The morphological phenotype of beta-amyloid plaques and associated neuritic changes in Alzheimer's disease

We have utilised laser confocal microscopy to categorise beta-amyloid plaque types that are associated with preclinical and end-stage Alzheimer's disease and to define the neurochemistry of dystrophic neurites associated with various forms of plaques. Plaques with a spherical profile were defined as either diffuse, fibrillar or dense-cored using Thioflavin S staining or immunolabelling for beta-amyloid. Confocal analysis demonstrated that fibrillar plaques had a central mass of beta-amyloid with compact spoke-like extensions leading to a confluent outer rim. Dense-cored plaques had a compacted central mass surrounded by an outer sphere of beta-amyloid. Diffuse plaques lacked a morphologically identifiable substructure, resembling a ball of homogeneous labelling. The relative proportion of diffuse, fibrillar and dense-cored plaques was 53, 22 and 25% in preclinical and 31, 49 and 20% in end-stage Alzheimer's disease cases, respectively. Plaque-associated dystrophic neurites in preclinical cases were immunolabeled for neurofilament proteins whereas, in end-stage cases, these abnormal neurites were variably labelled for tau and/or neurofilaments. Double labelling demonstrated that the proportion of diffuse, fibrillar and dense-cored plaques that were neuritic was 12, 47 and 82% and 24, 82 and 76% in preclinical and end-stage cases, respectively. Most dystrophic neurites in Alzheimer's disease cases were labelled for either neurofilaments or tau, however, confocal analysis determined that 30% of neurofilament-labelled bulb-like or elongated neurites had a core of tau immunoreactivity. These results indicate that all morphologically defined beta-amyloid plaque variants were present in both early and late stages of Alzheimer's disease. However, progression to clinical dementia was associated with both a shift to a higher proportion of fibrillar plaques that induced local neuritic alterations and a transformation of cytoskeletal proteins within associated abnormal neuronal processes. There data indicate key pathological changes that may be subject to therapeutic intervention to slow the progression of Alzheimer's disease.

The cause of neuronal degeneration in Alzheimer's disease

Alzheimer's disease is associated with a specific pattern of pathological changes in the brain that result in neurodegeneration and the progressive development of dementia. Pathological hallmarks common to the disease include beta-amyloid plaques, dystrophic neurites associated with plaques and neurofibrillary tangles within nerve cell bodies. The exact relationship between these pathological features has been elusive, although it is clear that beta-amyloid plaques precede neurofibrillary tangles in neocortical areas. Examination of the brains of individuals in the preclinical stage of the disease have shown that the earliest form of neuronal pathology associated with beta-amyloid plaques resembles the cellular changes that follow structural injury to axons. Thus, the development of beta-amyloid plaques in the brain may cause physical damage to axons, and the abnormally prolonged stimulation of the neuronal response to this kind of injury ultimately results in the profound cytoskeletal alterations that underlie neurofibrillary pathology and neurodegeneration. Therapeutically, inhibition of the neuronal reaction to physical trauma may be a useful neuroprotective strategy in the earliest stages of Alzheimer's disease.

Functional binding interaction identified between the axonal CAM L1 and members of the ERM family

A yeast two-hybrid library was screened using the cytoplasmic domain of the axonal cell adhesion molecule L1 to identify binding partners that may be involved in the regulation of L1 function. The intracellular domain of L1 bound to ezrin, a member of the ezrin, radixin, and moesin (ERM) family of membrane-cytoskeleton linking proteins, at a site overlapping that for AP2, a clathrin adaptor. Binding of bacterial fusion proteins confirmed this interaction. To determine whether ERM proteins interact with L1 in vivo, extracellular antibodies to L1 were used to force cluster the protein on cultured hippocampal neurons and PC12 cells, which were then immunolabeled for ERM proteins. Confocal analysis revealed a precise pattern of codistribution between ERMs and L1 clusters in axons and PC12 neurites, whereas ERMs in dendrites and spectrin labeling remained evenly distributed. Transfection of hippocampal neurons grown on an L1 substrate with a dominant negative ERM construct resulted in extensive and abnormal elaboration of membrane protrusions and an increase in axon branching, highlighting the importance of the ERM-actin interaction in axon development. Together, our data indicate that L1 binds directly to members of the ERM family and suggest this association may coordinate aspects of axonal morphogenesis.

