CdSe/ZnS Quantum Dots-Labeled Mesenchymal Stem Cells for Targeted Fluorescence Imaging of Pancreas Tissues and Therapy of Type 1 Diabetic Rats
- Haoqi Liu†1,
- Wei Tang†1,
- Chao Li2,
- Pinlei Lv3,
- Zheng Wang3,
- Yanlei Liu2,
- Cunlei Zhang2,
- Yi Bao1,
- Haiyan Chen1,
- Xiangying Meng4,
- Yan Song1,
- Xiaoling Xia1,
- Fei Pan2,
- Daxiang Cui2Email author and
- Yongquan Shi1Email author
© Liu et al. 2015
Received: 7 February 2015
Accepted: 28 May 2015
Published: 13 June 2015
Mesenchymal stem cells (MSCs) have been used for therapy of type 1 diabetes mellitus. However, the in vivo distribution and therapeutic effects of transplanted MSCs are not clarified well. Herein, we reported that CdSe/ZnS quantum dots-labeled MSCs were prepared for targeted fluorescence imaging and therapy of pancreas tissues in rat models with type 1 diabetes. CdSe/ZnS quantum dots were synthesized, their biocompatibility was evaluated, and then, the appropriate concentration of quantum dots was selected to label MSCs. CdSe/ZnS quantum dots-labeled MSCs were injected into mouse models with type 1 diabetes via tail vessel and then were observed by using the Bruker In-Vivo F PRO system, and the blood glucose levels were monitored for 8 weeks. Results showed that prepared CdSe/ZnS quantum dots owned good biocompatibility. Significant differences existed in distribution of quantum dots-labeled MSCs between normal control rats and diabetic rats (p < 0.05). The ratios of the fluorescence intensity (RFI) analysis showed an accumulation rate of MSCs in the pancreas of rats in the diabetes group, and was about 32 %, while that in the normal control group rats was about 18 %. The blood glucose levels were also monitored for 8 weeks after quantum dots-labeled MSC injection. Statistical differences existed between the blood glucose levels of the diabetic rat control group and MSC-injected diabetic rat group (p < 0.01), and the MSC-injected diabetic rat group displayed lower blood glucose levels. In conclusion, CdSe/ZnS-labeled MSCs can target in vivo pancreas tissues in diabetic rats, and significantly reduce the blood glucose levels in diabetic rats, and own potential application in therapy of diabetic patients in the near future.
KeywordsMesenchymal stem cell CdSe/ZnS quantum dots Fluorescence imaging Cell labeling Type 1 diabetes mellitus Rat model
The amount of diabetic patients has been increasing rapidly. According to the latest report, in China, the proportion of diabetic patients has reached to 11.6 %, accounting for one third of global diabetic patients. How to prevent and treat diabetes has become a challengeable problem. Type 1 diabetes mellitus is characterized by the permanent destruction of pancreatic β cells. Symptomatic individuals typically have lost more than 80 % of β cell population, resulting in essentially no insulin production and an inability to regulate plasma glucose levels properly .
Up to date, regarding the therapy of type 1 diabetes, some methods have been reported. For example, cell-based treatments with the aim of recovering pancreatic β cell function, including the pancreas transplantation, islet transplantation, and stem cell transplantation, focusing on replacing damaged β cell populations, should be the ideal potential therapeutic pathways . Among the cell-based treatments, the current gold standard is whole-pancreas transplantation. Recent pancreas transplants have demonstrated sustained exogenous insulin independence over 2 years post-transplant, evidenced by normal HbA1c values. However, pancreas transplantation is a major surgical procedure with mortality rates ranging from 1 to 3 %, and the complications of the procedure required long-term immunosuppression . In order to overcome these shortcomings of whole-organ transplantation, pancreatic islet cells transplanting has been actively explored. In 2000, a corticosteroid-free immunosuppressive regimen, the novel Edmonton protocol, enabled seven patients to remain insulin-independent for an average of 11.9 months . However, these results are quite difficult to reproduce, and nine-year islet graft survival rates are below 10 %. Therefore, looking for new therapeutic method may be a good choice.
