Thrombin receptor activation stimulates astrocyte proliferation and reversal of stellation by distinct pathways: involvement of tyrosine phosphorylation

Treatment of cultured type-1 astrocytes with thrombin leads to cell proliferation and reversal of stellation. The half-maximal concentrations of thrombin required for each response are 500 and 2 pM, respectively. To test whether they might be mediated by different receptors, we examined the contribution of the G protein-coupled thrombin receptor to these responses in purified rat astrocytes by using the agonist peptide SFLLRNP. In the absence of added growth factors, SFLLRNP fully mimicked the effects of thrombin at half-maximal concentrations of 30 microM for an increase in cell number and DNA synthesis and 100 nM for the reversal of stellation. The role of protein tyrosine phosphorylation in these events was investigated using antiphosphotyrosine antibodies. Thrombin and SFLLRNP at concentrations at least 10-fold greater than those required for half-maximal reversal of stellation but below those required for mitogenesis induced an identical pattern of tyrosine phosphorylation on several proteins of 55-65, 106, 110-115, and 120-130 kDa. The response was rapid (< 1 min) and transient with a peak response after approximately 2 min. The specific tyrosine kinase inhibitor herbimycin A did not affect thrombin- or SFLLRNP-mediated reversal of stellation at concentrations of up to 1 microM. In contrast, 1 microM herbimycin fully inhibited the ability of thrombin and SFLLRNP to increase cell number and stimulate DNA synthesis. Furthermore, this inhibition by 1 microM herbimycin A corresponded to inhibition of receptor-induced tyrosine phosphorylation. Thus, cell proliferation but not reversal of stellation is dependent on thrombin receptor-activated tyrosine kinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Protease nexin-1, a potent thrombin inhibitor, is reduced around cerebral blood vessels in Alzheimer's disease

The clotting protease thrombin might contribute to the pathophysiology of central nervous system (CNS) injury and certain diseases by its ability to retract processes on neurons and astrocytes and to stimulate astrocyte proliferation. Protease nexin-1 (PN-1) is a 43 kDa thrombin inhibitor found predominantly in the brain where much of it resides around capillaries and large blood vessels. This location of PN-1 prompted the hypothesis that it may play a protective role against extravasated thrombin released following cerebrovascular injury or under certain pathological conditions. Recent studies indicated that the levels of PN-1 are markedly reduced in the postmortem brains of patients with Alzheimer's disease (AD). It was suggested that this reduction in PN-1 levels was due to the sequestration of PN-1 by extravasated thrombin. In the present study we examined the specific nature of this reduction by immunohistochemical staining of sections from control and AD brains using PN-1 specific antibodies. We show that the levels of PN-1 immunoreactivity around blood vessels and the number of blood vessels exhibiting PN-1 immunoreactivity were markedly reduced in the brains of patients with AD compared to age-matched controls; this reduction was reflected by a decrease in the levels of PN-1 activity and PN-1 protein. Thus an imbalance between PN-1 and thrombin may be a contributing factor in the pathology of AD.

Thrombin receptor activation induces secretion and nonamyloidogenic processing of amyloid beta-protein precursor

The amyloid beta-protein (A beta) and protease nexin-2/amyloid beta-protein precursor (PN-2/A beta PP) are major constituents of senile plaques and cerebrovascular deposits in individuals with Alzheimer's disease and related disorders. It has been suggested that the coagulation protease thrombin may process A beta PP in a manner leading to the formation of A beta. Here we investigated the effects of thrombin on the secretion and processing of PN-2/A beta PP and the production of A beta in a cellular system. Incubation of glioblastoma cells with thrombin (1-5 nM) resulted in the accumulation of abnormally processed, carboxyl-terminal-truncated forms of secreted PN-2/A beta PP (approximately 85 kDa) in the culture medium. Higher concentrations of thrombin (> 10 nM) also increased the levels of secreted PN-2/A beta PP in cultured untransfected glioblastoma cells and glioblastoma cells that were stably transfected to overproduce the 695 isoform of A beta PP. Increased secretion of PN-2/A beta PP required the proteolytic activity of thrombin, was induced by activation of the thrombin receptor by agonist peptides, and required activation of protein kinase C. Incubation of the untransfected and transfected glioblastoma cells with thrombin led to decreased levels of soluble A beta in the culture medium consistent with previously suggested mechanisms regarding the secretion of PN-2/A beta PP. Although the present studies suggest that thrombin does not directly contribute to A beta formation, its proteolysis of secreted PN-2/A beta PP may disrupt regions near the carboxyl terminus of the secreted proteins that account for their neuroprotective and cell adhesive properties.

