Abstract

Background

The pathological features of adenomyosis include the presence of active endometrial glands and stroma surrounded by an enlarged and proliferative myometrial layer, accompanied by fibrosis and neovascularization. However, the exact etiology of this distinct pathological condition remains unclear. Pericytes (PCs), commonly referred to as Rouget cells, were initially characterized in the nineteenth century by Eberth and Rouget as flattened cells with protrusions interspersed among endothelial cells (ECs) and the basement membrane of capillaries.^1,2 As these cellular protrusions are closely associated with or integrate into the surface of the endothelial basement membrane, they are essential for modulating blood flow, ensuring vascular permeability, maintaining homeostasis, and aiding the tissue repair process. Given the involvement of PCs in numerous disease-related pathological and physiological processes, their regulation has emerged as a focal point of research. In this review, a comprehensive overview of the current research on the pathophysiology of adenomyosis was presented, with a particular focus on elucidating the potential correlation of and future implications associated with PCs in the development of adenomyosis.

Origin

The exact process of PC formation remains a topic of discussion. Contemporary literature spanning studies on organs such as the brain, ³ thymus, ⁴ stomach, ⁵ lungs, ⁶ and liver ⁷ and structures such as coronary arteries ⁸ postulate that PC development within organs may encompass epithelial–mesenchymal transition (EMT). This results in their migration to participate as mesenchymal constituents along with other cells such as fibroblasts and vascular smooth muscle cells (VSMCs). ⁹ Furthermore, evolving research has delineated the differentiation of PCs from human-induced pluripotent stem cells.^10,11

Classification

PCs are an eclectic ensemble of cells characterized by varied morphologies and protein profiles. Distinct patterns of protuberances were observed by Zimmermann across capillary beds, with circumferential protuberances located near arterioles, longitudinal protuberances located centrally, and stellate protuberances located near venules. The author classified these variations using the PC nomenclature. Based on their specific location within the capillaries, PCs are categorized into the precapillary, midcapillary, and postcapillary subclasses. ¹² Morphologically, PCs can be classified as sheath PCs, reticular PCs, and thin-stranded PCs.^13,14 The distinct protein expression profiles among these subtypes have been determined via antibody labeling and immunofluorescence techniques.^15,16 The organ-centric categorization scheme comprises brain, retinal, muscle, renal, hepatic, placental, and endometrial PCs. For example, brain PCs play a crucial role in the establishment of the blood–brain barrier, ¹⁷ as well as in the development of the central nervous system, maintenance of immune equilibrium, and regulation of white matter function. ¹⁸ Retinal PCs provide structural support to the vascular system by enveloping EC-aligned vessels, thereby increasing stability and tension to facilitate unobstructed blood flow. ¹⁹ Within kidneys, PCs underpin angiogenesis, regulate medullary and cortical blood dynamics, and function as progenitors during renal fibrosis, metamorphosing into interstitial myofibroblasts. ²⁰ In the liver, PCs are referred to as hepatic stellate cells ²¹ and play a pivotal role in liver fibrosis under pathophysiological conditions.

Markers

With advances in microscopy, multicolor flow cytometry, and lineage mapping, our comprehension of PC attributes and potential has expanded. ²² Common markers used to define PCs in the literature are platelet-derived growth factor receptor β (PDGFRβ),^23–25 CD146, ²⁶ neural glial antigen 2 (NG2),^27–30 desmin, ³¹ nestin, SUSD2, ³² regulator of G-protein signaling 5 (RGS5), ³³ and α-smooth muscle actin (αSMA). ³⁴ Meirelles ³⁵ analyzed the gene expression profile of noncultured, highly purified human adipose tissue PCs, and validated the expression of transcripts known to be PC markers by visually comparing their expression levels with those in other cell types; these markers included PDGFRA, PDGFRB, CXCL12, ANPEP, KCNJ8, and ANGPT1, which may also be expressed in other cell types. When acknowledging the limitations of using protein markers to identify specific subtypes or distinguish cellular states, it is important to note that RNA expression levels do not always correlate with protein expression levels within the cell. An unequivocal molecular beacon to singularly identify PCs and distinguish them from vascular smooth muscle or other mesenchymal cells has not yet been identified. For instance, PCs share PDGFR-β expression with VSMCs, neurons, and neural progenitor cells; ³⁶ NG2 expression with VSMCs, oligodendrocyte progenitors, and neuronal progenitors; ³⁷ and α-SMA expression with myofibroblasts, cardiomyocytes, and VSMCs. ³⁸ In light of this, Crisan et al. devised a targeted flow cytometry protocol for identifying human PCs, ³⁹ distinguishing them via the absence of hematopoietic markers (CD45) and endothelial markers (CD31/CD34) and the presence of CD146.

