Plasminogen activators and synaptic plasticity

Abstract

In addition to its role in the vascular system, plasminogen activator (PA) is involved in neural development, excitotoxic cell death, and has been implicated in aspects of cerebral synaptic remodeling associated with cerebellar motor learning, visual cortex ocular dominance columns, and hippocampal & corticostriatal LTP. During a complex motor task cerebellar granule neurons show a rapid and transient induction of mRNA for the extracellular protease tissue plasminogen activator (tPA). This induction of tPA is cerebellar specific, and is not seen in the cerebella of exercised or stressed mice, and is distinct from simple performance phenomena. Knock-out mice lacking the tPA gene show a significant reduction in both rate and extent of learning the complex motor task. Furthermore, blocking tPA activity by infusion of PAI-1 or tPA-STOP during training dramatically impairs motor learning. Thus, tPA plays and important role in motor learning, in which tPA may facilitate synaptic plasticity. We have explored the possibility that PA may also play a role in synaptic plasticity in the spinal cord. The crossed phrenic phenomenon (CPP) describes respiratory functional plasticity that arises following spinal cord injury; whereby, phrenic motoneuron drive to the diaphragm is restored following activation of functionally ineffective medullary respiratory neuron synapses on phrenic motoneurons (PMN). Synaptic remodeling is thought to occur during the characteristic delay period following spinal cord injury before the CPP becomes functional. The mechanisms underlying this synaptic plasticity are not well-defined. Our ultimate aim is to understand the underlying molecular mechanisms of this functional recovery using a mouse model amenable to a molecular genetic approach, and ours is the first report of CPP in mice. Using electromyographic (EMG) recordings from the diaphragm, we examined the inter-operative delay time between spinal cord hemisection and contralateral phrenicotomy required for diaphragm response, as compared to animal death from asphyxia at zerotime. For each animal, a spinal cord hemisection was performed on the left side at C2. A contralateral phrenicotomy was performed the next day (overnight animals), 6–8 hours, 4–5 hours, or 1–2 hours post-hemisection. As the inter-operative delay was reduced, the proportion of mice displaying the CPP decreased from 100% for overnight animals, 94% in 6–8 h, 82% in 4–5 h, to 76% for 1–2 h mice. A critical 1–2 hr window is required for this synaptic plasticity. In situ hybridization shows that uPA and tPA mRNAs are rapidly induced in C4-5 ventral spinal cord neurons in the ipsilateral phrenic nucleus compared to the contralateral PMN and sham controls, with markedly elevated tPA protein at 1hr. post-hemisection. This specific and concomitant induction of PA suggests a role in CPP spinal cord plasticity, which may ultimately lead to therapeutic uses for PA in spinal cord injury.