Stem cell transplantation for diabetes therapy may offer a promising possibility that deserves to be explored. Stem cell transplantation has some advantages, such as the easy operation, short operation time, and low invasiveness for patients relatively compared with whole-organ transplantation . Up to date, mesenchymal stem cells (MSCs) have been actively investigated for their therapeutic values on different kinds of diseases such as diabetes, injury, gastric cancer, etc. The benefit of MSCs, including multi-lineage differentiation and immunosuppressive capability, suggests a role of MSC therapy for tissue regeneration . However, the distribution and therapeutic effects of transplanted MSCs in diabetic patients are not clarified well.
In recent years, nanotechnology has become an emerging field of interest. Interaction studies between nanomaterials and stem cells have made great advances. The importance of nanotechnology to the fundamental developments in stem cell-based therapies for injuries and degenerative diseases has been recognized. In particular, the effects of the structure and properties of nanomaterials on the tracing of stem cells have become a new interdisciplinary frontier in regeneration medicine and material science.
Quantum dots (QDs) are one kind of nanomaterial which have broad application prospects in cellular imaging, immunoassays, DNA hybridization, and optical barcoding, due to its significant advantages including good photostability, strong fluorescence intensity, and various emission wavelengths. As QDs were broadly used for molecular imaging, QDs biocompatibility attracts more and more attention from the scientific field. Exploration of quantum dots biosafety has become a hotspot. In another aspect, QDs with near-infrared region (NIR) emission have also received increasing attention as an alternative method for overcoming these shortcomings [7, 8].
In our previous study, we have synthesized CdSe/ZnS QDs with near-infrared region . Herein, we evaluated the biosafety of CdSe/ZnS QDs and investigated the in vivo biodistribution and the effects of CdSe/ZnS QDs-labeled MSCs on plasma glucose levels in type 1 diabetes rat models. Our study lays foundation for MSC clinical application for diabetes therapy in the near future.
All animal experiments were approved by the Institutional Animal Care and Use Committee of Shanghai Jiao Tong University (NO. SYXK2007-0025).
Synthesis and Characterization of CdSe/ZnS QDs
CdSe/ZnS QDs were synthesized according to our previous report . Briefly, with the nitrogen protection, 79-mg selenium powder was dissolved in 50-ml liquid paraffin in a three-neck flask and the temperature was increased to 200 °C gradually. After vigorous stirring for 1 h, the mixture was cooled to 80 °C. In another flask, 1.28-g CdO and 11.4-g stearic acid were dissolved in 10-ml liquid paraffin at 160 °C with stirring and protection of nitrogen. When the mixture turned to bright yellow, the cooled solution of Se precursors was rapidly injected into the hot flask containing Cd precursors and the mixture temperature was quickly increased to 200 °C for 90 min. The molar ratio of CdO/Se/stearic acid in liquid paraffin was 1:1:4, and the crude QD products were purified by chloroform and ethanol. In order to improve the optical properties of QDs, an additional semiconductor shell (zinc sulfide (ZnS)) should be coated on CdSe nanocrystals. For the ZnS shell, equal molar ratios of (TMS)2S and ZnEt2 as precursors of Zn and S and TOP/TOPO were used, and 90 °C was used for shell growth. The final core–shell product was repurified and redispersed into aliquot chloroform for later use. About 10 ml of deionized water was added to the solution to prevent evaporation of chloroform for long-period storage.
Primary Culture and Identification of Rat MSCs
MSCs were isolated according to our previous reports and were cultured with Dulbecco’s modified Eagle’s medium (DMEM; Gibco, Shanghai, China) with 10 % fetal bovine serum (FBS; Hyclone, Thermo Scientific, Logan, UT, USA), 100 U/ml penicillin, and 100 mg/ml streptomycin (Gibco) at 37 °C in a humidified 5 % CO2 incubator. The MSC medium was changed once every 2 days. The primary cells were cultured for 4 ~ 5 days until they reached confluence and were defined as passage“0”. All experiments were performed with the MSCs from passage 3–6. In order to identify MSCs, passage 4 MSCs were detached by 0.25 % trypsin/EDTA and washed twice with phosphate buffered saline (PBS). About 2 × 105 cells were incubated with an appropriate concentration of FITC-conjugated CD29 monoclonal antibody (BioLegend, San Diego, CA, USA), phycoerythrin (PE)-conjugated CD31 monoclonal antibody (BioLegend), APC-conjugated CD45 (BioLegend), and PE-conjugated CD90 (BioLegend) monoclonal antibody for 40 min at 4 °C in a dark environment. MSCs were washed with PBS buffer, were centrifuged for 5 min, and then resuspended in PBS. Quantitative fluorescence analysis was carried out using FACSCalibur cytometer (Becton Dickinson, San Diego, CA, USA) and CellQuest software.
