Increased Synthesis of Spermidine as a Result of Upregulation of Arginase I Promotes Axonal Regeneration in Culture andIn Vivo

Abstract

Adult spinal axons do not spontaneously regenerate after injury. However, if the peripheral branch of dorsal root ganglion neurons is lesioned before lesioning the central branch of the same neurons in the dorsal column, these central axons will regenerate and, if cultured, are not inhibited from extending neurites by myelin-associated inhibitors of regeneration such as myelin-associated glycoprotein (MAG). This effect can be mimicked by elevating cAMP and is transcription dependent. The ability of cAMP to overcome inhibition by MAG in culture involves the upregulation of the enzyme arginase I (Arg I) and subsequent increase in synthesis of polyamines such as putrescine. Now we show that a peripheral lesion also induces an increase in Arg I expression and synthesis of polyamines. We also show that the conditioning lesion effect in overcoming inhibition by MAG is initially dependent on ongoing polyamine synthesis but, with time after lesion, becomes independent of ongoing synthesis. However, if synthesis of polyamines is blocked in vivo the early phase of good growth after a conditioning lesion is completely blocked and the later phase of growth, when ongoing polyamine synthesis is not required during culture, is attenuated. We also show that putrescine must be converted to spermidine both in culture and in vivo to overcome inhibition by MAG and that spermidine can promote optic nerve regeneration in vivo. These results suggest that spermidine could be a useful tool in promoting CNS axon regeneration after injury.

Acid Fibroblast Growth Factor and Peripheral Nerve Grafts Regulate Th2 Cytokine Expression, Macrophage Activation, Polyamine Synthesis, and Neurotrophin Expression in Transected Rat Spinal Cords

“…Treatment with polyamines also promotes axon regeneration in the peripheral nerves (Gilad et al, 1996). Another in vivo study reports that spermidine can promote optic nerve regeneration (Deng et al, 2009). After spinal cord injury of rats, M1 macrophages predominate in sites of trauma and exert neurotoxic effects, but the transient M2 macrophage response promotes repair and axonal growth (Kigerl et al, 2009).…”

Cyclic AMP and Polyamines Overcome Inhibition by Myelin-Associated Glycoprotein through eIF5A-Mediated Increases in p35 Expression and Activation of Cdk5

“…The first is that viral overexpression of p35 would likely not be a viable approach for clinical applications, from both a technical and safety standpoint, and so alternative means of inducing p35 expression would be required. These could include gene-based therapies or, most logically, exogenous administration of spermidine, which has been shown previously to promote axonal regeneration in the optic nerve (Deng et al, 2009). Importantly, spermidine has been shown to cross the blood-brain barrier following cerebral ischemia (Diler et al, 2002), and so it is possible that entry of spermidine into the CNS would be facilitated after traumatic injury.…”

“…We have also demonstrated that arginase I expression and polyamine synthesis are required for the ability of cAMP to reverse myelin-mediated inhibition and that treatment with exogenous putrescine overcome inhibition by MAG in vitro . We subsequently showed that putrescine must be converted to spermidine to have this effect and, most importantly, that spermidine can promote regeneration of retinal ganglion cell axons following optic nerve injury (Deng et al, 2009). In this study, we report that the ability of putrescine to overcome inhibition by MAG and myelin arises from spermidine's function as substrate for the hypusine modification of elongation initiation factor 5A (eIF5A).…”

“…We have also shown that the polyamine putrescine can overcome inhibition by MAG when administered in vitro Deng et al, 2009), but the molecular events underlying its effects have not yet been defined. In this study, we now show that the ability of both dbcAMP and polyamines to overcome MAG-mediated inhibition is dependent on Cdk5 activity.…”

“…Hypusination is essential for eIF5A function, as mutation of the hypusine residue abolishes the ability of eIF5A to interact with the ribosome (Jao and Chen, 2006). Interestingly, our laboratory has shown that putrescine must be converted to spermidine to overcome inhibition by MAG and promote axonal regeneration in vivo (Deng et al, 2009), and we have therefore hypothesized that hypusination of eIF5A is required for polyamine-induced expression of p35 and the ability of polyamines to overcome inhibition by MAG.…”