Upregulation of stathmin (p19) gene expression in adult rat brain during injury-induced synapse formation

Developmental Neurotoxicity of the Harmful Algal Bloom Toxin Domoic Acid: Cellular and Molecular Mechanisms Underlying Altered Behavior in the Zebrafish Model

BACKGROUND

Harmful algal blooms (HABs) produce potent neurotoxins that threaten human health, but current regulations may not be protective of sensitive populations. Early life exposure to low levels of the HAB toxin domoic acid (DomA) produces long-lasting behavioral deficits in rodent and primate models; however, the mechanisms involved are unknown. The zebrafish is a powerful in vivo vertebrate model system for exploring cellular processes during development and thus may help to elucidate mechanisms of DomA developmental neurotoxicity.

OBJECTIVES

We used the zebrafish model to investigate how low doses of DomA affect the developing nervous system, including windows of susceptibility to DomA exposure, structural and molecular changes in the nervous system, and the link to behavioral alterations.

METHODS

To identify potential windows of susceptibility, DomA (0.09-0.18 ng) was delivered to zebrafish through caudal vein microinjection during distinct periods in early neurodevelopment. Following exposure, structural and molecular targets were identified using live imaging of transgenic fish and RNA sequencing. To assess the functional consequences of exposures, we quantified startle behavior in response to acoustic/vibrational stimuli.

RESULTS

Larvae exposed to DomA at 2 d postfertilization (dpf), but not at 1 or 4 dpf, showed consistent deficits in startle behavior at 7 dpf, including lower responsiveness and altered kinematics. Similarly, myelination in the spinal cord was disorganized after exposure at 2 dpf but not 1 or 4 dpf. Time-lapse imaging revealed disruption of the initial stages of myelination. DomA exposure at 2 dpf down-regulated genes required for maintaining myelin structure and the axonal cytoskeleton.

DISCUSSION

These results in zebrafish reveal a developmental window of susceptibility to DomA-induced behavioral deficits and identify altered gene expression and disrupted myelin structure as possible mechanisms. The results establish a zebrafish model for investigating the mechanisms of developmental DomA toxicity, including effects with potential relevance to exposed sensitive human populations. https://doi.org/10.1289/EHP6652.

Influence of a genetic variant of the neuronal growth associated protein Stathmin 1 on cognitive and affective control processes: an event-related potential study

Stathmin 1 (STMN1) is a neuronal growth associated protein (NGAP) that is involved in microtubule dynamics and plays an important role in neurite outgrowth and synaptic plasticity. It is highly expressed in the amygdala, but also in different areas of the neocortex including the frontal lobe. Based on previous findings regarding an impact of STMN1 on fear processing, the present study aimed at extending the evidence concerning its functional role to include the domain of executive (frontal lobe) functions. To this end, a group of 59 healthy volunteers stratified for the single-nucleotide polymorphism rs182455 of the STMN1 gene was examined by means of three experimental paradigms probing different aspects of cognitive-affective functioning. Event-related potential measures of cognitive response control, emotional interference processing, and action monitoring were analyzed. STMN1 genotype significantly affected the NoGo-anteriorization (NGA)-a neurophysiological marker of cognitive response control associated with medial prefrontal cortex activation-as well as the modulation of the P300 by the valence of emotional Stroop stimuli. In both cases, carriers of the rs182455 C-allele showed altered cognitive-affective processing; effects appeared to be more pronounced in females. Our findings indicate a functional impact of STMN1 on cognitive and affective control processes, thereby complementing previous evidence on its role in fear processing. Based on these results, an influence of STMN1 should be considered in studies aiming at the etiopathogenesis of a broad range of neuropsychiatric disorders with dysfunctional networking, including neurodegenerative disorders as well as schizophrenia, autism spectrum disorders, anxiety disorders, depression, and ADHD.

Cytoskeleton-associated proteins are enriched in human embryonic-stem cell-derived neuroectodermal spheres

The ability to generate neural lineages from human embryonic stem cells (hESCs) in a controlled manner would further investigation of human neurogenesis and development of potential cell therapeutic applications to treat neurological diseases; however, generating such neural stem cells (NSCs) remains a challenge. In an attempt to characterize the cellular mechanisms involved in hESC differentiation into neuroprogenitor cells, we performed 2-DE using protein extracts from hESC-derived embryoid bodies (EBs) and neuroectodermal spheres (NESs) bearing neuroprogenitors. Of 47 differentially expressed protein spots, 28 nonredundant spots were shown to be upregulated in the NESs; these protein spots included neurogenesis-related proteins (TAF1, SEPT2, NPH3, and CRABP), as expected. Interestingly, 6 of these 28 protein spots were cytoskeleton-associated proteins (CSAP) such as Fascin-1, Cofilin-1, and Stathmin-1. Western-blot analyses confirmed the increased levels of these proteins in the NESs. Furthermore, immunostaining analysis showed that both Fascin-1 and Stathmin-1 were preferentially expressed in the inner rims of neural rosettes, which are characteristic features of neuroprogenitors in culture. We also confirmed prominent expression of Fascin-1 in (sub-)ventricular zone in E15.5 mouse fetal brain. Our results suggest that, in addition to the induction of those genes involved in neural development, hESC differentiation into the NES is associated with a marked reorganization of the cellular cytoskeleton.

