Spectrin Breakdown Products (SBDPs) as Potential Biomarkers for Neurodegenerative Diseases

Subcortical cytoskeleton periodicity throughout the nervous system

Superresolution fluorescence microscopy recently revealed a ~190 nm periodic cytoskeleton lattice consisting of actin, spectrin, and other proteins underneath the membrane of cultured hippocampal neurons. Whether the periodic cytoskeleton lattice is a structural feature of all neurons and how it is modified when axons are ensheathed by myelin forming glial cells is not known. Here, STED nanoscopy is used to demonstrate that this structure is a commonplace of virtually all neuron types in vitro. To check how the subcortical meshwork is modified during myelination, we studied sciatic nerve fibers from adult mice. Periodicity of both actin and spectrin was uncovered at the internodes, indicating no substantial differences between unmyelinated and myelinated axons. Remarkably, the actin/spectrin pattern was also detected in glial cells such as cultured oligodendrocyte precursor cells. Altogether our work shows that the periodic subcortical cytoskeletal meshwork is a fundamental characteristic of cells in the nervous system and is not a distinctive feature of neurons, as previously thought.

Submembranous cytoskeletons stabilize nodes of Ranvier

Rapid action potential propagation along myelinated axons requires voltage-gated Na(+) (Nav) channel clustering at nodes of Ranvier. At paranodes flanking nodes, myelinating glial cells interact with axons to form junctions. The regions next to the paranodes called juxtaparanodes are characterized by high concentrations of voltage-gated K(+) channels. Paranodal axoglial junctions function as barriers to restrict the position of these ion channels. These specialized domains along the myelinated nerve fiber are formed by multiple molecular mechanisms including interactions between extracellular matrix, cell adhesion molecules, and cytoskeletal scaffolds. This review highlights recent findings into the roles of submembranous cytoskeletal proteins in the stabilization of molecular complexes at and near nodes. Axonal ankyrin-spectrin complexes stabilize Nav channels at nodes. Axonal protein 4.1B-spectrin complexes contribute to paranode and juxtaparanode organization. Glial ankyrins enriched at paranodes facilitate node formation. Finally, disruption of spectrins or ankyrins by genetic mutations or proteolysis is involved in the pathophysiology of various neurological or psychiatric disorders.

Caspase-3/7-mediated Cleavage of β2-spectrin is Required for Acetaminophen-induced Liver Damage

The ubiquitously expressed β2-spectrin (β2SP, SPTBN1) is the most common non-erythrocytic member of the β-spectrin gene family. Loss of β2-spectrin leads to defects in liver development, and its haploinsufficiency spontaneously leads to chronic liver disease and the eventual development of hepatocellular cancer. However, the specific role of β2-spectrin in liver homeostasis remains to be elucidated. Here, we reported that β2-spectrin was cleaved by caspase-3/7 upon treatment with acetaminophen which is the main cause of acute liver injury. Blockage of β2-spectrin cleavage robustly attenuated β2-spectrin-specific functions, including regulation of the cell cycle, apoptosis, and transcription. Cleaved fragments of β2-spectrin were physiologically active, and the N- and C-terminal fragments retained discrete interaction partners and activity in transcriptional regulation and apoptosis, respectively. Cleavage of β2-spectrin facilitated the redistribution of the resulting fragments under conditions of liver damage induced by acetaminophen. In contrast, downregulation of β2-spectrin led to resistance to acetaminophen-induced cytotoxicity, and its insufficiency in the liver promoted suppression of acetaminophen-induced liver damage and enhancement of liver regeneration.

CONCLUSIONS

β2-Spectrin, a TGF-β mediator and signaling molecule, is cleaved and activated by caspase-3/7, consequently enhancing apoptosis and transcriptional control to determine cell fate upon liver damage. These findings have extended our knowledge on the spectrum of β2-spectrin functions from a scaffolding protein to a target and transmitter of TGF-β in liver damage.

