Neuroprotective signal transduction in model motor neurons exposed to thrombin: G-protein modulation effects on neurite outgrowth, Ca2+ mobilization,and apoptosis

Thrombin, the ultimate protease in the blood coagulation cascade, mediates its known cellular effects by unique proteolytic activation of G-protein-co upled protease-activated receptors (PARs), such as PAR1, PAR3, and PAR4 and a "tethered ligand" mechanism. PAR1 is variably expressed in subpopulation s of neurons and largely determines thrombin's effects on morphology, calci um mobilization, and caspase-mediated apoptosis, In spinal cord motoneurons , PARI expression correlates with transient thrombin-mediated [Ca2+](i) flu x, receptor cleavage, and elevation of rest [Ca2+](i) activating intracellu lar proteases, At nanomolar concentrations, thrombin retracts neurites via PAR1 activation of the monomeric, 21 kDa Ras G-protein RhoA, which is also involved in neuroprotection at lower thrombin concentrations. Such results suggest potential downstream targets for thrombin's injurious effects. Cons equently, we employed several G-protein-specific modulators prior to thromb in exposure in an attempt to uncouple both heterotrimeric and monomeric G-p roteins from motoneuronal PAR1. Cholera toxin, stimulating Gs, and lovastat in, which blocks isoprenylation of Rho, reduced thrombin-induced calcium mo bilization. In contrast, pertussis toxin and mastoparan, inhibiting or stim ulating G(o)/G(i), were found to exacerbate thrombin action. Effects on neu ronal rounding and apoptosis were also detected, suggesting therapeutic uti lity may result from interference with downstream components of thrombin si gnaling pathways in human motor neuron disorders, and possibly other neurod egenerative diseases. (C) 2001 John Wiley & Sons. Inc.*