Macrophage- and pluripotent-like reparative Muse cells are unique endogenous stem cells distinct from other somatic stem cells

Abstract

Muse cells are endogenous reparative stem cells with dual characteristics: pluripotent-like and macrophage-like. They can be identified by the pluripotent surface marker stage-specific embryonic antigen-3-positive (SSEA-3 (+)) cells in the bone marrow, peripheral blood, and various organs, including the umbilical cord and amnion. Muse cells can differentiate into ectodermal, endodermal, and mesodermal lineage cells, self-renew, and selectively migrate to damaged sites by sensing one of the universal tissue damage signals, sphingosine-1-phosphate (S1P). At these sites, they phagocytose damaged/apoptotic cells and differentiate into the same cell type as the phagocytosed cells. In this manner, Muse cells replace damaged/apoptotic cells with healthy, functioning cells, thereby repairing tissues. Due to their specific immunosuppressive and immunotolerant mechanism, clinical trials have been conducted for acute myocardial infarction (AMI), subacute ischemic stroke, epidermolysis bullosa, amyotrophic lateral sclerosis (ALS), cervical spinal cord injury, neonatal hypoxic-ischemic encephalopathy (HIE), and COVID-19 acute respiratory distress syndrome. These trials involved the intravenous injection of ∼1.5 × 10⁷ donor Muse cells without human leukocyte antigen (HLA) matching or immunosuppressant treatment, and they demonstrated safety and therapeutic efficacy. Thus, donor Muse cell treatment does not require gene manipulation, differentiation induction, or surgical intervention. These unique characteristics distinguish Muse cells from other somatic stem cells, such as mesenchymal stem cells, VSEL stem cells, and marrow-isolated adult multi-lineage inducible (MIAMI) cells.

Keywords: HLA; clinical trials; immunotolerance; phagocytosis; pluripotent; sphingosine-1-phosphate.

FIGURE 1

Similarity and difference between BM-Muse cells and monocytes/macrophages. Both are similar in the reserve site (the BM), size, morphology, response to S1P, phagocytosis-related receptor expression, and phagocytic activity but different in the S1P receptor subtype, repertoire of phagocytosis-related receptors, and differentiation ability. Monocytes/macrophages are professional phagocytes that clean apoptotic cells and cell debris, while Muse cells differentiate into the same cell type as the phagocytosed damaged/apoptotic cells.

FIGURE 2

Selective migration of Muse cells to the site of damage following intravenous administration. A rabbit AMI model 3 days after intravenous injection of nano-lantern-Muse and -non-Muse MSCs exhibited specific homing of Muse cells to the post-infarct heart, while non-Muse cells were trapped in the lung and rarely homed to the heart tissue (Yamada et al., 2018).

FIGURE 3

(A) Differentiation of Muse cells into the same cell type as the phagocytosed apoptotic cells. (B) Molecular mechanism of how Muse cells recycle signals from the up-taken damaged/apoptotic cells necessary for differentiation, such as transcription factors. (C) Single-cell RNA sequencing of human Muse cells (original Muse cells) after phagocytosing apoptotic cell fragments of mouse hepatic (Hepatic), mouse cardiac (Cardiac), and rat neural (Neuronal) cells at 7 days (Wakao et al., 2022).

FIGURE 4

Mechanism of pluripotency maintenance in Muse cells. (A) In naive Muse cells, let-7 maintains the expression of pluripotency genes and inhibits proliferation and glycolysis by inhibiting the expression of IGF1R and IRS2 to repress the PI3K-AKT pathway. The PI3K-AKT pathway negatively controls the expression of KLF4, which promotes the expression of POU5F1(OCT3/4), SOX2, and NANOG. The PI3K-AKT pathway directly inhibits proliferation and glycolysis. The MEK/ERK pathway, which seems not affected by let-7, suppresses the PI3K-AKT pathway by reducing the phosphorylation level of AKT. This pathway also inhibits the expression of KLF4, but it virtually does not affect the expression of pluripotency genes and is suggested to suppress senescence and maintain self-renewal of Muse cells. (B) Effect of let-7 knockdown (KD) on the PI3K-AKT and MEK/ERK pathways in Muse cells. The PI3K-AKT pathway is more activated than in naive Muse cells. Increased AKT phosphorylation inhibits the expression of KLF4, which leads to the downregulation of POU5F1(OCT3/4), SOX2, and NANOG. On the other hand, cell proliferation and glycolysis were promoted. The MEK/ERK pathway reduces the phosphorylation level of AKT and also suppresses cell senescence (Li et al., 2024).

FIGURE 5

Strategy for Muse cell therapy. 1) HLA-mismatched donor Muse cells can be directly administered to patients without immunosuppressants due to specific immunotolerance; 2) since Muse cells selectively migrate to the damage site by sensing S1P produced by damaged cells, intravenous injection is a more efficient method of delivering them to the damaged site than a surgical approach; 3) unlike ES and iPS cells, Muse cells are already pluripotent-like and do not require differentiation induction because they can differentiate into the same cell type as the damaged/apoptotic cell through phagocytosis; and 4) Muse cells remain in the homed tissue for an extended period without rejection, and therefore, their bystander effects such as anti-apoptotic and anti-fibrosis effects and neovascularization are long-lasting (Minatoguchi et al., 2024).