Reduction of glutamate-induced excitotoxicity in murine primary neurons involving calpain inhibition

Excessive glutamate secretion leads to excitotoxicity, which has been shown to underlie neurodegenerative disorders. Excitotoxicity is in part exerted by overactivation of calpains, which promote neuronal cell death via induction of limited proteolysis of the cellular proteins p35, regulatory subunit of cyclin-dependent kinase 5, and αII-spectrin. We used primary murine neuronal cells in a model of glutamate toxicity. The protease inhibitor α1-antitrypsin was able to prevent glutamate toxicity as determined by MTT assay and immunofluorescence. Calpain and caspase 3 activity were reduced following α1-antitrypsin treatment, as assessed by calpain and caspase 3 activity assays. In addition we could observe a modulation of cleavage of the calpain/caspase substrates αII-spectrin and p35 in Western blots. In summary, α1-antitrypsin shows inhibitory effects on excitotoxicity of primary neurons involving the inhibition of calpain activity. The advantage of using α1-antitrypsin is that the substance is already in clinical use for the treatment of patients with hereditary α1-antitrypsin deficiency. Further experiments are required in animal models of neurodegenerative disorders to assess the suitability of this substance in patients suffering from Alzheimer's disease or Parkinson's disease.

AMPA receptor desensitization is the determinant of AMPA receptor mediated excitotoxicity in purified retinal ganglion cells

The ionotropic glutamate receptors (iGLuR) have been hypothesized to play a role in neuronal pathogenesis by mediating excitotoxic death. Previous studies on iGluR in the retina have focused on two broad classes of receptors: NMDA and non-NMDA receptors including the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) and kainate receptor. In this study, we examined the role of receptor desensitization on the specific excitotoxic effects of AMPAR activation on primary retinal ganglion cells (RGCs). Purified rat RGCs were isolated from postnatal day 4-7 Sprague-Dawley rats. Calcium imaging was used to identify the functionality of the AMPARs and selectivity of the s-AMPA agonist. Phosphorylated CREB and ERK1/2 expression were performed following s-AMPA treatment. s-AMPA excitotoxicity was determined by JC-1 mitochondrial membrane depolarization assay, caspase 3/7 luciferase activity assay, immunoblot analysis for α-fodrin, and Live (calcein AM)/Dead (ethidium homodimer-1) assay. RGC cultures of 98% purity, lacking Iba1 and GFAP expression were used for the present studies. Isolated prenatal RGCs expressed calcium permeable AMPAR and s-AMPA (100 μM) treatment of cultured RGCs significantly increased phosphorylation of CREB but not that of ERK1/2. A prolonged (6 h) AMPAR activation in purified RGCs using s-AMPA (100 μM) did not depolarize the RGC mitochondrial membrane potential. In addition, treatment of cultured RGCs with s-AMPA, both in the presence and absence of trophic factors (BDNF and CNTF), did not increase caspase 3/7 activities or the cleavage of α-fodrin (neuronal apoptosis marker), as compared to untreated controls. Lastly, a significant increase in cell survival of RGCs was observed after s-AMPA treatment as compared to control untreated RGCs. However, preventing the desensitization of AMPAR with the treatment with either kainic acid (100 μM) or the combination of s-AMPA and cyclothiazide (50 μM) significantly reduced cell survivability. Activation of the AMPAR in RGCs does not appear to activate a signaling cascade to apoptosis, suggesting that RGCs in vitro are not susceptible to AMPA excitotoxicity as previously hypothesized. Conversely, preventing AMPAR desensitization through differential agonist activation caused AMPAR mediated excitotoxicity. Activation of the AMPAR in increasing CREB phosphorylation was dependent on the presence of calcium, which may help explain this action in increasing RGC survival.

