RESUMEN
Mutations in presenilin 1 (PS-1) and presenilin 2 (PS-2) have been linked to early onset, autosomal dominant Alzheimer's disease. Neither the normal function(s) of the presenilins nor their role(s) in mediating the devastating neurological and pathological changes associated with Alzheimer's Disease, however, are well understood. The results of the experiments described here demonstrate that expression of wild-type PS-1 or PS-2 increases outward K+ current densities in HEK-293 cells relative to untransfected or mock-transfected cells. Western blot analysis reveals that there is a marked increase in full-length, rather than processed, presenilins in transiently transfected HEK-293 cells, suggesting that full-length PS-1 (or PS-2) underlies the observed increases in outward K+ current densities. Consistent with this hypothesis, EXpression of an N-terminal proteolytic fragment of PS-1 is without effects on the membrane properties of HEK-293 cells. Mean outward K+ current densities are also shown to be increased in HEK-293 cells expressing the exon 9 splice site PS-1 mutation (deltaex9/PS-1), a mutant that does not undergo proteolytic processing. In HEK- 293 cells transiently transfected with a missense (G209V) PS-1 mutant, however, mean K+ current densities were not significantly different from controls. Expression of wild-type PS-1 in neonatal rat ventricular myocytes also results in increased outward K+ currents, whereas no detectable effects on membrane currents were seen in PS-1-transfected COS-7 cells. These results suggest that the presenilins do not actually form K+ channels, but rather that these proteins upregulate functional K+ channel expression either directly by associating with K+ channel pore-forming subunits or indirectly by increasing the synthesis, assembly, and/or transport of these subunits. The observation that PS-1 and PS-2 are highly expressed in neurons, localized to the endoplasmic reticulum, suggests that the presenilins could regulate neuronal K+ channel expression; mutations in PS-1/PS-2 would then be expected to result in profound changes in neuronal excitability and contribute to the cognitive decline commonly associated with Alzheimer's Disease.