Infusion of Cerebrospinal Fluid from Young Mice Reverses Memory Loss in Aged Mice

Aging causes changes in synaptic plasticity, myelination, vasculature, and cognitive function [1]. Accumulating evidence highlights that improvement of the systemic environment benefits brain function and alleviates brain aging. Such improvements include heterochronic parabiosis, transplantation of bone marrow, young blood, or fecal microbiota, and plasma transfer experiments [2–5]. In a previous study, colleagues from Stanford University found that young blood and umbilical cord plasma rejuvenate the aged brain, slow down aging-related processes, and preserve cognitive function [3, 4]. However, the existence of the blood-brain barrier limits the rejuvenation potential of young blood or umbilical cord plasma.

Unlike blood or plasma, cerebrospinal fluid (CSF) constitutes the immediate environment of various brain cells and provides the neurons with nourishing compounds. Therefore, it would be a more promising new approach for treating neurodegenerative diseases if CSF from young animals could restore memory in aged animals. In a recent study, an exciting finding published in Nature by the same group from Stanford University School of Medicine showed that young CSF restores oligodendrogenesis and improves memory in aged mice [1].

The authors first asked whether young CSF can protect against aging-related learning and memory impairments [1]. Young mouse CSF and artificial CSF were separately infused into aged mice. Interestingly, animals with young mouse CSF exhibited better fear memory than mice with artificial CSF, suggesting a potential role of young CSF in alleviating aging-related cognitive decline. Given the essential role of the hippocampus in learning and memory, bulk RNA sequencing was applied to measure the hippocampal transcriptome from animals with young mouse CSF or artificial CSF. Notably, comparison of gene expression between animals infused with young CSF or artificial CSF identified the upregulation of oligodendrocyte, oligodendrocyte differentiation, and major myelin protein component-related genes. These findings support the hypothesis that oligodendrocytes may be a primary cellular target for young CSF infusion. Practically, even an acute single injection of young mouse CSF induced the upregulation of oligodendrocyte marker genes compared with artificial mouse CSF or aged mouse CSF infusion. Next, the authors injected BrdU or EdU to label cellular proliferation after young CSF infusion. Importantly, they found that the young mouse CSF and human CSF infusion significantly increased oligodendrocyte progenitor cell (OPC) proliferation in the hippocampus. Notably, the hippocampal myelination was enhanced in the young CSF infusion group, in line with the increased OPC proliferation, suggesting that young CSF may contain cytokines that promote OPC proliferation and differentiation. The primary rat OPC culture system was then applied to demonstrate this concept. As expected, the young human CSF-treated OPCs exhibited dose-dependent proliferation and prominent differentiation, confirming a role of young CSF in regulating OPC proliferation and differentiation.

To elucidate the mechanism underlying young CSF-mediated effects on OPCs, the authors next used metabolic sequencing of RNA (SLAMseq) to analyze the labeled nascent mRNA from young CSF-treated OPCs in vitro. They found that serum response factor (SRF), a transcription factor involved in cellular motility, proliferation, and differentiation, was the top changed gene following exposure to young human CSF. Interestingly, the target genes of SRF and actin cytoskeleton transcripts increased following the response of SRF, suggesting activation of the SRF pathway and modulation of the actin cytoskeleton.

Previous studies reported that SRF-regulation of the actin cytoskeleton is essential in cellular proliferation and differentiation [6]. The authors then analyzed whether SRF expression following young CSF exposure modulates the OPC actin cytoskeleton. SiR-actin (a fluorescent probe) and the actin filament dye phalloidin were applied to measure the actin cytoskeleton in living and fixed OPCs, and detected increased cellular actin filament levels in the young CSF-treated group.

To further confirm the role of SRF in young CSF-mediated effects, SRF-knockout OPCs and wild-type OPCs were exposed to culture medium with young CSF. As expected, the young CSF-mediated effects on OPCs were abolished in the SRF-knockout cells. Moreover, the authors found that SRF-positive OPCs in the CA1 region of the hippocampus were significantly decreased in the aged mouse, suggesting the indispensable role of SRF in young CSF-mediated effects. Furthermore, the authors asked whether young CSF activates the SRF pathway in vivo. Acute injection of young CSF into the lateral ventricle of the aged brain activated SRF signaling, in line with the in vitro findings. These findings show that SRF was imperative for the young CSF-mediated effects on OPCs in vitro and in vivo.