Our "energy-Ca(2+) signaling deficits" hypothesis and its explanatory potential for key features of Alzheimer's disease

Sporadic Alzheimer's disease (sAD) has not been explained by any current theories, so new hypotheses are urgently needed. We proposed that "energy and Ca(2+) signaling deficits" are perhaps the earliest modifiable defects in brain aging underlying memory decline and tau deposits (by means of inactivating Ca(2+)-dependent protease calpain). Consistent with this hypothesis, we now notice that at least eight other known calpain substrates have also been reported to accumulate in aging and AD. Thus, protein accumulation or aggregation is not a "pathogenic" event, but occurs naturally and selectively to a peculiar family of proteins, and is best explained by calpain inactivation. Why are only calpain substrates accumulated and how can they stay for decades in the brain without being attacked by many other non-specific proteases there? We believe that these long-lasting puzzles can be explained by calpain's unique properties, especially its unusual specificity and exclusivity in substrate recognition, which can protect the substrates from other proteases' attacks after calpain inactivation. Interestingly, our model, in essence, may also explain tau phosphorylation and the formation of amyloid plaques. Our studies suggest that α-secretase is an energy-/Ca(2+)-dual dependent protease and is also the primary determinant for Aβ levels. Therefore, β- and γ-secretases can only play secondary roles and, by biological laws, they are unlikely to be "positively identified". This study thus raises serious questions for policymakers and researchers and these questions may help explain why sAD can remain an enigma today.

Alpha II Spectrin breakdown products in immature Sprague Dawley rat hippocampus and cortex after traumatic brain injury

After traumatic brain injury (TBI), proteolysis of Alpha II Spectrin by Calpain 1 produces 145 Spectrin breakdown products (SBDPs) while proteolysis by Caspase 3 produces 120 SBDPs. 145 and 120 SBDP immunoblotting reflects the relative importance of caspase-dependent apoptosis or calpain-dependent excitotoxic/necrotoxic cell death in brain regions over time. In the adult rat, controlled cortical impact (CCI) increased 120 SBDPs in the first hours, lasting a few days, and increased 145 SBDPs within the first few days lasting up to 14 days after injury. Little is known about SBDPs in the immature brain after TBI. Since development affects susceptibility to apoptosis after TBI, we hypothesized that CCI would increase 145 and 120 SBDPs in the immature rat brain relative to SHAM during the first 3 and 5 days, respectively. SBDPs were measured in hippocampi and cortices at post injury days (PID) 1, 2, 3, 5, 7 and 14 after CCI or SHAM surgery in the 17 day old Sprague Dawley rat. 145 SBDPs increased in both brain tissues ipsilateral to injury during the first 3 days, while changes in contralateral tissues were limited to PID2 cortex. 145 SBDPs elevations were more marked and enduring in hippocampus than in cortex. Against expectations, 120 SBDPs only increased in PID1 hippocampus and PID2 cortex. 145 SBDPs elevations occurred early after CCI, similar to previous studies in the adult rat, but resolved more quickly. The minimal changes in 120 SBDPs suggest that calpain-dependent, but not caspase-dependent, cell death predominates in the 17 day old rat after CCI.

Quantitative studies of caspase-3 catalyzed αII-spectrin breakdown

Under various physiological and patho-physiological conditions, spectrin breakdown reactions generate several spectrin breakdown products (SBDPs)-in particular SBDPs of 150 kDa (SBDP150) and 120 kDa (SBDP120). Recently, numerous studies have shown that reactions leading to SBDPs are physiologically relevant, well regulated, and complex. Yet molecular studies on the mechanism of the SBDP formation are comparatively scarce. We have designed basic systems to allow us to follow the breakdown of αII-spectrin model proteins by caspase-3 in detail with gel electrophoresis, fluorescence and mass spectrometry methods. Amongst the predicted and reported sites, our results show that caspase-3 cleaves after residues D1185 and D1478, but not after residues D888, D1340 and D1475. We also found that the cleavage at these two sites is independent of each other. It may be possible to inhibit one site without affecting the other site. Cleavage after residue D1185 in intact αII-spectrin leads to SBDP150, and cleavage after D1478 site leads to SBDP120. Our results also show that the cleavage after the D1185 residue is unusually efficient, with a kcat/KM value of 40,000 M(-1) s(-1), and the cleavage after the D1478 site is more similar to most of the other reported caspase-3 substrates, with a kcat/KM value of 3000 M(-1) s(-1). We believe that this study lays out a methodology and foundation to study caspase-3 catalyzed spectrin breakdown to provide quantitative information. Molecular understanding may lead to better understanding of brain injuries and more precise and specific biomarker development.

