Storage and release of tissue plasminogen activator by sympathetic axons in resistance vessel walls

Chemical sympathectomy attenuates inflammation, glycocalyx shedding and coagulation disorders in rats with acute traumatic coagulopathy

Acute traumatic coagulopathy (ATC) may trigger sympathoadrenal activation associated with endothelial damage and coagulation disturbances. Overexcitation of sympathetic nerve in this state would disrupt sympathetic-vagal balance, leading to autonomic nervous system dysfunction. The aim of this study was to evaluate the autonomic function in ATC and its influence on inflammation, endothelial and coagulation activation. Male Sprague-Dawley rats were randomly assigned to sham, ATC control (ATCC) and ATC with sympathectomy by 6-hydroxydopamine (ATCS) group. Sham animals underwent the same procedure without trauma and bleeding. Following trauma and hemorrhage, rats underwent heart rate variability (HRV) test, which predicts autonomic dysfunction through the analysis of variation in individual R-R intervals. Then, rats were euthanized at baseline, and at 0, 1 and 2 h after shock and blood gas, conventional coagulation test and markers of inflammation, coagulation, fibrinolysis, endothelial damage and catecholamine were measured. HRV showed an attenuation of total power and high frequency, along with a rise of low frequency and low frequency : high frequency ratio in the ATC rats, which both were reversed by sympathectomy in the ATCS group. Additionally, sympathetic denervation significantly suppressed the increase of proinflammatory cytokines, tumor necrosis factor-α and the fibrinolysis markers including tissue-type plasminogen activator and plasmin-antiplasmin complex. Serum catecholamine, soluble thrombomodulin and syndecan-1 were also effectively inhibited by sympathectomy. These data indicated that autonomic dysfunction in ATC involves both sympathetic activation and parasympathetic inhibition. Moreover, sympathectomy yielded anti-inflammatory, antifibrinolysis and endothelial protective effects in rats with ATC. The role of autonomic neuropathy in ATC should be explored further.

Human tissue-type plasminogen activator

Tissue-type plasminogen activator (t-PA ) plays an important role in the removal of intravascular fibrin deposits and has several physiological roles and pathological activities in the brain. Its production by many other cell types suggests that t-PA has additional functions outside the vascular and central nervous system. Activity of t-PA is regulated at the level of its gene transcription, its mRNA stability and translation, its storage and regulated release, its interaction with cofactors that enhance its activity, its inhibition by inhibitors such as plasminogen activator inhibitor type 1 or neuroserpin, and its removal by clearance receptors. Gene transcription of t-PA is modulated by a large number of hormones, growth factors, cytokines or drugs and t-PA gene responses may be tissue-specific. The aim of this review is to summarise current knowledge on t-PA function and regulation of its pericellular activity, with an emphasis on regulation of its gene expression.

Peptidergic regulation of plasminogen activator inhibitor-1 gene expression in vivo

BACKGROUND

The mechanisms by which PAI-1 biosynthesis is altered during stress have not been fully elucidated. Studies suggest a major role for neuro-peptidergic modulation of the stress response by PACAP (pituitary adenylate cyclase-activating polypeptide), a member of the VIP/secretin/glucagon family.

OBJECTIVE

We tested the hypothesis that PACAP regulates PAI-1 biosynthesis during stress in vivo.

METHODS

PAI-1 gene expression was monitored by RT-PCR in adrenal glands harvested from C57BL/6J mice that were unstressed, or subjected to restraint stress for 2 h, or treated with PACAP.

RESULTS

PAI-1 mRNA expression was markedly increased in adrenals from stressed mice. Restraint stress resulted in much smaller increments in adrenal tPA mRNA, suggesting that local adrenal tPA/PAI-1 biosynthetic balance is markedly altered by stress. The observed increases in PAI-1mRNA during stress were substantially blunted (55 ± 4%, P < 0.001) by pretreatment with the specific PACAP receptor antagonist, PACAP6-38, compared with pretreatment with vehicle. Administration of the agonist PACAP1-38 alone resulted in a dose-dependent increase in tissue PAI-1 mRNA. PACAP1-38 administration also resulted in substantial increases in plasma PAI-1 antigen and active PAI-1 concentrations that were significantly greater in male mice than in female mice.

