Effect of Neurotrophic Factors on Neuronal Stem Cell Death

Autophagy for the quality control of adult hippocampal neural stem cells

Autophagy plays an important role in neurodegeneration, as well as in normal brain development and function. Recent studies have also implicated autophagy in the regulation of stemness and neurogenesis in neural stem cells (NSCs). However, little is known regarding the roles of autophagy in NSC biology. It has been shown that in addition to cytoprotective roles of autophagy, pro-death autophagy, or ׳autophagic cell death (ACD),' regulates the quantity of adult NSCs. A tight regulation of survival and death of NSCs residing in the neurogenic niches through programmed cell death (PCD) is critical for maintenance of adult neurogenesis. ACD plays a primary role in the death of adult hippocampal neural stem (HCN) cells following insulin withdrawal. Despite the normal apoptotic capability of HCN cells, they are committed to death by autophagy following insulin withdrawal, suggesting the existence of a unique regulatory program that controls the mode of cell death. We propose that dual roles of autophagy for maintenance of NSC pluripotency, as well as for elimination of defective NSCs, may serve as a combined NSC quality control mechanism. This article is part of a Special Issue entitled SI:Autophagy.

Activity Dependency and Aging in the Regulation of Adult Neurogenesis

Age and activity might be considered the two antagonistic key regulators of adult neurogenesis. Adult neurogenesis decreases with age but remains present, albeit at a very low level, even in the oldest individuals. Activity, be it physical or cognitive, increases adult neurogenesis and thereby seems to counteract age effects. It is, thus, proposed that activity-dependent regulation of adult neurogenesis might contribute to some sort of "neural reserve," the brain's ability to compensate functional loss associated with aging or neurodegeneration. Activity can have nonspecific and specific effects on adult neurogenesis. Mechanistically, nonspecific stimuli that largely affect precursor cell stages might be related by the local microenvironment, whereas more specific, survival-promoting effects take place at later stages of neuronal development and require the synaptic integration of the new cell and its particular synaptic plasticity.

Aucubin promotes neurite outgrowth in neural stem cells and axonal regeneration in sciatic nerves

Aucubin is an iridoid glycoside with a wide range of biological activities, including anti-inflammatory, anti-microbial, anti-algesic as well as anti-tumor activities. Recently, it has been shown that aucubin prevents neuronal death in the hippocampal CA1 region in rats with diabetic encephalopathy. In addition, it has protective effects on H₂O₂-induced apoptosis in PC12 cells. We have shown here that aucubin promotes neuronal differentiation and neurite outgrowth in neural stem cells cultured primarily from the rat embryonic hippocampus. We also investigated whether aucubin facilitates axonal elongation in the injured peripheral nervous system. Aucubin promoted lengthening and thickness of axons and re-myelination at 3 weeks after sciatic nerve injury. These results indicate that administration of aucubin improved nerve regeneration in the rat model of sciatic nerve injury, suggesting that aucubin may be a useful therapeutic compound for the human peripheral nervous system after various nerve injuries.

Incretin mimetics as pharmacologic tools to elucidate and as a new drug strategy to treat traumatic brain injury

Traumatic brain injury (TBI), either as an isolated injury or in conjunction with other injuries, is an increasingly common event. An estimated 1.7 million injuries occur within the USA each year and 10 million people are affected annually worldwide. Indeed, nearly one third (30.5%) of all injury-related deaths in the USA are associated with TBI, which will soon outpace many common diseases as the major cause of death and disability. Associated with a high morbidity and mortality and no specific therapeutic treatment, TBI has become a pressing public health and medical problem. The highest incidence of TBI occurs in young adults (15-24 years age) and in the elderly (≥75 years of age). Older individuals are particularly vulnerable to these types of injury, often associated with falls, and have shown increased mortality and worse functional outcome after lower initial injury severity. In addition, a new and growing form of TBI, blast injury, associated with the detonation of improvised explosive devices in the war theaters of Iraq and Afghanistan, are inflicting a wave of unique casualties of immediate impact to both military personnel and civilians, for which long-term consequences remain unknown and may potentially be catastrophic. The neuropathology underpinning head injury is becoming increasingly better understood. Depending on severity, TBI induces immediate neuropathologic effects that, for the mildest form, may be transient; however, with increasing severity, these injuries cause cumulative neural damage and degeneration. Even with mild TBI, which represents the majority of cases, a broad spectrum of neurologic deficits, including cognitive impairments, can manifest that may significantly influence quality of life. Further, TBI can act as a conduit to longer term neurodegenerative disorders. Prior studies of glucagon-like peptide-1 (GLP-1) and long-acting GLP-1 receptor agonists have demonstrated neurotrophic/neuroprotective activities across a broad spectrum of cellular and animal models of chronic neurodegenerative (Alzheimer's and Parkinson's diseases) and acute cerebrovascular (stroke) disorders. In view of the mechanisms underpinning these disorders as well as TBI, we review the literature and recent studies assessing GLP-1 receptor agonists as a potential treatment strategy for mild to moderate TBI.

