Towards a Better Treatment Option for Parkinson’s Disease: A Review of Adult Neurogenesis

The motor symptoms of Parkinson’s disease (PD) are caused by degeneration of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) of midbrain. Given the fact that current treatment options are mostly symptomatic and based on increasing DA level in the nigrostriatal system, it is generally believed the most effective and long-lasting treatment for PD motor symptoms will be replacing SNc DA cells, either by endogenous repair (i.e. neurogenesis) or cell transplantation. While cell transplantation is hindered by failure of acquisition and maintenance of the DA phenotype by transplanted cells, hope rests upon non-invasive cell replacement therapy (CRT) with endogenous neural stem cells, which have the potential to give rise to new neurons including DA neurons. Understanding underlying mechanisms and signalling pathways of neurogenesis in the adult brain could shed light on obstacles to achieve effective CRTs and better treatments for PD. This paper first reviews different therapeutic strategies in context of PD along with their advantages and disadvantages followed by an extensive review of adult neurogenesis.     

Efficient regeneration by activation of neurogenesis in homeostatically quiescent regions of the adult vertebrate brain

In contrast to mammals, salamanders and teleost fishes can efficiently repair the adult brain. It has been hypothesised that constitutively active neurogenic niches are a prerequisite for extensive neuronal regeneration capacity. Here, we show that the highly regenerative salamander, the red spotted newt, displays an unexpectedly similar distribution of active germinal niches with mammals under normal physiological conditions. Proliferation zones in the adult newt brain are restricted to the forebrain, whereas all other regions are essentially quiescent. However, ablation of midbrain dopamine neurons in newts induced ependymoglia cells in the normally quiescent midbrain to proliferate and to undertake full dopamine neuron regeneration. Using oligonucleotide microarrays, we have catalogued a set of differentially expressed genes in these activated ependymoglia cells. This strategy identified hedgehog signalling as a key component of adult dopamine neuron regeneration. These data show that brain regeneration can occur by activation of neurogenesis in quiescent brain regions.     

Oct4-induced reprogramming is required for adult brain neural stem cell differentiation into midbrain dopaminergic neurons

Neural stem cells (NSCs) lose their competency to generate region-specific neuronal populations at an early stage during embryonic brain development. Here we investigated whether epigenetic modifications can reverse the regional restriction of mouse adult brain subventricular zone (SVZ) NSCs. Using a variety of chemicals that interfere with DNA methylation and histone acetylation, we showed that such epigenetic modifications increased neuronal differentiation but did not enable specific regional patterning, such as midbrain dopaminergic (DA) neuron generation. Only after Oct-4 overexpression did adult NSCs acquire a pluripotent state that allowed differentiation into midbrain DA neurons. DA neurons derived from Oct4-reprogrammed NSCs improved behavioural motor deficits in a rat model of Parkinson's disease (PD) upon intrastriatal transplantation. Here we report for the first time the successful differentiation of SVZ adult NSCs into functional region-specific midbrain DA neurons, by means of Oct-4 induced pluripotency.     

Lesion-induced neurogenesis in the hypothalamus is involved in behavioral recovery in adult ring doves

Although neurogenesis in the brain of adult vertebrates is region dependent, lesion induces generation of new neurons in non-neurogenic brain regions. These findings raise the question of the role of new neurons in brain repair and functional recovery. We addressed this question by applying previous observations that electrolytic lesion induced neurogenesis in the ventromedial nucleus (VMN) of the hypothalamus in adult ring doves. Such lesions disrupted the male's courtship behavior, which could be reinstated after rehabilitation with a female. We investigated whether lesion-induced newborn neurons in the VMN facilitate the recovery of courtship behavior in the lesioned birds. We conducted systematic observations of cytological, morphological, and neuroanatomical changes in the lesioned VMN, and concurrently we monitored behavioral changes. Using a multitude of specific cell markers, we found a well-circumscribed cellular zone that proliferated actively. This highly proliferative zone initially appeared along the periphery of the lesion site, where cells had high levels of expression of neuronal, glial, and neurovascular markers. As newborn neurons matured at the lesion site, the necrosis gradually decreased, whereas a downsized proliferative zone relocated to a region ventral to the VMN. Some of the mature neurons were found to project to the midbrain vocal nuclei. Restoration of these projection neurons coincided with the recovery of courtship vocalization. Finally, we found that a social factor, that is, when the male doves were cohoused with a mate, facilitated neurogenesis and behavioral recovery. These results suggest that lesion-induced neurogenesis contributes to behavioral recovery in adult animals.     

