中枢神经系统干细胞研究进展

近年来，中枢神经系统干细胞的研究方法和其在临床应用方面的价值日益成为神经干细胞研究的焦点，本文对于神经干细胞的发现，最新的神经干细胞研究技术以及神经干细胞在中枢神经系统退变性疾病，缺血损伤和肿瘤治疗等方面的研究进展作一概述。     

"Eat me" and "don't eat me" signals govern the innate immune response and tissue repair in the CNS: emphasis on the critical role of the complement system

A full innate immune system (e.g. complement system, scavenger receptors, Toll-like receptors (TLR)) has been described in the CNS and is thought to be an extremely efficient army designed to fight against invading pathogens and toxic cell debris such as apoptotic cells and amyloid fibrils. The binding of soluble or secreted innate immune molecules on pathogen-associated molecular patterns (PAMPs) as well as apoptotic cell-associated molecular patterns (ACAMPs) provide several "eat me" signals to promote the safe disposal of the intruders by professional and amateur phagocytes. These patterns are deciphered by receptors (pattern recognition receptors, PRRs; e.g. CR3) that control phagocytosis and associated inflammatory response depending on the meaning of these signals. Importantly, in order to avoid excessive collateral damage of surrounding cells, it is increasingly evident that "don't eat me" signals (coined herein as self-associated molecular patterns, SAMPs; e.g. complement regulatory proteins, CD200) are of paramount importance to signal a robust anti-inflammatory response and promote tissue repair. Further knowledge of the innate immune response in the CNS will greatly help to delineate the novel therapeutic routes to protect from CNS inflammation and neurodegeneration.     

Brain regeneration in physiology and pathology: the immune signature driving therapeutic plasticity of neural stem cells

Regenerative processes occurring under physiological (maintenance) and pathological (reparative) conditions are a fundamental part of life and vary greatly among different species, individuals, and tissues. Physiological regeneration occurs naturally as a consequence of normal cell erosion, or as an inevitable outcome of any biological process aiming at the restoration of homeostasis. Reparative regeneration occurs as a consequence of tissue damage. Although the central nervous system (CNS) has been considered for years as a "perennial" tissue, it has recently become clear that both physiological and reparative regeneration occur also within the CNS to sustain tissue homeostasis and repair. Proliferation and differentiation of neural stem/progenitor cells (NPCs) residing within the healthy CNS, or surviving injury, are considered crucial in sustaining these processes. Thus a large number of experimental stem cell-based transplantation systems for CNS repair have recently been established. The results suggest that transplanted NPCs promote tissue repair not only via cell replacement but also through their local contribution to changes in the diseased tissue milieu. This review focuses on the remarkable plasticity of endogenous and exogenous (transplanted) NPCs in promoting repair. Special attention will be given to the cross-talk existing between NPCs and CNS-resident microglia as well as CNS-infiltrating immune cells from the circulation, as a crucial event sustaining NPC-mediated neuroprotection. Finally, we will propose the concept of the context-dependent potency of transplanted NPCs (therapeutic plasticity) to exert multiple therapeutic actions, such as cell replacement, neurotrophic support, and immunomodulation, in CNS repair.     

Potential therapeutic clue of skin-derived progenitor cells following cytokine-mediated signal overexpressed in injured spinal cord

Recent evidence indicates that neural progenitor cell characters can be found in the population of adult skin-derived progenitor cells (SPCs). They have the ability to proliferate actively in vitro as spheres in suspension and they contain neural stem cells and several chemokines. Spheres derived from adult skin tissues have a higher capacity to differentiate into neurons in vitro. We report here that intravenous infusion of SPCs from adult skin ameliorated spinal cord lesions and improved motor function in laboratory mice with a spinal cord injury (SCI). After 4-5 weeks, transplanted SPCs survived and migrated into the injured region of the SCI very efficiently, and migrated cells were partially differentiated into neurons and glial cells. Behavioral and ultrastructural tissue analysis revealed that locomotor functions and remyelinated tissue lesions of SPCs engrafted onto SCI mice were restored significantly compared to those of the control group. Efficient migration of SPCs into SCI lesions suggests that SCI-induced chemotaxic factors facilitate the migration of SPCs. Also, we verified that SCI-induced chemotaxic factors play an important role in proliferation, migration, and differentiation of engrafted SPCs. In transplantation paradigms, the interaction between the SCI microenvironment and engrafted cells will be very important in promoting host injury repair through the induction of cell migration, proliferation, and differentiation. Finally, adult SPCs can behave as a multipotent population, suggesting potential clinical applications for SCI therapy.     

