All or none block of single Na+ channels by tetrodotoxin

Effects of DDT on Amyloid Precursor Protein Levels and Amyloid Beta Pathology: Mechanistic Links to Alzheimer's Disease Risk

BACKGROUND

The interaction of aging-related, genetic, and environmental factors is thought to contribute to the etiology of late-onset, sporadic Alzheimer's disease (AD). We previously reported that serum levels of p,p'-dichlorodiphenyldichloroethylene (DDE), a long-lasting metabolite of the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT), were significantly elevated in patients with AD and associated with the risk of AD diagnosis. However, the mechanism by which DDT may contribute to AD pathogenesis is unknown.

OBJECTIVES

This study sought to assess effects of DDT exposure on the amyloid pathway in multiple in vitro and in vivo models.

METHODS

Cultured cells (SH-SY5Y and primary neurons), transgenic flies overexpressing amyloid beta (Aβ), and C57BL/6J and 3xTG-AD mice were treated with DDT to assess impacts on the amyloid pathway. Real time quantitative polymerase chain reaction, multiplex assay, western immunoblotting and immunohistochemical methods were used to assess the effects of DDT on amyloid precursor protein (APP) and other contributors to amyloid processing and deposition.

RESULTS

Exposure to DDT revealed significantly higher APP mRNA and protein levels in immortalized and primary neurons, as well as in wild-type and AD-models. This was accompanied by higher levels of secreted Aβ in SH-SY5Y cells, an effect abolished by the sodium channel antagonist tetrodotoxin. Transgenic flies and 3xTG-AD mice had more Aβ pathology following DDT exposure. Furthermore, loss of the synaptic markers synaptophysin and PSD95 were observed in the cortex of the brains of 3xTG-AD mice.

DISCUSSION

Sporadic Alzheimer's disease risk involves contributions from genetic and environmental factors. Here, we used multiple model systems, including primary neurons, transgenic flies, and mice to demonstrate the effects of DDT on APP and its pathological product Aβ. These data, combined with our previous epidemiological findings, provide a mechanistic framework by which DDT exposure may contribute to increased risk of AD by impacting the amyloid pathway. https://doi.org/10.1289/EHP10576.

Tetrodotoxin: a brief history

Tetrodotoxin (TTX), contained in puffer, has become an extremely popular chemical tool in the physiological and pharmacological laboratories since our discovery of its channel blocking action in the early 1960s. This brief review describes the history of discovery of TTX action on sodium channels, and represents a story primarily of my own work. TTX inhibits voltage-gated sodium channels in a highly potent and selective manner without effects on any other receptor and ion channel systems. TTX blocks the sodium channel only from outside of the nerve membrane, and is due to binding to the selectivity filter resulting in prevention of sodium ion flow. It does not impairs the channel gating mechanism. More recently, the TTX-resistant sodium channels have been discovered in the nervous system and received much attention because of their role in pain sensation. TTX is now known to be produced not by puffer but by bacteria, and reaches various species of animals via food chain.(Communicated by Masanori OTSUKA, M.J.A.).

Post-repolarization block of cloned sodium channels by saxitoxin: the contribution of pore-region amino acids

Sodium channels expressed in oocytes exhibited isoform differences in phasic block by saxitoxin (STX). Neuronal channels (rat IIa co-expressed with beta 1 subunit, Br2a + beta 1) had slower kinetics of phasic block for pulse trains than cardiac channels (RHI). After the membrane was repolarized from a single brief depolarizing step, a test pulse at increasing intervals showed first a decrease in current (post-repolarization block) then eventual recovery in the presence of STX. This block/unblock process for Br2a + beta 1 was 10-fold slower than that for RHI. A model accounting for these results predicts a faster toxin dissociation rate and a slower association rate for the cardiac isoform, and it also predicts a shorter dwell time in a putative high STX affinity conformation for the cardiac isoform. The RHI mutation (Cys374-->Phe), which was previously shown to be neuronal-like with respect to high affinity tonic toxin block, was also neuronal-like with respect to the kinetics of post-repolarization block, suggesting that this single amino acid is important for conferring isoform-specific transition rates determining post-repolarization block. Because the same mutation determines both sensitivity for tonic STX block and the kinetics of phasic STX block, the mechanisms accounting for tonic block and phasic block share the same toxin binding site. We conclude that the residue at position 374, in the putative pore-forming region, confers isoform-specific channel kinetics that underlie phasic toxin block.

