3.1 Generation and characterization of ProliHHs.
Previous researches showed that ProliHHs were bi-phenotypical cells which expressed genes of mature hepatocytes and liver progenitors[15]. To some extent, ProliHHs could replace primary human hepatocytes for drug safety evaluation and cell therapy[16]. However, there were a lot of gene expression and function differences among PHH and different passages of ProliHHs. Thus, a quick and simple approach to identify cells stages is urgently needed. Here, PHH were induced to ProliHHs by HM medium and hypoxia cultured for 7d. PHH could expand more than 250 folds to ProliHHs at P4. PHH and P1were polygonal, P4 partly were long strips. (Fig. 1A). SOX9, as a biomarker for liver progenitor cells, mRNA expression was gradually increased from PHH to ProliHHs (P1 and P4). The nuclear receptors FXR and CAR, mature hepatic genes, expressed in PHH, P1 and P4. Interestingly, the FXR level of P4 was higher than PHH. The efflux transporter MRP2 mRNA level was downregulated both P1 and P4 relative to PHH. Although ProliHHs expressions of Phase I drug metabolizing enzymes, including CYP3A4 and CYP2B6, were lower than PHH. The CYP2B6 gene of P4 showed nearly 1/3 recovery compared to PHH. Phase II drug metabolizing enzymes, such as UGT1A1 and UGT2B7, were also expression of P1 and P4. In particular, the UGT1A1 level was significantly higher than PHH (Fig. 1B). In brief, from gene expression perspective, ProliHHs only maintained part of drug metabolic and transport genes, the cells expressed more and more liver progenitor biomarkers from P1 to P4.
3.2 Raman spectroscopy and classification analysis.
Although the mRNA and protein levels could characterize the differences between PHH and different passages of ProliHHs, those methods cost much time while get limited information. Therefore, we evaluated the potential of collected Raman spectroscopy to monitor cell to cell heterogeneity. PHH, ProliHHs P1 and P4 were respectively examined at least 600 single spectrum (Table 1). The fingerprint region, including more than 90% of the cellular peaks, was collected to identify cell stages[32]. The averaged spectra of each type cell were showed in Fig. 2A. Principal component analysis (Fig. S1) and linear discriminant analysis (Fig. 2B) were used to reduce dimension and highlight the spectral signatures on different cell stages. One Raman sample was represented as a point, at least 600 samples to reflect each type cells in different figures. Obviously, LDA was able to discriminate cell populations better than PCA. The significant wavenumbers in LD1 and LD2 contributed to differences among three groups were provided in (Fig. S2). This suggests the ability of Raman spectroscopy to classify cells.
Table 1
Numbers of Raman spectra acquired from PHH, ProliHHs P1 and P4
Group
|
Number of Spectra
|
PHH
|
620
|
ProliHHs P1
|
624
|
ProliHHs P4
|
606
|
3.3 Machine learning models.
In addition to extract significant wavenumbers, it is suitable to establish Raman dataset by machine learning models. The entire Raman spectral (1850) was randomly divided into two parts, 75% to establish the training set, 25% to verify the model. First, the single model KNN, LDA, PLS, Linear-SVM, RBF-SVM and RF were respectively constructed to classify by tenfold cross-validation with five repetitions and the corresponding parameters were showed in Table S2. The overall accuracy was 83.8% in Linear-SVM model. Then, the fitted results of above six models were stacked to form a two-layer machine learning model, which improved the prediction accuracy to 84.6% (Table 2). The sensitivities were 75.6%, 88.7%, 89.7% and specificity were 90.2%, 95.5%, 81.2% for PHH, P1 and P4.
Table 2
Machine learning by stacked (KNN, LDA, PLS, Linear-SVM, RBF-SVM, RF) model to identify cells. Overall accuracy at 84.6%.
|
Reference
|
|
P1
|
P4
|
PHH
|
Model prediction
|
|
|
|
ProliHHs P1
|
118
|
14
|
16
|
ProliHHs P4
|
14
|
134
|
0
|
PHH
|
24
|
3
|
139
|
Sensitivity(%)
|
75.6
|
88.7
|
89.7
|
Specificity(%)
|
90.2
|
95.5
|
81.2
|
3.4 Potential biomarkers of cell identification.
There were numbers of differences in Raman bands which may reflect cell biochemical components changes from PHH to different stages of ProliHHs (P1 and P4) (Fig. 3). The results suggested significant differences among spectral bands of cell clusters located at 480 cm-1 (glycogen), 831 cm-1 (tyrosine), 840-860 cm-1 (polysaccharide), 1003 cm-1 (phenylalanine), 1080 cm-1 (amide II, typical phospholipids), 1172 cm-1 (C-H in plane bending mode of tyrosine), 1206 cm-1 (hydroxyproline),1265 cm-1 ( -helix, collagen, tryptophan), 1300 cm-1 (lipids), 1337 cm-1 (Amide III), 1440 cm-1 (CH2 and CH3 formation vibrations of lipids), 1658 cm-1 (Amide I of proteins), 1744 cm-1 (carbonyl feature of lipid spectra) respectively. To further quantify the changes in 3 groups, relative Raman bands area were integrated (Fig. 4 A-M). The peak at 480 cm-1 (P 0.001) and 831 cm-1 (P 0.001) were significant difference between PHH and ProliHHs (P1 and P4). The Raman bands area at 840-860 cm-1 (P 0.0001), 1080 cm-1 (P 0.0001), 1265 cm-1 (P 0.001), 1300 cm-1 (P 0.0001), 1440 cm-1 (P 0.0001), 1658 cm-1(P 0.0001) and 1744 cm-1(P 0.0001) were significantly decreased following ProliHHs derived and passaged. In contrast, the Raman bands area at 1003 cm-1 (P 0.0001), 1206 cm-1 (P 0.0001), 1337 cm-1 (P 0.0001) were significantly increased. Although the band at 1172 cm-1 was no change between PHH and P1, P4 was higher than the others.
The Raman bands changes at 1003 cm-1, 1206 cm-1 and 1300 cm-1 were identified with reactive oxygen species, hydroxyproline and triglyceride levels by the corresponding kit. The ROS levels significant down regulated from PHH to ProliHHs (P1 and P4), while there was no difference between P1 and P4 (Fig. 5A). TG concentration gradually decreased from PHH to ProliHHs (Fig. 5B). Hydroxyproline concentration increased during the dedifferentiation process, however there was no statistical difference (Fig. 5C). These findings indicated the potential biomarkers for cell quality control which is essential in cell therapy.