Unilateral ischemic sensorimotor cortical damage induces contralesional synaptogenesis and enhances skilled reaching with the ipsilateral forelimb in adult male rats

Unilateral damage to the forelimb representation area of the sensorimotor cortex (SMC) results in a compensatory reliance on the unimpaired (ipsilateral to the lesion) forelimb as well as reorganization of neuronal structure and connectivity in the contralateral motor cortex. Recently, male rats with unilateral electrolytic SMC lesions were found to have enhanced skilled reaching performance with the ipsilesional forelimb compared with sham-operated controls. The present study was performed to determine whether these behavioral findings are replicable using an ischemic lesion and whether there is a link between the enhanced learning and synaptogenesis in motor cortical layer V opposite the trained limb and lesion, as assessed using stereological methods for light and electron microscopy. Rats were given a sham operation or an endothelin-1 (ET-1) induced ischemic SMC lesion. They were then trained for 20 days on a skilled reaching task with the unimpaired limb or received control procedures. As with previous findings using electrolytic lesions, rats with unilateral ischemic SMC lesions performed significantly better using the unimpaired forelimb than did sham-operates. Lesions, but not training, significantly increased the total number of motor cortical layer V synapses per neuron as well as the number of perforated and multisynaptic bouton (MSB) synapses per neuron compared with shams. Thus, in addition to a net increase in synapses, the improved reaching ability was coupled with an increase in synapse subtypes that have previously been linked to enhanced synaptic efficacy. The failure to induce synaptogenesis in layer V with reach training alone is in contrast to previous findings and may be related to training intensity.

Unilateral ischemic sensorimotor cortical damage in female rats: forelimb behavioral effects and dendritic structural plasticity in the contralateral homotopic cortex

Previous studies in male rats with unilateral sensorimotor cortical (SMC) damage have demonstrated dendritic structural plasticity in the contralateral homotopic cortex and an enhancement of skilled reaching performance in the forelimb ipsilateral to the lesion compared to sham-operated rats. The purpose of this study was to determine if these findings could be replicated in an ischemic lesion model in female rats. Female rats were given sham operations or unilateral ischemic (endothelin-1 induced) damage in the forelimb representation area of the SMC opposite their preferred forelimb. Animals then received either 20 consecutive days of training on a skilled reaching task with the non-preferred/unimpaired forelimb or no-training control procedures. The surface density of dendrites immunoreactive (IR) for microtubule-associated protein 2 (MAP2) was then measured in the motor cortex opposite the trained limb and/or lesion. Female rats with sufficiently large, but not very small, lesions performed better with the unimpaired forelimb than sham-operated rats on the reaching task. The post-lesion reaching performance was not found to be significantly dependent upon estrous stage at the time of surgery, in agreement with previous studies that failed to find sex or sex-hormone effects after other types of SMC damage. Additionally, there were major laminar-dependent increases in the surface density of MAP2 IR dendrites in the cortex opposite lesions and trained limbs. These findings in female rats are consistent with the dendritic and behavioral changes previously found in male rats. They extend these previous findings by indicating that lesion size is an important variable in the enhancement of reaching performance.

Facilitation of motor skill learning by callosal denervation or forced forelimb use in adult rats

Unilateral forelimb sensorimotor cortex lesions in adult rats produce a compensatory hyper-reliance on the forelimb ipsilateral to the lesion and temporally related glial and neural plasticity in the contralateral homotopic cortex. Recently, we found that these lesions enhance acquisition of a motor skills task with the ipsilateral, non-impaired, forelimb in comparison to shams. This effect might be related to a denervation-induced facilitation of neuroplastic changes in the motor cortex opposite the lesion and/or to the lesion-induced hyper-reliance on the non-impaired forelimb. The present study assessed whether increased forelimb use, denervation of motor cortical callosal afferents, or a combination of the two influences acquisition of a skilled reaching task. Adult rats with partial corpus callosum transections or sham procedures were either forced to rely on one forelimb or permitted normal forelimb use for 8 days. Rats were then trained for 14 days with their previously non-preferred forelimb (and the forced-use limb) on a unilateral pellet retrieval task. Compared to shams, transections produced a greater acquisition rate and asymptotic performance level on the task. Forced-use improved reaching performance relative to controls, but this effect was less enduring than the improvements produced by transections alone. The addition of forced-use to transections did not further enhance performance. These findings suggest that denervation-induced changes are likely to be a major contributor to the enhanced learning observed after unilateral sensorimotor cortex lesions.