CONCLUSIONS

We conclude that adrenal PAI-1 mRNA expression is markedly increased by stress, and that the PACAP peptidergic signaling pathway plays a major role in mediating the stress-induced increase in PAI-1 biosynthesis.

The plasminogen activation system and the regulation of catecholaminergic function

The local environment of neurosecretory cells contains the major components of the plasminogen activation system, including the plasminogen activators, tissue plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), as well as binding sites for t-PA, the receptor for u-PA (uPAR), and also the plasminogen activator inhibitor, PAI-1. Furthermore, these cells express specific binding sites for plasminogen, which is available in the circulation and in interstitial fluid. Colocalization of plasminogen and its activators on cell surfaces provides a mechanism for promoting local plasminogen activation. Plasmin is retained on the cell surface where it is protected from its inhibitor, α₂-antiplasmin. In neurosecretory cells, localized plasmin activity provides a mechanism for extracellular processing of secreted hormones. Neurotransmitter release from catecholaminergic cells is negatively regulated by cleavage products formed by plasmin-mediated proteolysis. Recently, we have identified a major plasminogen receptor, Plg-R_KT. We have found that Plg-R_KT is highly expressed in chromaffin cells of the adrenal medulla as well as in other catecholaminergic cells and tissues. Plg-R_KT-dependent plasminogen activation plays a key role in regulating catecholaminergic neurosecretory cell function.

Ambient temperature affects thrombotic potential at rest and following exercise

INTRODUCTION

During exercise, ischemic risk increases, possibly due to changes in coagulation and fibrinolytic activity. Previous research suggests ambient temperature affects resting thrombotic potential, but the effect of heat and cold on hemostasis during exercise is unknown. The purpose of this study was to assess changes in coagulation and fibrinolysis during maximal exercise in hot and cold temperatures, and to compare those responses to exercise under temperate conditions.

MATERIALS & METHODS

Fifteen healthy men completed maximal exercise tests in hot (30°C), temperate (20°C) and cold (5° - 8°C) temperatures. Blood samples were obtained before and immediately after exercise and analyzed for concentrations of thrombin-antithrombin III (TAT), active tissue plasminogen activator (tPA) and plasminogen activator inhibitor-1 (PAI-1). Results were analyzed by ANOVA.

RESULTS

A main effect of time was observed for TAT (temperate=1.71 ± 0.82 - 2.61 ± 0.43 ng/ml, hot=1.81 ± 0.73 - 2.62 ± 0.67 ng/ml, cold=2.33 ± 0.65 - 2.89 ± 0.81 ng/ml, PRE to POST, respectively) and tPA activity (temperate=0.72 ± 0.44 - 2.71 ± 0.55 IU/ml, hot=0.72 ± 0.38 - 2.64 ± 0.61 IU/ml, cold=0.86 ± 0.45 - 2.65 ± 0.77 IU/ml, PRE to POST, respectively). A trend was observed for the PAI-1 response to exercise (temperate=14.5 ± 23.7 - 12.3 ± 20.2I U/ml, hot=15.1 ± 26.5 - 10.0 ± 15.1 IU/ml, cold=10.5 ± 10.4 - 7.9 ± 9.7 IU/ml, PRE to POST, respectively, p=0.08). TAT concentrations were significantly higher in cold compared to temperate and hot conditions.

CONCLUSION

Coagulation potential is elevated during exposure to cold temperatures. These data suggest that risk of an ischemic event may be elevated in the cold.

The novel plasminogen receptor, plasminogen receptor(KT) (Plg-R(KT)), regulates catecholamine release