Significance of the cell adhesion molecules and sialic acid in neurodegeneration

The central nervous system (CNS) is a complex and precise mechanism that controls the most highest functions of the body. All of them depend on the cellular and molecular interactions called by neurobiologists "cellular plasticity". The CNS is a flexible structure but its regeneration after damage is strongly limited. Better understanding of cellular and molecular basis of brain repair can open new way in the development of therapeutic tools for neurodegeneration. Among many molecules that participate in the formation of neuronal networks, neural cell adhesion molecule (NCAM) and its sialylated derivative seem to play crucial role in the life of brain. In particular, polysialylated cell adhesion molecule (PSA-NCAM) is proposed to participate in the neuroprotective response in neurodegeneration by reducing of AMPA/NMDA receptors sensitivity to glutamate and facilitating disconnection of cell-cell interactions. These mechanisms protect from excitotoxic damage and promote dendritic/spine re-growth. This review briefly focuses on the expression and role of PSA-NCAM in neurodegenerative diseases and its potential application in therapy.     

Cell adhesion molecules in gene and cell therapy approaches for nervous system repair

The inability of the central nervous system (CNS) to efficiently repair damages results in severe functional impairment after trauma or neurodegenerative/demyelinating diseases. Regeneration failure is attributed to inhibitory molecules creating a nonpermissive environment for axonal regrowth, and dictates the necessity for the development of novel therapeutic strategies. An emerging approach for improving regeneration is the use of gene therapy to manipulate cell adhesion molecule expression in experimental animal models of degeneration. Alternatively, cell transplantation to replace lost neurons and the grafting of myelinating cells to repair demyelinating lesions are promising approaches for treating CNS injuries and demyelination. Schwann cells (SCs), oligodendrocyte progenitors, olfactory ensheathing cells and embryonic and neural stem cells have been shown to form myelin after transplantation into the demyelinated CNS. The repair capacity of the peripheral nervous system (PNS) is much higher, but there is still a limit to the amount of nerve loss that can be bridged after injury, and longer nerve gaps call for the use of conduits populated with living cells. In both cases, the interaction of grafted cells with the host environment is of paramount importance for the incorporation and functional integration of these cells and the manipulation of cell adhesion molecules is an attractive approach towards achieving this goal. In this review we summarize data from the recent literature regarding the manipulation of cell adhesion molecule expression towards CNS and PNS repair and discuss the prospects for future therapeutic applications.     

Cross-talk between neural stem cells and immune cells: the key to better brain repair?

Systemic or intracerebral delivery of neural stem and progenitor cells (NSPCs) and activation of endogenous NSPCs hold much promise as potential treatments for diseases in the human CNS. Recent studies have shed new light on the interaction between the NSPCs and cells belonging to the innate and adaptive arms of the immune system. According to these studies, the immune cells can be both beneficial and detrimental for cell genesis from grafted and endogenous NSPCs in the CNS, and the NSPCs exert their beneficial effects not only by cell replacement but also by immunomodulation and trophic support. The cross-talk between immune cells and NSPCs and their progeny seems to determine both the efficacy of endogenous regenerative responses and the mechanism of action as well as the fate and functional integration of grafted NSPCs. Better understanding of the dialog between NSPCs and innate and adaptive immune cells is crucial for further development of effective strategies for CNS repair.     