Regulation of protease nexin-1 target protease specificity by collagen type IV

Recent studies have shown that serine protease inhibitors can be regulated in their activity, specificity, and location by glycoprotein or extracellular matrix (ECM) co-factors. Protease nexin-1 (PN-1) is a member of the serpin superfamily of serine protease inhibitors which can rapidly inhibit thrombin, urokinase, and plasmin. PN-1 binds tightly to and is regulated by the ECM. This interaction accelerates the inhibition of thrombin by PN-1 and blocks urokinase and plasmin inhibition by PN-1. Previous work showed that heparan sulfate proteoglycan is largely responsible for the acceleration of thrombin inhibition by PN-1. Our current studies were directed at identifying ECM component(s) that decreased the ability of PN-1 to inhibit urokinase and plasmin. These studies showed that collagen type IV decreased the formation of SDS-stable complexes between urokinase or plasmin and PN-1 without affecting formation of complexes between thrombin and PN-1. The second order rate constant for inhibition of urokinase by PN-1 was markedly decreased with increasing collagen type IV, whereas the second order rate constant for inhibition of thrombin by PN-1 was unaffected by addition of collagen type IV. Other ECM components (collagen type I, vitronectin, fibronectin, and heat-denatured collagen type IV) did not affect complex formation or the rate of inhibition of proteases by PN-1, indicating that these effects were specific to collagen type IV. Binding of PN-1 to immobilized collagen type IV was demonstrated using an enzyme-linked immunosorbent assay; the concentration of PN-1 necessary to obtain 50% saturation of the immobilized collagen type IV binding sites was approximately 15 nM. Collagen type IV was also copurified with PN-1 from fibroblast-conditioned medium. These results demonstrate a novel regulation of serpin specificity in which an ECM co-factor decreased the inhibition of certain proteases by the serpin without affecting the inhibition of its target protease.

Co-distribution of protease nexin-1 and protease nexin-2 in brains of non-human primates

The protease nexins are protease inhibitors which regulate key blood coagulation proteases and which appear to be involved in certain physiological and pathological processes in the brain. Protease nexin-1 (PN-1), a potent inhibitor of thrombin, can regulate processes on cultured neurons and astrocytes. Protease nexin-2 (PN-2), a potent inhibitor of coagulation factor XIa, is identical to the secreted form of the Alzheimer's amyloid beta-protein precursor. In the present studies, PN-1 and PN-2 were analyzed in different tissues of monkey using monoclonal antibodies for either quantitative immunoblotting or specific [125I]protease-binding assays. PN-1 was detected only in brain. PN-2 was most abundant in brain, followed by testis and to a lesser extent kidney. Other tissues examined including spinal cord, heart, pancreas, spleen, liver, lung and muscle were essentially devoid of both PN-1 and PN-2. Within the brain, the levels of PN-1 and PN-2 were highest in the parietal cortex and lowest in the cerebellum and brainstem. The thalamus and striatum contained intermediate amounts of both proteins. Aged Cebus monkey cerebral cortical tissue contained slightly lower levels of PN-1 than did the middle-aged or young monkey tissue. The co-distribution of PN-1 and PN-2 in brain, their relative abundance in brain cortex, and previous studies on their functions suggest that in the brain they may participate in the regulation of blood coagulation and cell growth and differentiation.

Protease nexin-1, a thrombin inhibitor, is regulated by interleukin-1 and dexamethasone in normal human fibroblasts

Thrombin participates in several regulatory events following injury as a result of its effects on blood coagulation and cell migration, proliferation, and differentiation. Protease nexin-1 (PN-1) is a potent thrombin inhibitor in the extracellular environment. Since injury-related factors are known to regulate the synthesis and secretion of PN-1, the inhibitor may serve to modulate the actions of thrombin during injury. Here we report the molecular mechanisms that underlie this regulation. In normal human fibroblasts, interleukin-1 (IL-1) beta stimulated the synthesis and secretion of PN-1. The stimulation correlated with an increase in steady-state levels of PN-1 mRNA. Treatment of cells with both cycloheximide and IL-1 reduced the levels of PN-1 mRNA. Nuclear run-on assays indicated that IL-1 modestly increased the rate of PN-1 transcription. However, experiments with actinomycin D demonstrated that IL-1 significantly increased the half-life of the PN-1 mRNA. In contrast, dexamethasone (DXM) repressed the synthesis and secretion of PN-1 from fibroblasts. This effect correlated with a decrease in PN-1 mRNA. A sustained decrease in PN-1 mRNA was also seen when cells were treated with cycloheximide and DXM. In nuclear run-on assays, DXM functioned as a transcriptional repressor of PN-1 synthesis. Treatment of cells with actinomycin D showed that DXM did not affect mRNA stability. Thus, our experiments demonstrate that IL-1 and DXM, which function biologically in different fashions, regulate the synthesis of PN-1 by separate molecular mechanisms. While DXM directly regulates PN-1 at the level of transcription, IL-1 in the presence of ongoing protein synthesis regulates PN-1 production predominantly in a post-transcriptional fashion by increasing the half-life of the PN-1 mRNA.

