ROLE OF INNER MEMBRANE POTENTIAL IN TARGETING MITOCHONDRIA TO PRIMORDIAL GERM CELLS

Which mitochondria are transmitted to progeny? A subset of inactivated mitochondria, or the most active ones? The need for a functional mitochondrial genome, protected from oxidative damage induced by mitochondrial activity, is at the base of the 'inactive theory' [1,2]. By contrast, for the 'active theory' mitochondrial activity is an indication of correct functioning, and it may be the phenotype under selection for the inheritance mechanism [3,4]. Inherited mitochondria appear to be a specific organelle subset. Mitochondria of the Balbiani body (Bb), a distinctive structure found in oocytes of many organisms, are taken up by embryo primordial germ cells (PGCs) together with germ line determinants [5]. The selective accumulation in the Bb of mitochondria with high inner membrane potential (Δψm) is consistent with the active theory [6]. Mitochondrial trafficking is tightly associated with the ability of mitochondria to produce ATP [7], so mitochondria with high Δψm, indicating mtDNA integrity, would have a better chance to attach to microtubules, be recruited into the Bb, and be preferentially carried into PGCs [4]. Moreover, it appears that the timing of germ line specification can influence this process. In animals with an early mechanism of germ line specification, the segregation of the most active mitochondria in gonadic presumptive blastomeres has to take place contextually, in some cases allowing sperm mitochondria segregation into PCGs [4]. In animals in which germ line specification is driven by inductive signals at a later developmental stage, if sperm mitochondria are not degraded they are segregated early into specific blastomeres, preventing their spread in the embryo [4]. In summary, Δψm may allow the most active mitochondria to reach specific locations, but the different timing of germ line specification can influence the outcome of the segregation mechanism.