Absence of Ret signaling in mice causes progressive and late degeneration of the nigrostriatal system

Support of ageing neurons by endogenous neurotrophic factors such as glial cell line–derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) may determine whether the neurons resist or succumb to neurodegeneration. GDNF has been tested in clinical trials for the treatment of Parkinson disease (PD), a common neurodegenerative disorder characterized by the loss of midbrain dopaminergic (DA) neurons. BDNF modulates nigrostriatal functions and rescues DA neurons in PD animal models. The physiological roles of GDNF and BDNF signaling in the adult nigrostriatal DA system are unknown. We generated mice with regionally selective ablations of the genes encoding the receptors for GDNF (Ret) and BDNF (TrkB). We find that Ret, but not TrkB, ablation causes progressive and adult-onset loss of DA neurons specifically in the substantia nigra pars compacta, degeneration of DA nerve terminals in striatum, and pronounced glial activation. These findings establish Ret as a critical regulator of long-term maintenance of the nigrostriatal DA system and suggest conditional Ret mutants as useful tools for gaining insights into the molecular mechanisms involved in the development of PD.     

Lesion‐induced neurogenesis in the hypothalamus is involved in behavioral recovery in adult ring doves

Although neurogenesis in the brain of adult vertebrates is region dependent, lesion induces generation of new neurons in non‐neurogenic brain regions. These findings raise the question of the role of new neurons in brain repair and functional recovery. We addressed this question by applying previous observations that electrolytic lesion induced neurogenesis in the ventromedial nucleus (VMN) of the hypothalamus in adult ring doves. Such lesions disrupted the male's courtship behavior, which could be reinstated after rehabilitation with a female. We investigated whether lesion‐induced newborn neurons in the VMN facilitate the recovery of courtship behavior in the lesioned birds. We conducted systematic observations of cytological, morphological, and neuroanatomical changes in the lesioned VMN, and concurrently we monitored behavioral changes. Using a multitude of specific cell markers, we found a well‐circumscribed cellular zone that proliferated actively. This highly proliferative zone initially appeared along the periphery of the lesion site, where cells had high levels of expression of neuronal, glial, and neurovascular markers. As newborn neurons matured at the lesion site, the necrosis gradually decreased, whereas a downsized proliferative zone relocated to a region ventral to the VMN. Some of the mature neurons were found to project to the midbrain vocal nuclei. Restoration of these projection neurons coincided with the recovery of courtship vocalization. Finally, we found that a social factor, that is, when the male doves were cohoused with a mate, facilitated neurogenesis and behavioral recovery. These results suggest that lesion‐induced neurogenesis contributes to behavioral recovery in adult animals. © 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain

Post-embryonic neurogenesis is a fundamental feature of the vertebrate brain. However, the level of adult neurogenesis decreases significantly with phylogeny. In the first part of this review, a comparative analysis of adult neurogenesis and its putative roles in vertebrates are discussed. Adult neurogenesis in mammals is restricted to two telencephalic constitutively active zones. On the contrary, non-mammalian vertebrates display a considerable amount of adult neurogenesis in many brain regions. The phylogenetic differences in adult neurogenesis are poorly understood. However, a common feature of vertebrates (fish, amphibians and reptiles) that display a widespread adult neurogenesis is the substantial post-embryonic brain growth in contrast to birds and mammals. It is probable that the adult neurogenesis in fish, frogs and reptiles is related to the coordinated growth of sensory systems and corresponding sensory brain regions. Likewise, neurons are substantially added to the olfactory bulb in smell-oriented mammals in contrast to more visually oriented primates and songbirds, where much fewer neurons are added to the olfactory bulb. The second part of this review focuses on the differences in brain plasticity and regeneration in vertebrates. Interestingly, several recent studies show that neurogenesis is suppressed in the adult mammalian brain. In mammals, neurogenesis can be induced in the constitutively neurogenic brain regions as well as ectopically in response to injury, disease or experimental manipulations. Furthermore, multipotent progenitor cells can be isolated and differentiated in vitro from several otherwise silent regions of the mammalian brain. This indicates that the potential to recruit or generate neurons in non-neurogenic brain areas is not completely lost in mammals. The level of adult neurogenesis in vertebrates correlates with the capacity to regenerate injury, for example fish and amphibians exhibit the most widespread adult neurogenesis and also the greatest capacity to regenerate central nervous system injuries. Studying these phenomena in non-mammalian vertebrates may greatly increase our understanding of the mechanisms underlying regeneration and adult neurogenesis. Understanding mechanisms that regulate endogenous proliferation and neurogenic permissiveness in the adult brain is of great significance in therapeutical approaches for brain injury and disease.     

