Mapping current research and identifying hotspots on mesenchymal stem cells in cardiovascular disease

Mesenchymal stem cells (MSCs) have important research value and broad application prospects in the cardiovascular disease. This study provides information on the latest progress, evolutionary path, frontier research hotspots, and future research developmental trends in this field. A knowledge map was generated by CiteSpace and VOSviewer analysis software based on data obtained from the literature on MSCs in the cardiovascular field. The USA and China ranked at the top in terms of the percentage of articles, accounting for 34.306% and 28.550%, respectively. The institution with the highest number of research publications in this field was the University of Miami, followed by the Chinese Academy of Medical Sciences and Harvard University. The research institution with the highest ACI value was Harvard University, followed by the Mayo Clinic and the University of Cincinnati. The top three subjects in terms of the number of published articles were cell biology, cardiovascular system cardiology, and research experimental medicine. The journal with the most publications in this field was Circulation Research, followed by Scientific Reports and Biomaterials. The direction of research on MSCs in the cardiovascular system was divided into four parts: (1) tissue engineering, scaffolds, and extracellular matrix research; (2) cell transplantation, differentiation, proliferation, and signal transduction pathway research; (3) assessment of the efficacy of stem cells from different sources and administration methods in the treatment of acute myocardial infarction, myocardial hypertrophy, and heart failure; and (4) exosomes and extracellular vesicles research. Tissue research is the hotspot and frontier in this field. MSC research has presented a gradual upward trend in the cardiovascular field. Multidisciplinary intersection is a characteristic of this field. Engineering and materials disciplines are particularly valued and have received attention from researchers. The progress in multidisciplinary research will provide motivation and technical support for the development of this field.


Introduction
Mesenchymal stem cells (MSCs) are widely perceived as a class of adult pluripotent stem cells with multiple differentiation potentials, which derived from mesoderm and neuroectoderm and do not express hematopoieticrelated markers [1]. The important biological characteristics of MSCs include its low level of expression of human leukocyte antigen class I molecules and CD40, CD40 ligand, CD80, or CD86, which is required to induce effector T cells [2]. MSCs have low immunogenicity and immunoregulatory effects, which can affect every cell of the immune system through cell-cell interactions and paracrine effects [3]. Based on these special biological characteristics, MSCs have important research value and broad application prospects.
Recent studies have shown that the use of MSCs has made great progress in cardiovascular basic and clinical research. MSCs induce the differentiation of cardiomyocytes and vascular endothelial cells. Bonnet found that BMSCs induced in vitro can express platelet-derived growth factor receptor (PDGFR), smooth muscle myosin heavy chain 11 (SMMHC11), and myoglobulin light chain 2 (MLC2), which is similar to that observed in aortic smooth muscle cells, and BMSCs have similar electrophysiological activity and contraction ability [4]. MSCs inhibit myocardial fibrosis; for example, MSC exosomes reduce fibrosis of the heart by inhibiting the proliferation of fibroblasts, promoting the synthesis of metalloproteinases, and stimulating angiogenesis in the infarct area [5]. MSCs also promote angiogenesis. Rahbarghazi transplanted MSCs into infarcted myocardium in rabbits and found that the surrounding area of infarcted myocardium mainly differentiated into cardiomyocytes, endothelial cells, and smooth muscle cells, and the microvessel density significantly increased [6]. MSCs can effectively treat myocardial infarction, dilated cardiomyopathy, heart failure, and other conditions. For example, Lee et al. proved that intravenous injection of bone marrow MSCs was safe, mild, and effective and had a long-lasting effect [7]. Chin et al. confirmed that autologous bone marrow-derived MSCs were safe and effective in treating DCM [8]. Bartunek et al. found that MSC therapy did not produce myocardial toxicity, which significantly increased the left ventricular ejection fraction, reduced the end-systolic volume, and increased the 6-min walking distance of HF patients [9].
Bibliometrics uses the literature system and bibliometric characteristics as the research object and conducts quantitative and qualitative analysis of the literature [10]. It allows the quantitative measurement of the profile distribution as well as the relationships and clustering of studies [11]. In addition to describing and predicting the development of a particular research area, this type of analysis can also compare the contributions of different countries, institutions, journals, and scholars [12]. This type of analysis technology is playing an increasingly important role in developing guidelines and evaluating research trends [13]. Many scholars have used this method of literature analysis in various fields of medicine, such as spinal surgery research [14], health information research [15], biological signaling molecule research [16], neurogenetics research [17], and endocrine disease research [18].
This research is based on data regarding the literature on MSCs in the cardiovascular field, which uses Cite-Space and VOSviewer to form a corresponding knowledge map and recognize the knowledge base. The study provides information on the latest progress, evolutionary path, frontier research hotspots, and future research developmental trends in this field.

