Optimizing Stem Cell Differentiation

“Stem cell differentiation techniques are both easier and more complex than ever before,” says Natasha Lewis, Ph.D., Senior Scientist at Sartorius. Protocols are now well-established enough to provide a clearly defined path for scientists, which allows for standardization and repeatability. Simultaneously, technology continues “to push the field into new and exciting areas.” Extracellular matrices and bioprinting facilitate the transition from 2D to 3D cultures, directing stem cells, “not just to a particular type, but into functional tissues that contain multiple cell populations” mimicking organs. This means better disease modeling and drug testing, and even the potential for transplantation. That said, “one of the major challenges is achieving high efficiency and consistency in the differentiation process.”

Tracey Long, Ph.D., Senior Marketing Manager at Bio-Rad Laboratories, says induced pluripotent stem cells (iPSCs) complement the shift to 3D cultures. “Researchers have gained the opportunity to investigate more complex processes with stem cells cultivated in these settings, leading to the development of models for studying developmental biology, disease pathology, and drug screening.” CRISPR editing, chemical induction methods, and machine learning, along with high-throughput screening, add additional layers of precision to stem cell manipulation, according to Kevin Su, Scientist at MilliporeSigma.

The possibilities seem endless, but scientists must be vigilant when working with stem cells, especially iPSCs, or risk deceptive data and a loss of resources. Here, we’ll look at how to improve differentiation outcomes by embracing fundamentals while leveraging the latest developments.

The endless potential and frustration of iPSCs

“iPSC technology allows the reprogramming of adult cells, such as skin and blood cells, into an embryonic stem cell-like state. This allows for the generation of patient-specific cells for disease modeling, drug screening, and potential use in regenerative medicine,” explains Su. While a conduit rife with potential, these cells can be challenging and require rigorous attention. “iPSCs can be tricky to work with as they show significant variability in their ability to differentiate into specific cell types, especially between cell lines,” notes Lewis.

iPSCs run the risk of contamination, from either undifferentiated cells or other cell types. Su says this gives reason for concern when it comes to use in the clinic, as tumor formation, or other tissue development, down the line following transplantation could occur. Similarly, genetic and epigenetic changes during the cellular reprogramming process and prolonged time in culture can affect the ability to differentiate. There can also be issues related to immunogenicity and host compatibility, adds Su.

Long explains that it can be difficult to discern the stage of the cells during the differentiation process. And both she and Lewis agree, compared to their in vivo counterparts, full functional maturation may not occur and they may not be able to perform the same cellular tasks, particularly with differentiated cells derived from iPSCs.

“While iPSCs hold great promise for modeling diseases, some complex conditions may not be fully recapitulated in vitro due to the lack of the complete cellular environment and interactions that occur in living organisms,” underscores Lewis. But this does not discount iPSC value in biomedical research. Long points out the immense potential iPSCs hold, notably “through the generation of 3D organoids that closely mimic organ structure and function.” These include engineered heart tissue cells (EHTs), lung models, intestinal organoids, and mini-brains for neurological studies.

Tips for improving cell culture experiments

With stem cell differentiation, particularly iPSCs, combining optimization and quality control is the best chance for success. Experts advise honing in on cell culture basics and tweaking differentiation conditions and materials where needed. For instance, avoid over-passaging iPSCs: “Although, in theory, iPSCs have the potential to self-renew indefinitely, this often leads to chromosomal instability which can significantly affect the differentiation potential of these cells,” cautions Sreethu Sankar, Project Manager at Proteintech.

Fong Cheng Pan, Ph.D., Scientist at MilliporeSigma, distills down some key tips when working with iPSCs; she advises against “mishandling iPSCs by over-pipetting or using harsh dissociation reagents, inconsistent maintenance of high-quality iPSC cultures for seeding and differentiation, initiating differentiation before iPSCs reach optimal confluency, using growth factors from different batches with variable concentrations and activities across lots and suppliers, and utilizing various batches of Matrigel for coating, as each batch exhibits differences in extracellular matrix concentration.”

Long advises spending time prior to culturing to carefully select the optimal somatic source for iPSCs, and considering their reprogramming ease and efficiency in response to transcription factors. Once decided, attention should shift to “assess[ing] cell health, morphology, gene expression, and functionality.”

Consistent culture conditions are also key. “Cell culture media, extracellular matrix, and growth factor quality drives how successful the differentiation is,” notes Sankar. For example, using a chemically defined media, such as NutriStem^® hPSC XF Medium from Sartorius, will alleviate the variability and contamination risk that comes from using animal-derived products. Lewis comments Sartorius offers only research-use non-animal derived cytokines and growth factors.

“Defined media formulations containing only well-characterized components facilitate standardization, scalability, and regulatory compliance, particularly for clinical applications,” says Pan.

Sankar thinks the importance of growth factors is often overlooked. “For instance, growth factors derived from bacterial cells will not have the right post-translational modifications such as glycosylation to give the best bioactivity or stability in cell culture medium.” Sankar suggests trying Proteintech’s HumanKind growth factors derived from human embryonic kidney (HEK293) cells, so they have the correct human modifications. “This often leads to a greater cell differentiation”

There are limits to what the right media conditions can do, however. “In some instances, we find that differentiation of stem cells in a 3D environment may be necessary to provide the cells with all the cues required for a particular cell type,” informs Lewis. In this case, “advanced biomaterials that better mimic the natural extracellular matrix can provide the necessary mechanical and biochemical conditions for effective differentiation. These 3D culture systems can enhance cell-cell and cell-matrix interactions, which are crucial for proper differentiation and maturation.”