Neurotransmitter release by catecholaminergic cells is negatively regulated by prohormone cleavage products formed from plasmin-mediated proteolysis. Here, we investigated the expression and subcellular localization of Plg-R(KT), a novel plasminogen receptor, and its role in catecholaminergic cell plasminogen activation and regulation of catecholamine release. Prominent staining with anti-Plg-R(KT) mAb was observed in adrenal medullary chromaffin cells in murine and human tissue. In Western blotting, Plg-R(KT) was highly expressed in bovine adrenomedullary chromaffin cells, human pheochromocytoma tissue, PC12 pheochromocytoma cells, and murine hippocampus. Expression of Plg-R(KT) fused in-frame to GFP resulted in targeting of the GFP signal to the cell membrane. Phase partitioning, co-immunoprecipitation with urokinase-type plasminogen activator receptor (uPAR), and FACS analysis with antibody directed against the C terminus of Plg-R(KT) were consistent with Plg-R(KT) being an integral plasma membrane protein on the surface of catecholaminergic cells. Cells stably overexpressing Plg-R(KT) exhibited substantial enhancement of plasminogen activation, and antibody blockade of non-transfected PC12 cells suppressed plasminogen activation. In functional secretion assays, nicotine-evoked [(3)H]norepinephrine release from cells overexpressing Plg-R(KT) was markedly decreased (by 51 ± 2%, p < 0.001) when compared with control transfected cells, and antibody blockade increased [(3)H]norepinephrine release from non-transfected PC12 cells. In summary, Plg-R(KT) is present on the surface of catecholaminergic cells and functions to stimulate plasminogen activation and modulate catecholamine release. Plg-R(KT) thus represents a new mechanism and novel control point for regulating the interface between plasminogen activation and neurosecretory cell function.

Coagulation, fibrinolysis and cytokine responses to intramedullary nailing of the femur: an experimental study in pigs comparing traditional reaming and reaming with a one-step reamer-irrigator-aspirator system

INTRODUCTION

Operations in trauma patients represent a second insult and the extent of the surgical procedures influences the magnitude of the inflammatory response. Our hypothesis was that a reamer-irrigator-aspirator (RIA) system would cause a lesser inflammatory response than traditional reaming (TR).

MATERIALS AND METHODS

Coagulation, fibrinolysis and cytokine responses were studied in Norwegian landrace pigs during and after intramedullary nailing (IMN) with two different reaming systems using ELISA and chromogenic peptide substrate assays. The TR (n=8) and the RIA (n=7) reaming systems were compared to a control group (n=7). The animals were followed for 72 h. Arterial, mixed venous and femoral vein blood were withdrawn simultaneously peroperatively and until 2 h after the nail was inserted for demonstration of local, pulmonary and systemic activation of the cascade systems. At 6 h, 24 h, 48 h and 72 h postoperatively arterial blood samples were withdrawn.

RESULTS

Significantly procedure-related increased levels were found for thrombin-antithrombin (TAT) and tissue plasminogen activator (t-PA) in the TR group and TAT in the RIA group. The local and the pulmonary activation of coagulation and fibrinolysis were more pronounced in the TR than in the RIA group, the difference reached significance for plasminogen activator inhibitor-1 (PAI-1) (arterial blood). The cytokine response, mainly represented by IL-6 increase, was more pronounced in the TR than the RIA group, and was significant for IL-6 in femoral vein blood. The arterial levels of IL-6 exceeded the mixed venous levels indicating an additional pulmonary activation of IL-6. Two animals in the TR group, who died of pulmonary embolism (PE) prior to planned study end point, had a more pronounced response compared to the rest of the TR group.

CONCLUSION

A procedure-related coagulation and fibrinolytic response was demonstrated in both reaming groups, with more pronounced response in the TR than in the RIA group. Elevated levels of cytokines were demonstrated related to reaming and nailing, with significantly higher IL-6 levels in the TR than in the RIA group.

The anti-fibrinolytic SERPIN, plasminogen activator inhibitor 1 (PAI-1), is targeted to and released from catecholamine storage vesicles