Notably, PCs exhibit substantial morphological and marker heterogeneity across tissues, with all markers exhibiting dynamic expression ³⁹ that is potentially modulated based on developmental trajectories, pathological stimuli, and ex vivo culture. Therefore, in tissue sections and embedded samples, PC identification is contingent upon a pristine tissue morphology, counterstaining for endothelial markers, and combinations of at least two PC markers. ⁹ Crisan's immunohistochemical exploration of human specimens underscored the heterogeneity in NG2 and αSMA expression across perivascular types. ⁴⁰ For instance, pericapillary PCs are characterized as NG2+/αSMA-, venular PCs as NG2-/αSMA+, and arteriolar PCs as NG2+/αSMA+, whereas CD146 and PDGFRβ expression remains uniform across these types.

Stem cell characteristics

PCs can differentiate into different lineages depending on their location and physiological state within various tissues. ⁴¹ These cells have been demonstrated to differentiate into neurons, ⁴² skeletal muscle fibroblasts, adipocytes, ⁴³ follicular dendritic cells, ⁴⁴ chondrocytes, ⁴⁵ and osteocytes. ⁴⁶ In animal models, PCs have been shown to improve cardiac function postmyocardial infarction, ⁴⁷ participate in tissue regeneration, and serve as niche cells for stem cells, performing specialized functions such as hematopoiesis. ⁴⁸ They also expedite wound healing in injured tissues ⁴⁹ and promote fibrous tissue formation. Tissue-specific PCs play a pivotal role in maintaining the distinct functions of various organs, such as the storage of vitamin A in the liver, ⁵⁰ production of renin in the kidneys, ⁵¹ and maintenance of the blood–brain barrier.

Role in angiogenesis

PCs are integral to microvessels and envelop capillary ECs. They influence the biological properties of capillary ECs through direct interactions and paracrine signaling, acting as pivotal regulators ensuring the stability and quiescence of capillary ECs.^52,53

PCs play a vital role in regulating angiogenesis. Lindahl observed that embryos from platelet-derived growth factor-β (PDGF-β)-knockout mice exhibited lethality, ⁵⁴ with >95% of the PCs lost in the mice, leading to highly aberrant vascular development and hemorrhage. Angiogenesis refers to the process by which existing vascular ECs (ECs) form new vessel sprouts within mature or embryonic tissues. ⁵⁵ The nascent vessel sprouts attract PCs by secreting growth factors such as PDGF-β. ⁵⁶ During early angiogenesis, PCs precede ECs, guiding them to form regular vessels. In the later stages of angiogenesis, PCs exert inhibitory effects on EC growth by activating specific signaling pathways or releasing vascular active substances. This process mediates vessel maturation, improves capillary barriers, and prevents excessive proliferation of ECs in growth factor-rich environments. ⁵⁵ For instance, when recruited PCs overexpress Tie2, ⁵⁷ TGF-β1, ⁵⁸ etc., they significantly negatively impact EC growth However, as the abundance of growth factors decreases, PCs can also promote EC growth, thereby influencing various physiological functions of vessels. Mäe's ⁵⁹ study on PC-deficient PDGFβ^ret/ret mice showed that the loss of 70%–80% of PCs led to excessive angiogenesis and EC transformation toward a venous phenotype, accompanied by overexpression of angiopoietin-2 (Angpt2).

Under pathological conditions, PC signaling pathway dysregulation or excessive detachment of PCs could induce disease development. For instance, Gong ⁶⁰ discovered that after stroke, VEGFR1 was significantly upregulated in PCs. In vitro studies confirmed the critical role of PC-derived VEGFR1 in endothelial vessel formation and PC migration. In proliferative diabetic retinopathy, excessive PC detachment is a hallmark of disease progression. ⁶¹ Clinically, a high percentage of hexokinase (HK2)-positive PCs in vessels is correlated with a lower overall survival rate in patients with non-small cell lung cancer and hepatocellular carcinoma. Treatment with HK2 inhibitors can induce PC-driven tumor vascular remodeling, potentially improving drug delivery and inhibiting tumor growth. ⁶² Precise regulation of ECs by PCs ensures the appropriate differentiation of neovessels under pathological conditions such as wound healing, aiding in vascular adventitial maturation to counteract vascular injury and reduce vascular remodeling. ⁶³

Regulation of blood flow

The VSMCs surrounding cerebral arteries and arterioles play roles in vascular resistance, while PCs located on adjacent capillaries may possess similar regulatory functions. Hall demonstrated that, under pathological conditions, ischemia induces capillary constriction regulated by PCs; when PCs are in a rigid state, their death can permanently constrict capillaries of the blood–brain barrier. Therefore, PCs are the primary cells modulating cerebral blood flow. ⁶⁴ Stefan conducted four-dimensional (x-y-z-t) imaging on microvessels exposed to vasoactive molecules and confirmed the crucial role of precapillary sphincters and PCs as sensors and effectors for ECs or brain-derived vascular signals in primary capillaries. ¹⁴ However, strategies for differentiating between PCs and VSMCs remain controversial. Hartmann et al. ⁶⁵ utilized selective optical ablation or activation of PCs to demonstrate that PCs can modulate capillary diameter and affect blood flow. In contrast to the rapid adjustments typical of arteriolar smooth muscles, the action in capillaries was relatively slow. Similarly, Almaça ⁶⁶ observed in pancreatic islets that PCs are innervated by sympathetic axons, helping regulate local blood flow by modulating the diameter of islet capillaries. Targeted regulation of PCs by drugs, such as the use of drugs to inhibit PC contraction, can improve cerebral microcirculation post-ischemic stroke. ⁶⁷ Research on brain injuries caused by the novel coronavirus (SARS-CoV-2) revealed that the virus spike protein (S protein) binds to the transmembrane receptor angiotensin-converting enzyme 2 (ACE2) to infect host cells. ⁶⁸ The expression of ACE2 in the cerebral vascular PCs of patients increases upon exposure to the S protein, resulting in PC elongation and contraction. This subsequently triggers an immune response that ultimately leads to brain damage. ⁶⁹

