Analysis of the amyloid precursor protein role in neuritogenesis reveals a biphasic SH-SY5Y neuronal cell differentiation model

Degree of differentiation impacts neurobiological signature and resistance to hypoxia of SH-SY5Y cells

Objective.SH-SY5Y cells are valuable neuronalin vitromodels for studying patho-mechanisms and treatment targets in brain disorders due to their easy maintenance, rapid expansion, and low costs. However, the use of various degrees of differentiation hampers appreciation of results and may limit the translation of findings to neurons or the brain. Here, we studied the neurobiological signatures of SH-SY5Y cells in terms of morphology, expression of neuronal markers, and functionality at various degrees of differentiation, as well as their resistance to hypoxia. We compared these to neurons derived from human induced pluripotent stem cells (hiPSCs), a well-characterized neuronalin vitromodel.Approach.We cultured SH-SY5Y cells and neurons derived from hiPSCs on glass coverslips or micro-electrode arrays. We studied expression of mature neuronal markers, electrophysiological activity, and sensitivity to hypoxia at various degrees of differentiation (one day up to three weeks) in SH-SY5Y cells. We used hiPSC derived neurons as a reference.Main results.Undifferentiated and shortly differentiated SH-SY5Y cells lacked neuronal characteristics. Expression of neuronal markers and formation of synaptic puncta increased during differentiation. Longer differentiation was associated with lower resistance to hypoxia. At three weeks of differentiation, MAP2 expression and vulnerability to hypoxia were similar to hiPSC-derived neurons, while the number of synaptic puncta and detected events were significantly lower. Our results show that at least three weeks of differentiation are necessary to obtain neurobiological signatures that are comparable to those of hiPSC-derived neurons, as well as similar sensitivities to metabolic stress. Significance.This indicates that extended differentiation protocols should be used to study neuronal characteristics and to model brain disorders with SH-SY5Y cells. We provided insights that may offer the basis for the utilization of SH-SY5Y cells as a more relevant neuronal model in the study of brain disorders.

Customizing 3D Structures of Vertically Aligned Carbon Nanotubes to Direct Neural Stem Cell Differentiation

Neural tissue-related illnesses have a high incidence and prevalence in society. Despite intensive research efforts to enhance the regeneration of neural cells into functional tissue, effective treatments are still unavailable. Here, a novel therapeutic approach based on vertically aligned carbon nanotube forests (VA-CNT forests) and periodic VA-CNT micropillars produced by thermal chemical vapor deposition is explored. In addition, honeycomb-like and flower-like morphologies are created. Initial viability testing reveals that NE-4C neural stem cells seeded on all morphologies survive and proliferate. In addition, free-standing VA-CNT forests and capillary-driven VA-CNT forests are created, with the latter demonstrating enhanced capacity to stimulate neuritogenesis and network formation under minimal differentiation medium conditions. This is attributed to the interaction between surface roughness and 3D-like morphology that mimics the native extracellular matrix, thus enhancing cellular attachment and communication. These findings provide a new avenue for the construction of electroresponsive scaffolds based on CNTs for neural tissue engineering.

β-III Tubulin Levels Determine the Neurotoxicity Induced by Colchicine-Site Binding Agent Indibulin

Indibulin, a microtubule-depolymerizing agent, produces minimal neurotoxicity in animals. It is also less cytotoxic toward differentiated neuronal cells than undifferentiated cells. We found that the levels of β-III tubulin, acetylated tubulin, and polyglutamylated tubulin were significantly increased in differentiated neuroblastoma cells (SH-SY5Y). Since neuronal cells express β-tubulin isotypes differently from other cell types, we explored the binding of indibulin to different β-tubulin isotypes. Our molecular docking analysis suggested that indibulin binds to β-III tubulin with lower affinity than to other β-tubulin isotypes. We therefore studied the implications of different β-tubulin isotypes on the cytotoxic effects of indibulin, colchicine, and vinblastine in differentiated SH-SY5Y cells. Upon depletion of β-III tubulin in the differentiated cells, the toxicity of indibulin and colchicine significantly increased, while sensitivity to vinblastine was unaffected. Using biochemical, bioinformatics, and fluorescence spectroscopic techniques, we have identified the binding site of indibulin on tubulin, which had not previously been established. Indibulin inhibited the binding of colchicine and C12 (a colchicine-site binder) to tubulin and also increased the dissociation constant of the interaction between tubulin and colchicine. Indibulin did not inhibit the binding of vinblastine or taxol to tubulin. Interestingly, indibulin antagonized colchicine treatment but synergized with vinblastine treatment in a combination study performed in MDA-MB-231 cells. The results indicate that indibulin is a colchicine-site binder and that the efficacy of colchicine-site binders is affected by the β-III tubulin levels in the cells.

