Notes on Induced Pluripotent Stem Cells: Discovery and Characterization

Introduction to Pluripotency and Stem Cells

• Early Embryonic Development: At the $4-$cell, $8-$cell, and morula stages, cells possess a solid, bordered structure.

• Blastocyst Stage: This stage features a large, fluid-filled cavity called a blastocyst. The outer layer of cells, the trophectoderm, surrounds the blastocoele and has distinct differentiation characteristics.

• Embryonic Stem Cells (ESCs): These are crucial for development and research, known for their pluripotency.

Discovery of Reprogramming Factors

• Historical Context: Researchers in $2006$ (referencing specific paper by Yamanaka) discovered that fully differentiated cells can be reprogrammed.

• Initial Discovery: Culturing fully differentiated cells in the presence of $4$ factors (Oct$3/4$, c-Myc, Klf$4$, and Sox$2$) can revert them into a skin-cell-like, pluripotent state, referred to as induced Pluripotent Stem Cells (iPSCs).

• Candidate Gene Selection: The initial experiment began with $24$ candidate genes. These genes were chosen based on criteria such as: high expression levels within embryonic stem cells, known roles in inducing pluripotency, and involvement in the self-renewal processes of embryonic stem cells.

• Narrowing Down Factors: Researchers tested the effect of these $24$ genes on pluripotency and systematically reduced the number of factors to identify the minimal set required for inducing pluripotency.

Figure 1: Initial Reprogramming and Selection System

• Selection System - $FBX15$ Reporter:

  • The $FBX15$ gene promoter is active specifically in pluripotent stem cells ($ES$ cells).

  • A eta-geo (beta-galactosidase and neomycin resistance) knock-in construct was placed under the $FBX15$ promoter.

  • $G418$ (an antibiotic, neomycin analog) was used. Only cells that activated the $FBX15$ promoter (i.e., became pluripotent) would express the neomycin resistance gene and survive in the presence of $G418$. Normal cells, lacking $FBX15$ promoter activity, would die.

  • This system ensures that only reprogrammed (pluripotent) cells are selected for further study.

• Experimental Setup: $24$ candidate transcription factors were introduced into mouse embryonic fibroblasts (MEFs) using viral infection.

• Panel B (Colony Formation):

  • Mock Transfection: No growth observed.

  • $24$ Factors: Formation of $22$ colonies, $5$ of which exhibited morphology similar to embryonic stem cells.

• Panel C (Cell Morphology):

  • Mouse Embryonic Fibroblasts (MEFs): Control cells, appearing flat and elongated, typical of connective tissue cells.

  • Normal Embryonic Stem Cells (ESCs): Serve as a positive control, characterized by a rounded shape, compact colonies, and large nucleoli.

  • iPS MEF24 (Induced Pluripotent Stem Cells from MEFs with $24$ factors): Showed morphology similar to ESCs, indicating successful reprogramming.

  • Naming Convention: iPS MEF X refers to induced pluripotent stem cells from mouse embryonic fibroblasts, where $X$ is the number of factors used for transfection.

• Cell Proliferation (Growth Curve):

  • MEFs (represented by triangles) have a limited lifespan and do not survive beyond $37$ days in culture.

  • ESCs and iPSCs demonstrated sustained proliferation, continuing to divide beyond this timeframe, akin to immortalized cells.

• Panel E (Reverse Transcription PCR (RT-PCR) Analysis):

  • Purpose: To detect the expression levels of $mRNA$ of key pluripotency genes.

  • Method: $mRNA$ is converted to $cDNA$ via reverse transcriptase, then amplified using PCR.

  • Findings: ESCs displayed robust expression (visible bands) for all tested pluripotency markers (Nanog, Eras, Oct3/4, Sox2). Among the iPS MEF24 colonies, colonies $2$ and $6$ showed the most similar gene expression profiles to ESCs.

• Panel F (Bisulfite Sequencing Analysis):

  • Purpose: To assess DNA methylation status at promoter regions, which correlates with gene transcriptional activity (methylation typically leads to inactivation).

  • Findings:

    • ESCs exhibited low methylation (represented by open circles) at the promoters of $Oct3/4$, $Nanog$, and $FBX15$, indicating active transcription.

