Besides the same change, those of the tg la mice showed a more distinct change in voltage dependence of activation and inactivation, being shifted in the depolarizing direction by ϳ10 mV, with a broader voltage dependence of inactivation. In the recombinant expression system, the tg channel with a missense mutation (P601L) and one form of the two possible tg la aberrant splicing products, tg la (short) channel, showed a significant reduction in current density, while the other form of the tg la channels, tg la (long), had a current density comparable to the normal control. On the other hand, the shift in voltage dependence of activation and inactivation was observed only for the tg la (long) channel. Comparison of properties of the native and recombinant mutant channels suggests that single tottering mutations are directly responsible for the neuropathic phenotypes of reduction in current density and deviations in gating behavior, which lead to neuronal death and cerebellar atrophy.

Characterization of mammalian homologues of Drosophila transient receptor potential protein (TRP) is an important clue to understand molecular mechanisms underlying Ca2؉ influx activated in response to stimulation of G q protein-coupled receptors in vertebrate cells. Here we have isolated cDNA encoding a novel seventh mammalian TRP homologue, TRP7, from mouse brain. TRP7 showed abundant RNA expression in the heart, lung, and eye and moderate expression in the brain, spleen, and testis. TRP7 recombinantly expressed in human embryonic kidney cells exhibited distinctive functional features, compared with other TRP homologues. Basal influx activity accompanied by reduction in Ca 2؉ release from internal stores was characteristic of TRP7-expressing cells but was by far less significant in cells expressing TRP3, which is structurally the closest to TRP7 in the TRP family. TRP7 induced Ca 2؉ influx in response to ATP receptor stimulation at ATP concentrations lower than those necessary for activation of TRP3 and for Ca 2؉ release from the intracellular store, which suggests that the TRP7 channel is activated independently of Ca 2؉ release. In fact, TRP7 expression did not affect capacitative Ca 2؉ entry induced by thapsigargin, whereas TRP7 greatly potentiated Mn 2؉ influx induced by diacylglycerols without involvement of protein kinase C. Nystatin-perforated and conventional whole-cell patch clamp recordings from TRP7-expressing cells demonstrated the constitutively activated and ATP-enhanced inward cation currents, both of which were initially blocked and then subsequently facilitated by extracellular Ca 2؉ at a physiological concentration. Impairment of TRP7 currents by internal perfusion of the Ca 2؉ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid revealed an essential role of intracellular Ca 2؉ in activation of TRP7, and their potent activation by the diacylglycerol analogue suggests that the TRP7 channel is a new member of diacylglycerol-activated cation channels. Relative permeabilities indicate that TRP7 is slightly selective to divalent cations. Thus, our findings reveal an interesting correspondence of TRP7 to the background and receptor stimulation-induced cation currents in various native systems.

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SummaryParkinson disease (PD) is a progressive neurological disease caused by selective degeneration of dopaminergic (DA) neurons in the substantia nigra. Although most cases of PD are sporadic cases, familial PD provides a versatile research model for basic mechanistic insights into the pathogenesis of PD. In this study, we generated DA neurons from PARK2 patient-specific, isogenic PARK2 null and PARK6 patient-specific induced pluripotent stem cells and found that these neurons exhibited more apoptosis and greater susceptibility to rotenone-induced mitochondrial stress. From phenotypic screening with an FDA-approved drug library, one voltage-gated calcium channel antagonist, benidipine, was found to suppress rotenone-induced apoptosis. Furthermore, we demonstrated the dysregulation of calcium homeostasis and increased susceptibility to rotenone-induced stress in PD, which is prevented by T-type calcium channel knockdown or antagonists. These findings suggest that calcium homeostasis in DA neurons might be a useful target for developing new drugs for PD patients.