The hTH-GFP Reporter Rat Model for the Study of Parkinson's Disease

Parkinson disease (PD) is the second leading neurodegenerative disease in the US. As there is no known cause or cure for PD, researchers continue to investigate disease mechanisms and potential new therapies in cell culture and in animal models of PD. In PD, one of the most profoundly affected neuronal populations is the tyrosine hydroxylase (TH)-expressing dopaminergic (DA) neurons of the substantia nigra pars compacta (SNpc). These DA-producing neurons undergo degeneration while neighboring DA-producing cells of the ventral tegmental area (VTA) are largely spared. To aid in these studies, The Michael J. Fox Foundation (MJFF) partnered with Thomas Jefferson University and Taconic Inc. to generate new transgenic rat lines carrying the human TH gene promoter driving EGFP using a 11 kb construct used previously to create a hTH-GFP mouse reporter line. Of the five rat founder lines that were generated, three exhibited high level specific GFP fluorescence in DA brain structures (ie. SN, VTA, striatum, olfactory bulb, hypothalamus). As with the hTH-GFP mouse, none of the rat lines exhibit reporter expression in adrenergic structures like the adrenal gland. Line 12141, with its high levels of GFP in adult DA brain structures and minimal ectopic GFP expression in non-DA structures, was characterized in detail. We show here that this line allows for anatomical visualization and microdissection of the rat midbrain into SNpc and/or VTA, enabling detailed analysis of midbrain DA neurons and axonal projections after toxin treatment in vivo. Moreover, we further show that embryonic SNpc and/or VTA neurons, enriched by microdissection or FACS, can be used in culture or transplant studies of PD. Thus, the hTH-GFP reporter rat should be a valuable tool for Parkinson''s disease research.     

Targeted methylation sequencing reveals dysregulated Wnt signaling in Parkinson disease

Parkinson disease(PD) is a progressive neurodegenerative movement disorder. Both environmental and genetic factors play important roles in PD etiology. A number of environmental toxins cause parkinsonism in human and animal models. Genetic studies of rare early onset familial PD cases resulted in identification of disease-linked mutations in multiple genes. Nevertheless, the potential interaction between environment and genetics in PD pathogenesis remains largely unknown. We hypothesized that environmental factors induce abnormal epigenetic regulation that is involved in the pathogenesis of both familial and sporadic PD. We determined the global methylation status of 80,000e110,000 Cp G sites in each of the five sporadic PD patient brains and five age and postmodern interval matched control brains utilizing bisulfite padlock sequencing. Multiple genes involved in neurogenesis, particularly the ones in the Wnt signaling pathway, were hypermethylated in PD brains compared to their matched control brains. Consistent with the DNA methylation changes, marked reduction of protein expression was observed for four Wnt and neurogenesis related genes(FOXC1, NEURG2, SPRY1, and CTNNB1) in midbrain dopaminergic(DA) neurons of PD. The treatment of low concentration of 1-methyl-4-phenylpyridinium(MPPt) for cells resulted in downregulation of Wnt related genes. The study revealed an important link between the epigenetic disregulation of Wnt signaling and the pathogenesis and progression of PD.     

Naringin protects the nigrostriatal dopaminergic projection through induction of GDNF in a neurotoxin model of Parkinson's disease

This study investigated the effect of naringin, a major flavonoid in grapefruit and citrus fruits, on the degeneration of the nigrostriatal dopaminergic (DA) projection in a neurotoxin model of Parkinson's disease (PD) in vivo and the potential underlying mechanisms focusing on the induction of glia-derived neurotrophic factor (GDNF), well known as an important neurotrophic factor involved in the survival of adult DA neurons. 1-Methyl-4-phenylpyridinium (MPP⁺) was unilaterally injected into the medial forebrain bundle of rat brains for a neurotoxin model of PD in the presence or absence of naringin by daily intraperitoneal injection. To ascertain whether naringin-induced GDNF contributes to neuroprotection, we further investigated the effects of intranigral injection of neutralizing antibodies against GDNF in the MPP⁺ rat model of PD. Our observations demonstrate that naringin could increase the level of GDNF in DA neurons, contributing to neuroprotection in the MPP⁺ rat model of PD, with activation of mammalian target of rapamycin complex 1. Moreover, naringin could attenuate the level of tumor necrosis factor-α in microglia increased by MPP⁺-induced neurotoxicity in the substantia nigra. These results indicate that naringin could impart to DA neurons the important ability to produce GDNF as a therapeutic agent against PD with anti-inflammatory effects, suggesting that naringin is a beneficial natural product for the prevention of DA degeneration in the adult brain.     

