The role of cellular prion protein (PrPc) in HIV neuropathogenesis

Abstract

HIV-1 enters the CNS very early after peripheral infection, mainly through infected monocytes that traverse the blood brain barrier. Infection of the CNS results in neuroinflammation and neurotoxicity that will lead to cognitive impairment in 40-60% of infected people despite cART. HIV neuropathogenesis is characterized by chemokine and cytokine elaboration and neurotoxic viral protein and host factor production that will collectively lead to neuronal injury and loss. During this process, the expression and localization of adhesion molecules in CNS cells is dysregulated resulting in functional changes that contribute to the pathogenesis of HAND.;PrPc, cellular prion protein, is an adhesion molecule that is constitutively expressed on CNS cells including neurons, astrocytes, brain microvascular endothelial cells (BMVECs), macrophages and microglia. Previously, our laboratory found that the shed form of PrPc (sPrPc) is increased in the CSF of HIV positive individuals who have cognitive impairments as compared to infected individuals with normal cognitive functioning. In this work we examine the role of both cell associated and sPrPc in HIV neuropathogenesis.;We found that CCL2 and TNF-alpha, cytokines that are increased in the CNS of HIV infected people, induced increased shedding of PrPc from primary human astrocytes. CCL2 and TNF-alpha also increased astrocyte expression of the active form of ADAM10, a cell associated metalloprotease. We showed that increased PrPc shedding induced by these cytokines is mediated by active ADAM10, by using a specific ADAM10 inhibitor. To determine the functional consequence of increased sPrPc, we treated astrocytes with recombinant PrPc(recPrPc) and found increased secretion of CCL2, CXCL12, and IL-8, cytokines that are involved in monocyte recruitment across the endothelium. We also showed that supernatants of astrocytes treated with recPrPc contained factors produced in response to the treatment that increased uninfected and HIV infected monocytes chemotaxis. RecPrPc treatment of astrocytes also caused decreased glutamate uptake in astrocytes, suggesting a mechanism by which sPrPc contributes to neurotoxicity and death.;We also examined how cell associated PrPc on BMVEC is regulated during neuroinflammation. PrPc is expressed at junctions of BMVECs and is important for the transmigration of monocytes across endothelium. To examine how inflammation affects BMVEC PrPc, we treated cells with TNF-alpha and VEGF and examined PrPc levels. These factors have been implicated in increasing BBB permeability and dysregulating the expression of adhesion molecules. We found that both TNF-alpha and VEGF decreased total PrPc protein as well as mRNA levels indicating they regulate the de novo synthesis of this protein. We studied how decreased PrPc affects BMVEC barrier integrity, one of the processes that is dysregulated during HIV neuropathogenesis. To study the effect of PrPc loss from BMVEC, we used shRNA to downregulate this protein. We showed that endothelial monolayers composed of cells lacking PrP c exhibited increased permeability as determined by a FITC dextran permeability assay indicating that PrPc is essential in maintaining BBB integrity. We propose that one possible mechanism by which TNF-alpha and VEGF disrupt BBB permeability is by downregulating PrPc.;Collectively our data show that: 1) exposure of human astrocytes to CCL2 and TNF-alpha increases PrPc shedding 2) sPrPc contributes to neuroinflammation and toxicity 3) TNFalpha and VEGF downregulate cell associate PrPc from BMVEC 4) loss of PrPc causes increased endothelial monolayer permeability. Therefore, we propose that sPrPc and cell associated PrPc have different roles, with sPrPc contributing to neuroinflammation and toxicity and cell associated PrPc being essential for BBB integrity.