Methylazoximethanol acetate-induced cell death in the granule cell layer of the developing mouse cerebellum is associated with caspase-3 activation, but does not depend on the tissue-type plasminogen activator

Fine-tuning of neurogenesis is essential for the evolutionary expansion of the cerebral cortex

We used several animal models to study global and regional cortical surface expansion: The lissencephalic mouse, gyrencephalic normal ferrets, in which the parietal cortex expands more than the temporal cortex, and moderately lissencephalic ferrets, showing a similar degree of temporal and parietal expansion. We found that overall cortical surface expansion is achieved when specific events occur prior to surpragranular layer formation. (1) The subventricular zone (SVZ) shows substantial growth, (2) the inner SVZ contains an increased number of outer radial glia and intermediate progenitor cells expressing Pax6, and (3) the outer SVZ contains a progenitor cell composition similar to the combined VZ and inner SVZ. A greater parietal expansion is also achieved by eliminating the latero-dorsal neurogenic gradient, so that neurogenesis displays a similar developmental degree between parietal and temporal regions. In contrast, mice or lissencephalic ferrets show more advanced neurogenesis in the temporal region. In conclusion, we propose that global and regional cortical surface expansion rely on similar strategies consisting in altering the timing of neurogenic events prior to the surpragranular layer formation, so that more progenitor cells, and ultimately more neurons, are produced. This hypothesis is supported by findings from a ferret model of lissencephaly obtained by transiently blocking neurogenesis during the formation of layer IV.

Tissue-type plasminogen activator is an extracellular mediator of Purkinje cell damage and altered gait

Purkinje neurons are a sensitive and specialised cell type important for fine motor movement and coordination. Purkinje cell damage manifests as motor incoordination and ataxia - a prominent feature of many human disorders including spinocerebellar ataxia and Huntington's disease. A correlation between Purkinje degeneration and excess cerebellar levels of tissue-type plasminogen activator (tPA) has been observed in multiple genetically-distinct models of ataxia. Here we show that Purkinje loss in a mouse model of Huntington's disease also correlates with a 200% increase in cerebellar tPA activity. That elevated tPA levels arise in a variety of ataxia models suggests that tPA is a common mediator of Purkinje damage. To address the specific contribution of tPA to cerebellar dysfunction we studied the T4 mice line that overexpresses murine tPA in postnatal neurons through the Thy1.2 gene promoter, which directs preferential expression to Purkinje cells within the cerebellum. Here we show that T4 mice develop signs of cerebellar damage within 10 weeks of birth including atrophy of Purkinje cell soma and dendrites, astrogliosis, reduced molecular layer volume and altered gait. In contrast, T4 mice displayed no evidence of microgliosis, nor any changes in interneuron density, nor alteration in the cerebellar granular neuron layer. Thus, excess tPA levels may be sufficient to cause targeted Purkinje cell degeneration and ataxia. We propose that elevated cerebellar tPA levels exert a common pathway of Purkinje cell damage. Therapeutically lowering cerebellar tPA levels may represent a novel means of preserving Purkinje cell integrity and motor coordination across a wide range of neurodegenerative diseases.

Neurodegeneration in schizophrenia

The neurodegenerative aspect of schizophrenia presupposes gene-environmental interactions involving chromosomal abnormalities and obstetric/perinatal complications that culminate in predispositions that impart a particular vulnerability for drastic and unpredictable precipitating factors, such as stress or chemical agents. The notion of a neurodevelopmental progression to the disease state implies that early developmental insults, with neurodegenerative proclivities, evolve into structural brain abnormalities involving specific regional circuits and neurohumoral agents. This neurophysiological orchestration is expressed in the dysfunctionality observed in premorbid signs and symptoms arising in the eventual diagnosis, as well as the neurobehavioral deficits reported from animal models of the disorder. The relative contributions of perinatal insults, neonatal ventral hippocampus lesion, prenatal methylazoxymethanol acetate and early traumatic experience, as well as epigenetic contributions, are discussed from a neurodegenerative view of the essential neuropathology. It is implied that these considerations of factors that exert disruptive influences upon brain development, or normal aging, operationalize the central hub of developmental neuropathology around which the disease process may gain momentum. Nonetheless, the status of neurodegeneration in schizophrenia is somewhat tenuous and it is possible that brain imaging studies on animal models of the disorder, which may describe progressive alterations to cortical, limbic and ventricular structures similar to those of schizophrenic patients, are necessary to resolve the issue.

