Seven Steps to Stellate Cells

C57BL/6 mice should be used at ~20 weeks of age or older. The use of male mice weighing 25-30g is recommended. The yield of stellate cells can be increased by feeding mice a vitamin A-enriched diet for 2 months prior to stellate cell isolation. Approximately 2×10⁵ hepatic stellate cells can be purified from the liver of one C57BL/6 mouse, whereas the yield of stellate cells from Balb/c mice is considerably higher. The following protocol is adjusted to 5 mice. Animal care and experimentation were performed in accordance with approved Institutional Animal Care and Use Committee protocols.

1. In situ perfusion of mouse livers with digestive enzymes

1. Warm the SC1 buffer and enzyme perfusion solutions in a 37°C water bath.
2. Mount a winged infusion set onto the silicone tube of a peristaltic pump.
3. Calibrate the pump with PBS to obtain a laminar flow of 6.5ml/min, which is the flow rate that will be used for the in situ perfusion of the liver. Equilibrate the silicone tube with SC1 solution.
4. Anesthetize the mouse by intraperitoneal injection of Ketamine (90 mg/kg) and Xylazine (10 mg/kg) solution and test for loss of reflexes to ensure deep narcotization.
5. Fix the mouse in supine position on a suitable base.
6. Use scissors and forceps to perform a longitudinal incision in the abdominal skin and expose the peritoneum.
7. Carefully open the peritoneum and move the intestines out of the abdominal cavity to the left side of the animal in order to expose the portal vein.
8. Insert the cannula of the infusion set into the portal vein. For this step, the use of a stereomicroscope is recommended.
9. Start the perfusion of the liver with 30ml SC1 solution. Open the vena cava inferior immediately after initiating the flow. Successful flushing of the liver is indicated by a loss of color of the liver tissue.
10. Perfuse the liver with 30ml Pronase E solution. Upon successful digestion, the liver lobes will swell, and the lobules will appear distinct through the capsule.
11. Perfuse the liver with 30ml Collagenase P solution. The liver at this point has lost its shape and looks atonic and amorphous.
12. Carefully separate the liver from the diaphragm and surrounding organs and store it in 70ml SC2 solution on ice.
13. Repeat the procedure for additional mice.

2. In vitro digestion of mouse livers

All further working steps should be carried out under sterile conditions in a laminar flow hood.

1. Cut the livers into pieces of approximately 2x2x2mm³ using sharp scissors.
2. Combine the suspension of 70ml SC2 including the liver pieces with 50ml of Pronase E-Collagenase P solution and add 1ml of DNase I solution.
3. Digest the livers for 20min at 37°C while stirring.

3. Density gradient centrifugation

1. Filter the cell suspension through 70μm cell strainers into 6 50ml Falcon tubes and add up to 50ml with SC2 buffer. Centrifuge for 10min at 600g and 4°C.
2. Carefully aspirate 40ml of the supernatant, then add 150μl DNase I solution to each tube and resuspend the cells.
3. Pool the cell suspensions into 4 50ml Falcon tubes and wash with GBSS-B, centrifuge for 10min at 600g and 4°C.
4. Carefully aspirate as much of the supernatant as possible without disturbing the pellet and add 150μl DNase I solution to each tube. Resuspend the cell pellets in 10ml GBSS-B per tube.
5. Pool the cells into 2 50ml Falcon tubes and add GBSS-B to a total volume of 36ml per tube. Add 14ml Nycodenz solution to each tube and mix well.
6. Transfer 10ml of the cell suspension into one 12ml gradient centrifugation tube, resulting in 10 tubes in total. Gently overlay the cell suspension with 1.5ml GBSS-B per tube.
7. Centrifuge the gradients for 15min at 1500g and 4°C without brake. Subsequently, hepatocytes will be pelleted at the bottom of the tube whereas stellate cells are found in the interphase as a white ring.
8. Carefully harvest the interphase containing the stellate cells and wash them with GBSS-B; centrifuge for 10min at 600g and 4°C.
9. Aspirate the supernatant and resuspend the cells in 20ml DMEM supplemented with 10% heat-inactivated FBS, 1% Penicillin/Streptomycin, 2mM L-Glutamine, 1mM sodium pyruvate, and 10mM HEPES. Transfer the cells into tissue culture flasks with a concentration of 2×10⁴ cells/cm². Incubate the cells at 37°C and 5% CO₂.
10. Change the media as soon as the hepatic stellate cells are adherent (approximately 2h after the preparation) to wash off dead cells and cell debris.
11. On the following day, the stellate cells should develop their characteristic star-shaped morphology with perinuclear vitamin A-storing lipid vesicles.
12. For subsequent experiments, hepatic stellate cells can easily be detached from non-coated plastic surfaces using mild enzymatic solutions such as Accutase, e.g.

4. Representative Results:

Following the preparation of hepatic stellate cells using the protocol provided, the purity of the isolated population can be tested considering three major characteristics of this cell type, such as star-like shape, perinuclear lipid droplets, and expression of glial fibrillary acidic protein (GFAP). Representative pictures for the characteristic appearance of hepatic stellate cells 2h after cell isolation, as well as on day 1 and 3 of in vitro culture are depicted in Figure 1. Figure 2 shows a representative immunofluorescence staining for GFAP in hepatic stellate cells, which have been cultured for 3 days following cell isolation. Hepatic stellate cells differentiate into myofibroblastic cells during in vitro culture. A characteristic hallmark of those activated stellate cells is the expression of alpha smooth muscle actin (αSMA). Figure 3 shows immunofluorescence staining for aSMA and myosin IIA in activated stellate cells on day 7 of in vitro culture.

Figure 1. Characteristic morphology of hepatic stellate cells. Stellate cells were isolated from mouse livers using the protocol provided. The images depict stellate cells 2h after cell isolation (a, b), as well as on day 1 (c) and day 3 (d) of in vitro culture. Hepatic stellate cells exhibit high amounts of lipid vesicles at perinuclear sites and acquire their distinctive astral-like morphology during the first days of in vitro culture (Magnification 200x).

Figure 2. Hepatic stellate cells specifically express GFAP in the liver. Stellate cells were isolated and immunofluorescently stained for GFAP (red) on day 3 of in vitro culture. Cell nuclei are depicted in blue (Hoechst stain).

Figure 3. Liver stellate cells differentiate into myofibroblasts. Hepatic stellate cells isolated from C57BL/6 mice were cultured for 7 days. Subsequently, they were transferred to chamber slides and stained for aSMA (shown in red) and myosin IIA (depicted in green). The cell nuclei were counterstained with Hoechst (blue).

Table 1. Buffers and enzyme solutions required for isolation of hepatic stellate cells. The pH of all buffers should be adjusted to 7.3-7.4. Furthermore, sterile filtration of all buffers is recommended. It is important to adapt the amount of enzyme used according to the given enzymatic activity.