Tissue regeneration entails replenishing of damaged cells, appropriate cell differentiation and inclusion of regenerated cells into functioning tissues. In adult humans, the capacity of the injured spinal cord and muscle to self-repair is limited. In contrast, the amphibian larva can regenerate its tail after amputation with complete recovery of muscle, notochord and spinal cord. The cellular and molecular mechanisms underlying this phenomenon are still unclear. Here we show that upon injury muscle cell precursors exhibit Ca(2+) transients that depend on Ca(2+) release from ryanodine receptor-operated stores. Blockade of these transients impairs muscle regeneration. Furthermore, inhibiting Ca(2+) transients in the regenerating tail prevents the activation and proliferation of muscle satellite cells, which results in deficient muscle replenishment. These findings suggest that Ca(2+)-mediated activity is critical for the early stages of muscle regeneration, which may lead to developing effective therapies for tissue repair.

Preservation of mitochondrial integrity is critical for maintaining cellular homeostasis. Mitophagy is a mitochondria-specific type of autophagy which eliminates damaged mitochondria thereby contributing to mitochondrial quality control. Depolarization of the mitochondrial membrane potential is an established mechanism for inducing mitophagy, mediated through PINK1 stabilization and Parkin recruitment to mitochondria. Hexokinase-II (HK-II) which catalyzes the first step in glucose metabolism, also functions as a signaling molecule to regulate cell survival, and a significant fraction of cellular HK-II is associated with mitochondria (mitoHK-II). We demonstrate here that pharmacological interventions and adenoviral expression of a mitoHK-II dissociating peptide which reduce mitoHK-II levels lead to robust increases in mitochondrial Parkin and ubiquitination of mitochondrial proteins in cardiomyocytes and in a human glioblastoma cell line 1321N1, independent of mitochondrial membrane depolarization or PINK1 accumulation. MitoHK-II dissociation-induced mitophagy was demonstrated using Mito-Keima in cardiomyocytes and in 1321N1 cells. Subjecting cardiomyocytes or the in vivo heart to ischemia leads to modest dissociation of mitoHK-II. This response is potentiated by expression of the mitoHK-II dissociating peptide, which increases Parkin recruitment to mitochondria and, importantly, provides cardioprotection against ischemic stress. These results suggest that mitoHK-II dissociation is a physiologically relevant cellular event that is induced by ischemic stress, the enhancement of which protects against ischemic damage. The mechanism which underlies the effects of mitoHK-II dissociation can be attributed to the ability of Bcl2-associated athanogene 5 (BAG5), an inhibitor of Parkin, to localize to mitochondria and form a molecular complex with HK-II. Overexpression of BAG5 attenuates while knockdown of BAG5 sensitizes the effect of mitoHK-II dissociation on mitophagy. We suggest that HK-II, a glycolytic molecule, can function as a sensor for metabolic derangements at mitochondria to trigger mitophagy, and modulating the intracellular localization of HK-II could be a novel way of regulating mitophagy to prevent cell death induced by ischemic stress.

Skeletal muscle-specific stem cells are pivotal for tissue development and regeneration. Muscle plasticity, inherent in these processes, is also essential for daily life activities. Great advances and efforts have been made in understanding the function of the skeletal muscle-dedicated stem cells, called muscle satellite cells, and the specific signaling mechanisms that activate them for recruitment in the repair of the injured muscle. Elucidating these signaling mechanisms may contribute to devising therapies for muscular injury or disease. Here we review the studies that have contributed to our understanding of how calcium signaling regulates skeletal muscle development, homeostasis and regeneration, with a focus on the calcium dynamics and calcium-dependent effectors that participate in these processes.

Mitophagy, a mitochondria-specific form of autophagy, removes dysfunctional mitochondria and is hence an essential process contributing to mitochondrial quality control. PTEN-induced kinase 1 (PINK1) and the E3 ubiquitin ligase Parkin are critical molecules involved in stress-induced mitophagy, but the intracellular signaling mechanisms by which this pathway is regulated are unclear. We tested the hypothesis that signaling through RhoA, a small GTPase, induces mitophagy via modulation of the PINK1/Parkin pathway as a protective mechanism against ischemic stress. We demonstrate that expression of constitutively active RhoA as well as sphingosine-1-phosphate induced activation of endogenous RhoA in cardiomyocytes result in an accumulation of PINK1 at mitochondria. This is accompanied by translocation of Parkin to mitochondria and ubiquitination of mitochondrial proteins leading to recognition of mitochondria by autophagosomes and their lysosomal degradation. Expression of RhoA in cardiomyocytes confers protection against ischemia, and this cardioprotection is attenuated by siRNA-mediated PINK1 knockdown. In vivo myocardial infarction elicits increases in mitochondrial PINK1, Parkin, and ubiquitinated mitochondrial proteins. AAV9-mediated RhoA expression potentiates these responses and a concurrent decrease in infarct size is observed. Interestingly, induction of mitochondrial PINK1 accumulation in response to RhoA signaling is neither mediated through its transcriptional upregulation nor dependent on depolarization of the mitochondrial membrane, the canonical mechanism for PINK1 accumulation. Instead, our results reveal that RhoA signaling inhibits PINK1 cleavage, thereby stabilizing PINK1 protein at mitochondria. We further show that active RhoA localizes at mitochondria and interacts with PINK1, and that the mitochondrial localization of RhoA is regulated by its downstream effector protein kinase D. These findings demonstrate that RhoA activation engages a unique mechanism to regulate PINK1 accumulation, induce mitophagy and protect against ischemic stress, and implicates regulation of RhoA signaling as a potential strategy to enhance mitophagy and confer protection under stress conditions.

Nervous system development relies on the generation of neurons, their differentiation and establishment of synaptic connections. These events exhibit remarkable plasticity and are regulated by many developmental cues. Here, we review the mechanisms of three classes of these cues: morphogenetic proteins, electrical activity, and the environment. We focus on second messenger dynamics and their role as integrators of the action of diverse cues, enabling plasticity in the process of neural development.