Consequences of mitofusin ablation in cardiac myocytes. A genetic study in mice

Mitofusin-1 (Mfn-1) and mitofusin-2 (Mfn-2) are membrane-embedded mechanoenzymes involved in the remodelling and merging of the mitochondrial biomembrane. In differentiated cardiac myocytes, mitochondria occupy a third of the cell's volume and express both Mfn-1 and Mfn-2. The present thesis was aimed at exploring the roles of Mfn-1 and Mfn-2 specifically in cardiac myocytes using loss-of-function approaches in mice. We individually ablated either Mfn-1 or Mfn-2 specifically in cardiac myocytes. Ultramicroscopic analysis conducted in hearts of Mfn-1 KO or Mfn-2 KO mice revealed significant alterations in mitochondrial structure. Nevertheless, these knockout mice had normal heart function and a normal lifespan. Furthermore, Mfn-1 and Mfn-2 deficient mitochondria exhibited normal respiratory function in vitro. We also tested the susceptibility of Mfn-1 and Mfn-2 mitochondria against stress and unexpectedly found that the absence of these proteins conferred resistance to mitochondrial permeability transition (MPT). MPT reflects the loss of membrane integrity in mitochondria and is strongly associated with cell death. Using isolated adult cardiac myocytes we were able to demonstrate that the cell death in either Mfn-1 KO or Mfn-2 KO cells was delayed, consistent with the idea that MPT is attenuated in the absence of these proteins. We also utilized Mfn-2 KO mice to demonstrate that loss of Mfn-2 was associated with protection against cardiac ischemia/reperfusion injury, a stress model strongly linked to MPT. This work suggested for the first time that both Mfn-1 and Mfn-2 have important roles in the process of MPT. To incorporate these novel findings in context with the well-known role of mitofusins in membrane merging, I propose a working model where mitochondrial membrane fusion proceeds through formation of transient lipidic pores that compromise mitochondrial membrane integrity and serve as hotspots for MPT in conditions of stress. Lastly, we generated and characterized mice double-knockout (DKO) for Mfn-1 and Mfn-2. These mice are born in the expected ratios but undergo aberrant cardiac remodelling during the first week of their life and eventually succumb. The DKO mitochondria present multiple morphological and molecular abnormalities. This latter work illustrates that Mfn-1 and Mfn-2 operate interchangeably to regulate the early postnatal development of cardiac myocytes.