Second, optimal mitochondrial dynamics appear to support enhanced neurite growth. In nonneuronal cells, mitochondrial morphology, biogenesis, distribution, and signaling change with the state of the cell.
65,66 Our results suggested mitochondrial dynamics also change with the state of neurite growth. Suppressing mitochondrial fission pharmacologically with acute Mdivi-1 (e.g.,
Figs. 5a–d) or increasing mitochondrial fusion genetically with Mfn-2 overexpression suppressed neurite growth (not shown) in embryonic and postnatal RGCs. Similarly, neurites with enhanced growth potential, for example E20 RGCs
1 or KLF6-overexpressing RGCs,
57 had smaller, less dense polarized mitochondria, and similar mtDNA replication rates in the distal neurite and growth cone. Conversely, mitochondria in neurites with suppressed neurite growth potential, for example KLF4-overexpressing and P5 RGCs, deviated from these dynamics. In KLF4-overexpressing RGCs, though mitochondria were smaller, mtDNA replication was suppressed in distal neurites, suggesting dysfunctional mitochondrial biogenesis and likely reduced bioenergetics, which depends on proper mtDNA replication in neurons.
67 In P5 RGCs, polarized mitochondrial areas, density, and Δψ
m were greater than in E20 RGCs over the first 2 DIV before decreasing dramatically by 3 DIV. Δψ
m is dynamic, fluctuating regionally and globally,
55 in growth cones,
68 in dendrites,
52 during axogenesis,
63 during mitochondrial transport,
53 and in response to synaptic activity,
53 and growth factors and guidance cues.
68 We extended these findings by showing Δψ
m also is dynamic in RGCs with differing intrinsic neurite growth potentials.