Mitofusin-2 Regulates Platelet Mitochondria and Function

BACKGROUND

Single-nucleotide polymorphisms linked with the rs1474868 T allele (MFN2 [mitofusin-2] T/T) in the human mitochondrial fusion protein MFN2 gene are associated with reduced platelet MFN2 RNA expression and platelet counts. This study investigates the impact of MFN2 on megakaryocyte and platelet biology.

METHODS

Mice with megakaryocyte/platelet deletion of Mfn2 (Mfn2-/- [Mfn2 conditional knockout]) were generated using Pf4-Cre crossed with floxed Mfn2 mice. Human megakaryocytes were generated from cord blood and platelets isolated from healthy subjects genotyped for rs1474868. Ex vivo approaches assessed mitochondrial morphology, function, and platelet activation responses. In vivo measurements included endogenous/transfused platelet life span, tail bleed time, transient middle cerebral artery occlusion, and pulmonary vascular permeability/hemorrhage following lipopolysaccharide-induced acute lung injury.

RESULTS

Mitochondria was more fragmented in megakaryocytes derived from Mfn2-/- mice and from human cord blood with MFN2 T/T genotype compared with control megakaryocytes. Human resting platelets of MFN2 T/T genotype had reduced MFN2 protein, diminished mitochondrial membrane potential, and an increased rate of phosphatidylserine exposure during ex vivo culture. Platelet counts and platelet life span were reduced in Mfn2-/- mice accompanied by an increased rate of phosphatidylserine exposure in resting platelets, especially aged platelets, during ex vivo culture. Mfn2-/- also decreased platelet mitochondrial membrane potential (basal) and activated mitochondrial oxygen consumption rate, reactive oxygen species generation, calcium flux, platelet-neutrophil aggregate formation, and phosphatidylserine exposure following dual agonist activation. Ultimately, Mfn2-/- mice showed prolonged tail bleed times, decreased ischemic stroke infarct size after cerebral ischemia-reperfusion, and exacerbated pulmonary inflammatory hemorrhage following lipopolysaccharide-induced acute lung injury. Analysis of MFN2 SNPs in the iSPAAR study (Identification of SNPs Predisposing to Altered ALI Risk) identified a significant association between MFN2 and 28-day mortality in patients with acute respiratory distress syndrome.

CONCLUSIONS

Mfn2 preserves mitochondrial phenotypes in megakaryocytes and platelets and influences platelet life span, function, and outcomes of stroke and lung injury.

Abstract MP248: Sequestrosome1/p62 Regulates Redox Homeostasis Via The Nrf2-keap1 Pathway In The Murine Heart

Sequestosome1 (p62) is a multifunctional signaling molecule and an autophagy adaptor protein. Previous work demonstrated that mice with whole-body p62 knockout recapitulated many detrimental features of aging. Of note, these mice developed late onset obesity and systemic abnormalities that could have contributed to their aging phenotype. Multiple studies have also shown that cardiac dysfunction can be linked to an increase in oxidative stress. The Nrf2-Keap1 pathway is critical for protection against oxidative stress and p62 has been shown to interact with Keap1, thus allowing Nrf2 activation to induce anti-oxidant responses. However, the role of p62 in the heart is not well known. We tested the hypothesis that p62 plays an important homeostatic role in the heart through the regulation of redox homeostasis via the Nrf2-Keap1 pathway. Wild-type and cardiomyocytes-specific p62 knockout (cp62 KO) mice at 8 weeks and 60 weeks of age were used. At 8 weeks, cp62KO mice exhibited mild but significant contractile dysfunction compared to the wild-type controls. By 60 weeks, the KO mice developed cardiac hypertrophy, fibrosis and increased oxidative stress. cp62 KO hearts had decreased Nrf2 nuclear translocation and activation as evidenced by a 50% (p<0.005) reduction in the expression of the Nrf2 target glutathione S-transferase A4 ( Gsta2 ) gene. These findings were further validated by transcriptomic analysis followed by KEGG pathway analysis, which indicated that redox pathways were altered in the 60-week p62 null hearts. To examine the mechanisms involved in p62 regulation of Nrf2-Keap signaling, we utilized rat cardiac H9c2 myoblasts. Loss of p62 using p62 siRNA in H9c2 cells resulted in decreased Nrf2 levels and increased oxidative stress. These pathological consequences of suppressing p62 could be attributed to increased Nrf2 degradation via the proteasome. Together, these results reveal a previously uncharacterized role for p62 in the maintenance of cardiac redox signaling in the mouse heart.