Piezo1/2 channels

Piezo1/2 Mechanosensitive Channels

Overview

Piezo channels (Piezo1 and Piezo2) are large-conductance mechanosensitive ion channels that represent the most recently characterized and mechanistically well-defined class of MS channels relevant to ultrasound neuromodulation. Unlike K2P channels — which are background leak channels with graded tension sensitivity — Piezo channels are rapidly activating, inactivating, non-selective cation channels that generate transient inward currents upon membrane mechanical stimulation. Their discovery (Coste et al. 2010, Nobel Prize 2021 to Patapoutian) established a molecular identity for the long-hypothesized fast mechanotransducer in touch and proprioception, and their subsequent identification in CNS neurons opened a new line of investigation into ultrasound neuromodulation mechanisms.

1. Molecular Structure and Biophysical Properties

Piezo channels are homotrimeric complexes of three identical subunits, each contributing to form a propeller-shaped structure with a central ion-conducting pore. Each subunit contains 38 transmembrane helices, making Piezo1 (₂₅₀₀ amino acids) one of the largest known ion channel proteins. The structural organization is directly relevant to mechanosensitivity:

• Peripheral blade domains: each subunit forms a curved blade that interacts with the surrounding lipid bilayer over a large membrane footprint (₇₀₀ nm²). Membrane tension flattens the blades, transmitting force to the central pore via an intrinsic lever mechanism
• Central pore: non-selective cation channel permeable to Na⁺, K⁺, and Ca²⁺, with a slight Ca²⁺ preference (PCa/PNa ~10 for Piezo1)
• Beam and anchor domains: connect peripheral blades to the central pore, acting as mechanical transmission elements

The large membrane footprint means Piezo channels sense membrane tension rather than local curvature or pressure directly — they are activated when the surrounding bilayer is laterally stretched, regardless of the source of that tension (osmotic stress, mechanical indentation, acoustic radiation pressure, or cavitation-driven bilayer area oscillation).

Key biophysical parameters:

2. Expression Pattern in CNS and Relevance to Neuromodulation

Piezo1 is expressed broadly across the CNS, including cortical neurons, hippocampal neurons, astrocytes, and microglia. Its expression in non-sensory CNS neurons was initially surprising — its function in these contexts is still being characterized. In neurons, Piezo1 activation produces:

• Transient inward Na⁺/Ca²⁺ current → membrane depolarization
• Ca²⁺ influx triggering downstream signaling cascades (CAMKII, synaptic plasticity)
• At sufficient amplitude, AP generation

Piezo2 expression is more restricted in the CNS, with highest levels in dorsal root ganglia (DRG) sensory neurons, Merkel cells, and proprioceptors. Limited but non-zero expression has been reported in some cortical and thalamic neurons. Piezo2 inactivates faster than Piezo1 and has been primarily studied in the context of touch and proprioception rather than ultrasound neuromodulation.

For LIFU neuromodulation of cortical and thalamic targets (the focus of this project), Piezo1 is the primary relevant isoform.

3. Methods of Study

Research on Piezo channels in the ultrasound context has employed a range of complementary experimental approaches:

Patch-clamp electrophysiology

The gold standard for characterizing Piezo channel gating. Cell-attached and excised patch configurations allow direct measurement of single-channel and whole-cell Piezo currents under controlled mechanical stimulation (membrane indentation via piezoelectric probe, cell inflation via pressure pipette, or membrane stretch via suction). Key advantage: direct measurement of gating kinetics and ion selectivity. Limitation: difficult to apply ultrasound directly in patch-clamp configuration without mechanical artifacts.

GsMTx4 pharmacological blockade

GsMTx4 is a spider venom peptide (from Grammostola spatulata) that selectively inhibits mechanosensitive channels gated by membrane tension, including Piezo1, Piezo2, and some TREK channels. It inserts into the outer leaflet of the bilayer and modifies the local membrane tension environment around the channel, shifting the activation threshold. In ultrasound studies, GsMTx4 application and washout allows attribution of ultrasound-evoked currents to tension-gated MS channels. Zhu et al. (2023) used GsMTx4 to demonstrate that a significant fraction of LIFU-evoked Ca²⁺ transients in cortical neurons was Piezo1-dependent.

Calcium imaging (GCaMP / Fluo-4)

Piezo1's Ca²⁺ permeability makes it detectable via fluorescent Ca²⁺ indicators. GCaMP (genetically encoded) and Fluo-4 (synthetic dye) have been used to image Piezo1-mediated Ca²⁺ transients in neurons during LIFU application. Zhu et al. (2023) combined GCaMP imaging with Piezo1 knockout (KO) and GsMTx4 to establish Piezo1's contribution to ultrasound-evoked Ca²⁺ signaling in vivo in mouse cortex.