Defective neurogenesis resulting from DNA ligase IV deficiency requires Atm

Ataxia telangiectasia results from mutations of ATM and is characterized by severe neurodegeneration and defective responses to DNA damage. Inactivation of certain DNA repair genes such as DNA ligase IV results in massive neuronal apoptosis and embryonic lethality in the mouse, indicating the occurrence of endogenously formed DNA double-strand breaks during nervous system development. Here we report that Atm is required for apoptosis in all areas of the DNA ligase IV-deficient developing nervous system. However, Atm deficiency failed to rescue deficits in immune differentiation in DNA ligase IV-null mice. These data indicate that ATM responds to endogenous DNA lesions and functions during development to eliminate neural cells that have incurred genomic damage. Therefore, ATM could be important for preventing accumulation of DNA-damaged cells in the nervous system that might eventually lead to the neurodegeneration observed in ataxia telangiectasia.     

Boosting T-cell immunity as a therapeutic approach for neurodegenerative conditions: The role of innate immunity

Our research group has been working for more than a decade on the cross-talk between the immune and the nervous systems. Due to the unique nature of the central nervous system (CNS) as an immune privileged site, it was commonly believed that the nervous system functions optimally without any immune intervention, and that any immune cell infiltration to the CNS is a sign of pathology. However, since the immune system constitutes the body's major defense and repair mechanism, it seemed unreasonable that the CNS would have completely lost the need for assistance from this system. This insight prompted us to revisit the entire question of whether immune cells are required for recovery from CNS injuries. We subsequently made numerous fundamental observations that led us to formulate a unified theory linking all neurodegenerative conditions; thus, we suggested that “T-cell immunity to self maintains the self,” at least in the CNS. According to this view, immunity to self (“protective autoimmunity”) provides a pivotal role in maintenance, protection, and repair of the healthy and diseased CNS. We further showed that the T cells mediate their effect, at least under pathological conditions, by controlling the recruitment of blood-borne monocytes, which play a crucial local role that cannot be replaced by the resident microglia. Boosting of such a T cell response specific for brain proteins, while carefully choosing the antigen, the carrier, timing, dosing, and regimen should be considered as a way of augmenting a physiological repair mechanism needed to ameliorate disease conditions while restoring equilibrium needed for protection, repair and renewal; such therapy is not intended to modify a single mediator of a single disease, but rather, would serve as an approach for adjusting the levels of the immune response needed to restore a lost balance.     

From Stem Cells to Oligodendrocytes: Prospects for Brain Therapy

Multiple sclerosis is an autoimmune disease that destroys myelin-forming oligodendrocytes of the CNS. While the damage can be partially controlled using anti-inflammatory cytokines and steroids, endogenous repair is insufficient to replace lost cells. Until now cell replenishment (transplant therapy) has been viewed as unlikely to succeed due to allograft rejection in this sensitized immune environment. However, advances in stem cell biology give new hope for deriving patient-specific, autologous oligodendrocytes which may tip the balance to favor repair. The challenge will be to engineer these cells to respond to cues that can target their migration into lesions for brain and spinal cord repair.     

Intense Immune Suppression for Systemic Lupus—The Role of Hematopoietic Stem Cells

The treatment of severe autoimmune diseases has been recently revitalized by the introduction of intense immune suppression with immune ablative intent followed by three different procedures. These are allogeneic hematopoietic stem cell transplantation (HSCT), autologous HSCT (using either marrow or peripheral blood), and intense immune suppression without stem cell support. Current trials suggest that high dose immune suppressive therapy with or without autologous hematopoietic stem cell support can induce remission of previously refractory disease. Follow-up is too brief to determine if intense immune suppression, and more specifically autologous HSCT, will ultimately cure SLE. It is conceivable that an allogeneic source of stem cells from a normal donor (e.g. HLA matched sibling) will be required to achieve a cure. It is also possible that autologous HSCT, even if not curative, may prolong the life of patients with otherwise high-risk features. In carefully selected patients, the potential benefits of this procedure may outweigh the risks.     

