Ataxia telangiectasia mutated impacts insulin-like growth factor 1 signalling in skeletal muscle

ATM Protein Kinase: Old and New Implications in Neuronal Pathways and Brain Circuitry

Despite that the human autosomal recessive disease ataxia telangiectasia (A-T) is a rare pathology, interest in the function of ataxia-telangiectasia mutated protein (ATM) is extensive. From a clinical point of view, the role of ATM in the central nervous system (CNS) is the most impacting, as motor disability is the predominant symptom affecting A-T patients. Coherently, spino-cerebellar neurodegeneration is the principal hallmark of A-T and other CNS regions such as dentate and olivary nuclei and brain stem are implicated in A-T pathophysiology. Recently, several preclinical studies also highlighted the involvement of ATM in the cerebral cortex and hippocampus, thus extending A-T symptomatology to new brain areas and pathways. Here, we review old and recent evidence that largely demonstrates not only the historical ATM account in DNA damage response and cell cycle regulation, but the multiple pathways through which ATM controls oxidative stress homeostasis, insulin signalling pathways, epigenetic regulation, synaptic transmission, and excitatory–inhibitory balance. We also summarise recent evidence on ATM implication in neurological and cognitive diseases beyond A-T, bringing out ATM as new pathological substrate and potential therapeutic target.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

Atrophy, oxidative switching and ultrastructural defects in skeletal muscle of the ataxia telangiectasia mouse model

Ataxia telangiectasia is a rare, multi system disease caused by ATM kinase deficiency. Atm-knockout mice recapitulate premature aging, immunodeficiency, cancer predisposition, growth retardation and motor defects, but not cerebellar neurodegeneration and ataxia. We explored whether Atm loss is responsible for skeletal muscle defects by investigating myofiber morphology, oxidative/glycolytic activity, myocyte ultrastructural architecture and neuromuscular junctions. Atm-knockout mice showed reduced muscle and fiber size. Atrophy, protein synthesis impairment and a switch from glycolytic to oxidative fibers were detected, along with an increase of in expression of slow and fast myosin types (Myh7, and Myh2 and Myh4, respectively) in tibialis anterior and solei muscles isolated from Atm-knockout mice. Transmission electron microscopy of tibialis anterior revealed misalignments of Z-lines and sarcomeres and mitochondria abnormalities that were associated with an increase in reactive oxygen species. Moreover, neuromuscular junctions appeared larger and more complex than those in Atm wild-type mice, but with preserved presynaptic terminals. In conclusion, we report for the first time that Atm-knockout mice have clear morphological skeletal muscle defects that will be relevant for the investigation of the oxidative stress response, motor alteration and the interplay with peripheral nervous system in ataxia telangiectasia.

Inflammation, a significant player of Ataxia-Telangiectasia pathogenesis?

INTRODUCTION

Ataxia-Telangiectasia (A-T) syndrome is an autosomal recessive neurodegenerative disorder characterized by cerebellar ataxia, oculocutaneous telangiectasia, immunodeficiency, chromosome instability, radiosensitivity, and predisposition to malignancy. There is growing evidence that A-T patients suffer from pathologic inflammation that is responsible for many symptoms of this syndrome, including neurodegeneration, autoimmunity, cardiovascular disease, accelerated aging, and insulin resistance. In addition, epidemiological studies have shown A-T heterozygotes, somewhat like deficient patients, are susceptible to ionizing irradiation and have a higher risk of cancers and metabolic disorders.

AREA COVERED

This review summarizes clinical and molecular findings of inflammation in A-T syndrome.

CONCLUSION

Ataxia-Telangiectasia Mutated (ATM), a master regulator of the DNA damage response is the protein known to be associated with A-T and has a complex nuclear and cytoplasmic role. Loss of ATM function may induce immune deregulation and systemic inflammation.

Ataxia telangiectasia syndrome: moonlighting ATM

INTRODUCTION

Ataxia-telangiectasia (A-T) a multisystem disorder primarily characterized by cerebellar degeneration, telangiectasia, immunodeficiency, cancer susceptibility and radiation sensitivity. Identification of the gene defective in this syndrome, ataxia-telangiectasia mutated gene (ATM), and further characterization of the disorder together with a greater insight into the function of the ATM protein have expanded our knowledge about the molecular pathogenesis of this disease. Area covered: In this review, we have attempted to summarize the different roles of ATM signaling that have provided new insights into the diverse clinical phenotypes exhibited by A-T patients. Expert commentary: ATM, in addition to DNA repair response, is involved in many cytoplasmic roles that explain diverse phenotypes of A-T patients. It seems accumulation of DNA damage, persistent DNA damage response signaling, and chronic oxidative stress are the main players in the pathogenesis of this disease.