The role of NAD metabolism in neuronal differentiation

BACKGROUND

Nicotinamide adenine dinucleotide (NAD) metabolism is involved in redox and non-redox reactions that regulate several processes including differentiation of cells of different origins. Here, the role of NAD metabolism in neuronal differentiation, which remains elusive so far, was investigated.

MATERIAL AND METHODS

A protein-protein interaction network between neurotrophin signaling and NAD metabolic pathways was built. Expression of NAD biosynthetic enzymes in SH-SY5Y cells during retinoic acid (RA)/brain derived neurotrophic factor (BDNF) differentiation, was evaluated. The effects of NAD biosynthetic enzymes QPRT and NAPRT inhibition in neurite outgrowth, cell viability, NAD availability and histone deacetylase (HDAC) activity, were analysed in RA- and BDNF-differentiated cells.

RESULTS

Bioinformatics analysis revealed the interaction between NAD biosynthetic enzyme NMNAT1 and NTRK2, a receptor activated by RA/BDNF sequential treatment. Differences were found in the expression of NAD biosynthetic enzymes during neuronal differentiation, namely, increased QPRT gene expression along the course of RA/BDNF treatment and NAPRT protein expression after a 5-day treatment with RA. QPRT inhibition in BDNF-differentiated SH-SY5Y cells resulted in less neuritic length per cell, decreased expression of the neuronal marker β-III Tubulin and also decreased NAD⁺ levels and HDAC activity. NAPRT inhibition had no effect in neuritic length per cell, NAD⁺ levels and HDAC activity. Of note, NAD supplementation along with RA, but not with BDNF, resulted in considerable cell death.

CONCLUSIONS

Taken together, our results show the involvement of NAD metabolism in neuronal differentiation, specifically, the importance of QPRT-mediated NAD biosynthesis in BDNF-associated SH-SY5Y differentiation and suggest additional roles for NAPRT beyond NAD production in RA-differentiated cells.

Capacitive interdigitated system of high osteoinductive/conductive performance for personalized acting-sensing implants

Replacement orthopedic surgeries are among the most common surgeries worldwide, but clinically used passive implants cannot prevent failure rates and inherent revision arthroplasties. Optimized non-instrumented implants, resorting to preclinically tested bioactive coatings, improve initial osseointegration but lack long-term personalized actuation on the bone-implant interface. Novel bioelectronic devices comprising biophysical stimulators and sensing systems are thus emerging, aiming for long-term control of peri-implant bone growth through biointerface monitoring. These acting-sensing dual systems require high frequency (HF) operations able to stimulate osteoinduction/osteoconduction, including matrix maturation and mineralization. A sensing-compatible capacitive stimulator of thin interdigitated electrodes and delivering an electrical 60 kHz HF stimulation, 30 min/day, is here shown to promote osteoconduction in pre-osteoblasts and osteoinduction in human adipose-derived mesenchymal stem cells (hASCs). HF stimulation through this capacitive interdigitated system had significant effects on osteoblasts' collagen-I synthesis, matrix, and mineral deposition. A proteomic analysis of microvesicles released from electrically-stimulated osteoblasts revealed regulation of osteodifferentiation and mineralization-related proteins (e.g. Tgfb3, Ttyh3, Itih1, Aldh1a1). Proteomics data are available via ProteomeXchange with the identifier PXD028551. Further, under HF stimulation, hASCs exhibited higher osteogenic commitment and enhanced hydroxyapatite deposition. These promising osteoinductive/conductive capacitive stimulators will integrate novel bioelectronic implants able to monitor the bone-implant interface and deliver personalized stimulation to peri-implant tissues.

