Altered gene expression for calpain/calpastatin system in motor neuron degeneration (Mnd) mutant mouse brain and spinal cord

Increased immunoreactivities of cleaved αII-spectrin and cleaved caspase-3 in the aged dog spinal cord

The activation of caspase-3 is considered to be a reliable marker for apoptotic cell death, and a 120-kDa fragment of αII-spectrin is generated by caspase-3 mediated cleavage of this structural protein. In the present study, we compared cleaved αII-spectrin (120-kDa) and cleaved caspase-3-immunoreactive cells and their protein levels in the cervical (C₅-C₆) and lumbar (L₃-L₄) levels of the spinal cord in adult (1-2 year-old) and aged (10-12 year-old) dogs (German shepherds). Weak cleaved αII-spectrin and cleaved caspase-3 immunoreactivity was found in neurons of the adult group; however, their immunoreactivity was distinctively increased in the neuronal cytoplasm in the aged group compared to those in the adult group, although the distribution pattern of their neurons was similar between the adult and age group. In addition, cleaved αII-spectrin and cleaved caspase-3 levels in the aged spinal cord were markedly increased compared to those in the adult group. These findings suggest that the increases of cleaved αII-spectrin and cleaved caspase-3 immunoreactivity may be related to aging of the spinal cord in dogs.

Exposure of neurons to excitotoxic levels of glutamate induces cleavage of the RNA editing enzyme, adenosine deaminase acting on RNA 2, and loss of GLUR2 editing

AMPA receptors are glutamate receptors that are tetramers of various combinations of GluR1-4 subunits. AMPA receptors containing GluR1, 3 and 4 are Ca2+ permeable, however, AMPA receptors containing even a single subunit of GluR2 are Ca2+ impermeable. Most AMPA receptors are Ca2+ impermeable due to the presence of GluR2. GluR2 confers special properties on AMPA receptors through the presence of arginine at the pore apex; other subunits (GluR1, 3, 4) contain glutamine at the pore apex and allow Ca2+ influx. Normally, an RNA editing step changes DNA-encoded glutamine to arginine, introduces arginine in the GluR2 pore apex. GluR2 RNA editing is carried out by an RNA-dependent adenosine deaminase (ADAR2). Loss of GluR2 editing leads to the formation of highly excitotoxic AMPA channels [Mahajan and Ziff (2007) Mol Cell Neurosci 35:470-481] and is shown to contribute to loss of motor neurons in amyotrophic lateral sclerosis (ALS). Relatively higher levels of Ca2+-permeable AMPA receptors are found in motor neurons and this has been correlated with lower GluR2 mRNA levels. However, the reason for loss of GluR2 editing is not known. Here we show that exposure of neurons to excitotoxic levels of glutamate leads to specific cleavage of ADAR2 that leads to generation of unedited GluR2. We demonstrate that cleaved ADAR2 leads to a decrease or loss of GluR2 editing, which will further result in high Ca2+ influx and excitotoxic neuronal death.

Axonal degeneration in motor neuron disease

Growing evidence from animal models and patients with amyotrophic lateral sclerosis (ALS) suggests that distal axonal degeneration begins very early in this disease, long before symptom onset and motor neuron death. The cause of axonal degeneration is unknown, and may involve local axonal damage, withdrawal of trophic support from a diseased cell body, or both. It is increasingly clear that axons are not passive extensions of their parent cell bodies, and may die by mechanisms independent of cell death. This is supported by studies in which protection of motor neurons in models of ALS did not significantly improve symptoms or prolong lifespan, likely due to a failure to protect axons. Here, we will review the evidence for early axonal degeneration in ALS, and discuss possible mechanisms by which it might occur, with a focus on oxidative stress. We contend that axonal degeneration may be a primary feature in the pathogenesis of motor neuron disease, and that preventing axonal degeneration represents an important therapeutic target that deserves increased attention.