To explore the upstream inducers of SRF in the CSF, the authors listed 35 potential SRF inducers according to the previously published CSF proteomic dataset and tested these inducers with HEK293 cells transfected with the serum response element (SRE)-GFP reporter. Because the binding of SRF to SRE plays an indispensable role in SRF-induced cellular proliferation and differentiation, the transfected HEK293 cell with an SRE-GFP reporter was considered an ideal model to screen for potential SRF inducers [6]. Fibroblast growth factor 17 (Fgf17), a brain-enriched protein, was chosen for further analysis because of the most robust dose-dependent responses of SRE-GFP reporters following Fgf17 addition. Further analysis confirmed that Fgf17 promotes the proliferation and differentiation of primary OPCs by adding Fgf17 directly into the cell culture medium. To determine the role of Fgf17 in young CSF-mediated effects, Fgf17 was injected into the aged brain. The authors found that the Fgf17 administration also induced hippocampal OPC proliferation and significantly improved memory function, suggesting that Fgf17 is necessary for the young CSF-mediated effects on OPCs in the hippocampus. Finally, the anti-Fgf17 blocking antibody was delivered by intracerebroventricular injection to confirm the role of Fgf17 in normal memory function. Indeed, the memory function and neuronal plasticity were impaired in the animals with the anti-Fgf17 injection. Anti-Fgf17 blocking antibodies inhibited the proliferation of cultured OPCs in the young CSF or Fgf17 exposure group, in line with the in vivo results. These findings suggest that Fgf17 contributes to young CSF-mediated effects on OPCs and the improved memory function in aged mice.

As shown in Fig. 1, the authors demonstrated that infusing CSF from young mice or humans promotes OPC proliferation and hippocampal myelination, which finally improves aged animals’ memory. Mechanistic studies have identified the SRF pathway as an essential mediator of the effects of young CSF on OPCs. Furthermore, the authors found that Fgf17 is the essential SRF inducer for improving cognitive function and other beneficial effects on myelination. Although this study cannot exclude the possibility that young CSF directly affects the functions of astrocytes and mature oligodendrocytes for better neuronal and cognitive function in aged animals, their study provides a novel concept that the improved CSF contact with brain cells could benefit neuronal health and cognitive function. This offers a new connection between brain cells and the environment in which they live and supplies a potential therapeutic strategy to prevent or rescue cognitive deficits in neurodegenerative diseases. Based on these exciting findings, several interesting questions arise and deserve further analysis in future studies. First, cellular senescence has been widely studied in the aged brain and neurodegenerative diseases [7]. Senescent OPCs and oligodendrocytes contribute to diminished remyelination and cognitive impairment in neurodegenerative disease [8]. Therefore, it would be interesting to investigate the effect of young CSF on cellular senescence, especially on senescent OPCs, in aged animals. Second, both neurodegenerative and neuropsychiatric diseases display prominent demyelination [9, 10]. The effects of transfusing young CSF on demyelination in neurodegenerative and neuropsychiatric diseases remain unknown. Expanding this finding to disease models may provide a potential therapeutic strategy for neurodegenerative diseases and neuropsychiatric diseases. Third, this Nature paper found that CSF infusions induce EdU incorporation in other glial cells, including microglia and astrocytes. Therefore, it is worth exploring the effects of CSF and SRF signaling on other glial cells in aged mice. Finally, physical exercise is a widely studied non-invasive approach that induces a CSF response [11]. Transfusing CSF from exercised animals to sedentary animals would help clarify a novel underlying mechanism of the beneficial effects of exercise on the brain. Looking deeper into the mechanism underlying the beneficial effects young CSF and investigating the effects of transfusing CSF under different conditions may provide a potential approach for various brain diseases.

Fig. 1.

Young CSF infusion improves memory in aged mice. Young CSF or Fgf17 infusion improves OPC proliferation and hippocampal myelination in aged animals, which benefits memory recall. Fgf17 in the young CSF is the essential SRF inducer for improving cognitive function and other beneficial effects on myelination. Fgf17-induced SRF binds to SRE to promote OPC proliferation and differentiation. The infusion of artificial CSF or anti-Fgf17 blocking antibody does not have beneficial effects in aged mice.

Conflict of interest

The authors declare no competing financial interests.