Transitional correlation between inner-membrane potential and ATP levels of neuronal mitochondria

https://www.nature.com/articles/s41598-018-21109-2

R. Suzuki, K. Hotta and K. Oka

In this paper, the authors simultaneously measure mitochondrial membrane potential (Δψ) and mitochondrial ATP production (ATPmito) in dorsal root ganglion neurons from rat embryos. They measure the dynamics of Δψ and ATPmito, as well as their correlation, during physiological neuronal activity and focus on the following questions:

Is there a relation between Δψ, ATPmito and 

• mitochondrial size?
• mitochondrial transport velocity?
• mitochondrial transport direction?

 Furthermore, they ask the question

• What happens to Δψ and ATPmito during mitochondrial fusion and fission events? 20 fusion events and 20 fission events were investigated.

Some of the Results:

• Δψ and ATPmito were compared among anterogradely transported, retrogradely transported, and stationary mitochondria in axons. Retrogradely transported mitochondria had slightly lower Δψ compared to anterogradely transported mitochondria.
• No correlation was found between Δψ, ATPmito and mitochondrial velocity and transported distance.
• Post-fusion mitochondrial membrane potential Δψ seemed to be higher than the average of the pre-fusion potentials (i.e. Δψfused > 0.5 (Δψpre-1 + Δψpre-2)).*
• ATPmito was higher in the post-fusion mitochondrion compared to the average of the two pre-fusion mitochondria
•  The two post-fission mitochondria tended to have different values for
  Δψ and their average was typically lower than the pre-fission potential (again, no information on size was provided as discussed below*).
• No changes in ATPmito were observed upon fission
• Mitochondrial density was higher in growth cones (an extension of a developing or regenerating neurite seeking its synaptic target) compared to axons. 
• Average ATPmito levels were slightly lower in growth cones, though integrated ATP levels (over all mitochondria) were higher. 
• Average Δψ was higher in growth cones compared to axons
• Higher ATP levels in growth cones led to faster elongation of the axon, though no correlation between elongation speed and Δψ was found.
• ATPmito tends to follow a change in Δψ (i.e. the change in Δψ occurs first). 
  ATPmito and Δψ are not necessarily always correlated.
• Various other results were obtained which you can find by reading the paper!

*We note that no information was provided regarding mitochondrial size. If one of the pre-fusion mitochondria is much larger than the other, we might expect the membrane potential of the former to have more influence on the final potential of the post-fusion mitochondrion. In this case, one would not expect  Δψfused to be the arithmetic average of the two pre-fusion potentials.