The benefits of adding insult to injury

Spinal axon regeneration induced by elevation of cyclic AMP

Myelin inhibitors, including MAG, are major impediments to CNS regeneration. However, CNS axons of DRGs regenerate if the peripheral branch of these neurons is lesioned first. We show that 1 day post-peripheral-lesion, DRG-cAMP levels triple and MAG/myelin no longer inhibit growth, an effect that is PKA dependent. By 1 week post-lesion, DRG-cAMP returns to control, but growth on MAG/myelin improves and is now PKA independent. Inhibiting PKA in vivo blocks the post-lesion growth on MAG/myelin at 1 day and attenuates it at 1 week. Alone, injection of db-cAMP into the DRG mimics completely a conditioning lesion as DRGs grow on MAG/myelin, initially, in a PKA-dependent manner that becomes PKA independent. Importantly, DRG injection of db-cAMP results in extensive regeneration of dorsal column axons lesioned 1 week later. These results may be relevant to developing therapies for spinal cord injury.

Central nervous system regeneration: mission impossible?

1. Attempts to induce clinical repair after central nervous system injury, such as spinal cord damage, are likely to involve several protocols because eliciting a regenerative response from an injured central neuron is a complex task. Future treatments, applied when a window of opportunity exists, address the requirements for regeneration. 2. Application of trophic support to the lesion site for axotomized neurons aims to initiate and maintain a cell body response conducive to axonal regrowth. 3. Surgical intervention may provide a bridge across the injury site that contains either Schwann cells or olfactory bulb ensheathing cells derived from the patient's own tissue. 4. The application of antibodies may block the inhibitory action of myelin-associated molecules and other glial elements. 5. Gene therapy may induce the correct cascade of guidance molecules to be released at appropriate times. 6. Physical rehabilitation may ensure that muscle wastage is reduced and encourages functional reconnection.

Axon regeneration: Vaccinating against spinal cord injury

Myelin is a potent inhibitor of axon regeneration, but has been viewed as just one of many factors that prevent regeneration after injury. So it comes as a surprise that immunization against myelin has been found to allow extensive axon regeneration after injury, without apparent autoimmune-induced demyelination.