Altered synapses and gliotransmission in Alzheimer's disease and AD model mice

Amyloid-β (Aβ) plaque accumulation in Alzheimer's disease (AD) is associated with glutamatergic synapse loss, but less is known about its effect on inhibitory synapses. Here, we demonstrate that vesicular γ-aminobutyric acid (GABA) transporter (VGAT) presynaptic bouton density is unaffected in human preclinical and end-stage AD and in APP/PS1 transgenic (TG) mice. Conversely, excitatory vesicular glutamate transporter 1 (VGlut1) boutons are significantly reduced in end-stage AD cases and less reduced in preclinical AD cases and TGs. Aged TGs also show reduced protein levels of VGlut1 and synaptophysin but not VGAT or glutamate decarboxylase (GAD). These findings indicate that GABAergic synapses are preserved in human AD and mouse TGs. Synaptosomes isolated from plaque-rich TG cortex had significantly higher GAD activity than those from plaque-free cerebellum or the cortex of wild-type littermates. Using tissue fractionation, this increased activity was localized to glial synaptosomes, suggesting that Aβ plaques stimulate increased astrocyte GABA synthesis.

Neurochemical diversity of dystrophic neurites in the early and late stages of Alzheimer's disease

We examined the neurochemical and morphological diversity of abnormal neurites associated with beta-amyloid plaque formation in the early and late stages of Alzheimer's disease. Preclinical Alzheimer's disease was characterised by the presence of abnormal neurites containing either neurofilament or chromogranin A immunoreactivity. All clustered dystrophic neurites in these cases were associated with beta-amyloid plaques. Neurofilament immunoreactive dystrophic neurites in preclinical Alzheimer's disease could be further subclassified into bulb- and ring-like structures, and these abnormal neurites contained both phosphorylated and dephosphorylated neurofilament epitopes. Dystrophic neurites in Alzheimer's disease could be subdivided into predominantly neurofilament, tau, or chromogranin A immunolabeled forms. Some neurofilament immunoreactive neurites had a core region labeled for tau. The neurofilaments of the dystrophic neurites in Alzheimer's disease had the same complement of phosphorylation- and dephosphorylation-dependent epitopes as observed in preclinical cases. Therefore, an abnormal accumulation of variably phosphorylated neurofilaments represent the earliest cytoskeletal alteration associated with dystrophic neurite formation. Furthermore, these data indicate that dystrophic neurites may "mature" through neurofilament-abundant forms to the neurites containing the profoundly altered filaments labeled for tau. The precise morphological and neurochemical changes associated with dystrophic neurite formation suggests that beta-amyloid plaques are causing physical damage to surrounding axons. The resultant axonal sprouting and profound cytoskeletal alterations would follow the chronic stimulation of the stereotypical reaction to such physical trauma.

Alpha-synuclein is upregulated in neurones in response to chronic oxidative stress and is associated with neuroprotection

Chronic oxidative stress has been linked to the neurodegenerative changes characteristic of Parkinson's disease, particularly alpha-synuclein accumulation and aggregation. However, it remains contentious whether these alpha-synuclein changes are cytotoxic or neuroprotective. The current study utilised long-term primary neural culture techniques with antioxidant free media to study the cellular response to chronic oxidative stress. Cells maintained in antioxidant free media were exquisitely more vulnerable to acute exposure to hydrogen peroxide, yet exposure of up to 10 days in antioxidant free media did not lead to morphological alterations in neurones or glia. However, a subpopulation of neurones demonstrated a significant increase in the level of alpha-synuclein expressed within the cell body and at synaptic sites. This subset of neurones was also more resistant to apoptotic changes following exposure to antioxidant free media relative to other neurones. These data indicate that increased alpha-synuclein content is associated with neuroprotection from relatively low levels of oxidative stress.