Genetic approaches (knockout, knockdown, overexpression)

Piezo1/2 KO mice, siRNA knockdown, and Cre-dependent conditional KO have been used to isolate Piezo-specific contributions. Comparison of ultrasound-evoked responses in WT vs. Piezo1 KO animals allows clean attribution of effects. Limitation: compensatory upregulation of other MS channels in KO animals may confound interpretation.

Cryo-EM structural studies

High-resolution cryo-EM structures of Piezo1 (Saotome et al. 2018, Guo & MacKinnon 2017) established the propeller architecture and provided the structural basis for the lever-mediated tension-gating mechanism. These structures enable molecular dynamics simulations of Piezo1 gating under membrane tension.

AFM and optical tweezers

Atomic force microscopy and optical tweezers have been used to apply controlled mechanical forces to membranes expressing Piezo channels, characterizing the force-displacement relationships and activation thresholds at the single-molecule level.

4. Key Studies and Results

Zhu et al. (2023) — In vivo Piezo1-dependent Ca²⁺ transients under LIFU

This is the most directly relevant study for ultrasound neuromodulation. Zhu et al. applied LIFU (500 kHz, 100–400 kPa, various DC) to mouse somatosensory cortex in vivo and measured neuronal Ca²⁺ transients using GCaMP6s. Key findings:

• LIFU evoked robust Ca²⁺ transients in cortical neurons that were significantly reduced by GsMTx4 and absent in Piezo1 conditional KO neurons — establishing Piezo1 as a necessary mediator of LIFU-evoked Ca²⁺ signaling
• The Ca²⁺ transients were not abolished by TTX (Na⁺ channel blocker), indicating that Piezo1-mediated Ca²⁺ entry is partially independent of action potential generation — suggesting direct Ca²⁺ influx through Piezo1 rather than secondary Ca²⁺ entry through voltage-gated Ca²⁺ channels
• Intensity dependence: Ca²⁺ transient amplitude increased monotonically with pressure amplitude, with a threshold around 100 kPa — consistent with Boltzmann tension-gating predictions
• Frequency dependence: responses were larger at 500 kHz than at 1 MHz, suggesting frequency-dependent membrane tension generation rather than a simple pressure-amplitude effect
• Piezo1-dependent Ca²⁺ transients were observed at intensities below the NICE/SONIC cavitation threshold, suggesting that Piezo1 activation may be operative in a parameter regime where intramembrane cavitation is negligible

Sorum et al. (2024) — Quantitative tension-gating characterization

Sorum et al. provided a rigorous quantitative characterization of Piezo1 tension-gating using excised membrane patches with controlled lateral tension. This study is essential for parameterizing the Boltzmann gating model:

\[P_{open}(T) = \frac{1}{1 + \exp\left(-\frac{\Delta A (T - T_{1/2})}{k_B \Theta}\right)}\]

Where:

• T = membrane tension (mN/m)
• \(T_{1/2}\) = half-activation tension (₁.5 mN/m for Piezo1)
• \(\Delta A\) = gating area (₂₀ nm²)
• \(k_B \Theta\) = thermal energy at physiological temperature

Key findings: Piezo1 activates at tensions well within the range generated by SONIC-predicted bilayer area oscillations at therapeutic LIFU pressures (0.1–1 MPa), supporting the physical plausibility of Piezo1 activation under LIFU. The rapid inactivation (₂₀ ms time constant) means that sustained sonication produces intermittent rather than sustained Piezo1-mediated currents — a consideration for protocol design.

Prieto et al. (2020) — Piezo channel contribution in peripheral ultrasound stimulation

Prieto et al. investigated MS channel contributions to ultrasound-evoked responses in peripheral neurons (DRG) using a combination of pharmacological blockade and computational modeling. While focused on peripheral rather than CNS neurons, this study established:

• Both Piezo1 and Piezo2 contribute to ultrasound-evoked depolarization in sensory neurons
• Piezo2's faster inactivation makes it more sensitive to high-frequency (PRF > 500 Hz) pulsed protocols
• Combined Piezo1 + Piezo2 blockade reduced ultrasound-evoked firing by ~60–70%, with the remaining ~30–40% attributable to other MS channels (TREK-1, TRAAK) or cavitation-driven capacitive currents
• The relative contribution of Piezo vs. K2P channels is intensity-dependent: at low intensities, Piezo channels dominate excitation; at higher intensities, K2P-mediated effects become more prominent

