Binding of alphaII spectrin to 14-3-3beta is involved in NCAM-dependent neurite outgrowth

14-3-3 proteins-a moonlight protein complex with therapeutic potential in neurological disorder: in-depth review with Alzheimer's disease

Alzheimer's disease (AD) affects millions of people worldwide and is a gradually worsening neurodegenerative condition. The accumulation of abnormal proteins, such as tau and beta-amyloid, in the brain is a hallmark of AD pathology. 14-3-3 proteins have been implicated in AD pathology in several ways. One proposed mechanism is that 14-3-3 proteins interact with tau protein and modulate its phosphorylation, aggregation, and toxicity. Tau is a protein associated with microtubules, playing a role in maintaining the structural integrity of neuronal cytoskeleton. However, in the context of Alzheimer's disease (AD), an abnormal increase in its phosphorylation occurs. This leads to the aggregation of tau into neurofibrillary tangles, which is a distinctive feature of this condition. Studies have shown that 14-3-3 proteins can bind to phosphorylated tau and regulate its function and stability. In addition, 14-3-3 proteins have been shown to interact with beta-amyloid (Aβ), the primary component of amyloid plaques in AD. 14-3-3 proteins can regulate the clearance of Aβ through the lysosomal degradation pathway by interacting with the lysosomal membrane protein LAMP2A. Dysfunction of lysosomal degradation pathway is thought to contribute to the accumulation of Aβ in the brain and the progression of AD. Furthermore, 14-3-3 proteins have been found to be downregulated in the brains of AD patients, suggesting that their dysregulation may contribute to AD pathology. For example, decreased levels of 14-3-3 proteins in cerebrospinal fluid have been suggested as a biomarker for AD. Overall, these findings suggest that 14-3-3 proteins may play an important role in AD pathology and may represent a potential therapeutic target for the disease. However, further research is needed to fully understand the mechanisms underlying the involvement of 14-3-3 proteins in AD and to explore their potential as a therapeutic target.

Pathways to Parkinson's disease: a spotlight on 14-3-3 proteins

14-3-3s represent a family of highly conserved 30 kDa acidic proteins. 14-3-3s recognize and bind specific phospho-sequences on client partners and operate as molecular hubs to regulate their activity, localization, folding, degradation, and protein-protein interactions. 14-3-3s are also associated with the pathogenesis of several diseases, among which Parkinson's disease (PD). 14-3-3s are found within Lewy bodies (LBs) in PD patients, and their neuroprotective effects have been demonstrated in several animal models of PD. Notably, 14-3-3s interact with some of the major proteins known to be involved in the pathogenesis of PD. Here we first provide a detailed overview of the molecular composition and structural features of 14-3-3s, laying significant emphasis on their peculiar target-binding mechanisms. We then briefly describe the implication of 14-3-3s in the central nervous system and focus on their interaction with LRRK2, α-Synuclein, and Parkin, three of the major players in PD onset and progression. We finally discuss how different types of small molecules may interfere with 14-3-3s interactome, thus representing a valid strategy in the future of drug discovery.

New Partners Identified by Mass Spectrometry Assay Reveal Functions of NCAM2 in Neural Cytoskeleton Organization

Neuronal cell adhesion molecule 2 (NCAM2) is a membrane protein with an important role in the morphological development of neurons. In the cortex and the hippocampus, NCAM2 is essential for proper neuronal differentiation, dendritic and axonal outgrowth and synapse formation. However, little is known about NCAM2 functional mechanisms and its interactive partners during brain development. Here we used mass spectrometry to study the molecular interactome of NCAM2 in the second postnatal week of the mouse cerebral cortex. We found that NCAM2 interacts with >100 proteins involved in numerous processes, including neuronal morphogenesis and synaptogenesis. We validated the most relevant interactors, including Neurofilaments (NEFs), Microtubule-associated protein 2 (MAP2), Calcium/calmodulin kinase II alpha (CaMKIIα), Actin and Nogo. An in silico analysis of the cytosolic tail of the NCAM2.1 isoform revealed specific phosphorylation site motifs with a putative affinity for some of these interactors. Our results expand the knowledge of NCAM2 interactome and confirm the key role of NCAM2 in cytoskeleton organization, neuronal morphogenesis and synaptogenesis. These findings are of interest in explaining the phenotypes observed in different pathologies with alterations in the NCAM2 gene.

