Soluble or bound laminin elicit in human neuroblastoma cells short- or long-term potentiation of a K⁺ inwardly rectifying current: Relevance to neuritogenesis

Changes in the resting potential (V_REST) and in the underlying ionic conductances were measured by the patch-clamp technique in SH-SY5Y human neuroblastoma cells exposed to substrate-bound or soluble Laminin (bLN; sLN), as compared to integrin-independent substrates (polylysine (PL); bovine serum albumin (BSA)). While PL and BSA were ineffective, both forms of LN caused an early (5-15 min) activation of a peculiar type of Inwardly Rectifying K⁺ current (I_IR) characterised by a voltage-dependent inactivation in the range of membrane potentials around -50/0 mV. I_IR was blocked by Cs⁺ ions and by the antiarrhythmic drug E-4031, a specific inhibitor of the HERG-codified channels. In cells adherent to bLN, I_IR potentiation (85%) persisted for 90-120 min and was accompanied by a similar, but transient, increase in the leakage conductance (G_L). Successively, the persistence of a high I_IR conductance and the decrease of G_L progressively bring V_REST from -12 to ∼ -30 mV in about 120 min. On the other hand, in cells adherent to PL, exposure to sLN produced a similar but not persistent activation of I_IR, without any increase in G_L. this caused a rapid, transient hyperpolarisation of V_REST. The effects of bLN and sLN were mimicked by antibodies raised against the integrin β₁ subunit, suggesting a specific integrin-mediated mechanism. In fact, when bound to the culture dishes, these antibodies simultaneously activated the I_IR and G_L, whereas in soluble form they only activated I_IR. Cells adherent to bLN emitted neurites, a process impaired by the block of I_IR by E-4031 and Cs^-. On the whole data suggest that the integrin-mediated activation of I_IR plays a crucial role in the commitment to neuritogenesis of neuroblastoma cells, independently on the effects of this activation on V_REST.