Autologous adult rodent neural progenitor cell transplantation represents a feasible strategy to promote structural repair in the chronically injured spinal cord

Adult neural progenitor cells (NPCs) represent an attractive source for cell-based regenerative strategies in CNS disease. In animal models of spinal cord injury, syngenic adult NPCs, which were isolated from pooled post-mortem CNS tissue and co-transplanted together with fibroblasts, have been shown to promote substantial structural repair. The autologous transplantation of adult NPCs represents a major advantage compared with other sources of neural stem/progenitor cells. However, the feasibility of autologous NPC generation from a single biopsy in a relevant preclinical CNS disease model has yet to be demonstrated. To investigate this matter, adult Wistar rats underwent a cervical spinal cord lesion, which was followed by a minimal subventricular zone aspiration biopsy 2 days later. NPCs were isolated and propagated separately for each animal for the following 8 weeks. Thereafter, they were co-transplanted with simultaneously harvested skin fibroblasts in an autologous fashion into the cervical spinal cord lesion site. A total of 4 weeks later, graft survival, tissue replacement and axonal regeneration were assessed histologically. Animals receiving either allogenic NPCs combined with fibroblasts or autologous pure fibroblast grafts served as control groups. Within 8 weeks after the biopsy more than 3 million NPCs could be generated from a single aspiration biopsy, which displayed a differentiation pattern indistinguishable from syngenic NPC grafts. NPCs within autologous co-grafts readily survived, replaced cystic lesion defects completely and differentiated exclusively into glial phenotypes, thus paralleling previous findings with syngenic NPCs. The delayed transplantation 8 weeks after the spinal cord lesion elicited substantial axonal regeneration. These findings demonstrate that the therapeutic strategy to induce structural repair by transplanting adult autologous NPCs, after the successful propagation from a small brain biopsy into an acute CNS disease model, such as spinal cord injury, is feasible at the preclinical level.     

Intracompartmental delivery of CNTF as therapy for Huntington's disease and retinitis pigmentosa

Ciliary neurotrophic factor (CNTF) is a cytokine with neurotrophic activity across a broad spectrum of peripheral and central nervous system (CNS) cells. While its therapeutic potential for CNS diseases has been clear for sometime, the blood brain barrier (BBB) hinders the systemic delivery of CNTF and direct bolus injections are not suitable due to the short half-life of CNTF. One means of overcoming the BBB while providing continuous delivery of CNTF is with immunoisolated cellular implants that produce and deliver CNTF directly to the region of interest. Cells can be protected from host rejection by encapsulating, or surrounding, them within an immunoisolatory, semipermeable membrane that admits oxygen and required nutrients and releases bioactive cell secretions, but restricts passage of larger cytotoxic agents from the host immune defense system. The selective membrane eliminates the need for chronic immunosuppression of the host and allows the implanted cells to be obtained from nonhuman sources. In this review we discuss cell immunoisolation for Huntington's disease and retinitis pigmentosa. These two indications are highlighted because of extensive pre-clinical data supporting the general concept and recent clinical data that both strengthens the theoretical role of CNTF for treating neurodegeneration and justifies additional clinical evaluation in these and other diseases.     

Leptomeningeal-derived doublecortin-expressing cells in poststroke brain

Increasing evidence indicates that neural stem/progenitor cells (NSPCs) reside in many regions of the central nervous system (CNS), including the subventricular zone (SVZ) of the lateral ventricle, subgranular zone of the hippocampal dentate gyrus, cortex, striatum, and spinal cord. Using a murine model of cortical infarction, we recently demonstrated that the leptomeninges (pia mater), which cover the entire cortex, also exhibit NSPC activity in response to ischemia. Pial-ischemia-induced NSPCs expressed NSPC markers such as nestin, formed neurosphere-like cell clusters with self-renewal activity, and differentiated into neurons, astrocytes, and oligodendrocytes, although they were not identical to previously reported NSPCs, such as SVZ astrocytes, ependymal cells, oligodendrocyte precursor cells, and reactive astrocytes. In this study, we showed that leptomeningeal cells in the poststroke brain express the immature neuronal marker doublecortin as well as nestin. We also showed that these cells can migrate into the poststroke cortex. Thus, the leptomeninges may participate in CNS repair in response to brain injury.     