14-3-3 mitigates alpha-synuclein aggregation and toxicity in the in vivo preformed fibril model

Alpha-synuclein (αsyn) is the key component of proteinaceous aggregates termed Lewy Bodies that pathologically define a group of disorders known as synucleinopathies, including Parkinson's Disease (PD) and Dementia with Lewy Bodies. αSyn is hypothesized to misfold and spread throughout the brain in a prion-like fashion. Transmission of αsyn necessitates the release of misfolded αsyn from one cell and the uptake of that αsyn by another, in which it can template the misfolding of endogenous αsyn upon cell internalization. 14-3-3 proteins are a family of highly expressed brain proteins that are neuroprotective in multiple PD models. We have previously shown that 14-3-3θ acts as a chaperone to reduce αsyn aggregation, cell-to-cell transmission, and neurotoxicity in the in vitro pre-formed fibril (PFF) model. In this study, we expanded our studies to test the impact of 14-3-3s on αsyn toxicity in the in vivo αsyn PFF model. We used both transgenic expression models and adenovirus associated virus (AAV)-mediated expression to examine whether 14-3-3 manipulation impacts behavioral deficits, αsyn aggregation, and neuronal counts in the PFF model. 14-3-3θ transgene overexpression in cortical and amygdala regions rescued social dominance deficits induced by PFFs at 6 months post injection, whereas 14-3-3 inhibition by transgene expression of the competitive 14-3-3 peptide inhibitor difopein in the cortex and amygdala accelerated social dominance deficits. The behavioral rescue by 14-3-3θ overexpression was associated with delayed αsyn aggregation induced by PFFs in these brain regions. Conversely, 14-3-3 inhibition by difopein in the cortex and amygdala accelerated αsyn aggregation and reduction in NECAB1-positive neuron counts induced by PFFs. 14-3-3θ overexpression by AAV in the substantia nigra (SN) also delayed αsyn aggregation in the SN and partially rescued PFF-induced reduction in tyrosine hydroxylase (TH)-positive dopaminergic cells in the SN. 14-3-3 inhibition in the SN accelerated nigral αsyn aggregation and enhanced PFF-induced reduction in TH-positive dopaminergic cells. These data indicate a neuroprotective role for 14-3-3θ against αsyn toxicity in vivo.

The role of spectrin in cell adhesion and cell-cell contact

Spectrins are proteins that are responsible for many aspects of cell function and adaptation to changing environments. Primarily the spectrin-based membrane skeleton maintains cell membrane integrity and its mechanical properties, together with the cytoskeletal network a support cell shape. The occurrence of a variety of spectrin isoforms in diverse cellular environments indicates that it is a multifunctional protein involved in numerous physiological pathways. Participation of spectrin in cell–cell and cell–extracellular matrix adhesion and formation of dynamic plasma membrane protrusions and associated signaling events is a subject of interest for researchers in the fields of cell biology and molecular medicine. In this mini-review, we focus on data concerning the role of spectrins in cell surface activities such as adhesion, cell–cell contact, and invadosome formation. We discuss data on different adhesion proteins that directly or indirectly interact with spectrin repeats. New findings support the involvement of spectrin in cell adhesion and spreading, formation of lamellipodia, and also the participation in morphogenetic processes, such as eye development, oogenesis, and angiogenesis. Here, we review the role of spectrin in cell adhesion and cell–cell contact.

Impact statement

This article reviews properties of spectrins as a group of proteins involved in cell surface activities such as, adhesion and cell–cell contact, and their contribution to morphogenesis. We show a new area of research and discuss the involvement of spectrin in regulation of cell–cell contact leading to immunological synapse formation and in shaping synapse architecture during myoblast fusion. Data indicate involvement of spectrins in adhesion and cell–cell or cell–extracellular matrix interactions and therefore in signaling pathways. There is evidence of spectrin’s contribution to the processes of morphogenesis which are connected to its interactions with adhesion molecules, membrane proteins (and perhaps lipids), and actin. Our aim was to highlight the essential role of spectrin in cell–cell contact and cell adhesion.