    • MEFs showed heavy methylation (closed circles) at these promoters, indicating transcriptional inactivity.

    • iPS MEF24 colonies displayed varying degrees of demethylation, with some regions similar to ESCs and others remaining methylated, suggesting that the reprogramming was not entirely complete or uniform across all loci.

• Figure 1 Conclusion: This figure demonstrated that specific combinations of factors could induce ES-like programming and self-reproduction in MEFs. However, while iPSCs shared many characteristics with ESCs, they were not an exact match, showing differences in gene expression and epigenetic modifications.

Figure 2: Narrowing Down the Reprogramming Factors

• Factor Reduction Strategy: Researchers systematically removed factors from the initial $24$ candidates. Removing $10$ factors led to a drastic decrease in colony growth. This iterative process continued until $4$ factors were identified as absolutely essential for inducing pluripotency.

• Panel D (Cell Morphology Comparison):

  • iPS MEF4 (reprogrammed with $4$ factors) and iPS MEF10 (from Figure $1$) showed morphology very similar to native ESCs.

  • iPS MEF3 (reprogrammed with $3$ factors, highlighting the necessity of all four) exhibited an aberrant morphology, consistent with previous observations of poor colony formation.

• Panel E (RT-PCR for Endogenous Transcripts):

  • Crucial Detail: The experiment used primer sets designed to amplify endogenous (native) $mRNA$ transcripts, not those from the introduced transgenes.

  • Purpose: To determine if the native pluripotency genes were reactivated, rather than just relying on the expression of the introduced viral gene constructs.

  • Findings: iPS MEF4 and iPS MEF10 demonstrated $mRNA$ expression levels of Nanog, Sox2, and Oct3/4 more comparable to ESCs, whereas iPS MEF3 cells were notably deficient in the expression of many of these genes. This reinforced the critical role of all four identified factors.

Figure 3: Detailed Characterization of iPSCs (Molecular Markers and Epigenetics)

• Histone Modifications:

  • Chromatin Immunoprecipitation (ChIP) Analysis: Used to examine specific histone modifications associated with gene activity.

  • $H3K4$ dimethylation ($diMe$) is generally associated with active gene promoters.

  • $H3K9$ dimethylation ($diMe$) is typically associated with inactive gene promoters.

  • Findings ($Oct3/4$ and $Nanog$ Promoters):

    • ESCs displayed high levels of $H3K4diMe$ and low levels of $H3K9diMe$, indicating active pluripotency gene expression.

    • MEFs showed the inverse pattern, consistent with these genes being inactive.

    • iPSCs (specifically iPS MEF4 and iPS MEF10) exhibited histone modification patterns similar to ESCs.

  • Complementary Nature: The data also investigated histone acetylation, which is another mark of active transcription. High acetylation levels were observed in conjunction with high $H3K4diMe$ and low $H3K9diMe$, confirming an active transcriptional state in ESCs and iPSCs for these genes.

  • Figure 3 Conclusion: While histone modifications in iPSCs were largely similar to ESCs for key pluripotency loci, subtle differences remained, suggesting iPSCs are similar but not perfectly equivalent to ESCs.

• Panel F (Surface Marker Staining):

  • $SSEA-1$ Marker: Staining for $SSEA-1$, an embryonic stem cell surface marker.

  • Findings: Both ESCs and the successfully reprogrammed iPSCs (iPS MEF4 and iPS MEF10) showed positive staining for $SSEA-1$, further indicating their pluripotency and similarity to ESCs.

Figure 4: Global Gene Expression Analysis (Microarray)

• Microarray Methodology:

  • Purpose: To perform a global analysis of gene expression (transcriptome analysis), comparing iPSCs to ESCs and MEFs.

  • Distinction from RNA-Seq: Unlike RNA sequencing which surveys all RNA species, microarray uses a chip with predefined probes for a specific set of genes (e.g., $10,000$ to $15,000$ genes).

• Heatmap Interpretation (Gene Expression Patterns):

  • Group 1 Genes: These genes demonstrated similar upregulation in ESCs and all iPSCs tested (iPS MEF4, iPS MEF10, iPS MEF3), regardless of the specific combination of reprogramming factors used. These genes are characteristic of non-ESCs being reprogrammed.