Ataxia-telangiectasia (A-T): An emerging dimension of premature ageing

A-T is a prototype genome instability syndrome and a multifaceted disease. A-T leads to neurodegeneration - primarily cerebellar atrophy, immunodeficiency, oculocutaneous telangiectasia (dilated blood vessels), vestigial thymus and gonads, endocrine abnormalities, cancer predisposition and varying sensitivity to DNA damaging agents, particularly those that induce DNA double-strand breaks. With the recent increase in life expectancy of A-T patients, the premature ageing component of this disease is gaining greater awareness. The complex A-T phenotype reflects the ever growing number of functions assigned to the protein encoded by the responsible gene - the homeostatic protein kinase, ATM. The quest to thoroughly understand the complex A-T phenotype may reveal yet elusive ATM functions.

New Role of ATM in Controlling GABAergic Tone During Development

The capacity to guarantee the proper excitatory/inhibitory balance is one of the most critical steps during early development responsible for the correct brain organization, function, and plasticity. GABAergic neurons guide this process leading to the right structural organization, brain circuitry, and neuronal firing. Here, we identified the ataxia telangiectasia mutated (ATM), a serine/threonine protein kinase linked to DNA damage response, as crucial in regulating neurotransmission. We found that reduced levels of ATM in the hippocampal neuronal cultures produce an excitatory/inhibitory unbalance toward inhibition as indicated by the higher frequency of miniature inhibitory postsynaptic current events and an increased number of GABAergic synapses. In vivo, the increased inhibition still persists and, even if a higher excitation is also present, a reduced neuronal excitability is found as indicated by the lower action potential frequency generated in response to high-current intensity stimuli. Finally, we found an elevated extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in heterozygous hippocampi associated with lower expression levels of the ERK1/2 phosphatase PP1. Given that the neurodegenerative condition associated with genetic mutations in the Atm gene, ataxia telangiectasia, presents a variable phenotype with impairment in cognition, our molecular findings provide a logical frame for a more clear comprehension of cognitive defects in the pathology, opening to novel therapeutic strategies.

Proteomic Screening and Lasso Regression Reveal Differential Signaling in Insulin and Insulin-like Growth Factor I (IGF1) Pathways

Insulin and insulin-like growth factor I (IGF1) influence cancer risk and progression through poorly understood mechanisms. To better understand the roles of insulin and IGF1 signaling in breast cancer, we combined proteomic screening with computational network inference to uncover differences in IGF1 and insulin induced signaling. Using reverse phase protein array, we measured the levels of 134 proteins in 21 breast cancer cell lines stimulated with IGF1 or insulin for up to 48 h. We then constructed directed protein expression networks using three separate methods: (i) lasso regression, (ii) conventional matrix inversion, and (iii) entropy maximization. These networks, named here as the time translation models, were analyzed and the inferred interactions were ranked by differential magnitude to identify pathway differences. The two top candidates, chosen for experimental validation, were shown to regulate IGF1/insulin induced phosphorylation events. First, acetyl-CoA carboxylase (ACC) knock-down was shown to increase the level of mitogen-activated protein kinase (MAPK) phosphorylation. Second, stable knock-down of E-Cadherin increased the phospho-Akt protein levels. Both of the knock-down perturbations incurred phosphorylation responses stronger in IGF1 stimulated cells compared with insulin. Overall, the time-translation modeling coupled to wet-lab experiments has proven to be powerful in inferring differential interactions downstream of IGF1 and insulin signaling, in vitro.

Endocrine abnormalities in ataxia telangiectasia: findings from a national cohort

BACKGROUND

Ataxia telangiectasia (AT) is a genetic multisystem disorder, presenting with progressive ataxia, immune deficiency, and propensity toward malignancy. Endocrine abnormalities (growth retardation, reproductive dysfunction, and diabetes) have been described, however detailed information regarding this aspect is lacking. We aimed to characterize endocrine anomalies and growth patterns in a large cohort of AT patients.

METHODS

Retrospective study comprising all 52 patients (aged 2-26.2 y) followed at a national AT Clinic. Anthropometric and laboratory measurements were extracted from the charts.

RESULTS

Median height-SDS was already subnormal during infancy, remaining negative throughout follow up to adulthood. Height-SDS was more impaired than weight-SDS up to age 4 y, thereafter weight-SDS steadily decreased, resulting in progressively lower BMI-SDS. IGF-I-SDS was low (-1.53 ± 1.54), but did not correlate with height-SDS. Gonadal failure was present in all 13 females older than 10 y but only in one male. Two patients had diabetes and 10 had dyslipidemia. Vitamin D deficiency was observed in 52.2% of the evaluated patients.