Data collection
SCI-E and SSCI of the core database of the document information index database Web of Science were selected as the target databases for source document retrieval. The search formula was set to TS = ("cardiovascular" OR "heart" OR "circulation") AND TS = (mesenchymal stem cells), and the dates of the search were from January 1, 2010, to March 31, 2020, which resulted in a total of 3455 records. There were 8 types of documents among the 3455 records. As shown in Table 1, there were 2380 articles, which accounted for 72.187% of the total number of records, making articles the most common type of literature. Reviews ranked second, as there were 755 reviews, accounting for 22.900% of the total. The other 8 document types were meeting abstracts (98), editorial materials (55), book chapters (35), proceedings papers (32), early access (20), letters (5), corrections (3), and news items (1).

Data analysis
VOSviewer and CiteSpace were used to analyze the 3384 exported articles. VOSviewer constructs a map based on the cooccurrence matrix. The construction of the map is a three-step process. In the first step, the similarity matrix is calculated based on the cooccurrence matrix. In the second step, the VOS mapping technique is applied to the similarity matrix to construct a map. Finally, in the third step, the map is translated, rotated, and reflected [19]. The term cooccurrence graph in VOSviewer only includes terms that appear in the title and are abstracted at least 50 times under the binary count [19]. The purpose of the algorithm is to ensure that terms that occur more frequently have larger bubble images and that terms with high similarity are close to each other [20]. CiteSpace is a web-based Java application that focuses on detecting and analyzing the evolution of research frontiers and the relationship between research frontiers and their knowledge base. CiteSpace also examines the internal connections between different research frontiers [21]. It is used to capture keywords associated with strong citation bursts, which can be used as predictors of research frontiers.

Results
Temporal distribution map of the literature  Table 2, the number of articles published by the USA and China placed them at the top of the ranking, as each accounted for 34.306% and 28.550% of the total, respectively. The total number of studies conducted by both countries comprised more than half of the total, indicating that the two countries had high research interest in this field. The top three countries in terms of ACI values were the Netherlands (40.2288), Spain (34.2771), and the USA (32.5757), suggesting that these three countries had started to conduct research in this field earlier than other countries and that their research results were more mature.

As shown in
As shown in Fig. 2, countries with close cooperation can be mainly divided into three types. The green part: the USA and China showed the greatest cooperation with South Korea, Japan, Canada, and Australia. The brown part: Germany and Italy worked more closely with England, Netherlands, France, Switzerland, and Spain. The blue part: Singapore contacted with New  Zealand and Portugal closely, and Malaysia worked more frequently with India.

Distribution of authors and research institutions
As shown in Table 3 As shown in Table 4, the institution with the highest number of research publications in this field is the University of Miami with a quantity of 73, followed by the Chinese Academy of Medical Sciences with a quantity of 55 and Harvard University with a quantity of 52. The research institution with the highest ACI value in  Fig. 4, different colors mean clusters of intimate relationship. The University of Miami cooperated closely with Harvard University, Pittsburgh University, and Zhejiang University. Sun Yat-sen University cooperated closely with Fudan University, and so on. Table 5, the top three disciplines in terms of the number of published articles were cell biology (30.664%), cardiovascular system cardiology (20.534%), and research experimental medicine (20.140%). Additional disciplines represented in the literature were engineering (9.827%), materials science (9.160%), biochemistry and molecular biology (8.280%), biotechnology and applied microbiology (7.158%), pharmacology and pharmacy (6.946%), transplantation (4.974%), chemistry (3.063%), and other disciplines, indicating that the research performed in this field was broad and that the research methods were diverse.

As shown in
As shown in Table 6, the journal with the highest number of articles in this field was Circulation Research, followed by Scientific Reports (42)

