Jerome A. Staal

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

J. Neurochem. (2010) 112, 1147–1155.

Axonopathy and cytoskeletal disruption in degenerative diseases of the central nervous system

There has been growing interest in the axon as the initial focus of pathological change in a number of neurodegenerative diseases of the central nervous system. This review concentrates on three major neurodegenerative conditions--amyotrophic lateral sclerosis, multiple sclerosis and Alzheimers disease--with emphasis on key cellular changes that may underlie early axonal dysfunction and pathology and, potentially, the degeneration of neurons. In particular, this review will address recent data that indicate that the main pathological stimuli for these conditions, though often not definitively determined, result in an initial perturbation of the axon and its cytoskeleton, which then results in slow neuronal degeneration and loss of connectivity. The identification of a degenerative process initiated in the axon may provide new therapeutic targets for early intervention to inhibit the grim outcomes related to the progression of these diseases.

Characterization of Cortical Neuronal and Glial Alterations during Culture of Organotypic Whole Brain Slices from Neonatal and Mature Mice

Background Organotypic brain slice culturing techniques are extensively used in a wide range of experimental procedures and are particularly useful in providing mechanistic insights into neurological disorders or injury. The cellular and morphological alterations associated with hippocampal brain slice cultures has been well established, however, the neuronal response of mouse cortical neurons to culture is not well documented. Methods In the current study, we compared the cell viability, as well as phenotypic and protein expression changes in cortical neurons, in whole brain slice cultures from mouse neonates (P4–6), adolescent animals (P25–28) and mature adults (P50+). Cultures were prepared using the membrane interface method. Results Propidium iodide labeling of nuclei (due to compromised cell membrane) and AlamarBlue™ (cell respiration) analysis demonstrated that neonatal tissue was significantly less vulnerable to long-term culture in comparison to the more mature brain tissues. Cultures from P6 animals showed a significant increase in the expression of synaptic markers and a decrease in growth-associated proteins over the entire culture period. However, morphological analysis of organotypic brain slices cultured from neonatal tissue demonstrated that there were substantial changes to neuronal and glial organization within the neocortex, with a distinct loss of cytoarchitectural stratification and increased GFAP expression (p<0.05). Additionally, cultures from neonatal tissue had no glial limitans and, after 14 DIV, displayed substantial cellular protrusions from slice edges, including cells that expressed both glial and neuronal markers. Conclusion In summary, we present a substantial evaluation of the viability and morphological changes that occur in the neocortex of whole brain tissue cultures, from different ages, over an extended period of culture.

Selective vulnerability of non-myelinated axons to stretch injury in an in vitro co-culture system.

Diffuse axonal injury (DAI) is an evolving axonopathy commonly characterized clinically as widespread damage to the white matter tracts. In recent electrophysiological studies, researchers have proposed that myelinated and unmyelinated axons differ in their vulnerability and functional recovery following DAI. In this study we present for the first time an in vitro stretch-injury approach that utilizes a novel myelinating co-culture system to determine the differential response between myelinated and non-myelinated axon bundles to injury. In implementing this technique we demonstrate that myelinated axon bundles are less vulnerable to stretch injury compared to caliber-matched non-myelinated bundles. Interestingly, moderate axonal strain did not induce demyelination, but instead caused an increase in the proportion of degenerated myelin basic protein over time. Additionally, there were no significant differences in the expression of axonal swellings, which is indicative of disrupted axonal transport. In summary, we present an ideal in vitro model that permits further mechanistic investigations into the role of myelin and oligodendrocyte-neuron interactions in response to DAI.

Disruption of the ubiquitin proteasome system following axonal stretch injury accelerates progression to secondary axotomy

