Presynaptic perspective: Axonal transport defects in neurodevelopmental disorders

Disruption of synapse assembly and maturation leads to a broad spectrum of neurodevelopmental disorders. Presynaptic proteins are largely synthesized in the soma, where they are packaged into precursor vesicles and transported into distal axons to ensure precise assembly and maintenance of presynapses. Due to their morphological features, neurons face challenges in the delivery of presynaptic cargos to nascent boutons. Thus, targeted axonal transport is vital to build functional synapses. A growing number of mutations in genes encoding the transport machinery have been linked to neurodevelopmental disorders. Emerging lines of evidence have started to uncover presynaptic mechanisms underlying axonal transport defects, thus broadening the view of neurodevelopmental disorders beyond postsynaptic mechanisms. In this review, we discuss presynaptic perspectives of neurodevelopmental disorders by focusing on impaired axonal transport and disturbed assembly and maintenance of presynapses. We also discuss potential strategies for restoring axonal transport as an early therapeutic intervention.

Understanding the neuronal synapse and challenges associated with the mitochondrial dysfunction in mild cognitive impairment and Alzheimer's disease

Synaptic mitochondria are crucial for maintaining synaptic activity due to their high energy requirements, substantial calcium (Ca2+) fluctuation, and neurotransmitter release at the synapse. To provide a continuous energy supply, neurons use special mechanisms to transport and distribute healthy mitochondria to the synapse while eliminating the damaged mitochondria from the synapse. Along the neuron, mitochondrial membrane potential (ψ) gradient exists and is highest in the somal region. Lower ψ in the synaptic region renders mitochondria more vulnerable to oxidative stress-mediated damage. Secondly, mitochondria become susceptible to the release of cytochrome c, and mitochondrial DNA (mtDNA) is not shielded from the reactive oxygen species (ROS) by the histone proteins (unlike nuclear DNA), leading to activation of caspases and pronounced oxidative DNA base damage, which ultimately causes synaptic loss. Both synaptic mitochondrial dysfunction and synaptic failure are crucial factors responsible for Alzheimer's disease (AD). Furthermore, amyloid beta (Aβ) and hyper-phosphorylated Tau, the two leading players of AD, exaggerate the disease-like pathological conditions by reducing the mitochondrial trafficking, blocking the bi-directional transport at the synapse, enhancing the mitochondrial fission via activating the mitochondrial fission proteins, enhancing the swelling of mitochondria by increasing the influx of water through mitochondrial permeability transition pore (mPTP) opening, as well as reduced ATP production by blocking the activity of complex I and complex IV. Mild cognitive impairment (MCI) is also associated with decline in cognitive ability caused by synaptic degradation. This review summarizes the challenges associated with the synaptic mitochondrial dysfunction linked to AD and MCI and the role of phytochemicals in restoring the synaptic activity and rendering neuroprotection in AD.

Exploring New Mechanism of Depression from the Effects of Virus on Nerve Cells

Depression is a common neuropsychiatric disorder with long-term recurrent depressed mood, pain and despair, pessimism and anxiety, and even suicidal tendencies as the main symptoms. Depression usually induces or aggravates the development of other related diseases, such as sleep disorders and endocrine disorders. In today's society, the incidence of depression is increasing worldwide, and its pathogenesis is complex and generally believed to be related to genetic, psychological, environmental, and biological factors. Current studies have shown the key role of glial cells in the development of depression, and it is noteworthy that some recent evidence suggests that the development of depression may be closely related to viral infections, such as SARS-CoV-2, BoDV-1, ZIKV, HIV, and HHV6, which infect the organism and cause some degree of glial cells, such as astrocytes, oligodendrocytes, and microglia. This can affect the transmission of related proteins, neurotransmitters, and cytokines, which in turn leads to neuroinflammation and depression. Based on the close relationship between viruses and depression, this paper provides an in-depth analysis of the new mechanism of virus-induced depression, which is expected to provide a new perspective on the mechanism of depression and a new idea for the diagnosis of depression in the future.

Glycogen synthase kinase 3β (GSK3β) and presenilin (PS) are key regulators of kinesin-1-mediated cargo motility within axons

It has been a quarter century since the discovery that molecular motors are phosphorylated, but fundamental questions still remain as to how specific kinases contribute to particular motor functions, particularly in vivo, and to what extent these processes have been evolutionarily conserved. Such questions remain largely unanswered because there is no cohesive strategy to unravel the likely complex spatial and temporal mechanisms that control motility in vivo. Since diverse cargoes are transported simultaneously within cells and along narrow long neurons to maintain intracellular processes and cell viability, and disruptions in these processes can lead to cancer and neurodegeneration, there is a critical need to better understand how kinases regulate molecular motors. Here, we review our current understanding of how phosphorylation can control kinesin-1 motility and provide evidence for a novel regulatory mechanism that is governed by a specific kinase, glycogen synthase kinase 3β (GSK3β), and a scaffolding protein presenilin (PS).

HTT (huntingtin) and RAB7 co-migrate retrogradely on a signaling LAMP1-containing late endosome during axonal injury