Olfactory ensheathing cells promote neurite sprouting of injured axons in vitro by direct cellular contact and secretion of soluble factors

Olfactory ensheathing cells (OECs) represent an exciting possibility for promoting axonal regeneration within the injured spinal cord. A number of studies have indicated the ability of these cells to promote significant reactive sprouting of injured axons within the injured spinal cord, and in some cases restoration of functional abilities. However, the cellular and/or molecular mechanisms OECs use to achieve this are unclear. To investigate such mechanisms, we report for the first time the ability of OECs to promote post-injury neurite sprouting in an in vitro model of axonal injury. Using this model, we were able to differentiate between the direct and indirect mechanisms underlying the ability of OECs to promote neuronal recovery from injury. We noted that OECs appeared to act as a physical substrate for the growth of post-injury neurite sprouts. We also found that while post-injury sprouting was promoted most when OECs were allowed to directly contact injured neurons, physical separation using tissue culture inserts (1 mm pore size, permeable to diffusible factors but not cells) did not completely block the promoting properties of OECs, suggesting that they also secrete soluble factors which aid post-injury neurite sprouting. Furthermore, this in vitro model allowed direct observation of the cellular interactions between OECs and sprouting neurites using live-cell-imaging techniques. In summary, we found that OECs separately promote neurite sprouting by providing a physical substrate for growth and through the expression of soluble factors. Our findings provide new insight into the ability of OECs to promote axonal regeneration, and also indicate potential targets for manipulation of these cells to enhance their restorative ability.

Mild Axonal Stretch Injury In Vitro Induces a Progressive Series of Neurofilament Alterations Ultimately Leading to Delayed Axotomy

We report a new model of transient axonal stretch injury involving pressurized fluid deflection of bundles of axons, resulting in a transient 1-6% increase in original axon length to investigate the slow progression of axonal alterations that are characteristic of diffuse axonal injury (DAI). We found no discernable difference in axon bundle morphology or cytoskeletal neurofilament protein arrangement between unstretched and stretched axonal bundles at 24 h post-injury. However, by 48 h post-injury, there was a stereotypical response of stretched axons involving characteristic neurofilament alterations that bear similarities to in vivo neuronal responses associated with DAI that have been reported previously. For instance, neurofilament protein immunoreactivity (SMI-312) was increased in axons contained within 51% of all injured axon bundles at 48 h compared to surrounding unstretched axon bundles, suggestive of neurofilament compaction. Furthermore, axonal bundle derangement occurred in 25% of injured axon bundles, with individual fibres segregating from each other and becoming undulating and wavy. By 72 h post-stretch, 70% of injured axon bundles underwent secondary axotomy, becoming completely severed at the site of initial stretch injury. While these results suggest a temporal series of stereotypical responses of axons to injury, we were able to distinguish very clear differences between mildly (100-103% increase in original axonal length) injured and strongly injured (106%+) axons. For instance, mildly injured axons developed increased neurofilament immunoreactivtity (SMI-312) within 48 h, and the marked development of ring-like neurofilament immunoreactive structures within axonal bundles, which were rarely axotomized. Conversely, at more severe strain levels increased neurofilament immunoreactivity was less apparent, while axons often became distorted and disorganised within axonal bundles and eventually became completely disconnected. Almost no ring-like neurofilament structures were observed in these severely injured axonal bundles. This suggests that axons do not respond in a stereotypical manner to a transient stretch insult, and indeed that variable degrees of stretch injury activate different responses within axons, with dramatically different outcomes. Hence, it is possible that the cytoskeletal characteristics that we have used in this study may be useful parameters for discriminating between mildly and severely injured axons following TBI.

Initial calcium release from intracellular stores followed by calcium dysregulation is linked to secondary axotomy following transient axonal stretch injury

Acute axonal shear and stretch in the brain induces an evolving form of axonopathy and is a major cause of ongoing motor, cognitive and emotional dysfunction. We have utilized an in vitro model of mild axon bundle stretch injury, in cultured primary cortical neurons, to determine potential early critical cellular alterations leading to secondary axonal degeneration. We determined that transient axonal stretch injury induced an initial acute increase in intracellular calcium, principally derived from intracellular stores, which was followed by a delayed increase in calcium over 48 h post-injury (PI). This progressive and persistent increase in intracellular calcium was also associated with increased frequency of spontaneous calcium fluxes as well as cytoskeletal abnormalities. Additionally, at 48 h post-injury, stretch-injured axon bundles demonstrated filopodia-like sprout formation that preceded secondary axotomy and degeneration. Pharmacological inhibition of the calcium-activated phosphatase, calcineurin, resulted in reduced secondary axotomy (p < 0.05) and increased filopodial sprout length. In summary, these results demonstrate that stretch injury of axons induced an initial substantial release of calcium from intracellular stores with elevated intracellular calcium persisting over 2 days. These long-lasting calcium alterations may provide new insight into the earliest neuronal abnormalities that follow traumatic brain injury as well as the key cellular changes that lead to the development of diffuse axonal injury and secondary degeneration.

