Inosine induces axonal rewiring and improves behavioral outcome after stroke

Abstract

Cerebral infarct (stroke) often causes devastating and irreversible losses of function, in part because of the brain's limited capacity for anatomical reorganization. The purine nucleoside inosine has previously been shown to induce neurons to express a set of growthassociated proteins and to extend axons in culture and in vivo. We show here that in adult rats with unilateral cortical infarcts, inosine stimulated neurons on the undamaged side of the brain to extend new projections to denervated areas of the midbrain and spinal cord. This growth was paralleled by improved performance on several behavioral measures. C erebral ischemic infarct, or stroke, affects approximately 750,000 people annually in the U.S. alone. Depending on the locus of damage and other complicating factors, stroke can result in devastating losses in sensory, motor, and cognitive functions. A limited amount of synaptic reorganization is thought to occur spontaneously in the brain after stroke (1-5) or focal injury (6, 7) and can be enhanced by training (8, 9). However, there is little evidence that undamaged neurons can extend lengthy projections to reinnervate brain regions that have lost their normal inputs, nor are any clinical methods available to improve functional outcome by stimulating axonal reorganization after stroke.The purine nucleoside inosine enters cells via facilitated diffusion or can be synthesized readily from adenosine. In at least some neurons, inosine activates an intracellular signaling pathway that regulates the expression of multiple genes involved in axon outgrowth (10,11). In vivo, inosine treatment can promote extensive sprouting of the intact corticospinal tract (CST) into areas denervated by transecting the contralateral CST (12). Here, we have investigated whether inosine treatment can stimulate axonal growth and help improve behavioral outcome after stroke. The rat's sensorimotor cortex is required for fine motor control of the contralateral limbs (5,13,14). After creating a stroke that damaged most of the sensorimotor cortex on one side of the brain, we tested whether inosine could induce neurons in the intact hemisphere to grow new connections to denervated target areas and restore cortical control to the hemiparetic side. Although a number of neural pathways may be important in this regard, we have focused on the corticorubral tract and CST, two pathways that help mediate fine motor coordination (15,16). Our results demonstrate that inosine stimulates significant axonal reorganization after stroke and leads to improved performance on several sensorimotor tasks. MethodsSurgery. Male Sprague-Dawley rats (250-325 g, age range 8-10 weeks: Charles River Breeding Laboratories) were used throughout. In a pilot study done in collaboration with Cerebrotec (Boston), we investigated whether inosine treatment would improve behavioral outcome after stroke. Under halothane anesthesia, rats were placed on their sides, and a vertical incision was made midway between the right orbit and external auditory canal. The underly...

Manganese-Enhanced MRI of Brain Plasticity in Relation to Functional Recovery after Experimental Stroke

“…Although our study does not provide direct evidence of a relationship between functional recovery and neuroanatomical reorganization, previous studies have shown that the temporal pattern of functional recovery correlates with neuroanatomical alterations, such as peri-and contralesional synaptogenesis (Stroemer et al, 1995) and contralesional dendritic sprouting (Jones and Schallert, 1992) after cortical brain injury. Furthermore, pharmacological stimulation of neuronal sprouting has been shown to improve behavioral performance (Chen et al, 2002;Kawamata et al, 1997;Stroemer et al, 1998). The lack of a direct correlation between early functional recovery and neuroanatomical changes in our study may be explained by insensitivity of the neuroanatomical tracer techniques to detect initial changes in projection patterns.…”

Mst3b, a purine-sensitive Ste20-like protein kinase, regulates axon outgrowth

“…After either a unilateral transection of the corticospinal tract or a unilateral stroke, inosine causes layer five pyramidal cells on the undamaged side of the brain to extend axon collaterals that cross the midline into the denervated side of the spinal cord (10), leading to improved performance with the denervated forepaw (8). The possibility that Mst3b mediates these effects suggests that further studies on how this kinase is regulated may not only enhance our basic understanding of axon growth, but may also be of benefit in the treatment of CNS injury.…”

“…The purine nucleoside inosine stimulates axon outgrowth in certain types of neurons in culture and in vivo after CNS injury (6)(7)(8)(9)(10). The purine analog 6-thioguanine (6-TG), on the other hand, blocks outgrowth induced by neurotrophic factors (9,11,12), and this effect is paralleled by the inhibition of a previously unidentified 45-50 kDa serine-threonine protein kinase (13).…”

Secondary degeneration reduced by inosine after spinal cord injury in rats

“…In the recent years, inosine has been reported to have important effects on the injured nervous system. Thus, administration of inosine in vivo can induce massive sprouting of the normal corticospinal tract into the lesioned side of the cord, 14 stimulate significant axonal rewiring and improve functional outcome after stroke, 28 and protect axotomized retinal ganglion cells from optic nerve transection. 19,20 Inosine is also capable of protecting astrocytes and neurons against hypoxia, ischemia and zinc sulfate in vitro.…”