APP Binds to the EGFR Ligands HB-EGF and EGF, Acting Synergistically with EGF to Promote ERK Signaling and Neuritogenesis

The amyloid precursor protein (APP) is a transmembrane glycoprotein central to Alzheimer's disease (AD) with functions in brain development and plasticity, including in neurogenesis and neurite outgrowth. Epidermal growth factor (EGF) and heparin-binding EGF-like growth factor (HB-EGF) are well-described neurotrophic and neuromodulator EGFR ligands, both implicated in neurological disorders, including AD. Pro-HB-EGF arose as a putative novel APP interactor in a human brain cDNA library yeast two-hybrid screen. Based on their structural and functional similarities, we first aimed to verify if APP could bind to (HB-)EGF proforms. Here, we show that APP interacts with these two EGFR ligands, and further characterized the effects of APP-EGF interaction in ERK activation and neuritogenesis. Yeast co-transformation and co-immunoprecipitation assays confirmed APP interaction with HB-EGF. Co-immunoprecipitation also revealed that APP binds to cellular pro-EGF. Overexpression of HB-EGF in HeLa cells, or exposure of SH-SY5Y cells to EGF, both resulted in increased APP protein levels. EGF and APP were observed to synergistically activate the ERK pathway, crucial for neuronal differentiation. Immunofluorescence analysis of cellular neuritogenesis in APP overexpression and EGF exposure conditions confirmed a synergistic effect in promoting the number and the mean length of neurite-like processes. Synergistic ERK activation and neuritogenic effects were completely blocked by the EGFR inhibitor PD 168393, implying APP/EGF-induced activation of EGFR as part of the mechanism. This work shows novel APP protein interactors and provides a major insight into the APP/EGF-driven mechanisms underlying neurite outgrowth and neuronal differentiation, with potential relevance for AD and for adult neuroregeneration.

Bottom up proteomics identifies neuronal differentiation pathway networks activated by cathepsin inhibition treatment in neuroblastoma cells that are enhanced by concurrent 13-cis retinoic acid treatment

Neuroblastoma is the second most common pediatric cancer involving the peripheral nervous system in which stage IVS metastatic tumors regress due to spontaneous differentiation. 13-cis retinoic acid (13-cis RA) is currently used in the clinic for its differentiation effects and although it improves outcomes, relapse is seen in half of high-risk patients. Combinatorial therapies have been shown to be more effective in oncotherapy and since cathepsin inhibition reduces tumor growth, we explored the potential of coupling 13-cis RA with a cathepsin inhibitor (K777) to enhance therapeutic efficacy against neuroblastoma. Shotgun proteomics was used to identify proteins affected by K777 and dual (13-cis RA/K777) treatment in neuroblastoma SK-N-SH cells. Cathepsin inhibition was more effective in increasing proteins involved in neuronal differentiation and neurite outgrowth than 13-cis RA alone, but the combination of both treatments enhanced the neuronal differentiation effect. SIGNIFICANCE: As neuroblastoma can spontaneously differentiate, determining which proteins are involved in differentiation can guide development of more accurate diagnostic markers and more effective treatments. In this study, we established a differentiation proteomic map of SK-N-SH cells treated with a cathepsin inhibitor (K777) and K777/13-cis RA (dual). Bioinformatic analysis revealed these treatments enhanced neuronal differentiation and axonogenesis pathways. The most affected proteins in these pathways may become valuable biomarkers of efficacy of drugs designed to enhance differentiation of neuroblastoma [1].

