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GARP Overexpression During In Vitro Expansion May Provide a Boost for MSC-based Therapies

Review of “GARP is a key molecule for mesenchymal stromal cell responses to TGF‐β and fundamental to control mitochondrial ROS levels” from STEM CELLS Translational Medicine by Stuart P. Atkinson 

The need to in vitro expand mesenchymal stem cells (MSCs) for extended periods of time to generate therapeutically relevant numbers of cells may lead to the loss of proliferative capacity and therapeutically relevant functions; therefore, the optimization of culture conditions remains an important research goal. 

Recently, a team of researchers led by Per Anderson (Hospital Universitario Virgen de las Nieves, Granada, Spain) and Francisco Martin (University of Granada, Spain) reported in a STEM CELLS article that the glycoprotein A repetitions predominant (GARP) type I transmembrane protein [1, 2] binds to the latency‐associated peptide (LAP)/transforming growth factor (TGF)‐β1 complex on the surface of murine adipose-derived MSCs to regulate TGF‐β activation and their immunomodulatory capacity [3]. Furthermore, this study established that GARP silencing prompted a decrease in MSC proliferation, leading the authors to investigate the mechanisms involved and whether GARP overexpression could promote MSC proliferation and survival during long-term culture.

Their new findings, published recently in STEM CELLS Translational Medicine, now suggest that GARP-mediated regulation of TGF‐β activation/signaling can allow MSCs to safely proliferate under stress-inducing conditions, such as those occurring during extended in vitro culture periods [4].

Carrillo‐Gálvez et al. first explored the silencing of GARP in MSCs via the expression of short hairpin RNAs, finding that the modulation of cellular fitness-, cell cycle regulation-, and DNA repair-associated gene expression accompanied the expected decrease in proliferation. Furthermore, they also discovered that GARP loss in MSCs prompted an increase in TGF‐β activation, the induction of basal mitochondrial reactive oxygen species (ROS) generation, and an increase in the levels of DNA damage markers. The authors note that related studies found that TGF‐β1 signaling induced ROS production in murine bone marrow-derived MSCs, resulting in senescence [5] and apoptosis [6]. Interestingly, blocking TGF‐β signaling reduced ROS levels and DNA damage and increased MSC proliferation, thereby suggesting that GARP may function to prevent TGF‐β activation and inhibit the subsequent deleterious consequences.

Next, turning to evaluate GARP overexpression, the authors found that GARP mediated an increase in MSC proliferation and imbued them with increased resistance to DNA damage and apoptosis. However, GARP overexpression also led to an increase in TGF‐β activation when compared to control MSCs, suggesting that both the presence and absence of GARP give rise to TGF‐β responses with diametrically opposing effects on MSCs.

The authors believe that, in the absence of GARP, TGF‐β is secreted, targeted to the extracellular matrix, and activated, while the overexpression of GARP leads to TGF‐β activation at the cell surface in a cell‐cell contact‐dependent manner. They hope to fully decipher this mechanism in future research and delineate any potential roles for the three TGF‐β isoforms (TGF‐β1‐3), which bind to the same receptors but exhibit distinct functions in vivo due to their different tissue distributions and expression levels [7].

To find out more about how GARP overexpression in adipose-derived MSCs may permit for their safe expansion and effective use in regenerative therapies, stay tuned to the Stem Cells Portal!

 

References

  1. Dolan J, Walshe K, Alsbury S, et al., The Extracellular Leucine-Rich Repeat Superfamily; A Comparative Survey and Analysis of Evolutionary Relationships and Expression Patterns. BMC Genomics 2007;8:320.
  2. Ollendorff V, Szepetowski P, Mattei M-G, et al., New Gene in the Homologous Human 11q13–q14 and Mouse 7F Chromosomal Regions. Mammalian Genome 1992;2:195-200.
  3. Carrillo-Galvez AB, Cobo M, Cuevas-Ocaña S, et al., Mesenchymal Stromal Cells Express GARP/LRRC32 on their Surface: Effects on their Biology and Immunomodulatory Capacity. STEM CELLS 2015;33:183-195.
  4. Carrillo-Gálvez AB, Gálvez-Peisl S, González-Correa JE, et al., GARP is a key molecule for mesenchymal stromal cell responses to TGF-β and fundamental to control mitochondrial ROS levels. STEM CELLS Translational Medicine 2020;9:636-650.
  5. Wu J, Niu J, Li X, et al., TGF-β1 Induces Senescence of Bone Marrow Mesenchymal Stem Cells via Increase of Mitochondrial ROS Production. BMC Developmental Biology 2014;14:21.
  6. Zhang F, Ren T, and Wu J, TGF-beta1 Induces Apoptosis of Bone Marrow-derived Mesenchymal Stem Cells via Regulation of Mitochondrial Reactive Oxygen Species Production. Experimental and Therapeutic Medicine 2015;10:1224-1228.
  7. de Araújo Farias V, Carrillo-Gálvez AB, Martín F, et al., TGF-β and Mesenchymal Stromal Cells in Regenerative Medicine, Autoimmunity and Cancer. Cytokine & Growth Factor Reviews 2018;43:25-37.