ABBREVIATIONS

Atg5: Autophagy-related 5; Atg8a: Autophagy-related 8a; AL: autolysosome; AP: autophagosome; BAF1: bafilomycin A1; BDNF: brain derived neurotrophic factor; BMP: bone morphogenetic protein; Cyt-c-p: Cytochrome c proximal; CQ: chloroquine; DCTN1: dynactin 1; Dhc: dynein heavy chain; EE: early endosome; DYNC1I1: dynein cytoplasmic 1 intermediate chain 1; HD: Huntington disease; HIP1/Hip1: huntingtin interacting protein 1; HTT/htt: huntingtin; iNeuron: iPSC-derived human neurons; IP: immunoprecipitation; Khc: kinesin heavy chain; KIF5C: kinesin family member 5C; LAMP1/Lamp1: lysosomal associated membrane protein 1; LE: late endosome; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAP3K12/DLK: mitogen-activated protein kinase kinase kinase 12; MAPK8/JNK/bsk: mitogen-activated protein kinase 8/basket; MAPK8IP3/JIP3: mitogen-activated protein kinase 8 interacting protein 3; NGF: nerve growth factor; NMJ: neuromuscular junction; NTRK1/TRKA: neurotrophic receptor tyrosine kinase 1; NRTK2/TRKB: neurotrophic receptor tyrosine kinase 2; nuf: nuclear fallout; PG: phagophore; PtdIns3P: phosphatidylinositol-3-phosphate; puc: puckered; ref(2)P: refractory to sigma P; Rilpl: Rab interacting lysosomal protein like; Rip11: Rab11 interacting protein; RTN1: reticulon 1; syd: sunday driver; SYP: synaptophysin; SYT1/Syt1: synaptotagmin 1; STX17/Syx17: syntaxin 17; tkv: thickveins; VF: vesicle fraction; wit: wishful thinking; wnd: wallenda.

Axonal transport deficits in the pathogenesis of diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is a chronic and prevalent metabolic disease that gravely endangers human health and seriously affects the quality of life of hyperglycemic patients. More seriously, it can lead to amputation and neuropathic pain, imposing a severe financial burden on patients and the healthcare system. Even with strict glycemic control or pancreas transplantation, peripheral nerve damage is difficult to reverse. Most current treatment options for DPN can only treat the symptoms but not the underlying mechanism. Patients with long-term diabetes mellitus (DM) develop axonal transport dysfunction, which could be an important factor in causing or exacerbating DPN. This review explores the underlying mechanisms that may be related to axonal transport impairment and cytoskeletal changes caused by DM, and the relevance of the latter with the occurrence and progression of DPN, including nerve fiber loss, diminished nerve conduction velocity, and impaired nerve regeneration, and also predicts possible therapeutic strategies. Understanding the mechanisms of diabetic neuronal injury is essential to prevent the deterioration of DPN and to develop new therapeutic strategies. Timely and effective improvement of axonal transport impairment is particularly critical for the treatment of peripheral neuropathies.

Inhibition of glycogen synthase kinase 3 by lithium, a mechanism in search of specificity

Inhibition of Glycogen synthase kinase 3 (GSK3) is a popular explanation for the effects of lithium ions on mood regulation in bipolar disorder and other mental illnesses, including major depression, cyclothymia, and schizophrenia. Contribution of GSK3 is supported by evidence obtained from animal and patient derived model systems. However, the two GSK3 enzymes, GSK3α and GSK3β, have more than 100 validated substrates. They are thus central hubs for major biological functions, such as dopamine-glutamate neurotransmission, synaptic plasticity (Hebbian and homeostatic), inflammation, circadian regulation, protein synthesis, metabolism, inflammation, and mitochondrial functions. The intricate contributions of GSK3 to several biological processes make it difficult to identify specific mechanisms of mood stabilization for therapeutic development. Identification of GSK3 substrates involved in lithium therapeutic action is thus critical. We provide an overview of GSK3 biological functions and substrates for which there is evidence for a contribution to lithium effects. A particular focus is given to four of these: the transcription factor cAMP response element-binding protein (CREB), the RNA-binding protein FXR1, kinesin subunits, and the cytoskeletal regulator CRMP2. An overview of how co-regulation of these substrates may result in shared outcomes is also presented. Better understanding of how inhibition of GSK3 contributes to the therapeutic effects of lithium should allow for identification of more specific targets for future drug development. It may also provide a framework for the understanding of how lithium effects overlap with those of other drugs such as ketamine and antipsychotics, which also inhibit brain GSK3.

The ubiquitous microtubule-associated protein 4 (MAP4) controls organelle distribution by regulating the activity of the kinesin motor

Regulation of organelle transport by molecular motors along the cytoskeletal microtubules is central to maintaining cellular functions. Here, we show that the ubiquitous tau-related microtubule-associated protein 4 (MAP4) can bias the bidirectional transport of organelles toward the microtubule minus-ends. This is concurrent with MAP4 phosphorylation, mediated by the kinase GSK3β. We demonstrate that MAP4 achieves this bias by tethering the cargo to the microtubules, allowing it to impair the force generation of the plus-end motor kinesin-1. Consistent with this mechanism, MAP4 physically interacts with dynein and dynactin and, when phosphorylated, associates with the cargo-motor complex through its projection domain. Its phosphorylation coincides with the perinuclear accumulation of organelles, a phenotype that is rescued by abolishing the cargo-microtubule MAP4 tether or by the pharmacological inhibition of dynein, confirming the ability of kinesin to inch along, albeit inefficiently, in the presence of phosphorylated MAP4. These findings have broad biological significance because of the ubiquity of MAP4 and the involvement of GSK3β in multiple diseases, more specifically in cancer, where the MAP4-dependent redistribution of organelles may be prevalent in cancer cells, as we demonstrate here for mitochondria in lung carcinoma epithelial cells.

A stop or go switch: glycogen synthase kinase 3β phosphorylation of the kinesin 1 motor domain at Ser314 halts motility without detaching from microtubules

It is more than 25 years since the discovery that kinesin 1 is phosphorylated by several protein kinases. However, fundamental questions still remain as to how specific protein kinase(s) contribute to particular motor functions under physiological conditions. Because, within an whole organism, kinase cascades display considerable crosstalk and play multiple roles in cell homeostasis, deciphering which kinase(s) is/are involved in a particular process has been challenging. Previously, we found that GSK3β plays a role in motor function. Here, we report that a particular site on kinesin 1 motor domain (KHC), S314, is phosphorylated by GSK3β in vivo. The GSK3β-phosphomimetic-KHCS314D stalled kinesin 1 motility without dissociating from microtubules, indicating that constitutive GSK3β phosphorylation of the motor domain acts as a STOP. In contrast, uncoordinated mitochondrial motility was observed in CRISPR/Cas9-GSK3β non-phosphorylatable-KHCS314A Drosophila larval axons, owing to decreased kinesin 1 attachment to microtubules and/or membranes, and reduced ATPase activity. Together, we propose that GSK3β phosphorylation fine-tunes kinesin 1 movement in vivo via differential phosphorylation, unraveling the complex in vivo regulatory mechanisms that exist during axonal motility of cargos attached to multiple kinesin 1 and dynein motors.

