Identification of Microglial Signal Transduction Pathways Mediating a Neurotoxic Response to Amyloidogenic Fragments of β-Amyloid and Prion Proteins

Abstract

Microglial interaction with amyloid fibrils in the brains of Alzheimer's and prion disease patients results in the inflammatory activation of these cells. We observed that primary microglial cultures and the THP-1 monocytic cell line are stimulated by fibrillar beta-amyloid and prion peptides to activate identical tyrosine kinase-dependent inflammatory signal transduction cascades. The tyrosine kinases Lyn and Syk are activated by the fibrillar peptides and initiate a signaling cascade resulting in a transient release of intracellular calcium that results in the activation of classical PKC and the recently described calcium-sensitive tyrosine kinase PYK2. Activation of the MAP kinases ERK1 and ERK2 follows as a subsequent downstream signaling event. We demonstrate that PYK2 is positioned downstream of Lyn, Syk, and PKC. PKC is a necessary intermediate required for ERK activation. Importantly, the signaling response elicited by beta-amyloid and prion fibrils leads to the production of neurotoxic products. We have demonstrated in a tissue culture model that conditioned media from beta-amyloid- and prion-stimulated microglia or from THP-1 monocytes are neurotoxic to mouse cortical neurons. This toxicity can be ameliorated by treating THP-1 cells with specific enzyme inhibitors that target various components of the signal transduction pathway linked to the inflammatory responses.

The Spleen Tyrosine Kinase (Syk) Regulates Alzheimer Amyloid-β Production and Tau Hyperphosphorylation

“…Pharmacological inhibitors of Syk have been developed and represent potential immunomodulatory agents for the treatment of autoimmune and inflammatory conditions. Interestingly, A␤ has been shown to stimulate Syk activity resulting in microglial activation and neurotoxicity (65)(66)(67)(68)(69)(70).…”

Microglia, major player in the brain inflammation: their roles in the pathogenesis of Parkinson's disease

“…Although the mechanism by which A peptides cause enhanced expression of proinflammatory cytokines from microglia, is not fully understood there is evidence that A may interact with cell-surface receptors, including receptors for advanced glycosylated endproducts (RAGE) and scavenger receptors (El Khoury et al, 1996;Yan et al, 1996). Additionally, calcium-, protein kinase C, and protein tyrosine kinase-dependent second messenger pathways have been postulated in A receptor-mediated signal transduction (Lorton, 1997;Combs et al, 1999). A peptide activates microglia through these signal transduction pathways to induce the secretion of neurotoxic substances including TNF-and IL-1 (Mrak and Griffin, 2001;Smits et al, 2001).…”

CD45 Inhibits CD40L-induced Microglial Activation via Negative Regulation of the Src/p44/42 MAPK Pathway

“…Cross-linking of CD45 Suppresses CD40L-induced p44/42 MAPK Activity-It has been reported that Src kinases are involved in regulation of MAPK activation (10,11,36). We and others have shown that activation of MAPK, in particular p44/42 MAPK, is involved in TNF-␣ production in macrophages, monocytes, and microglia following activation of these cells with a variety of stimuli, including LPS and CD40 ligand (9,37,38).…”

“…Intracellularly, microglial activation induced by a variety of stimuli including CD40L, lipopolysaccharide (LPS), ␤-amyloid peptides, and prion, has been shown to involve activation of the mitogen-activated protein kinase (MAPK) module, ultimately leading to production of neurotoxic products by these cells (9,10). Additionally, it has been shown that members of the Src family, including the tyrosine kinase Lyn (10), regulate activation of MAPK in these cells.…”