Cerebrospinal fluid protein biomarker panel for assessment of neurotoxicity induced by kainic acid in rats

Glutamate excitotoxicity plays a key role in the etiology of a variety of neurological, psychiatric, and neurodegenerative disorders. The goal of this study was to investigate spatiotemporal distribution in the brain and cerebrospinal fluid (CSF) concentrations of ubiquitin C-terminal hydrolase-1 (UCH-L1), glial fibrillary acidic protein (GFAP), αII-spectrin breakdown products (SBDP150, SBDP145, and SBDP120), and their relationship to neuropathology in an animal model of kainic acid (KA) excitotoxicity. Triple fluorescent labeling and Fluoro-Jade C staining revealed a reactive gliosis in brain and specific localization of degenerating neurons in hippocampus and entorhinal cortex of KA-treated rats. Immunohistochemistry showed upregulation of GFAP expression in hippocampus and cortex beginning 24h post KA injection and peaking at 48h. At these time points concurrent with extensive neurodegeneration all SBDPs were observed throughout the brain. At 24h post KA injection, a loss of structural integrity was observed in cellular distribution of UCH-L1 that correlated with an increase in immunopositive material in the extracellular matrix. CSF levels of UCH-L1, GFAP, and SBDPs were significantly increased in KA-treated animals compared with controls. The temporal increase in CSF biomarkers correlated with brain tissue distribution and neurodegeneration. This study provided evidence supporting the use of CSF levels of glial and neuronal protein biomarkers to assess neurotoxic damage in preclinical animal models that could prove potentially translational to the clinic. The molecular nature of these biomarkers can provide critical information on the underlying mechanisms of neurotoxicity that might facilitate the development of novel drugs and allow physicians to monitor drug safety.

Age-related intraneuronal elevation of αII-spectrin breakdown product SBDP120 in rodent forebrain accelerates in 3×Tg-AD mice

Spectrins line the intracellular surface of plasmalemma and play a critical role in supporting cytoskeletal stability and flexibility. Spectrins can be proteolytically degraded by calpains and caspases, yielding breakdown products (SBDPs) of various molecular sizes, with SBDP120 being largely derived from caspase-3 cleavage. SBDPs are putative biomarkers for traumatic brain injury. The levels of SBDPs also elevate in the brain during aging and perhaps in Alzheimer’s disease (AD), although the cellular basis for this change is currently unclear. Here we examined age-related SBDP120 alteration in forebrain neurons in rats and in the triple transgenic model of AD (3×Tg-AD) relative to non-transgenic controls. SBDP120 immunoreactivity (IR) was found in cortical neuronal somata in aged rats, and was prominent in the proximal dendrites of the olfactory bulb mitral cells. Western blot and densitometric analyses in wild-type mice revealed an age-related elevation of intraneuronal SBDP120 in the forebrain which was more robust in their 3×Tg-AD counterparts. The intraneuronal SBDP120 occurrence was not spatiotemporally correlated with transgenic amyloid precursor protein (APP) expression, β-amyloid plaque development, or phosphorylated tau expression over various forebrain regions or lamina. No microscopically detectable in situ activated caspase-3 was found in the nuclei of SBDP120-containing neurons. The present study demonstrates the age-dependent intraneuronal presence of an αII-spectrin cleavage fragment in mammalian forebrain which is exacerbated in a transgenic model of AD. This novel neuronal alteration indicates that impairments in membrane protein metabolism, possibly due to neuronal calcium mishandling and/or enhancement of calcium sensitive proteolysis, occur during aging and in transgenic AD mice.