Tau isoform-specific enhancement of L-type calcium current and augmentation of afterhyperpolarization in rat hippocampal neurons.

Accumulation of tau is observed in dementia, with human tau displaying 6 isoforms grouped by whether they display either 3 or 4 C-terminal repeat domains (3R or 4R) and exhibit no (0N), one (1N) or two (2N) N terminal repeats. Overexpression of 4R0N-tau in rat hippocampal slices enhanced the L-type calcium (Ca) current-dependent components of the medium and slow afterhyperpolarizations (AHPs). Overexpression of both 4R0N-tau and 4R2N-tau augmented Ca1.2-mediated L-type currents when expressed in tsA-201 cells, an effect not observed with the third 4R isoform, 4R1N-tau. Current enhancement was only observed when the pore-forming subunit was co-expressed with Caβ3 and not Caβ2a subunits. Non-stationary noise analysis indicated that enhanced Ca channel current arose from a larger number of functional channels. 4R0N-tau and Caβ3 were found to be physically associated by co-immunoprecipitation. In contrast, the 4R1N-tau isoform that did not augment expressed macroscopic L-type Ca current exhibited greatly reduced binding to Caβ3. These data suggest that physical association between tau and the Caβ3 subunit stabilises functional L-type channels in the membrane, increasing channel number and Ca influx. Enhancing the Ca-dependent component of AHPs would produce cognitive impairment that underlie those seen in the early phases of tauopathies.