Ion Channel SUMOylation: a Novel Role for SUMO in Homeostatic Regulation of Multiple Ionic Conductances

Neurons can adjust their ionic currents to maintain a stable output. The homeostatic mechanisms that produce compensatory changes in ionic currents operate over multiple time scales. The rapid mechanisms that act over minutes are mostly unknown. We have been characterizing a fast homeostatic mechanism that stabilizes activity phase in the rhythmically active lateral pyloric neuron (LP) of the crustacean stomatogastric ganglion. LP activity phase is invariant. It is determined, in part, by the balance between the hyperpolarization-activated current (Ih) and the transient potassium current (IA). When LP IA is experimentally decreased, activity phase is initially disrupted, but then it recovers over minutes. This is because the decrease in IA modifies LP activity, which in turn alters cytosolic Ca2+ levels. Ca-dependent enzymes then mediate a reduction in LP Ih to restore the balance between the two conductances. We have been studying the molecular mechanisms that correlate LP IA and Ih in an activity- dependent fashion. We have found that neuronal activity adjusts the level of ion channel post- translational modification by Small Ubiquitin-like Modifier (SUMO), a peptide which when conjugated to target proteins alters their protein-protein interactions. Using a heterologous expression system, we showed that enhancing SUMOylation of HCN or Kv4 ion channels that mediate Ih and IA, respectively, produced opposite effects on the amplitudes of Ih and IA. We also demonstrated that a given change in activity produced the opposite effect on SUMOylation levels associated with each current. Thus, activity-dependent regulation of ion channel SUMOylation specified a positive correlation between the two currents. We have also demonstrated that activity-dependent regulation of ion channel SUMOylation is conditional; it only occurs in the presence of the appropriate modulatory tone. We showed this is because modulators, like dopamine, specify the targets of the SUMOylation machinery. In sum, we have discovered a novel mechanism that acts over minutes to correlate ionic conductances and thereby stabilize neuronal output