Received: 31 August 2018 | Revised: 29 April 2019 | Accepted: 3 May 2019 DOI: 10.1111/micc.12554 INVITED REVIEW The pericyte microenvironment during vascular development Laura B. Payne1 | Huaning Zhao1,2 | Carissa C. James1,3 | Jordan Darden1,3 | David McGuire1,3 | Sarah Taylor1 | James W. Smyth1,4,5 | John C. Chappell1,2,5 1Center for Heart and Reparative Medicine, Fralin Biomedical Research Abstract Institute, Roanoke, Virginia Vascular pericytes provide critical contributions to the formation and integrity of the 2 Department of Biomedical Engineering blood vessel wall within the microcirculation. Pericytes maintain vascular stability and Mechanics, Virginia Polytechnic State Institute and State University, Blacksburg, and homeostasis by promoting endothelial cell junctions and depositing extracellular Virginia matrix (ECM) components within the vascular basement membrane, among other 3Graduate Program in Translational Biology, Medicine, and Health, Virginia Polytechnic vital functions. As their importance in sustaining microvessel health within various Institute and State University, Blacksburg, tissues and organs continues to emerge, so does their role in a number of pathological Virginia conditions including cancer, diabetic retinopathy, and neurological disorders. Here, 4Department of Biological Sciences, College of Science, Virginia Polytechnic State we review vascular pericyte contributions to the development and remodeling of the Institute and State University, Blacksburg, microcirculation, with a focus on the local microenvironment during these processes. Virginia We discuss observations of their earliest involvement in vascular development and 5Department of Basic Science Education, Virginia Tech Carilion School of essential cues for their recruitment to the remodeling endothelium. Pericyte involve- Medicine, Roanoke, Virginia ment in the angiogenic sprouting context is also considered with specific attention to Correspondence crosstalk with endothelial cells such as through signaling regulation and ECM deposi- John C. Chappell, Fralin Biomedical Research tion. We also address specific aspects of the collective cell migration and dynamic Institute, 2 Riverside Circle, Roanoke, VA 24016. interactions between pericytes and endothelial cells during angiogenic sprouting. Email: [email protected] Lastly, we discuss pericyte contributions to mechanisms underlying the transition Funding information from active vessel remodeling to the maturation and quiescence phase of vascular This work was supported by NIH grants R00HL105779 and R56HL133826 and NSF development. grant 1752339 (to JCC). KEYWORDS endothelial cells, pericytes, vascular morphogenesis 1 | INTRODUCTION described these cells in the 1870s as perivascular cells that differed in morphology from vascular smooth muscle cells (vSMCs); peri- Vascular pericytes are essential components of the microcirculation. cytes appeared to be elongated along microvessels in perivascular These specialized cells wrap around and ensheath microvessels, pro- locations, while vSMCs were concentrically wrapped around the 1-3 moting endothelial cell junction stability and depositing extracellular endothelium. Despite being nearly ubiquitous in the microcircu- matrix (ECM), among many key functions. Eberth and Rouget first lation, research into vascular pericytes lagged behind their endothe- lial counterparts following their initial identification. More recently, however, pericytes have attracted significant attention across an Abbreviations: CNS, central nervous system; Col-IV, Type IV collagen; ECM, extracellular matrix; EGFR, epidermal growth factor receptor; HB-EGF, heparin-binding EGF; HSPGs, array of biological disciplines. Potential new functions are being re- heparan sulfate proteoglycans; NG2, neural-glial antigen-2; PDGF-BB, Platelet-derived ported for these cells in vascular development and tissue homeo- growth factor-BB; PDGFR , platelet-derived growth factor receptor- ; vBM, vascular β β stasis, as well as in a broad range of disease states.4,5 Contributions basement membrane; VEGF-A, vascular endothelial growth factor-A; vSMC, vascular smooth muscle cell; αSMA, α-smooth muscle actin. to microvessel stability, and to overall vascular barrier function, are Laura B. Payne and Huaning Zhao equally contributed to this work. Microcirculation. 