Differential mitochondrial roles for α-synuclein in DRP1-dependent fission and PINK1/Parkin-mediated oxidation

Mitochondria are highly dynamic organelles with strict quality control processes that maintain cellular homeostasis. Within axons, coordinated cycles of fission-fusion mediated by dynamin related GTPase protein (DRP1) and mitofusins (MFN), together with regulated motility of healthy mitochondria anterogradely and damaged/oxidized mitochondria retrogradely, control mitochondrial shape, distribution and size. Disruption of this tight regulation has been linked to aberrant oxidative stress and mitochondrial dysfunction causing mitochondrial disease and neurodegeneration. Although pharmacological induction of Parkinson's disease (PD) in humans/animals with toxins or in mice overexpressing α-synuclein (α-syn) exhibited mitochondrial dysfunction and oxidative stress, mice lacking α-syn showed resistance to mitochondrial toxins; yet, how α-syn influences mitochondrial dynamics and turnover is unclear. Here, we isolate the mechanistic role of α-syn in mitochondrial homeostasis in vivo in a humanized Drosophila model of Parkinson's disease (PD). We show that excess α-syn causes fragmented mitochondria, which persists with either truncation of the C-terminus (α-syn^1-120) or deletion of the NAC region (α-syn^ΔNAC). Using in vivo oxidation reporters Mito-roGFP2-ORP1/GRX1 and MitoTimer, we found that α-syn-mediated fragments were oxidized/damaged, but α-syn^1-120-induced fragments were healthy, suggesting that the C-terminus is required for oxidation. α-syn-mediated oxidized fragments showed biased retrograde motility, but α-syn^1-120-mediated healthy fragments did not, demonstrating that the C-terminus likely mediates the retrograde motility of oxidized mitochondria. Depletion/inhibition or excess DRP1-rescued α-syn-mediated fragmentation, oxidation, and the biased retrograde motility, indicating that DRP1-mediated fragmentation is likely upstream of oxidation and motility changes. Further, excess PINK/Parkin, two PD-associated proteins that function to coordinate mitochondrial turnover via induction of selective mitophagy, rescued α-syn-mediated membrane depolarization, oxidation and cell death in a C-terminus-dependent manner, suggesting a functional interaction between α-syn and PINK/Parkin. Taken together, our findings identify distinct roles for α-syn in mitochondrial homeostasis, highlighting a previously unknown pathogenic pathway for the initiation of PD.

Glycogen Synthase Kinase 3 (GSK3): Its Role and Inhibitors

Glycogen Synthase Kinase 3 (GSK3) is one of the Serine/Threonine protein kinases, which has gained a lot of attention for its role in a variety of pathways. It has two isoforms, GSK3α and GSK3β. However, GSK3β is highly expressed in different areas of the brain and has been implicated in Alzheimer's disease as it is involved in tau phosphorylation. Due to its high specificity concerning substrate recognition, GSK3 has been considered as an important target. In the last decade, several GSK3 inhibitors have been reported and two molecules are in clinical trials. This review collates the information published in the last decade about the role of GSK3 in Alzheimer's disease and progress in the development of its inhibitors. Using this collated information, medicinal chemists can strategize and design novel GSK3 inhibitors that could be useful in the treatment of Alzheimer's disease.

Defective Mitochondrial Pyruvate Flux Affects Cell Bioenergetics in Alzheimer's Disease-Related Models

Mitochondria are key organelles for brain health. Mitochondrial alterations have been reported in several neurodegenerative disorders, including Alzheimer's disease (AD), and the comprehension of the underlying mechanisms appears crucial to understand their relationship with the pathology. Using multiple genetic, pharmacological, imaging, and biochemical approaches, we demonstrate that, in different familial AD cell models, mitochondrial ATP synthesis is affected. The defect depends on reduced mitochondrial pyruvate oxidation, due to both lower Ca²⁺-mediated stimulation of the Krebs cycle and dampened mitochondrial pyruvate uptake. Importantly, this latter event is linked to glycogen-synthase-kinase-3β (GSK-3β) hyper-activation, leading, in turn, to impaired recruitment of hexokinase 1 (HK1) to mitochondria, destabilization of mitochondrial-pyruvate-carrier (MPC) complexes, and decreased MPC2 protein levels. Remarkably, pharmacological GSK-3β inhibition in AD cells rescues MPC2 expression and improves mitochondrial ATP synthesis and respiration. The defective mitochondrial bioenergetics influences glutamate-induced neuronal excitotoxicity, thus representing a possible target for future therapeutic interventions.

Mechanisms and Therapeutic Implications of GSK-3 in Treating Neurodegeneration

Neurodegenerative disorders are spreading worldwide and are one of the greatest threats to public health. There is currently no adequate therapy for these disorders, and therefore there is an urgent need to accelerate the discovery and development of effective treatments. Although neurodegenerative disorders are broad ranging and highly complex, they may share overlapping mechanisms, and thus potentially manifest common targets for therapeutic interventions. Glycogen synthase kinase-3 (GSK-3) is now acknowledged to be a central player in regulating mood behavior, cognitive functions, and neuron viability. Indeed, many targets controlled by GSK-3 are critically involved in progressing neuron deterioration and disease pathogenesis. In this review, we focus on three pathways that represent prominent mechanisms linking GSK-3 with neurodegenerative disorders: cytoskeleton organization, the mammalian target of rapamycin (mTOR)/autophagy axis, and mitochondria. We also consider the challenges and opportunities in the development of GSK-3 inhibitors for treating neurodegeneration.

