Potential therapeutics for myocardial ischemia-reperfusion injury. Focus on "Induction of cardioprotection by small netrin-1-derived peptides".

ACUTE MYOCARDIAL INFARCTION (MI) is a major cause of morbidity and mortality worldwide. MI occurs when a portion of the heart is deprived of oxygen due to blockage of a coronary artery; this injures heart muscle, leading to impaired cardiac function. In patients with MI, the treatment of choice for reducing acute myocardial ischemic injury and limiting MI size is the timely and effective restoration of blood perfusion. However, the process of reperfusion itself can paradoxically induce myocardial injury, known as myocardial ischemiareperfusion (I/R) injury, for which there is still no effective therapy (2). Experimental studies have identified several critical factors that act in concert to induce the detrimental effects of myocardial reperfusion injury. These factors are cytosolic and mitochondrial oxidative stress, intracellular Ca overload, mitochondrial permeability transition pore (MPTP) opening, and inflammation. Of these identified factors, reactive oxygen species (ROS) may be the most important, due to their functions in 1) inducing the opening of the MPTP; 2) mediating dysfunction of the sarcoplasmic reticulum, which contributes to Ca overload; and 3) acting as neutrophil chemoattractant. In a previous study, Zhang and Cai (10) reported that netrin-1 functions as a potent cardioprotective agent. Netrin-1 reduces I/R-induced myocardial injury via an antioxidative mechanism triggered by the netrin-1 receptor-mediated endothelial nitric oxide synthase (eNOS)/nitric oxide (NO) pathway (Fig. 1). Netrins are a family of secreted proteins that were first identified as guidance cues, directing cell and axon migration during neural development (4). In mammals, three secreted netrins, netrin 1, 3, and 4, and two membrane-tethered glycophosphatidylinositol-linked netrins, netrin G1 and G2, have been identified (4, 8). Netrins function through interactions with their canonical receptors. Netrin-1’s axonal functions have been linked to two classes of receptors, the DCC (deleted in colon cancer) family, including DCC and neogenin, and the Unc5 family (Unc5A through D). Besides their originally identified function in neural development, netrins have since been shown to play important roles in the development of various tissues and diseases, including morphogenesis of the vascular system, angiogenesis, atherosclerosis, and myocardial reperfusion injury. Netrin-1’s protection of myocardial I/R injury relies on the production of NO, which upregulates the accumulation of netrin-1’s receptor DCC and can further amplify netrin-1’s function (10) (Fig. 1). Therefore netrin-1-induced NO may be the key molecule underlying the marked reduction in infarct size and cardiac cell death. Netrin-1 reduction of infarct size was significantly attenuated in DCC / mice, indicating a key role of DCC in cardioprotection. Upon netrin-1 perfusion, its receptor DCC is activated, resulting in ERK1/2 activation, which leads to eNOSs1177 phosphorylation; activated eNOS then produces NO that mediates DCC upregulation, forming a positive feedback loop to amplify DCC activity and NO production. In three recent studies investigating roles and mechanisms of netrin-1 in cardioprotection, Cai’s group demonstrated that netrin-1 improves postinjury cardiac function in vivo via DCC/NO-dependent preservation of mitochondrial integrity, while attenuating autophagy (1). The authors demonstrated that netrin-1 preserves mitochondrial integrity via NO-dependent inhibition of NADPH oxidase 4 (NOX4) activity, recoupling of NOS, augmented NO bioavailability, and reduction of oxidative stress—ultimately preserving mitochondrial function (9). While investigating the mechanism underlying netrin-1-induced DCC accumulation, they discovered that NO upregulates DCC by inhibition of E3 ubiquitin ligase seven in absentia homolog (SIAH), which regulates DCC protein abundance through an ubiquitin-proteasome pathway (6). RNAi inhibition of SIAH proved to be beneficial in reducing infarct size and improving cardiac function by accumulation of DCC. Thus, the novel NO/SIAH/DCC signaling pathway is important for cardioprotection. In addition to posttranslational regulation, DCC may also be regulated by netrin-1 via transcriptional/posttranscriptional mechanisms (10). All the data above strongly support a novel role of netrin-1 in cardioprotection and its potential therapeutic application. However, the potential clinical application of netrin-1 is difficult to estimate because large protein molecules are generally unstable, difficult to purify, and costly to produce. Therefore, it is important to identify the minimal functional peptide that could preserve netrin-1 function in activating the receptor DCC and stimulating downstream signaling events. In this issue of the American Journal of Physiology-Cell Physiology, Cai’s group extend their previous finding and identify a 9-amino acid (aa) core sequence in the laminin V region of netrin-1 to be highly effective in cardioprotection (5). Structurally, netrins are laminin-related proteins. Netrin-1 has a laminin VI-like (Lam VI) domain, three cysteine-rich repeats similar to those of domain V of laminin subunits (Lam V, including V1, V2, and V3), and a COOH-terminal domain (C345C). Previous results have shown that the Lam VI and Lam V domains were involved in the association with receptor DCC (3). Lam V1, V2, and V3 domains were required primarily for the dorsal axon guidance activities of netrin-1 homolog UNC-6 in Caenorhabditis elegans (7). Therefore, these three domains in Lam V might contain the core element required for netrin-1 induced cardioprotection. Using synthesized peptides V1 (54 aa, residues 285-338), V2 (61 aa, residues 341–401), and V3 (48 aa, residues 404–451), the authors found that Address for reprint requests and other correspondence: M.-Z. Cui, Dept. of Biomedical and Diagnostic Sciences, College of Veterinary Medicine, Univ. of Tennessee, 2407 River Dr., Knoxville TN 37996 (e-mail: [email protected]). Am J Physiol Cell Physiol 309: C97–C99, 2015; doi:10.1152/ajpcell.00150.2015. Editorial Focus