Taking shape: regulating mitochondria morphology through alternative splicing and phosphorylation of fission factor proteins

Mitochondria are important cellular organelles whose functions include generation of ATP, sequestration and release of pro-apoptotic molecules and calcium buffering. Mitochondria function is tightly linked to organelle morphology, which exits in a dynamic spectrum between a highly interconnected/fused mitochondria network to a punctate/fragmented scattering of individual mitochondrion. A family of large GTPase enzymes modulates this spectrum, with fusion catalyzed through the actions of mitofusin 1 and 2 (Mfn1/2) on the outer mitochondria membrane (OMM) and optic atrophy 1 (Opa1) causing fusion of the inner mitochondria membrane (IMM). On the other end of the spectrum, fragmentation is catalyzed through the actions of dynamin-related protein 1 (Drp1). Drp1 is recruited from the cytosol to binding partners at the OMM, organizes into concentric spiral rings, undergoes GTP hydrolysis to constrict the ring and pinches mitochondrion in two.

While fragmentation is achieved through the action of only one GTPase enzyme, the mechanisms behind the complex regulation of Drp1 remain relatively obscure. In order to expand upon known Drp1 regulatory mechanisms, an examination of how both Drp1 splicing and Drp1 recruitment to the OMM contributes to protein regulation is necessary. Drp1 contains three alternatively spliced exons, resulting in the potential generation of eight protein isoforms. Each of these isoforms is capable of inducing mitochondrial fragmentation, however one exon arrangement (termed Drp1-x01) can also bind to microtubules within the cell. Characterization of the Drp1-x01 isoform at both the RNA and protein level indicate an important, yet incompletely characterized, role in immune system biology.

Drp1 is capable of interacting with several proteins localized at the OMM. Among these, mitochondria fission factor (Mff) has been implicated in the formation of Drp1 spirals and the eventual fragmentation process. Mff contains four alternatively spliced exons as well as several phosphorylation sites identified through nonbiased phosphoproteomic screens. Inclusion of alternative exons to the Mff structure decreases its ability to recruit Drp1 from the cytosol, while phosphomimetic substitutions to conserved serine residues enhances the Drp1::Mff interaction. Taken together, this suggests that regulation of mitochondrial fragmentation occurs at the pretranslational (alternative splicing) and the posttranslational (phosphorylation) level is critical for maintaining the complex, yet essential, balance between mitochondrial fission and fragmentation.