Functional and morphologic evidence of the presence of tissue-plasminogen activator in vascular nerves: implications for a neurologic control of vessel wall fibrinolysis and rigidity.

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Tissue plasminogen activator (t-PA) is expressed by hypothalamic and peripheral sympathetic neurons. The sympathetic axons that permeate artery walls have not been investigated as possible sources of intramural t-PA. The plasmin produced by such a system would locally activate both fibrinolysis and matrix metalloproteinases that regulate arterial collagen turnover. To assess this neural t-PA production, we investigated the capacity of rat cervical sympathetic ganglion neurons to synthesize and release t-PA, and the expression of the enzyme in carotid artery and the iris-choroid microvascular tissues that receive the ganglion axon distribution. Functional studies confirmed that (i) the ganglion neuron cell bodies synthesize t-PA mRNA, (ii) cultured ganglion carotid artery and iris-choroid microvascular explants predominantly release t-PA rather than urokinase, (iii) microvascular tissues release approximately 20 times more t-PA per milligram than carotid explants (which accords with the higher innervation density of small vessels), and (iv) removal of the endothelium did not cause major reductions in the t-PA release from carotid and microvascular explants. Immunolocalization studies then confirmed a strong expression of the enzyme within the ganglion axons, the carotid adventitia that receives these axons, and the predominantly sympathetic axon terminals in the iris-choroid microvasculature. These data indicate the existence of a previously undescribed system for the delivery of neural t-PA to vessel walls. The intramural production of plasmin induced by this system represents a novel principle for the regulation of arterial matrix flexibility, especially in the media of densely innervated small arteries and resistance arterioles involved in the pathogenesis of stroke, hypertension, and vascular aging. Thus, the data suggest an important new interface between neuroscience and vascular biology that merits further exploration.