Activity-driven dendritic remodeling requires microtubule-associated protein 1A

RREB1 regulates neuronal proteostasis and the microtubule network

Transcription factors play vital roles in neuron development; however, little is known about the role of these proteins in maintaining neuronal homeostasis. Here, we show that the transcription factor RREB1 (Ras-responsive element-binding protein 1) is essential for neuron survival in the mammalian brain. A spontaneous mouse mutation causing loss of a nervous system-enriched Rreb1 transcript is associated with progressive loss of cerebellar Purkinje cells and ataxia. Analysis of chromatin immunoprecipitation and sequencing, along with RNA sequencing data revealed dysregulation of RREB1 targets associated with the microtubule cytoskeleton. In agreement with the known role of microtubules in dendritic development, dendritic complexity was disrupted in Rreb1-deficient neurons. Analysis of sequencing data also suggested that RREB1 plays a role in the endomembrane system. Mutant Purkinje cells had fewer numbers of autophagosomes and lysosomes and contained P62- and ubiquitin-positive inclusions. Together, these studies demonstrate that RREB1 functions to maintain the microtubule network and proteostasis in mammalian neurons.

Sculpting the dendritic landscape: Actin, microtubules, and the art of arborization

Dendrites are intricately designed neuronal compartments that play a vital role in the gathering and processing of sensory or synaptic inputs. Their diverse and elaborate structures are distinct features of neuronal organization and function. Central to the generation of these dendritic arbors is the neuronal cytoskeleton. In this review, we delve into the current progress toward our understanding of how dendrite arbors are generated and maintained, focusing on the role of the actin and microtubule cytoskeleton.

N-degron-mediated degradation of the proteolytically activated form of PKC-theta kinase attenuates its pro-apoptotic function

Cellular signalling cues lead to the initiation of apoptotic pathways and often result in the activation of caspases which in turn cause the generation of proteolytically generated protein fragments with new or altered functions. Mounting number of studies reveal that the activity of these proteolytically activated protein fragments can be counteracted via their selective degradation by the N-degron degradation pathways. Here, we investigate the proteolytically generated fragment of the PKC theta kinase, where we demonstrate the first report on the stability of this pro-apoptotic protein fragment. We have determined that the pro-apoptotic cleaved fragment of PKC-theta is unstable in cells because its N-terminal lysine targets it for proteasomal degradation via the N-degron degradation pathway and this degradation is inhibited by mutating the destabilizing N-termini, knockdown of the UBR1 and UBR2 E3 ligases. Tellingly, we demonstrate that the metabolic stabilization of the cleaved fragment of PKC-theta or inhibition of the N-degron degradation augments the apoptosis-inducing effect of staurosporine in Jurkat cells. Notably, we have unveiled that the cleaved fragment of PKC theta, per se, can induce apoptotic cell death in Jurkat T-cell leukemia. Our results expand the functional scope of mammalian N-degron degradation pathways, and support the notion that targeting N-degron degradation machinery may have promising therapeutic implications in cancer cells.

The ion channel CALHM6 controls bacterial infection-induced cellular cross-talk at the immunological synapse

Membrane ion channels of the calcium homeostasis modulator (CALHM) family promote cell-cell crosstalk at neuronal synapses via ATP release, where ATP acts as a neurotransmitter. CALHM6, the only CALHM highly expressed in immune cells, has been linked to the induction of natural killer (NK) cell anti-tumour activity. However, its mechanism of action and broader functions in the immune system remain unclear. Here, we generated Calhm6-/- mice and report that CALHM6 is important for the regulation of the early innate control of Listeria monocytogenes infection in vivo. We find that CALHM6 is upregulated in macrophages by pathogen-derived signals and that it relocates from the intracellular compartment to the macrophage-NK cell synapse, facilitating ATP release and controlling the kinetics of NK cell activation. Anti-inflammatory cytokines terminate CALHM6 expression. CALHM6 forms an ion channel when expressed in the plasma membrane of Xenopus oocytes, where channel opening is controlled by a conserved acidic residue, E119. In mammalian cells, CALHM6 is localised to intracellular compartments. Our results contribute to the understanding of neurotransmitter-like signal exchange between immune cells that fine-tunes the timing of innate immune responses.

Protocol for Evaluating Neuronal Polarity in Murine Models

The establishment of polarity is crucial for the physiology and wiring of neurons. Therefore, monitoring the axo-dendritic specification allows the mechanisms and signals associated with development, growth, and disease to be explored. Here, we describe major and minor steps to study polarity acquisition, using primary cultures of hippocampal neurons isolated from embryonic rat hippocampi, for in vitro monitoring. Furthermore, we use in utero electroporated, GFP-expressing embryonic mouse brains for visualizing cortical neuron migration and polarization in situ. Some underreported after-protocol steps are also included.

For complete details on the use and execution of this protocol, please refer to Wilson et al. (2020).

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Dissection, isolation, and digestion of embryonic (E18.5) rat hippocampi

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Culturing isolated hippocampal neurons and monitoring polarity acquisition in vitro

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In utero electroporation of embryonic (E15.5) mouse brains with GFP plasmids

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Visualization of migration and polarization of E17.5–E18.5 cortical neurons in situ

Age- and memory- related differences in hippocampal gray matter integrity are better captured by NODDI compared to single-tensor diffusion imaging

Single-tensor diffusion imaging (DTI) has traditionally been used to assess integrity of white matter. For example, we previously showed that integrity of limbic white matter tracts declines in healthy aging and relates to episodic memory performance. However, multi-compartment diffusion models may be more informative about microstructural properties of gray matter. The current study examined hippocampal gray matter integrity using both single-tensor and multi-compartment (neurite orientation dispersion and density imaging, NODDI) diffusion imaging. Younger (20-38 years) and older (59-84 years) adults also completed the Mnemonic Similarity Task to measure mnemonic discrimination performance. Results revealed age-related declines in both single-tensor (lower fractional anisotropy, higher mean diffusivity) and multi-compartment (higher restricted, hindered and free diffusion) measures of hippocampal gray matter integrity. As expected, NODDI measures (hindered and free diffusion) captured more age-related variance than DTI measures. Moreover, mnemonic discrimination of highly similar lure items in memory was related to hippocampal gray matter integrity in younger but not older adults. These findings support the notion that age-related differences in gray matter integrity are better captured by multi-compartment versus single-tensor diffusion models and show that the relationship between mnemonic discrimination and hippocampal gray matter integrity is moderated by age.

