On a solution to the riddle of “amyloid made” versus “amyloid regulated” channels in cell membranes

1 September 2013

The paper recently presented by Lee et al. [1] impressively broadens the debate on the effects of soluble amyloid-β (Aβ) peptide oligomers applied at the cell surface of neurons and thus again fuels the question formulated in the headline. The authors, examining the impact of exogenously applied Aβ on signaling in neurons cultured on multi-electrode arrays, observed that subcytotoxic amounts of human Aβ1-42 perturbed synaptic transmissions within hours. The effect was prevented by the calcium chelator BAPTA, pointing to a requirement for calcium, while the application of antagonists of the NMDA receptor or L-type voltage-sensitive calcium channels, respectively, was without effect. Lee and colleges conclude that Aβ may induce calcium influx by either channel, or additional channels.

A synopsis of data from corresponding studies may allow a discrimination of "amyloid made" and "amyloid regulated" channels in the plasmalemma.

On the one hand, observations on varying channel phenotypes, set up by soluble Aβ peptides applied to liposomes or black membranes, i.e., "amyloid made" channels, have accumulated since the early nineties of last century. Channels of this type are often assumed to explain apoptogenic effects on living cells in cell physiologic settings [2-8]. Concerning their characteristics, "amyloid made" channels are thought to be Zn2+ sensitive pathways in the plasmalemma of neuronal as well as non-neuronal cells, involved in the induction of apoptosis by disturbing calcium homeostasis of cells. Noteworthy, there is a time factor concerning the effects of Aβ peptides being applied to cell surfaces. So, in the experiments by Mukhamedyarov et al. [2], the depolarizing action of Aβ on muscle fiber RMP was maximal after two hours time. This points to an indirect effect of Aβ peptides, not to direct cation channeling by " amyloid made" channels.

On the other hand, Aβ peptides, alternatively, can be assumed to work as regulators of endogenous cell membrane-standing channel molecules, i.e., "amyloid regulated" channels. Cell membrane-integrated type-1 VDAC is a candidate in this context. Concerning its involvement in apoptosis, opening of plasmalemmal type-1 VDAC has been shown to precede caspase activation in neuronal apoptosis, induced by staurosporin [9] or Aβ peptides [10]. VDAC-1 channels as well as Aβ peptides carry GxxxG motifs, established dimerization/aggregation peptide stretches. Based on this, I postulated plasmalemma-standing type-1 VDAC to be an "amyloid regulated" channel involved in the very first step of a type of extrinsic apoptosis. However, the model allows a new look into the pathogenesis of the Alzheimer syndrome and correlated phenotypes [11-13].

VDAC channels, broadly studied in liposomes and artificial planar bilayers, proved to be anion- or cation-selective, respectively; this is in relation to the regulatory status. Low molecular agonist (cholesterol, DIDS, ATP, polyamines, Ruthenium Red, Compound 48/80, Gd3+, Zn2+, Al3+) as well as BH4-BclXL peptides have been shown to work as VDAC-1 modulators. According to crystallographic studies, human type-1 VDAC forms β-barrels including 19 β-pleated sheets and an additional N-terminal helical stretch, which crosses the channel lumen and is engaged in channel regulation. The channel has been found in different compartments of vertebrate cells: outer mitochondrial membrane, endoplasmic reticulum, and cell membrane. In the plasmalemma, the channel is involved in cell volume regulation and thus apoptosis, playing its role in regulatory volume decrease and apoptotic volume decrease [13; http://www.futhin.de]. Concerning the relevance of calcium in VDAC regulation, see [14].

Is there a way to distinguish between “amyloid made” and “amyloid regulated” channels in cell membranes? A synopsis of data pointing to a time factor in the reaction of Aβ applied to cell surfaces with those of a report by Busche et al. [3], reporting on immediate effects of Aβ dimers at the plasmalemma of neuronal cells, may help to make a decision.

Busche and colleges [3] studied the activity patterns of hippocampal CA1 neurons in vivo in a mouse model of AD. They found that neuronal activity in the plaque-bearing CA1 region of older mice is profoundly altered. In the hippocampus of young mice, the authors observed a selective increase in hyperactive neurons before the formation of plaques, suggesting that soluble species of Aβ may underlie this impairment. Along this line, it was found that acute treatment with the γ-secretase inhibitor LY-411575 reduced soluble Aβ levels and rescued neuronal dysfunction. Finally, the study demonstrated that the direct application of soluble Aβ can induce neuronal hyperactivity in wild-type mice. To achieve this, the authors applied nanomolar concentrations of cross-linked dimers (Aβ40S2C6) to CA1 neurons in wild-type mice.

As shown in Figure 5 of Busche et al. [3], the dimers studied immediately elevated the rate of Ca2+ transients in CA1 neurons, indicating increased action potential firing. Heat-denatured dimers or local application of the peptide vehicle, respectively, did not alter neuronal activity. These results support and extend previous in vitro observations that soluble Aβ can acutely induce inward currents in hippocampal neurons, leading to increased action potential firing and intracellular Ca2+ elevations. The study identifies hippocampal hyperactivity as a very early functional impairment in AD transgenic mice and provides direct evidence that soluble Aβ is crucial for hippocampal hyperactivity.

Conclusion
Type-1 VDAC is well known in molecular terms. Cell membrane-standing type-1 VDAC is part of the cell volume regulatory system and thus apoptosis. VDAC-1 channels as well as Aβ peptides carry GxxxG motifs, which might interact to generate plasmalemma-standing "amyloid regulated" channel, putatively involved in the first step of a type of extrinsic apoptosis. Regarding the system wide distribution of different Aβ peptides forms in several body fluids, the proposed mechanism might be relevant for pathogenic processes in different cell types and the syndromes resting on those. Taken together, to look for immediate effects of Aβ peptides on cell surfaces might help to distinguish between "amyloid made" and "amyloid regulated" channels in the field.

Outlook
There are few reports on autoantibodies against VDAC-1 in correlation to different syndromes i.e., pancreas cancer [15,16], mixed connective tissue disease [17], and autism [18,19], with data pointing to its involvement in the necrotic process. Finally, VDAC-1 in cell membranes has been shown to be engaged in the ATP efflux from mouse epithelial cells [20] and human erythrocytes [21] as well. From here, it may pay to keep cell membrane-standing VDAC-1 as a candidate for "find me" signal pathways [22,23].

Friedrich P. Thinnes, Baumschulenweg 5, D-37083 Göttingen, Germany; futhin@t-online.de

References
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Comments

We agree that VDAC-1 channel activity may play a key role in early aspects of Abeta toxicity, including our observed alteration in spontaneous signaling in culture. We recognized a limitation of our study in that we did not simultaneously inhibit both NMDA and L-voltage-sensitive channels, and therefore included the somewhat generic acknowledgment that one or more additional channels could be involved. The points made by Dr. Thinnes are excellent, and further examination of a role of VDAC-1 activity is warranted.

Tom Shea