14-3-3 Proteins Reduce Cell-to-Cell Transfer and Propagation of Pathogenic α-Synuclein

α-Synuclein (αsyn) is the key protein that forms neuronal aggregates in the neurodegenerative disorders Parkinson's disease (PD) and dementia with Lewy bodies. Recent evidence points to the prion-like spread of αsyn from one brain region to another. Propagation of αsyn is likely dependent on release, uptake, and misfolding. Under normal circumstances, this highly expressed brain protein functions normally without promoting pathology, yet the underlying endogenous mechanisms that prevent αsyn spread are not understood. 14-3-3 proteins are highly expressed brain proteins that have chaperone function and regulate protein trafficking. In this study, we investigated the potential role of the 14-3-3 proteins in the regulation of αsyn spread using two models of αsyn spread. In a paracrine αsyn model, 14-3-3θ promoted release of αsyn complexed with 14-3-3θ. Despite higher amounts of released αsyn, extracellular αsyn showed reduced oligomerization and seeding capability, reduced internalization, and reduced toxicity in primary mixed-gender mouse neurons. 14-3-3 inhibition reduced the amount of αsyn released, yet released αsyn was more toxic and demonstrated increased oligomerization, seeding capability, and internalization. In the preformed fibril model, 14-3-3 θ reduced αsyn aggregation and neuronal death, whereas 14-3-3 inhibition enhanced αsyn aggregation and neuronal death in primary mouse neurons. 14-3-3s blocked αsyn spread to distal chamber neurons not exposed directly to fibrils in multichamber, microfluidic devices. These findings point to 14-3-3s as a direct regulator of αsyn propagation, and suggest that dysfunction of 14-3-3 function may promote αsyn pathology in PD and related synucleinopathies.SIGNIFICANCE STATEMENT Transfer of misfolded aggregates of α-synuclein from one brain region to another is implicated in the pathogenesis of Parkinson's disease and other synucleinopathies. This process is dependent on active release, internalization, and misfolding of α-synuclein. 14-3-3 proteins are highly expressed chaperone proteins that interact with α-synuclein and regulate protein trafficking. We used two different models in which toxicity is associated with cell-to-cell transfer of α-synuclein to test whether 14-3-3s impact α-synuclein toxicity. We demonstrate that 14-3-3θ reduces α-synuclein transfer and toxicity by inhibiting oligomerization, seeding capability, and internalization of α-synuclein, whereas 14-3-3 inhibition accelerates the transfer and toxicity of α-synuclein in these models. Dysfunction of 14-3-3 function may be a critical mechanism by which α-synuclein propagation occurs in disease.

Scale Invariant Disordered Nanotopography Promotes Hippocampal Neuron Development and Maturation with Involvement of Mechanotransductive Pathways

The identification of biomaterials which promote neuronal maturation up to the generation of integrated neural circuits is fundamental for modern neuroscience. The development of neural circuits arises from complex maturative processes regulated by poorly understood signaling events, often guided by the extracellular matrix (ECM). Here we report that nanostructured zirconia surfaces, produced by supersonic cluster beam deposition of zirconia nanoparticles and characterized by ECM-like nanotopographical features, can direct the maturation of neural networks. Hippocampal neurons cultured on such cluster-assembled surfaces displayed enhanced differentiation paralleled by functional changes. The latter was demonstrated by single-cell electrophysiology showing earlier action potential generation and increased spontaneous postsynaptic currents compared to the neurons grown on the featureless unnaturally flat standard control surfaces. Label-free shotgun proteomics broadly confirmed the functional changes and suggests furthermore a vast impact of the neuron/nanotopography interaction on mechanotransductive machinery components, known to control physiological in vivo ECM-regulated axon guidance and synaptic plasticity. Our results indicate a potential of cluster-assembled zirconia nanotopography exploitable for the creation of efficient neural tissue interfaces and cell culture devices promoting neurogenic events, but also for unveiling mechanotransductive aspects of neuronal development and maturation.