Involvement of calpain activation in neurodegenerative processes

One of the challenges in the coming years will be to better understand the mechanisms of neuronal cell death with the objective of developing adequate drugs for the treatment of neurodegenerative disorders. Caspases and calpains are among the best-characterized cysteine proteases activated in brain disorders. Likewise, during the last decade, extensive research revealed that the deregulation of calpains activity is a key cytotoxic event in a variety of neurodegenerative disorders. Moreover, interest in the role of calpain in neurodegenerative processes is growing due to implication of the involvement of cdk5 in neurodegenerative diseases. Since calpain inhibitors appear to not only protect brain tissue from ischemia, but also to prevent neurotoxicity caused by such neurotoxins as beta-amyloid or 3-nitropropionic acid, the currently available data suggest that calpain and cdk5 play a key role in neuronal cell death. It seems clear that the inappropriate activation of cysteine proteases occurs not only during neuronal cell death, but may also contribute to brain pathology in ischemia and traumatic brain disorders. Pharmacological modulation of calpain activation may, therefore, be useful in the treatment of neurodegenerative disorders. It is possible, although difficult, to develop synthetic inhibitors of cysteine proteases, specifically calpains. The inhibition of calpain activation has recently emerged as a potential therapeutic target for the treatment of neurodegenerative diseases.

The role of the N-methyl-D-aspartate receptor NR1 subunit in peripheral nerve injury-induced mechanical allodynia, glial activation and chemokine expression in the mouse

The N-methyl-d-aspartate receptor (NMDAR) has been strongly implicated in mechanisms of persistent pain states. The purpose of the present study was to determine whether the NMDAR NR-1, a key subunit in regulation of NMDAR channel complex is directly contributing to the onset and propagation of peripheral nerve injury-induced allodynia and whether N-methyl-d-aspartate (NMDA) signaling interacts with spinal chemokine (chemotactic cytokines) expression and glial activation. We used genetically engineered male mice that had their normal NR1 gene knocked out and expressed a modified NR1 gene at either normal level (NR1 +/+, wild type) or at a low level (NR1+/-, knock down). Each mouse underwent a peripheral nerve injury in which the lumbar 5 spinal segment (L5) nerve was transected. Mechanical allodynia was assessed using 0.008 and 0.015 g von Frey filaments on days 1, 3, 5, 7, 10, 14, 17 and 21 post-surgery. Mice were killed on day 21 and the harvested L5 spinal cord was analyzed for chemokine expression using RNAse protection assay. In a separate study, glial expression using immunohistochemistry was assessed in both groups 7 days following peripheral nerve injury. The NR1+/- mice displayed decreased mechanical allodynia in comparison to their wild type counterparts. However, even with dramatically impaired NMDA receptor signaling, there was still evidence of tactile hypersensitivity. Using the RPA analysis, we found decreases in mRNA chemokine expression in the NR1+/- mice as compared with NR1+/+ mice. There were no apparent differences in microglial or astrocytic expression between the wild type and knock down mice. These data provide important insights into the cascade of events involving the dynamic interaction between NMDAR function and spinal chemokine and glial production in neuropathic pain states. The results support the findings that chemokine signaling releases glutamate in the spinal cord.

Calcium-mediated proteolytic damage in white matter of hydrocephalic rats?

Hydrocephalus is a pathological dilatation of the cerebrospinal fluid (CSF)-containing ventricles of the brain. Damage to periventricular white matter is multifactorial with contributions by chronic ischemia and gradual physical distortion. Acute ischemic and traumatic brain injuries are associated with calcium-dependent activation of proteolytic enzymes. We hypothesized that hydrocephalus is associated with calcium ion accumulation and proteolytic enzyme activation in cerebral white matter. Hydrocephalus was induced in immature and adult rats by injection of kaolin into the cisterna magna and several different experimental approaches were used. Using the glyoxal bis (2-hydroxyanil) method, free calcium ion was detected in periventricular white matter at sites of histological injury. Western blot determinations showed accumulation of calpain I (mu-calpain) and immunoreactivity for calpain I was increased in periventricular axons of young hydrocephalic rats. Proteolytic cleavage of a fluorogenic calpain substrate was demonstrated in white matter. Immunoreactivity for spectrin breakdown products was detected in scattered callosal axons of young hydrocephalic rats. The findings support the hypothesis that periventricular white matter damage associated with experimental hydrocephalus is due, at least in part, to calcium-activated proteolytic processes. This may have implications for supplemental drug treatments of this disorder.