The beta-chemokines CCL2 and CCL7 are two novel differentiation factors for midbrain dopaminergic precursors and neurons

β-chemokines are secreted factors that regulate diverse functions in the adult brain, such as neuro-immune responses and neurotransmission, but their function in the developing brain is largely unknown. We recently found that the orphan nuclear receptor, Nurr1, up regulates CCL2 and CCL7 in neural stem cells, suggesting a possible function of β-chemokines in midbrain development. Here we report that two β-chemokines, CCL2 and CCL7, and two of their receptors, CCR1 and CCR2, are expressed and developmentally regulated in the ventral midbrain (VM). Moreover, we found that the expression of CCL7 was down regulated in the Nurr1 knockout mice, linking CCL7 to dopamine (DA) neuron development. When the function of CCL2 and CCL7 was examined, we found that they selectively enhanced the differentiation of Nurr1+ precursors into DA neurons, but not their survival or progenitor proliferation in primary precursor cultures. Moreover, both CCL2 and CCL7 promoted neuritogenesis in midbrain DA neuron cultures. Thus, our results show for the first time a function of β-chemokines in the developing brain and identify β-chemokines as novel class of pro-differentiation factors for midbrain DA neurons. These data also suggest that β-chemokines may become useful tools to enhance the differentiation of DA cell preparations for cell replacement therapy and drug discovery in Parkinson's disease (PD).     

Development of the mesencephalic dopaminergic neuron system is compromised in the absence of neurogenin 2

Neurogenin 2 (Ngn2) is a proneural gene involved in neuronal differentiation and subtype specification in various regions of the nervous system. In the ventral midbrain, Ngn2 is expressed in a spatiotemporal pattern that correlates with the generation of mesencephalic dopaminergic (mesDA) neurons. We show here that lack of Ngn2 impairs the development of mesDA neurons, such that less than half of the normal mesDA neuron number remain in Ngn2 mutant mice at postnatal stages. Analysis of Ngn2 mutant mice during mesDA neurogenesis show that medially located precursors are formed but are arrested in their differentiation at a stage when they have not yet acquired the characteristics of mesDA neuron precursors. Loss of Ngn2 function appears to specifically affect the generation of DA neurons, as the development of other types of neurons within the ventral midbrain is unaltered. Ngn2 is the first example of a gene expressed in progenitors in the ventricular zone of the mesDA neuron domain that is essential for proper mesDA neuron differentiation, and whose loss of function causes impaired mesDA neurogenesis without other major abnormalities in the ventral midbrain.     

Cell-autonomous FGF signaling regulates anteroposterior patterning and neuronal differentiation in the mesodiencephalic dopaminergic progenitor domain

The structure and projection patterns of adult mesodiencephalic dopaminergic (DA) neurons are one of the best characterized systems in the vertebrate brain. However, the early organization and development of these nuclei remain poorly understood. The induction of midbrain DA neurons requires sonic hedgehog (Shh) from the floor plate and fibroblast growth factor 8 (FGF8) from the isthmic organizer, but the way in which FGF8 regulates DA neuron development is unclear. We show that, during early embryogenesis, mesodiencephalic neurons consist of two distinct populations: a diencephalic domain, which is probably independent of isthmic FGFs; and a midbrain domain, which is dependent on FGFs. Within these domains, DA progenitors and precursors use partly different genetic programs. Furthermore, the diencephalic DA domain forms a distinct cell population, which also contains non-DA Pou4f1(+) cells. FGF signaling operates in proliferative midbrain DA progenitors, but is absent in postmitotic DA precursors. The loss of FGFR1/2-mediated signaling results in a maturation failure of the midbrain DA neurons and altered patterning of the midbrain floor. In FGFR mutants, the DA domain adopts characteristics that are typical for embryonic diencephalon, including the presence of Pou4f1(+) cells among TH(+) cells, and downregulation of genes typical of midbrain DA precursors. Finally, analyses of chimeric embryos indicate that FGF signaling regulates the development of the ventral midbrain cell autonomously.     