Regulation of protease nexin-1 synthesis and secretion in cultured brain cells by injury-related factors

The clotting protease thrombin might contribute to cell damage following brain injury by its ability to retract processes on neurons and astrocytes. Protease nexin-1 (PN-1), a potent inhibitor of thrombin, is localized around cerebral blood vessels where it may protect these cells from extravasated thrombin during injury or alteration of the blood-brain barrier. Here we examined the effects of several injury-related factors on the regulation of PN-1 in cultured brain cells. Interleukin-1, tumor necrosis factor-alpha, and transforming growth factor-beta stimulated the secretion of PN-1 by the neuroblastoma cell line SK-N-SH. This cell line comprises both neuronal and glial cells. Analyses using cloned derivatives of these two cell types showed that PN-1 was secreted by the glial cells; PN-1 secretion was stimulated 90-fold by interleukin-1, 15-fold by tumor necrosis factor-alpha, 10-fold by tumor growth factor-beta, and 4-fold by platelet-derived growth factor. Measurements of newly synthesised PN-1 demonstrated that these factors produced an equivalent stimulation of PN-1 synthesis. The neuronal cells secreted two thrombin-binding proteins distinct from PN-1. Interactions between these two cell types regulated the secretion of PN-1 and the two thrombin-binding proteins.

Aggregation of the amyloid precursor protein within degenerating neurons and dystrophic neurites in Alzheimer's disease

Using a monoclonal antibody raised against purified, native, human protease nexin-2/amyloid precursor protein, which recognizes an amino terminal epitope on the amyloid precursor protein and detects all major isoforms of amyloid precursor protein, we examined the localization of the amyloid precursor protein within Alzheimer's and aged control brains. Very light cytoplasmic neuronal amyloid precursor protein staining but no neuritic staining was visible in control brains. In the Alzheimer's brain, we detected numerous amyloid precursor protein-immunopositive neurons with moderate to strong staining in select regions. Many neurons also contained varying levels of discrete granular, intracellular accumulations of amyloid precursor protein, and a few pyramidal neurons in particular appeared completely filled with amyloid precursor protein granules. "Ghost"-like deposits of amyloid precursor protein granules arranged in pyramidal, plaque-like shapes were identified. We detected long, amyloid precursor protein-immunopositive neurites surrounding and entering plaques. Many contained swollen varicosities along their length or ended in bulbous tips. Amyloid precursor protein immunoreactivity in the Alzheimer's brain was primarily present as granular deposits (plaques). The amyloid precursor protein granules do not appear to co-localize within either astrocytes or microglia, as evidenced by double-labeling immunohistochemistry with anti-glial fibrillary acidic protein and anti-leukocyte common antigen antibodies or Rinucus cummunicus agglutin lectin. Amyloid precursor protein could occasionally be detected in blood vessels in Alzheimer's brains. The predominantly neuronal and neuritic localization of amyloid precursor protein immunoreactivity indicates a neuronal source for much of the amyloid precursor protein observed in Alzheimer's disease pathology, and suggests a time-course of plaque development beginning with neuronal amyloid precursor protein accumulation, then deposition into the extracellular space, subsequent processing by astrocytes or microglia, and resulting in beta-amyloid peptide accumulation in plaques.