Recent studies suggest a crucial role for plasminogen activator inhibitor-1 (PAI-1) in mediating stress-induced hypercoagulability and thrombosis. However, the mechanisms by which PAI-1 is released by stress are not well-delineated. Here, we examined catecholaminergic neurosecretory cells for expression, trafficking, and release of PAI-1. PAI-1 was prominently expressed in PC12 pheochromocytoma cells and bovine adrenomedullary chromaffin cells as detected by Northern blotting, Western blotting, and specific PAI-1 ELISA. Sucrose gradient fractionation studies and immunoelectron microscopy demonstrated localization of PAI-1 to catecholamine storage vesicles. Secretogogue stimulation resulted in corelease of PAI-1 with catecholamines. Parallel increases in plasma PAI-1 and catecholamines were observed in response to acute sympathoadrenal activation by restraint stress in mice in vivo. Reverse fibrin zymography demonstrated free PAI-1 in cellular releasates. Detection of high molecular weight complexes by Western blotting, consistent with PAI-1 complexed with t-PA, as well as bands consistent with cleaved PAI-1, suggested that active PAI-1 was present. Modulation of PAI-1 levels by incubating PC12 cells with anti-PAI-1 IgG caused a marked decrease in nicotine-mediated catecholamine release. In summary, PAI-1 is expressed in chromaffin cells, sorted into the regulated pathway of secretion (into catecholamine storage vesicles), and coreleased, by exocytosis, with catecholamines in response to secretogogues.

The effects of whole body vibration and exercise on fibrinolysis in men

Whole body vibration (WBV) is a novel modality that has been demonstrated to enhance muscular and cardiovascular functions reported to increase fibrinolytic activity. The purpose of this study was to examine the fibrinolytic response to WBV and exercise in men. Twenty healthy males (23.8 ± 0.9 years, 25.6 ± 0.2 kg m(-2)) participated in the study. Each subject performed three trials in randomized order separated by 1 week: exercise (X), vibration (V) and vibration + exercise (VX). Exercise sessions consisted of 15 min of unloaded squatting at a rate of 20 per minute. Vibration sessions were conducted on a WBV platform vibrating for 15 min. Tissue plasminogen activator (tPA) and plasminogen activator inhibitor (PAI-1) were assessed at baseline and immediately after each condition. The increase in tPA activity was significantly greater in VX (0.87 ± 0.35 to 3.21 ± 1.06 IU ml(-1)) compared to X (0.71 ± 0.36 to 2.4 ± 1.13 IU ml(-1)) or V (0.83 ± 0.25 to 1.00 ± 0.37 IU ml(-1)) conditions, and greater in the X condition compared to the V condition. PAI-1 activity decreased significantly more in the VX (6.54 ± 5.53 to 4.89 ± 4.13 IU ml(-1)) and X (9.76 ± 8.19 to 7.48 ± 7.11 IU ml(-1)) conditions compared to the V (5.68 ± 3.53 to 5.84 ± 3.52 IU ml(-1)) condition. WBV does not augment fibrinolytic activity in healthy men. However, WBV combined with squatting exercise increases fibrinolytic activity more than exercise alone.

Altered interaction between plasminogen activator inhibitor type 1 activity and sympathetic nerve activity with aging

BACKGROUND

It has been reported that sympathetic nerve activity (SNA) is associated with fibrinolysis, but the interaction between SNA and the fibrinolytic system with aging has not been elucidated in humans. The purpose of this study was to examine the effect of age-related SNA on the activity of plasminogen activator inhibitor type 1 (PAI-1) and tissue plasminogen activator (tPA) using muscle SNA (MSNA).

METHODS AND RESULTS

This study included 16 young subjects (mean age 26.1 years) and 10 aged subjects (mean age 56.9 years). Lower body negative pressure (LBNP) was performed at -40 mmHg for 30 min. LBNP significantly increased both tPA and PAI-1 activity (from 5.2+/-0.5 to 7.3+/-1.2 IU/ml and from 2.85+/-0.68 to 4.06+/-0.73 U/ml, p<0.01, respectively) in the aged group. In the young group, tPA activity tended to increase, whereas PAI-1 activity was unchanged. There was a correlation between MSNA and PAI-1 activity in the aged group (r=0.47, p<0.01).

CONCLUSIONS

SNA in an aging subject leads to an increase in the activity of PAI-1, which indicates that an altered interaction between SNA and PAI-1 activity contributes to increased cardiovascular events in the elderly population.