Genistein protects against amyloid-beta-induced toxicity in SH-SY5Y cells by regulation of Akt and Tau phosphorylation

Alzheimer's disease is a neurodegenerative disorder characterized by extracellular deposition of amyloid-β (Aβ) peptide and hyperphosphorylation of Tau protein, which ultimately leads to the formation of intracellular neurofibrillary tangles and cell death. Increasing evidence indicates that genistein, a soy isoflavone, has neuroprotective effects against Aβ-induced toxicity. However, the molecular mechanisms involved in its neuroprotection are not well understood. In this study, we have established a neuronal damage model using retinoic-acid differentiated SH-SY5Y cells treated with different concentrations of Aβ_25-35 to investigate the effect of genistein against Aβ-induced cell death and the possible involvement of protein kinase B (PKB, also termed Akt), glycogen synthase kinase 3β (GSK-3β), and Tau as an underlying mechanism to this neuroprotection. Differentiated SH-SY5Y cells were pre-treated for 24 hr with genistein (1 and 10 nM) and exposed to Aβ_25-35 (25 μM), and we found that genistein partially inhibited Aβ induced cell death, primarily apoptosis. Furthermore, the protective effect of genistein was associated with the inhibition of Aβ-induced Akt inactivation and Tau hyperphosphorylation. These findings reinforce the neuroprotective effects of genistein against Aβ toxicity and provide evidence that its mechanism may involve regulation of Akt and Tau proteins.

CD81 Promotes a Migratory Phenotype in Neuronal-Like Cells

Tetraspanins, such as CD81, can form lateral associations with each other and with other transmembrane proteins. These interactions may underlie CD81 functions in multiple cellular processes, such as adhesion, morphology, migration, and differentiation. Since CD81's role in neuronal cells' migration has not been established, we here evaluated effects of CD81 on the migratory phenotype of SH-SY5Y neuroblastoma cells. CD81 was found enriched at SH-SY5Y cell's membrane, co-localizing with its interactor filamentous-actin (F-actin) in migratory relevant structures of the leading edge (filopodia, stress fibers, and adhesion sites). CD81 overexpression increased the number of cells with a migratory phenotype, in a potentially phosphatidylinositol 3 kinase (PI3K)-Ak strain transforming (AKT) mediated manner. Indeed, CD81 also co-localized with AKT, a CD81-interactor and actin remodeling agent, at the inner leaflet of the plasma membrane. Pharmacologic inhibition of PI3K, the canonical AKT activator, led both to a decrease in the acquisition of a migratory phenotype and to a redistribution of intracellular CD81 and F-actin into cytoplasmic agglomerates. These findings suggest that in neuronal-like cells CD81 bridges active AKT and actin, promoting the actin remodeling that leads to a motile cell morphology. Further studies on this CD81-mediated mechanism will improve our knowledge on important physiological and pathological processes such as cell migration and differentiation, and tumor metastasis.

Functional analysis of juxta- and intra-membrane domains of murine APP by genome editing in Neuro2a cells

Amyloid-β precursor protein (APP) correlates with the pathogenesis of certain brain diseases, such as Alzheimer disease (AD). APP is cleaved by several enzymes to produce APP metabolites, including the amyloid beta peptide (Aβ), which accumulates in the brain of AD patients. However, the exact functions of APP metabolites remain elusive. In this study, using genome editing technology, we mutated juxta- and intra-membrane domains of murine APP in the mouse neuroblastoma cell line, Neuro2a. We identified several clones that expressed characteristic patterns of APP metabolites. Mutations in juxta- (deletion 673A), and intra-membrane (deletion 705-6LM) domains of APP, decreased overall levels of APP metabolites or decreased the level of α-secretase-cleaved carboxy-terminal fragment (αCTF), respectively. APP is known to influence neuronal differentiation; therefore, we used theses clones to dissect the function of APP metabolites during neuronal differentiation. One clone (CA), which expressed reduced levels of both FL-APP and αCTF, showed increased expression of the neuronal marker, β3-tubulin, and enhanced retinoic acid (RA)-induced neurite outgrowth. In contrast, a clone that expressed FL-APP, but was devoid of αCTF (CE), showed comparable expression of β3-tubulin and neurite outgrowth compared with normal Neuro2a cells. These data indicate that FL-APP is a suppressor of neurite outgrowth. Our data suggest a novel regulatory function of juxta- and intra-membrane domains on the metabolism and function of APP.

