During the last decades, a vast amount of publications have unraveled the importance of mitochondrial dynamics during cells’s life and death. Nowadays, it is clear that the control of mitochondrial shape through the balance of the opposite processes of fusion and fission is essential in mitochondrial functions such as respiration and cell death. However, many aspects of the mechanisms underlying mitochondrial dynamics are yet unclear. In particular, mitochondrial fission was though to be a two-‐component system, with Fis1 being the receptor for the translocation of Drp1 to the OMM. The discovery of Mff and MiDs as receptors for Drp1 in mammals challenged the relevance of Fis1 in mitochondrial fission. In this project, we found compelling evidence supporting the importance of Fis1 gene in mitochondrial morphology with consequences in mitochondrial functionality and cell death. Additionally, we report the unexpected discovery that Fis1 is alternatively spliced in three variants expressed in several tissues. While two of these variants play a role in mitochondrial fission, increased mFis1.2 levels cause mitochondrial elongation.
By mapping the relative expression of mFis1.2 in pathophysiological conditions, we found it increased during macroautophagy in a PKA-‐dependent manner to produce mitochondrial elongation. Hence, we have identifying a novel mechanism for mitochondrial morphology changes during autophagy that,
together with the already known mechanism of PKA-‐dependent inhibition of Drp1, helps to enhance respiration in conditions of nutrient deprivation.
Great emphasis has been placed in the many post-‐translational modification that Drp1 suffers (phosphorylation, ubiquitination and sumoylation) and how they modulate mitochondrial fission. On the other hand, Drp1 receptors have been treated as invariable proteins, ready to interact with Drp1 whenever it is activated. The results presented in this work highlight the relevance of the modulation of Fis1 variant expression, especially of mFis1.2, in the regulation of mitochondrial morphology. Likewise, other fission factors may be regulated in a similar manner. For example, Mff also has splice variants that may have different expression according to internal or external cues as we showed for mouse Fis1.
It is interesting to note that only a small deletion in the amino-‐terminus of mFsi1.2 induces an apposite effect on mitochondrial morphology, given to the coiled coil domain in this region a regulatory role in mitochondrial dynamics.
However, whether the coiled coil arm is important in the interaction of Fis1 with itself or with other mitochondrial shaping proteins still has to be investigated.
Additionally, human Fis1 gene is also able to produce four splice variants, some with differences in the coiled coil arm, however, nothing is known about their expression pattern and regulation nor about their role in mitochondrial morphology. Hence, it is possible that mitochondrial shape in human cells may also be regulated by Fis1 splice variants. To this regard, it is of paramount importance to direct studies in mouse and human cells aimed to address the
121 hierarchical interaction between the different Drp1 receptors and their splice variants in order to produce a specific mitochondrial shape in response to a given stimulus. Most likely, a detailed mapping of the mitochondrial fission transcriptome and interactome will contribute to a clearer view of how mitochondrial fragmentation is achieved.
Finally, mitochondria have a central role in cancer and neurodegenerative diseases and data in this work clearly show that mFis1 impinges not only in the efficiency of mitochondrial respiration but as well in the progression of cell death. Future work in animal models should clarify whether the modulation of mFis1 variants expression could have an impact in physiological processes with pathological consequences.
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