Modulation of sympathetic activity by tissue plasminogen activator is independent of plasminogen and urokinase

Sympathetic neurons synthesize, transport, and release tissue-type plasminogen activators (t-PAs) and urinary-type plasminogen activators (u-PAs). We reported that t-PA enhances sympathetic neurotransmission and exacerbates reperfusion arrhythmias. We have now assessed the role of u-PA and plasminogen. Neurogenic contractile responses to electrical field stimulation (EFS) were determined in vasa deferentia (VD) from mice lacking t-PA (t-PA(-/-)), plasminogen activator inhibitor-1 (PAI-1(-/-)), plasminogen (plgn(-/-)), u-PA (u-PA(-/-)), and wild-type (WT) controls. Similar levels of t-PA were present in VD and cardiac synaptosomes of WT, PAI-1(-/-), plgn(-/-), and u-PA(-/-) mice, whereas t-PA was undetectable in t-PA(-/-) tissues. EFS responses were potentiated and attenuated in VD from PAI-1(-/-) and t-PA(-/-) mice, respectively, but indistinguishable from WT responses in VD from plgn(-/-) and u-PA(-/-) mice. Moreover, t-PA inhibition with t-PA(stop) decreased EFS response in WT mice, whereas u-PA(stop) did not. VD responses to ATP, norepinephrine, and K(+) in t-PA(-/-), PAI-1(-/-), plgn(-/-), and u-PA(-/-) mice were similar to those in WT, whereas t-PA(stop) did not modify VD responses to norepinephrine in WT, t-PA(-/-), and PAI-1(-/-) mice, indicating a prejunctional site of action for t-PA-induced potentiation of sympathetic neurotransmission. Indeed, K(+)-induced norepinephrine exocytosis from cardiac synaptosomes was potentiated in PAI-1(-/-), attenuated in t-PA(-/-) and not different from WT in u-PA(-/-) and plgn(-/-) mice. Likewise, ATP exocytosis was decreased in t-PA(-/-) and attenuated by t-PA(stop) in WT mice. Thus, t-PA-induced enhancement of sympathetic neurotransmission is a prejunctional event associated with increased transmitter exocytosis and independent of u-PA and plasminogen availability. This novel t-PA action may be a potential therapeutic target in hyperadrenergic states.

Cell-surface actin binds plasminogen and modulates neurotransmitter release from catecholaminergic cells

An emerging area of research has documented a novel role for the plasminogen activation system in the regulation of neurotransmitter release. Prohormones, secreted by cells within the sympathoadrenal system, are processed by plasmin to bioactive peptides that feed back to inhibit secretagogue-stimulated release. Catecholaminergic cells of the sympathoadrenal system are prototypic prohormone-secreting cells. Processing of prohormones by plasmin is enhanced in the presence of catecholaminergic cells, and the enhancement requires binding of plasmin(ogen) to cellular receptors. Consequently, modulation of the local cellular fibrinolytic system of catecholaminergic cells results in substantial changes in catecholamine release. However, mechanisms for enhancing prohormone processing and cell-surface molecules mediating the enhancement on catecholaminergic cells have not been investigated. Here we show that plasminogen activation was enhanced >6.5-fold on catecholaminergic cells. Carboxypeptidase B treatment decreased cell-dependent plasminogen activation by approximately 90%, suggesting that the binding of plasminogen to proteins exposing C-terminal lysines on the cell surface is required to promote plasminogen activation. We identified catecholaminergic plasminogen receptors required for enhancing plasminogen activation, using a novel strategy combining targeted specific proteolysis using carboxypeptidase B with a proteomics approach using two-dimensional gel electrophoresis, radioligand blotting, and tandem mass spectrometry. Two major plasminogen-binding proteins that exposed C-terminal lysines on the cell surface contained amino acid sequences corresponding to beta/gamma-actin. An anti-actin monoclonal antibody inhibited cell-dependent plasminogen activation and also enhanced nicotine-dependent catecholamine release. Our results suggest that cell-surface-expressed forms of actin bind plasminogen, thereby promoting plasminogen activation and increased prohormone processing leading to inhibition of neurotransmitter release.