In order to demonstrate the multipotential property of our prepared MSCs, MSCs were further characterized by differentiation assays. Passage 3 MSCs were cultured in an adipogenic medium including DMEM, 5 % FBS, 1-μM dexamethasone (Sigma), 50-μM indomethacin (Sigma), 500-nM IBMX (Sigma), 5 μg/ml insulin (Sigma), 100 U/ml penicillin, and 100 mg/ml streptomycin for about 2 weeks . The medium was replaced every 3 days with a fresh induction medium. Subsequently, to evaluate cultures for adipogenic differentiation, the cells were washed with PBS, fixated with 2.5 % glutaraldehyde, and stained with Oil Red O (Sigma) for revealing lipid globules. In addition, passage 3 MSCs were cultured in DMEM supplemented with 5 % FBS, 50-μM ascorbic acid (Sigma), 10-nM dexamethasone, 100 U/ml penicillin, and 100 mg/ml streptomycin for about 2 weeks for osteogenic differentiation. Finally, the cells were evaluated and cultured for osteogenic differentiation with Alizarin Red S (Sigma).
Effects of CdSe/ZnS QDs on the Differentiation Ability of MSCs
In order to estimate the changes of MSCs’ differential ability under the condition of CdSe/ZnS, the differentiation experiments were carried out with or without the CdSe/ZnS QDs (16 μg/ml), and the morphology of the final differentiation cells were captured in microscopy and compared.
Cell Viability Assay
Toxicity test is determined by the cell viability after incubation in a culture medium containing different concentrations of CdSe/ZnS QDs. A controlled trial was conducted by culturing cells in a normal medium without QDs. We carried out this test by using Cell Counting Kit-8 (CCK-8) which is an upgrade alternative to MTT assay. This toxicity assay was performed in a 96-well plate with about 1 × 103 cells seeding in every well. After incubating in a culture medium containing different concentrations (concentration = the volume of QDs/the volume of medium, the unit is μl/ml)of QDs for 24 h, the CCK-8 reagent (10 μl) was added to each well and there, action was allowed to proceed for up to 4 h. The optical absorbance was measured at 450 nm by using the Thermo multiskan MK3 ELISA plate reader according to the protocol of the CCK-8 assay kit.
Transduction of Different Concentrations of QDs into MSCs
MSCs were cultured in the medium without FBS at 37 °C, in 5 % CO2 for 4 h, and different amounts of of QDs(100μg/ml) (5.0, 10.0, 15, and 20 μl/ml) were mixed in the transduction medium (DMEM, 10 % FBS, 100 U/ml penicillin/streptomycin) at 37 °C for 15 min, respectively; the medium was added into the MSC culture flask, incubated in a humidified 5 % CO2 incubator at 37 °C for 6 h. And then, the flask was washed three times with PBS and covered with new PBS. The cells were counted and prepared for quantum dot distribution test and cell transplantation.
The Distribution of the Quantum Dots in MSCs
Passage 3 MSCs were treated with a medium containing CdSe/ZnS QDs (15 μl/ml) for 4 h. Afterward, the cells were visualized under an inverted fluorescence microscope (Olympus IX71, Olympus, Shanghai, China). After incubation with QDs, the cells were fixed with paraformaldehyde for 15 min at room temperature. The labeled MSCs were also stained with 4′,6-diamidino-2-phenylindole (DAPI) in PBS (pH 7.4) for 5 min. And then, the cells were washed with PBS. The cells were observed with the fluorescence microscope. Fluorescence imaging microscopy was equipped with 372- and 510-nm wavelength excitation light filters.
