An invertebrate model for TMEM43-induced arrhythmogenic right ventricular cardiomyopathy (ARVC) type 5

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): DFG

Objectives

TMEM43 encodes for a transmembrane protein located in the ER membrane and the nuclear rim. TMEM43 has predicted four transmembrane domains. The missense variant TMEM43 p.S358L is fully penetrant in males and associated with arrhythmogenic right ventricular cardiomyopathy (ARVC) type 5. Even though, TMEM43 is a phyllogenetically conserved protein throughout different species, little is known on the molecular mechanism of the mutation. TMEM43 is highly homologous to the single CG8111 gene in Drosophila melanogaster. The clincal relevant amino acid serine at position p358 is situated in the third transmembrane domain, the same applies to the corresponding amino acid p.S333L in the Drosophila homologue. Here, we established the first invertebrate model to study the pathophysiological effects of the human variant in the Drosophila homologue CG8111.

Methods

To investigate the effect of this missense mutation in flies, we introduced the variant CG8111 p.S333L in Drosophila and overexpressed the mutant variant CG8111p.S333L in transgenic flies, either ubiquitously or in a tissue specific manner. Cardiac specific physiological effects of the mutant variant were examined by Semi-automatic Optical Heartbeat Analysis (SOHA) for heart rate, systolic and diastolic interval, fractional shortening and arrhythmicity index. Mutant and wildtype flies were further compared by mass-spectrometry for differentially regulated proteins. Metabolites were analysed by NMR accordingly.

Results

Our studies showed that in comparison to the wild type the ubiquitously overexpressed variant leads to lethal effects at pupal stages. Individuals died from reduced food uptake and excessive lipid droplet accumulation in adipocytes. CG8111p.S333L animals up-regulate proteins, involved in fatty acid metabolism pathways. Metabolomic data revealed increased amounts of fatty acids accumulation in mutant versus wild-type animals. Furthermore, heart specific overexpression of CG8111p.S333L was associated with arrhythmias in five week old male flies.

Conclusions

CG8111p.S333L overexpression is a lethal genotype in Drosophila. Our data suggest a metabolic dysfunction associated with the mutant form of CG8111p.S333L, which shares pathophysiological characteristics of the human condition in TMEM43 p.S358L carriers.

The ARVC causing TMEM43-p.S358L mutation has impact on the lipid metabolism

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 450999875

Background

Transmembrane protein 43 (TMEM43) is a phylogenetically highly conserved and ubiquitously expressed protein with unknown function. The protein contains four transmembrane domains located in the inner nuclear envelope and the endoplasmic reticulum (ER). A large acidic domain located between the first and second transmembrane domains, with unknown function, is exposed to the ER lumen. TMEM43-c.1073C>T is a rare fully penetrant mutation that leads to arrhythmogenic right ventricular cardiomyopathy type 5 (ARVC5). ARVC5 is associated with myocardial fibrosis, fibrofatty replacement and progressive loss of right ventricular myocardial tissue and severe arrhythmias causing sudden cardiac death (SCD) especially in males. The pathomechanism of ARVC5 is not well understood.

Methods

We isolated and cultivated primary dermal fibroblasts of three different individuals carrying TMEM43-c.1073C>T (p.S358L). Proteins were isolated from cell cultures and analysed by Orbitrap mass spectrometry. Proteome data of the mutation carriers were compared to TMEM43 wildtype control fibroblasts. Additionally, TMEM43-p.S358L and control fibroblasts were investigated by transmission electron microscopy (TEM).

Results

A total of 9 proteins were upregulated in fibroblasts expressing TMEM43-p.S358L (p<0.003). Of note, proteins involved in fatty acid metabolism, such as acyl-coenzym A (CoA) thioesterase 1 encoded by the gene ACOT1, and Malonate-CoA ligase encoded by ACSF3 were strongly upregulated (FC>3 or >2, respectively). ACOT1 catalyses hydrolysis acyl-CoAs to long chain fatty acids and CoA. ACSF3 catalyses the formation of thioesters between fatty acids and CoA. ACOT1 and ACSF3 regulate the intracellular levels of free fatty acids and are expressed in the cardiomyocytes of the human myocardium. TEM images of TMEM43-p.S358L carriers show an accumulation of multilamellar vesicles, which may support the dysregulation of the lipid metabolism.

Conclusion

The differentially regulated proteins in dermal fibroblasts and the accumulation of multilamellar vesicles suggest a dysregulation of the lipid metabolism by TMEM43-p.S358L, which may also have relevance for the development of myocardial fibrofatty replacement contributing to ARVC5.

The properties of alamethicin incorporated into planar lipid bilayers under the influence of microgravity

During evolution, life on earth had adapted to the gravity of 1g. Due to space flight, in the last decades the question arose what happens to the brain under microgravity on the molecular level. Ion channels among others are the molecular basis of brain function. Therefore, the investigation of ion channel function under microgravity seems to be a promising approach to gather knowledge on brain function during space flight. In a first step, the ion channel forming peptide Alamethicin was used as a model channel in an artificial membrane. It is well suitable for this kind of investigation, since its properties are well described under standard gravity. For that reason, changes due to microgravity can be detected easily. All experiments were performed in the German drop tower at ZARM-FAB, Bremen. A special set-up was constructed based on the bilayer technique introduced by Mueller and Rudin. All functions of this set-up can be observed and controlled remotely. In the first set of experiments, a dramatic change of electrical properties of Alamethicin under microgravity could be observed. Mainly, the pore frequency is significantly reduced.

Alpha7 and alpha8 nicotinic receptor subtypes immunopurified from chick retina have different immunological, pharmacological and functional properties

Nicotinic receptors are present in the chick retina, but their structure and functional characteristics are still unclear. Using anti-alpha7 and anti-alpha8 subunit-specific antibodies, we immunopurified the alpha7 and alpha8 subtypes of chick retina neuronal nicotinic receptors. When analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, the two purified subtypes consistently showed a similar peptide composition characterized by the presence of two major peptides of M(r) 58 +/- 1 and 54 +/- 1 kDa, and two minor peptides of 67 and 61 +/- 1 kDa. In the alpha7 subtype, the 58 kDa peptide was recognized by anti-alpha7 but not by anti-alpha8 antibodies; in the alpha8 subtype, the 58 kDa peptide was recognized only by anti-alpha8 antibodies. The alpha7 subtype had a single class of [125I]alpha-bungarotoxin binding sites with a K(D) value of 1.2 nM, whereas the purified alpha8 subtype had two classes of binding sites, one with a K(D) of 5.5 nM and the other with very high affinity (KD 52 pM), but present in only 8% of the receptors. Competition binding experiments also showed the presence on the alpha8 subtype of high- and low-affinity classes of binding sites; the affinity for cholinergic drugs of the former was greater than that of the single class present on the alpha7 subtype. When reconstituted in planar lipid bilayers, both subtypes formed ligand-gated cation channels with major conductance levels of 42 and 52 pS but with different lifetimes; the two channels were activated by agonists and blocked by d-tubocurarine and the glycinergic antagonist strychnine. In line with the binding data, the reconstituted alpha8 subtype had greater agonist sensitivity than the alpha7 subtype.