As summarized in Table , Nav channels are dangerously leaky in a multitude of “sick-cell excitable cell” conditions. Mechanical trauma and/or ischemia and/or inflammation, and/or various genetic diseases result in damaged excitable cell membranes. More specifically, at Nav-rich membranes, bilayer structure degradation renders the Nav channels leaky. A CNS trauma example is depicted in Figure A: stretch trauma has caused axolemmal blebbing at the node of Ranvier. The fully blebbed bilayer, though detached from the specialized nodal spectrin skeleton (Figure C), would still be well-endowed with Nav1.6 channels. Even in such egregiously bleb-damaged plasma membrane, Nav channels are capable of voltage-dependent gating (Milton and Caldwell, 1990
). In fact they gate “too well” (Figure B) in that they activate at inappropriately hyperpolarized (left-shifted) voltages (Shcherbatko et al., 1999
; Tabarean et al., 1999
; Wang et al., 2009
; Beyder et al., 2010
). A Nav channel whose operation becomes left-shifted in this manner is, in effect, leaky.
Sick excitable cells and their leaky, hyperactive, lethal Nav channels.
Figure 2 (A) From a stretch-traumatized optic nerve, the nodal and paranodal regions of a node of Ranvier (cartooned by tracing an electron micrograph, Figure A of Maxwell, 1996) showing intact (arrowheads) and severely blebbed yet unruptured (more ...)
By virtue of molecular coupling between the fast-mode activation and inactivation (availability) processes in a Nav channel (Banderali et al., 2010
), these two processes left shift in synchrony: if the activation Boltzmann [“m3∞
)” in Hodgkin–Huxley parlance] left shifts by, say, 7 (or 11 or 20…)
mV, then availability [“h∞
)”] left shifts by 7 (or 11 or 20…)
mV as well. This behavior of fast-mode Nav channels, shown for recombinant Nav1.6 channels in Figures A,B we term “coupled-left shift” (CLS). Crucially, as shown in Figure C, Nav-CLS yields a left-shifted steady-state window conductance [“m3h
)”]. Here, m3h
) is shown before (black) and after a 20-mV CLS (gray). Note that the Nav-CLS version of m3h
) now peaks at what would normally be a subthreshold voltage range (i.e., below the normal Vrest
). Even if only a fraction (say 10%) of the nodal membrane suffered damage, that fraction, by virtue of its left-shifted m3h
), should generate a “subthreshold persistent current” that would depolarize the adjacent (intact) Nav-rich membrane, causing the axon to fire ectopically. Because it facilitates rhythmic firing, a subthreshold persistent INa
is used by pacemaker neurons (Taddese and Bean, 2002
) and by the first node (beyond the axon initial segment) to elicit rhythmic firing (Kole, 2011
), but injury-induced Nav-CLS based subthreshold persistent INa
triggers pathological ectopic activity. We have modeled the consequences of Nav-CLS for axonal ion homeostasis and excitability (Boucher et al., 2012
). In a nutshell, Nav-CLS causes [Na+
] gradients to run down, it overtaxes the Na/K-ATPase and, depending on CLS pervasiveness and severity, produces subthreshold voltage oscillations, bursting, and diverse other manifestations of hyper -and hypo-excitability.
After finding that reversible bilayer stretch elicits reversible Nav-CLS in recombinant Nav1.5 (Morris and Juranka, 2007
), we turned to recombinant Nav1.6 (Wang et al., 2009
). There, irreversible bilayer damage due to “membrane stretch” (cell-attached patch clamp, with stretch- and bleb-inducing pipette aspiration) yields irreversible CLS (Figures B,C) whether or not Nav1.6 channels are co-expressed with β-subunits. Once the irreversible damage has “saturated” (typically producing >20
mV of irreversible CLS), further stretch of the bilayer elicits reversible Nav-CLS because now, pipette aspiration is indeed causing reversible thinning/disordering (i.e., stretching) of the bilayer. For Nav1.6, as for Nav1.5 (Morris and Juranka, 2007
; Banderali et al., 2010
) for comfortably non-lytic stretch stimuli, reversible Nav-CLS (Figure in Wang et al., 2009
) is small (typically <5
mV). When studied in fast-mode, Nav1.4 and Nav1.2 channels (co-expressed with β-subunits) behave the same way as fast-mode Nav1.6 (Catherine E. Morris and P. F. Juranka, unpublished observations). It is now known, however, that expressed in HEK cells Nav1.5 channels exhibit large substantially irreversible Nav-CLS (Beyder et al., 2010
). An interesting possible explanation for the isoform differences in oocytes is an idea that would be tested by immuno-biochemistry, namely that in oocytes, Nav1.5 traffics to bilayer subdomains with a high degree of disorderliness whereas Nav1.2, Nav1.4, and Nav1.6 traffic to highly ordered cholesterol-rich domains. It would, in fact, be worth exploiting this stark difference between Nav1.5 and the other Nav isoforms as a way to correlate channel-lipid interactions and kinetic behavior.
Figure 5 Modeling how to dissect fast and slow INa using sawtooth hysteresis. (A) For gni/gNa-fast=0.0075 (also in B), total INa(V,t) during negative (black) and positive-going (gray) voltage ramps of various speeds, normalized to peak INa for (more ...)
What appears to be irreversible Nav-CLS arising from metabolic (not mechanical) injury has been reported for the Nav channels of hippocampal neuronal somata subjected to prolonged epileptic discharge: the somatic Nav channels (presumably Nav1.2) show a measurably left-shifted Iwindow
associated with left-shifted (fast-mode) activation and availability (Sun et al., 2006
). Presumably these overworked neurons are ATP-depleted and their plasma membranes are beset by the usual chemical insults of ischemia. Skeletal muscles subject to inflammatory conditions appear to have a comparable type of Nav-leak (Haeseler et al., 2008
; Nikic et al., 2011
). A recurring theme here is that it will be necessary to revisit native-Nav channels linked to disease states in Table to establish whether CLS correlates broadly with increasing Nav-leak.