Neurofilament triplet proteins are restricted to a subset of neurons in the rat neocortex

The cellular localisation of neurofilament triplet subunits was investigated in the rat neocortex. A subset of mainly pyramidal neurons showed colocalisation of subunit immunolabelling throughout the neocortex, including labelling with the antibody SMI32, which has been used extensively in other studies of the primate cortex as a selective cellular marker. Neurofilament-labelled neurons were principally localised to two or three cell layers in most cortical regions, but dramatically reduced labelling was present in areas such as the perirhinal cortex, anterior cingulate and a strip of cortex extending from caudal motor regions through the medial parietal region to secondary visual areas. However, quantitative analysis demonstrated a similar proportion (10-20%) of cells with neurofilament triplet labelling in regions of high or low labelling. Combining retrograde tracing with immunolabelling showed that cellular content of the neurofilament proteins was not correlated with the length of projection. Double labelling immunohistochemistry demonstrated that neurofilament content in axons was closely associated with myelination. Analysis of SMI32 labelling in development indicated that content of this epitope within cell bodies was associated with relatively late maturation, between postnatal days 14 and 21. This study is further evidence of a cell type-specific regulation of neurofilament proteins within neocortical neurons. Neurofilament triplet content may be more closely related to the degree of myelination, rather than the absolute length, of the projecting axon.

Rho kinase activates ezrin-radixin-moesin (ERM) proteins and mediates their function in cortical neuron growth, morphology and motility in vitro

The ezrin-radixin-moesin (ERM) family of proteins contribute to cytoskeletal processes underlying many vital cellular functions. Their previously elucidated roles in non-neuronal cells are an indication of their potential importance in CNS neurons. The specific mechanisms of their activation are unknown, but are likely to depend on factors such as the cell type and biological context. For ERM proteins to become active they must be phosphorylated at a specific C-terminal threonine residue. In non-neuronal cells, several kinases, including the Rho GTPase family member Rho kinase, have been identified as capable of phosphorylating the C-terminal threonine. In these experiments we have investigated specifically the potential role of Rho kinase mediated ERM activation in cortical neurons, utilizing a new pharmacologic inhibitor of Rho kinase and quantitative analysis of aspects of neuronal functions potentially mediated by ERM proteins. Rho kinase inhibition significantly suppressed aspects of neuronal development including neurite initiation and outgrowth, as well as growth cone morphology, with a concomitant loss of phosphorylated ERM immunolabeling in areas associated with neuronal growth. The ability of the Rho kinase inhibitor to decrease the amount of pERM protein was shown by immunoblotting. Post-injury responses were negatively affected by Rho kinase inhibition, namely by a significant decrease in the number of regenerative neurites. We investigated a novel role for ERM proteins in neuron migration using a post-injury motility assay, where Rho kinase inhibition resulted in significant and drastic reduction in neuron motility and phosphorylated ERM immunolabeling. Thus, Rho kinase is an important activator of ERMs in mediating specific neuronal functions.

Localization of glutamate receptors in developing cortical neurons in culture and relationship to susceptibility to excitotoxicity

Overactivation of glutamate receptors leading to excitotoxicity has been implicated in the neurodegenerative alterations of a range of central nervous system (CNS) disorders. We have investigated the cell-type-specific changes in glutamate receptor localization in developing cortical neurons in culture, as well as the relationship between glutamate receptor subunit distribution with synapse formation and susceptibility to excitotoxicity. Glutamate receptor subunit clustering was present prior to the formation of synapses. However, different receptor types showed distinctive temporal patterns of subunit clustering, localization to spines, and apposition to presynaptic terminals. N-methyl-D-aspartate (NMDA) receptor subunit immunolabelling was present in puncta along dendrites prior to the formation of synapses, with relatively little localization to spines. Vulnerability to NMDA receptor-mediated excitotoxicity occurred before receptor subunits became localized in apposition to presynaptic terminals. Clustering of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors occurred concurrently with development of vulnerability to excitotoxicity and was related to localization of AMPA receptors at synapses and in spines. Different AMPA receptor subunits demonstrated cell-type-specific localization as well as distribution to spines, dendrites, and extrasynaptic subunit clusters. A subclass of neurons demonstrated substantial perineuronal synaptic innervation, and these neurons expressed relatively high levels of GluR1 and/or GluR4 at receptor puncta, indicating the presence of calcium-permeable AMPA receptors and suggesting alternative synaptic signalling mechanisms and vulnerability to excitotoxicity. These data demonstrate the relationship between glutamate receptor subunit expression and localization with synaptogenesis and development of neuronal susceptibility to excitotoxicity. These data also suggest that excitotoxicity can be mediated through extrasynaptic receptor subunit complexes along dendrites.