The plasminogen activator system modulates sympathetic nerve function

Sympathetic neurons synthesize and release tissue plasminogen activator (t-PA). We investigated whether t-PA modulates sympathetic activity. t-PA inhibition markedly reduced contraction of the guinea pig vas deferens to electrical field stimulation (EFS) and norepinephrine (NE) exocytosis from cardiac synaptosomes. Recombinant t-PA (rt-PA) induced exocytotic and carrier-mediated NE release from cardiac synaptosomes and cultured neuroblastoma cells; this was a plasmin-independent effect but was potentiated by a fibrinogen cleavage product. Notably, hearts from t-PA–null mice released much less NE upon EFS than their wild-type (WT) controls (i.e., a 76.5% decrease; P < 0.01), whereas hearts from plasminogen activator inhibitor-1 (PAI-1)–null mice released much more NE (i.e., a 275% increase; P < 0.05). Furthermore, vasa deferentia from t-PA–null mice were hyporesponsive to EFS (P < 0.0001) but were normalized by the addition of rt-PA. In contrast, vasa from PAI-1–null mice were much more responsive (P < 0.05). Coronary NE overflow from hearts subjected to ischemia/reperfusion was much smaller in t-PA–null than in WT control mice (P < 0.01). Furthermore, reperfusion arrhythmias were significantly reduced (P < 0.05) in t-PA–null hearts. Thus, t-PA enhances NE release from sympathetic nerves and contributes to cardiac arrhythmias in ischemia/reperfusion. Because the risk of arrhythmias and sudden cardiac death is increased in hyperadrenergic conditions, targeting the NE-releasing effect of t-PA may have valuable therapeutic potential.

New transgenic evidence for a system of sympathetic axons able to express tissue plasminogen activator (t-PA) within arterial/arteriolar walls

Sympathetic axons embedded in a few arterioles and vasa vasora were recently shown to store tissue plasminogen activator (t-PA) in vesicles. But the extension of such t-PA axons to arteries and arterioles throughout the organism has not been verified. Confirmation of this anatomy would identify a second significant source of vessel wall t-PA. To visualize fine embedded axons independent of endothelium, we created a transgenic mouse whose expressions of the t-PA promoter and enhanced green fluorescent protein are confined to sympathetic neurons and other neural crest derivatives. Confocal images reveal the extension of t-PA axons to arterioles serving heart, brain, kidney, lung, mesentery, and skin; plus aortic, carotid, and mesenteric artery walls. Ganglion neurons and adrenal chromaffin cells also show strong expressions. These new sightings confirm the existence of a system of t-PA axons that is prominent in arterioles, and compatible with the release of neural t-PA into their walls.

Stimulated Tissue Plasminogen Activator Release as a Marker of Endothelial Function in Humans

The initiation, modulation, and resolution of thrombus associated with eroded or unstable coronary plaques are critically dependent on the efficacy of endogenous fibrinolysis. This is dependent on the cellular function of the surrounding endothelium and vascular wall. In particular, the acute release of tissue plasminogen activator from the endothelium makes an important contribution to the defense against intravascular thrombosis. Here, we describe the rationale and methodology for, and clinical relevance of, assessing acute endothelial tissue plasminogen activator release in humans. The investigation of endothelial fibrinolytic function has the potential to provide major new insights into the pathophysiology of cardiovascular disease, and to shape future therapeutic interventions.

Cardiac fibrinolytic capacity is markedly increased after brief periods of local myocardial ischemia, but declines following successive periods in anesthetized pigs

BACKGROUND

Fibrinolysis in blood is mainly reflected by the activities of tissue plasminogen activator (tPA) and of plasminogen activator inhibitor-1 (PAI-1). The effect of myocardial ischemia on their activities in the coronary circulation is, however, not established.

OBJECTIVES

With an improved experimental model, we therefore examined the effect of a brief period of myocardial ischemia on their activities. Furthermore, the consequences of repeated periods of ischemia, mimicking the situations in patients with unstable angina, were investigated.

METHODS

In six anesthetized pigs, we occluded the distal left anterior descending coronary artery (LAD) four times for 10 min with 40 min intervals and determined the activities of tPA and PAI-1 in arterial and coronary venous blood. By simultaneously recording LAD flow, we could estimate cardiac release of these factors at baseline conditions and during reperfusion.

RESULTS

Neither net cardiac release of PAI-1 nor alterations in plasma PAI-1 levels were demonstrated during the experiment. However, a significant net release of tPA activity of 10.4 +/- 3.2 IU mL(-1) (P < 0.005) was recorded during baseline conditions. During reperfusion following the first period of ischemia, the cardiac release of tPA activity increased to a peak of 103 +/- 30-fold baseline release, but declined progressively after repeated periods of ischemia. After the fourth period, tPA release did not exceed an estimated baseline accumulation during ischemia and early reperfusion.

