Background
Microglia, a glial cell subtype, comprise the resident macrophage population located within the central nervous system (CNS). The precise origin of microglia has long remained the subject of debate [
1,
2]. Previous studies have demonstrated that microglia originate from progenitor cells in the embryonic yolk sac during early development and that embryonically derived microglia self-maintain until adulthood under normal conditions [
3,
4]. Nevertheless, it remains unclear whether these cells can continuously produce microglia in the adult CNS, even under pathological conditions. It has been proposed that some microglia originate from bone-marrow-derived hematopoietic cells or circulating monocytes [
5‐
7]. However, it remains controversial whether new microglia do indeed originate from bone marrow cells [
8], suggesting the potential for other sources of microglia within the adult CNS.
Mounting evidence suggests that progenitor cells localized to the adventitia (adventitial progenitor cells (APCs)) around the blood vessels may serve as multipotent resident vascular stem cells (VSCs) [
9] that contribute to vasculogenesis [
10‐
12]. A recent study by Psaltis and colleagues demonstrated that macrophage progenitors can derive from APCs located in the adult murine aorta [
13]. In addition to the APCs near larger vessels, vascular pericytes (PCs) located around capillaries are also strong candidate sources for the VSC population [
9,
14]. PCs exhibit the potential for differentiation into multiple different cell populations, including neural cells, adipocytes, chondroblasts, and osteoblasts [
15,
16]. Although it remains controversial whether PCs can produce microglia [
17‐
19], we recently demonstrated that PCs acquire multipotent stem cell activity in response to brain injuries such as ischemia/hypoxia and that these reactive PCs can differentiate into various lineages, including the neural and vasculogenic lineages [
20]. In addition, brain multipotent stem cells exhibit microglia-like cell phenotypes [
21,
22] and microglia have been described as arising from meningeal cells [
2,
23]. We demonstrated that substantial quantities of multipotent PCs were derived from the latter cells following ischemic stroke [
20,
24‐
27]. These findings led us to hypothesize that resident microglia might originate from ischemia-induced multipotent PCs following CNS injury.
In this study, we used a mouse model of cerebral infarction to investigate whether reactive PCs develop the traits of microglia-producing VSCs following ischemia.
Discussion
The multipotent stem cell activity of PCs, which allows them to differentiate into various cell types, including adipocytes, osteoblasts, chondrocytes, neural cells, and vascular cells, has been well documented [
16,
25,
26,
37‐
43]. Previous reports have shown that PCs can differentiate into immune cells such as dendritic cells [
44] and macrophage-like cells [
45]. However, the ability of PCs to produce microglia has remained unclear because most reported studies based their investigation on ultrastructural findings alone [
17‐
19,
23]. Although a recent report showed that brain PCs acquire a microglial phenotype after ischemia [
36], the precise mechanism was not elucidated. The current study clearly demonstrated that brain PCs acquire multipotent VSC activity following ischemic stroke and can therefore produce functional microglia.
VSCs are capable of differentiating into multiple cell lineages [
9,
14,
46]. Although the precise traits of these cells remain unclear, PCs located around capillaries are strong candidate VSCs, as are APCs located around larger vessels [
9]. APCs are Sca1
+, multipotent stem/progenitor cells that localize in the adventitia of blood vessels [
47]. Similar to the traits of multipotent PCs [
20], APCs exhibit the potential for multi-lineage differentiation into various cell populations, including adipocytes, osteoblasts, chondrocytes, myocytes, neural cells, and vascular cells [
10‐
12,
48]. In addition, a recent study by Psaltis and colleagues showed that APCs, but not bone marrow cells, give rise to macrophages that co-express αSMA [
13,
49]. Combined with the present results showing that αSMA was predominantly present at perivascular cells around large vessels, these findings suggest that APCs rather than PCs have the potential to produce macrophages. Furthermore, our current study showed that Iba1
+ microglia expressed PDGFRβ but not αSMA, suggesting that PCs rather than APCs have the potential to produce microglia. The precise relationship between APCs and PCs remains unclear. However, since APCs and PCs both express several markers and display similar traits [
14], both are likely to be multipotent VSCs that can produce cells of the microglia/macrophage lineage.