Ontogenic development of the TTX-sensitive and TTX-insensitive Na+ channels in neurons of the rat dorsal root ganglia

Developmental changes in the sensitivity of neurons to tetrodotoxin (TTX) were studied in relation to the cell size in rat dorsal root ganglia (DRG). Na+ currents were recorded from neurons of various stages of development. Two types of Na+ channels were identified on the basis of their sensitivity to TTX. One type was insensitive to a very high concentration (0.1 mM) of TTX, while the other type was blocked by a low concentration (1 nM) of TTX. These two types of Na+ channels were observed throughout the developmental stages examined from day 17 of gestation and adulthood. Thus, both types of Na+ channels are already established at the early stage of neuronal development and appear to be retained throughout the life-span of the DRG neuron. The concentration-response relationships for the block of TTX-sensitive Na+ current by TTX did not appreciably change during development. Although two types of Na+ channels had strikingly different kinetic properties, the kinetic properties of each channel type were basically similar throughout development. The TTX-sensitive Na+ channels were mainly concentrated in cells with large cell diameters throughout developmental stages examined. These large cells appear to correspond to the 'large-light' cells. On the contrary, the TTX-insensitive Na+ channels were found in smaller diameter cells which may correspond to the 'small-dark' cells. Thus, it is concluded that there are heterogeneous categories of neurons which have Na+ channels with different physiological and pharmacological properties. Since Na+ channels play a pivotal role in the action potential generation, these heterogeneity of DRG neurons appear to be instrumental in integrating the sensory signals.

Responsiveness of cardiac Na+ channels to antiarrhythmic drugs: the role of inactivation

Elementary Na+ currents were recorded at 9 degrees C in inside-out patches from cultured neonatal rat heart myocytes. In characterizing the sensitivity of cooled, slowly inactivating cardiac Na+ channels to several antiarrhythmic drugs including propafenone, lidocaine and quinidine, the study aimed to define the role of Na+ inactivation for open channel blockade. In concentrations (1-10 mumol/liter) effective to depress NPo significantly, propafenone completely failed to influence the open state of slowly inactivating Na+ channels. With 1 mumol/liter, tau open (at -45 mV) in cooled, (-)-DPI-modified, noninactivating Na+ channels proved to be drug resistant and could not be flicker-blocked by 10 mumol/liter propafenone. The same drug concentration induced in (-)-DPI-modified Na+ channels a discrete block with association and dissociation rate constants of 16.1 +/- 5.3 x 10(6) mol-1 sec-1 and 675 +/- 25 sec-1, respectively. Quinidine, known to have a considerable affinity for activated Na+ channels, in lower concentrations (5 mumol/liter) left tau open unchanged or reduced, in higher concentrations (10 mumol/liter) tau open only slightly to 81% of the predrug value whereas NPo declined to 30%, but repetitive blocking events during the conducting state could never be observed. Basically the same drug resistance of the open state was seen in cardiac Na+ channels whose open-state kinetics had been modulated by the cytoplasmic presence of F- ions. But in this case, propafenone reduced reopening and selectively abolished a long-lasting open state. This drug action is unlikely related to the inhibitory effect on NPo since hyperpolarization and the accompanying block attenuation did not restore the channel kinetics. It is concluded that cardiac Na+ channels cannot be flicker-blocked by antiarrhythmic drugs unless Na+ inactivation is removed.

Tetrodotoxin block of single germitrine-activated sodium channels in cultured rat cardiac cells