CONCLUSION

Our results suggest a primary growth abnormality in AT, rather than secondary to nutritional impairment or disease severity. Sex hormone replacement should be considered for female patients. Vitamin D levels should be followed and supplementation given if needed.

Liver Disease in Pediatric Patients With Ataxia Telangiectasia: A Novel Report

OBJECTIVE

Ataxia telangiectasia (A-T) is a rare genetic multiorgan disease. Although gastrointestinal involvement is known, hepatic involvement in A-T has not been investigated. We aimed to study the hepatic involvement in a large cohort of patients with A-T.

METHODS

A retrospective review of patients, studied from January 1986 to January 2015 at a National A-T Center. Clinical data including demographic, genetic, laboratory, nutritional, radiographic, and histological data were retrieved.

RESULTS

Fifty-three patients, 27 (49%) boys, age 14.6 ± 5.2 years (range 5.9-26.1 years), were included. Twenty-three patients (43.4%), age 9.9 ± 5.1 years, had consistently abnormal liver enzymes. The mean enzyme levels were alanine aminotransferase 76.8 ± 73.8 IU/L, aspartate aminotransferase 70 ± 50 IU/L, alkaline phosphatase 331 ± 134 IU/L, and gamma glutamyl transferase 114.7 ± 8 IU/L. Evaluation of other etiology of liver disease was negative. Ultrasonography revealed fatty liver in 9 of them (39%). Liver biopsy was performed in 2 patients, revealing mild-to-moderate steatosis in both, and fibrosis in 1 patient. Progression to advanced liver disease occurred in 2 of 23 (9%) patients within 2 to 5 years. Dyslipidemia was significantly associated with abnormal liver enzymes: 3 of 30 (10%) patients without abnormal liver enzymes versus 10 of 23 (45.5%) patients with abnormal liver enzymes, respectively (P < 0.05, Fisher exact test). No correlation was found between hepatic involvement and HbA1C, sex, presence of malignancy, or type of mutation.

CONCLUSIONS

Abnormal liver enzymes and fatty liver are common in patients with A-T and may progress to advanced liver disease at a young age. These findings are novel and implicate that patients with A-T with abnormal liver enzymes should be evaluated for the presence of liver disease.

Chloroquine increases phosphorylation of AMPK and Akt in myotubes

Aims

There are reports that ataxia telangiectasia mutated (ATM) can activate the AMP-activated protein kinase (AMPK) and also Akt, two kinases that play integral parts in cardioprotection and metabolic function. We hypothesized that chloroquine and resveratrol, both known ATM activators, would also activate AMPK and Akt.

Main methods

Phosphorylation of AMPK and Akt was assessed after C2C12 myotubes were exposed to chloroquine or resveratrol. Additional experiments were done in cells expressing shRNA against ATM or in the presence of the ATM inhibitor KU55933. The effects of chloroquine on intracellular calcium were assessed with the fluorescent probe Calcium Green-1 AM.

Key findings

0.5 mM chloroquine increased AMPK phosphorylation by nearly four-fold (P < 0.05), and 0.25 mM chloroquine roughly doubled Akt phosphorylation (P < 0.05). Chloroquine also increased autophosphorylation of ATM by ∼50% (P < 0.05). Resveratrol (0.15 mM) increased AMPK phosphorylation about three-fold (P < 0.05) but in contrast to chloroquine sharply decreased Akt phosphorylation. Chloroquine increased AMPK and Akt phosphorylation in myotubes expressing shRNA against ATM that reduced ATM protein levels by about 90%. Likewise, chloroquine-stimulated phosphorylation of AMPK and Akt and resveratrol-stimulated phosphorylation of AMPK were not altered by inhibition of ATM. Chloroquine decreased intracellular calcium by >50% concomitant with a decrease in glucose transport.