Preparation of Diabetes Rat Models
Type 1 diabetes SD rat models were prepared according to the previous report [4, 10]. Thirty specimens of eight-week-old male rats were lightly anesthetized. The rats were randomly divided into three groups: the normal control group (n = 10), diabetic control group (n = 10), and MSC treatment group (n = 10). Streptozocin (STZ, Sigma–Aldrich S1030) was dissolved in 0.1-M citrate at pH 4.5 and was immediately injected into the abdominal cavities of the diabetic control group and MSC treatment group at a dose of 60 mg/kg. The normal control group received the same dose of citrate buffer injection. One week later, fast plasma glucose (FPG) amounts of all rats were examined, and those rats with plasma glucose concentration FPG ≥ 16.5 mmol/l were considered as diabetic rats.
CdSe/ZnS QDs-Labeled MSCs for Targeted Fluorescence Imaging
At the seventh day after the diabetes rat models were prepared, the MSCs (5.0 × 106 cells/rat) labeled with QDs were injected into the rat models in the MSC treatment group via tail vein, while the normal control group rats received the same dose of QDs-labeled MSCs.
For in vivo imaging analysis, whole-animal imaging was performed with the Bruker In-Vivo F PRO system. The images of several time points (1, 3, 6, and 12 h) post-injection of MSCs were captured and analyzed to understand the migration and homing behavior of MSCs in type 1 diabetes rats. The excitation and emission filters were set to 410 and 700 nm (bandpass, ±15 nm), respectively. The collected images were analyzed with the image J software (NIH ImageJ; http://rsb.info.nih.gov/ij/), which employs spectral algorithms to separate autofluorescence from quantum dot signals. (The imaging condition is the same of in vivo imaging.)
Distribution of CdSe/ZnS QDs-Labeled MSCs in Rats with Type 1 Diabetes
For the ex vivo fluorescence imaging analysis, rats were sacrificed by cervical detachment method at 6 h after MSC injection (time point selection were accorded with the homing effects of MSCs, data not shown), five organs (heart, lungs, pancreas, spleen, and liver) of the normal control and MSC treatment groups were harvested, and the fluorescence images were captured by the Bruker In-Vivo F PRO system. The excitation and emission filters were set to 510 and 700 nm (bandpass, ±15 nm), respectively. The collected images were analyzed with the image J software.
The Measurement of Plasma Glucose Levels in Diabetes Rat Models
Eight-week-old rats were administered with STZ, which induced pancreatic injury and hyperglycemia. In order to investigate the therapeutic effects of MSCs (unlabeled with QDs), the level of blood glucoses from the 1st to the 8th week after MSC injection were measured by general methods (10 rats of each group, including the normal control, diabetes control, and treatment groups).
Numerical values are presented as the mean ± SD. Each experiment was repeated three times. Statistical significance was evaluated using unpaired Student’s t test for comparisons between the two groups; p values <0.05 were considered to be statistically significant. All statistical analyses were performed by the SPSS software package.
Results and Discussion
Preparation and Identification of MSCs
In order to confirm whether prepared MSCs could differentiate into two lineages such as osteogenic and adipogenic lineages, we finished the differentiation ability analysis of osteogenic and adipogenic differentiation induction.
Afterward, adipogenic differentiation of putative MSC lines induced by using adipo-inductive media resulted in massive lipid droplet accumulation as demonstrated by positive staining with Oil Red O (Fig. 3b). Lipid droplets were detectable even after 3 days, but a period of 2 weeks was necessary to accomplish maximal lipid accumulation.
In addition, the differentiating abilities of osteogenic and adipogenic lineages of MSCs were assessed in our study as previously described (Fig. 3) . These results indicate that the cultured cells possessed the characteristics of MSCs.