1. The open time of single Na+ channels in excised (outside-out) patches from cultured late-fetal rat ventricular myocytes was prolonged to several minutes by germitrine (0.5 mM) in order to analyse tetrodotoxin (TTX) blocking kinetics. 2. The germitrine modification appeared during depolarizing pulses that activated normal Na+ channels. Following repolarization to -100 mV, the modified Na+ channel remained activated for 136 +/- 186 s (mean +/- S.D., n = 54) with an open-channel current amplitude of -0.5 pA. The predominant open state with a mean open time of 0.13 s was interrupted by brief closing events lasting for milliseconds. Replacing extracellular Na+ by Cs+ decreased the current amplitude to -0.1 pA. 3. Extracellular superfusion with TTX (3 x 10(-7) M) of a single germitrine-activated Na+ channel induced full channel closures lasting seconds (blocked events) separated by channel reopenings (unblocked events) that were indistinguishable in terms of amplitude and gating kinetics from the germitrine-activated state in the absence of TTX. 4. Cumulative probability histograms of blocked and unblocked events (n greater than 140) collected during long-lasting germitrine modifications at 10(-7) and 3 x 10(-7) M-TTX are well described by single exponentials. The 3-fold increase in [TTX] decreased the time constant of the unblocked state, tau o, from 11.9 to 4.7 s, while the time constant of the blocked state, tau c, was not significantly altered from 8.6 to 9.7 s. A microscopic association rate constant of 7.7 x 10(5) M-1 s-1, dissociation rate constant of 0.11 s-1, and equilibrium dissociation constant of 1.4 x 10(-7) M (at -100 mV) were calculated (20 degrees C). 5. Increasing [TTX] to 10(-5) M decreased tau o to 86 ms. This argues against the existence of a slower conformational step interposed between the binding of TTX to an open channel and the resultant channel closure. 6. Setting the membrane potential to -50 or 0 mV subsequent to a germitrine modification at -100 mV did not significantly alter TTX (3 x 10(-7) M) blocking kinetics: tau o was 6.7 s at -50 mV and 5.2 s at 0 mV; tau c was 8.9 and 8.1 s, respectively. 7. These results suggest that blocked events correspond to the random times that a TTX molecule resides on the Na+ channel before it dissociates, and unblocked events correspond to the random waiting times of an unoccupied channel before it binds another toxin molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

Batrachotoxin-modified sodium channels in planar lipid bilayers. Characterization of saxitoxin- and tetrodotoxin-induced channel closures

The guanidinium toxin-induced inhibition of the current through voltage-dependent sodium channels was examined for batrachotoxin-modified channels incorporated into planar lipid bilayers that carry no net charge. To ascertain whether a net negative charge exists in the vicinity of the toxin-binding site, we studied the channel closures induced by tetrodotoxin (TTX) and saxitoxin (STX) over a wide range of [Na+]. These toxins carry charges of +1 and +2, respectively. The frequency and duration of the toxin-induced closures are voltage dependent. The voltage dependence was similar for STX and TTX, independent of [Na+], which indicates that the binding site is located superficially at the extracellular surface of the sodium channel. The toxin dissociation constant, KD, and the rate constant for the toxin-induced closures, kc, varied as a function of [Na+]. The Na+ dependence was larger for STX than for TTX. Similarly, the addition of tetraethylammonium (TEA+) or Zn++ increased KD and decreased kc more for STX than for TTX. These differential effects are interpreted to arise from changes in the electrostatic potential near the toxin-binding site. The charges giving rise to this potential must reside on the channel since the bilayers had no net charge. The Na+ dependence of the ratios KDSTX/KDTTX and kcSTX/kcTTX was used to estimate an apparent charge density near the toxin-binding site of about -0.33 e X nm-2. Zn++ causes a voltage-dependent block of the single-channel current, as if Zn++ bound at a site within the permeation path, thereby blocking Na+ movement. There was no measurable interaction between Zn++ at its blocking site and STX or TTX at their binding site, which suggests that the toxin-binding site is separate from the channel entrance. The separation between the toxin-binding site and the Zn++ blocking site was estimated to be at least 1.5 nm. A model for toxin-induced channel closures is proposed, based on conformational changes in the channel subsequent to toxin binding.

Preferential block of skeletal muscle sodium channels by geographutoxin II, a new peptide toxin from Conus geographus

The effects of geographutoxin II (GTX II), a novel polypeptide toxin isolated from the marine snail Conus geographus, on nerves and muscles were studied by current clamp and voltage clamp techniques. GTX II (5 X 10(-7) M) abolished the action potential of the guinea pig skeletal muscle without change in the resting potential. However, action potentials of the crayfish giant axon, mouse neuroblastoma N1E-115 cell and guinea pig cardiac muscle were not affected by GTX II even at concentrations higher than 1 X 10(-6) M. In the voltage clamped bullfrog skeletal muscle fiber, sodium currents were almost completely blocked by GTX II (1 X 10(-6) M), and slowly recovered after washout. The time course of sodium currents was not appreciably altered by GTX II. These results suggest that GTX II selectively blocks skeletal muscle sodium channels in much the same way as tetrodotoxin.