Recent Advances on the Role of GSK3β in the Pathogenesis of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a common neurodegenerative disease characterized by progressive motor neuron degeneration. Although several studies on genes involved in ALS have substantially expanded and improved our understanding of ALS pathogenesis, the exact molecular mechanisms underlying this disease remain poorly understood. Glycogen synthase kinase 3 (GSK3) is a multifunctional serine/threonine-protein kinase that plays a critical role in the regulation of various cellular signaling pathways. Dysregulation of GSK3β activity in neuronal cells has been implicated in the pathogenesis of neurodegenerative diseases. Previous research indicates that GSK3β inactivation plays a neuroprotective role in ALS pathogenesis. GSK3β activity shows an increase in various ALS models and patients. Furthermore, GSK3β inhibition can suppress the defective phenotypes caused by SOD, TDP-43, and FUS expression in various models. This review focuses on the most recent studies related to the therapeutic effect of GSK3β in ALS and provides an overview of how the dysfunction of GSK3β activity contributes to ALS pathogenesis.

Presenilin-2 and Calcium Handling: Molecules, Organelles, Cells and Brain Networks

Presenilin-2 (PS2) is one of the three proteins that are dominantly mutated in familial Alzheimer's disease (FAD). It forms the catalytic core of the γ-secretase complex-a function shared with its homolog presenilin-1 (PS1)-the enzyme ultimately responsible of amyloid-β (Aβ) formation. Besides its enzymatic activity, PS2 is a multifunctional protein, being specifically involved, independently of γ-secretase activity, in the modulation of several cellular processes, such as Ca2+ signalling, mitochondrial function, inter-organelle communication, and autophagy. As for the former, evidence has accumulated that supports the involvement of PS2 at different levels, ranging from organelle Ca2+ handling to Ca2+ entry through plasma membrane channels. Thus FAD-linked PS2 mutations impact on multiple aspects of cell and tissue physiology, including bioenergetics and brain network excitability. In this contribution, we summarize the main findings on PS2, primarily as a modulator of Ca2+ homeostasis, with particular emphasis on the role of its mutations in the pathogenesis of FAD. Identification of cell pathways and molecules that are specifically targeted by PS2 mutants, as well as of common targets shared with PS1 mutants, will be fundamental to disentangle the complexity of memory loss and brain degeneration that occurs in Alzheimer's disease (AD).

Excess Rab4 rescues synaptic and behavioral dysfunction caused by defective HTT-Rab4 axonal transport in Huntington's disease

Huntington's disease (HD) is characterized by protein inclusions and loss of striatal neurons which result from expanded CAG repeats in the poly-glutamine (polyQ) region of the huntingtin (HTT) gene. Both polyQ expansion and loss of HTT have been shown to cause axonal transport defects. While studies show that HTT is important for vesicular transport within axons, the cargo that HTT transports to/from synapses remain elusive. Here, we show that HTT is present with a class of Rab4-containing vesicles within axons in vivo. Reduction of HTT perturbs the bi-directional motility of Rab4, causing axonal and synaptic accumulations. In-vivo dual-color imaging reveal that HTT and Rab4 move together on a unique putative vesicle that may also contain synaptotagmin, synaptobrevin, and Rab11. The moving HTT-Rab4 vesicle uses kinesin-1 and dynein motors for its bi-directional movement within axons, as well as the accessory protein HIP1 (HTT-interacting protein 1). Pathogenic HTT disrupts the motility of HTT-Rab4 and results in larval locomotion defects, aberrant synaptic morphology, and decreased lifespan, which are rescued by excess Rab4. Consistent with these observations, Rab4 motility is perturbed in iNeurons derived from human Huntington's Disease (HD) patients, likely due to disrupted associations between the polyQ-HTT-Rab4 vesicle complex, accessory proteins, and molecular motors. Together, our observations suggest the existence of a putative moving HTT-Rab4 vesicle, and that the axonal motility of this vesicle is disrupted in HD causing synaptic and behavioral dysfunction. These data highlight Rab4 as a potential novel therapeutic target that could be explored for early intervention prior to neuronal loss and behavioral defects observed in HD.

Targeting Aggrephagy for the Treatment of Alzheimer's Disease

Alzheimer’s disease (AD) is one of the most common neurodegenerative diseases in older individuals with specific neuropsychiatric symptoms. It is a proteinopathy, pathologically characterized by the presence of misfolded protein (Aβ and Tau) aggregates in the brain, causing progressive dementia. Increasing studies have provided evidence that the defect in protein-degrading systems, especially the autophagy-lysosome pathway (ALP), plays an important role in the pathogenesis of AD. Recent studies have demonstrated that AD-associated protein aggregates can be selectively recognized by some receptors and then be degraded by ALP, a process termed aggrephagy. In this study, we reviewed the role of aggrephagy in AD development and discussed the strategy of promoting aggrephagy using small molecules for the treatment of AD.

The Non-amyloidal Component Region of α-Synuclein Is Important for α-Synuclein Transport Within Axons

Proper transport of the Parkinson's disease (PD) protein, α-synuclein (α-syn), is thought to be crucial for its localization and function at the synapse. Previous work has shown that defects in long distance transport within narrow caliber axons occur early in PD, but how such defects contribute to PD is unknown. Here we test the hypothesis that the NAC region is involved in facilitating proper transport of α-syn within axons via its association with membranes. Excess α-syn or fPD mutant α-synA53T accumulates within larval axons perturbing the transport of synaptic proteins. These α-syn expressing larvae also show synaptic morphological and larval locomotion defects, which correlate with the extent of α-syn-mediated axonal accumulations. Strikingly, deletion of the NAC region (α-synΔ71-82) prevented α-syn accumulations and axonal blockages, and reduced its synaptic localization due to decreased axonal entry and axonal transport of α-syn, due to less α-syn bound to membranes. Intriguingly, co-expression α-synΔ71-82 with full-length α-syn rescued α-syn accumulations and synaptic morphological defects, and decreased the ratio of the insoluble higher molecular weight (HMW)/soluble low molecular weight (LMW) α-syn, indicating that this region is perhaps important for the dimerization of α-syn on membranes. Together, our observations suggest that under physiological conditions, α-syn associates with membranes via the NAC region, and that too much α-syn perturbs axonal transport via aggregate formation, instigating synaptic and behavioral defects seen in PD.