Platelet protease nexin-2/amyloid beta-protein precursor. Possible pathologic and physiologic functions

The amyloid beta-protein and its parent protein, amyloid beta-protein precursor (APP), are major constituents of neuritic plaques and cerebrovascular deposits in Alzheimer's disease and Down's syndrome. We reported that the protease inhibitor protease nexin-2 (PN-2) is the secreted form of APP that contains the Kunitz protease inhibitor domain. Previous studies suggested that circulating forms of PN-2/APP exist. Recently, we reported that PN-2/APP is a platelet alpha granule protein and is secreted upon platelet activation. Subsequent studies revealed that platelets are the major circulating repository for PN-2/APP and may contribute to its deposition in Alzheimer's disease. Protease inhibition measurements demonstrated that PN-2/APP is a potent inhibitor of certain serine proteases, particularly intrinsic blood coagulation factor XIa. Together, these findings indicate that PN-2/APP regulates blood coagulation, and possibly other proteolytic events, at sites of vascular injury.

The predominant form of the amyloid beta-protein precursor in human brain is protease nexin 2

The amyloid beta protein and the amyloid beta-protein precursor (APP) are major constituents of senile plaques and cerebrovascular deposits in patients with Alzheimer disease and Down syndrome. Most human tissues contain mRNA that encodes forms of APP that contain the Kunitz protease inhibitor (KPI+) domain. A major 120-kDa protein corresponding to this KPI+ mRNA is also found in these tissues. This protein is identical to the protease inhibitor protease nexin 2. Brain contains an additional mRNA species that encodes a form of APP that lacks the KPI domain (KPI-). This latter mRNA has been suggested to encode a 105-kDa KPI- form of APP protein also found in brain. Using protease inhibitory functional assays, we show that both the 105-kDa and 120-kDa APP proteins in normal and Alzheimer disease brain contain the KPI domain. Moreover, KPI domain-specific precipitation assays reveal that KPI- forms of APP protein represent less than 14% of total brain APP. Lastly, an enriched fraction from total brain homogenate contains proteolytic activity that can process the purified 120-kDa KPI+ form of APP into a 105-kDa form, resulting in a high-molecular-mass doublet identical to that seen in brain. These findings indicate that although KPI- APP mRNA is abundant in brain, little corresponding protein is present. Thus, KPI+ APP protein (equivalent to protease nexin 2) is the predominant form of APP in human brain.

Protease nexin-2/amyloid beta-protein precursor in blood is a platelet-specific protein

The protease inhibitor, protease nexin-2 (PN-2), is the secreted form of the amyloid beta-protein precursor (APP) which contains the Kunitz protease inhibitor domain. PN-2/APP is an abundant platelet alpha-granule protein which is secreted upon platelet activation. PN-2/APP mRNA is present in cultured endothelial cells and the protein has been detected in plasma. In the present studies we quantitated PN-2/APP in platelets, plasma and several different cell types of the vasculature to identify the repository of the protein in the circulatory system. We report that PN-2/APP is predominantly a platelet protein in the vascular compartment. Lysates of unstimulated umbilical vein endothelial cells, granulocytes or monocytes contained little PN-2/APP based on sensitive functional protease binding and immunoblotting assays. Quantitative immunoblotting studies demonstrated that normal citrated-plasma contains less than or equal to 60 pM PN-2/APP. In contrast, platelets can contribute up to 30 nM PN-2/APP, indicating that they are the major source of the protein in blood.

Inhibitors of urokinase and thrombin in cultured neural cells

Recent studies have suggested important roles for certain proteases and protease inhibitors in the growth and development of the CNS. In the present studies, inhibitors of urokinase or thrombin in cultured neural cells and serum-free medium from the cells were identified by screening for components that formed sodium dodecyl sulfate-stable complexes with 125I-urokinase or 125I-thrombin. Rinsed glioblastoma possessed two components that complexed 125I-urokinase. One was type 1 plasminogen activator inhibitor (PAI-1), because the 125I-urokinase-containing complexes were immunoprecipitated with anti-PAI-1 antibodies. The other component formed complexes with 125I-urokinase that were not recognized by antibodies to PAI-1 or protease nexin-1 (PN-1). Its identity is unknown. In addition to these cell-bound components, the glioblastoma cells also secreted two inhibitors that formed complexes with 125I-urokinase; one was PAI-1, and the other was PN-1. The secreted PN-1 also formed complexes with 125I-thrombin. It was the only thrombin inhibitor detected in these studies. Human neuroblastoma cells did not contain components that formed detectable complexes with either 125I-urokinase or 125I-thrombin. However, human neuroblastoma cells did contain very low levels of PN-1 mRNA and PN-1 protein. Added PN-1 bound to the surface of both glioblastoma and neuroblastoma cells. This interaction accelerated the inhibition of thrombin by PN-1 and blocked the ability of PN-1 to form complexes with 125I-urokinase. Thus, cell-bound PN-1 was a specific thrombin inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Protease nexin-1. Localization in the human brain suggests a protective role against extravasated serine proteases