The pathogenic activation of calpain: a marker and mediator of cellular toxicity and disease states

Over-activation of calpain, a ubiquitous calcium-sensitive protease, has been linked to a variety of degenerative conditions in the brain and several other tissues. Dozens of substrates for calpain have been identified and several of these have been used to measure activation of the protease in the context of experimentally induced and naturally occurring pathologies. Calpain-mediated cleavage of the cytoskeletal protein spectrin, in particular, results in a set of large breakdown products (BDPs) that are unique in that they are unusually stable. Over the last 15 years, measurements of BDPs in experimental models of stroke-type excitotoxicity, hypoxia/ischemia, vasospasm, epilepsy, toxin exposure, brain injury, kidney malfunction, and genetic defects, have established that calpain activation is an early and causal event in the degeneration that ensues from acute, definable insults. The BDPs also have been found to increase with normal ageing and in patients with Alzheimer's disease, and the calpain activity may be involved in related apoptotic processes in conjunction with the caspase family of proteases. Thus, it has become increasingly clear that regardless of the mode of disturbance in calcium homeostasis or the cell type involved, calpain is critical to the development of pathology and therefore a distinct and powerful therapeutic target. The recent development of antibodies that recognize the site at which spectrin is cleaved has greatly facilitated the temporal and spatial resolution of calpain activation in situ. Accordingly, sensitive spectrin breakdown assays now are utilized to identify potential toxic side-effects of compounds and to develop calpain inhibitors for a wide range of indications including stroke, cerebral vasospasm, and kidney failure.

Long-term neuroprotective effects of glycosaminoglycans-IGF-I cotreatment in the motor neuron degeneration (mnd) mutant mouse

This study shows that cotreatment with insulin-like growth factor-I (IGF-I) and glycosaminoglycans (GAGs) prevents the onset of neuromuscular deficit in the m/m mutant mouse. These mice show a mid-to-late-life onset of progressive paralysis of the hind limb, that is correlated with altered innervation and reduced nerve-evoked isometric twitch tension in the extensor digitorum longus (EDL) muscle. Almost 50% of EDL nerve endings are negative for antisynaptophysin staining, while retrograde labelling with beta-cholera-toxin coupled to type IV horseradish and quantitative histological analysis show a small loss of EDL and lumbar cord motor neurons. At 10 months of age also forelimb function evaluated as grip strength is significantly reduced. Animals treated either with glycosaminoglycans alone or with IGF-I alone at low and high doses showed only a partial improvement of their condition. However, cotreatment of m/m mice with IGF-I and GAGs fully prevented the neuromuscular abnormalities, the twitch tension loss, the motor neuron decrease and the reduction of forelimb grip strength.

A putative mechanism of demyelination in multiple sclerosis by a proteolytic enzyme, calpain

