Integration analysis was conducted on two microarray datasets with comparable levels of osteogenic induction to identify differentially expressed genes associated with MSC osteogenic differentiation
We integrated two microarray datasets, GSE37558 and GSE28205, from the GEO database. These datasets contained sequencing samples with comparable levels of osteogenic induction. Specifically, we utilized the GSE37558 dataset for 12-day osteogenic induction and the GSE28205 dataset for 14-day induction.
We utilized the screening criteria of a relaxed threshold of a false discovery rate (FDR)<=0.25 and a P value<=0.05 to identify genes that were differentially expressed in the aforementioned microarray datasets and subsequently determined their intersection through Venn diagram analysis. A total of 1156 genes exhibiting differential expression were identified in both datasets, as illustrated in Figure 1 (A and B).
In addition, we utilized a log2(fold change)>=1.5 as the cutoff value to identify 169 differentially expressed genes, consisting of 100 down-regulated and 69 up-regulated genes, as depicted in Figure 1C. These identified genes will be subjected to further scientific scrutiny.
The exploration of genes continuously differentially expressed during osteogenic induction in MSCs was investigated by integrating microarray data from the GSE37558 dataset
To ensure a comprehensive analysis of transcriptome dynamics during osteogenic differentiation, we examined a microarray dataset (GSE37558) comprising MSC samples treated with time-course osteogenic induction.
We used the preceding time point of osteogenic induction as a reference for subsequent induction time points and analyzed the GSE37558 dataset. By applying screening criteria with an FDR<=0.25 and a P value<=0.05, we simultaneously identified 549 differentially expressed genes across Day 0 to 2, Day 2 to 8, and Day 8 to 25 during osteogenic induction. The results are presented in Figure 2A and B.
In addition, we utilized a log2(fold change)>=1.5 as the cutoff value to identify 121 differentially expressed genes, consisting of 84 up-regulated and 27 down-regulated genes, as depicted in Figure 2C. These identified genes will be subjected to further scientific scrutiny.
Integrated time-course analysis of differentially expressed genes within a single batch of microarray data and across multiple batches of microarray datasets optimized screening for continuously differentially expressed genes during osteogenic induction in MSCs
After accounting for batch effects and improving data accuracy, we integrated the aforementioned analysis results to identify a total of 124 genes that exhibited differential expression across both microarray batches conducted by two distinct research groups. The corresponding data are presented in Figure 3A.
We subsequently applied a screening criterion of LOGFC>=1.5 to identify 65 genes, including 49 genes that exhibited significantly high expression throughout the osteogenic induction process and 16 genes that showed significantly low expression during this period. The corresponding data are presented in Figure 3B.
The results mentioned above indicated that the 49 identified genes might play a crucial role in regulating the dynamics of MSC osteogenic differentiation. Follow-up investigations were also conducted to validate these candidates and elucidate their biological functions in this process.
Identification of key regulators that govern the dynamic progression of MSC osteogenic differentiation through integration of their expression levels in human tissues through the HUMAN PROTEIN ATLAS database
We focused on the aforementioned candidates, which were identified as potential key regulators of MSC osteogenic differentiation. We assessed the expression of these genes in human tissues using the HUMAN PROTEIN ATLAS database, which provides comprehensive data on gene expression levels at both the RNA and protein levels across 45 human tissue types.
Numerous studies have demonstrated the pivotal role of bone cells, including mesenchymal stem cells and hematopoietic stem cells, in regulating cell types and maintaining tissue homeostasis within the lineage of stem cells [27, 28]. The skeletal system harbors these intricate cell lineages derived from this series of cells, which dictate their differentiation into the osteogenic lineage and further determine the homeostasis of both skeletal and marrow tissues [29].
Therefore, we focused on hematopoietic tissues related to the blood and immune system in the HUMAN PROTEIN ATLAS database, which provides information on RNA and protein expression levels in these tissues. A total of 13 potential candidate genes were identified that are highly expressed in some or all of these tissues, and their information is presented in Table 1.
Table 1 clearly shows that four factors, PTBP1, H2AFZ, TTPAL (C20ORF121), and BCL6, exhibit high expression levels in most of the tissues associated with the blood and immune system (three are highly expressed in all four tissues, while one exhibits medium expression). These factors showed significant changes according to the microarray data obtained via transcriptomic sequencing. These findings suggest that these four factors could be considered crucial potential targets responsible for controlling the dynamic process of MSC osteogenic differentiation. Next, we performed biological experiments to confirm the relative molecular functions of the genes.
Isolation of bone mesenchymal stem cells and identification of potential candidate genes through qRT‒PCR analysis at different time points during osteogenic induction
Bone marrow mesenchymal stem cells (BMSCs) are among the most commonly used stem cells in cell therapy and tissue engineering [30, 31]. They are a type of multilineage progenitor cell that possesses self-renewal capacity and can differentiate into various types of mesoblastic cells, including osteoblasts, chondrocytes, adipocytes, etc. [32]. Therefore, we isolated BMSCs from bone marrow stromal cells that exhibited the most prominent stem cell characteristics and subsequently conducted biological experiments to validate the potential candidate targets identified through bioinformatics analysis.
We initially isolated BMSCs using methods described in the literature. Subsequently, we confirmed the characteristics of the stem cells through ARS, Alcian blue, and oil red O staining under osteogenic, chondrogenic, and adipogenic induction conditions. The results demonstrated that the BMSCs utilized in this study possessed multilineage differentiation potential, as shown in Figure 4A.
We induced BMSCs with osteogenic differentiation medium and collected RNA at different time points (Day-7, Day-14, Day-21) to detect the expression levels of biomarker RNAs for the following biomarkers of osteogenesis: ALP and BGLAP. The results demonstrated up-regulation of these biomarkers, indicating the successful design of the osteogenic differentiation experiment, as shown in Figure 4B.
In the subsequent step, we evaluated the RNA expression of these four potential genes that were optimized by bioinformatic database analysis at various induction time points to validate the accuracy of our bioinformatic analysis. We observed sustained high expression of BCL6 and TTPAL during the induction period in a time-dependent manner, as depicted in Figure 4C. Conversely, PTBP1 and H2AFZ exhibited continuous decreases in expression throughout the induction period in a time-dependent manner, as shown in Figure 4D.
These experimental results were consistent with previous findings from Gene Expression Omnibus (GEO) database mining, thus laying a solid foundation for further research.
Identifying the molecular function of dynamically regulating osteogenic induction in MSCs by overexpressing four potential genes via lentiviral packaging technology
We employed lentiviral packaging technology to overexpress four candidate genes, PTBP1, H2AFZ, BCL6, and TTPAL, in 293T cells. The generated virus was then used to infect BMSCs via the 293T supernatant. After a transduction period of 48 hours, we collected RNA from the BMSCs and assessed the infection efficacy using qRT‒PCR. As shown in Figure 5A, successful infection was observed.
We conducted experiments to investigate the regulatory factors involved in osteogenesis and utilized lentiviruses containing these four genes to infect BMSCs. The osteogenic ability was evaluated by ALP assay at day-7, with a control group was infected with the vector virus for comparison. As shown in Figure 5B, BCL6 and TTPAL both enhance osteogenic potential. Conversely, as demonstrated in Figure 5C, PTBP1 and H2AFZ appeared to inhibit osteogenesis.
These findings suggest that BCL6 and TTPAL serve as positive regulators of osteogenic lineage differentiation in BMSCs, while PTBP1 and H2AFZ act as negative regulators.