Consistent with our previous studies [
20,
50], the current study demonstrated that PCs derived from the post-ischemic brain expressed various stem/undifferentiated cell markers as well as multipotency. Why do reactive PCs acquire multipotency following an ischemic stroke? Under normal conditions, PCs are quiescent cells that cycle slowly. However, upon stimulation, PCs proliferate, migrate, and differentiate into various cell types [
25,
26,
51]. These characteristics suggest that PCs alter their phenotypes under pathologic conditions. In support of this concept, our previous and current studies demonstrated that brain PCs cultured under conditions of OGD and in a MET-like manner can be reprogrammed to become multipotent stem cells that express various stem/undifferentiated cell markers, including nestin, c-myc, Klf4, and Sox2 [
20,
50]. These findings indicate that the properties of PCs under normal and pathological conditions are completely distinct and that reactive, but not quiescent, PCs are likely multipotent VSCs that can produce cells of various lineages.
In the present study, we found that PC-OGD that formed cell clusters further increased their expression of stem cell markers, such as Sox2 and nestin. In addition, PDGFRβ
+ iPCs showed that pericytic markers were predominantly expressed in the peripheral zones of cell clusters but not in the cores. Although we do not know the exact reason for this phenomenon, the current study showed that pericytic marker expression was downregulated during MET that occurred following ischemia/hypoxia [
20]. Therefore, MET most likely occurs within the hypoxic cores of cell clusters rather than in the peripheral zones.
Previous studies have shown that within brains under pathologic conditions, the pericytic marker NG2 was expressed in microglia determined to have multipotency [
22,
52]. Consistent with these reports, we found that some NG2
+ cells within ischemic areas expressed the microglial marker Iba1 (data not shown). Although it remains unclear whether NG2
+ PCs can transform into microglia under these conditions, the present study showed that reactive PCs expressing PDGFRβ acquire stemness and can produce microglia. These results indicate that it is possible that some NG2
+ microglia originate from PCs following injury. Together, these finding suggest that reactive PCs acquire not only stemness but also hematopoietic potential since microglia/macrophage-like cells have been reported to be derived from hematopoietic lineage cells, including hematopoietic stem cells [
53,
54]. Why do reactive PCs acquire hematopoietic potential? Although adult brain PCs lack angiogenic properties under normal conditions, multipotent PCs do exhibit vasculogenic traits [
40,
43,
55]. In addition, APCs expressed both PC (PDGFRβ, NG2) and hematopoietic stem cell (CD34) markers [
40,
43,
55]. Furthermore, we recently demonstrated that following ischemia/hypoxia, adult brain PCs display a complex angioblastic phenotype that includes the expression of various hematopoietic stem cell markers such as CD34 and CD144 in addition to their original mesenchymal properties [
20]. These cells acquired angioblastic traits along with enhanced expression of pluripotent markers such as Klf4 [
20], which promotes angioblastic lineage reprogramming [
56]. Combined with the finding that PDGFRβ is expressed in early hematopoietic precursors during development [
57], these findings suggest that ischemia/hypoxia may convert normal PCs into reactive PCs with a mesenchymoangioblastic phenotype, which is typically observed in immature PCs during development [
57,
58]. Nevertheless, the precise traits and subtypes of multipotent PCs with hematopoietic potential should be determined, ideally through future studies that include pericyte genetic lineage labeling experiments.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
RS, MK, and TH contributed to the collection and assembly of data and data analysis and interpretation. AN-D, AT, YT, AN, and SK-O contributed to the collection and assembly of data. HY contributed to the conception and design and data analysis and interpretation. TM contributed to the conception and design, financial support, and data analysis and interpretation. TN contributed to the conception and design, financial support, data analysis and interpretation, and manuscript writing. All authors read and approved the final manuscript.