Modulation of Human Adipose Stem Cells' Neurotrophic Capacity Using a Variety of Growth Factors for Neural Tissue Engineering Applications: Axonal Growth, Transcriptional, and Phosphoproteomic Analyses In Vitro

We report on a potential strategy involving the exogenous neurotrophic factors (NTF) for enhancing the neurotrophic capacity of human adipose stem cells (ASC) in vitro. For this, ASC were stimulated for three days using NTF, i.e., nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT3), NT4, glial cell-derived neurotrophic factor (GDNF), and ciliary neurotrophic factor (CNTF). The resulting conditioned medium (CM) as well as individual NTF exhibited distinct effects on axonal outgrowth from dorsal root ganglion (DRG) explants. In particular, CM derived from NT3-stimulated ASC (CM-NT3-ASC) promoted robust axonal outgrowth. Subsequent transcriptional analysis of DRG cultures in response to CM-NT3-ASC displayed significant upregulation of STAT-3 and GAP-43. In addition, phosphoproteomic analysis of NT3-stimulated ASC revealed significant changes in the phosphorylation state of different proteins that are involved in cytokine release, growth factors signaling, stem cell maintenance, and differentiation. Furthermore, DRG cultures treated with CM-NT3-ASC exhibited significant changes in the phosphorylation levels of proteins involved in tubulin and actin cytoskeletal pathways, which are crucial for axonal growth and elongation. Thus, the results obtained at the transcriptional, proteomic, and cellular level reveal significant changes in the neurotrophic capacity of ASC following NT3 stimulation and provide new options for improving the axonal growth-promoting potential of ASC in vitro.

Non-photoreceptor Expression of Tulp1 May Contribute to Extensive Retinal Degeneration in Tulp1-/- Mice

Mutations in tubby like protein 1 gene (TULP1) are causative of early-onset recessive inherited retinal degenerations (IRDs); similarly, the Tulp1-/- mouse is also characterized by a rapid IRD. Tulp1 mRNA and protein expression was analyzed in wild type mouse retinas and expression data sets (NCBI) during early postnatal development. Comparative histology was undertaken in Tulp1-/-, rhodopsin-/- (Rho-/-) and retinal degeneration slow-/- (Rds-/-) mouse retinas. Bioinformatic analysis of predicted TULP1 interactors and IRD genes was performed. Peak expression of Tulp1 in healthy mouse retinas was detected at p8; of note, TULP1 was detected in both the outer and inner retina. Bioinformatic analysis indicated Tulp1 expression in retinal progenitor, photoreceptor and non-photoreceptor cells. While common features of photoreceptor degeneration were detected in Tulp1-/-, Rho-/-, and Rds-/- retinas, other alterations in bipolar, amacrine and ganglion cells were specific to Tulp1-/- mice. Additionally, predicted TULP1 interactors differed in various retinal cell types and new functions for TULP1 were suggested. A pilot bioinformatic analysis indicated that in a similar fashion to Tulp1, many other IRD genes were expressed in both inner and outer retinal cells at p4-p7. Our data indicate that expression of Tulp1 extends to multiple retinal cell types; lack of TULP1 may lead to primary degeneration not only of photoreceptor but also non-photoreceptor cells. Predicted interactors suggest widespread retinal functions for TULP1. Early and widespread expression of TULP1 and some other IRD genes in both the inner and outer retina highlights potential hurdles in the development of treatments for these IRDs.

Autophagy lipidation machinery regulates axonal microtubule dynamics but is dispensable for survival of mammalian neurons

Neurons maintain axonal homeostasis via employing a unique organization of the microtubule (MT) cytoskeleton, which supports axonal morphology and provides tracks for intracellular transport. Abnormal MT-based trafficking hallmarks the pathology of neurodegenerative diseases, but the exact mechanism regulating MT dynamics in axons remains enigmatic. Here we report on a regulation of MT dynamics by AuTophaGy(ATG)-related proteins, which previously have been linked to the autophagy pathway. We find that ATG proteins required for LC3 lipid conjugation are dispensable for survival of excitatory neurons and instead regulate MT stability via controlling the abundance of the MT-binding protein CLASP2. This function of ATGs is independent of their role in autophagy and requires the active zone protein ELKS1. Our results highlight a non-canonical role of ATG proteins in neurons and suggest that pharmacological activation of autophagy may not only promote the degradation of cytoplasmic material, but also impair axonal integrity via altering MT stability.

In neurons, the microtubule cytoskeleton provides support and directionality of axons. Here, the authors report that microtubule dynamics in axons may be regulated by the autophagy proteins (ATGs) independently of their known role in autophagy.

The microtubule skeleton and the evolution of neuronal complexity in vertebrates

The evolution of a highly developed nervous system is mirrored by the ability of individual neurons to develop increased morphological complexity. As microtubules (MTs) are crucially involved in neuronal development, we tested the hypothesis that the evolution of complexity is driven by an increasing capacity of the MT system for regulated molecular interactions as it may be implemented by a higher number of molecular players and a greater ability of the individual molecules to interact. We performed bioinformatics analysis on different classes of components of the vertebrate neuronal MT cytoskeleton. We show that the number of orthologs of tubulin structure proteins, MT-binding proteins and tubulin-sequestering proteins expanded during vertebrate evolution. We observed that protein diversity of MT-binding and tubulin-sequestering proteins increased by alternative splicing. In addition, we found that regions of the MT-binding protein tau and MAP6 displayed a clear increase in disorder extent during evolution. The data provide evidence that vertebrate evolution is paralleled by gene expansions, changes in alternative splicing and evolution of coding sequences of components of the MT system. The results suggest that in particular evolutionary changes in tubulin-structure proteins, MT-binding proteins and tubulin-sequestering proteins were prominent drivers for the development of increased neuronal complexity.