Significance

These ATM-independent effects of chloroquine on AMPK and Akt and the additional effect to decrease intracellular calcium are likely to partially underlie the positive metabolic effects of chloroquine that have been reported in the literature.

mTOR kinase-dependent, but raptor-independent regulation of downstream signaling is important for cell cycle exit and myogenic differentiation

Myogenic differentiation in the C2C12 myoblast model system reflects a concerted and controlled activation of transcription and translation following the exit of cells from the cell cycle. Previously we have shown that the mTORC1 signaling inhibitor, RAD001, decreased protein synthesis rates, delayed C2C12 myoblast differentiation, decreased p70S6K activity but did not affect the hypermodification of 4E-BP1. Here we have further investigated the modification of 4E-BP1 during the early phase of differentiation as cells exit the cell cycle, using inhibitors to target mTOR kinase and siRNAs to ablate the expression of raptor and rictor. As predicted, inhibition of mTOR kinase activity prevented p70S6K, 4E-BP1 phosphorylation and was associated with an inhibition of myogenic differentiation. Surprisingly, extensive depletion of raptor did not affect p70S6K or 4E-BP1 phosphorylation, but promoted an increase in mTORC2 activity (as evidenced by increased Akt Ser473 phosphorylation). These data suggest that an mTOR kinase-dependent, but raptor-independent regulation of downstream signaling is important for myogenic differentiation.

Linear growth and endocrine function in children with ataxia telangiectasia

INTRODUCTION

Ataxia telangiectasia (AT) is a rare, genetic, primary immune deficiency disease characterized by immunodeficiency and neurological manifestations, with an increased tendency to infection, malignancy, and autoimmune diseases. Both growth delay and endocrine abnormalities are occasionally reported in these patients.

PATIENTS AND METHODS

We studied growth parameters height (Ht), weight, body mass index (BMI) and calculated the Ht standard deviation scores (HtSDS) of 13 patients (age 7.7 ± 3.5 years-age range: 3-14.5 years) with AT in relation to their mid-parental Ht SDS (MPHtSDS). We measured their serum calcium (Ca), phosphorus (PO4), alkaline phosphatase, alanine transferase (ALT), serum ferritin, creatinine and albumin concentrations. Endocrine investigations included the assessment of serum free thyroxine (FT4), thyrotropin (TSH), insulin-like growth factor-I (IGF-I) and morning cortisol. Complete blood count and serum immunoglobulins (IgG, IgM and IgA antibodies) were also measured. Growth data were correlated to hormonal and immune data.

RESULTS

About 31% of patients with AT had short stature (HtSDS <-2). However, their MPHtSDS denoted that their short stature was familial because four out of 13 had MPHtSDS <-2. They had low BMI, and two of them had low serum albumin and IGF-I, denoting malnutrition or disturbed growth hormone secretion. Elevated serum ALT and ferritin in some patients suggest immune-related inflammation in the liver. 30% of patients had high TSH, two of them had low FT4 diagnosing overt (15%) and sub-clinical (15%) hypothyroidism. Anti-thyroid peroxidase antibodies were high in two out of 13 patients denoting immune-related thyroid aggression. Eight out of 13 patients had Vitamin D deficiency (<20 ng/ml) however, their serum Ca and PO4 levels were in the normal range. One adolescent girl (14.5 years) had hyper-gonadotropic hypogonadism (low estradiol and high follicle stimulating hormone). All patients had normal 8 AM cortisol and renal function. None of the growth parameters were correlated with the IgG, IgM or IgA levels.

IN SUMMARY

Patients with AT had a high prevalence of growth retardation and endocrine dysfunction in the form of low IGF-I, overt and subclinical hypothyroidism and hypogonadism. Physicians should be aware of these possible endocrinopathies for an early diagnosis and proper treatment.

Impaired insulin-stimulated glucose transport in ATM-deficient mouse skeletal muscle

There are reports that ataxia telangiectasia mutated (ATM) plays a role in insulin-stimulated Akt phosphorylation, although this is not the case in some cell types. Because Akt plays a key role in insulin signaling, which leads to glucose transport in skeletal muscle, the predominant tissue in insulin-stimulated glucose disposal, we examined whether insulin-stimulated Akt phosphorylation and (or) glucose transport would be decreased in skeletal muscle of mice lacking functional ATM, compared with muscle from wild-type mice. We found that in vitro insulin-stimulated Akt phosphorylation was normal in soleus muscle from mice with 1 nonfunctional allele of ATM (ATM+/-) and from mice with 2 nonfunctional alleles (ATM-/-). However, insulin did not stimulate glucose transport or the phosphorylation of AS160 in ATM-/- soleus. ATM protein level was markedly higher in wild-type extensor digitorum longus (EDL) than in wild-type soleus. In EDL from ATM-/- mice, insulin did not stimulate glucose transport. However, in contrast to findings for soleus, insulin-stimulated Akt phosphorylation was blunted in ATM-/- EDL, concomitant with a tendency for insulin-stimulated phosphatidylinositol 3-kinase activity to be decreased. Together, the findings suggest that ATM plays a role in insulin-stimulated glucose transport at the level of AS160 in muscle comprised of slow and fast oxidative-glycolytic fibers (soleus) and at the level of Akt in muscle containing fast glycolytic fibers (EDL).