  • Group 2 Genes: Showed upregulation in ESCs, iPS MEF4, and iPS MEF10, but notably not in iPS MEF3. This highlights the distinct gene expression profile of iPS MEF3 cells and the requirement of all four factors for full reprogramming.

  • Group 3 Genes: These genes were highly expressed only in ESCs but not in any of the iPSCs. This represents a clear transcriptional difference between ESCs and iPSCs, indicating that not all ES cell-specific genes are fully reactivated during reprogramming.

• Figure 4 Conclusion: While iPSCs share substantial similarities in gene expression with ESCs, they are distinct entities. iPSCs do express many characteristics of ESCs, but ESCs often exhibit higher levels of gene upregulation, and some ES-specific genes are not fully reactivated in iPSCs.

Figure 5: In Vivo and In Vitro Pluripotency Assays

• Pluripotency Definition: A pluripotent cell must be able to generate all three germ layers (ectoderm, mesoderm, endoderm) in the context of an embryo or a teratoma.

• Teratoma Formation (In Vivo Gold Standard):

  • Method: iPSCs are injected into immunodeficient mice.

  • Findings (Panels A and B): The formation of a teratoma (a tumor containing differentiated tissues from all three germ layers, e.g., neural tissue from ectoderm, cartilage from mesoderm, gut-like structures from endoderm) confirms the pluripotency of the iPSCs.

• Embryoid Body (EB) Formation (In Vitro Differentiation):

  • Method: ESCs and iPSCs (from MEF3, MEF4, MEF10) were induced to form 3D aggregates (embryoid bodies) in vitro, which then undergo spontaneous differentiation into various cell types representing all three germ layers.

  • Findings (Panel C): This experiment further validated iPSCs' ability to differentiate, showing varying degrees of differentiation potential across the different iPS MEF lines. Each experiment served as crucial validation during the development of the iPSC system.

Figure 6: Reprogramming Adult Fibroblasts and Genetic Stability

• Reprogramming Adult Cells: This figure presents a pivotal finding that iPSCs can be generated from adult, fully differentiated cells, specifically tail-tip fibroblasts (TTFs).

• iPS TTF Clones: These clones expressed ES cell marker genes, similar to the iPS MEF cells. While not all genes were expressed identically to ESCs, certain iPS TTF clones showed very similar expression profiles.

  • Significance: This demonstrates that pluripotency networks can be reactivated in adult somatic cells, opening avenues for patient-specific cell therapies.

• Panel D (Teratoma Formation from Adult-Derived iPSCs):

  • Findings: Teratomas derived from iPS TTF cells contained tissues from all three germ layers, confirming that these adult-derived iPSCs were truly pluripotent and capable of generating diverse cell types.

• Karyotyping (Genetic Stability):

  • Purpose: To analyze the chromosomal structure of iPSCs to ensure genetic normality (euploidy) and rule out chromosomal abnormalities often seen in cancer cells.

  • Findings (Panel D, Karyotype Image): The iPSCs (from tail-tip fibroblasts) displayed normal karyotypes, indicating they maintained genetic integrity and were not chromosomally aberrant, which is essential for therapeutic applications.

Overall Conclusion of the Paper

• iPSCs are highly similar to ESCs in many aspects of pluripotency but are not absolutely identical. Distinct differences in gene expression and epigenetic modification persist.

• The study definitively showed that four specific transcription factors (Oct$3/4$, Sox$2$, c-Myc, and Klf$4$) are sufficient to revert fully differentiated adult cells (e.g., skin fibroblasts) back to an ES-like pluripotent state.

• These induced pluripotent stem cells (iPSCs) possess the capability to differentiate into all three embryonic germ layers (ectoderm, mesoderm, endoderm) both in vitro and in vivo, making them functionally pluripotent.

• Importantly, these $4$ inducing factors are distinct from factors previously known to be essential solely for maintaining pluripotency in ESCs.

General Note to Students: Active participation in discussions and asking questions is highly encouraged for a deeper understanding of the material. Do not hesitate to clarify any points.