Upregulation of TrkB by forskolin facilitated survival of MSC and functional recovery of memory deficient model rats

Mesenchymal stem cells (MSCs) are effective vectors in delivering a gene of interest into degenerating brain. In ex vivo gene therapy, viability of transplanted MSCs is correlated with the extent of functional recovery. It has been reported that BDNF facilitates survival of MSCs but dividing MSCs do not express the BDNF receptor, TrkB. In this study, we found that the expression of TrkB is upregulated in human MSCs by the addition of forskolin (Fsk), an activator of adenylyl cyclase. To increase survival rate of MSCs and their secretion of tropic factors that enhance regeneration of endogenous cells, we pre-exposed hMSCs with Fsk and transduced with BDNF-adenovirus before transplantation into the brain of memory deficient rats, a degenerating brain disease model induced by ibotenic acid injection. Viability of MSCs and expression of a GABA synthesizing enzyme were increased. The pre-treatment improved learning and memory, as detected by the behavioral tests including Y-maze task and passive avoidance test. These results suggest that TrkB expression of hMSCs elevates the neuronal regeneration and efficiency of BDNF delivery for treating degenerative neurological diseases accompanying memory loss.

Functions of neurotrophins and growth factors in neurogenesis and brain repair

The identification and isolation of multipotent neural stem and progenitor cells in the brain, giving rise to neurons, astrocytes, and oligodendrocytes initiated many studies in order to understand basic mechanisms of endogenous neurogenesis and repair mechanisms of the nervous system and to develop novel therapeutic strategies for cellular regeneration therapies in brain disease. A previous review (Trujillo et al., Cytometry A 2009;75:38-53) focused on the importance of extrinsic factors, especially neurotransmitters, for directing migration and neurogenesis in the developing and adult brain. Here, we extend our review discussing the effects of the principal growth and neurotrophic factors as well as their intracellular signal transduction on neurogenesis, fate determination and neuroprotective mechanisms. Many of these mechanisms have been elucidated by in vitro studies for which neural stem cells were isolated, grown as neurospheres, induced to neural differentiation under desired experimental conditions, and analyzed for embryonic, progenitor, and neural marker expression by flow and imaging cytometry techniques. The better understanding of neural stem cells proliferation and differentiation is crucial for any therapeutic intervention aiming at neural stem cell transplantation and recruitment of endogenous repair mechanisms.

Berberine promotes axonal regeneration in injured nerves of the peripheral nervous system

Berberine, an isoquinoline alkaloid component of Coptidis Rhizoma (goldenthread) extract, has been reported to have therapeutic potential for central nervous system disorders such as Alzheimer's disease, cerebral ischemia, and schizophrenia. We have previously shown that berberine promotes the survival and differentiation of hippocampal precursor cells. In a memory-impaired rat model induced by ibotenic acid injection, the survival of pyramidal and granular cells was greatly increased in the hippocampus by berberine administration. In the present study, we investigated the effects of berberine on neurite outgrowth in the SH-SY5Y neuronal cell line and axonal regeneration in the rat peripheral nervous system (PNS). Berberine enhanced neurite extension in differentiating SH-SY5Y cells at concentrations of 0.25-3 μg/mL. In an injury model of the rat sciatic nerve, we examined the neuroregenerative effects of berberine on axonal remyelination by using immunohistochemical analysis. Four weeks after berberine administration (20 mg/kg i.p. once per day for 1 week), the thickness of remyelinated axons improved approximately 1.4-fold in the distal stump of the injury site. Taken together, these results indicate that berberine promotes neurite extension and axonal regeneration in injured nerves of the PNS.

The application of platelet-rich plasma may be a novel treatment for central nervous system diseases

As a potential biological product, platelet-rich plasma (PRP) has been widely utilized in the areas of oral and maxillofacial reconstruction, bone and soft tissue restoration and wound healing. A recent study reported that the application of PRP on interrupted sciatic nerve could promote remyelinization of peripheral nerve. This renovated a notion that the application of PRP might extend to the nervous system. Most central nervous system (CNS) diseases have a series of common pathological changes in the later period of diseases which induce neurons and glia apoptosis and aggravate neurological dysfunction. It has been demonstrated that the potent restorative function of PRP is mainly based on neurotrophic capacity of preparation rich in growth factors (PRGFs) and scaffolding effect of platelet-rich gel (PRG), all of which could be certified to ameliorate the pathological process of CNS diseases. In view of this, we propose a hypothesis that the application of PRP and its derivatives might provide a novel therapeutic approach for CNS diseases, especially for traumatic brain or spinal cord injury, autoimmune diseases and neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis.

