Abstract
D
Although of clear and crystalline appearance, the CSF is rich in proteins, microRNAs, and exosomes. In recent years, it has been shown that by harboring these molecules, the CSF provides instructive signals to NSCs during development. Therefore, with this molecular composition and the ability to be an independent circulatory system, the CSF directly affects multiple cellular compartments in the central nervous system (CNS). In fact, the CSF interacts with the ependyma and most notably with the subventricular zone (SVZ) niche, which is an organized group of cells that line the ventricular wall in the brain. In this study, the NSCs intermix with other progenitors in a stereotypical manner, leading to the migration of newly born neuroblasts to the olfactory bulb in mice. In the human SVZ, NSCs, which are remnants of radial glial cells, exhibit a slightly similar organization and express glial fibrillary acidic protein (GFAP) as a marker. These NSCs are in direct contact with the CSF in human and mouse through protrusions of their membranes that contain cilia, which have been suggested to function as sensors, detecting molecules in the CSF. More notably, ablation of these cells by targeting cilia proteins resulted in decreased neurogenesis [2] in mice, which demonstrates the ability of NSCs to sense CSF signals.
The CSF contains the insulin-like growth factor (IGF)-2, which promotes NSC survival, an effect that is more robust in embryonic than adult CSF [3]. Furthermore, the migration of neuroblasts parallels the CSF flow, which requires the beating of ependymal cilia. This generates a concentration gradient of the chemorepellent molecule SLIT2, which propels the migration of neuroblasts toward the olfactory bulb in mice [4]. Other cells, such as meningeal and endothelial cells, can support the survival and differentiation of NSCs, due to secretion of molecules in the CSF that affect NSC function, including stromal cell-derived factor-1, sonic hedgehog, and neurotrophin-3.
The CSF–NSC interactions are also important during diseases. In multiple sclerosis (MS), especially progressive MS, the patient's CSF reduced the proliferation of NSCs and affected their differentiation in vitro [5]. Furthermore, hydrocephalus, the accumulation of CSF fluid with distention of the ventricular walls, modifies the composition of the CSF with inflammatory markers [6] and the SVZ, leading to reduction of proliferation of NSCs. In contrast, in other diseases, the CSF composition leads to increased proliferation, for instance, IGF-2 is elevated in the CSF of patients with glioblastoma multiforme, stimulating stem cell proliferation [3], suggesting that CSF proteins can induce the growth of these SVZ cells in brain tumors. The repercussions of this mechanism in the pathology of glioblastoma are not clear, however, excessive NSC proliferation leading to SVZ hyperplasia can contribute to brain tumor formation.
The impact of the CSF on NSCs has been mostly studied in relation to proliferation; however, the effects of the CSF on migration, a critical property of stem cells, are unknown. In this issue of Stem Cell and Development, Zhu et al. studied the role of CSF on human stem cell migration in vitro by using an innovative nanopatterned surface model. The authors demonstrated that exposure of human neural and mesenchymal stem cells to human CSF in vitro increased the expression of the C-X-C chemokine receptor type 4 (CXCR4), a bona fide chemokine for many stem cells during development and injury [7]. Zhu et al. demonstrated that the human CSF not only promoted proliferation and inhibited apoptosis of human stem cells from neural and mesenchymal origin but also increased their migratory capacity in human CSF. They further demonstrated through blocking experiments that the IGF-1 was directly responsible for the effects of human CSF on reduction of apoptosis, enhanced migration capacity, and the upregulation of CXCR4 expression in both stem cell types [8].
The work by Zhu et al. is contributing to the understanding of the sensing mechanisms of human NSCs to the CSF. Our future goal should be to measure the impact of the CSF in stem cells located in the human SVZ during injury, as a surrogate marker or endophenotype of the effects of the diseased CNS on NSCs and progenitors located outside the SVZ. Achieving this goal may facilitate finding new ways to activate endogenous repair in neurological diseases. Traditionally, we have studied the CSF in search for molecules that are increased or decreased in diseases to establish them as potential biomarkers. However, the work by Zhu et al. is adding to the mounting evidence that multiple CSF molecules may act in a combinatorial manner changing NSC function in development and disease, suggesting to us that studying combinatorial effects rather than focusing only in highly increased molecules in the CSF will more closely mimic the impact of the CSF in the SVZ niche in vivo.
Due to abilities of the CSF to contain a mix of growth factors [3], its positive effect on migration [8], as well as being a contained circulatory system reaching the entire brain, the CSF is proposed as a route for delivery of stem cells. However, the use of CSF as delivery of NSCs is under investigation, and there are issues that we need to resolve, for instance, whether injecting stem cells in the CSF is a reasonable and viable alternative to intracranial or intravenous (IV) injections. There are some recognized disadvantages of injecting cells directly to the brain, such as the potential of associated morbidity. In addition, some diseases are multifocal, where multiple injections may be needed, and the IV route may be a reasonable alternative; however, we still do not know how many cells effectively reach the CNS after an IV injection since NSCs can get trapped in the lungs.
A critical point is that NSCs injected in the CSF may not migrate directly into the parenchyma. Therefore, the benefits of this route may be relegated to the paracrine effects of the stem cells in the CSF, rather than the NSC ability to interact directly with diseased cells. One important issue is the possibility that injected human stem cells in the CSF can lead to brain tumors. It has been shown that NSC transplantation into the CSF in a patient with ataxia telangiectasia resulted in a brain tumor [9]. Therefore, exploiting the capacity of the CSF to direct the migration of NSCs to affected zones of the brain may be problematic. These issues are gaining more attention as Phase I clinical trials are under way investigating the injection of human NSCs into the CSF compartment for a variety of neurological disorders, including amyotrophic lateral sclerosis and MS.
The study of CSF–NSC interactions is an emerging and promising area of stem cell research. The work by Zhu et al. and others is eliciting pressing and critical questions that will spark the interest and eventually generate future studies to address the translational potential of the CSF effects on NSCs to improve neurological diseases.