Tissue-type plasminogen activator–mediated shedding of astrocytic low-density lipoprotein receptor–related protein increases the permeability of the neurovascular unit

Abstract

The low-density lipoprotein receptorrelated protein (LRP) is a member of the LDL receptor gene family that binds several ligands, including tissue-type plasminogen activator (tPA). tPA is found in blood, where its primary function is as a thrombolytic enzyme, and in the central nervous system where it mediates events associated with cell death. Cerebral ischemia induces changes in the neurovascular unit (NVU) that result in brain edema. We investigated whether the interaction between tPA and LRP plays a role in the regulation of the permeability of the NVU during cerebral ischemia. We found that the ischemic insult induces shedding of LRP's ectodomain from perivascular astrocytes into the basement membrane. This event associates with the detachment of astrocytic end-feet processes and the formation of areas of perivascular edema. The shedding of LRP's ectodomain is significantly decreased in tPA deficient (tPA ؊/؊ ) mice, is increased by incubation with tPA, and is inhibited by the receptor-associated protein (RAP). IntroductionThe low-density lipoprotein receptor-related protein (LRP) is a member of the LDL receptor gene family composed of a 515-kDa heavy chain noncovalently bound to an 85-kDa light chain containing a transmembrane and a cytoplasmic domain. 1 LRP mediates the internalization of apoE-enriched lipoprotein particles, 2 ␣-2-macroglobulin-protease complexes, 3 and several other ligands, including plasminogen activators, proteinase-inhibitor complexes, clotting factors, and the amyloid precursor protein (APP). 1 LRP has also been implicated in cellular signal transduction pathways 4 and neurotransmission. 5 Like other signaling receptors such as Notch 6 and APP, 7 LRP undergoes a ␥-secretase-likedependent cleavage of its cytoplasmic site with release of its intramembranous domain. 8 This process is preceded by shedding of the receptor's ectodomain which may increase substrate availability for the enzymes that are required for the cleavage of the intramembranous or cytosolic sites. 8,9 Tissue-type plasminogen activator (tPA) is a ligand for LRP, 10,11 on binding to LRP induces a transient tyrosine phosphorylation of its cytoplasmic domain, and induces increased synthesis of 13 Animal studies have demonstrated that following the onset of cerebral ischemia there is an increase in endogenous tPA activity within the ischemic tissue, [14][15][16] and that either genetic deficiency of tPA 14,17 or its inhibition with neuroserpin 15,18 are associated with neuronal survival and decrease in the volume of the ischemic lesion.The neurovascular unit (NVU) is a dynamic structure consisting of endothelial cells, the basal lamina, astrocytic end-feet processes, pericytes, and neurons. 19,20 One of the functions of the NVU is to form a barrier, known as the blood-brain barrier (BBB), that regulates the entry of selected molecules from the blood into the central nervous system (CNS). 21,22 During cerebral ischemia the permeability of the NVU increases, resulting in the development of cerebral edema, 23,24 which is a ...

A shed form of LDL receptor–related protein–1 regulates peripheral nerve injury and neuropathic pain in rodents

“…Cleavage of the β-chain releases a shed form of the receptor with an apparently intact α-chain. A soluble or shed form of LRP1 with an intact α-chain (sLRP-α) is present in human plasma (25) and has been identified at the blood-brain barrier in ischemia (17). Given the diverse activities of membrane-anchored LRP1, it is reasonable to hypothesize that LRP1 shedding alters cell physiology by loss of function.…”

“…Finally, LRP1 directly regulates cell signaling and gene transcription by its capacity to bind signaling adaptor proteins (1,16). In the CNS, LRP1 regulates the permeability of the blood-brain barrier (17,18) and clears β-amyloid peptide from the neuropil (19).…”

Unveiling an exceptional zymogen: the single-chain form of tPA is a selective activator of NMDA receptor-dependent signaling and neurotoxicity

“…2,4 tPA has also critical functions in the brain parenchyma either as an enzyme, a cytokine-like molecule or a neuromodulator with roles in cell migration, neuronal plasticity and neurodegeneration. [5][6][7] Neurons 8 and glial cells 9,10 secrete tPA in the brain parenchyma where it can controls some plasminogendependent effects. For instance, it was proposed that the tPA-mediated generation of plasmin controls the degradation of the extracellular matrix to promote neuronal death.…”

“…[11][12][13] There are also evidences that tPA mediates a number of plasminogen-independent functions on non-fibrin substrates. These substrates, binding proteins or receptors for tPA include the platelet-derived growth factor-C, 14 the low density lipoprotein receptor-related protein, 10 annexin-II, 9,15 the chemokine macrophage chemoattractant protein, macrophage chemoattractant protein-1 16 and the N-methyl-D-aspartate receptor (NMDAR). 6 It is now well established that the interaction of tPA with the NMDAR leads to an increased calcium influx, activation of Erk(½) and neuronal death.…”

NR2D-containing NMDA receptors mediate tissue plasminogen activator-promoted neuronal excitotoxicity

“…[1][2][3] However, tPA not only activates plasminogen, but rather acts through several modalities 4 by interacting with the low-density lipoproteinrelated receptor protein (LRP), [5][6][7][8] annexin-II 9 or N-methyl-Daspartate receptors (NMDAR). 1,[10][11][12] These interactions mediate several potentially damaging effects of tPA, including potentiation of NMDAR-mediated signalling and excitotoxicity.…”