The presenilin loop region is essential for glycogen synthase kinase 3 β (GSK3β) mediated functions on motor proteins during axonal transport

Neurons require intracellular transport of essential components for function and viability and defects in transport has been implicated in many neurodegenerative diseases including Alzheimer's disease (AD). One possible mechanism by which transport defects could occur is by improper regulation of molecular motors. Previous work showed that reduction of presenilin (PS) or glycogen synthase kinase 3 beta (GSK3β) stimulated amyloid precursor protein vesicle motility. Excess GSK3β caused transport defects and increased motor binding to membranes, while reduction of PS decreased active GSK3β and motor binding to membranes. Here, we report that functional PS and the catalytic loop region of PS is essential for the rescue of GSK3β-mediated axonal transport defects. Disruption of PS loop (PSΔE9) or expression of the non-functional PS variant, PSD447A, failed to rescue axonal blockages in vivo. Further, active GSK3β associated with and phosphorylated kinesin-1 in vitro. Our observations together with previous work that showed that the loop region of PS interacts with GSK3β propose a scaffolding mechanism for PS in which the loop region sequesters GSK3β away from motors for the proper regulation of motor function. These findings are important to uncouple the complex regulatory mechanisms that likely exist for motor activity during axonal transport in vivo.

Presenilin-mediated cleavage of APP regulates synaptotagmin-7 and presynaptic plasticity

Mutations of the intramembrane protease presenilin (PS) or of its main substrate, the amyloid precursor protein (APP), cause early-onset form of Alzheimer disease. PS and APP interact with proteins of the neurotransmitter release machinery without identified functional consequences. Here we report that genetic deletion of PS markedly decreases the presynaptic levels of the Ca²⁺ sensor synaptotagmin-7 (Syt7) leading to impaired synaptic facilitation and replenishment of synaptic vesicles. The regulation of Syt7 expression by PS occurs post-transcriptionally and depends on γ-secretase proteolytic activity. It requires the substrate APP as revealed by the combined genetic invalidation of APP and PS1, and in particular the APP-Cterminal fragments which interact with Syt7 and accumulate in synaptic terminals under pharmacological or genetic inhibition of γ-secretase. Thus, we uncover a role of PS in presynaptic mechanisms, through APP cleavage and regulation of Syt7, that highlights aberrant synaptic vesicle processing as a possible new pathway in AD.

Mutations in presenilin, which cleaves amyloid precursor protein, cause familial Alzheimer’s Disease. Here, the authors show that loss of presenilin leads to loss of synaptotagmin 7, leading to impaired presynaptic release.

Ethanol stimulates the in vivo axonal movement of neuropeptide dense-core vesicles in Drosophila motor neurons

Proper neuronal function requires essential biological cargoes to be packaged within membranous vesicles and transported, intracellularly, through the extensive outgrowth of axonal and dendritic fibers. The precise spatiotemporal movement of these cargoes is vital for neuronal survival and, thus, is highly regulated. In this study we test how the axonal movement of a neuropeptide-containing dense-core vesicle (DCV) responds to alcohol stressors. We found that ethanol induces a strong anterograde bias in vesicle movement. Low doses of ethanol stimulate the anterograde movement of neuropeptide-DCV while high doses inhibit bi-directional movement. This process required the presence of functional kinesin-1 motors as reduction in kinesin prevented the ethanol-induced stimulation of the anterograde movement of neuropeptide-DCV. Furthermore, expression of inactive glycogen synthase kinase 3 (GSK-3β) also prevented ethanol-induced stimulation of neuropeptide-DCV movement, similar to pharmacological inhibition of GSK-3β with lithium. Conversely, inhibition of PI3K/AKT signaling with wortmannin led to a partial prevention of ethanol-stimulated transport of neuropeptide-DCV. Taken together, we conclude that GSK-3β signaling mediates the stimulatory effects of ethanol. Therefore, our study provides new insight into the physiological response of the axonal movement of neuropeptide-DCV to exogenous stressors. Cover Image for this Issue: doi: 10.1111/jnc.14165.

Insulin signaling regulates a functional interaction between adenomatous polyposis coli and cytoplasmic dynein

Diabetes is linked to an increased risk for colorectal cancer, but the mechanistic underpinnings of this clinically important effect are unclear. Here we describe an interaction between the microtubule motor cytoplasmic dynein, the adenomatous polyposis coli tumor suppressor protein (APC), and glycogen synthase kinase-3β (GSK-3β), which could shed light on this issue. GSK-3β is perhaps best known for glycogen regulation, being inhibited downstream in an insulin-signaling pathway. However, the kinase is also important in many other processes. Mutations in APC that disrupt the regulation of β-catenin by GSK-3β cause colorectal cancer in humans. Of interest, both APC and GSK-3β interact with microtubules and cellular membranes. We recently demonstrated that dynein is a GSK-3β substrate and that inhibition of GSK-3β promotes dynein-dependent transport. We now report that dynein stimulation in intestinal cells in response to acute insulin exposure (or GSK-3β inhibition) is blocked by tumor-promoting isoforms of APC that reduce an interaction between wild-type APC and dynein. We propose that under normal conditions, insulin decreases dynein binding to APC to stimulate minus end-directed transport, which could modulate endocytic and secretory systems in intestinal cells. Mutations in APC likely impair the ability to respond appropriately to insulin signaling. This is exciting because it has the potential to be a contributing factor in the development of colorectal cancer in patients with diabetes.

