Fission and selective fusion govern mitochondrial segregation and elimination by autophagy.

Accumulation of depolarized mitochondria within beta-cells has been associated with oxidative damage and development of diabetes. To determine the source and fate of depolarized mitochondria, individual mitochondria were photolabeled and tracked through fusion and fission. Mitochondria were found to go through frequent cycles of fusion and fission in a 'kiss and run' pattern. Fission events often generated uneven daughter units: one daughter exhibited increased membrane potential (delta psi(m)) and a high probability of subsequent fusion, while the other had decreased membrane potential and a reduced probability for a fusion event. Together, this pattern generated a subpopulation of non-fusing mitochondria that were found to have reduced delta psi(m) and decreased levels of the fusion protein OPA1. Inhibition of the fission machinery through DRP1(K38A) or FIS1 RNAi decreased mitochondrial autophagy and resulted in the accumulation of oxidized mitochondrial proteins, reduced respiration and impaired insulin secretion. Pulse chase and arrest of autophagy at the pre-proteolysis stage reveal that before autophagy mitochondria lose delta psi(m) and OPA1, and that overexpression of OPA1 decreases mitochondrial autophagy. Together, these findings suggest that fission followed by selective fusion segregates dysfunctional mitochondria and permits their removal by autophagy.

Mitochondrial networking protects beta-cells from nutrient-induced apoptosis.

OBJECTIVE: Previous studies have reported that beta-cell mitochondria exist as discrete organelles that exhibit heterogeneous bioenergetic capacity. To date, networking activity, and its role in mediating beta-cell mitochondrial morphology and function, remains unclear. In this article, we investigate beta-cell mitochondrial fusion and fission in detail and report alterations in response to various combinations of nutrients. RESEARCH DESIGN AND METHODS: Using matrix-targeted photoactivatable green fluorescent protein, mitochondria were tagged and tracked in beta-cells within intact islets, as isolated cells and as cell lines, revealing frequent fusion and fission events. Manipulations of key mitochondrial dynamics proteins OPA1, DRP1, and Fis1 were tested for their role in beta-cell mitochondrial morphology. The combined effects of free fatty acid and glucose on beta-cell survival, function, and mitochondrial morphology were explored with relation to alterations in fusion and fission capacity. RESULTS: beta-Cell mitochondria are constantly involved in fusion and fission activity that underlies the overall morphology of the organelle. We find that networking activity among mitochondria is capable of distributing a localized green fluorescent protein signal throughout an isolated beta-cell, a beta-cell within an islet, and an INS1 cell. Under noxious conditions, we find that beta-cell mitochondria become fragmented and lose their ability to undergo fusion. Interestingly, manipulations that shift the dynamic balance to favor fusion are able to prevent mitochondrial fragmentation, maintain mitochondrial dynamics, and prevent apoptosis. CONCLUSIONS: These data suggest that alterations in mitochondrial fusion and fission play a critical role in nutrient-induced beta-cell apoptosis and may be involved in the pathophysiology of type 2 diabetes.

Initial experience with dorsal venous arch arterialization for limb salvage.

Surgical reconstruction for patients with symptomatic lower extremity arterial occlusive disease is successful when a suitable distal target vessel is present. In patients with unreconstructable disease, practical surgical options are at a minimum. We report our initial experience with dorsal venous arch arterialization (DVAA) for limb salvage. Patients with a lower extremity arteriogram and tibia/plantar artery duplex scan demonstrating unreconstructable occlusive disease were evaluated for DVAA. The venous arch valve lysis technique consisted of retrograde balloon catheter, arterial dilator disruption, and direct valvulectomy. Outcome variables included patency, limb salvage rate, and toe pressure alterations. Initial results suggest that DVAA may be a viable option for end-stage limb salvage. Application of the DVAA appears to be more suitable for patients with symptoms secondary to atherosclerosis than those with Buerger's disease.