Expression of FasL and its interaction with Fas are mediated by c-Jun N-terminal kinase (JNK) pathway in 6-OHDA-induced rat model of Parkinson disease

Our previous studies and those of others have strongly suggested that c-Jun N-terminal kinase (JNK) signaling pathway plays a critical role in 6-hydroxydopamine (6-OHDA)-induced dopaminergic neuron injury in the substantia nigra. However, the downstream mechanism that accounts for the proapoptotic actions of JNK in 6-OHDA lesion remains to be investigated in detail. Fas, a member of the tumor necrosis factor receptor family with proapoptotic functions, was reported to be elevated within the striatum and substantia nigra pars compacta (SNc) of Parkinson's disease (PD) patients. In the present study, we examined the changes in the protein level of Fas ligand (FasL) and its interaction with Fas in a rat model of PD. We demonstrate that the expression of FasL and not Fas was increased after 6-OHDA lesion; additionally, the interaction of FasL and Fas was increased due to 6-OHDA lesion. This indicates that the 6-OHDA-induced activation of Fas signaling pathway is mediated by JNK and that FasL may be a promising target in the therapeutic approach for PD patients.

Adult Hippocampal Neurogenesis, Synaptic Plasticity and Memory: Facts and Hypotheses

The demonstration that progenitor cells in regions of the adult mammalian brain such as the dentate gyrus of the hippocampus can undergo mitosis and generate new cells that differentiate into functionally integrated neurons throughout life has marked a new era in neuroscience. In recent years, a wide range of investigations has been directed at understanding the physiological mechanisms and functional relevance of this form of brain plasticity. Our current knowledge of adult hippocampal neurogenesis indicates that the production of new cells in the brain follows a multi-step process during which newborn cells are submitted to various regulatory factors that influence cell proliferation, maturation, fate determination and survival. As details of the dynamics of morphological maturation and functional integration of newborn neurons in corticohippocampal circuits have become clearer, an increasing number of studies have examined how environmental and/or behavioural factors can modulate neurogenesis and affect hippocampal-dependent learning and memory. In this article we present an overview of recent literature that relates neurogenesis to hippocampal function on the basis of correlative studies investigating the modulation of neurogenesis by learning and behavioural experience, and the consequences of the loss of hippocampal neurogenesis for memory function. We also highlight experimental evidence that immature neurons exhibit unique electrophysiological characteristics and therefore may constitute a specific cell population particularly inclined to undergo activity-dependent plasticity. Moreover, we review recent work that reveals an unsuspected mechanistic link between synaptic plasticity and the proliferation and survival of new hippocampal neurons. From the present background of research, we argue that the incorporation of functional adult-generated neurons into existing neural networks provides a higher capacity for plasticity, which may favour the encoding and storage of certain types of memories. Depending on their birth date and maturation stage, new neurons might be implicated in the encoding/storage process of the task at hand or may help future learning experience. Finally, we highlight critical issues to be addressed in order to decipher the exact contribution of newly generated neurons to cognitive functions.

Proteomic Analysis of the Genotoxicant Methylazoxymethanol (MAM)-Induced Changes in the Developing Cerebellum

The genotoxicant methylazoxymethanol (MAM) is a widely used developmental neurotoxin, and its glucoside is an etiological factor for western Pacific amyotrophic lateral sclerosis-parkinsonism-dementia complex (ALS/PDC). Identification of global protein expression changes that occur in response to MAM in the developing cerebellum could provide valuable insight into the potential mechanisms involved in the neurodegeneration process. We have utilized fluorescence 2-dimensional differential gel electrophoresis (2D-DIGE), to determine the protein expression changes that occur during normal cerebellar development and in response to MAM. Three day-old postnatal C57BL/6 mice (PND3) received a single injection of MAM, and the cerebella of postnatal day 4 (PND4) and day 22 (PND22) were analyzed. Approximately, 1400 unique spots were matched and quantified in all samples. Comparison of PND4 and PND22 developing cerebellum showed that a significant fraction of the proteome (approximately 68%) changes at this stage. The immediate response of the developing cerebellum to MAM was minimal (approximately 10%). However, significant differences (27%) were noted 14 days after MAM exposure. In contrast, the transcriptome changes were more pronounced at 24 h compared to 14 days. MAM targeted several proteins networks including transport (e.g., alpha-synuclein), cytoskeletal (e.g., beta-tubulin, vimentin), and mitochondrial (e.g., Atp5b) proteins. Immunochemistry confirmed several of the changes in protein expression (alpha-synuclein). Comparison with gene expression changes revealed that the temporal changes observed in the transcriptome and proteome are not correlative. These studies demonstrate for the first time the potential networks involved during neuronal development and neurodegenerative processes that are perturbed by MAM.