Protease nexin-1 (PN-1) is a potent thrombin inhibitor that is identical to the glia-derived neurite-promoting factor or glia-derived nexin. Here we report immunocytochemical studies of adult human cerebral cortex that revealed the presence of strong immunoreactivity for PN-1 in capillaries and in the smooth muscle cells of arteries and arterioles. Expression of PN-1 was also abundant in astroglial processes in the parenchyma and in perivascular astroglial endfeet of human cerebral cortex. In situ hybridization with an 35S-labeled RNA antisense probe for PN-1 resulted in significant labeling of astrocytes and blood vessels. Because thrombin is known to cause retraction of neurites and modification of astrocytic morphology at low concentrations, PN-1 around blood vessels may play a major protective role against extravasation of thrombin and possibly other serine protease into the human brain.

Immunopurification and protease inhibitory properties of protease nexin-2/amyloid beta-protein precursor

Protease nexin-2 (PN-2) is a protease inhibitor that is synthesized and secreted by a variety of extravascular cells including human fibroblasts. It forms sodium dodecyl sulfate-stable complexes with trypsin, the epidermal growth factor binding protein and the gamma-subunit of nerve growth factor. Recently we reported that PN-2 is the secreted form of the amyloid beta-protein precursor (APP) and is a potent inhibitor of chymotrypsin. Here we describe a two-step procedure to purify PN-2/APP using a monoclonal antibody immunoaffinity column. We also quantitated the protease inhibitory properties of purified PN-2/APP on a number of serine proteases. PN-2/APP was a potent inhibitor of coagulation factor XIa with a Ki = 2.9 x 10(-10). The inhibition of factor XIa by PN-2/APP was augmented by heparin and resulted in a Ki = 5.5 x 10(-11) M. Trypsin and chymotrypsin were also effectively inhibited with a Ki = 4.2 x 10(-10) and 1.6 x 10(-9), respectively. PN-2/APP also inhibited the epidermal growth factor binding protein, the gamma-subunit of nerve growth factor, and chymase and plasmin to a lesser extent. In view of recent findings that PN-2/APP is contained in alpha-granules of platelets and is secreted upon platelet activation, the potent inhibition of factor XIa suggests that PN-2/APP may play a regulatory role in the coagulation pathway at vascular wound sites. In addition, these studies define biochemical activities of PN-2/APP which may be involved in regulating proteases that lead to the generation and deposition of the beta-protein in neurodegenerative lesions associated with Alzheimer's disease and Down's syndrome.

Protease nexin-II (amyloid beta-protein precursor): a platelet alpha-granule protein

Protease nexin-II (PN-II) [amyloid beta-protein precursor (APP)] and the amyloid beta-protein are major constituents of neuritic plaques and cerebrovascular deposits in individuals with Alzheimer's disease and Down syndrome. Both the brain and the circulation have been implicated as sources of these molecules, although they have not been detected in blood. Human platelets have now been found to contain relatively large amounts of PN-II/APP. Platelet PN-II/APP was localized in platelet alpha-granules and was secreted upon platelet activation. Because PN-II/APP is a potent protease inhibitor and possesses growth factor activity, these results implicate PN-II/APP in wound repair. In certain disease states, alterations in platelet release and processing and clearance of PN-II/APP and its derived fragments could lead to pathological accumulation of these proteins.

Reciprocal modulation of astrocyte stellation by thrombin and protease nexin-1

When cultured astroglia are treated with agents that elevate intracellular cyclic AMP, they become process-bearing stellate cells and resemble differentiated astrocytes in vivo. Thrombin rapidly reversed the stellation induced by dibutyryl cyclic AMP, forskolin, or isoproterenol in cultured rat astrocytes; half-maximal and maximal effects occurred at 0.5 and 8 pM, respectively. The proteolytic activity of thrombin was required for stellation reversal, as thrombin derivatized at its catalytic site serine with a diisopropylphospho group was inactive. Two thrombin inhibitors, protease nexin-1 and hirudin, blocked and reversed the effect of thrombin. The stellation reversal effect of thrombin was specific, as 300-1,000-fold higher concentrations of other serine proteinases, including plasmin, urokinase, trypsin, and T cell serine proteinase-1, were ineffective. Thrombin is a mitogen for astrocytes at concentrations in excess of 30 pM. Thrombin increased both cell number and ornithine decarboxylase activity, an early marker for mitogenic stimulation, in astrocyte cultures. The lowest thrombin concentrations that completely reversed astrocyte stellation, however, did not increase ornithine decarboxylase activity. Moreover, several other mitogens for astrocytes did not reverse dibutyryl cyclic AMP-induced stellation. Thus, the stellation reversal effect of thrombin is distinct from the mitogenic response.

