Proteins and small molecules for cellular regenerative medicine

Abstract

Regenerative medicine seeks to understand tissue development and homeostasis and build on that knowledge to enhance regeneration of injured tissues. By replenishing lost functional tissues and cells, regenerative medicine could change the treatment paradigm for a broad range of degenerative and ischemic diseases. Multipotent cells hold promise as potential building blocks for regenerating lost tissues, but successful tissue regeneration will depend on comprehensive control of multipotent cells-differentiation into a target cell type, delivery to a desired tissue, and integration into a durable functional structure. At each step of this process, proteins and small molecules provide essential signals and, in some cases, may themselves act as effective therapies. Identifying these signals is thus a fundamental goal of regenerative medicine. In this review we discuss current progress using proteins and small molecules to regulate tissue regeneration, both in combination with cellular therapies and as monotherapy.

Figure 1.

Proteins and small molecules contribute to multiple approaches to cellular regenerative medicine. This cartoon depicts strategies for organ regeneration using the example of the heart following a myocardial infarction. Approaches to restore heart function using regenerative therapies include 1) transplantation of stem cells, 2) transplantation of differentiated cells produced from stem cells ex vivo, 3) recruitment of stem cells to the site of injury, 4) proliferation of adult cardiomyocytes, and 5) transdifferentiation of other somatic cells into cardiomyocytes. As detailed in the main text, proteins and small molecules can play a key role in each of these approaches.

Figure 2.

Transforming growth factor (TGF)-β family signaling regulates cell fate decisions. Ligands of the TGF-β superfamily (including bone morphogenetic proteins, activin, and nodal) bind to surface receptors that phosphorylate ligand-specific SMAD (SMA/mothers against decapentaplegic) proteins (SMAD2/3 or Smad1/5/8). These Smad proteins then form a complex with SMAD4 and translocate to the nucleus to activate gene expression. The small molecules of the IDE (induce definitive endoderm) family activate phosphorylation of SMAD2/3 and increase signaling by activin and nodal. Mouse embryonic stem cells treated with IDE become definitive endoderm. The small molecule dorsomorphin inhibits phosphorylation of SMAD1/5/8 and decreases signaling by bone morphogenetic protein ligands. Treatment of mouse embryonic stem cells with dorsomorphin promotes differentiation into cardiomyocytes. Pharmacological modulation of TGF-β signaling control can specify stem cell fate.

Figure 3.

Wnt signaling regulates fate decisions in embryonic stem cells. A: soluble Wnt ligand binds to the frizzled receptor and lipoprotein receptor-related protein (LRP) coreceptor. This promotes translocation of β-catenin to the nucleus and activation of gene expression. Glycogen synthase kinase 3β (GSK3β) inhibits β-catenin activity by promoting its degradation. The small molecule 6-bromoindirubin-3′-oxime (BIO) inhibits GSK3β and thus increases the activity of β-catenin.

Figure 4.

Stromal cell-derived factor (SDF-1) recruits stem cells to sites of injury and is regulated by degradation. SDF-1 binds to the cell-surface receptor CXCR4 (C-X-C chemokine receptor 4) on embryonic and hematopoietic stem cells to recruit these cells to sites of injury. SDF-1 is degraded by extracellular peptidases such as dipeptidyl peptidase IV (DDPIV). Inhibition of DPPIV with diprotin A decreases SDF-1 degradation and increases SDF-1 activity. Similarly, engineering of SDF-1 to be resistant to protease activity (SDF-1*) increases its activity. SDF-1 plays an important role in homing of stem cells, and its activity is increased by pharmacological and protein engineering strategies to reduce its degradation.