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Antibodies: Replacing Reprogramming Factors and Exploring Pluripotency

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Review of “Replacing reprogramming factors with antibodies selected from combinatorial antibody libraries” from Nature Biotechnology by Stuart P. Atkinson

Following the first report of somatic cell reprogramming [1], new studies have reported a multitude of new and improved means to generate induced pluripotent stem cells (iPSCs). More modern methodologies have replaced the exogenous expression of transcription factors with small-molecule drug cocktails; however, this strategy may result in unwanted off-target effects [2, 3] and, therefore, there still exists room for improvements.

In a new Nature Biotechnology study, a team of researchers from the laboratory of Kristin K Baldwin (Scripps Research Institute, La Jolla, California, USA) report on an exciting new reprogramming strategy that has permitted the exploration of the inner workings of pluripotency: triggering membrane-to-nucleus signaling pathways employing antibodies [4].

Blanchard et al. sought to discover if specific antibodies could replace any of the traditional factors (Oct4, Sox2, Klf4 and c-Myc [OSKM]) exploited to reprogram mouse embryonic fibroblasts. To screen immense numbers (~100 million) of antibodies, the study employed integrative lentivi¬ral libraries encoding secreted or membrane-tethered single-chain antibodies [5]. If a colony of iPSCs arises during reprogramming, a simple PCR experiment provides the identity of the reprogramming-enhancing antibody in question.

Excitingly, screening experiments identified several antibodies that replaced either Sox2 and c-Myc or Oct4, and the authors chose to study a Sox2-replacing antibody (SoxAb2) in the hope of uncovering a novel path¬way to pluripotency. In-depth analyses established that SoxAb2 bound to and antagonized Basp1 (brain acid soluble protein 1), a calmodulin-binding membrane-associated protein not previously implicated in pluripotency. In agreement with this finding, shRNA-mediated inhibition of Basp1 expression promoted reprogramming and highlighted a novel pluripotency-associated pathway.

So how does Basp1 inhibition promote reprogramming? Exhaustive investigation discovered that Basp1 normally represses Wilm’s tumor suppressor 1 (Wt1) activity in fibroblasts; however, Basp1 inhibition (by SoxAb2 or Basp1 shRNA) increases nuclear Wt1 activity and induces the expression of two widely accepted pluripotency-associated factors (Lin28a and Esrrb) before Sox2 expression.

Overall, this exciting study demonstrates the feasibility of antibodies as reprogramming factors and tools to explore pluripotency. The authors note that the future may bring iPSC reprogramming solely with specific engineered antibodies, offering advantages such as synchronous reprogramming, reduced off-target effects, reduced genome instability, and improved safety and reproducibility. 

To keep in touch with antibody-mediated reprogramming and new pluripotency networks, stay tuned to the Stem Cells Portal.

References

  1. Takahashi K and Yamanaka S, Induction of Pluripotent Stem Cells from Mouse Embryonic and Adult Fibroblast Cultures by Defined Factors. Cell;126:663-676.
  2. Arkin MR, Tang Y, and Wells JA, Small-molecule inhibitors of protein-protein interactions: progressing toward the reality. Chem Biol 2014;21:1102-14.
  3. Laraia L, McKenzie G, Spring DR, et al., Overcoming Chemical, Biological, and Computational Challenges in the Development of Inhibitors Targeting Protein-Protein Interactions. Chem Biol 2015;22:689-703.
  4. Blanchard JW, Xie J, El-Mecharrafie N, et al., Replacing reprogramming factors with antibodies selected from combinatorial antibody libraries. Nat Biotech 2017;35:960-968.
  5. Zhang H, Yea K, Xie J, et al., Selecting agonists from single cells infected with combinatorial antibody libraries. Chem Biol 2013;20:734-41.