Proteomic and immunochemical characterization of a role for stathmin in adult neurogenesis

Stathmin is a developmentally regulated cytosolic protein expressed at high levels in the brain. Two-dimensional differential in-gel electrophoresis and mass spectroscopy of proteins expressed in immature and mature cultures from embryonic rat cerebral cortex identified stathmin among several differentially expressed proteins, consistent with a possible role in neurogenesis. Stathmin immunohistochemistry in adult rodent brain revealed prominent expression in neuroproliferative zones and neuronal migration pathways, a pattern that resembles the expression of doublecortin, which is implicated in neuronal migration. Stathmin immunoreactivity was also associated with neurons undergoing ectopic chain migration into the ischemic striatum and cerebral cortex following focal cerebral ischemia. Reducing the expression of stathmin or doublecortin with an antisense oligonucleotide inhibited the migration of new neurons from the subventricular zone to the olfactory bulb via the rostral migratory stream. These results suggest a role for stathmin in the migration of newborn neurons in the adult brain.

Synapse replacement in the striatum of the adult rat following unilateral cortex ablation

Defining the selective pattern of synapse replacement that occurs in different areas of the damaged brain is essential for predicting the limits of functional compensation that can be achieved after various types of brain injury. Here we describe the time course of dendritic reorganization, spine loss and recovery, and synapse replacement in the striatum following a unilateral cortex ablation. We found that the time course for the transient loss and recovery of dendritic spines on medium spiny I (MSI) neurons, the primary postsynaptic target for corticostriatal axons, paralleled the time course for the removal of degenerating axon terminals from the neuropil and the formation of new synapses on MSI neurons. Reinnervation of the deafferented striatum occurred chiefly by axon terminals that formed asymmetric synapses with dendritic spines of MSI neurons, and the mean density of asymmetric synapses recovered to 86% of the sham-operated rat value by 30 days postlesion. In addition, the synaptic circuitry of the reconstructed striatum was characterized by an increase in the number of multiple synaptic boutons (MSBs), i.e., presynaptic axon terminals that make contact with more than one dendritic spine. Whether the postsynaptic contacts of MSBs are formed with the dendritic spines of the same or a different parent dendrite in the striatum is unknown. Nevertheless, these data suggest that the formation of MSBs is an essential part of the compensatory response to the loss of input from the ipsilateral cortex following the aspiration lesion and may serve to modulate activity-dependent adaptive changes in the reconstructed striatum that can lead to functional recovery.

Importance of behavioral manipulations and measures in rat models of brain damage and brain repair

The relevance of careful behavioral measures and manipulations in animal research on neural plasticity and brain damage has become increasingly clear. Recent research in adult rats indicates that an understanding of neural restructuring after brain damage requires an understanding of how it is influenced by postinjury behavioral experiences. Other research indicates that optimizing pharmacological and other treatments for brain damage may require their combination with rehabilitative training. Assessing the efficacy of a treatment approach in animal models requires the use of sensitive behavioral measures of functional outcome. In research on restorative plasticity after brain damage, procedures for handling and housing rats should promote the quality of behavioral measures and manipulations.

SCG10-related neuronal growth-associated proteins in neural development, plasticity, degeneration, and aging

Neuronal growth-associated proteins (nGAPs) are in general neuron-specific gene products whose expression correlates tightly with neuronal process outgrowth and/or regeneration, and are mostly good downstream targets of neurotrophin stimulation. Expression of genes encoding nGAPs such as GAP-43, SCG10, and stathmin is upregulated following lesioning of cortical and hippocampal regions of the adult rat brain. In the brains of aged animals, however, the magnitude of the response is reduced, whereas the time course of the response is mostly unchanged when compared with that for brains of young ones. Expression of GAP-43 and stathmin is reduced by aging, and is also changed in age-related neurodegenerative conditions such as Alzheimer's disease in humans. Certain nGAPs are induced during long-term potentiation (LTP) and also during critical periods of song-learning and ocular dominance column formation in birds and cats, respectively. Recent evidence further supports the idea that functional synaptic modulation is often associated with remodeling of synaptic structures. These results suggest that neurotrophin-responsive nGAPs serve as molecular markers of neuronal plasticity during development and aging, and that the neuronal plasticity decreases, at least in certain neuronal circuits, in the aged brain and neurodegenerative diseases. Recent findings on the roles of stathmin and SCG10-related proteins in microtubule destabilization and its functional block by phosphorylation further support the importance of the SCG10 family proteins in neuronal cytoskeletal regulation, particularly as to microtubule dynamics. We summarize here a decade of research on SCG10 and its related molecules with special interests to brain aging and disease.