14-3-3 Proteins regulate mutant LRRK2 kinase activity and neurite shortening

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common known cause of inherited Parkinson's disease (PD), and LRRK2 is a risk factor for idiopathic PD. How LRRK2 function is regulated is not well understood. Recently, the highly conserved 14-3-3 proteins, which play a key role in many cellular functions including cell death, have been shown to interact with LRRK2. In this study, we investigated whether 14-3-3s can regulate mutant LRRK2-induced neurite shortening and kinase activity. In the presence of 14-3-3θ overexpression, neurite length of primary neurons from BAC transgenic G2019S-LRRK2 mice returned back to wild-type levels. Similarly, 14-3-3θ overexpression reversed neurite shortening in neuronal cultures from BAC transgenic R1441G-LRRK2 mice. Conversely, inhibition of 14-3-3s by the pan-14-3-3 inhibitor difopein or dominant-negative 14-3-3θ further reduced neurite length in G2019S-LRRK2 cultures. Since G2019S-LRRK2 toxicity is likely mediated through increased kinase activity, we examined 14-3-3θ's effects on LRRK2 kinase activity. 14-3-3θ overexpression reduced the kinase activity of G2019S-LRRK2, while difopein promoted the kinase activity of G2019S-LRRK2. The ability of 14-3-3θ to reduce LRRK2 kinase activity required direct binding of 14-3-3θ with LRRK2. The potentiation of neurite shortening by difopein in G2019S-LRRK2 neurons was reversed by LRRK2 kinase inhibitors. Taken together, we conclude that 14-3-3θ can regulate LRRK2 and reduce the toxicity of mutant LRRK2 through a reduction of kinase activity.

Switching responses: spatial and temporal regulators of axon guidance

The ability of the axonal growth cone to switch between attraction and repulsion in response to guidance cues in the extracellular environment during nervous system development is fundamental to the precise wiring of complex neural circuits. Regulation of cell-surface receptors by means of transcriptional control, local translation, trafficking and proteolytic processing are powerful mechanisms to regulate the response of the growth cone. Important work has also revealed how intracellular signalling pathways, including calcium and cyclic nucleotide signalling, can alter the directional response elicited by a particular cue. Here, we describe how these multiple regulatory mechanisms influence growth cone turning behaviour. We focus on recent evidence that suggests a significant role for 14-3-3 adaptor proteins in modifying growth cone turning behaviour and mediating directional polarity switches during development. Characterizing how 14-3-3 s regulate growth cone signalling will provide invaluable insight into nervous system development and may facilitate the identification of novel targets for promoting nerve regeneration following injury.

A conserved sequence in calmodulin regulated spectrin-associated protein 1 links its interaction with spectrin and calmodulin to neurite outgrowth

Calmodulin regulated spectrin-associated protein 1 (CAMSAP1) is a vertebrate microtubule-binding protein, and a representative of a family of cytoskeletal proteins that arose with animals. We reported previously that the central region of the protein, which contains no recognized functional domain, inhibited neurite outgrowth when over-expressed in PC12 cells [Baines et al., Mol. Biol. Evol. 26 (2009), p. 2005]. The CKK domain (DUF1781) binds microtubules and defines the CAMSAP/ssp4 family of animal proteins (Baines et al. 2009). In the central region, three short well-conserved regions are characteristic of CAMSAP-family members. One of these, CAMSAP-conserved region 1 (CC1), bound to both βIIΣ1-spectrin and Ca²⁺/calmodulin in vitro. The binding of Ca²⁺/calmodulin inhibited spectrin binding. Transient expression of CC1 in PC12 cells inhibited neurite outgrowth. siRNA knockdown of CAMSAP1 inhibited neurite outgrowth in PC12 cells or primary cerebellar granule cells: this could be rescued in PC12 cells by wild-type CAMSAP1-enhanced green fluorescent protein, but not by a CC1 mutant. We conclude that CC1 represents a functional region of CAMSAP1, which links spectrin-binding to neurite outgrowth.

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.

Proteomic atlas of the human olfactory bulb

The olfactory bulb (OB) is the first site for the processing of olfactory information in the brain and its deregulation is associated with neurodegenerative disorders. Although different efforts have been made to characterize the human brain proteome in depth, the protein composition of the human OB remains largely unexplored. We have performed a comprehensive analysis of the human OB proteome employing protein and peptide fractionation methods followed by LC-MS/MS, identifying 1529 protein species, corresponding to 1466 unique proteins, which represents a 7-fold increase in proteome coverage with respect to previous OB proteome descriptions from translational models. Bioinformatic analyses revealed that protein components of the OB participated in a plethora of biological process highlighting hydrolase and phosphatase activities and nucleotide and RNA binding activities. Interestingly, 631 OB proteins identified were not previously described in protein datasets derived from large-scale Human Brain Proteome Project (HBPP) studies. In particular, a subset of these differential proteins was mainly involved in axon guidance, opioid signaling, neurotransmitter receptor binding, and synaptic plasticity. Taken together, these results increase our knowledge about the molecular composition of the human OB and may be useful to understand the molecular basis of the olfactory system and the etiology of its disorders.