The ubiquitin proteasome system (UPS) plays a vital role in the regulation of protein degradation. Ubiquitination of proteins has been implicated in the pathological cascade associated with neuronal degeneration in both neurodegenerative disease and following acquired central nervous system (CNS) injury. In the present study, we have investigated the role of the UPS following mild to moderate in vitro axonal stretch injury to mature primary cortical neurons, a model of the evolving axonal pathology characteristic of diffuse axonal injury following brain trauma. Transient axonal stretch injury in this model does not involve primary axotomy. However, delayed accumulation of ubiquitin in neuritic swellings at 48 h post-injury (PI) was present in axonal bundles, followed by approximately 60% of axonal bundles progressing to secondary axotomy at 72 h PI. This delayed accumulation of ubiquitin was temporally and spatially associated with cytoskeletal damage. Pharmacological inhibition of the UPS with both MG132 and lactacystin prior to axonal injury resulted in a significant (p < 0.05) increase in the number of axonal bundles progressing to secondary axotomy at 48 and 72 h PI. These results demonstrate that, following mild to moderate transient axonal stretch injury, UPS activity may assist structural reorganization within axons, potentially impeding secondary axotomy. Protein ubiquitination in the axon may therefore have a protective role relative to the diffuse axonal changes that follow traumatic brain injury.

Cytoskeletal changes during development and aging in the cortex of neurofilament light protein knockout mice

The neurofilament light (NFL) subunit is considered as an obligate subunit polymer for neuronal intermediate filaments comprising the neurofilament (NF) triplet proteins. We examined cytoskeletal protein levels in the cerebral cortex of NFL knockout (KO) mice at postnatal day 4 (P4), 5 months, and 12 months of age compared with age‐matched wild‐type (WT) mice of a similar genetic background (C57BL/6). The absence of NFL protein resulted in a significant reduction of phosphorylated and dephosphorylated NFs (NF‐P, NF‐DP), the medium NF subunit (NFM), and the intermediate filament α‐internexin (INT) at P4. At 5 months, NF‐DP, NFM, and INT remained significantly lower in knockouts. At 12 months, NF‐P was again significantly decreased, and INT significantly increased, in KOs compared with wild type. In addition, protein levels of class III neuron‐specific β‐tubulin and microtubule‐associated protein 2 were significantly increased in NFL KO mice at P4, 5 months, and 12 months, whereas β‐actin levels were significantly decreased at P4. Immunocytochemical studies demonstrated that NF‐DP accumulated abnormally in the perikarya of cortical neurons by 5 months of age in NFL KO mice. Neurons that lacked NF triplet proteins, such as calretinin‐immunolabeled nonpyramidal cells, showed no alterations in density or cytoarchitectural distribution in NFL KO mice at 5 months relative to WT mice, although calretinin protein levels were decreased significantly after 12 months in NFL KO mice. These findings suggest that a lack of NFL protein alters the expression of cytoskeletal proteins and disrupts other NF subunits, causing intracellular aggregation but not gross structural changes in cortical neurons or cytoarchitecture. The data also indicate that changes in expression of other cytoskeletal proteins may compensate for decreased NFs. J. Comp. Neurol. 521:1817–1827, 2013.

Focal demyelination associated with amyloid plaque formation in Alzheimer's disease

Background: We have investigated the alterations in myelin associated with Aß plaques, a major pathological hallmark of Alzheimer’s disease (AD), both in human tissue and relevant transgenic mice models. Methods: Human tissue examined included neocortical samples from pathologically aged (Braak Stage III) and end-stage AD. Transgenic animal tissue examined included Tg2576(APPSwe670/671) and APP/PS1 (APPSwe x PS1M146L)) lines as well as age-matched wildtype mice (n 1⁄4 5, for each type). Myelination was assessed using FluoroMyelin or a modified Gallyas silver stain. FluoroMyelin staining was combined with immunofluorescence for Aß and/or axonal/dystrophic neurite markers. Results: We determined that fibrillar Aß pathology in the grey matter of the neocortex was associated with focal demyelination in human preclinical and end-stage AD cases, as well as in two mouse transgenic models of AD, as compared to age-matched control tissue. This demyelination was most pronounced at the core of Aß plaques. In both human and transgenic mice, plaque-free neocortical regions did not demonstrate significant demyelination compared to controls. Dystrophic neurites associated with the plaques were also demyelinated. Diffuse plaques were not associated with demyelination. In both human cases and transgenic mice, plaque-free neocortical grey matter did not demonstrate significant demyelination compared to controls. Conclusions: Fibrillar Aß plaque formation results in focal demyelination in the neocortex. Furthermore, alterations in axons that result in dystrophic neurite formation involve a loss of myelin. This demyelination confirms that fibrillar plaques are sites of substantial neuropil disruption in AD.