Hippocampal stimulation promotes intracellular Tip60 dynamics with concomitant genome reorganization and synaptic gene activation

Genomic reorganizations mediating the engagement of target genes to transcription factories (TFs), characterized as specialized nuclear subcompartments enriched in hyperphosphorylated RNA polymerase II (RNAPII) and transcriptional regulators, act as an important layer of control in coordinating efficient gene transcription. However, their presence in hippocampal neurons and potential role in activity-dependent coregulation of genes within the brain remains unclear. Here, we investigate whether the well-characterized role for the histone acetyltransferase (HAT) Tip60 in mediating epigenetic control of inducible neuroplasticity genes involves TF associated chromatin reorganization in the hippocampus. We show that Tip60 shuttles into the nucleus following extracellular stimulation of rat hippocampal neurons with concomitant enhancement of Tip60 binding and activation of specific synaptic plasticity genes. Multicolor three-dimensional (3D) DNA fluorescent in situ hybridization (DNA-FISH) reveals that hippocampal stimulation mobilizes these same synaptic plasticity genes and Tip60 to RNAPII-rich TFs. Our data support a model by which external hippocampal stimulation promotes intracellular Tip60 HAT dynamics with concomitant TF associated genome reorganization to initiate Tip60mediated synaptic gene activation.

Proteomic Identification of Pathways Responsible for the Estradiol Therapeutic Window in AD Animal Models

Benefits and risks were reported for hormone therapy (HT) to prevent chronic disease, including Alzheimer's disease (AD). While the Women's Health Initiative (WHI) found no protective effect of HT on the cognitive function of women whose treatment was initiated far past the onset of menopause, other studies showed reduced risk of AD with midlife treatment, versus increased risk of AD with late treatment. These suggest a critical window during which estradiol must be administered to prevent cognitive decline and AD in women. Our published work supports this, by demonstrating that early and long-term estradiol treatment improves cognitive function and reduce Aβ accumulation in AD mouse models with estradiol deficiency, while there is no effect of late and short-term estradiol treatment on AD neuropathogenesis. However, little is known about the molecular mechanisms underlying the critical window and whether different protein networks are responsible for the brain estradiol deficiency-associated risk of AD in females. In this study, we used proteomics to identify target protein pathways that are activated during the estradiol therapeutic window in AD mouse model. Our results showed that different signaling pathways were involved in the regulatory effects of estradiol on MAP1A and hemoglobin α. Estradiol treatment increased the level of MAP1A through the phosphorylation of ERK1/2 and increased the level of hemoglobin α through the phosphorylation of AKT. This study has provided molecular insights into the "critical window" theory and identifies specific target proteins of therapeutic responsiveness that may lead to improved treatment strategies and optimal estradiol therapy.

Mouse induced pluripotent stem cells-derived Alzheimer's disease cerebral organoid culture and neural differentiation disorders

Alzheimer's disease (AD) is a progressive neurodegenerative disease, characterized by cognitive impairment. However, the pathogenesis of AD are very complicated, and the theories of Aβ and neurofibrillary tangles cannot explain all pathological alterations and clinical symptoms. Here, we used three-dimensional (3D) neural organoids culture derived from mouse induced pluripotent stem cells (iPSCs) to investigate the pathological mechanisms of AD. In this study, AD cerebral organoids were generated by overexpressing familial AD mutations (APP and PS1 genes) in mouse induced pluripotent stem cells, so that the early pathogenesis of AD could be investigated well with protein and cellular phenotype analyses. The results showed that AD cerebral organoids appeared some AD pathological alterations, and high levels of Aβ and p-Tau were induced as well. Furthermore, the number of GFAP-positive astrocytes and glutamatergic excitatory neurons increased significantly, but the number of GABAergic interneurons decreased. In conclusion, we suggest that cerebral organoids are a suitable AD model for scientific study, and that will provide us a novel insight into the understanding of the pathogenesis of AD.

Ancestral Stress Alters Lifetime Mental Health Trajectories and Cortical Neuromorphology via Epigenetic Regulation

Experiences during early development are powerful determinants of lifetime mental health. Here we investigated if ancestral stress regulates the brain’s epigenetic memory to alter neuromorphology and emotionality in the remote F4 progeny. Pregnant female rat dams of the parental F0 generation were exposed to stress on gestational days 12–18. To generate a transgenerational stress lineage, their pregnant daughters (F1), grand-daughters (F2) and great-grand-daughters (F3) remained undisturbed. To generate a multigenerational stress lineage, pregnant dams of each generation (F1–F3) were stressed. A lineage of non-stress controls (F0–F3) was also produced. Multigenerational stress exceeded the impact of transgenerational stress by increasing anxiety-like behaviours and stress response in young and middle-aged F4 males but not females. Functional changes were accompanied by reduced spine density in the male medial prefrontal cortex with opposite effects in the orbital frontal cortex. Ancestral stress regulated cortical miR-221 and miR-26 expression and their target genes, thus downregulating ntrk2 and map1a genes in males while downregulating crh and upregulating map1a genes in females. These miRNA-dependent pathways are candidates for developmental programming of lifetime mental health. Thus, multigenerational stress in particular determines sexually dimorphic predisposition to stress vulnerability and generates a phenotype resembling symptoms of post-traumatic stress disorder.

Deafferentation-induced alterations in mitral cell dendritic morphology in the adult zebrafish olfactory bulb

The removal of afferent input to the olfactory bulb by both cautery and chemical olfactory organ ablation in adult zebrafish results in a significant decrease in volume of the ipsilateral olfactory bulb. To examine the effects of deafferentation at a cellular level, primary output neurons of the olfactory bulb, the mitral cells, were investigated using retrograde tract tracing with fluorescent dextran using ex vivo brain cultures. Morphological characteristics including the number of major dendritic branches, total length of dendritic branches, area of the dendritic arbor, overall dendritic complexity, and optical density of the arbor were used to determine the effects of deafferentation on mitral cell dendrites. Following 8 weeks of permanent deafferentation there were significant reductions in the total length of dendritic branches, the area of the dendritic arbor, and the density of fine processes in the dendritic tuft. With 8 weeks of chronic, partial deafferentation there were significant reductions in all parameters examined, including a modified Sholl analysis that showed significant decreases in overall dendritic complexity. These results show the plasticity of mitral cell dendritic structures in the adult brain and provide information about the response of these output neurons following the loss of sensory input in this key model system.