Linking mitochondrial dysfunction, metabolic syndrome and stress signaling in Neurodegeneration

Mounting evidence suggests a link between metabolic syndrome (MetS) such as diabetes, obesity, non-alcoholic fatty liver disease in the progression of Alzheimer's disease (AD), Parkinson's disease (PD) and other neurodegenerative diseases (NDDs). For instance, accumulated Aβ oligomer is enhancing neuronal Ca²⁺ release and neural NO where increased NO level in the brain through post translational modification is modulating the level of insulin production. It has been further confirmed that irrespective of origin; brain insulin resistance triggers a cascade of the neurodegeneration phenomenon which can be aggravated by free reactive oxygen species burden, ER stress, metabolic dysfunction, neuorinflammation, reduced cell survival and altered lipid metabolism. Moreover, several studies confirmed that MetS and diabetic sharing common mechanisms in the progression of AD and NDDs where mitochondrial dynamics playing a critical role. Any mutation in mitochondrial DNA, exposure of environmental toxin, high-calorie intake, homeostasis imbalance, glucolipotoxicity is causative factors for mitochondrial dysfunction. These cumulative pleiotropic burdens in mitochondria leads to insulin resistance, increased ROS production; enhanced stress-related enzymes that is directly linked MetS and diabetes in neurodegeneration. Since, the linkup mechanism between mitochondrial dysfunction and disease phenomenon of both MetS and NDDs is quite intriguing, therefore, it is pertinent for the researchers to identify and implement the therapeutic interventions for targeting MetS and NDDs. Herein, we elucidated the pertinent role of MetS induced mitochondrial dysfunction in neurons and their consequences in NDDs. Further, therapeutic potential of well-known biomolecules and chaperones to target altered mitochondria has been comprehensively documented. This article is part of a Special Issue entitled: Oxidative Stress and Mitochondrial Quality in Diabetes/Obesity and Critical Illness Spectrum of Diseases - edited by P. Hemachandra Reddy.

Axonal Transport and Neurodegeneration: How Marine Drugs Can Be Used for the Development of Therapeutics

Unlike virtually any other cells in the human body, neurons are tasked with the unique problem of transporting important factors from sites of synthesis at the cell bodies, across enormous distances, along narrow-caliber projections, to distally located nerve terminals in order to maintain cell viability. As a result, axonal transport is a highly regulated process whereby necessary cargoes of all types are packaged and shipped from one end of the neuron to the other. Interruptions in this finely tuned transport have been linked to many neurodegenerative disorders including Alzheimer's (AD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS) suggesting that this pathway is likely perturbed early in disease progression. Therefore, developing therapeutics targeted at modifying transport defects could potentially avert disease progression. In this review, we examine a variety of potential compounds identified from marine aquatic species that affect the axonal transport pathway. These compounds have been shown to function in microtubule (MT) assembly and maintenance, motor protein control, and in the regulation of protein degradation pathways, such as the autophagy-lysosome processes, which are defective in many degenerative diseases. Therefore, marine compounds have great potential in developing effective treatment strategies aimed at early defects which, over time, will restore transport and prevent cell death.

Dynein, kinesin and morphological changes in optic nerve axons in a rat model with cerebrospinal fluid pressure reduction: the Beijing Intracranial and Intraocular Pressure (iCOP) study

PURPOSE

To examine the influence of experimentally reduced cerebrospinal fluid pressure (CSFP) as compared to elevated intraocular pressure (IOP) on axonal morphology and axonal motor proteins in retinal ganglion cells (RGCs).

METHODS

The experimental study included 39 rats which underwent cerebrospinal fluid drainage for 6 hr, 30 rats which unilaterally underwent IOP elevation for 6 hr and 30 rats in a control group. Six hours after baseline, the animals were killed and the eyes were histologically and immunohistochemically examined.

RESULTS

In experimental models in the high-IOP group and the low-CSFP group as compared to the control group, RGC axons became abnormally dilated and accumulated vesicles. Both groups as compared to the control group showed an accumulation of dynein IC (intermediate chain) at the optic nerve head and retina and a reduction in kinesin HC (heavy chain) immunoreactivity in the optic nerve fibre axons. As a corollary, Western blot analysis revealed an elevation of dynein IC protein levels in the optic nerve head and retina and a decrease in kinesin HC protein levels in the optic nerve.

CONCLUSIONS

Experimental models with an acute IOP rise or with an acute CSFP reduction showed similar morphologic changes in the retinal ganglion cell axons and similar immunohistochemical changes in the axonal motor proteins kinesin HC and dynein IC. It supports the hypothesis that an experimental model with an acute reduction in CSFP as well as an experimental model with an acute rise in IOP may share similarities in the process of optic nerve damage.

Non-Catalytic Roles of Presenilin Throughout Evolution

Research into Alzheimer's disease pathology and treatment has often focused on presenilin proteins. These proteins provide the key catalytic activity of the γ-secretase complex in the cleavage of amyloid-β precursor protein and resultant amyloid tangle deposition. Over the last 25 years, screening novel drugs to control this aberrant proteolytic activity has yet to identify effective treatments for the disease. In the search for other mechanisms of presenilin pathology, several studies have demonstrated that mammalian presenilin proteins also act in a non-proteolytic role as a scaffold to co-localize key signaling proteins. This role is likely to represent an ancestral presenilin function, as it has been described in genetically distant species including non-mammalian animals, plants, and a simple eukaryotic amoeba Dictyostelium that diverged from the human lineage over a billion years ago. Here, we review the non-catalytic scaffold role of presenilin, from mammalian models to other biomedical models, and include recent insights using Dictyostelium, to suggest that this role may provide an early evolutionary function of presenilin proteins.