Amyloid Precursor Protein Dimerisation Reduces Neurite Outgrowth

The amyloid precursor protein (APP) undergoes extensive metabolism, and its transport and proteolytic processing can be modulated by its ability to form a homodimer. We have investigated the functional consequences of stabilised APP dimer expression in cells by studying the engineered dimerisation of the APP_L17C (residue 17 in Aβ sequence) construct, which is associated with a 30% increase in APP dimer expression, on APP's neurite outgrowth promoting activity. Overexpression of APP_L17C in SH-SY5Y cells decreased neurite outgrowth upon retinoic acid differentiation as compared to overexpressing APP_WT cells. The APP_L17C phenotype was rescued by replacing the APP_L17C media with conditioned media from APP_WT cells, indicating that the APP_L17C mutant is impairing the secretion of a neuritogenic promoting factor. APP_L17C had altered transport and was localised in the endoplasmic reticulum. Defining the molecular basis of the APP_L17C phenotype showed that RhoA GTPase activity, a negative regulator of neurite outgrowth, was increased in APP_L17C cells. RhoA activity was decreased after APP_WT conditioned media rescue. Moreover, treatment with the RhoA inhibitor, Y27632, restored a wild-type morphology to the APP_L17C cells. Small RNAseq analysis of APP_L17C and APP_WT cells identified several differentially expressed miRNAs relating to neurite outgrowth. Of these, miR-34a showed the greatest decrease in expression. Lentiviral-mediated overexpression of miR-34a rescued neurite outgrowth in APP_L17C cells to APP_WT levels and changed RhoA activation. This study has identified a novel link between APP dimerisation and its neuritogenic activity which is mediated by miR-34a expression.

NeuronRead, an open source semi-automated tool for morphometric analysis of phase contrast and fluorescence neuronal images

Neurons are specialized cells of the Central Nervous System whose function is intricately related to the neuritic network they develop to transmit information. Morphological evaluation of this network and other neuronal structures is required to establish relationships between neuronal morphology and function, and may allow monitoring physiological and pathophysiologic alterations. Fluorescence-based microphotographs are the most widely used in cellular bioimaging, but phase contrast (PhC) microphotographs are easier to obtain, more affordable, and do not require invasive, complicated and disruptive techniques. Despite the various freeware tools available for fluorescence-based images analysis, few exist that can tackle the more elusive and harder-to-analyze PhC images. To surpass this, an interactive semi-automated image processing workflow was developed to easily extract relevant information (e.g. total neuritic length, average cell body area) from both PhC and fluorescence neuronal images. This workflow, named 'NeuronRead', was developed in the form of an ImageJ macro. Its robustness and adaptability were tested and validated on rat cortical primary neurons under control and differentiation inhibitory conditions. Validation included a comparison to manual determinations and to a golden standard freeware tool for fluorescence image analysis. NeuronRead was subsequently applied to PhC images of neurons at distinct differentiation days and exposed or not to DAPT, a pharmacological inhibitor of the γ-secretase enzyme, which cleaves the well-known Alzheimer's amyloid precursor protein (APP) and the Notch receptor. Data obtained confirms a neuritogenic regulatory role for γ-secretase products and validates NeuronRead as a time- and cost-effective useful monitoring tool.

A mobile APP for sharing contacts on your cell: An Editorial Highlight for 'The physiological role of the amyloid precursor protein as an adhesion molecule in the developing nervous system' on page 11

The Review highlighted in this Editorial followed a CAEN Return Home Grant.

BRI2 Processing and Its Neuritogenic Role Are Modulated by Protein Phosphatase 1 Complexing

BRI2 is a ubiquitously expressed type II transmembrane phosphoprotein. BRI2 undergoes proteolytic processing into secreted fragments and during the maturation process it suffers post-translational modifications. Of particular relevance, BRI2 is a protein phosphatase 1 (PP1) interacting protein, where PP1 is able to dephosphorylate the former. Further, disruption of the BRI2:PP1 complex, using BRI2 PP1 binding motif mutants, leads to increased BRI2 phosphorylation levels. However, the physiological function of BRI2 remains elusive; although findings suggest a role in neurite outgrowth and neuronal differentiation. In the work here presented, BRI2 expression during neuronal development was investigated. This increases during neuronal differentiation and an increase in its proteolytic processing is also evident. To elucidate the importance of BRI2 phosphorylation for both proteolytic processing and neuritogenesis, SH-SY5Y cells were transfected with the BRI2 PP1 binding motif mutant constructs. For the first time, it was possible to show that BRI2 phosphorylation is an important regulatory mechanism for its proteolytic processing and its neuritogenic role. Furthermore, by modulating BRI2 processing using an ADAM10 inhibitor, a dual role for BRI2 in neurite outgrowth is suggested: phosphorylated full-length BRI2 appears to be important for the formation of neuritic processes, and BRI2 NTF promotes neurite elongation. This work significantly contributed to the understanding of the physiological function of BRI2 and its regulation by protein phosphorylation. J. Cell. Biochem. 118: 2752-2763, 2017. © 2017 Wiley Periodicals, Inc.