A Case for Microtubule Vulnerability in Amyotrophic Lateral Sclerosis: Altered Dynamics During Disease

Amyotrophic lateral sclerosis (ALS) is an aggressive multifactorial disease converging on a common pathology: the degeneration of motor neurons (MNs), their axons and neuromuscular synapses. This vulnerability and dysfunction of MNs highlights the dependency of these large cells on their intracellular machinery. Neuronal microtubules (MTs) are intracellular structures that facilitate a myriad of vital neuronal functions, including activity dependent axonal transport. In ALS, it is becoming increasingly apparent that MTs are likely to be a critical component of this disease. Not only are disruptions in this intracellular machinery present in the vast majority of seemingly sporadic cases, recent research has revealed that mutation to a microtubule protein, the tubulin isoform TUBA4A, is sufficient to cause a familial, albeit rare, form of disease. In both sporadic and familial disease, studies have provided evidence that microtubule mediated deficits in axonal transport are the tipping point for MN survivability. Axonal transport deficits would lead to abnormal mitochondrial recycling, decreased vesicle and mRNA transport and limited signaling of key survival factors from the neurons peripheral synapses, causing the characteristic peripheral "die back". This disruption to microtubule dependant transport in ALS has been shown to result from alterations in the phenomenon of microtubule dynamic instability: the rapid growth and shrinkage of microtubule polymers. This is accomplished primarily due to aberrant alterations to microtubule associated proteins (MAPs) that regulate microtubule stability. Indeed, the current literature would argue that microtubule stability, particularly alterations in their dynamics, may be the initial driving force behind many familial and sporadic insults in ALS. Pharmacological stabilization of the microtubule network offers an attractive therapeutic strategy in ALS; indeed it has shown promise in many neurological disorders, ALS included. However, the pathophysiological involvement of MTs and their functions is still poorly understood in ALS. Future investigations will hopefully uncover further therapeutic targets that may aid in combating this awful disease.

Synapse Dysfunction of Layer V Pyramidal Neurons Precedes Neurodegeneration in a Mouse Model of TDP-43 Proteinopathies

TDP-43 is a major protein component of pathological neuronal inclusions that are present in frontotemporal dementia and amyotrophic lateral sclerosis. We report that TDP-43 plays an important role in dendritic spine formation in the cortex. The density of spines on YFP+ pyramidal neurons in both the motor and somatosensory cortex of Thy1-YFP mice, increased significantly from postnatal day 30 (P30), to peak at P60, before being pruned by P90. By comparison, dendritic spine density was significantly reduced in the motor cortex of Thy1-YFP::TDP-43A315T transgenic mice prior to symptom onset (P60), and in the motor and somatosensory cortex at symptom onset (P90). Morphological spine-type analysis revealed that there was a significant impairment in the development of basal mushroom spines in the motor cortex of Thy1-YFP::TDP-43A315T mice compared to Thy1-YFP control. Furthermore, reductions in spine density corresponded to mislocalisation of TDP-43 immunoreactivity and lowered efficacy of synaptic transmission as determined by electrophysiology at P60. We conclude that mutated TDP-43 has a significant pathological effect at the dendritic spine that is associated with attenuated neural transmission.

α-Internexin immunoreactivity reflects variable neuronal vulnerability in Alzheimer's disease and supports the role of the β-amyloid plaques in inducing neuronal injury

This study investigated the role of alpha-internexin in the neuronal alterations associated with beta-amyloid plaque formation in Alzheimer's disease (AD). Cortical neurons could be defined by their variable content of neurofilament (NF) triplet and alpha-internexin proteins, with a distinct population of supragranular pyramidal cells containing alpha-internexin alone. Both NF triplet and alpha-internexin were localized to reactive axonal structures in physically damaged neurons in experimental trauma models. Similarly, NF triplet and alpha-internexin immunoreactive neurites were localized to plaques densely packed with beta-amyloid fibrils in preclinical AD cases, indicating that certain plaques may cause structural injury or impediment of local axonal transport. However, alpha-internexin, and not NF triplet, ring-like reactive neurites were present in end-stage AD cases, indicating the relatively late involvement of neurons that selectively contain alpha-internexin. These results implicate the expression of specific intermediate filament proteins in a distinct hierarchy of differential neuronal vulnerability to AD.

