Stuck in traffic: an emerging theme in diseases of the nervous system

Presynaptic protein homeostasis and neuronal function

Proteome integrity is maintained by a coordinated network of molecular chaperones, by protein degradation machineries and by their regulators. Numerous human pathologies are considered as diseases of compromised protein homeostasis (proteostasis), including neurodegeneration. These are characterized by the accumulation of neuronal protein aggregates and by synaptic defects followed by loss of connectivity and cell death. While this suggests that synaptic terminals are particularly sensitive to proteostasis imbalance, our understanding of protein turnover mechanisms and regulation at the synapse remains limited. Recent reports show that different proteolytic pathways act at synapses, including several forms of autophagy. The role of chaperones in controlling the balance between synaptic protein refolding and degradation and how this complex network regulates neuronal function also begins to be unraveled.

BK channel agonist represents a potential therapeutic approach for lysosomal storage diseases

Efficient lysosomal Ca²⁺ release plays an essential role in lysosomal trafficking. We have recently shown that lysosomal big conductance Ca²⁺-activated potassium (BK) channel forms a physical and functional coupling with the lysosomal Ca²⁺ release channel Transient Receptor Potential Mucolipin-1 (TRPML1). BK and TRPML1 forms a positive feedback loop to facilitate lysosomal Ca²⁺ release and subsequent lysosome membrane trafficking. However, it is unclear whether the positive feedback mechanism is common for other lysosomal storage diseases (LSDs) and whether BK channel agonists rescue abnormal lysosomal storage in LSDs. In this study, we assessed the effect of BK agonist, NS1619 and NS11021 in a number of LSDs including NPC1, mild cases of mucolipidosis type IV (ML4) (TRPML1-F408∆), Niemann-Pick type A (NPA) and Fabry disease. We found that TRPML1-mediated Ca²⁺ release was compromised in these LSDs. BK activation corrected the impaired Ca²⁺ release in these LSDs and successfully rescued the abnormal lysosomal storage of these diseases by promoting TRPML1-mediated lysosomal exocytosis. Our study suggests that BK channel activation stimulates the TRPML1-BK positive reinforcing loop to correct abnormal lysosomal storage in LSDs. Drugs targeting BK channel represent a potential therapeutic approach for LSDs.

Normal Molecular Specification and Neurodegenerative Disease-Like Death of Spinal Neurons Lacking the SNARE-Associated Synaptic Protein Munc18-1

The role of synaptic activity during early formation of neural circuits is a topic of some debate; genetic ablation of neurotransmitter release by deletion of the Munc18-1 gene provides an excellent model to answer the question of whether such activity is required for early circuit formation. Previous analysis of Munc18-1(-/-) mouse mutants documented their grossly normal nervous system, but its molecular differentiation has not been assessed. Munc18-1 deletion in mice also results in widespread neurodegeneration that remains poorly characterized. In this study, we demonstrate that the early stages of spinal motor circuit formation, including motor neuron specification, axon growth and pathfinding, and mRNA expression, are unaffected in Munc18-1(-/-) mice, demonstrating that synaptic activity is dispensable for early nervous system development. Furthermore, we show that the neurodegeneration caused by Munc18-1 loss is cell autonomous, consistent with apparently normal expression of several neurotrophic factors and normal GDNF signaling. Consistent with cell-autonomous degeneration, we demonstrate defects in the trafficking of the synaptic proteins Syntaxin1a and PSD-95 and the TrkB and DCC receptors in Munc18-1(-/-) neurons; these defects do not appear to cause ER stress, suggesting other mechanisms for degeneration. Finally, we demonstrate pathological similarities to Alzheimer's disease, such as altered Tau phosphorylation, neurofibrillary tangles, and accumulation of insoluble protein plaques. Together, our results shed new light upon the neurodegeneration observed in Munc18-1(-/-) mice and argue that this phenomenon shares parallels with neurodegenerative diseases.

SIGNIFICANCE STATEMENT

In this work, we demonstrate the absence of a requirement for regulated neurotransmitter release in the assembly of early neuronal circuits by assaying transcriptional identity, axon growth and guidance, and mRNA expression in Munc18-1-null mice. Furthermore, we characterize the neurodegeneration observed in Munc18-1 mutants and demonstrate that this cell-autonomous process does not appear to be a result of defects in growth factor signaling or ER stress caused by protein trafficking defects. However, we find the presence of various pathological hallmarks of Alzheimer's disease that suggest parallels between the degeneration in these mutants and neurodegenerative conditions.