Preparation and Characterization of CdSe/ZnS Quantum Dots
Effects of CdSe/ZnS QDs on the Differential Ability of MSCs
Effects of CdSe/ZnS Quantum Dots on Viability of MSCs
Preparation and Observation of QDs-Labeled MSCs
The Measurement of Blood Glucose Levels
In Vivo Imaging and Distribution of QDs-Labeled MSCs
Biodistribution of QDs-Labeled MSCs
After 6 h of MSC transplantation, in order to analyze the accumulation ratio of QDs, the ratios of fluorescence intensity (RFI) analysis was performed in five organs (Fig. 11b). The RFI results showed that the RFI of MSCs in the pancreas of diabetic rats was about 32 %, while that in the normal control group rats was about 18 % (p < 0.05, triplicate of each groups). However, the RFI of the liver in the normal control group was much higher than that of the diabetes group (p < 0.05, triplicate of each group). These data suggest that in vivo MSCs can migrate and enter into the pancreas tissues actively, can be introduced for pathological imaging of the pancreas tissue, and deduce the therapeutic function initiatively.
Previous studies have revealed that MSCs are capable of reducing glucose levels in animals or subjects with type 1 diabetes . The underlying mechanism of the therapeutic effect of MSCs on hyperglycemia might involve islet regeneration . Several studies highly indicate that the unique immunomodulatory effects of MSCs seemed to be closely associated with decreasing hyperglycemia. MSCs also exerted anti-inflammatory effects that might be important in maintaining peripheral tolerance [14, 15].
In our study, the blood glucose of the normal control group and MSC treatment group changed significantly at 2 months after MSC injection. The FPG of the MSC treatment group dropped more than 5 mmol/l, while the diabetic control group and normal control group make no difference at the beginning of the study, and this phenemenon was similar to previous reports [16, 17].
In order to clarify the potential mechanism, we prepared QDs-labeled MSCs and observed their in vivo distribution. QDs own obvious advantages compared with traditional fluorescent dye molecules . Specifically, QDs have a broad absorption range from ultraviolet (UV) to visible light, but exhibit distinct narrow emission spectrum, resistant to photochemical degradation; therefore, QDs are very suitable for tracking MSCs and monitoring cell biological changes in vivo [19–21].
We selected SD rats to prepare a diabetes model [22–24]. With the help of the Bruker In-Vivo F PRO system, the transplanted MSCs were observed to accumulate in the liver in the normal control animals, few MSCs in the pancreas tissues, and the RFI analysis showed that the accumulation rate of MSCs in the liver was about 41 %. While in diabetic rats, the accumulation rate of the transplanted MSCs in the liver was decreased to about 32 %, and the accumulation rate in the pancreas increased to about 32 %. Accordingly, distribution difference between the two groups should be closely associated with pancreas injury, which also confirms that MSCs can target and accumulate to the injured organ [10, 25–30]. Our results also indirectly suggest that quantum dots do not alter the self-replication and differentiation potential of MSCs [7, 31, 32] and exhibit great potential in tracking MSCs in vivo .
Regarding the molecular mechanism of MSC homing, the transplanted MSCs were observed to accumulate in the liver in normal control, which may be caused by Kupffer cells with strong phagocytosis in hepatic sinusoid [31, 34]. The accumulation of transplanted MSCs in the pancreas of diabetic rats may be closely associated with chemokines [35, 36]. Our previous studies confirmed that CCL19/CCR7 and CXCL12/CXCR4 axis loops played key roles in the targeting of MSCs to in vivo gastric cancer [37–39]. Therefore, we predict that CCL19/CCR7 and CXCL12/CXCR4 axis loops also may play key roles in the targeting of MSCs to in vivo injured pancreas. The concrete mechanism is under way.
We investigated the behavior and organ-specific accumulation of transplanted MSCs labeled with QDs in a rat model of type 1 diabetes. Using the Bruker In-Vivo F PRO system, the accumulation rate of MSCs in the pancreas of rats in the diabetes group was increased compared with the normal control group. The MSC-injected diabetic rat group displayed lower blood glucose levels. Our results highly indicate that QDs-labeled MSCs can target in vivo pancreatic tissues of diabetic rats, and decrease the blood glucose levels, and then, in vivo transplanted MSCs may own clinical therapeutic prospect to diabetic patients.
This work is supported by the National Natural Scientific Fund (Nos. 81170728, 81300672, 81225010, and 81100586) and Shanghai Science and Technology Fund (No. 11NM0504200), and 863 project of China (no. 2012AA022703 and 2014AA020700).
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