The yin and yang of KV channels in cerebral small vessel pathologies

Abstract

Cerebral SVDs encompass a group of genetic and sporadic pathological processes leading to brain lesions, cognitive decline, and stroke. There is no specific treatment for SVDs, which progress silently for years before becoming clinically symptomatic. Here, we examine parallels in the functional defects of PAs in CADASIL, a monogenic form of SVD, and in response to SAH, a common type of hemorrhagic stroke that also targets the brain microvasculature. Both animal models exhibit dysregulation of the voltage-gated potassium channel, K_V 1, in arteriolar myocytes, an impairment that compromises responses to vasoactive stimuli and impacts CBF autoregulation and local dilatory responses to neuronal activity (NVC). However, the extent to which this channelopathy-like defect ultimately contributes to these pathologies is unknown. Combining experimental data with computational modeling, we describe the role of K_V 1 channels in the regulation of myocyte membrane potential at rest and during the modest increase in extracellular potassium associated with NVC. We conclude that PA resting membrane potential and myogenic tone depend strongly on K_V 1.2/1.5 channel density, and that reciprocal changes in K_V channel density in CADASIL and SAH produce opposite effects on extracellular potassium-mediated dilation during NVC.

Keywords: CADASIL; cerebral blood flow; cerebral small vessel disease; subarachnoid hemorrhage; voltage-gated potassium channel.

Figure 1. K_V channels exert a tonic dilatory influence on the diameter of intracerebral arterioles

(A) Families of K_V currents from an isolated arteriolar smooth muscle cell elicited by voltage pulses from −70 mV to +50 mV in the presence of 100 nM iberiotoxin to inhibit large conductance (BK) currents. (B) steady-state activation (circles) and inactivation (triangles) properties of K_V currents measured from isolated arteriolar smooth muscle cells. Solid lines, Boltzmann fits to the data. (C) Typical recording of the internal diameter of a pressurized parenchymal arteriole (40 mm Hg) showing the constriction caused by the perfusion of the K_V blocker 4-AP, 1 and 5 mM. A and B are from (58) and C is from (17).

Figure 2. Steady state K_V current density in PA SMCs from normal and diseased animal models

Steady state K_V current densities for CTL, CADASIL (TgNotch3^R169C), and SAH models are fitted using a linear equation $I_{\mathit{\text{Kv}}}=\frac{1}{c_m}{\mathit{\text{mG}}}_{\mathit{\text{Kv}}}(V_m-E_K)$ with a Boltzmann-type activation term $m=\frac{1}{1+\text{exp}(-(\frac{V_m-V_{\mathit{\text{Kv}},0.5}}{k_{\mathit{\text{kv}}}}))},$, from experimental data (17) and Koide & Wellman unpublished data. C_m is the membrane capacitance; V_m is the membrane potential; G_Kv is the whole-cell conductance of K_V channels; E_K is the reversal potential for K⁺. At physiological membrane potentials pA differences in K_V currents are predicted (Figure inset). Model parameters: G_kv = 1.6 [nS]; V_Kv,0.5 = 6 [mV]; k_kv = 14 [mV] for control; G_kv = 0.8 [nS]; V_Kv,0.5 = 6 [mV]; k_kv = 14 [mV] for SAH; and G_kv = 3.2 [nS]; V_Kv,0.5 = 2.6 [mV]; k_kv = 15.8 [mV] for CADASIL model; C_m = 12.8 [pF]; [K⁺]_i = 150 [mM]; [K⁺]_o = 3 [mM].

Figure 3. Relationship between myogenic tone and membrane potential

(A) Values of membrane potential and myogenic tone at different intravascular pressure (mm Hg) from CADASIL (TgNotch3^R169C, blue triangles) and SAH (red circles) animals are consistent with the linear regression obtained from CTL animals (black triangles represent CTL mice from (17) and black circles represent CTL rats from (6)) showing a similar relationship between tone and membrane potential. (B) A detailed model of SMC membrane potential and Ca²⁺ dynamics was adapted from (102) and modified by incorporating the K_V1 current of PA SMCs from CTL animals (Figure 2), while adjusting other transmembrane currents (K_IR, NSC, VDCC, NaK, PMCA) to produce resting V_m and Ca²⁺ concentration in agreement with experimental data (17,103). The effect of altered K_V1 channel density in CADASIL (blue triangles) and SAH (red triangles) was examined assuming all other model parameters remain the same as in CTL (black circles). (C) The effect of increasing pressure was simulated by depolarizing SMC membrane through increasing Na⁺ permeability (P_Na). Model simulations, in agreement with the corresponding experiments in (A), show differences between CADASIL and SAH animals in V_m (bottom) and Ca²⁺ (top) as pressure increases and highlight the inhibitory role of K_V channels and the effect K_V channel density on myogenic tone. Parameters as in reference (102) except: P_VDCC=6.3×10⁻⁵ cm/s; P_NaNSC=1.23×10⁻⁶ cm/s; I_PMCA=8.58 pA; I_NaK=7.76 pA/pF; G_KIR = 0.5 nS/(mM)^0.5; G_Na,leak=0.12 nS; C_m = 12.8 pF.

Figure 4. Effect of PA SMC K_V current density and the interplay with K_IR current on V_m dynamics at rest and during potassium challenge

(A) Combined contribution of K_IR and K_v currents in healthy and diseased models during rest and [K⁺]_o stimulus. Activation of K_IR current by [K⁺]_o and hyperpolarization is accounted: $I_{\mathit{\text{Kir}}}=\frac{G_{\mathit{\text{Kir}},\mathit{\text{max}}}(V_m-E_K)}{1+\text{exp}(\frac{(V_m-V_{K_{\mathit{\text{IR}}},0.5})}{k_{\mathit{\text{Kir}}}})};G_{\mathit{\text{Kir}},\mathit{\text{max}}}=G_{\mathit{\text{Kir}}}{[K^{+}]}_o^{0.5}$ where G_kir,max is the maximal K_IR conductance. Solid lines show the sum of the two currents at rest, and dashed lines are during elevation of [K⁺]_o from 3 to 8 mM. The shaded regions show the range of voltages within which K_IR current increases more than K_V current decreases during the K⁺ stimulus, i.e. the resting V_m window where the K⁺ challenge will result in hyperpolarization. As K_V current density increases (from SAH, red lines; to CTL black lines; to CADASIL, blue lines) the window shrinks in size and shifts to more hyperpolarized potentials. (B) Representative simulation using the model of PA SMCs from Figure 3. SMCs from CTL (black line), CADASIL (blue line) and SAH (red line) conditions hyperpolarize following an increase in extracellular K⁺, [K⁺]_o, from 3 mM to 8 mM. The change in membrane potential is less for CADASIL as a result of the more hyperpolarized resting V_m prior to the K⁺ challenge. G_KIR = 0.76 [nS/(mM)^0.5]; k_KIR = 7 [mV]; V_KIR,0.5= E_K+12 [mV].