CONCLUSIONS

In this porcine model, a substantial local increase in fibrinolytic capacity was observed after brief periods of ischemia, but declined subsequently by repeated periods of ischemia.

Distribution of sympathetic tissue plasminogen activator (tPA) to a distant microvasculature

Tissue plasminogen activator (tPA) is the predominant plasminogen activator present in the vascular and nervous systems. Prior studies of the two have emphasized different tPA sources; respectively, endothelium and neurons. A closer relationship is now suggested by evidence that the peripheral sympathetic nervous system synthesizes and infuses enzymatically active tPA into small artery walls and the microcirculation. TPA may thus be the only known neural product able to effect degradation of the artery wall extracellular matrix. This brief review considers historical and current indications for the existence of such an autonomically controlled system and some physiologic implications. Immunohistochemical tPA expression in small arteries and arterioles is more prominent in the outer wall sympathetic axon plexus than in endothelium. Its presence in nerve filaments beneath the seldom-studied adventitia was obscured in earlier localizations. The systemic impact of a neural distribution is suggested by a 60% reduction of blood tPA activity after chemical sympathectomy. TPA-bearing axons extend outward from ganglion neuron cell bodies to reach even thin-walled vasa vasora and uveal microvessels. Ganglion cell bodies synthesize and package tPA in vesicles for the long axoplasmic transport. Densely innervated intact vessels release much greater amounts of tPA in vitro than do larger vessels, indicating a high neuron tPA production capacity and a large storage reservoir available within axon networks. The influence of an autonomically controlled plasmin production within small artery walls on regulation of blood pressure and capillary perfusion awaits further investigation. Its possible role in the pathogenesis of vessel wall matrix degradations in aging, hypertension, and diabetes may also merit further consideration.

Stimulated release of tissue plasminogen activator from artery wall sympathetic nerves: implications for stress-associated wall damage

Recurrent stress is clinically associated with early onset hypertension and coronary artery disease. A mechanism linking emotion to pathogenic remodeling of the artery wall has not been identified. Stress stimulates acute regulated release of tissue plasminogen activator (t-PA) into the circulation, which is presently attributed to the vascular endothelium. Sympathetic neurons also synthesize t-PA and axonally transport it to the arterial smooth muscle. Unlike release by the endothelium, a stress-stimulated sympathetic discharge would potentially accelerate degradation of the wall matrix by plasmin. To assess whether sympathetic axons are the principal source of acute stress-induced arterial release of t-PA, we compared the output from small densely innervated and large sparsely innervated isolated artery segments before and after sympathetic stimulation, and after ablations. Following phenylephrine infusion densely-innervated microvessels in uveal eyecups were released over 60-fold greater amounts of active t-PA per milligram than the sparsely innervated aorta; and ten-fold more than carotid artery segments. Mesenteric artery release was 4.8-fold greater than release by the carotid artery. In vivo, uveal release of t-PA increased more than three-fold within one minute following superior cervical sympathetic ganglion electrical stimulation, and after phenylephrine, or nicotine infusions of the anterior chamber. Circulating levels of t-PA fell 70% following chemical sympathectomy. We propose that sympathetic nerves are the primary source of stress-induced release of t-PA into and from the densely innervated resistance arteries and arterioles, where dysregulated plasmin-induced proteolysis could damage the wall matrix.

New functions for an old enzyme: nonhemostatic roles for tissue-type plasminogen activator in the central nervous system

Tissue-type plasminogen activator (tPA) is a highly specific serine proteinase that activates the zymogen plasminogen to the broad-specificity proteinase plasmin. Tissue-type plasminogen activator is found not only in the blood, where its primary function is as a thrombolytic enzyme, but also in the central nervous system (CNS), where it promotes events associated with synaptic plasticity and acts as a regulator of the permeability of the neurovascular unit. Tissue-type plasminogen activator has also been associated with pathological events in the CNS such as cerebral ischemia and seizures. Neuroserpin is an inhibitory serpin that reacts preferentially with tPA and is located in regions of the brain where either tPA message or tPA protein are also found, indicating that neuroserpin is the selective inhibitor of tPA in the CNS. There is a growing body of evidence demonstrating the participation of tPA in a number of physiological and pathological events in the CNS, as well as the role of neuroserpin as the natural regulator of tPA's activity in these processes. This review will focus on nonhemostatic roles of tPA in the CNS with emphasis on its newly described function as a regulator of permeability of the neurovascular unit and on the regulatory role of neuroserpin in these events.