5. Boltzmann Tension-Gating Model

For integration into the unified HH framework, Piezo1 gating is described by the Boltzmann tension-gating model with rapid inactivation:

Activation (tension-dependent, rapid):

\[m_{\infty}(T) = \frac{1}{1 + \exp\left(-\frac{\Delta A_m (T - T_{1/2,m})}{k_B \Theta}\right)}\]

Inactivation (voltage- and time-dependent):

\[\frac{dh}{dt} = \frac{h_{\infty}(V_m) - h}{\tau_h}\]

\[h_{\infty}(V_m) = \frac{1}{1 + \exp\left(\frac{V_m - V_{1/2,h}}{k_h}\right)}\]

Piezo1 current:

\[I_{Piezo1} = \bar{g}_{Piezo1} \cdot m_{\infty}(T) \cdot h \cdot (V_m - E_{Piezo1})\]

Where $EPiezo1} \approx 0$ mV (non-selective cation reversal potential near 0 mV at physiological ion concentrations) and $\bar{g{Piezo1}$ is the maximal Piezo1 conductance density (cell-type specific).

The membrane tension $T$ is linked to the SONIC framework output via:

\[T = K_A \cdot \frac{\Delta A_m}{A_m}\]

Where $\Delta A_m / A_m$ is the relative membrane area change from SONIC bubble dynamics and $K_A \approx 0.24$ N/m is the bilayer area expansion modulus.

6. Parameter Regimes for Piezo1 Relevance

Piezo1 activation under LIFU is most relevant under the following conditions:

7. Interaction with Other Mechanisms

Piezo1 → Ca²⁺ → K2P activation: Piezo1-mediated Ca²⁺ influx elevates intracellular Ca²⁺, which can secondarily activate Ca²⁺-sensitive K⁺ channels (SK, BK) and shift K2P channel gating — creating a delayed inhibitory response following initial Piezo1-mediated excitation. This sequential excitation-then-inhibition dynamic may contribute to the observed transient nature of LIFU-evoked responses.

Piezo1 → Ca²⁺ → flexoelectric sign reversal: As established in Badawe & Khraiche (2023), Piezo1-mediated Ca²⁺ influx elevating $[\text{Ca}^{2+}]_i$ can shift the flexoelectric coefficient toward inhibitory values — creating a cross-mechanism interaction between the MS channel and flexoelectric frameworks.

Piezo1 vs. NICE/SONIC: at sub-cavitation intensities, Piezo1 may be the dominant excitation mechanism; at supra-cavitation intensities, the NICE capacitive current dominates and Piezo1 inactivation limits its contribution. The two mechanisms operate in partially complementary parameter regimes.

8. Strengths and Limitations

Strengths

• Direct experimental confirmation in CNS neurons (Zhu et al. 2023) — strongest empirical support among all proposed FUS mechanisms
• Molecularly identified: specific protein, known structure, characterized pharmacology (GsMTx4 blockade)
• Quantitative gating model available: Sorum et al. (2024) Boltzmann parameters enable direct integration into HH framework
• Accounts for sub-cavitation excitation: operative at intensities below NICE/SONIC threshold, extending the mechanistic account to lower acoustic amplitudes
• Natural Ca²⁺ signaling link: Piezo1 Ca²⁺ permeability connects acoustic stimulation directly to downstream signaling cascades relevant for plasticity and network modulation

Limitations

• Rapid inactivation limits sustained contribution: Piezo1 inactivates on a ~20 ms timescale, meaning sustained sonication produces diminishing Piezo1-mediated currents — single-pulse protocols are more effective for Piezo1 engagement
• CNS expression levels poorly quantified: absolute Piezo1 conductance density \(\bar{g}_{Piezo1}\) in cortical and thalamic neuron subtypes is not precisely known
• Tension generation mechanism under debate: whether LIFU generates sufficient membrane tension via radiation pressure alone (without cavitation) remains unresolved; estimates range from sub-threshold to supra-threshold depending on assumptions about acoustic coupling to the bilayer
• Inactivation mechanism incompletely understood: the molecular basis of Piezo1 inactivation is not fully characterized, making quantitative modeling of recovery kinetics uncertain
• Piezo2 contribution to CNS neuromodulation unclear: Piezo2 expression in cortical/thalamic neurons is low; its role in CNS LIFU responses is not established
• No cell-type-specific expression data for modeling: differential Piezo1 expression across RS, FS, LTS, TC, TRN, STN is not quantified at the conductance level