Highly cited literature analysis
As shown in Table 7, the article "Pericytes: Developmental, Physiological, and Pathological Perspectives, Problems, and Promises" was cited the most often. Armulik discussed the important roles of pericytes in the processes of organismal development and vascular homeostasis and their relationship with MSCs [22]. The second most cited article was "Comparison of Allogeneic vs Autologous Bone Marrow-Derived Mesenchymal Stem Cells Delivered by Transendocardial Injection in Patients with Ischemic Cardiomyopathy: The POSEIDON Randomized Trial". In this article, Hare et al. confirmed that intracardiac injection of allogeneic and autologous MSCs could treat ischemic cardiomyopathy effectively and relatively safely [23].
The third most cited article was "Whole-Organ Tissue Engineering: Decellularization and Recellularization of Three-Dimensional Matrix". In this article, Badylak explained that the combination of three-dimensional bioscaffold materials and cell transplantation was a promising tissue engineering strategy and a method for  the regeneration of functional organs for medical replacement [24]. The above articles could be regarded as constituting an important theoretical basis and providing clinical evidence for research in this field. According to the types of the articles, 8 of the most highly cited articles were reviews, and 7 were monographs. Based on the publication dates of the most highly cited articles, the most highly cited articles were published from 2011 to 2013, followed by 2015 to 2016. These periods can be regarded as representing the two stages of the development of this field. Based on the numbers of cooperating institutions and countries, there were 10 articles involving more than three institutions and 6 articles involving groups in at least two countries.

Research hotspots and frontier analysis Research hotspot analysis
Keywords reflect the core content of the article and can be used to identify the evolving research frontiers related to the knowledge field [25]. As shown in Table 8, in addition to mesenchymal stem cells and heart, the keywords with a high frequency of occurrence were heart transplantation (582), differentiation (535), myocardial infarction (482), in vitro (473), therapy (472), and progenitor cells (458).  As shown in Fig. 5, in the keyword cooccurrence network map, the thicker the connection between the nodes is, the more frequently the two keywords appear together. The keywords formed 4 clusters, which represented the four major research directions in the field.

Combined evolutionary path
In Fig. 6, the year corresponding to each of the keywords is the first year it appeared in the analyzed data set. The transformation between nodes could reveal the evolution of MSCs in the cardiovascular research hotspot. From 2010 to 2012, cardiovascular MSC research began to focus on apoptosis, left ventricular function, proliferation, bone marrow cells, and endothelial cells. In 2013-2015, endothelial progenitor cells, extracellular matrix, ischemic cardiomyopathy, and tissue received increased attention in the field. From 2016 to 2017, the field turned to research on injury and oxidative stress. Tissue engineering, exosomes, and inflammation became the new focus in 2018-2020.

Research frontier identification
In Table 9, the timeline is depicted as a blue line, while burst detection is shown as a red segment on the blue timeline that indicates the start year, end year, and duration of the burst. In particular, we are interested in the key words with research significance, which reflect the evolutionary trend of this field.
Endothelial progenitor cells showed the strongest burst strength, followed by ischemic cardiomyopathy, cardiovascular disease, and endothelial cells. The terms progenitor cells, marrow stromal cells, and heart function appeared for the first time recently but persisted for a shorter period of time. The burst times of the terms delivery, ischemic heart, and ischemic cardiomyopathy were consistent. Tissue is the current research frontier in this field and is currently within the burst period.