Tissue-type plasminogen activator and neuroserpin: a well-balanced act in the nervous system?

Tissue-type plasmingen activator (tPA) is a highly specific serine proteinase that activates the zymogen plasminogen to the broad-specificity proteinase plasmin. tPA is found in the blood, where its primary function is as a thrombolytic enzyme, as well as in the central nervous system (CNS), where it promotes events associated with synaptic plasticity and cell death in a number of settings, such as cerebral ischemia and seizures. Neuroserpin is a fully inhibitory serine proteinase inhibitor (serpin) that reacts preferentially with tPA, and is located in regions of the brain where either tPA message or tPA protein are also found, suggesting that neuroserpin is the selective inhibitor of tPA in the CNS. There is a growing body of evidence demonstrating the participation of tPA in a number of physiologic and pathologic events in the CNS, and the role of neuroserpin as the natural regulator of tPA's activity in these processes.

Tissue-type plasminogen activator induces opening of the blood-brain barrier via the LDL receptor-related protein

The regulation of cerebrovascular permeability is critical for normal brain homeostasis, and the "breakdown" of the blood-brain barrier (BBB) is associated with the development of vasogenic edema and intracranial hypertension in a number of neurological disorders. In this study we demonstrate that an increase in endogenous tissue-type plasminogen activator (tPA) activity in the perivascular tissue following cerebral ischemia induces opening of the BBB via a mechanism that is independent of both plasminogen (Plg) and MMP-9. We also show that injection of tPA into the cerebrospinal fluid in the absence of ischemia results in a rapid dose-dependent increase in vascular permeability. This activity is not seen with urokinase-type Plg activator (uPA) but is induced in Plg-/- mice, confirming that the effect is Plg-independent. However, the activity is blocked by antibodies to the LDL receptor-related protein (LRP) and by the LRP antagonist, receptor-associated protein (RAP), suggesting a receptor-mediated process. Together these studies demonstrate that tPA is both necessary and sufficient to directly increase vascular permeability in the early stages of BBB opening, and suggest that this occurs through a receptor-mediated cell signaling event and not through generalized degradation of the vascular basement membrane.

Enhanced tissue plasminogen activator synthesis by the sympathetic neurons that innervate aging vessels

We investigated the source of the increased release of tissue plasminogen activator (t-PA) into the circulation that occurs during natural aging. Both the basal release and the acute stress-associated release induced by sympathetic stimulations are greater in older subjects. It is widely assumed that the source of these increases is vascular endothelium. However, the sympathetic neurons that densely innervate resistance vessel walls were recently shown to synthesize and transport active t-PA to axon terminals in vascular smooth muscle, suggesting an alternative source. These fine t-PA-bearing axons lie in the seldom-studied deep adventitia of vessel walls, where they are less visible than endothelium in tissue sections. Using Northern blot analysis, we observed that t-PAmRNA synthesis is increased 54% in the ganglion parent neuron cell bodies that innervate aged vessels. The t-PA release from isolated, aged ganglia in cultures was twofold greater than that from younger controls. In addition, aged whole-artery explants showed a 20% greater basal and a 50% greater acute release of stored t-PA in vitro. In vivo levels of active t-PA were 33% greater in the blood and 40% greater in the aqueous humor. These results are consistent with an increased infusion of the active t-PA protease from sympathetic axon terminals into the vessel wall extracellular matrix and the blood during natural aging, in addition to the basal endothelial release. We suggest that the cumulative impact of an accelerated plasmin production and matrix degradation within vessel walls, especially during repetitive stress, may play an unrecognized role in the pathogenesis of vascular aging. The possibility that increased sympathetic nervous system plasminogenesis influences the aging process in nonvascular tissues also deserves further investigation.