Dystrophic neurites in TgCRND8 and Tg2576 mice mimic human pathological brain aging

The morphology and neurochemistry of beta-amyloid (A beta) plaque-associated dystrophic neurites present in TgCRND8 and Tg2576 mice was demonstrated to be strikingly similar to that observed in pathologically aged human cases, but not in Alzheimer's disease (AD) cases. Specifically, pathologically aged cases and both transgenic mouse lines exhibited alpha-internexin- and neurofilament-triplet-labelled ring- and bulb-like dystrophic neurites, but no classical hyperphosphorylated-tau dystrophic neurite pathology. In contrast, AD cases demonstrated abundant classical hyperphosphorylated-tau-labelled dystrophic neurites, but no neurofilament-triplet-labelled ring-like dystrophic neurites. Importantly, quantitation demonstrated that the A beta plaques in TgCRND8 mice were highly axonopathic, and localised displacement or clipping of apical dendrite segments was also associated with A beta plaques in both transgenic mouse models. These results suggest that neuronal pathology in these mice represent an accurate and valuable model for understanding, and developing treatments for, the early brain changes of AD.

ERMs colocalize transiently with L1 during neocortical axon outgrowth

L1 is a member of the Ig superfamily of cell adhesion molecules (CAMs) that functions in many aspects of neuronal development including axonal outgrowth and neuronal migration. These functions require coordination between L1 and the actin cytoskeleton. Because CAMs and the cytoskeleton do not bind directly, membrane-cytoskeletal linkers (MCLs) such as ankyrin are thought to be crucial to their interactions, but data from a knockout mouse suggest that ankyrin is not necessary for the earliest events attributed to L1 function. Recent findings in hippocampal cell culture show that members of the ERM family of proteins (ezrin, radixin, and moesin) can also serve as MCLs between L1 and actin in neurons. Here, we demonstrate that ERM proteins are expressed in extending neuronal processes in the intermediate zone of the developing cortex, a region that is densely packed with migrating neurons and growing axons. ERMs and L1 are codistributed extensively over a transient time course that coincides with rapid axon growth and cortical expansion. This codistribution is strong at embryonic day 17 and 19 but diminishes by postnatal day 0, at which time ankyrin-L1 codistribution increases dramatically. These findings suggest that in the developing neocortex, ERMs are the predominant MCL for L1 during migration and axon extension, neither of which requires ankyrin function. Furthermore, these data suggest that there is a developmentally regulated switch in MCL function in the developing brain.

Focal damage to the adult rat neocortex induces wound healing accompanied by axonal sprouting and dendritic structural plasticity

Accumulating evidence indicates that damage to the adult mammalian brain evokes an array of adaptive cellular responses and may retain a capacity for structural plasticity. We have investigated the cellular and architectural alterations following focal experimental brain injury, as well as the specific capacity for structural remodeling of neuronal processes in a subset of cortical interneurons. Focal acute injury was induced by transient insertion of a needle into the neocortex of anesthetized adult male Hooded-Wistar rats and thy1 green fluorescent protein (GFP) mice. Immunohistochemical, electron microscopy, and bromodeoxyuridine cell proliferation studies demonstrated an active and evolving response of the brain to injury, indicating astrocytic but not neuronal proliferation. Immunolabeling for the neuron-specific markers phosphorylated neurofilaments, α-internexin and calretinin at 7 days post injury (DPI) indicated phosphorylated neurofilaments and α-internexin but not calretinin immunopositive axonal sprouts within the injury site. However, quantitative studies indicated a significant realignment of horizontally projecting dendrites of calretinin-labeled interneurons at 14 DPI. This remodeling was specific to calretinin immunopositive interneurons and did not occur in a subpopulation of pyramidal neurons expressing GFP in the injured mouse cortex. These data show that subclasses of cortical interneurons are capable of adaptive structural remodeling.