Regulation of Axonal Transport by Protein Kinases

The intracellular transport of organelles, proteins, lipids, and RNA along the axon is essential for neuronal function and survival. This process, called axonal transport, is mediated by two classes of ATP-dependent motors, kinesins, and cytoplasmic dynein, which carry their cargoes along microtubule tracks. Protein kinases regulate axonal transport through direct phosphorylation of motors, adapter proteins, and cargoes, and indirectly through modification of the microtubule network. The misregulation of axonal transport by protein kinases has been implicated in the pathogenesis of several nervous system disorders. Here, we review the role of protein kinases acting directly on axonal transport and discuss how their deregulation affects neuronal function, paving the way for the exploitation of these enzymes as novel drug targets.

Beyond γ-secretase activity: The multifunctional nature of presenilins in cell signalling pathways

The presenilins are the catalytic subunit of the membrane-embedded tetrameric γ-secretase protease complexes. More that 90 transmembrane proteins have been reported to be γ-secretase substrates, including the widely studied amyloid precursor protein (APP) and the Notch receptor, which are precursors for the generation of amyloid-β peptides and biologically active APP intracellular domain (AICD) and Notch intracellular domain (NICD). The diversity of γ-secretase substrates highlights the importance of presenilin-dependent γ-secretase protease activities as a regulatory mechanism in a range of biological systems. However, there is also a growing body of evidence that supports the existence of γ-secretase-independent functions for the presenilins in the regulation and progression of an array of cell signalling pathways. In this review, we will present an overview of current literature that proposes evolutionarily conserved presenilin functions outside of the γ-secretase complex, with a focus on the suggested role of the presenilins in the regulation of Wnt/β-catenin signalling, protein trafficking and degradation, calcium homeostasis and apoptosis.

Imaging organelle transport in primary hippocampal neurons treated with amyloid-β oligomers

We describe a strategy for fluorescent imaging of organelle transport in primary hippocampal neurons treated with amyloid-β (Aβ) peptides that cause Alzheimer's disease (AD). This method enables careful, rigorous analyses of axonal transport defects, which are implicated in AD and other neurodegenerative diseases. Moreover, we present and emphasize guidelines for investigating Aβ-induced mechanisms of axonal transport disruption in the absence of nonspecific, irreversible cellular toxicity. This approach should be accessible to most laboratories equipped with cell culture facilities and a standard fluorescent microscope and may be adapted to other cell types.

Stress-Induced CDK5 Activation Disrupts Axonal Transport via Lis1/Ndel1/Dynein

Axonal transport is essential for neuronal function, and defects in transport are associated with multiple neurodegenerative diseases. Aberrant cyclin-dependent kinase 5 (CDK5) activity, driven by the stress-induced activator p25, also is observed in these diseases. Here we show that elevated CDK5 activity increases the frequency of nonprocessive events for a range of organelles, including lysosomes, autophagosomes, mitochondria, and signaling endosomes. Transport disruption induced by aberrant CDK5 activation depends on the Lis1/Ndel1 complex, which directly regulates dynein activity. CDK5 phosphorylation of Ndel1 favors a high affinity Lis1/Ndel/dynein complex that blocks the ATP-dependent release of dynein from microtubules, inhibiting processive motility of dynein-driven cargo. Similar transport defects observed in neurons from a mouse model of amyotrophic lateral sclerosis are rescued by CDK5 inhibition. Together, these studies identify CDK5 as a Lis1/Ndel1-dependent regulator of transport in stressed neurons, and suggest that dysregulated CDK5 activity contributes to the transport deficits observed during neurodegeneration.

GSK-3β Phosphorylation of Cytoplasmic Dynein Reduces Ndel1 Binding to Intermediate Chains and Alters Dynein Motility

Glycogen synthase kinase 3 (GSK‐3) has been linked to regulation of kinesin‐dependent axonal transport in squid and flies, and to indirect regulation of cytoplasmic dynein. We have now found evidence for direct regulation of dynein by mammalian GSK‐3β in both neurons and non‐neuronal cells. GSK‐3β coprecipitates with and phosphorylates mammalian dynein. Phosphorylation of dynein intermediate chain (IC) reduces its interaction with Ndel1, a protein that contributes to dynein force generation. Two conserved residues, S87/T88 in IC‐1B and S88/T89 in IC‐2C, have been identified as GSK‐3 targets by both mass spectrometry and site‐directed mutagenesis. These sites are within an Ndel1‐binding domain, and mutation of both sites alters the interaction of IC's with Ndel1. Dynein motility is stimulated by (i) pharmacological and genetic inhibition of GSK‐3β, (ii) an insulin‐sensitizing agent (rosiglitazone) and (iii) manipulating an insulin response pathway that leads to GSK‐3β inactivation. Thus, our study connects a well‐characterized insulin‐signaling pathway directly to dynein stimulation via GSK‐3 inhibition.

Dendritic and axonal mechanisms of Ca2+ elevation impair BDNF transport in Aβ oligomer-treated hippocampal neurons

Intracellular Ca²⁺ dysregulation and transport disruption precede cell death in Alzheimer's disease. Mechanisms of AβO-induced Ca²⁺ elevation are identified that regulate the onset, severity, and spatiotemporal progression of BDNF transport defects. The results challenge dogmatic views on mechanisms of AβO toxicity and subcellular sites of action.

Disruption of fast axonal transport (FAT) and intracellular Ca²⁺ dysregulation are early pathological events in Alzheimer's disease (AD). Amyloid-β oligomers (AβOs), a causative agent of AD, impair transport of BDNF independent of tau by nonexcitotoxic activation of calcineurin (CaN). Ca²⁺-dependent mechanisms that regulate the onset, severity, and spatiotemporal progression of BDNF transport defects from dendritic and axonal AβO binding sites are unknown. Here we show that BDNF transport defects in dendrites and axons are induced simultaneously but exhibit different rates of decline. The spatiotemporal progression of FAT impairment correlates with Ca²⁺ elevation and CaN activation first in dendrites and subsequently in axons. Although many axonal pathologies have been described in AD, studies have primarily focused only on the dendritic effects of AβOs despite compelling reports of presynaptic AβOs in AD models and patients. Indeed, we observe that dendritic CaN activation converges on Ca²⁺ influx through axonal voltage-gated Ca²⁺ channels to impair FAT. Finally, FAT defects are prevented by dantrolene, a clinical compound that reduces Ca²⁺ release from the ER. This work establishes a novel role for Ca²⁺ dysregulation in BDNF transport disruption and tau-independent Aβ toxicity in early AD.

