Manuela Lavorato

Congenital myopathy results from misregulation of a muscle Ca2+ channel by mutant Stac3

Significance Skeletal muscle contractions are regulated by a process called excitation–contraction (EC) coupling, and defects in it are associated with numerous human myopathies. Recently, stac3 (SH3 and cysteine-rich domain 3) was identified as a key regulator of EC coupling and a STAC3 mutation as causal for the debilitating Native American myopathy (NAM). We now show that Stac3 controls EC coupling by regulating Ca2+ channels in muscles. Both the NAM mutation and a mutation that leads to the loss of Stac3 decrease the amount, organization, stability, and voltage sensitivity of Ca2+ channels. Furthermore, we find evidence that the NAM allele of STAC3 is linked to malignant hyperthermia, a common pharmacogenic disorder. These findings define critical roles for Stac3 in muscle contraction and human disease. Skeletal muscle contractions are initiated by an increase in Ca2+ released during excitation–contraction (EC) coupling, and defects in EC coupling are associated with human myopathies. EC coupling requires communication between voltage-sensing dihydropyridine receptors (DHPRs) in transverse tubule membrane and Ca2+ release channel ryanodine receptor 1 (RyR1) in the sarcoplasmic reticulum (SR). Stac3 protein (SH3 and cysteine-rich domain 3) is an essential component of the EC coupling apparatus and a mutation in human STAC3 causes the debilitating Native American myopathy (NAM), but the nature of how Stac3 acts on the DHPR and/or RyR1 is unknown. Using electron microscopy, electrophysiology, and dynamic imaging of zebrafish muscle fibers, we find significantly reduced DHPR levels, functionality, and stability in stac3 mutants. Furthermore, stac3NAM myofibers exhibited increased caffeine-induced Ca2+ release across a wide range of concentrations in the absence of altered caffeine sensitivity as well as increased Ca2+ in internal stores, which is consistent with increased SR luminal Ca2+. These findings define critical roles for Stac3 in EC coupling and human disease.

Increased mitochondrial nanotunneling activity, induced by calcium imbalance, affects intermitochondrial matrix exchanges.

Significance Nanotunnels are long, thin mitochondrial extensions that have been implied in direct long-distance (1 to >5 µm) communication between mitochondria of cardiac myocytes. The engineered RyR2A4860G+/− mutation, resulting in loss of function of the sarcoplasmic reticulum calcium release channel and arrhythmia, induces a striking increase in the frequency of long-distance intermitochondrial communication via nanotunnels without involvement of obvious mitochondrial migration. We use this model for exploring the significance of mitochondrial nanotunneling in myocardium and the contribution of microtubules to the formation of these unusual organelle extensions using EM tomography and live confocal imaging. This study constitutes an approach to arrhythmia investigations that focuses on a new target: the mitochondria. Exchanges of matrix contents are essential to the maintenance of mitochondria. Cardiac mitochondrial exchange matrix content in two ways: by direct contact with neighboring mitochondria and over longer distances. The latter mode is supported by thin tubular protrusions, called nanotunnels, that contact other mitochondria at relatively long distances. Here, we report that cardiac myocytes of heterozygous mice carrying a catecholaminergic polymorphic ventricular tachycardia-linked RyR2 mutation (A4860G) show a unique and unusual mitochondrial response: a significantly increased frequency of nanotunnel extensions. The mutation induces Ca2+ imbalance by depressing RyR2 channel activity during excitation–contraction coupling, resulting in random bursts of Ca2+ release probably due to Ca2+ overload in the sarcoplasmic reticulum. We took advantage of the increased nanotunnel frequency in RyR2A4860G+/− cardiomyocytes to investigate and accurately define the ultrastructure of these mitochondrial extensions and to reconstruct the overall 3D distribution of nanotunnels using electron tomography. Additionally, to define the effects of communication via nanotunnels, we evaluated the intermitochondrial exchanges of matrix-targeted soluble fluorescent proteins, mtDsRed and photoactivable mtPA-GFP, in isolated cardiomyocytes by confocal microscopy. A direct comparison between exchanges occurring at short and long distances directly demonstrates that communication via nanotunnels is slower.

De novo reconstitution reveals the proteins required for skeletal muscle voltage-induced Ca2+ release

Significance Excitation–contraction coupling (ECC) in vertebrate skeletal muscle depends upon specialized junctions at which CaV1.1, a voltage-gated channel in the plasma membrane, interacts with RyR1, a calcium release channel in the sarcoplasmic reticulum. Because calcium flux via CaV1.1 is unnecessary for ECC, CaV1.1 is thought to “conformationally couple” to RyR1. Studies of muscle cells with gene knockouts have shown that additional, junctional proteins are also necessary, but are unable to reveal whether or not these complete the set of required proteins. Here, we show that conformational coupling can be conferred upon tsA201 cells by expressing five junctional proteins (CaV1.1, RyR1, β1a, Stac3, and junctophilin2), thus establishing a minimum set of proteins supporting CaV1.1–RyR1 conformational coupling in skeletal muscle. Skeletal muscle contraction is triggered by Ca2+ release from the sarcoplasmic reticulum (SR) in response to plasma membrane (PM) excitation. In vertebrates, this depends on activation of the RyR1 Ca2+ pore in the SR, under control of conformational changes of CaV1.1, located ∼12 nm away in the PM. Over the last ∼30 y, gene knockouts have revealed that CaV1.1/RyR1 coupling requires additional proteins, but leave open the possibility that currently untested proteins are also necessary. Here, we demonstrate the reconstitution of conformational coupling in tsA201 cells by expression of CaV1.1, β1a, Stac3, RyR1, and junctophilin2. As in muscle, depolarization evokes Ca2+ transients independent of external Ca2+ entry and having amplitude with a saturating dependence on voltage. Moreover, freeze-fracture electron microscopy indicates that the five identified proteins are sufficient to establish physical links between CaV1.1 and RyR1. Thus, these proteins constitute the key elements essential for excitation–contraction coupling in skeletal muscle.

Skeletal muscle microalterations in patients carrying Malignant Hyperthermia-related mutations of the e-c coupling machinery

We have compared the ultrastructure of skeletal muscle biopsies from patients that have survived a [Malignant Hyperthermia, MH] episode and siblings that test positive for MH susceptibility with those from siblings that tested negatives. The aim is to establish whether life long exposure to the MH-related mutation effects may result in subtle abnormalities even in the absence of active episodes and/or clinically detectable deficiencies. Although a specific ultrastructural signature for MH mutants cannot be demonstrated, an MH related pattern of minor alterations does exist. These include the tendency for micro damage to the contractile apparatus and a higher than normal level of mitochondrial abnormalities.