EVALUATION OF INTRA-ARTICULAR COLLOIDAL GOLD AU 198 IN THE TREATMENT OF PRESISTENT KNEE EFFUSIONS.

Abstract

Listen

Translate article

Inhibitory molecules associated with CNS myelin, such as myelin-associated glycoprotein (MAG), represent major obstacles to axonal regeneration following CNS injury. Our laboratory has shown that elevating levels of intracellular cAMP, via application of the nonhydrolyzable analog dibutyryl cAMP (dbcAMP), can block the inhibitory effects of MAG and myelin. We have also shown that elevation of cAMP results in upregulation of arginase I and increased polyamine synthesis. Treatment with putrescine or spermidine blocks myelin-mediated inhibition of neurite outgrowth, but the mechanism underlying this effect has not yet been elucidated. Here we show that cyclin-dependent kinase 5 (Cdk5) is required for dbcAMP and putrescine to overcome MAG-mediated inhibition. The ability of dbcAMP and putrescine to overcome inhibition by MAG is abolished in the presence of roscovitine, a Cdk inhibitor that has greater selectivity for Cdk5, and expression of dominant negative Cdk5 abolishes the ability of dbcAMP or putrescine to enhance neurite outgrowth in the presence of MAG. Importantly, dbcAMP and putrescine increase expression of p35, the neuron-specific activator of Cdk5, and rat DRG neurons transduced with HSV overexpressing p35 can overcome inhibition by MAG. The upregulation of p35 by putrescine is also reflected in increased localization of p35 to neurites and growth cones. Last, we show that putrescine upregulates p35 expression by serving as a substrate for hypusine modification of eIF5A, and that this hypusination is necessary for putrescine9s ability to overcome inhibition by MAG. Our findings reveal a previously unknown mechanism by which polyamines may encourage regeneration after CNS injury. <b>SIGNIFICANCE STATEMENT</b> This study describes a novel mechanism for the activation of Cdk5 in neurons and a new role for Cdk5 in promoting axonal growth in the presence of myelin-associated inhibitors. We show that administration of the polyamine putrescine leads to hypusination of the translation initiation factor eIF5A, increased translation of p35, and activation of Cdk5 in cultured cerebellar neurons, and that each of these events is required for putrescine9s ability to overcome inhibition by myelin-associated glycoprotein. These findings have implications for the development of new treatments for spinal cord and traumatic brain injury and provide new insight into the functions of eIF5A and Cdk5 in the nervous system.