Brian Glancy, Lisa M. Hartnell, Daniela Malide, Zu-Xi Yu, Christian A. Combs, Patricia S. Connelly, Sriram Subramaniam, and Robert S. Balaban
Cells need energy, especially muscle cells. Mitochondria generate part of this energy and to be able to do this they need to be supplied with various resources. Muscle cells can be quite big, so these resources (that enter the cell at its periphery) need to diffuse through the cell towards the mitochondria, which might take a long time. Is there a better, more efficient way of producing energy in large muscle cells?
In this paper they show that the mitochondrial network provides a conductive pathway for energy distribution.
Mitochondria use their Electron Transport Chain protein complexes to pump protons across their membrane, which creates an electrochemical gradient in which energy is stored. Their ATP synthase then uses this stored energy to make ATP. If many mitochondria are connected in the cell (i.e. if they all have an electrically continuous inner membrane) then a mitochondrion at one end of the cell can use the potential energy created by a mitochondrion at the other end of the cell, to generate ATP. The conduction of electric potential along the mitochondria can be faster than all kinds of resources needing to diffuse through the cell. This idea was first proposed by a Russian scientist Владимир Скулачёв (Vladimir Skulachev).
Here they show that the proteins involved in generating the electrochemical gradient are mainly found at the cell periphery (where the resources enter the cell), while proteins involved in using this energy to create ATP are found in the cell's interior. They also show that the mitochondria are indeed electrically connected to each other. The mitochondria in muscle cells are organized in a way that facilitates energy conduction.
The question remains whether this mitochondrial conductivity plays a role in all cells, or only in cells that are very energy demanding. Skin cells for example, hardly seem to need a fast conducting mitochondrial network. Nevertheless, mitochondrial fusion and network forming is seen in a variety of cells, so the fusion of mitochondria probably has other uses as well.