Regulation of Histone Acetylation by Autophagy in Parkinson Disease

Parkinson disease (PD) is the most common age-dependent neurodegenerative movement disorder. Accumulated evidence indicates both environmental and genetic factors play important roles in PD pathogenesis, but the potential interaction between environment and genetics in PD etiology remains largely elusive. Here, we report that PD-related neurotoxins induce both expression and acetylation of multiple sites of histones in cultured human cells and mouse midbrain dopaminergic (DA) neurons. Consistently, levels of histone acetylation are markedly higher in midbrain DA neurons of PD patients compared to those of their matched control individuals. Further analysis reveals that multiple histone deacetylases (HDACs) are concurrently decreased in 1-methyl-4-phenylpyridinium (MPP⁺)-treated cells and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mouse brains, as well as midbrain tissues of human PD patients. Finally, inhibition of histone acetyltransferase (HAT) protects, whereas inhibition of HDAC1 and HDAC2 potentiates, MPP⁺-induced cell death. Pharmacological and genetic inhibition of autophagy suppresses MPP⁺-induced HDACs degradation. The study reveals that PD environmental factors induce HDACs degradation and histone acetylation increase in DA neurons via autophagy and identifies an epigenetic mechanism in PD pathogenesis.     

Can mesenchymal stem cells reduce vulnerability of dopaminergic neurons in the substantia nigra to oxidative insult in individuals at risk to Parkinson's disease?

PD (Parkinson's disease) is characterized by the selective loss of DA (dopaminergic) neurons in the substantia nigra of the midbrain region, but not in the ventral tegmental area and other catecholaminergic cell group areas. The aetiology of PD is attributed both to environmental and genetic causes, and certain population of individuals may be classified as at risk of developing PD later in life. However, there are as yet no therapy regimens that can help to delay or prevent the onset of the disease to realize long-term benefits from this early diagnosis. In PD, a vicious cycle gets initiated in the substantia nigra, because of which susceptible neurons continue to degenerate whereas damaged neurons do not get enough support for regeneration. This happens primarily because of the local environment of oxidative damage brought about by the dual presence of dopamine and high levels of iron, decline in cellular detoxification systems and low density of glial cells surrounding the DA neurons in the mesencephalic region. To enhance the defence mechanism of the substantia nigra in this situation, it is necessary to combat the oxidative insult while providing trophic factors for the survival and regeneration of the damaged neurons. In light of in vitro and in vivo studies, MSCs (mesenchymal stem cells) as candidates for cell-based therapies in PD have greater scope than as mere replacement of cell type, since they can be used as a cellular system for the detoxification of ROS (reactive oxygen species) as well as a supplier of neurotrophic factors to modulate the local environment. Building on progress in unravelling the multipronged effect of MSCs, we therefore hypothesize that MSCs could be used as a prophylactic strategy to delay or prevent the onset of PD in at-risk individuals, and to slow down the progression of the disease.     

Towards brain repair: Insights from teleost fish

In the adult mammalian brain, the ability to minimize secondary cell death after injury, and to repair nervous tissue through generation of new neurons, is severely compromised. By contrast, certain taxa of non-mammalian vertebrates possess an enormous potential for regeneration. Examination of one of these taxa, teleost fish, has revealed a close link between this phenomenon and constitutive adult neurogenesis. Key factors mediating successful regeneration appear to be: elimination of damaged cells by apoptosis, instead of necrosis; activation of mechanisms that prevent the occurrence of secondary cell death; increased production of new neurons that replace neurons lost to injury; and activation of developmental mechanisms that mediate directed migration of the new cells to the site of injury, the differentiation of the young cells, and their integration into the existing neural network. Comparative analysis has suggested that constitutive adult neurogenesis is a primitive vertebrate trait, the main function of which has been to ensure a numerical matching between muscle fibers/sensory receptor cells and central elements involved in motor control/processing of sensory information associated with these peripheral elements. It is hypothesized that, when in the course of the evolution of mammals a major shift in the growth pattern from hyperplasia to hypertrophy took place, the number of neurogenic brain regions and new neurons markedly decreased. As a consequence, the potential for neuronal regeneration was greatly reduced, but remnants of neurogenic areas have persisted in the adult mammalian brain in form of quiescent stem cells. It is likely that the study of regeneration-competent taxa will provide important information on how to activate intrinsic mechanisms for successful brain regeneration in humans.