In autoimmune demyelinating diseases such as multiple sclerosis (MS), the degradation of myelin proteins results in destabilization of the myelin sheath. Thus, proteases have been implicated in myelin protein degradation, and recent studies have demonstrated increased expression and activity of a calcium-activated neutral proteinase (calpain) in experimental allergic encephalomyelitis, the corresponding animal model of MS. In the present study, calpain activity and expression (at translational and transcriptional levels) were evaluated in white matter from human patients with MS and Parkinson's and Alzheimer's diseases and compared with that of white matter from normal controls. Western blot analysis revealed that levels of the active form of calpain and calpain-specific degradation products (fodrin) were increased by 90.1% and 52.7%, respectively, in MS plaques compared with normal white matter. Calpain translational expression was up-regulated by 462.5% in MS plaques compared with controls, although levels of the specific endogenous inhibitor, calpastatin, were not altered significantly. At the transcriptional level, no significant changes in calpain or calpastatin expression were detected by reverse transcription-PCR. Using double immunofluorescent labeling, increased calpain expression was observed in reactive astrocytes, activated T cells, and activated mononuclear phagocytes in and adjacent to demyelinating lesions. Calpain activity and translational expression were not increased significantly in white matter from patients with Parkinson's or Alzheimer's diseases compared with that of normal controls. Because calpain degrades all major myelin proteins, the increased activity and expression of this proteinase may play a critical role in myelinolysis in autoimmune demyelinating diseases such as MS.

Calpain activity and expression are increased in splenic inflammatory cells associated with experimental allergic encephalomyelitis

Since calcium-activated neutral proteinase (calpain) activity and expression are significantly increased in activated glial/inflammatory cells in the central nervous system of animals with autoimmune demyelinating diseases, this enzyme may also play a role in peripheral organ systems in these diseases. In this study, the activity and expression of calpain and the endogenous inhibitor, calpastatin, were evaluated at transcriptional and translational levels in spleens of Lewis rats with acute experimental allergic encephalomyelitis (EAE) prior to the onset of clinical symptoms. Calpain activity and translational expression were increased by 475.5% and 44.3% respectively, on day 4 post-induction in adjuvant controls and animals with EAE. These levels remained elevated compared to normal controls on days 8 and 12. Calpastatin translational expression was similarly increased at these time points although transcriptional expression was not significantly altered at any time following induction of EAE. Likewise, transcriptional expression of mu-calpain was unchanged following induction, while small increases in m-calpain transcriptional expression were observed on days 2 and 8. Most calpain expression was observed in activated splenic macrophages at day 8 post-induction even though activated T cells were also calpain positive. In spinal cords of animals with EAE, calpain expression was significantly increased in rats with severe disease compared to those exhibiting only mild symptoms at day 12 post-induction. Thus, prior to symptomatic EAE, increased calpain activity and expression in peripheral lymphoid organs may play an important role in T cell migration and subsequent disease progression.

Biochemical and pharmacological evidence of a functional role of AMPA receptors in motor neuron dysfunction in mnd mice

We studied ionotropic glutamate receptor subtypes and the effect of chronic treatment with NBQX [6-nitro-7-sulphamoyl-benzo(F)quinoxaline-2,3-dione], a selective (rs)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist, in the spinal cord of mnd mice. NBQX (8 mg/kg daily i.p. for 3 weeks starting from 24 weeks old) significantly improved the behavioural scores (hind leg extension reflex, cage rung grasping and gait) in mnd mice, measured after the last drug injection, and increased the number of mice with 'normal' gait (from 50% to 90%, P < 0.05). Receptor binding autoradiography of the competitive N-methyl-D-aspartate (NMDA) antagonist, [3H]CGP 39653, of [3H]AMPA and [3H]kainic acid in spinal cord sections, measured after 1 week of drug washout, were not significantly different in control and mnd mice, and were not modified by NBQX. GluR2/3 immunoreactivity, assessed using Western blotting, was significantly enhanced (by 59%, P < 0.01) in the spinal cord but not in the brain of 28-week-old mnd mice compared to age-matched control mice. NBQX treatment increased GluR2/3 immunoreactivity in the spinal cord of control mice and mnd mice by 327 +/- 74% (P < 0.01) and 212 +/- 52% (P < 0.01), respectively. The changes in GluR2/3 subunits may involve adaptive mechanisms of the receptor and play some role in the protective effect of NBQX. These findings suggest that selective antagonism of ionotropic non-NMDA receptors may be of value in the treatment of motor neuron disease.