Disruption of axonal transport perturbs bone morphogenetic protein (BMP)--signaling and contributes to synaptic abnormalities in two neurodegenerative diseases

Formation of new synapses or maintenance of existing synapses requires the delivery of synaptic components from the soma to the nerve termini via axonal transport. One pathway that is important in synapse formation, maintenance and function of the Drosophila neuromuscular junction (NMJ) is the bone morphogenetic protein (BMP)-signaling pathway. Here we show that perturbations in axonal transport directly disrupt BMP signaling, as measured by its downstream signal, phospho Mad (p-Mad). We found that components of the BMP pathway genetically interact with both kinesin-1 and dynein motor proteins. Thick vein (TKV) vesicle motility was also perturbed by reductions in kinesin-1 or dynein motors. Interestingly, dynein mutations severely disrupted p-Mad signaling while kinesin-1 mutants showed a mild reduction in p-Mad signal intensity. Similar to mutants in components of the BMP pathway, both kinesin-1 and dynein motor protein mutants also showed synaptic morphological defects. Strikingly TKV motility and p-Mad signaling were disrupted in larvae expressing two human disease proteins; expansions of glutamine repeats (polyQ77) and human amyloid precursor protein (APP) with a familial Alzheimer's disease (AD) mutation (APPswe). Consistent with axonal transport defects, larvae expressing these disease proteins showed accumulations of synaptic proteins along axons and synaptic abnormalities. Taken together our results suggest that similar to the NGF-TrkA signaling endosome, a BMP signaling endosome that directly interacts with molecular motors likely exist. Thus problems in axonal transport occurs early, perturbs BMP signaling, and likely contributes to the synaptic abnormalities observed in these two diseases.

Silencing [Formula: see text] Rescues Tau Pathologies and Memory Deficits through Rescuing PP2A and Inhibiting GSK-3β Signaling in Human Tau Transgenic Mice

Increase of inhibitor-2 of protein phosphatase-2A [Formula: see text] is associated with protein phosphatase-2A (PP2A) inhibition and tau hyperphosphorylation in Alzheimer's disease (AD). Down-regulating [Formula: see text] attenuated amyloidogenesis and improved the cognitive functions in transgenic mice expressing amyloid precursor protein (tg2576). Here, we found that silencing [Formula: see text] by hippocampal infusion of [Formula: see text] down-regulated [Formula: see text] (~45%) with reduction of tau phosphorylation/accumulation, improvement of memory deficits, and dendritic plasticity in 12-month-old human tau transgenic mice. Silencing [Formula: see text] not only restored PP2A activity but also inhibited glycogen synthase kinase-3β (GSK-3β) with a significant activation of protein kinase A (PKA) and Akt. In HEK293/tau and N2a/tau cells, silencing [Formula: see text] by [Formula: see text] also significantly reduced tau hyperphosphorylation with restoration of PP2A activity and inhibition of GSK-3β, demonstrated by the decreased GSK-3β total protein and mRNA levels, and the increased inhibitory phosphorylation of GSK-3β at serine-9. Furthermore, activation of PKA but not Akt mediated the inhibition of GSK-3β by [Formula: see text] silencing. We conclude that targeting [Formula: see text] can improve tau pathologies and memory deficits in human tau transgenic mice, and activation of PKA contributes to GSK-3β inhibition induced by silencing [Formula: see text]in vitro, suggesting that [Formula: see text] is a promising multiple target of AD.

Spatial and temporal characteristics of normal and perturbed vesicle transport

Efficient intracellular transport is essential for healthy cellular function and structural integrity, and problems in this pathway can lead to neuronal cell death and disease. To spatially and temporally evaluate how transport defects are initiated, we adapted a primary neuronal culture system from Drosophila larval brains to visualize the movement dynamics of several cargos/organelles along a 90 micron axonal neurite over time. All six vesicles/organelles imaged showed robust bi-directional motility at both day 1 and day 2. Reduction of motor proteins decreased the movement of vesicles/organelles with increased numbers of neurite blocks. Neuronal growth was also perturbed with reduction of motor proteins. Strikingly, we found that all blockages were not fixed, permanent blocks that impeded transport of vesicles as previously thought, but that some blocks were dynamic clusters of vesicles that resolved over time. Taken together, our findings suggest that non-resolving blocks may likely initiate deleterious pathways leading to death and degeneration, while resolving blocks may be benign. Therefore evaluating the spatial and temporal characteristics of vesicle transport has important implications for our understanding of how transport defects can affect other pathways to initiate death and degeneration.

Axonal transport and neurodegenerative disease: vesicle-motor complex formation and their regulation

The process of axonal transport serves to move components over very long distances on microtubule tracks in order to maintain neuronal viability. Molecular motors - kinesin and dynein - are essential for the movement of neuronal cargoes along these tracks; defects in this pathway have been implicated in the initiation or progression of some neurodegenerative diseases, suggesting that this process may be a key contributor in neuronal dysfunction. Recent work has led to the identification of some of the motor-cargo complexes, adaptor proteins, and their regulatory elements in the context of disease proteins. In this review, we focus on the assembly of the amyloid precursor protein, huntingtin, mitochondria, and the RNA-motor complexes and discuss how these may be regulated during long-distance transport in the context of neurodegenerative disease. As knowledge of these motor-cargo complexes and their involvement in axonal transport expands, insight into how defects in this pathway contribute to the development of neurodegenerative diseases becomes evident. Therefore, a better understanding of how this pathway normally functions has important implications for early diagnosis and treatment of diseases before the onset of disease pathology or behavior.