within each group of patients, the two forms of LGMD present distinctive clinical features. The various phenotypes and courses can be attributed to specific pathogenetic mechanisms and might suggest differential therapeutic strategies.

Frequency of LGMD gene mutations in Italian patients with distinct clinical phenotypes.

BACKGROUND: The frequency of various limb-girdle muscular dystrophy (LGMD) molecular diagnoses has previously been investigated only in cohorts of patients presenting LGMD phenotype. METHODS: A total of 550 muscle biopsies underwent multiple protein screening (including calpain-3 functional assay) and extensive gene mutation analysis to examine the frequency of LGMD subtypes in patients with distinct clinical phenotypes (severe childhood-onset LGMD, adult-onset LGMD, distoproximal myopathy, and asymptomatic hyperCKemia). RESULTS: The percentage of molecularly ascertained cases directly relates with the degree of clinical involvement: 60% of total LGMD (77% of childhood-onset, 46% of adult-onset, 66% of distoproximal myopathy) and 14% of hyperCKemia. The higher number of molecular diagnoses in severe phenotypes might suggest that genes selected for our screening are those more frequently associated with severe LGMD, and that the hyperCKemia group includes heterogeneous diagnoses. The probability of obtaining a molecular diagnosis increases when a protein defect is found in a muscle biopsy: in such cases, we diagnosed 87% of LGMD and 76% of hyperCKemia. CONCLUSIONS: Diagnosing 77% of childhood-onset limb-girdle muscular dystrophy (LGMD) and 60% of total LGMD is an important result. The missing identification of gene mutations in about 40% of patients with typical LGMD phenotype suggests that unknown genetic or nongenetic etiologies are still to be recognized. Dysferlin, caveolin-3, and emerin protein defects invariably corresponded to primary disorders (100%), whereas a lower correlation was found for sarcoglycans (77%) and calpain-3 (84%). The different efficiency of genetic diagnosis after the identification of a protein defect in the various disorders is possibly due to different pathogenetic effects of mutations.

Correlations between clinical severity, genotype and muscle pathology in limb girdle muscular dystrophy type 2A.

BACKGROUND: Limb girdle muscular dystrophy type 2A (LGMD2A) is characterised by wide variability in clinical features and rate of progression. Patients with two null mutations usually have a rapid course, but in the remaining cases (two missense mutations or compound heterozygote mutations) prognosis is uncertain. METHODS: We conducted what is to our knowledge the first systematic histopathological, biochemical and molecular investigation of 24 LGMD2A patients, subdivided according to rapid or slow disease progression, to determine if some parameters could correlate with disease progression. RESULTS: We found that muscle histopathology score and the extent of regenerating and degenerating fibres could be correlated with the rate of disease course when the biochemical and molecular data do not offer sufficient information. Comparison of clinical and muscle histopathological data between LGMD2A and four other types of LGMD (LGMD2B-E) also gave another important and novel result. We found that LGMD2A has significantly lower levels of dystrophic features (ie degenerating and regenerating fibres) and higher levels of chronic changes (ie lobulated fibres) compared with other LGMDs, particularly LGMD2B. These results might explain the observation that atrophic muscle involvement seems to be a clinical feature peculiar to LGMD2A patients. CONCLUSIONS: Distinguishing patterns of muscle histopathological changes in LGMD2A might reflect the effects of a disease-specific pathogenetic mechanism and provide clues complementary to genetic data.

Screening of calpain-3 autolytic activity in LGMD muscle: a functional map of CAPN3 gene mutations.

BACKGROUND: The diagnosis of calpainopathy is obtained by identifying calpain-3 protein deficiency or CAPN3 gene mutations. However, in many patients with limb girdle muscular dystrophy type 2A (LGMD2A), the calpain-3 protein quantity is normal because loss-of-function mutations cause its enzymatic inactivation. The identification of such patients is difficult unless a functional test suggests pursuing a search for mutations. MATERIALS AND METHODS: A functional in vitro assay, which was able to test calpain-3 autolytic function, was used to screen a large series of muscle biopsy specimens from patients with unclassified LGMD/hyperCKaemia who have previously shown normal calpain-3 protein quantity. RESULTS: Of 148 muscle biopsy specimens tested,17 samples (11%) had lost normal autolytic function. CAPN3 gene mutations were identified in 15 of 17 patients (88%), who account for about 20% of the total patients with LGMD2A diagnosed in our series. CONCLUSIONS: The loss of calpain-3 autolytic activity is highly predictive of primary calpainopathy, and the use of this test as part of calpainopathy diagnosis would improve the rate of disease detection markedly. This study provides the first evidence of the pathogenetic effect of specific CAPN3 gene mutations on the corresponding protein function in LGMD2A muscle and offers new insights into the structural-functional relationship of the gene and protein regions that are crucial for the autolytic activity of calpain-3.

Epilepsy and limb girdle muscular dystrophy type 2A: double trouble, serendipitous finding or new phenotype?

