TCF7l2, a nuclear marker that labels premyelinating oligodendrocytes and promotes oligodendroglial lineage progression

Abstract

Clinical and basic neuroscience research is greatly benefited from the identification and characterization of lineage specific and developmental stage-specific markers. In the glial research community, histological markers that specifically label newly differentiated premyelinating oligodendrocytes are still scarce. Premyelinating oligodendrocyte markers, especially those of nuclear localization, enable researchers to easily quantify the rate of oligodendrocyte generation regardless of developmental ages. We propose that the transcription factor 7-like 2 (TCF7l2, mouse gene symbol Tcf7l2) is a useful nuclear marker that specifically labels newly generated premyelinating oligodendrocytes and promotes oligodendroglial lineage progression. Here, we highlight the controversial research history of TCF7l2 expression and function in oligodendroglial field and discuss previous experimental data justifying TCF7l2 as a specific nuclear marker for premyelinating oligodendrocytes during developmental myelination and remyelination. We conclude that TCF7l2 can be used alone or combined with pan-oligodendroglial lineage markers to identify newly differentiated or newly regenerated oligodendrocytes and quantify the rate of oligodendrocyte generation.

Keywords: histological markers; myelinating oligodendrocytes; oligodendrocyte progenitor cells; oligodendrocytes; premyelinating oligodendrocytes.

FIGURE 1

Commonly used stage-specific markers of oligodendroglial lineage cells. (a) Schematic diagram depicting the expression dynamics of commonly used protein markers. (b) Triple immunostaining of Sox10 (or Olig2), PDGFRα, and CC1 identifying OPCs (Sox10⁺PDGFRα⁺ or Olig2⁺PDGFRα⁺) and OLs (Sox10⁺CC1⁺ or Olig2⁺CC1⁺) in the same sections of the murine spinal cord at postnatal day 8 (P8). Red arrows point to meningeal layers. (c) Immunostaining of myelin basic protein (MBP) in the external capsule (EC) of the murine brain at P8 and P14. Note that MBP is located in cell bodies (arrowheads) and proximal processed of newly generated premyelinating OLs

FIGURE 2

Relative expression of the mRNA transcripts of premyelinating OL-enriched genes along the 12 maturation stages of oligodendroglial lineage cells clustered by scRNA-seq (Marques et al., 2016). COP, differentiation-committed OPC; MFOL, myelin forming oligodendrocytes; MOL, mature oligodendrocytes; NFOL, newly formed oligodendrocytes; OPC, oligodendrocyte progenitor cells. The figure panels are retrieved from the single cell RNA-seq (www.linnarssonlab.org/oligodendrocytes) (Marques et al., 2016). Note that Tcf7l2 mRNA transcript shows a more transient and restricted expression pattern along oligodendroglial lineage progression and is absent from OPCs and mature myelinating OLs. Enpp6 mRNA transcript shows a bi-phasic expression: One peak in newly formed OLs and the other in mature OLs. Gpr17 is also expressed in OPCs

FIGURE 3

TCF7l2-expressing cells belong to oligodendroglial lineage in the murine CNS, a notion illustrated by pan-oligodendroglial lineage marker Sox10. (a–c) Double fluorescence immunohistochemistry of TCF7l2 and Sox10 in the murine spinal cord at P1, P7, and P28. (d) Quantification of Sox10⁺ and Sox10⁺TCF7l2⁺ cells in the white matter of the spinal cord. (e) The antibody specificity of TCF7l2 immunoreactive signals was validated by Cre-loxP-mediated TCF7l2 knockout in Cnp-Cre:Tcf7l2^fl/fl double transgenic mice; P5 spinal cord was used for IHC. (d, e) Adapted from a previous paper (Hammond et al., 2015). TCF7l2 antibody was rabbit monoclonal antibody purchased from Cell Signaling Technology (clone # C48H11, catalog #2569)

FIGURE 4

TCF7l2 is highly expressed in thalamic neurons in the forebrain. Fluorescent immunostaining showing abundant TCF7l2 expression in thalamic neurons at early postnatal and adult ages. TCF7l2 antibody, Cell Signaling #2569 (rabbit clone C48H11). Scale bar = 100 μm

FIGURE 5

TCF7l2⁺ OLs are myelin protein-expressing cells that are characterized by premyelinating morphology. (a) IHC of TCF7l2 and Plp mRNA in situ hybridization (IHC/ISH) on the sections of P7 spinal cord. Arrowheads, double positive cells. (b) IHC of TCF7l2 and Mbp mRNA in situ hybridization (IHC/ISH) on the sections of P7 spinal cord. Arrowheads, double positive cells. (c) Double fluorescent IHC of TCF7l2 and MBP in the spinal cord of mouse embryonic 18.5 days. (d–e) Double fluorescent IHC of TCF7l2 and MBP (d) or CNP (e) in the dorsal column of P2 mouse spinal cord. The boxed areas in (d) and (e) were shown as orthogonal views of confocal single optic slice at the right. (f–g) Double fluorescent IHC of TCF7l2 and MBP in the corpus callosum of P6 (f) and P8 (g) mice. The data of panel (a–e) and the concept underlying panels (f–g) are from previous paper (Hammond et al., 2015). Arrowheads in all panels point to double positive OLs. Blue, DAPI nuclear counterstaining. Scale bars in (f–g), 20 μm. TCF7l2 antibody, Cell Signaling #2569 (rabbit clone C48H11)

FIGURE 6

TCF7l2 is co-expressed with adenomatous polyposis coli. Double fluorescent IHC of TCF7l2 and APC in P4 cerebellum (a), P8 external capsule of the forebrain (b), and P3 spinal cord (c). Note TCF7l2 and APC were overlapped in same cell populations. (d) Sparse TCF7l2⁺ cells were observed and co-labeled with APC in the white matter of P60 spinal cord. Scale bar = 20 μm. The concept underlying the figure panels are published in the previous papers (Lang et al., 2013). APC antibody, Santa Cruz Biotechnology (catalog # sc-896); TCF7l2 antibody, Cell Signaling #2569 (rabbit clone C48H11).

FIGURE 7

Proposed molecular mechanism of TCF7l2-regulated oligodendroglial differentiation and maturation. (a) TCF7l2 recruits the Wnt inhibitor Kaiso to dampen Wnt/β-catenin signaling activity during early differentiation and Sox10 to enhance myelin gene expression during late maturation (Zhao et al., 2016). (b) TCF7l2 is co-expressed in the same population of OLs that are positive for APC (Lang et al., 2013) (also see Figure 6), a crucial regulator for β-catenin degradation. TCF7l2 represses autocrine BMP4-mediated signaling to promote early OL differentiation (Zhang et al., 2021)