Injectable MnSr-doped brushite bone cements with improved biological performance

Combining Mn and Sr co-doping β-TCP powder with sucrose addition in the setting liquid enhances injectability, mechanical and biological performance of brushite-forming cements, renders them promising for minimally invasive surgery applications.

Enhanced Neuroplasticity by the Metabolic Enhancer Piracetam Associated with Improved Mitochondrial Dynamics and Altered Permeability Transition Pore Function

The mitochondrial cascade hypothesis of dementia assumes mitochondrial dysfunction leading to reduced energy supply, impaired neuroplasticity, and finally cell death as one major pathomechanism underlying the continuum from brain aging over mild cognitive impairment to initial and advanced late onset Alzheimer's disease. Accordingly, improving mitochondrial function has become an important strategy to treat the early stages of this continuum. The metabolic enhancer piracetam has been proposed as possible prototype for those compounds by increasing impaired mitochondrial function and related aspects like mechanisms of neuroplasticity. We here report that piracetam at therapeutically relevant concentrations improves neuritogenesis in the human cell line SH-SY5Y over conditions mirroring the whole spectrum of age-associated cognitive decline. These effects go parallel with improvement of impaired mitochondrial dynamics shifting back fission and fusion balance to the energetically more favorable fusion site. Impaired fission and fusion balance can also be induced by a reduction of the mitochondrial permeability transition pore (mPTP) function as atractyloside which indicates the mPTP has similar effects on mitochondrial dynamics. These changes are also reduced by piracetam. These findings suggest the mPTP as an important target for the beneficial effects of piracetam on mitochondrial function.

New cosurface capacitive stimulators for the development of active osseointegrative implantable devices

Non-drug strategies based on biophysical stimulation have been emphasized for the treatment and prevention of musculoskeletal conditions. However, to date, an effective stimulation system for intracorporeal therapies has not been proposed. This is particularly true for active intramedullary implants that aim to optimize osseointegration. The increasing demand for these implants, particularly for hip and knee replacements, has driven the design of innovative stimulation systems that are effective in bone-implant integration. In this paper, a new cosurface-based capacitive system concept is proposed for the design of implantable devices that deliver controllable and personalized electric field stimuli to target tissues. A prototype architecture of this system was constructed for in vitro tests, and its ability to deliver controllable stimuli was numerically analyzed. Successful results were obtained for osteoblastic proliferation and differentiation in the in vitro tests. This work provides, for the first time, a design of a stimulation system that can be embedded in active implantable devices for controllable bone-implant integration and regeneration. The proposed cosurface design holds potential for the implementation of novel and innovative personalized stimulatory therapies based on the delivery of electric fields to bone cells.

Transcriptomic Profiling Discloses Molecular and Cellular Events Related to Neuronal Differentiation in SH-SY5Y Neuroblastoma Cells