Adrenomedullin, a Novel Target for Neurodegenerative Diseases

Neurodegenerative diseases represent a heterogeneous group of disorders whose common characteristic is the progressive degeneration of neuronal structure and function. Although much knowledge has been accumulated on the pathophysiology of neurodegenerative diseases over the years, more efforts are needed to understand the processes that underlie these diseases and hence to propose new treatments. Adrenomedullin (AM) is a multifunctional peptide involved in vasodilation, hormone secretion, antimicrobial defense, cellular growth, and angiogenesis. In neurons, AM and related peptides are associated with some structural and functional cytoskeletal proteins that interfere with microtubule dynamics. Furthermore, AM may intervene in neuronal dysfunction through other mechanisms such as immune and inflammatory response, apoptosis, or calcium dyshomeostasis. Alterations in AM expression have been described in neurodegenerative processes such as Alzheimer's disease or vascular dementia. This review addresses the current state of knowledge on AM and its possible implication in neurodegenerative diseases.

Mechanisms regulating dendritic arbor patterning

The nervous system is populated by diverse types of neurons, each of which has dendritic trees with strikingly different morphologies. These neuron-specific morphologies determine how dendritic trees integrate thousands of synaptic inputs to generate different firing properties. To ensure proper neuronal function and connectivity, it is necessary that dendrite patterns are precisely controlled and coordinated with synaptic activity. Here, we summarize the molecular and cellular mechanisms that regulate the formation of cell type-specific dendrite patterns during development. We focus on different aspects of vertebrate dendrite patterning that are particularly important in determining the neuronal function; such as the shape, branching, orientation and size of the arbors as well as the development of dendritic spine protrusions that receive excitatory inputs and compartmentalize postsynaptic responses. Additionally, we briefly comment on the implications of aberrant dendritic morphology for nervous system disease.

ReMAPping the microtubule landscape: How phosphorylation dictates the activities of microtubule-associated proteins

Classical microtubule-associated proteins (MAPs) were originally identified based on their co-purification with microtubules assembled from mammalian brain lysate. They have since been found to perform a range of functions involved in regulating the dynamics of the microtubule cytoskeleton. Most of these MAPs play integral roles in microtubule organization during neuronal development, microtubule remodeling during neuronal activity, and microtubule stabilization during neuronal maintenance. As a result, mutations in MAPs contribute to neurodevelopmental disorders, psychiatric conditions, and neurodegenerative diseases. MAPs are post-translationally regulated by phosphorylation depending on developmental time point and cellular context. Phosphorylation can affect the microtubule affinity, cellular localization, or overall function of a particular MAP and can thus have profound implications for neuronal health. Here we review MAP1, MAP2, MAP4, MAP6, MAP7, MAP9, tau, and DCX, and how each is regulated by phosphorylation in neuronal physiology and disease. Developmental Dynamics 247:138-155, 2018. © 2017 Wiley Periodicals, Inc.

Structure of the PSD-95/MAP1A complex reveals a unique target recognition mode of the MAGUK GK domain

The PSD-95 family of membrane-associated guanylate kinases (MAGUKs) are major synaptic scaffold proteins and play crucial roles in the dynamic regulation of dendritic remodelling, which is understood to be the foundation of synaptogenesis and synaptic plasticity. The guanylate kinase (GK) domain of MAGUK family proteins functions as a phosphor-peptide binding module. However, the GK domain of PSD-95 has been found to directly bind to a peptide sequence within the C-terminal region of neuronal-specific microtubule-associated protein 1A (MAP1A), although the detailed molecular mechanism governing this phosphorylation-independent interaction at the atomic level is missing. In the present study, we determine the crystal structure of PSD-95 GK in complex with the MAP1A peptide at 2.6-Å resolution. The complex structure reveals that, unlike a linear and elongated conformation in the phosphor-peptide/GK complexes, the MAP1A peptide adopts a unique conformation with a stretch of hydrophobic residues far from each other in the primary sequence clustering and interacting with the ‘hydrophobic site’ of PSD-95 GK and a highly conserved aspartic acid of MAP1A (D2117) mimicking the phosphor-serine/threonine in binding to the ‘phosphor-site’ of PSD-95 GK. We demonstrate that the MAP1A peptide may undergo a conformational transition upon binding to PSD-95 GK. Further structural comparison of known DLG GK-mediated complexes reveals the target recognition specificity and versatility of DLG GKs.

Dynamic Palmitoylation Targets MAP6 to the Axon to Promote Microtubule Stabilization during Neuronal Polarization

Microtubule-associated proteins (MAPs) are main candidates to stabilize neuronal microtubules, playing an important role in establishing axon-dendrite polarity. However, how MAPs are selectively targeted to specific neuronal compartments remains poorly understood. Here, we show specific localization of microtubule-associated protein 6 (MAP6)/stable tubule-only polypeptide (STOP) throughout neuronal maturation and its role in axonal development. In unpolarized neurons, MAP6 is present at the Golgi complex and in secretory vesicles. As neurons mature, MAP6 is translocated to the proximal axon, where it binds and stabilizes microtubules. Further, we demonstrate that dynamic palmitoylation, mediated by the family of α/β Hydrolase domain-containing protein 17 (ABHD17A-C) depalmitoylating enzymes, controls shuttling of MAP6 between membranes and microtubules and is required for MAP6 retention in axons. We propose a model in which MAP6's palmitoylation mediates microtubule stabilization, allows efficient organelle trafficking, and controls axon maturation in vitro and in situ.