Tissue injury repair and fibrosis

Excessive accumulation of extracellular matrix (ECM) components, including type I collagen, fibronectin, and hyaluronic acid, leads to fibrosis in injured tissues. Uncontrolled fibrosis inflicts permanent tissue damage and can potentially cause disease. The important role of PCs in organ fibrosis primarily manifests in their provision of a source of myofibroblasts. ⁷⁰ The Glast-CreER transgenic mouse model developed by Göritz et al. ⁷¹ allows recombination of R26R-yellow fluorescent protein specifically in type A spinal cord intrinsic vascular PCs within the R26R subgroup while designating the remaining PCs as type B. Dias ⁷² used in vivo lineage tracing to discover that type A PCs were the origin cells of the matrix myofibroblasts in fibrotic scars post-penetrative spinal injury. Type A PCs were recruited to injured tissues, where they formed fibrotic scar tissue and hindered the regeneration of nerve axons. Birbrair ⁷³ also identified two subtypes of PCs in skeletal muscle interstitial tissue. In vitro experiments revealed that type 1 and type 2 PCs were fibrotic and muscular, respectively. Type 1 PCs differentiated into adipocytes and myofibroblasts, which participated in the fibrosis process, while type 2 PCs promoted the regeneration of blood vessels and muscles. Kramann traced the genetic lineage of vascular periphery Gli1⁺ MSC-like cells (i.e. PCs) and confirmed that they were one of the primary sources of myofibroblasts in various organs. Yamaguchi ⁷⁴ conducted in vitro studies on human lung cells that transformed into PC-like cells in the absence of TGF-β signals and into myofibroblast-like cells (PC–myofibroblast transformation) in the presence of TGF-β signals. In idiopathic pulmonary fibrosis patients, lung lesions were found to be consistently located between the KRT7-expressing lung periepithelium and the CD34-positive capillary endothelium, in which the cells also expressed PDGFRβ and NG2, suggesting that myofibroblasts likely originated from PCs. Using lineage tracing, Humphreys found that in renal fibrosis, PCs detached from vessel walls, migrated, acquired a myofibroblast-like phenotype, differentiated into myofibroblasts, were activated into fibroblasts, and acted as collagen-producing cells. ⁷⁵ Similar studies have been conducted on the impact of hepatic stellate cells on liver fibrosis, ²¹ the role of PCs in systemic sclerosis, ⁷⁶ etc. Blocking signaling pathways involved in PC differentiation to myofibroblasts may offer effective treatment strategy for early fibrosis. By driving PC mobilization to improve tissue regeneration, for example, by utilizing photobiomodulation to facilitate skin regeneration, ⁷⁷ employing PC patches for wound healing promotion, ⁷⁸ and promoting the formation of peri-infarction oligodendrocytes and astrocyte proliferation mediated by PDGFR in postacute ischemic stroke for fibrotic repair, ⁷⁹ functional recovery can be facilitated.

Pathophysiological characteristics of adenomyosis and the potential mechanism of PCs in adenomyosis

Adenomyosis is a prevalent gynecological condition, but a complete understanding of its etiology remains elusive. The current prevailing hypothesis is that invasion of the interstitium into the inner layer of the myometrium, accompanied by infiltration, is promoted by microenvironmental factors that stimulate smooth muscle cell growth. The hypertrophy and proliferation of the myometrial layer are believed to stem from metaplasia of stromal cells. Additional hypotheses that have been proposed to explain adenomyosis pathogenesis include EMT, embryonic remnants, ⁸⁰ vascular PC pluripotency ⁸¹ and adipocyte activation. ⁸² Given the limited research on the role of PCs in adenomyosis, we comprehensively analyzed the existing data to elucidate the relationship between vascular PCs and adenomyosis.

Conclusions

PCs, possessing stem cell potential, are extensively distributed around blood vessels throughout the human body. A substantial body of evidence from both human and animal studies has unequivocally demonstrated the pivotal roles played by PCs in angiogenesis, modulation of blood flow, and facilitation of tissue repair processes. Considering that adenomyosis is characterized by angiogenesis, ReTIAR, fibrosis, and EMT, coupled with limited evidence of mesenchymal changes and the relatively unexplored role of PCs in the initiation and progression of adenomyosis, further comprehensive investigations are warranted to gain deeper insights into this area.

Author biographies

Footnotes

References