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Genetically Encoded Voltage Indicator (GEVI)-ASAP5
ASAP5 is a cutting-edge Genetically Encoded Voltage Indicator (GEVI) used in neuroscience to monitor the electrical activity of neurons in real time.
Developed by Michael Lin’s lab at Stanford University (released via Neuron in late 2024), it represents a major iteration in the ASAP (Accelerated Sensor of Action Potentials) family of molecular biosensors.
How It Works
Structurally, ASAP5 is a fusion protein engineered by combining a voltage-sensing domain (VSD) from a potassium channel with a circularly permuted green fluorescent protein (cpGFP).
- The Mechanism: When a neuron fires or experiences subthreshold voltage fluctuations, the change in membrane potential alters the physical conformation of the voltage-sensing domain. This directly changes the chromophore environment of the attached GFP, causing a rapid shift in fluorescence intensity.
- Negative Tuning: Like earlier iterations (ASAP1, ASAP3), ASAP5 is a negatively tuned sensor. This means it is highly fluorescent at the resting membrane potential (around $-70\text{ mV}$) and dims rapidly upon depolarization (when the cell fires an action potential).
Why It Matters: Key Advancements
While calcium indicators (like GCaMPs) are widely used to track neural activity, they are limited by slow kinetics and cannot detect subthreshold voltage shifts or high-frequency firing rates. GEVIs solve this, and ASAP5 introduces several critical optimizations:
- Unitary Synaptic Event Sensitivity: ASAP5 was specifically engineered via targeted mutagenesis of its predecessor (ASAP3) to achieve significantly enhanced sensitivity. It is sharp enough to detect subtle, single synaptic inputs (unitary events) and subthreshold changes that previous indicators missed.
- Fast Kinetics: It tracks fast, single-trial action potentials with exceptional temporal resolution, allowing researchers to capture the exact timing of spikes.
- Dendritic Imaging: Because of its high baseline brightness and sensitivity, it is highly effective for localized imaging in thin neuronal structures like dendrites, where electrical signals are integrated.
Delivery and Experimental Use
In laboratory settings, the ASAP5 gene sequence is typically cloned into mammalian expression plasmids downstream of a cell-type-specific promoter—such as the pan-neuronal human synapsin (hSyn) promoter or elongation factor 1-alpha (EF1a) for Cre-dependent (Dio) lines.
These constructs are then packaged into viral vectors (most commonly rAAVs) to transduce specific neural populations in vitro or in vivo for optical recording under 1-photon or 2-photon microscopy.
The diagram is organized into four key sections:
- 1. ASAP5 Structural Blueprint: This shows how the sensor is engineered. It fuses a customized voltage-sensing domain (VSD) from a potassium channel (which senses membrane potential changes) to a circularly permuted GFP (cpGFP). A flexible linker connects them, ensuring that movement in the VSD directly influences the GFP’s light output.
- 2. Functional Cycle (Resting vs. Active States): This illustrates the negative tuning mechanism. At a resting membrane potential (-70 mV), the VSD sensor (specifically the S4 helix) is in a ‘down’ position, which stabilizes the cpGFP in a bright state (High Fluorescence). When the neuron depolarizes (during an action potential, +30 mV), the VSD moves to an ‘up’ position, causing a conformational shift that quenches the cpGFP (Low Fluorescence/Dim).
- 3. Ultra-Fast Sensitivity: This panel uses temporal traces to compare ASAP5 to its predecessor (ASAP3). You can see that ASAP5 offers a much larger and sharper change in fluorescence signal (ΔF/F) for the same single action potential, demonstrating its improved sensitivity.
- 4. Comparative Advantage: This final section contrasts ASAP5 with standard calcium indicators (like GCaMP7s). While calcium indicators provide slow, broad signals, ASAP5 is fast enough to track distinct, milliseconds-long electrical spikes. It is also uniquely sensitive enough to resolve tiny, subthreshold fluctuations, such as a unitary synaptic event that GCaMP would miss entirely.
Refer to our pre-made AAV, AAV9-Syn-ASAP5-WPRE, which is currently in stock.
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