Dendritic Actin Cytoskeleton: Structure, Functions, and Regulations

Actin is a versatile and ubiquitous cytoskeletal protein that plays a major role in both the establishment and the maintenance of neuronal polarity. For a long time, the most prominent roles that were attributed to actin in neurons were the movement of growth cones, polarized cargo sorting at the axon initial segment, and the dynamic plasticity of dendritic spines, since those compartments contain large accumulations of actin filaments (F-actin) that can be readily visualized using electron- and fluorescence microscopy. With the development of super-resolution microscopy in the past few years, previously unknown structures of the actin cytoskeleton have been uncovered: a periodic lattice consisting of actin and spectrin seems to pervade not only the whole axon, but also dendrites and even the necks of dendritic spines. Apart from that striking feature, patches of F-actin and deep actin filament bundles have been described along the lengths of neurites. So far, research has been focused on the specific roles of actin in the axon, while it is becoming more and more apparent that in the dendrite, actin is not only confined to dendritic spines, but serves many additional and important functions. In this review, we focus on recent developments regarding the role of actin in dendrite morphology, the regulation of actin dynamics by internal and external factors, and the role of F-actin in dendritic protein trafficking.

Microtubule dynamics and the neurodegenerative triad of Alzheimer's disease: The hidden connection

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is, on a histopathological level, characterized by the presence of extracellular amyloid plaques composed of the protein fragment Aβ, and intracellular neurofibrillary tangles, which contain the microtubule-associated protein tau in a hyperphosphorylated state. In AD defects in microtubule (MT) assembly and organization have also been reported; however, it is unclear whether MT abnormalities have a causal and early role in the disease process or represent a common end point downstream of the neurodegenerative cascade. Recent evidence indicates that microtubule-stabilizing drugs prevent axonopathy in animal models of tauopathies and reverse Aβ-induced loss of synaptic connectivity in an ex vivo model of amyloidosis. This could suggest that MT dysfunction connects some of the degenerative events and provides a useful target to simultaneously prevent several neurodegenerative processes in AD. Here, we describe how changes in the structure and dynamics of MTs are involved in the different aspects of the neurodegenerative triad of AD. We discuss evidence that MTs are affected both by tau-dependent and tau-independent mechanisms but appear to be regulated in a distinct way in different neuronal compartments. We argue that modulation of MT dynamics could be of potential benefit but needs to be precisely controlled in a cell and compartment-specific manner to avoid harmful side effects. This article is part of the series "Beyond Amyloid".

Activity-Dependent Regulation of Distinct Transport and Cytoskeletal Remodeling Functions of the Dendritic Kinesin KIF21B

The dendritic arbor is subject to continual activity-dependent remodeling, requiring a balance between directed cargo trafficking and dynamic restructuring of the underlying microtubule tracks. How cytoskeletal components are able to dynamically regulate these processes to maintain this balance remains largely unknown. By combining single-molecule assays and live imaging in rat hippocampal neurons, we have identified the kinesin-4 KIF21B as a molecular regulator of activity-dependent trafficking and microtubule dynamicity in dendrites. We find that KIF21B contributes to the retrograde trafficking of brain-derived neurotrophic factor (BDNF)-TrkB complexes and also regulates microtubule dynamics through a separable, non-motor microtubule-binding domain. Neuronal activity enhances the motility of KIF21B at the expense of its role in cytoskeletal remodeling, the first example of a kinesin whose function is directly tuned to neuronal activity state. These studies suggest a model in which KIF21B navigates the complex cytoskeletal environment of dendrites by compartmentalizing functions in an activity-dependent manner.

Specific age-related molecular alterations in the cerebellum of Down syndrome mouse models

Down syndrome, or trisomy 21, has been modeled with various trisomic and transgenic mice to help understand the consequences of an altered gene dosage in brain development and function. Though Down syndrome has been associated with premature aging, little is known about the molecular and cellular alterations that target brain function. To help identify alterations at specific ages, we analyzed the cerebellum of Ts1Cje mice, trisomic for 77 HSA21 orthologs, at three ages-young (4 months), middle-age (12 months), and old (17 months)-compared to age-matched controls. Quantification of neuronal and glial markers (n=11) revealed increases in GFAP, with an age effect, and S100B, with age and genotype effects. The genotype effect on S100B with age was unexpected as Ts1Cje has only two copies of the S100b gene. Interestingly, the different increase in GFAP observed between Ts1Cje (trisomic segment includes Pcp4 gene) and controls was magnified in TgPCP4 mice (1 extra copy of the human PCP4 gene) at the same age. S100B increase was not found in the TgPCP4 confirming a difference of regulation with aging for GFAP and S100B and excluding the calcium signaling regulator, Pcp4, as a potential candidate for increase of S100B in the Ts1Cje. To understand these differences, comparison of GFAP and S100B immunostainings at young and middle-age were performed. Immunohistochemical detection of differences in GFAP and S100B localization with aging implicate S100B+ oligodendrocytes as a new phenotypic target in this specific aging process.

Defects in Synaptic Plasticity, Reduced NMDA-Receptor Transport, and Instability of Postsynaptic Density Proteins in Mice Lacking Microtubule-Associated Protein 1A

Microtubule-associated protein 1A (MAP1A) is a member of the major non-motor microtubule-binding proteins. It has been suggested that MAP1A tethers NMDA receptors (NRs) to the cytoskeleton by binding with proteins postsynaptic density (PSD)-93 and PSD-95, although the function of MAP1A in vivo remains elusive. The present study demonstrates that mouse MAP1A plays an essential role in maintaining synaptic plasticity through an analysis of MAP1A knock-out mice. The mice exhibited learning disabilities, which correlated with decreased long-term potentiation and long-term depression in the hippocampal neurons, as well as a concomitant reduction in the extent of NR-dependent EPSCs. Surface expression of NR2A and NR2B subunits also decreased. Enhanced activity-dependent degradation of PSD-93 and reduced transport of NR2A/2B in dendrites was likely responsible for altered receptor function in neurons lacking MAP1A. These data suggest that tethering of NR2A/2B with the cytoskeleton through MAP1A is fundamental for synaptic function.

SIGNIFICANCE STATEMENT

This work is the first report showing the significance of non-motor microtubule-associated protein in maintaining synaptic plasticity thorough a novel mechanism: anchoring of NMDA receptors to cytoskeleton supports transport of NMDA receptors and stabilizes postsynaptic density scaffolds binding to NMDA receptors. Newly generated mutant mice lacking MAP1A exhibited learning disabilities and reduced synaptic plasticity attributable to disruptions of the anchoring machinery.