Tissue-type plasminogen activator rescues neurones from serum deprivation-induced apoptosis through a mechanism independent of its proteolytic activity

Although the mechanism of action of tissue-type plasminogen activator (tPA) in excitotoxic necrosis is well documented, whether this serine protease can influence the apoptotic cascade remains a subject of debate. Here, we report that tPA protects cultured cortical neurones against apoptotic cell death induced by serum deprivation, an effect associated with a reduction of caspase-3 activation. Interestingly, blocking tPA proteolytic activity by either tPA stop or neuroserpin did not prevent this neuroprotection. Similarly, prevention of the interaction between tPA and its receptor low-density lipoprotein receptor-related protein (LRP) could not alter tPA anti-apoptotic activity. Interestingly, the survival-promoting effect of tPA was blocked by the phosphatidylinositol-3 (PI-3) kinase inhibitor, LY294002, but not by the mitogen-activated protein (MAP) kinase inhibitor, U0126. In conclusion, the present demonstration of an anti-apoptotic effect of tPA, independent of its enzymatic activity, reveals an additional level of complexity in our understanding of this critical mediator of brain physiology and pathology.

Molecular networks perturbed in a developmental animal model of brain injury

Methylazoxymethanol (MAM) is widely used as a developmental neurotoxin and exposure to its glucoside (i.e., cycasin) is associated with the prototypical neurological disorder western Pacific ALS/PDC. However, the specific molecular targets that play a key role in MAM-induced brain injury remain unclear. To reveal potential molecular networks targeted by MAM in the developing nervous system, we examined characteristic phenotypic changes (DNA damage, cytoarchitecture) induced by MAM and their correlation with gene expression differences using microarray assays (27,648 genes). Three day-old postnatal C57BL/6 mice (PND3) received a single injection of MAM and the cerebellum and cerebral cortex of PND4, 8, 15, and 22 mice were analyzed. DNA damage was detected in both the cerebellum (N7-mGua, TUNEL labeling) and cerebral cortex (N7-mGua) of PND4 mice, but progressive disruption of the cytoarchitecture was restricted to the cerebellum. A majority (>75%) of the genes affected (cerebellum 636 genes, cortex 1080 genes) by MAM were developmentally regulated, with a predominant response early (PND4) in the cerebellum and delayed (PND8 and 15) in the cerebral cortex. The genes and pathways (e.g., proteasome) affected by MAM in the cerebellum are distinct from cortex. The genes perturbed in the cerebellum reflect critical cellular processes such as development (17%), cell cycle (7%), protein metabolism (12%), and transcriptional regulation (9%) that could contribute to the observed cytoarchitectural disruption of the cerebellum. This study demonstrates for the first time that specific genes and molecular networks are affected by MAM during CNS development. Further investigation of these targets will help to understand how disruption of these developmental programs could contribute to chronic brain injury or neurodegenerative disease.

Cerebellar granule cells as a model to study mechanisms of neuronal apoptosis or survival in vivo and in vitro

Granule cells of the cerebellum constitute the largest homogeneous neuronal population of mammalian brain. Due to their postnatal generation and the feasibility of well characterized primary in vitro cultures, cerebellar granule cells are a model of election for the study of cellular and molecular correlates of mechanisms of survival/apoptosis and neurodegeneration/neuroprotection. The present review mainly deals with recent data on mechanisms and factors promoting survival or apoptotic elimination of cerebellar granule neurons, with a particular focus on the molecular correlates at the level of gene expression and induction of cellular signal pathways. The in vivo development is first analysed with particular reference to the role played by several neurotrophic factors and by the NMDA subtype of glutamate receptor. Then, mechanisms of survival/apoptosis are examined in the model of primary in vitro cultures, where the role of neurotrophins acting on cerebellar granule cells is followed by the large deal of data coming from the paradigm of potassium/serum withdrawal. The role of some key genes of the Bcl family, of some kinase systems and of transcriptional factors is primarily highlighted. Furthermore, the involvement of mitochondria, free radicals and proteases of the caspase family is considered. Finally, the use of cerebellar granule neurons in primary culture to experimentally address the issue of neurodegeneration and pharmacological neuroprotection is considered, with some comments on models at the borderline between necrosis and apoptosis, such as the excitotoxic neuronal damage. The overlapping of cellular signal pathways activated in granule neurons by apparently unrelated stimuli, such as neurotrophins and neurotransmitters/neuromodulators is stressed to put into light the special 'trophic' role played by activity in neurons. Finally, the advantage of designing and performing conceptually equivalent experiments on cerebellar granule neurons during development in vivo and in vitro, is stressed. On the basis of the reviewed material, it is concluded that cerebellar granule neurons have acquired a special position in modern neuroscience as one of the most reliable models for the study of neural development, function and pathology.