Determining the extent of differentiation, as well as the purity of the population (and validating it consists of the correct cell type) requires reliable quality control processes. Lewis suggests using the live-cell imaging system, Incucyte^® Live-Cell Analysis Platform, for morphological analysis “throughout the differentiation process and [to] help us make on-the-fly decisions based on real-time data,” as well as marker expression analysis using the iQue^® Advanced Flow Cytometry Platform. Long explains immunofluorescence and immunohistochemistry are often used to visualize specific markers of differentiated cells, and gene expression following CRISPR editing. Gene expression analysis through methods like real-time PCR “provides insights into differentiation and stem cell lineage,” and “western blotting offers easy confirmation of protein production and can even be multiplexed for key stem cell factors such as SOX2, NANOG, and CD44.”

Long also stresses the importance of functional analysis to gauge activity states of differentiated cells, such as enzyme assays for secreted proteins, electrophysiology, calcium imaging and drug responsiveness assays. “These methods collectively enable a comprehensive assessment of stem cell differentiation outcomes and aid in refining differentiation protocols for various applications.”

Su advises that researchers look at both the population and single-cell level. “Multi-omics approaches, including genomics, transcriptomics, proteomics, and metabolomics, enable comprehensive profiling of cellular states and dynamics during differentiation. Systems-level analyses provide valuable insights into the molecular mechanisms underlying cell fate decisions and identify key regulators and signaling pathways that can be targeted to modulate differentiation trajectories.” Meanwhile single-cell RNA sequencing and single-cell mass spectroscopy can characterize “cellular heterogeneity and lineage trajectories at high resolution. By profiling individual cells throughout the differentiation process, scientists can identify rare cell populations, transitional states, and regulatory networks governing cell fate determination, facilitating the optimization of differentiation protocols.”

In addition, the use of machine learning and data integration with large-scale datasets, omics profiling, and imaging analysis, can “uncover complex patterns, correlations, and hidden variables in multidimensional data, guiding the design of more effective differentiation protocols,” continues Su.

To the future and beyond

In addition to AI and machine learning, newer technology is further advancing stem cell differentiation protocols. ATAC-Seq is a powerful tool for evaluating stem cell differentiation, according to Long. “By using Tn5 transposase to cut the open chromatin regions and insert sequencing adaptors, it generates ready-to-sequence libraries and provides information in differentially accessible chromatin regions, which can help identify unique cell subsets and states during stem cell differentiation.” She points out that while RNA-Seq focuses on gene expression, ATAC-Seq provides insights into the fate of genes. Thus, integrating both datasets can unveil gene regulation mechanisms and cellular heterogeneity during cell development. Long also notes that bioactive-carrying extracellular vesicles (EVs) such as exosomes and microvesicles may be another avenue through which scientists can modulate differentiation. “Harnessing the therapeutic potential of EVs or engineering stem cells to enhance EV secretion and cargo delivery may serve as a novel strategy to improve differentiation outcomes and promote tissue regeneration in regenerative medicine applications.”

Lewis believes scientists shouldn’t shy away from new technology: “Being flexible and innovative is, after all, part of being a scientist, even though it can be hard to find the time to improve established systems.” Luckily, with the right technology, this can be made a lot easier.

Stem Cell Differentiation Tips

• Optimization 
  • Focus on cell culture basics and tweak differentiation conditions and materials
  • Avoid over-passaging iPSCs to prevent chromosomal instability
  • Handle iPSCs carefully to avoid mishandling during culture and differentiation
  • Consistently maintain high-quality iPSC cultures for seeding and differentiation
• Selection of Somatic Source and Assessment
  • Carefully select the optimal somatic source for iPSCs
  • Assess cell health, morphology, gene expression, and functionality before culturing
• Culture Conditions
  • Utilize chemically defined media to alleviate variability and contamination risks
  • Ensure consistent culture conditions with well-characterized components
  • Pay attention to the quality of growth factors, ensuring they have the correct human modifications
• Advanced Biomaterials and 3D Culture Systems
  • Employ advanced biomaterials for effective differentiation
  • Use 3D culture systems to enhance cell-cell and cell-matrix interactions
• Quality Control Processes
  • Employ live-cell imaging and flow cytometry for morphological and marker expression analysis
  • Utilize immunofluorescence, immunohistochemistry, and gene expression analysis for validation
  • Conduct functional analysis to gauge activity states of differentiated cells
• Multi-Omics Approaches
  • Employ genomics, transcriptomics, proteomics, and metabolomics for comprehensive profiling
  • Utilize systems-level analyses to identify key regulators and signaling pathways
• Single-Cell Analysis
  • Employ single-cell RNA sequencing and mass spectroscopy to characterize cellular heterogeneity and lineage trajectories
  • Profile individual cells throughout the differentiation process to optimize protocols
• Integration with AI and Machine Learning
  • Use machine learning and data integration to uncover complex patterns and correlations
  • Guide the design of more effective differentiation protocols
• Advancements in Technology
  • Utilize ATAC-Seq for evaluating epigenetic mechanisms such as chromatin accessibility
  • Explore bioactive-carrying extracellular vesicles (EVs) for modulating differentiation and tissue regeneration
  • Embrace new technology to improve established systems, with a focus on flexibility and innovation