Posttranslational Modifications Regulate the Postsynaptic Localization of PSD-95

The postsynaptic density (PSD) consists of a lattice-like array of interacting proteins that organizes and stabilizes synaptic receptors, ion channels, structural proteins, and signaling molecules required for normal synaptic transmission and synaptic function. The scaffolding and hub protein postsynaptic density protein-95 (PSD-95) is a major element of central chemical synapses and interacts with glutamate receptors, cell adhesion molecules, and cytoskeletal elements. In fact, PSD-95 can regulate basal synaptic stability as well as the activity-dependent structural plasticity of the PSD and, therefore, of the excitatory chemical synapse. Several studies have shown that PSD-95 is highly enriched at excitatory synapses and have identified multiple protein structural domains and protein-protein interactions that mediate PSD-95 function and trafficking to the postsynaptic region. PSD-95 is also a target of several signaling pathways that induce posttranslational modifications, including palmitoylation, phosphorylation, ubiquitination, nitrosylation, and neddylation; these modifications determine the synaptic stability and function of PSD-95 and thus regulate the fates of individual dendritic spines in the nervous system. In the present work, we review the posttranslational modifications that regulate the synaptic localization of PSD-95 and describe their functional consequences. We also explore the signaling pathways that induce such changes.

Microtubule-binding protein doublecortin-like kinase 1 (DCLK1) guides kinesin-3-mediated cargo transport to dendrites

In neurons, the polarized distribution of vesicles and other cellular materials is established through molecular motors that steer selective transport between axons and dendrites. It is currently unclear whether interactions between kinesin motors and microtubule-binding proteins can steer polarized transport. By screening all 45 kinesin family members, we systematically addressed which kinesin motors can translocate cargo in living cells and drive polarized transport in hippocampal neurons. While the majority of kinesin motors transport cargo selectively into axons, we identified five members of the kinesin-3 (KIF1) and kinesin-4 (KIF21) subfamily that can also target dendrites. We found that microtubule-binding protein doublecortin-like kinase 1 (DCLK1) labels a subset of dendritic microtubules and is required for KIF1-dependent dense-core vesicles (DCVs) trafficking into dendrites and dendrite development. Our study demonstrates that microtubule-binding proteins can provide local signals for specific kinesin motors to drive polarized cargo transport.

Building Blocks of Functioning Brain: Cytoskeletal Dynamics in Neuronal Development

Neural connectivity requires proper polarization of neurons, guidance to appropriate target locations, and establishment of synaptic connections. From when neurons are born to when they finally reach their synaptic partners, neurons undergo constant rearrangment of the cytoskeleton to achieve appropriate shape and polarity. Of particular importance to neuronal guidance to target locations is the growth cone at the tip of the axon. Growth-cone steering is also dictated by the underlying cytoskeleton. All these changes require spatiotemporal control of the cytoskeletal machinery. This review summarizes the proteins that are involved in modulating the actin and microtubule cytoskeleton during the various stages of neuronal development.

What has electron microscopy contributed to Alzheimer's research?

Electron microscopy has enlarged the visual horizons of the morphological alterations in Alzheimer's disease (AD). Study of the mitochondria and Golgi apparatus in early cases of AD revealed the principal role that these important organelles play in the drama of pathogenic dialog of AD, substantially affecting energy production and supply, and protein trafficking in neurons and glia. In addition, study of the morphological alterations of the dendritic arbor, dendritic spines and neuronal synapses, which are associated with mitochondrial damage, may reasonably interpret the clinical phenomena of the irreversible decline of the mental faculties and an individual's personality changes. Electron microscopy also reveals the involvement of microvascular alterations in the etiopathogenic background of AD.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

Mutations in the microtubule-associated protein 1A (Map1a) gene cause Purkinje cell degeneration

The structural microtubule-associated proteins (MAPs) are critical for the organization of neuronal microtubules (MTs). Microtubule-associated protein 1A (MAP1A) is one of the most abundantly expressed MAPs in the mammalian brain. However, its in vivo function remains largely unknown. Here we describe a spontaneous mouse mutation, nm2719, which causes tremors, ataxia, and loss of cerebellar Purkinje neurons in aged homozygous mice. The nm2719 mutation disrupts the Map1a gene. We show that targeted deletion of mouse Map1a gene leads to similar neurodegenerative defects. Before neuron death, Map1a mutant Purkinje cells exhibited abnormal focal swellings of dendritic shafts and disruptions in axon initial segment (AIS) morphology. Furthermore, the MT network was reduced in the somatodendritic and AIS compartments, and both the heavy and light chains of MAP1B, another brain-enriched MAP, was aberrantly distributed in the soma and dendrites of mutant Purkinje cells. MAP1A has been reported to bind to the membrane-associated guanylate kinase (MAGUK) scaffolding proteins, as well as to MTs. Indeed, PSD-93, the MAGUK specifically enriched in Purkinje cells, was reduced in Map1a(-/-) Purkinje cells. These results demonstrate that MAP1A functions to maintain both the neuronal MT network and the level of PSD-93 in neurons of the mammalian brain.

Shotgun proteomics reveals possible mechanisms for cognitive impairment in Mucopolysaccharidosis I mice

Mucopolysaccharidosis type I (MPS I) is due to deficient alpha-L-iduronidase (IDUA) which leads to storage of undegraded glycosaminoglycans (GAG). The severe form of the disease is characterized by mental retardation of unknown etiology. Trying to unveil the mechanisms that lead to cognitive impairment in MPS I, we studied alterations in the proteome from MPS I mouse hippocampus. Eight-month old mice presented increased LAMP-1 expression, GAG storage in neurons and glial cells, and impaired aversive and non-aversive memory. Shotgun proteomics was performed and 297 proteins were identified. Of those, 32 were differentially expressed. We found elevation in proteins such as cathepsins B and D; however their increase did not lead to cell death in MPS I brains. Glial fibrillary acid protein (GFAP) was markedly elevated, and immunohistochemistry confirmed a neuroinflammatory process that could be responsible for neuronal dysfunction. We didn't observe any differences in ubiquitin expression, as well as in other proteins related to protein folding, suggesting that the ubiquitin system is working properly. Finally, we observed alterations in several proteins involved in synaptic plasticity, including overexpression of post synaptic density-95 (PSD95) and reduction of microtubule-associated proteins 1A and 1B. These results together suggest that the cognitive impairment in MPS I mice is not due to massive cell death, but rather to neuronal dysfunction caused by multiple processes, including neuroinflammation and alterations in synaptic plasticity.