Neurite outgrowth activity of protease nexin-1 on neuroblastoma cells requires thrombin inhibition

Protease nexin-1 (PN-1) is a protein proteinase inhibitor recently shown to be identical with the glial-derived neurite-promoting factor or glial-derived nexin. It has been shown to promote neurite outgrowth in neuroblastoma cells and in sympathetic neurons. The present experiments were designed to further test the hypothesis that this activity on neuroblastoma cells is due to its ability to complex and inhibit thrombin. It has been suggested that PN-1:thrombin complexes might mediate the neurite outgrowth activity of PN-1. However, the present studies showed that such complexes, unlike free PN-1, did not promote neurite outgrowth. The neurite outgrowth activity of PN-1 was only detected in the presence of thrombin or serum (which contains thrombin). PN-1 did not affect the rate or extent of neurite outgrowth that occurred when neuroblastoma cells were placed in serum-free medium. Retraction of neurites by thrombin was indistinguishable in cells whose neurites had been extended in the presence or absence of PN-1. The neurite-promoting activity of PN-1 was inhibited by an anti-PN-1 monoclonal antibody, which blocks its capacity to complex serine proteinases. The plasma thrombin inhibitor, antithrombin III, stimulated neurite outgrowth but only when its thrombin inhibitory activity was accelerated by heparin. The neurite outgrowth activity of both antithrombin III and PN-1 corresponded to their inhibition of thrombin. Together, these observations show that PN-1 promotes neurite outgrowth from neuroblastoma cells by inhibiting thrombin and suggest that this depends on the ability of thrombin to retract neurites.

Protease nexin-1, an antithrombin with neurite outgrowth activity, is reduced in Alzheimer disease

Protease nexin-1 (PN-1) is a cell-secreted protein that inhibits certain proteases, particularly thrombin, by forming SDS-stable complexes with the catalytic site serine of the protease. PN-1 was recently shown to be identical to a glial-derived neurite-promoting factor/glial-derived nexin present in rat brain. Its neurite outgrowth activity depends on inhibition of thrombin, presumably because thrombin brings about neurite retraction. Here we show that human brain contains PN-1 and that PN-1 activity in brains of individuals with Alzheimer disease (AD) was only 14% of control values (total of 14 AD patients and 7 control individuals). PN-1 activity in the hippocampus, a region with marked neuropathology in AD, was 15% of control values (10 AD patients and 4 control individuals). Western blot analysis indicated a large decrease in free PN-1 protein and an increase in PN-1-containing complexes that comigrated with PN-1-thrombin complexes. Northern blot analysis indicated that PN-1 mRNA levels were about equal in brains from AD patients and control individuals. Thus these results suggest that the decreases in PN-1 activity and free PN-1 protein are due to formation of PN-1-protease complexes.

Protease nexin-II, a potent antichymotrypsin, shows identity to amyloid beta-protein precursor

Protease nexin-II (PN-II) is a protease inhibitor that forms SDS-resistant inhibitory complexes with the epidermal growth factor (EGF)-binding protein, the gamma-subunit of nerve growth factor, and trypsin. The properties of PN-II indicate that it has a role in the regulation of certain proteases in the extracellular environment. Here we describe more of the amino-acid sequence of PN-II and its identity to the deduced sequence of the amyloid beta-protein precursor (APP). Amyloid beta-protein is present in neuritic plaques and cerebrovascular deposits in individuals with Alzheimer's disease and Down's syndrome. A monoclonal antibody against PN-II (designated mAbP2-1) recognized PN-II in immunoblots of serum-free culture medium from human glioblastoma cells and neuroblastoma cells, as well as in homogenates of normal and Alzheimer's disease brains. In addition, mAbP2-1 stained neuritic plaques in Alzheimer's disease brain. PN-II was a potent inhibitor of chymotrypsin with an inhibition constant Ki of 6 x 10(-10)M. Together, these data demonstrate that PN-II and APP are probably the same protein. The regulation of extracellular proteolysis by PN-II and the deposition of at least parts of the molecule in senile plaques is consistent with previous reports that implicate altered proteolysis in the pathogenesis of Alzheimer's disease.