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. 2018 Mar 20;22(12):3351-3361.
doi: 10.1016/j.celrep.2018.02.081.

Silk Fibroin Films Facilitate Single-Step Targeted Expression of Optogenetic Proteins

Free PMC article

Silk Fibroin Films Facilitate Single-Step Targeted Expression of Optogenetic Proteins

Skyler L Jackman et al. Cell Rep. .
Free PMC article


Optical methods of interrogating neural circuits have emerged as powerful tools for understanding how the brain drives behaviors. Optogenetic proteins are widely used to control neuronal activity, while genetically encoded fluorescent reporters are used to monitor activity. These proteins are often expressed by injecting viruses, which frequently leads to inconsistent experiments due to misalignment of expression and optical components. Here, we describe how silk fibroin films simplify optogenetic experiments by providing targeted delivery of viruses. Films composed of silk fibroin and virus are applied to the surface of implantable optical components. After surgery, silk releases the virus to transduce nearby cells and provide localized expression around optical fibers and endoscopes. Silk films can also be used to express genetically encoded sensors in large cortical regions by using cranial windows coated with a silk/virus mixture. The ease of use and improved performance provided by silk make this a promising approach for optogenetic studies.

Keywords: 2-photon calcium imaging; biomaterials; cranial windows; in vivo imaging; optical fiber implants; optogenetics; silk; stereotaxic injections; tapered optical fibers; viral vectors.

Conflict of interest statement


The authors declare no competing interests.


Figure 1
Figure 1. Silk Fibroin Is a Vehicle that Allows AAV Delivery that Is Restricted to the Tip of Optical Fibers
(A) In vivo optogenetic applications typically require two surgeries: first, a viral vector is injected into the target region to drive opsin expression, and second, an optical fiber is implanted to deliver light. (A1) Light will fail to drive activity in the target region if the viral injection and the implant are not aligned. (A2) If excess virus is delivered, then light may drive activity outside the target region. (B) A suspension of silk fibroin and viral vector (silk/AAV) was applied to fiber implants and dried to produce films that release AAV after implantation. (C) Large droplets deposited onto fibers from above often dried along the outside cladding. (D) Large droplets deposited from below did not coat the outside of the fiber but dried into large and irregular shapes that were prone to break off during implantation. (E) Sequential deposition of small volumes led to compact and mechanically stable dried films. (C–E) For visualization, silk was mixed with a red fluorescent dye. (F) Representative GFP expression in the striatum 14 days after implantation of a fiber coated with silk/AAV as in (E). (G) Example result from a fiber coated with AAV without silk. (H) Comparison of expression following implantation of fibers coated with silk/AAV or AAV alone. (I) Fibers showing silk/AAV labeled with red dye (left) and GFP expression patterns in the striatum following fiber implantation (right) are shown for indicated quantities of silk/AAV. (J) Area of expression versus virus coated on implant. Data are presented as mean ± SEM. See also Figure S1.
Figure 2
Figure 2. Silk/AAV-Coated Optical Fibers Reliably Drive Expression near the Fiber Tip and Produce Reliable Light-Evoked Behavior
(A) Fibers coated with AAV-ChR2-YFP and silk were targeted to the anterior hypothalamic nucleus (AHN) with a coronal slice showing representative ChR2-YRP expression in a mouse 4 weeks after implantation. (B) Raster plot of jumps elicited by light activation (blue, 20 Hz, 1 ms for 60 s) in 11 mice. (C) Average jumps per minute elicited by optogenetic stimulation. (D) Optogenetic stimulation elicited jumping in all mice, but much lower intensities were needed to elicit jumping for implants accurately targeted to the AHN. All data are presented as mean ± SEM. See also Movie S1.
Figure 3
Figure 3. Tapered Optical Fibers Coated with Silk/AAV Can Drive Expression along Fibers and Can Be Used to Produce Reliable Light-Evoked Behaviors
(A) Bright-field (left) and fluorescent image (middle) of a tapered fiber uniformly coated with silk. Green fluorescent dye was added to visualize the coating. Right: fluorescent image of coronal brain slice with GFP expression in the striatum 2 weeks after implantation of an AAV-GFP coated fiber. (B) Left: a fiber coated with silk/AAV containing green dye on the fiber tip and red dye up the shaft. Right: example implant site with a fiber used to express both RFP and GFP. (C) Left: anatomy of a brain slice and (right) ChR2-YFP expression in the motor cortex following implantation of a tapered fiber. (D) Stimulation of the motor cortex using a silk/AAV-ChR2-coated fiber resulted in robust turning behavior, as shown for four successive stimulations in the same mouse. (E) Optical stimulation (blue, 20 Hz 5 ms) with silk/AAV-coated tapered fibers in the right motor cortex reliably increased the speed of locomotion in mice. (F) Optical stimulation turned mice to the left. All data are presented as mean ± SD. See also Movie S2.
Figure 4
Figure 4. Expression of Calcium Indicators for In Vivo Mini-endoscope Imaging Is Facilitated by Coating Imaging Optical Fibers with AAV-Silk
(A) Implant schematic. 1 mm diameter endoscope lenses were coated with an AAV-GCaMP6 + silk mixture and implanted into the striatum. Raw image from an imaging session is shown in the inset. (B) Processed image from inset in (A) showing cells and ROIs (areas circled with different colors). (C) Example calcium transients from ROIs indicated in (B).
Figure 5
Figure 5. Widespread Cortical Expression of Fluorescent Proteins Can Be Achieved by Coating Cranial Windows with Silk and AAV
(A) The typical approach for in vivo two-photon imaging requires multiple virus injections. These time-consuming injections often result in non-uniform expression patterns. (B) A mixture of silk and an AAV was dried on the surface of the cranial window to release viral vector onto the brain after implantation. (C) Fluorescent images of GFP expression driven by a window that was coated with AAV-HI-EGFP/silk. AAV-HI-EGFP results in fluorescent labeling of somata. Left: whole brain image. Middle: in-vivo two-photon image. Right: GFP fluores-cence of a coronal section following brain removal and sectioning. (D) As in (C), except with a coating that contained AAV-HI-EGFP only.
Figure 6
Figure 6. Silk/AAV-Coated Cranial Windows Allow Widespread Imaging of Neuronal Activity Using GCaMP6f
(A) Silk+AAV-Syn-GCaMP6f was coated on an imaging window and implanted over the cortex. (B) GCaMP6f fluorescence is shown for acute slices cut following removal of brains. (C) Silk/AAV-coated cranial windows resulted in expression of GCaMP6f across multiple layers of cortex. Expression was highest in durectomized animals with imaging windows coated with silk and AAV. Data are presented as mean ± SD. (D) Left: fluorescence image through an imaging window for silk/AAV-coated windows implanted over a full durectomy. Middle: 2-photon image taken 85 μm below the cortical surface. Right: GCaMP fluorescence imaged in the ROIs highlighted in the same color. See also Figure S2 and Movie S3.

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