Adolescent nicotine-induced dendrite remodeling in the nucleus accumbens is rapid, persistent, and D1-dopamine receptor dependent

Chronic nicotine exposure during adolescence induces dendritic remodeling of medium spiny neurons (MSNs) in the nucleus accumbens (NAcc) shell. While nicotine-induced dendritic remodeling has frequently been described as persistent, the trajectory of dendrite remodeling is unknown. Specifically, no study to date has characterized the structural plasticity of dendrites in the NAcc immediately following chronic nicotine, leaving open the possibility that dendrite remodeling emerges gradually over time. Further, the neuropharmacological mechanisms through which nicotine induces dendrite remodeling are not well understood. To address these questions, rats were co-administered chronic nicotine (0.5 mg/kg) and the D1-dopamine receptor (D1DR) antagonist SCH-23390 (0.05 mg/kg) subcutaneously every other day during adolescence. Brains were then processed for Golgi-Cox staining either 1 day or 21 days following drug exposure and dendrites from MSNs in the NAcc shell digitally reconstructed in 3D. Spine density was also measured at both time points. Our morphometric results show (1) the formation of new dendritic branches and spines 1 day following nicotine exposure, (2) new dendritic branches, but not spine density, remains relatively stable for at least 21 days, (3) the co-administration of SCH-23390 completely blocked nicotine-induced dendritic remodeling of MSNs at both early and late time points, suggesting the formation of new dendritic branches in response to nicotine is D1DR-dependent, and (4) SCH-23390 failed to block nicotine-induced increases in spine density. Overall this study provides new insight into how nicotine influences the normal trajectory of adolescent brain development and demonstrates a persistent form of nicotine-induced neuroplasticity in the NAcc shell that develops rapidly and is D1DR dependent.

Molecular mechanisms of dendrite stability

In the developing brain, dendrite branches and dendritic spines form and turn over dynamically. By contrast, most dendrite arbors and dendritic spines in the adult brain are stable for months, years and possibly even decades. Emerging evidence reveals that dendritic spine and dendrite arbor stability have crucial roles in the correct functioning of the adult brain and that loss of stability is associated with psychiatric disorders and neurodegenerative diseases. Recent findings have provided insights into the molecular mechanisms that underlie long-term dendrite stabilization, how these mechanisms differ from those used to mediate structural plasticity and how they are disrupted in disease.

Regulation of spine density and morphology by IQGAP1 protein domains

IQGAP1 is a scaffolding protein that regulates spine number. We now show a differential role for IQGAP1 domains in spine morphogenesis, in which a region of the N-terminus that promotes Arp2/3-mediated actin polymerization and branching stimulates spine head formation while a region that binds to Cdc42 and Rac is required for stalk extension. Conversely, IQGAP1 rescues spine deficiency induced by expression of dominant negative Cdc42 by stimulating formation of stubby spines. Together, our observations place IQGAP1 as a crucial regulator of spine number and shape acting through the N-Wasp Arp2/3 complex, as well as upstream and downstream of Cdc42.

Plastic changes in the astrocyte GLUT1 glucose transporter and beta-tubulin microtubule protein following voluntary exercise in mice

Glucose, the predominant energy substrate of the central and peripheral nervous system, is delivered to neurons via a family of facilitative glucose transporters (GLUT). The majority of glucose is transported to the brain via glucose transporter 1 (GLUT1) located on epithelial cells of capillaries and on the astrocytes that wrap around them. Changes in neuronal activity are linked to increases in glucose demand and local cerebral glucose utilization. Current research has indicated a corresponding change in GLUT1 expression in response to increased metabolic demand in operant tasks. The purpose of this study was to examine, in the mouse brain, the effects of neuronal activation induced by voluntary running on the plastic expression of vascular GLUT1 and neuronal plasticity as measured by the microtubule protein beta-tubulin III (Tuj). The results showed that access to a running wheel for 48h induced plastic changes in the expression of GLUT1, Tuj and GLUT1-associated estimate of astrocyte vascular endfeet in motor regions. The results tend to support the plastic association between mechanisms of energy supply and plastic reorganization of neurons following a new training experience.

The role of small GTPases in neuronal morphogenesis and polarity

The highly dynamic remodeling and cross talk of the microtubule and actin cytoskeleton support neuronal morphogenesis. Small RhoGTPases family members have emerged as crucial regulators of cytoskeletal dynamics. In this review we will comprehensively analyze findings that support the participation of RhoA, Rac, Cdc42, and TC10 in different neuronal morphogenetic events ranging from migration to synaptic plasticity. We will specifically address the contribution of these GTPases to support neuronal polarity and axonal elongation.

Inherited balanced translocation t(9;17)(q33.2;q25.3) concomitant with a 16p13.1 duplication in a patient with schizophrenia

We report two rare genetic aberrations in a schizophrenia patient that may act together to confer disease susceptibility. A previously unreported balanced t(9;17)(q33.2;q25.3) translocation was observed in two schizophrenia-affected members of a small family with diverse psychiatric disorders. The proband also carried a 1.5 Mbp microduplication at 16p13.1 that could not be investigated in other family members. The duplication has been reported to predispose to schizophrenia, autism and mental retardation, with incomplete penetrance and variable expressivity. The t(9;17) (q33.2;q25.3) translocation breakpoint occurs within the open reading frames of KIAA1618 on 17q25.3, and TTLL11 (tyrosine tubulin ligase like 11) on 9q33.2, causing no change in the expression level of KIAA1618 but leading to loss of expression of one TTLL11 allele. TTLL11 belongs to a family of enzymes catalyzing polyglutamylation, an unusual neuron-specific post-translational modification of microtubule proteins, which modulates microtubule development and dynamics. The 16p13.1 duplication resulted in increased expression of NDE1, encoding a DISC1 protein partner mediating DISC1 functions in microtubule dynamics. We hypothesize that concomitant TTLL11-NDE1 deregulation may increase mutation load, among others, also on the DISC1 pathway, which could contribute to disease pathogenesis through multiple effects on neuronal development, synaptic plasticity, and neurotransmission. Our data illustrate the difficulties in interpreting the contribution of multiple potentially pathogenic changes likely to emerge in future next-generation sequencing studies, where access to extended families will be increasingly important.