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

The world's human population ages rapidly thanks to the great advance in modern medicine. While more and more body system diseases become treatable and curable, age-related neurodegenerative diseases remain poorly understood mechanistically, and are desperately in need of preventive and therapeutic interventions. Biomarker development consists of a key part of concerted effort in combating neurodegenerative diseases. In many chronic neurodegenerative conditions, neuronal damage/death occurs long before the onset of disease symptoms, and abnormal proteolysis may either play an active role or be a companying event of neuronal injury. Increased spectrin cleavage yielding elevated spectrin breakdown products (SBDPs) by calcium-sensitive proteases such as calpain and caspases has been established in conditions associated with acute neuronal damage such as traumatic brain injury (TBI). Here we review literature regarding spectrin expression and metabolism in the brain, and propose a potential use of SBDPs as biomarkers for neurodegenerative diseases such as Alzheimer's diseases.

Cell organization, growth, and neural and cardiac development require αII-spectrin

Spectrin α2 (αII-spectrin) is a scaffolding protein encoded by the Spna2 gene and constitutively expressed in most tissues. Exon trapping of Spna2 in C57BL/6 mice allowed targeted disruption of αII-spectrin. Heterozygous animals displayed no phenotype by 2 years of age. Homozygous deletion of Spna2 was embryonic lethal at embryonic day 12.5 to 16.5 with retarded intrauterine growth, and craniofacial, neural tube and cardiac anomalies. The loss of αII-spectrin did not alter the levels of αI- or βI-spectrin, or the transcriptional levels of any β-spectrin or any ankyrin, but secondarily reduced by about 80% the steady state protein levels of βII- and βIII-spectrin. Residual βII- and βIII-spectrin and ankyrins B and G were concentrated at the apical membrane of bronchial and renal epithelial cells, without impacting cell morphology. Neuroepithelial cells in the developing brain were more concentrated and more proliferative in the ventricular zone than normal; axon formation was also impaired. Embryonic fibroblasts cultured on fibronectin from E14.5 (Spna2(-/-)) animals displayed impaired growth and spreading, a spiky morphology, and sparse lamellipodia without cortical actin. These data indicate that the spectrin-ankyrin scaffold is crucial in vertebrates for cell spreading, tissue patterning and organ development, particularly in the developing brain and heart, but is not required for cell viability.

14-3-3 proteins in neurodegeneration

Among the first reported functions of 14-3-3 proteins was the regulation of tyrosine hydroxylase (TH) activity suggesting a possible involvement of 14-3-3 proteins in Parkinson's disease. Since then the relevance of 14-3-3 proteins in the pathogenesis of chronic as well as acute neurodegenerative diseases, including Alzheimer's disease, polyglutamine diseases, amyotrophic lateral sclerosis and stroke has been recognized. The reported function of 14-3-3 proteins in this context are as diverse as the mechanism involved in neurodegeneration, reaching from basal cellular processes like apoptosis, over involvement in features common to many neurodegenerative diseases, like protein stabilization and aggregation, to very specific processes responsible for the selective vulnerability of cellular populations in single neurodegenerative diseases. Here, we review what is currently known of the function of 14-3-3 proteins in nervous tissue focussing on the properties of 14-3-3 proteins important in neurodegenerative disease pathogenesis.

Spectrin-based skeleton as an actor in cell signaling

This review focuses on the recent advances in functions of spectrins in non-erythroid cells. We discuss new data concerning the commonly known role of the spectrin-based skeleton in control of membrane organization, stability and shape, and tethering protein mosaics to the cellular motors and to all major filament systems. Particular effort has been undertaken to highlight recent advances linking spectrin to cell signaling phenomena and its participation in signal transduction pathways in many cell types.