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-coupled protease-activated receptors (PARs), such as PAR1 , PAR3, and PAR4, and a "tethered ligand" mechanism. PAR1 is variably expressed in subpopulations of neurons and largely determines thrombin's effects on morphology, calcium mobilization, and caspase-mediated apoptosis. In spinal cord motoneurons, PAR1 expression correlates with transient thrombin-mediated [Ca²⁺]_i flux, receptor cleavage, and elevation of rest [Ca²⁺]_i activating intracellular 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. Consequently, we employed several G-protein-specific modulators prior to thrombin exposure in an attempt to uncouple both heterotrimeric and monomeric G-proteins from motoneuronal PAR1. Cholera toxin, stimulating Gs, and lovastatin, which blocks isoprenylation of Rho, reduced thrombin-induced calcium mobilization. In contrast, pertussis toxin and mastoparan, inhibiting or stimulating G_o/G_i, were found to exacerbate thrombin action. Effects on neuronal rounding and apoptosis were also detected, suggesting therapeutic utility may result from interference with downstream components of thrombin signaling pathways in human motor neuron disorders, and possibly other neurodegenerative diseases.