An early-onset congenic strain of the motor neuron degeneration (mnd) mouse

The mouse mutant motor neuron degeneration (mnd/mnd) has been proposed as a model of neuronal ceroid lipofuscinosis (NCL) on the basis of widespread abnormal accumulating lipopigment and neuronal and retinal degeneration. Clinically, the mutant on a C57Bl/6 genetic background shows a progressive motor abnormality starting by 6 months of age, with death prior to 12 months. When mnd is outcrossed to the AKR/J genetic background, ca. 40% of the mnd/mnd F2 progeny show early onset (onset by 4.5-5 months and death by 7 months). A congenic strain of mnd has now been produced by eight generations of backcross onto the AKR background. Mice on this background show average onset at 4 months, and most are moribund prior to 5.5 months. The early onset appears to correlate with levels of abnormal accumulating material, and should prove useful in elucidating NCL neurodegenerative mechanisms.

Pathophysiological role of calpain in experimental demyelination

Calcium-activated neutral proteinase (calpain) has been extensively studied over the past three decades such that many enzymatic and structural properties of this enzyme are well understood. However, the pathophysiological roles of calpain remain poorly defined. In addition to recent studies delineating a role for calpain in various pathological conditions, this proteinase has been implicated in the degradation of myelin proteins in autoimmune demyelinating diseases such as multiple sclerosis and experimental allergic encephalomyelitis (EAE). In EAE, calpain translational expression is significantly increased in activated glial/inflammatory cells that participate in myelinolysis while calpain substrates (axonal and myelin proteins) are lost. Thus, since all major myelin proteins are calpain substrates, early studies suggest calpain may play an important role in demyelination of the central nervous system.

Batten disease: four genes and still counting

The neuronal ceroid lipofuscinoses (NCLs, also known as Batten disease) are the most common childhood neurodegenerative disease. They are a group of inherited neurodegenerative disorders characterized by the accumulation of autofluorescent storage material in many cell types. Clinical features include seizures, psychomotor deterioration, and blindness, the ages and order of onset of which differ for each NCL type. An increasing number of subtypes caused by mutations in different genes are now recognized. With the advent of molecular genetics the basic genetic defect underlying each NCL phenotype is being determined, thus shedding light on the molecular basis of the NCLs and opening the way for the development of effective treatment. Four genes have been identified to date. The function of two of these is known and suggests that the primary defect in the NCLs lies in lysosomal proteolysis, the first example of this type of disease. However, since the function of the other two genes remains elusive, and at least four more genes remain to be identified, the molecular basis underlying the NCLs may be more complex than originally predicted.

In vitro generation of an active calmodulin-independent phosphodiesterase from brain calmodulin-dependent phosphodiesterase (PDE1A2) by m-calpain

In the present study we have shown that bovine brain 60-kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme (CaMPDE - PDE1A2) is proteolyzed by a Ca2+-dependent cysteine protease, m-calpain. The proteolysis of PDE1A2 by m-calpain results in its conversion to a totally calmodulin (CaM)-independent form accompanied by degradation of PDE1A2 into a 45-kDa catalytic fragment and a 15-kDa fragment. The activity of PDE1A2 is unaffected by the presence or absence of CaM during cleavage, suggesting that the interaction between CaM and PDE1A2 does not alter substrate recognition by calpain. Furthermore, we provide evidence, based on the studies of CaM overlay and phosphorylation, that the cleavage site is not present either in the CaM-binding domain or phosphorylation site. N-terminal sequence analysis of the 45-kDa fragment indicated that cleavage occurs between residues 126Gln and 127Ala, and eliminates the CaM-dependent activity of carboxy termini PDE1A2. The present findings suggest that limited proteolysis in the brain through calpains could be an alternate mechanism for activating CaMPDE(s) and for regulating intracellular levels of cAMP.