The behavioral and dendritic growth effects of focal sensorimotor cortical damage depend on the method of lesion induction

Using different models of focal cortical injury in adult rats, the neural structural and behavioral outcomes of unilateral lesions of the forelimb representation of the sensorimotor cortex (SMC) were assessed. Lesions were produced using either electrolytic, aspiration, or combined ('electroaspiration') techniques. Measurements of dendritic arborization in layer V of the motor cortex opposite the lesion revealed a growth of pyramidal neuron dendritic processes following electrolytic lesions in comparison to shams. This effect was not found in either the aspiration or electroaspiration lesion groups. Behaviorally, animals in all lesion groups developed a hyper-reliance on the forelimb ipsilateral to the lesion and proportionate disuse of the contralateral (impaired) forelimb for postural support behaviors. In comparison to sham-operated animals, the initial asymmetries in behaviors expressed during movement were similar between lesion groups, but were less enduring following electrolytic lesions than following aspiration and electroaspiration lesions. Furthermore, both aspiration lesion groups had more prevalent adduction of the impaired forelimb than the electrolytic-only lesion rats. Thus, cortical aspiration resulted in more severe and enduring forelimb impairments than the electrolytic lesions, despite similar lesion sizes, as assessed using cortical volume measures. These findings suggest that the aspiration lesion procedures, at least as performed in the present study, exacerbate the behavioral effects of focal cortical injury and limit compensatory plasticity in the contralateral cortex.

Perforant path lesion induces up-regulation of stathmin messenger RNA, but not SCG10 messenger RNA, in the adult rat hippocampus

In this study, we performed in situ hybridization analysis of the expression pattern of two growth-associated proteins, stathmin and SCG10, in the hippocampus after unilateral lesion of the perforant pathway, the main excitatory input from the entorhinal cortex to the hippocampus. Stathmin is one of the major neural-enriched cytosolic phosphoproteins and a potential target of cyclic-AMP-dependent kinases [Jin L. W. et al. (1996) Neurobiol. Aging 17, 331-341; Leighton I. A. et al. (1993) Molec. Cell Biochem. 127/128, 151-156]. Three days after the lesion, stathmin messenger RNA was up-regulated ipsilaterally in the hilus, in the granule cell layer of the dentate gyrus and in the pyramidal cell layer of the CA1 region. Simultaneously, the hilar region of the contralateral dentate gyrus showed an increased stathmin messenger RNA expression. This altered expression pattern was observed until 15 days after lesion. Stathmin messenger RNA expression returned to a normal level until 21 days after lesion in all regions analysed. SCG10, a membrane-bound neuronal growth-associated protein belonging to the SCG10/stathmin gene family, did not show any alteration of messenger RNA expression after perforant path lesion. The temporal changes of stathmin messenger RNA expression in the ipsilateral hippocampus correspond well to the process of reactive synaptogenesis. The enhanced messenger RNA expression in the hilar region of the contralateral dentate gyrus might suggest a role in neurite elongation, since this region is the origin of commissural fibres involved in the sprouting response in the deafferented hippocampus. The present study provides evidence that the induction of specific growth-associated proteins is differentially regulated in the hippocampus.

Transient increase in rab 3A and synaptobrevin immunoreactivity after mild hypoxia in neonatal rats

1. In the present work we describe the short term effects of mild neonatal hypoxia on the synapse as assessed by the immunoreactivity (IR) of two synaptic proteins: rab 3A and synaptobrevin (VAMP). 2. Using the sensitive methodology of immunoblotting, we measured rab 3A and VAMP-IR in homogenates from the cerebral cortex, hippocampus, and corpus striatum of control (breathing room air) and hypoxiated (breathing 95.5% N2-6.5% O2 for 70 min) 4-day-old rats at 1, 2, and 6 h after the end of the hypoxia. Immunostaining with examination by light microscopy was performed using the synaptic protein-specific antibodies on fixed brain sections from animals belonging to the same litter and submitted to hypoxia. 3. A transient increase of VAMP-IR was observed in the hippocampus and corpus striatum, and for rab 3A in the striatum, 1 h after initiating reoxygenation. At the following time points the values returned to control levels. This effect was less clearly observed in the immunostained sections. 4. Mild hypoxia has an effect on sensitive brain regions, eliciting an increase in the IR of at least two proteins involved in the synaptic vesicle cycle. The transient nature of this effect possibly indicates the activation of endogenous neuroprotective mechanisms.