Autosomal recessive limb girdle muscular dystrophies (LGMD) type 2A are a group of disorders characterised by progressive involvement of proximal limb girdle muscles and caused by changes in the CAPN3 gene. Involvement of tissues other than the skeletal muscle has not been reported so far. Here we describe the unusual association of LGMD2A and idiopathic generalised epilepsy in a 14-year-old girl.

Extensive scanning of the calpain-3 gene broadens the spectrum of LGMD2A phenotypes.

BACKGROUND: The limb girdle muscular dystrophies (LGMD) are a heterogeneous group of Mendelian disorders highlighted by weakness of the pelvic and shoulder girdle muscles. Seventeen autosomal loci have been so far identified and genetic tests are mandatory to distinguish among the forms. Mutations at the calpain 3 locus (CAPN3) cause LGMD type 2A. OBJECTIVE: To obtain unbiased information on the consequences of CAPN3 mutations. PATIENTS: 530 subjects with different grades of symptoms and 300 controls. METHODS: High throughput denaturing HPLC analysis of DNA pools. RESULTS: 141 LGMD2A cases were identified, carrying 82 different CAPN3 mutations (45 novel), along with 18 novel polymorphisms/variants. Females had a more favourable course than males. In 94% of the more severely affected patient group, the defect was also discovered in the second allele. This proves the sensitivity of the approach. CAPN3 mutations were found in 35.1% of classical LGMD phenotypes. Mutations were also found in 18.4% of atypical patients and in 12.6% of subjects with high serum creatine kinase levels. CONCLUSIONS: A non-invasive and cost-effective strategy, based on the high throughput denaturing HPLC analysis of DNA pools, was used to obtain unbiased information on the consequences of CAPN3 mutations in the largest genetic study ever undertaken. This broadens the spectrum of LGMD2A phenotypes and sets the carrier frequency at 1:103.

Molecular diagnosis in LGMD2A: mutation analysis or protein testing?

Limb girdle muscular dystrophy (LGMD) type 2A (LGMD2A) is caused by mutations in the CAPN3 gene encoding for calpain-3, a muscle specific protease. While a large number of CAPN3 gene mutations have already been described in calpainopathy patients, the diagnosis has recently shifted from molecular genetics towards biochemical assay of defective protein. However, an estimate of sensitivity and specificity of protein analysis remains to be established. Thus, we first correlated protein and molecular data in our large LGMD2A patient population. By a preliminary immunoblot screening for calpain-3 protein of 548 unclassified patients with various phenotypes (LGMD, myopathy, or elevated levels of serum creatine kinase [hyperCKemia]), we selected 208 cases for CAPN3 gene mutation analysis: 69 had protein deficiency and 139 had normal expression. Mutation search was conducted using SSCP, denaturing high performance liquid chromatography (DHPLC), amplification refractory mutation system (ARMS-PCR), and direct sequencing methods. We identified 58 LGMD2A mutant patients: 46 (80%) had a variable degree of protein deficiency and 12 (20%) had normal amount of calpain-3. We calculated that the probability of having LGMD2A is very high (84%) when patients show a complete calpain-3 deficiency and progressively decreases with the amount of protein; this new data offers an important tool for genetic counseling when only protein data are available. A total of 37 different CAPN3 gene mutations were detected, 10 of which are novel. In our population, 87% of mutant alleles were concentrated in seven exons (exons 1, 4, 5, 8, 10, 11, and 21) and 61% correspond to only eight mutations, indicating the regions where future molecular analysis could be restricted. This study reports the largest collection of LGMD2A patients so far in which both protein and gene mutations were obtained to draw genotype-protein-phenotype correlations and provide insights into a critical protein domain.

Morphological changes in late onset acid Maltase deficient patients with splicing gene mutation.

Clinical and morphological features have been studied in 11 late onset Acid Maltase Deficient (AMD) patients. All patients have been diagnosed on biochemical evidence of acid maltase deficiency on muscle biopsy. Molecular studies showed a heterozygous mutation (IVS1-13 T > G transversion resulting in aberrant splicing) in the GSDII gene, which is the most common mutation in late onset AMD patients. Morphological features in muscle biopsy showed a vacuolar myopathy and Golgi apparatus proliferation within fibres. The peripheral areas of autophagic vacuoles were positive for caveolin-3 and dystrophin, documenting an extensive membrane turnover. The ultrastructural study of muscle biopsy showed randomly distributed or isolated vesicles sometimes derived from the Golgi apparatus. In subsarcolemmal region, lipofucsin bodies and abnormal mitochondria with crystalline inclusions were observed. Primary and secondary lysosomes were typically filled with glycogen. These data suggest a predominant role of Golgi in vesicle proliferation and extensive intra-fibral membrane remodelling. The pathological changes observed are selective for muscle fibres (mostly in type 1) and for muscle groups (mainly proximal). An attractive hypothesis for the variability of clinical phenotype in adult and infantile onset AMD patients is that in the former, an aberrant transcript of smaller size may have originated from alternative splicing (exon 2 skipping). A residual enzyme activity is detectable in muscle, but the intracellular processing of the enzyme precursor from Golgi to the mature form in lysosomes might be blocked.