2019;00:1–11. wileyonlinelibrary.com/journal/micc © 2019 John Wiley & Sons Ltd | 1 2 | PAYNE ET AL. well -accepted roles for pericytes. Intriguing nuances within these proximity to the endothelium as another criteria for positive identifi- more “canonical” roles are still being discovered in health6-8 and in cation of vascular pericytes.4 French et al70 utilized this approach in 9-12 diseases such as proliferative diabetic retinopathy, cancer and the developing mouse yolk sac and found PDGFRβ-positive pericytes metastatic progression,13,14 and Alzheimer's disease.15 Pericyte adjacent to distinct Tie2-expressing endothelial cells as early as E8.5. contractility, or the modulation of microvessel diameter, is an area Recently, Jung et al coupled a powerful double-labeled pericyte of ongoing investigation, particularly in the central nervous system transgenic mouse line (Pdgfrβ-EGFP and Cspg4-DsRed) with endo- (CNS).16-23 In addition, the tissue regeneration capacity of pericytes, thelial immunostaining. The authors visualized vascular-associated acting as a pool of perivascular mesenchymal stem cells,24-26 has also pericytes in the brain at E10.5, with increasing abundance through- been described as a potential role for these cells.27-29 This particu- out the remainder of embryonic and early postnatal development.71 lar role may be context- and/or model-dependent however.27,30-39 Previous studies have suggested that the endothelium acquires These established and emerging functions for pericytes, among pericyte associations primarily, if not exclusively, after endothelial others, are still being elucidated and have been the subject of many sprouting events begin to establish basic vascular networks, perhaps insightful reviews.4,40-46 We acknowledge the wide breadth of in- allowing greater plasticity in endothelial remodeling.66,72-74 We, and triguing studies that have addressed pericyte biology thus far. Here, others,75-78 have found that mouse embryonic stem cells give rise we narrow the focus of this review on pericytes in their engagement not only to primitive endothelial cell networks, but also to presump- with the vascular endothelium during blood vessel development and tive pericytes (or pericyte precursors) during the earliest stages of remodeling. We give a specific consideration to the elements within cardiovascular development.64 Pericytes seem to emerge at approx- the pericyte microenvironment that are critical for their contribu- imately the same time as, or even prior to, endothelial cell differenti- tions to these processes. ation, homing to endothelium engaged in both vasculogenic (Payne, L.B. and Chappell, J.C., Unpublished observations) and angiogenic processes.64,75-78 Nevertheless, whether pericytes, or their precur- 2 | PERICYTE ORIGINS AND sors, directly engage with the developing endothelium to actively co- IDENTIFICATION ordinate the formation of primitive blood vessels remains an ongoing area of investigation. Insight from these studies could shed light on During development, pericytes arise from a wide range of embry- the involvement of pericytes in pathological conditions associated onic and extra-embryonic regions. Neural crest and primordial mes- with vessel dysmorphogenesis and mis-patterning such as arterio- enchyme are the most commonly described origins, though specific venous malformations and cerebral cavernous malformations.58,79,80 tissues and organs likely derive vascular pericytes from unique cellu- lar niches.23,43,47,48 Because of their physical proximity and proposed functional overlap, pericyte lineage has frequently been inferred 3 | PERICYTE RECRUITMENT AND from tracing the differentiation of vSMCs.4 While pericytes and CONTRIBUTION TO EARLY VESSEL vSMCs may share a common derivation to a certain point, these cells FORMATION ultimately occupy unique regions of the vasculature and are mor- phologically and functionally distinct.49 Additional validation studies As endothelial cells form primitive tubes and more complex vascular will therefore be necessary, especially as pericyte-specific tools and networks, they simultaneously release molecular cues that facilitate markers continue to evolve alongside the advancement of single- pericyte recruitment to the developing vessel wall.66,81,82 Platelet- cell analysis and next-generation sequencing techniques.50-55 Volz derived growth factor-BB (PDGF-BB) is one of the most potent regu- et al56 for example recently observed vascular pericytes present in lators of pericyte recruitment, investment, and retention. Genetic the mouse heart as early as embryonic day 11.5 (E11.5), and these loss or mutation of this ligand (eg, altering its ECM retention motif) cells provided a source for coronary artery vSMCs. Interestingly, and its primary signaling receptor tyrosine kinase, PDGFRβ, lead this elegant study found that Notch pathway signals, known to be to substantial loss of pericytes and subsequent vascular complica- important for vSMC differentiation and development,57-61 were not tions.8,66,83-87 In an in vitro model of vasculogenic tube formation, required for the differentiation, recruitment, or retention of micro- endothelial cells also secrete high levels