Neural stem cell differentiation is mediated by integrin beta4 in vitro

Neural stem cells are capable of differentiating into three major neural cell types, but the underlying molecular mechanisms remain unclear. Here, we investigated the mechanism by which integrin beta4 modulates mouse neural stem cell differentiation in vitro. Inhibition of endogenous integrin beta4 by RNA interference inhibited the cell differentiation and the expression of fibroblast growth factor receptor 2 but not fibroblast growth factor receptor 1 or fibroblast growth factor receptor 3. Overexpression of integrin beta4 in neural stem cells promoted neural stem cell differentiation. Furthermore, integrin beta4-induced differentiation of neural stem cells was attenuated by SU5402, the inhibitor of fibroblast growth factor receptors. Finally, we investigated the role of integrin beta4 in neural stem cell survival: knockdown of integrin beta4 did not affect survival or apoptosis of neural stem cells. These data provide evidence that integrin beta4 promotes differentiation of mouse neural stem cells in vitro possibly through fibroblast growth factor receptor 2.

Zuckerkandl's organ improves long-term survival and function of neural stem cell derived dopaminergic neurons in Parkinsonian rats

Transplantation of neural stem cells (NSC) derived dopamine (DA) neurons has emerged as an alternative approach to fetal neural cell transplantation in Parkinson's disease (PD). However, similar to fetal neural cell, survival of these neurons following transplantation is also limited due to limited striatal reinnervation (graft with dense neuronal core), limited host-graft interaction, poor axonal outgrowth, lack of continuous neurotrophic factors supply and principally an absence of cell adhesion molecules mediated appropriate developmental cues. In the present study, an attempt has been made to increase survival and function of NSC derived DA neurons, by co-grafting with Zuckerkandl's organ (a paraneural organ that expresses neurotrophic factors as well as cell adhesion molecules); to provide continuous NTF support and developmental cues to transplanted DA neurons in the rat model of PD. 24 weeks post transplantation, a significant number of surviving functional NSC derived DA neurons were observed in the co-transplanted group as evident by an increase in the number of tyrosine hydroxylase immunoreactive (TH-IR) neurons, TH-IR fiber density, TH-mRNA expression and TH-protein level at the transplantation site (striatum). Significant behavioral recovery (amphetamine induced stereotypy and locomotor activity) and neurochemical recovery (DA-D2 receptor binding and DA and DOPAC levels at the transplant site) were also observed in the NSC+ZKO co-transplanted group as compared to the NSC or ZKO alone transplanted group. In vivo results were further substantiated by in vitro studies, which suggest that ZKO increases the NSC derived DA neuronal survival, differentiation, DA release and neurite outgrowth as well as protects against 6-OHDA toxicity in co-culture condition. The present study suggests that long-term and continuous NTF support provided by ZKO to the transplanted NSC derived DA neurons, helped in their better survival, axonal arborization and integration with host cells, leading to long-term functional restoration in the rat model of PD.

Adult hippocampal neurogenesis and aging

The demographic changes in the foreseeable future stress the need for research on successful cognitive aging. Advancing age constitutes a primary risk factor for disease of the central nervous system most notably neurodegenerative disorders. The hippocampus is one of the brain regions that is prominently affected by neurodegeneration and functional decline even in what is still considered "normal aging". Plasticity is the basis for how the brain adapts to changes over time. The discovery of adult hippocampal neurogenesis has added a whole new dimension to research on structural plasticity in the adult and aging hippocampus. In this article, we briefly summarize and discuss recent findings on the regulation of adult neurogenesis with relevance to aging. Aging is an important co-variable for many regulatory mechanisms affecting adult neurogenesis but so far, only few studies have specifically addressed this interaction. We hypothesize that adult neurogenesis contributes to a neural reserve, i.e. the maintained potential for structural plasticity that allows compensation in situations of functional losses with aging. As such we propose that adult neurogenesis might contribute to the structural correlates of successful aging.