Immune response after central nervous system injury

Traumatic injuries of the central nervous system (CNS) affect millions of people worldwide, and they can lead to severely damaging consequences such as permanent disability and paralysis. Multiple factors can obstruct recovery after CNS injury. One of the most significant is the progressive neuronal death that follows the initial mechanical impact, leading to the loss of undamaged cells via a process termed secondary neurodegeneration. Efforts to define treatments that limit the spread of damage, while important, have been largely ineffectual owing to gaps in the mechanistic understanding that underlies the persisting neuronal cell death. Inflammation, with its influx of immune cells that occurs shortly after injury, has been associated with secondary neurodegeneration. However, the role of the immune system after CNS injury is far more complex. Studies have indicated that the immune response after CNS injury is detrimental, owing to immune cell-produced factors (e.g., pro-inflammatory cytokines, free radicals, neurotoxic glutamate) that worsen tissue damage. Our lab and others have also demonstrated the beneficial immune response that occurs after CNS injury, with the release of growth factors such as brain-derived growth factor (BDNF) and interleukin (IL-10) and the clearance of apoptotic and myelin debris by immune cells^1–4. In this review, we first discuss the multifaceted roles of the immune system after CNS injury. We then speculate on how advancements in single-cell RNA technologies can dramatically change our understanding of the immune response, how the spinal cord meninges serve as an important site for hosting immunological processes critical for recovery, and how the origin of peripherally recruited immune cells impacts their function in the injured CNS.     

The promise of stem cell and regenerative therapies for multiple sclerosis

The regenerative capacity of the adult central nervous system (CNS) is severely limited and although partial regeneration can be observed in the CNS of multiple sclerosis (MS) patients, these attempts at repair have been universally unsuccessful in preventing the accumulation of irreversible neurological deficits. Novel therapies to treat MS must therefore take into account the need for both immunomodulation and neuroprotection and, as such, multifaceted treatment strategies are required. Two complimentary approaches that aim to regenerate an incapacitated CNS have recently emerged. Firstly, targeting degraded myelin growth inhibitory molecules released as a consequence of the inflammatory process provides a unique opportunity to manipulate the microenvironment of the degenerating CNS. Proof of concept studies have established that this therapeutic approach has tremendous potential in regenerating damaged axons as demonstrated in models of spinal cord injury (SCI) and experimental autoimmune encephalomyelitis (EAE), an animal model for MS. In addition, stem cell based therapies offer a means of modulating inflammatory immune cells and promoting tissue repair as shown in a number of allogeneic transplant and autoimmune settings. This review attempts to summarise some of these approaches.     

Absence of core autophagy gene expression in an ex vivo central nervous system model infected with Listeria monocytogenes

Recent studies have suggested that autophagy can act as a protective immune mechanism against Listeria monocytogenes infection. L. monocytogenes is a Gram-positive, facultative intracellular bacterium that causes invasive diseases in humans and animals, particularly in the central nervous system (CNS). Human listeriosis of the CNS can manifest in many ways, including meningitis and brain abscesses. The initial line of defence against bacterial colonisation is provided by microglia, resident phagocytes of the CNS parenchyma. Microglial cells are also well known for clearing dead and dying neural cells after injury, and therefore play a key role in infectious diseases and neurodegeneration.     

Bmi-1 promotes neural stem cell self-renewal and neural development but not mouse growth and survival by repressing the p16Ink4a and p19Arf senescence pathways

Bmi-1 is required for the post-natal maintenance of stem cells in multiple tissues including the central nervous system (CNS) and peripheral nervous system (PNS). Deletion of Ink4a or Arf from Bmi-1(-/-) mice partially rescued stem cell self-renewal and stem cell frequency in the CNS and PNS, as well as forebrain proliferation and gut neurogenesis. Arf deficiency, but not Ink4a deficiency, partially rescued cerebellum development, demonstrating regional differences in the sensitivity of progenitors to p16Ink4a and p19Arf. Deletion of both Ink4a and Arf did not affect the growth or survival of Bmi-1(-/-) mice or completely rescue neural development. Bmi-1 thus prevents the premature senescence of neural stem cells by repressing Ink4a and Arf, but additional pathways must also function downstream of Bmi-1.