Gabriela J. Blaszczyk

Fibrinogen is a blood‐derived protein involved in coagulation and can make its way into the central nervous system (CNS) following breakdown of the blood–brain barrier. This molecule has been implicated in multiple sclerosis (MS), a disease marked by inflammation and demyelination in the CNS. However, the effect of this molecule has not been studied on human myelinating cells. This study examines how fibrinogen influences human oligodendrocyte (OL) lineage cells at various stages of development. Using induced pluripotent stem cell‐derived (iPSC) OL precursors and human primary OLs, we examined the effects of fibrinogen on cell differentiation, viability, and myelination‐related function. Here we show the differential effect of fibrinogen, based on OL‐lineage stage. While fibrinogen induced aberrant differentiation of early lineage OLs, by inhibiting their maturation and inducing an astrocytic phenotype, on mature OLs fibrinogen was found to promote myelination capacity, as shown by ensheathment assays as well as on the RNA level. These effects were associated with the activation of bone morphogenetic protein (BMP) signaling, both in early and mature OLs. We further found BMP signaling enrichment in OLs to be correlated with the inflammatory activity of an MS lesion and confirmed fibrinogen deposition on OLs in situ. Unlike previous rodent studies, these findings indicate that fibrinogen has a lineage‐dependent effect, where it may be inhibitory earlier in the lineage while promoting OL function in later stages. Understanding this dual role will provide insight into remyelination failure in MS and highlights the importance of timing and target in future therapeutic strategies.

Summary: Cellular heterogeneity is a common issue in differentiation protocols of oligodendrocytes (OLs) from human induced pluripotent stem cells. Our previous work described a novel method to generate OLs and highlighted the presence of glial progenitors. Here, we unravel the glial heterogeneity and characterize the response of isolated subpopulations to differentiation. This study provides a novel tool for studying the dynamics of glial development in vitro and on a transcriptomic level.

Background and Objectives: Oligodendrocytes (OL) and their myelin-forming processes are targeted and lost during the disease course of Multiple Sclerosis (MS), targeted by infiltrating leukocytes and their effector cytokines. Myelin repair is considered to be dependent on recruitment and differentiation of oligodendrocyte precursor cells (OPCs). The basis of failure of re-myelination during the disease course of MS remains to be defined. The aim of this study is to determine the impact of pro-inflammatory molecules tumor necrosis factor ⍰ (TNF⍰) and interferon gamma (IFN _γ ) on the differentiation of human OPCs. Methods: We generated human OPCs from induced pluripotent stem cells with a reporter gene under the OL-specific transcription factor SOX10. We treated the cells in vitro with TNF⍰ or IFN _γ and evaluated effects in terms of cell viability, expression of OL-lineage markers, and co- expression of astrocyte markers. To relate our findings to the molecular properties of OPCs as found in the MS brain we re-analyzed publicly available single nuclear RNAseq datasets. Results: Our analysis indicated that both TNF⍰ and IFN _γ decreased the proportion of cells differentiating into the OL-lineage; consistent with previous reports. We now observe the TNF⍰ effect is linked to aberrant OPC differentiation. A subset of O4+, reporter-positive cells co- expressed the astrocytic marker Aquaporin-4 (AQP4). On the transcriptomic level, the cells acquire an astrocyte-like signature alongside a conserved reactive phenotype. Analysis of single- nuclear RNAseq datasets from human MS brain revealed a subset of OPCs expressing an astrocytic signature. Discussion: In the context of MS, these results imply that OPCs are present but inhibited from differentiating along the OL-lineage, with a subset acquiring a reactive and stem-cell like phenotype, reducing their capacity to contribute towards repair. These findings help define a potential basis for the impaired myelin repair in MS and provide a prospective route for regenerative treatment.