Mesencephalic human neural progenitor cells transplanted into the neonatal hemiparkinsonian rat striatum differentiate into neurons and improve motor behaviour

Neural stem cell transplantation is a promising strategy for the treatment of neurodegenerative diseases. To evaluate the differentiation potential of human neural progenitor cells (hNPCs) as a prerequisite for clinical trials, we intracerebrally transplanted in vitro expanded fetal mesencephalic hNPCs into hemiparkinsonian rats. On postnatal day one (P1), 17 animals underwent a unilateral intraventricular 6-hydroxydopamine injection into the right lateral ventricle. At P3, animals (n = 10) received about 100,000 hNPCs (1 microL) in the right striatum. Five weeks after birth, animals underwent behaviour tests prior to fixation, followed by immunohistochemistry on brain slices for human nuclei, glial fibrillary acidic protein, S100beta, neuronal nuclei antigen, neuron-specific enolase and tyrosine hydroxylase. Compared with the apomorphine-induced rotations in the lesioned-only group (7.4 +/- 0.5 min(-1)), lesioned and successfully transplanted animals (0.3 +/- 0.1 min(-1)) showed a significant therapeutic improvement. Additionally, in the cylinder test, the lesioned-only animals preferred to use the ipsilateral forepaw. Conversely, the lesioned and transplanted animals showed no significant side bias similar to untreated control animals. Transplanted human nuclei-immunoreactive cells were found to survive and migrate up to 2000 microm into the host parenchyma, many containing the pan-neuronal markers neuronal nuclei antigen and neuron-specific enolase. In the striatum, tyrosine hydroxylase-immunoreactive somata were also found, indicating a dopaminergic differentiation capacity of transplanted hNPCs in vivo. However, the relative number of tyrosine hydroxylase-immunoreactive neurons in vivo seemed to be lower than in corresponding in vitro differentiation. To minimize donor tissue necessary for transplantation, further investigations will aim to enhance dopaminergic differentiation of transplanted cells in vivo.

Identification of aromatase-positive radial glial cells as progenitor cells in the ventricular layer of the forebrain in zebrafish

Compared with other vertebrates, the brain of adult teleost fish exhibits two unique features: it exhibits unusually high neurogenic activity and strongly expresses aromatase, a key enzyme that converts aromatizable androgens into estrogens. Until now, these two features, high neurogenic and aromatase activities, have never been related to each other. Recently, it was shown that aromatase is expressed in radial glial cells of the forebrain and not in neurons. Here, we further document that Aromatase B is never detected in cells expressing the markers of postmitotic neurons, Hu and acetylated tubulin. By using a combination of bromodeoxyuridine (BrdU) treatment and immunohistochemical techniques, we demonstrate for the first time to our knowledge that aromatase-positive radial cells actively divide to generate newborn cells in many forebrain regions. Such newborn cells can further divide, as shown by BrdU-proliferating cell nuclear antigen double staining. We also demonstrate that, over time, newborn cells move away from the ventricles, most likely by migrating along the radial processes. Finally, by using antisera to Hu and acetylated tubulin, we further document that some of the newborn cells derived from radial glia differentiate into neurons. These data provide new evidence for the mechanism of neurogenesis in the brain of adult fish. In addition, given that estrogens are well-known neurotrophic and neuroprotective factors affecting proliferation, apoptosis, migration, and differentiation, the expression of aromatase in the neural stem cells of the adult strongly demonstrates that the fish brain is an outstanding model for studying the effects of estrogens on adult neurogenesis and brain repair.

Mechanisms of cell death of neural progenitor cells caused by trophic support deprivation

Cell death of neural progenitor cells is the primary problem limiting the value of neural progenitor cell-based therapy for central nervous system disorders. However, little is known about the mechanism of cell death of neural progenitor cells. In this study, we investigated the mechanisms of cell death of a multipotent cell line, MEB5, caused by deprivation of epidermal growth factor (EGF). When EGF was removed from the culture medium, the total number of viable MEB5 cells reduced, and nuclear condensation and elevation of caspase-3-like enzyme activity were observed in MEB5 cells. Treatment with a broad-range caspase inhibitor reduced cell death in a concentration-dependent manner, indicating that MEB5 cells undergo caspase-mediated apoptotic cell death caused by EGF deprivation. We also investigated the effects of glutamate receptor antagonists, antioxidants and nitric oxide synthase inhibitor on EGF deprivation-induced cell death. N-methyl-D-aspartate (NMDA) glutamate receptor antagonists, alpha-amino-3-hydrozy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptor antagonist and nitric oxide synthase inhibitor failed to reduce cell death. In contrast, two antioxidants with different chemical structures reduced cell death in a concentration-dependent manner. The production of reactive oxygen species was detected in MEB5 cells after EGF deprivation by monitoring dichlorodihydrofluorescein fluorescence as a marker of reactive oxygen species-related radicals. Our results suggest that oxidative stress triggers caspase-mediated apoptosis of neural progenitor cells by trophic support deprivation.