Discussion
This paper used the information visualization software CiteSpace and VOSviewer to carry out bibliometric analysis of the literature on MSCs in the cardiovascular field published from 2000 to 2020. The analysis assessed the spatial and temporal distribution, author contribution, core literature, research hotspots, and research frontier analysis. We used keyword cooccurrence analysis to (1) The research of MSCs in the cardiovascular field showed a zigzag upward trend.
Stem cell therapy has great potential for use in future regenerative medicine treatment; however, it has some risks and limitations, such as the type of stem cells used, cell proliferation ability, differentiation status, drug delivery route, drug delivery site, and the ability for survival of transplanted cells, which will affect the therapeutic effect. The therapeutic characteristics, medical ethics, and possible teratogenicity have made the study of MSCs highly controversial [74]. For example, Hegyi et al. proposed that MSCs might form primary cardiac sarcomas and develop into tumors with multiple lineage differentiation [75]. Huang et al. reported that transplantation and differentiation of MSCs led to progressive ventricular dysfunction and other diseases [76]. In view of the risks of research and the limitations of technology, researchers are cautious in carrying out the research work.
(2) The study of cardiovascular MSCs in Europe and America started first, and Asian countries have paid increasing attention to this area of research.
European and American countries started research in this field earlier than other countries, and their research is therefore more mature. For example, Harvard University, the Mayo Clinic, and University of Cincinnati in the USA have obtained a large number of high-quality research results. Harvard University has mainly studied 3D microcapsules [77] and engineered three-layer scaffolds [78]. The Mayo Clinic has explored the effect of MSCs on left ventricular assist device (LVAD) implantation [79], left ventricular remodeling [80], and heart failure [80] through clinical trials while developing microfiber stents [81] and vascular biomaterials [82]. The paracrine effect of nuclear casein kinase on MSC and MSCderived extracellular vesicles (EVs) has been studied [83]. The University of Cincinnati explored the cardiac protection mechanism of paracrine MSCs, which involves iPS cells (MiPS) [84], the Wnt11 signaling pathway [85], CXCR4 factor expression [86], the suicide gene [87], and the clusterin Akt/GATA-4 pathway [88]. Table 9 Top 15 keywords with the strongest citation bursts For the past decade, Asian countries have also paid attention to MSC research. For example, the Chinese Academy of Medical Sciences and Sun Yat-sen University in China are the leading research institutions with high achievements in the field. The Chinese Academy of Medical Sciences examined the time-distribution characteristics of MSCs in the myocardium and other organs [89]. The mechanism by which statins and Chinese medicines regulate SDF-1/CXCR4 [90], JAK-STAT [91], RhoA/ROCK [48], and AMPK/eNOS [92] in MSCs has been studied. The Chinese Academy of Medical Sciences also explored the improvement of cardiac function by MSCs regulated by matrix-derived factor 1 (SDF1a) [93] and CXC chemokine receptor 4 (CXCR4) [94]. Sun Yatsen University researched the effects of exons (Exo) [95], the TGF-β superfamily [96], the long noncoding RNA brave heart (lncRNA-Bvht) [97], apelin [98], and granulocyte colony stimulating factor (G-CSF) [99] on the proliferation, differentiation, and vascularization of MSCs. The functions of the transcription factors islet-1 (ISL1) [100] and platelet fibrin (PRF) [101] in regulating the repair of myocardial infarction by MSCs have been explored. Although basic research on cardiovascular stem cells in China is at the forefront of efforts around the world, progress on clinical studies in China is stagnant, which may be related to the lack of efficient scientific approval systems and strict regulatory policies.
(3) Multidisciplinary intersection provides power and technical support for the development of this field.
The literature published in this field is mainly focused on cell biology, cardiovascular system cardiology, and research experimental medicine, as well as engineering, materials science, chemistry, biophysics, and other disciplines, which reflects that multidisciplinary intersection is a characteristic of research in this field. Engineering and materials disciplines, such as bioengineering [102], tissue engineering [103], genetic engineering [104,105], and biomaterials [106], have received special attention from scholars. The development of these related disciplines will aid in breaking through the limitations of the technical conditions of research in this field.
(4) Tissue research is a hot spot and frontier area in this field.
Tissue research refers to tissue repair and tissue engineering. Heart failure caused by ischemic and nonischemic cardiomyopathy is due to the progressive and complex process of myocardial remodeling. Local compensatory changes at the genetic, molecular, cellular, and interstitial levels are accompanied by ventricular dilatation and the impairment of systolic and diastolic function. The consequence is that billions of cardiomyocytes replaced by fibrous tissue, and the cardiomyocytes and vascular cells are severely injured [107]. Although it has been confirmed that the adult heart contains a small number of active circulating cells, resident stem cells, and progenitor cells [108], its inherent ability for selfregeneration is unable to compensate for the loss in cell quality due to heart failure. At present, heart transplantation is the only treatment that can completely restore heart function for patients with advanced heart failure. However, due to the shortage of donors, this treatment is often limited. The need to cell therapy has aroused great interest in regenerative medicine.
MSCs have advantages in differentiating into cardiomyocytes [109], protection of cardiac muscle cells from apoptosis [110], promotion of vascular endothelial cell recovery [111], angiogenesis [112], and myocardial tissue repair [113], which is the hot spot of cardiovascular regeneration medicine research.
Because MSCs have low retention rate and poor survival rate, they need to be combined with various forms of bioactive tissue structure and effectively integrated into target tissue, which in order to regenerate cardiac tissue and improve cardiac function, such as polyurethane patch [114], elastic mold [115] based on type I collagen and matrix gel, fibrin patch [116], and Graphene [117]. Tissue engineering technology is the key strategy to repair myocardial tissue.

Conclusion
MSCs have important research value and broad application prospects in the field of cardiovascular. With the help of information visualization technology, we have obtained a more in-depth understanding of the study progression, evolutionary path, frontier hot spots, and future trends of the research of MSCs in cardiovascular disease over the past 10 years. Multinational cooperation and multidisciplinary intersection are the characteristics and trend of the development in the field, and tissue engineering will be the focus of future research.

Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Ethics approval and consent to participate Not applicable.

Consent for publication
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