Fibrinogen is a blood-derived protein involved in coagulation, and can make its way into the central nervous system (CNS) following breakdown of the blood-brain barrier. This molecule has been implicated in multiple sclerosis (MS), a disease marked by inflammation and demyelination in the CNS, as well as other neurological disorders. However, the effect of this molecule has not been studied on human myelinating cells. This study examines how fibrinogen influences human oligodendrocyte (OL) lineage cells at various stages of development. Using induced pluripotent stem cell-derived (iPSC) OL precursors and human primary OLs, we examined the effects of fibrinogen on cell differentiation, viability and myelination-related function. Here we show that fibrinogen induces an aberrant differentiation of early lineage OLs, by inhibiting their maturation and inducing an astrocytic phenotype, as seen in previous studies. On mature OLs, fibrinogen was found to promote myelination capacity as shown by ensheathment assays as well as on the RNA level. These effects were associated with the activation of BMP signalling, both in early and mature OLs. Transcriptomic analysis of human MS brain tissue shows similar pro-myelination changes in a subset of OLs, suggesting in vivo relevance. These findings indicate that fibrinogen has a lineage-dependent effect, where it may be inhibitory earlier in the lineage while promoting OL function in later stages. Understanding this dual role will provide insight into remyelination failure in MS and highlights the importance of timing and target in future therapeutic strategies. Significance Statement: In multiple sclerosis (MS), the blood protein fibrinogen leaks into the brain and has been shown to interfere with myelin repair. This study demonstrates that fibrinogen has opposite effects on human oligodendrocyte-lineage cells depending on their stage of maturation. While it blocks the differentiation of early-stage cells, it enhances the functional capacity of mature oligodendrocytes. These findings help explain why remyelination may fail in MS and suggest that fibrinogen could both hinder and support repair, depending on the cell context. This dual role has important implications for developing stage-specific therapies for MS.

Oligodendrocytes (OLs) are key players in the central nervous system, critical for the formation and maintenance of the myelin sheaths insulating axons, ensuring efficient neuronal communication. In the last decade, the use of human induced pluripotent stem cells (iPSCs) has become essential for recapitulating and understanding the differentiation and role of OLs in vitro. Current methods include overexpression of transcription factors for rapid OL generation, neglecting the complexity of OL lineage development. Alternatively, growth factor‐based protocols offer physiological relevance but struggle with efficiency and cell heterogeneity. To address these issues, we created a novel SOX10‐P2A‐mOrange iPSC reporter line to track and purify oligodendrocyte precursor cells. Using this reporter cell line, we analyzed an existing differentiation protocol and shed light on the origin of glial cell heterogeneity. Additionally, we have modified the differentiation protocol, toward enhancing reproducibility, efficiency, and terminal maturity. Our approach not only advances OL biology but also holds promise to accelerate research and translational work with iPSC‐derived OLs.

Oligodendrocytes (OLs) are key players in the central nervous system, critical for the formation and maintenance of the myelin sheaths insulating axons, ensuring efficient neuronal communication. In the last decade, the use of human induced pluripotent stem cells (iPSCs) has become essential for recapitulating and understanding the differentiation and role of OLs in vitro . Current methods include overexpression of transcription factors for rapid OL generation, neglecting the complexity of OL lineage development. Alternatively, growth factor-based protocols offer physiological relevance but struggle with efficiency and cell heterogeneity. To address these issues, we created a novel SOX10-P2A-mOrange iPSC reporter line to track and purify oligodendrocyte precursor cells (OPCs). Using this reporter cell line, we analyzed an existing differentiation protocol and shed light on the origin of glial cell heterogeneity. Additionally, we have modified the differentiation protocol, towards enhancing reproducibility, efficiency, and terminal maturity. Our approach not only advances OL biology but holds promise to accelerate research and translational work with iPSC-derived OLs. Main Points: The differentiation of iPSCs in Oligodendrocyte Precursor Cells (OPCs) and Oligodendrocytes (OLs) is a notoriously difficult technique and often displays variable efficiency and cellular heterogeneity. We engineered a novel reporter line carrying the fluorescent protein mOrange under the control of the OL-specific transcription factor SOX10 to track, purify and characterize OLs. By experimenting with diverse differentiation media, we improved the generation of SOX10-positive cells. Consequently, these cells exhibited increased consistency and effectiveness in evolving into myelinating OLs.