The application of umbilical cord‐derived MSCs in cardiovascular diseases

Abstract Transplantation of stem cells is a promising, emerging treatment for cardiovascular diseases in the modern era. Mesenchymal stem cells (MSCs) derived from the umbilical cord are one of the most promising cell sources because of their capacity for differentiation into cardiomyocytes, endothelial cells and vascular smooth muscle cells in vitro/in vivo. In addition, umbilical cord‐derived MSCs (UC‐MSCs) secrete many effective molecules regulating apoptosis, fibrosis and neovascularization. Another important and specific characteristic of UC‐MSCs is their low immunogenicity and immunomodulatory properties. However, the application of UC‐MSCs still faces some challenges, such as low survivability and tissue retention in a harmful disease environment. Gene engineering and pharmacological studies have been implemented to overcome these difficulties. In this review, we summarize the differentiation ability, secretion function, immunoregulatory properties and preclinical/clinical studies of UC‐MSCs, highlighting the advantages of UC‐MSCs for the treatment of cardiovascular diseases.

neonatal tissues and have been intensively investigated for their use in regenerative medicine. MSCs are widely used in clinical practice because they have fewer ethical issues and safety concerns than other stem cells such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). 14 Researchers have established many robust, practical, reproducible and low-cost protocols for MSC isolation and storage. [15][16][17][18][19] One kind of MSC widely used in regeneration medicine is umbilical cord-derived MSCs (UC-MSCs). The umbilical cord is a perinatal tissue containing 2 umbilical arteries and 1 umbilical vein, which are embedded with Wharton's jelly. MSCs can be isolated from the umbilical cord by enzymatic digestion methods. 20 MSCs derived from UC/Wharton's jelly can differentiate into three germ layers and migrate to damaged tissue. 21,22 A study conducted by Vidal et al. demonstrated that the umbilical cord may be preferable for MSC banking purposes in research and tissue engineering compared with bone marrow. 23 In addition, UC-MSCs present a more primitive phenotype and exhibit longer telomeres, which enables them to achieve a greater proliferative ability and less cellular senescence. 24 However, stem cell transplantation still faces challenges due to the low ability of some features, including the viability of cells, hom-

| D IFFERENTIATI ON AB ILIT Y OF UC-MSC S
One of the most important factors leading to CVDs is the loss of a large number of cardiomyocytes and their limited intrinsic capacity to regenerate damaged tissue. Cardiomyocytes are essential for ensuring the contraction of the chambers and efficient blood flow throughout the body. 25,26 Vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) are the major cell types in blood vessels. 27 UC-MSCs play an important role in cardiac regenerative medicine due to their ability to differentiate into cardiomyocytes, VSMCs and ECs, which are summarized in (Figure 1). [28][29][30] We also summarize the studies that induced UC-MSCs into cardiovascular lineages in Table 1, which will be described in the following paragraphs.

| INDUC TI ON OF UC-MSC S INTO C ARDIOMYOC Y TE S
MSCs, as an alternative cell source for use in clinic, have been investigated for use in regenerative medicine since their initial description in 1995. 31 The safety and efficacy of MSCs have been shown in several studies on cardiac regeneration. 28,29 In the cardiomyocyte induction process, some markers have been identified to indicate the differentiation stage: initiation or maturity. NKX2.5, myocyte enhancer factor 2C (Mef2c) and GATA4 as early differentiation markers indicate the initiation of cardiomyocyte differentiation.
Cardiac troponin T (cTnT), heavy chain cardiac myosin (MYH6) and  38 Szaraz and colleagues cocultured UC-MSCs with rat cardiomyocytes, which were used as a feeder layer, and they induced s spontaneous contraction of cardiomyocytes from human UC-MSCs. They found higher expression of the Cx43 gene and Mef2c protein in UC-MSCs than BMSCs. Cardiomyocytespecific regulatory and structural protein genes such as cTnT and MY6H were also upregulated in the coculture system. Inspiringly, the authors observed spontaneous contraction within 1 week. 38 BMSCs formed myotube structures in two to three days and spontaneous contractions were observed in five to seven days with MyoD-positive muscle markers in vitro. 39 The above results reinforce the concept that UC-MSCs are a novel and promising source of autologous cells for cardiac tissue engineering.

| PAR ACRINE EFFEC TS OF UC-MSC S
Although UC-MSCs differentiate into cardiovascular-related cells, their ability and specialized target cell numbers are limited. It is widely accepted that the majority of UC-MSC-derived myocardial recovery in vivo depends on paracrine effect. 40,41 UC-MSCs showed higher TGF-β3 and hepatocyte growth factor (HGF) gene expression than BMSCs, which greatly influences the paracrine effects of UC-MSCs. 33 The paracrine effects of UC-MSCs, including neovascularization and antiapoptosis effects, anti-fibrosis effects and immunoregulatory effects, are summarized in ( Figure 2) and will be described in the following paragraphs (Table 2).

| NEOVA SCUL ARIZ ATION AND ANTIAP OP TOS IS EFFEC TS
Hypoxia-ischaemia and apoptosis of cardiomyocytes are common situations that occur in CVDs. Neovascularization is the process by which new vascular structures assemble that support blood and oxygen for the heart. Vasculogenesis occurring exclusively during embryogenesis and angiogenesis in adults is now recognized as two primary mechanisms that contribute to neovascularization in a single microenvironment. 45  Other studies have shown that MSCs constitutively express COX1, COX2, TGFβ and IL-10 and release a large amount of PGE2 to inhibit the proliferation of splenocytes. 57,58 From the results above, it is speculated that the mechanisms used by UC-MSCs seem to be paracrine effects.

| CULTURE AND MOD IFI C ATI ON OF UC-MSC S
It has been demonstrated that MSCs derived from perinatal tissues have a high proliferative, antiapoptosis and anti-senescence function, making them more suitable for use in the clinic. 59 To enhance the survival, paracrine and therapeutic effects of UC-MSCs, a series of studies was conducted, as summarized in Table 3 and presented in the following paragraphs ( Figure 2).

| PROB LEMS AND PROS PEC TS
The major mechanism of MSC transplantation for myocardial repair is the paracrine effect, and the immunomodulation effect is one of the most important specific characteristics of UC-MSCs, which is more significant than that of other tissue-derived MSCs. Many

CO N FLI C T O F I NTE R E S T
The authors have declared that no competing interest exists.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no new data were created or analyzed in this study.