Local synaptic signaling enhances the stochastic transport of motor-driven cargo in neurons

The tug-of-war model of motor-driven cargo transport is formulated as an intermittent trapping process. An immobile trap, representing the cellular machinery that sequesters a motor-driven cargo for eventual use, is located somewhere within a microtubule track. A particle representing a motor-driven cargo that moves randomly with a forward bias is introduced at the beginning of the track. The particle switches randomly between a fast moving phase and a slow moving phase. When in the slow moving phase, the particle can be captured by the trap. To account for the possibility that the particle avoids the trap, an absorbing boundary is placed at the end of the track. Two local signaling mechanisms--intended to improve the chances of capturing the target--are considered by allowing the trap to affect the tug-of-war parameters within a small region around itself. The first is based on a localized adenosine triphosphate (ATP) concentration gradient surrounding a synapse, and the second is based on a concentration of tau--a microtubule-associated protein involved in Alzheimer's disease--coating the microtubule near the synapse. It is shown that both mechanisms can lead to dramatic improvements in the capture probability, with a minimal increase in the mean capture time. The analysis also shows that tau can cause a cargo to undergo random oscillations, which could explain some experimental observations.

The von Economo neurons in frontoinsular and anterior cingulate cortex in great apes and humans

The von Economo neurons (VENs) are large bipolar neurons located in frontoinsular (FI) and anterior cingulate cortex in great apes and humans, but not other primates. We performed stereological counts of the VENs in FI and LA (limbic anterior, a component of anterior cingulate cortex) in great apes and in humans. The VENs are more numerous in humans than in apes, although one gorilla approached the lower end of the human range. We also examined the ontological development of the VENs in FI and LA in humans. The VENs first appear in small numbers in the 36th week post-conception, are rare at birth, and increase in number during the first 8 months after birth. There are significantly more VENs in the right hemisphere than in the left in FI and LA in postnatal brains of apes and humans. This asymmetry in VEN numbers may be related to asymmetries in the autonomic nervous system. The activity of the inferior anterior insula, which contains FI, is related to physiological changes in the body, decision-making, error recognition, and awareness. The VENs appear to be projection neurons, although their targets are unknown. We made a preliminary study of the connections of FI cortex based on diffusion tensor imaging in the brain of a gorilla. The VEN-containing regions connect to the frontal pole as well as to other parts of frontal and insular cortex, the septum, and the amygdala. It is likely that the VENs in FI are projecting to some or all of these structures and relaying information related to autonomic control, decision-making, or awareness. The VENs selectively express the bombesin peptides neuromedin B (NMB) and gastrin releasing peptide (GRP) which are also expressed in another population of closely related neurons, the fork cells. NMB and GRP signal satiety. The genes for NMB and GRP are expressed selectively in small populations of neurons in the insular cortex in mice. These populations may be related to the VEN and fork cells and may be involved in the regulation of appetite. The loss of these cells may be related to the loss of satiety signaling in patients with frontotemporal dementia who have damage to FI. The VENs and fork cells may be morphological specializations of an ancient population of neurons involved in the control of appetite present in the insular cortex in all mammals. We found that the protein encoded by the gene DISC1 (disrupted in schizophrenia) is preferentially expressed by the VENs. DISC1 has undergone rapid evolutionary change in the line leading to humans, and since it suppresses dendritic branching it may be involved in the distinctive VEN morphology.

Unique post-translational modifications in specialized microtubule architecture

Microtubules (MTs) play specialized roles in a wide variety of cellular events, e.g. molecular transport, cell motility, and cell division. Specialized MT architectures, such as bundles, axonemes, and centrioles, underlie the function. The specialized function and highly organized structure depend on interactions with MT-binding proteins. MT-associated proteins (e.g. MAP1, MAP2, and tau), molecular motors (kinesin and dynein), plus-end tracking proteins (e.g. CLIP-170), and MT-severing proteins (e.g. katanin) interact with MTs. How can the MT-binding proteins know temporospatial information to associate with MTs and to properly play their roles? Post-translational modifications (PTMs) including detyrosination, polyglutamylation, and polyglycylation can provide molecular landmarks for the proteins. Recent efforts to identify modification-regulating enzymes (TTL, carboxypeptidase, polyglutamylase, polyglycylase) and to generate genetically manipulated animals enable us to understand the roles of the modifications. In this review, we present recent advances in understanding regulation of MT function, structure, and stability by PTMs.

Axonal mRNAs: characterisation and role in the growth and regeneration of dorsal root ganglion axons and growth cones

We have developed a compartmentalised culture model for the purification of axonal mRNA from embryonic, neonatal and adult rat dorsal root ganglia. This mRNA was used un-amplified for RT-qPCR. We assayed for the presence of axonal mRNAs encoding molecules known to be involved in axon growth and guidance. mRNAs for beta-actin, beta-tubulin, and several molecules involved in the control of actin dynamics and signalling during axon growth were found, but mRNAs for microtubule-associated proteins, integrins and cell surface adhesion molecules were absent. Quantification of beta-actin mRNA by means of qPCR showed that the transcript is present at the same level in embryonic, newborn and adult axons. Using the photoconvertible reporter Kaede we showed that there is local translation of beta-actin in axons, the rate being increased by axotomy. Knock down of beta-actin mRNA by RNAi inhibited the regeneration of new axon growth cones after in vitro axotomy, indicating that local translation of actin-related molecules is important for successful axon regeneration.