Organelle transport is vital for the development and maintenance of axons, in which the distances between sites of organelle biogenesis, function, and recycling or degradation can be vast. Movement of mitochondria in axons can serve as a general model for how all organelles move: mitochondria are easy to identify, they move along both microtubule and actin tracks, they pause and change direction, and their transport is modulated in response to physiological signals. However, they can be distinguished from other axonal organelles by the complexity of their movement and their unique functions in aerobic metabolism, calcium homeostasis and cell death. Mitochondria are thus of special interest in relating defects in axonal transport to neuropathies and degenerative diseases of the nervous system. Studies of mitochondrial transport in axons are beginning to illuminate fundamental aspects of the distribution mechanism. They use motors of one or more kinesin families, along with cytoplasmic dynein, to translocate along microtubules, and bidirectional movement may be coordinated through interaction between dynein and kinesin-1. Translocation along actin filaments is probably driven by myosin V, but the protein(s) that mediate docking with actin filaments remain unknown. Signaling through the PI 3-kinase pathway has been implicated in regulation of mitochondrial movement and docking in the axon, and additional mitochondrial linker and regulatory proteins, such as Milton and Miro, have recently been described.

/pdf/the-axonal-transport-of-mitochondria-3x5ew6t008.pdf

The axonal transport of mitochondria

Mitochondria have a number of essential roles in neuronal function. Their complex mobility patterns within neurons are characterized by frequent changes in direction. Mobile mitochondria can become stationary or pause in regions that have a high metabolic demand and can move again rapidly in response to physiological changes. Defects in mitochondrial transport are implicated in the pathogenesis of several major neurological disorders. Research into the mechanisms that regulate mitochondrial transport is thus an important emerging frontier.

/pdf/mitochondrial-transport-in-neurons-impact-on-synaptic-400ibpyp4x.pdf

Mitochondrial transport in neurons: impact on synaptic homeostasis and neurodegeneration

Mitochondrial dysfunction as a cause of optic neuropathies

Oligodendrocytes and Schwann cells are highly specialized glial cells that wrap axons with a multilayered myelin membrane for rapid impulse conduction. Investigators have recently identified axonal signals that recruit myelin-forming Schwann cells from an alternate fate of simple axonal engulfment. This is the evolutionary oldest form of axon-glia interaction, and its function is unknown. Recent observations suggest that oligodendrocytes and Schwann cells not only myelinate axons but also maintain their long-term functional integrity. Mutations in the mouse reveal that axonal support by oligodendrocytes is independent of myelin assembly. The underlying mechanisms are still poorly understood; we do know that to maintain axonal integrity, mammalian myelin-forming cells require the expression of some glia-specific proteins, including CNP, PLP, and MAG, as well as intact peroxisomes, none of which is necessary for myelin assembly. Loss of glial support causes progressive axon degeneration and possibly local inflammation, both of which are likely to contribute to a variety of neuronal diseases in the central and peripheral nervous systems.

Axon-Glial Signaling and the Glial Support of Axon Function

A biomechanical paradigm for axonal insult within the optic nerve head in aging and glaucoma.

Mitochondrial accumulation has been reported at sites of degenerating nerve fibres. The reason for focal accumulation of mitochondria within axons has not previously been investigated. The aim of this study was to quantitate changes in mitochondrial concentration within demyelinated axons and determine if these changes were due to obstruction of the free passage of organelles within the fibre. The average number of mitochondria per unit area of axon within axons of normal cat optic nerve and optic nerve in which demyelination had been induced with anti-galactocerebroside (GC) was determined using transmission electron microscopy. Increased numbers of mitochondria were demonstrated at the site of demyelination, peaking at approximately two and a half times the value for normal non-lesioned nerves at 6-7 days after anti-GC administration [0.66+/-0.06 (SD) mitochondria per microm2 of axon compared to 0.28+/-0.04 in controls; P < 0.05]. The mitochondrial concentration gradually decreased over the next 20 days towards normal values. The concentration of mitochondria within non-myelinated axons of cat retina (0.55+/-0.18) was also determined and shown to be comparable to the values for the demyelinated optic axons (0.66+/-0.06). It is proposed that the accumulation of mitochondria within demyelinated axons reflects the adaptive process of the axon during the early restoration of impulse conduction rather than the obstruction of the free passage of organelles within the fibre.

Focal accumulation of intra-axonal mitochondria in demyelination of the cat optic nerve.

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Focal accumulation of intra-axonal mitochondria in demyelination of the cat optic nerve.