Cyclosporin-A treatment attenuates delayed cytoskeletal alterations and secondary axotomy following mild axonal stretch injury

Following central nervous system trauma, diffuse axonal injury and secondary axotomy result from a cascade of cellular alterations including cytoskeletal and mitochondrial disruption. We have examined the link between intracellular changes following mild/moderate axonal stretch injury and secondary axotomy in rat cortical neurons cultured to relative maturity (21 days in vitro). Axon bundles were transiently stretched to a strain level between 103% and 106% using controlled pressurized fluid. Double-immunohistochemical analysis of neurofilaments, neuronal spectrin, alpha-internexin, cytochrome-c, and ubiquitin was conducted at 24-, 48-, 72-, and 96-h postinjury. Stretch injury resulted in delayed cytoskeletal damage, maximal at 48-h postinjury. Accumulation of cytochrome-c and ubiquitin was also evident at 48 h following injury and colocalized to axonal regions of cytoskeletal disruption. Pretreatment of cultures with cyclosporin-A, an inhibitor of calcineurin and the mitochondrial membrane transitional pore, reduced the degree of cytoskeletal damage in stretch-injured axonal bundles. At 48-h postinjury, 20% of untreated cultures demonstrated secondary axotomy, whereas cyclosporin A-treated axon bundles remained intact. By 72-h postinjury, 50% of control preparations and 7% of cyclosporin A-treated axonal bundles had progressed to secondary axotomy, respectively. Statistical analyses demonstrated a significant (p < 0.05) reduction in secondary axotomy between treated and untreated cultures. In summary, these results suggest that cyclosporin-A reduces progressive cytoskeletal damage and secondary axotomy following transient axonal stretch injury in vitro.

Sequence of cellular changes following localized axotomy to cortical neurons in glia-free culture

This study utilizes an in vitro model of localized physical injury to axons to examine the specific responses of neocortical neurons to trauma in isolation from glia cell types. The neuronal response to axotomy was closely linked with nerve cell maturity. Cultures grown for 14 days in vitro showed no accumulation of either neurofilaments or, the axonal sprouting marker, GAP43, within injured axons following injury. In older cultures (21 days in vitro), however, temporally distinct axonal changes were evident following transection of axonal bundles. At 12 h postinjury, these included extensive accumulation of neurofilaments into ring-like structures within the cut stumps and an increase in punctate GAP43 labelling throughout the damaged area. At 24 h postinjury, bulb-like accumulations of neurofilaments were also present within the transected axons. Finally at 3 days postinjury, distinct GAP43 and neurofilament immunolabeled axons, and GAP43 immunopositive growth cones, emanated from the cut stump. These results indicate that injured axons of mature neurons undergo a defined series of reactive changes, ultimately culminating in a sprouting response, which occur independently of the presence or effects of glial cell populations.

Mislocalisation of TDP-43 to the cytoplasm causes cortical hyperexcitability and reduced excitatory neurotransmission in the motor cortex

Amyotrophic lateral sclerosis (ALS) is a chronic neurodegenerative disease pathologically characterised by mislocalisation of the RNA-binding protein TAR-DNA-binding protein 43 (TDP-43) from the nucleus to the cytoplasm. Changes to neuronal excitability and synapse dysfunction in the motor cortex are early pathological changes occurring in people with ALS and mouse models of disease. To investigate the effect of mislocalised TDP-43 on the function of motor cortex neurons we utilised mouse models that express either human wild-type (TDP-43^WT ) or nuclear localisation sequence-deficient TDP-43 (TDP-43^ΔNLS ) on an inducible promoter that enriches expression to forebrain neurons. Pathophysiology was investigated through immunohistochemistry and whole-cell patch-clamp electrophysiology. Thirty days expression of TDP-43^ΔNLS in adult mice did not cause any changes in the number of CTIP2-positive neurons in the motor cortex. However, at this time-point, the expression of TDP-43^ΔNLS drives intrinsic hyperexcitability in layer V excitatory neurons of the motor cortex. This hyperexcitability occurs concomitantly with a decrease in excitatory synaptic input to these cells and fluctuations in both directions of ionotropic glutamate receptors. This pathophysiology is not present with TDP-43^WT expression, demonstrating that the localisation of TDP-43 to the cytoplasm is crucial for the altered excitability phenotype. This study has important implications for the mechanisms of toxicity of one of the most notorious proteins linked to ALS, TDP-43. We provide the first evidence that TDP-43 mislocalisation causes aberrant synaptic function and a hyperexcitability phenotype in the motor cortex, linking some of the earliest dysfunctions to arise in people with ALS to mislocalisation of TDP-43.

