Research Article Summary
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University of Florida and Kansas researchers 3D print a microfluidic chip to engineer gene-editing extracellular vesicles
Title
CRISPR-Cas9 Engineered Extracellular Vesicles for the Treatment of Dominant Progressive Hearing Loss
Authors
Xiaoshu Pan, Peixin Huang, Samantha S. Ali, et al.
Journal
bioRxiv (preprint), 2023
DOI Link
Summary
Pan et al. built a 3D printed microfluidic device that loads CRISPR gene-editing tools into extracellular vesicles (EVs), creating a fast, low-cost way to manufacture gene therapy carriers capable of restoring hearing in a mouse model of genetic hearing loss.
Summary Author
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Title
CRISPR-Cas9 Engineered Extracellular Vesicles for the Treatment of Dominant Progressive Hearing Loss
Authors
Xiaoshu Pan, Peixin Huang, Samantha S. Ali, et al.
Journal
bioRxiv (preprint), 2023
DOI Link
Key Results at a Glance
30 µm
Print Resolution
Achieved when 3D printing the device’s microfluidic mold
10x
Loading Efficiency
Improvement over standard bulk electroporation
1,000x
Processing Throughput
Increase in EV processing speed versus conventional methods
20 dB
Hearing Threshold Imporvement
Average recovery measured six months after treatment
Objective
Sensorineural hearing loss affects more than 450 million people worldwide, and up to half of all cases can be traced to a genetic cause. One well studied example is a mutation in the Myo7a gene, which powers the sensory hair cells inside the inner ear. Left uncorrected, this mutation drives a progressive, dominant form of hearing loss that gradually worsens over a person’s lifetime.
Gene editing tools like CRISPR-Cas9 can directly correct the mutated allele, but safely delivering the editing machinery into inner ear cells has been a major hurdle. Viral vectors such as AAVs are the current standard for gene delivery, but they can only carry small genetic payloads, well under the size needed for a full gRNA:Cas9 system, and they carry a risk of prolonged, unwanted activity once inside cells. Lipid nanoparticles (LNPs) are an alternative, but their safety profile inside the delicate inner ear is still not well established.
Extracellular vesicles (EVs), naturally occurring nanoscale particles released by the body’s own cells, have emerged as a promising, biocompatible carrier for gene editing cargo. What has held EVs back from clinical use is loading efficiency: existing methods for packing large CRISPR components into EVs are slow, inconsistent, and difficult to scale up. This study set out to close that gap by engineering a device capable of loading CRISPR ribonucleoprotein (RNP) complexes into EVs at high efficiency and high throughput, then testing whether the resulting engineered EVs could reverse hearing loss in a living animal model.
Methodology and Design
To build their delivery platform, the researchers needed a microfluidic device that could generate thousands of uniform, droplet-sized reaction chambers and apply a gentle electric pulse to each one. Rather than relying on a cleanroom microfabrication facility, the team designed the channel pattern in CAD software, 3D printed a resin mold of that pattern, and used the mold to cast the final device in PDMS (polydimethylsiloxane), a flexible, optically clear silicone widely used for lab-on-a-chip devices. The result was a droplet generator and electroporation chamber that could be produced quickly and cheaply, without specialized cleanroom equipment.
Figure 1.
This single printed device carried the therapy pipeline through three connected stages:
μDES Chip
RNP-EVs
Shaker-1 Mice
Inside the μDES chip, a stream of extracellular vesicles mixed with pre-assembled gRNA:Cas9 ribonucleoprotein (RNP) complexes was broken into a continuous flow of uniform water-in-oil droplets, each acting as its own tiny electroporation chamber. A low-voltage electric field, as little as 10 to 30V compared to the 1,000+ V typically needed for bulk cuvette electroporation, opened brief pores in the EV membrane as each droplet passed through, letting the RNP cargo diffuse inside without overheating or damaging the vesicle. The chip generated droplets at roughly 700 per minute, translating to about 30 mL of processed EVs per hour, a scale far beyond what conventional bench electroporation can achieve.
The resulting RNP-loaded EVs (RNP-EVs) were then tested as a delivery vehicle for allele-specific gene editing. Using two guide RNAs designed to target the mutant Myo7a allele responsible for Shaker-1 hearing loss, the team confirmed in fibroblast cells that RNP-EVs could selectively cut the mutant sequence while leaving the healthy allele largely intact.
Finally, the RNP-EVs were injected into the inner ear of Shaker-1 mice, a model for dominant progressive hearing loss, through the posterior semicircular canal. This route reached both outer and inner hair cells, the cells responsible for sound sensing, and treated animals were monitored for six months to assess gene editing, hearing recovery, and safety.
On the fabrication side, the resin mold was printed at 30 µm resolution with a 50 µm layer thickness and a 40% power setting, then washed in ethanol to remove uncured resin and UV cured before PDMS casting. After demolding, the PDMS layer was surface-activated and bonded to a glass slide to seal the channels, and L-shaped platinum-iridium electrodes were inserted at the electroporation sites before a final low-temperature bake locked everything in place.
Results
Printed at 30 µm resolution, the μDES platform delivered a 10-fold increase in loading efficiency and more than a 1,000-fold increase in processing throughput compared to conventional bulk electroporation, while using only 10 to 30V instead of the 1,000+ V standard methods require. Encapsulation stayed consistent across eight independent replicates, and the resulting RNP-EVs retained the same size, surface protein markers, and overall structure as untreated, native EVs, showing that the loading process did not compromise vesicle integrity.
In fibroblast cells carrying the Shaker-1 mutation, RNP-EVs achieved efficient, allele-specific editing of the mutant Myo7a sequence, with the lead guide RNAs cutting the mutant allele in roughly 82% of edited sequences while leaving the wild-type allele largely untouched.
In living Shaker-1 mice, a single injection of RNP-EVs into the inner ear reached both outer and inner hair cells within two hours, a pattern that lipid nanoparticle-delivered RNPs did not replicate. Six months after treatment, mice receiving RNP-EVs showed roughly a 20 dB improvement in hearing threshold compared to their untreated ear, approaching the hearing ability of genetically normal mice. Markers of oxidative stress in the inner ear, a hallmark of progressive hearing damage, dropped to near-undetectable levels in treated animals, and no safety concerns were observed over the full six-month monitoring period.
“We developed a novel Microfluidic Droplet-based EV Electroporation System (µDES).”
— Pan et al. CRISPR-Cas9 Engineered Extracellular Vesicles for the Treatment of Dominant Progressive Hearing Loss. bioRxiv. 2023
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