Human SH-SY5Y neuroblastoma cells are widely utilized in in vitro studies to dissect out pathogenetic mechanisms of neurodegenerative disorders. These cells are considered as neuronal precursors and differentiate into more mature neuronal phenotypes under selected growth conditions. In this study, in order to decipher the pathways and cellular processes underlying neuroblastoma cell differentiation in vitro, we performed systematic transcriptomic (RNA-seq) and bioinformatic analysis of SH-SY5Y cells differentiated according to a two-step paradigm: retinoic acid treatment followed by enriched neurobasal medium. Categorization of 1989 differentially expressed genes (DEGs) identified in differentiated cells functionally linked them to changes in cell morphology including remodelling of plasma membrane and cytoskeleton, and neuritogenesis. Seventy-three DEGs were assigned to axonal guidance signalling pathway, and the expression of selected gene products such as neurotrophin receptors, the functionally related SLITRK6, and semaphorins, was validated by immunoblotting. Along with these findings, the differentiated cells exhibited an ability to elongate longer axonal process as assessed by the neuronal cytoskeletal markers biochemical characterization and morphometric evaluation. Recognition of molecular events occurring in differentiated SH-SY5Y cells is critical to accurately interpret the cellular responses to specific stimuli in studies on disease pathogenesis.

The Neuroprotective Properties of the Amyloid Precursor Protein Following Traumatic Brain Injury

Despite the significant health and economic burden that traumatic brain injury (TBI) places on society, the development of successful therapeutic agents have to date not translated into efficacious therapies in human clinical trials. Injury to the brain is ongoing after TBI, through a complex cascade of primary and secondary injury events, providing a valuable window of opportunity to help limit and prevent some of the severe consequences with a timely treatment. Of note, it has been suggested that novel treatments for TBI should be multifactorial in nature, mimicking the body's own endogenous repair response. Whilst research has historically focused on the role of the amyloid precursor protein (APP) in the pathogenesis of Alzheimer's disease, recent advances in trauma research have demonstrated that APP offers considerable neuroprotective properties following TBI, suggesting that APP is an ideal therapeutic candidate. Its acute upregulation following TBI has been shown to serve a beneficial role following trauma and has lead to significant advances in understanding the neuroprotective and neurotrophic functions of APP and its metabolites. Research has focused predominantly on the APP derivative sAPPα, which has consistently demonstrated neuroprotective and neurotrophic functions both in vitro and in vivo following various traumatic insults. Its neuroprotective activity has been narrowed down to a 15 amino acid sequence, and this region is linked to both heparan binding and growth-factor-like properties. It has been proposed that APP binds to heparan sulfate proteoglycans to exert its neuroprotective action. APP presents us with a novel therapeutic compound that could overcome many of the challenges that have stalled development of efficacious TBI treatments previously.

BRI2 and BRI3 are functionally distinct phosphoproteins

Three BRI protein family members have been identified. Among these are BRI3 and BRI2, the latter is associated with Familial Danish and Familial British dementias. 'In silico' sequence analysis identified putative PP1 binding sites in BRI2 and BRI3. This is singularly important, given that protein phosphorylation is a major mechanism regulating intracellular processes. Protein phosphatase 1 (PP1) interacting proteins (PIPs) are fundamental in determining substrate specificity and subcellular localization of this phosphatase. More than 200 PIPs have thus far been reported. Both BRI2 and BRI3 are type II transmembrane glycoproteins relevant in neuronal systems. Using Myc-BRI2 and Myc-BRI3, wild type and PP1 binding mutant constructs, it was possible to show, for the first time, that in fact BRI2 and BRI3 bind PP1. The complexes BRI2:PP1 and BRI3:PP1 were validated in vitro and in vivo. The subcellular distribution of BRI2 and BRI3 is similar; both localize to the perinuclear area and Golgi apparatus in non-neuronal cells. However, in SH-SY5Y cells, BRI2 and BRI3 could also be detected in elongated cellular projections ('processes') and in rat cortical neurons both are broadly distributed throughout the cell body, neuritis and the nucleus. Consistently, co-localization of BRI2 and BRI3 with PP1 was evident. The functional significance of these complexes is apparent given that both BRI proteins are substrates of PP1, thus simultaneously this is the first report of BRI2 and BRI3 as phosphoproteins. Moreover, we show that when BRI2 is phosphorylated a significant increase in neuronal outgrowth and differentiation is evident. Interestingly, the Alzheimer's amyloid precursor protein (APP), forms a trimeric complex composed of PP1 and Fe65, with PP1 having the capacity to dephosphorylate APP at Thr668 residue. The emerging consensus appears to be that PP1 containing complexes are crucial in regulating signaling events underlying neuropathological conditions.