Calretinin and Neuropeptide Y interneurons are differentially altered in the motor cortex of the SOD1G93A mouse model of ALS

Increasing evidence indicates an excitatory/inhibitory imbalance may have a critical role in the pathogenesis of amyotrophic lateral sclerosis (ALS). Impaired inhibitory circuitry is consistently reported in the motor cortex of both familial and sporadic patients, closely associated with cortical hyperexcitability and ALS onset. Inhibitory network dysfunction is presumably mediated by intra-cortical inhibitory interneurons, however, the exact cell types responsible are yet to be identified. In this study we demonstrate dynamic changes in the number of calretinin- (CR) and neuropeptide Y-expressing (NPY) interneurons in the motor cortex of the familial hSOD1^G93A ALS mouse model, suggesting their potential involvement in motor neuron circuitry defects. We show that the density of NPY-populations is significantly decreased by ~17% at symptom onset (8 weeks), and by end-stage disease (20 weeks) is significantly increased by ~30%. Conversely, the density of CR-populations is progressively reduced during later symptomatic stages (~31%) to end-stage (~36%), while CR-expressing interneurons also show alteration of neurite branching patterns at symptom onset. We conclude that a differential capacity for interneurons exists in the ALS motor cortex, which may not be a static phenomenon, but involves early dynamic changes throughout disease, implicating specific inhibitory circuitry.

The microtubule-stabilizing drug Epothilone D increases axonal sprouting following transection injury in vitro

Neuronal cytoskeletal alterations, in particular the loss and misalignment of microtubules, are considered a hallmark feature of the degeneration that occurs after traumatic brain injury (TBI). Therefore, microtubule-stabilizing drugs are attractive potential therapeutics for use following TBI. The best-known drug in this category is Paclitaxel, a widely used anti-cancer drug that has produced promising outcomes when employed in the treatment of various animal models of nervous system trauma. However, Paclitaxel is not ideal for the treatment of patients with TBI due to its limited blood-brain barrier (BBB) permeability. Herein we have characterized the effect of the brain penetrant microtubule-stabilizing agent Epothilone D (Epo D) on post-injury axonal sprouting in an in vitro model of CNS trauma. Epo D was found to modulate axonal sprout number in a dose dependent manner, increasing the number of axonal sprouts generated post-injury. Elevated sprouting was observed when analyzing the total population of injured neurons, as well as in selective analysis of Thy1-YFP-labeled excitatory neurons. However, we found no effect of Epo D on axonal sprout length or outgrowth speed. These findings indicate that Epo D specifically affects injury-induced axonal sprout generation, but not net growth. Our investigation demonstrates that primary cultures of cortical neurons are tolerant of Epo D exposure, and that Epo D significantly increases their regenerative response following structural injury. Therefore Epo D may be a potent therapeutic for enhancing regeneration following CNS injury. This article is part of a Special Issue entitled 'Traumatic Brain Injury'.

Identifying the primary site of pathogenesis in amyotrophic lateral sclerosis - vulnerability of lower motor neurons to proximal excitotoxicity

There is a desperate need for targeted therapeutic interventions that slow the progression of amyotrophic lateral sclerosis (ALS). ALS is a disorder with heterogeneous onset, which then leads to common final pathways involving multiple neuronal compartments that span both the central and peripheral nervous system. It is believed that excitotoxic mechanisms might play an important role in motor neuron death in ALS. However, little is known about the mechanisms by which excitotoxicity might lead to the neuromuscular junction degeneration that characterizes ALS, or about the site at which this excitotoxic cascade is initiated. Using a novel compartmentalised model of site-specific excitotoxin exposure in lower motor neurons in vitro, we found that spinal motor neurons are vulnerable to somatodendritic, but not axonal, excitotoxin exposure. Thus, we developed a model of somatodendritic excitotoxicity in vivo using osmotic mini pumps in Thy-1-YFP mice. We demonstrated that in vivo cell body excitotoxin exposure leads to significant motor neuron death and neuromuscular junction (NMJ) retraction. Using confocal real-time live imaging of the gastrocnemius muscle, we found that NMJ remodelling preceded excitotoxin-induced NMJ degeneration. These findings suggest that excitotoxicity in the spinal cord of individuals with ALS might result in a die-forward mechanism of motor neuron death from the cell body outward, leading to initial distal plasticity, followed by subsequent pathology and degeneration.