FAM21 directs SNX27-retromer cargoes to the plasma membrane by preventing transport to the Golgi apparatus

The endosomal network maintains cellular homeostasis by sorting, recycling and degrading endocytosed cargoes. Retromer organizes the endosomal sorting pathway in conjunction with various sorting nexin (SNX) proteins. The SNX27–retromer complex has recently been identified as a major endosomal hub that regulates endosome-to-plasma membrane recycling by preventing lysosomal entry of cargoes. Here, we show that SNX27 directly interacts with FAM21, which also binds retromer, within the Wiskott–Aldrich syndrome protein and SCAR homologue (WASH) complex. This interaction is required for the precise localization of SNX27 at an endosomal subdomain as well as for recycling of SNX27-retromer cargoes. Furthermore, FAM21 prevents cargo transport to the Golgi apparatus by controlling levels of phosphatidylinositol 4-phosphate, which facilitates cargo dissociation at the Golgi. Together, our results demonstrate that the SNX27–retromer–WASH complex directs cargoes to the plasma membrane by blocking their transport to lysosomes and the Golgi.

Endosomes maintain cellular homeostasis by sorting, recycling and degrading endocytosed cargoes. Here the authors show that the SNX27-retromer-WASH complex acts as a hub to direct cargoes to the plasma membrane by blocking their transport to lysosomes and Golgi apparatus.

Analysis of Signaling Endosome Composition and Dynamics Using SILAC in Embryonic Stem Cell-Derived Neurons

Neurons require efficient transport mechanisms such as fast axonal transport to ensure neuronal homeostasis and survival. Neurotrophins and their receptors are conveyed via fast axonal retrograde transport of signaling endosomes to the soma, where they elicit transcriptional responses. Despite the essential roles of signaling endosomes in neuronal differentiation and survival, little is known about their molecular identity, dynamics, and regulation. Gaining a better mechanistic understanding of these organelles and their kinetics is crucial, given the growing evidence linking vesicular trafficking deficits to neurodegeneration. Here, we exploited an affinity purification strategy using the binding fragment of tetanus neurotoxin (HCT) conjugated to monocrystalline iron oxide nanoparticles (MIONs), which in motor neurons, is transported in the same carriers as neurotrophins and their receptors. To quantitatively assess the molecular composition of HCT-containing signaling endosomes, we have developed a protocol for triple Stable Isotope Labeling with Amino acids in Cell culture (SILAC) in embryonic stem cell-derived motor neurons. After HCT internalization, retrograde carriers were magnetically isolated at different time points and subjected to mass-spectrometry and Gene Ontology analyses. This purification strategy is highly specific, as confirmed by the presence of essential regulators of fast axonal transport in the make-up of these organelles. Our results indicate that signaling endosomes undergo a rapid maturation with the acquisition of late endosome markers following a specific time-dependent kinetics. Strikingly, signaling endosomes are specifically enriched in proteins known to be involved in neurodegenerative diseases and neuroinfection. Moreover, we highlighted the presence of novel components, whose precise temporal recruitment on signaling endosomes might be essential for proper sorting and/or transport of these organelles. This study provides the first quantitative proteomic analysis of signaling endosomes isolated from motor neurons and allows the assembly of a functional map of these axonal carriers involved in long-range neuronal signaling.

Association between Rare Variants in AP4E1, a Component of Intracellular Trafficking, and Persistent Stuttering

Stuttering is a common, highly heritable neurodevelopmental disorder characterized by deficits in the volitional control of speech. Whole-exome sequencing identified two heterozygous AP4E1 coding variants, c.1549G>A (p.Val517Ile) and c.2401G>A (p.Glu801Lys), that co-segregate with persistent developmental stuttering in a large Cameroonian family, and we observed the same two variants in unrelated Cameroonians with persistent stuttering. We found 23 other rare variants, including predicted loss-of-function variants, in AP4E1 in unrelated stuttering individuals in Cameroon, Pakistan, and North America. The rate of rare variants in AP4E1 was significantly higher in unrelated Pakistani and Cameroonian stuttering individuals than in population-matched control individuals, and coding variants in this gene are exceptionally rare in the general sub-Saharan West African, South Asian, and North American populations. Clinical examination of the Cameroonian family members failed to identify any symptoms previously reported in rare individuals carrying homozygous loss-of-function mutations in this gene. AP4E1 encodes the ε subunit of the heterotetrameric (ε-β4-μ4-σ4) AP-4 complex, involved in protein sorting at the trans-Golgi network. We found that the μ4 subunit of AP-4 interacts with NAGPA, an enzyme involved in the synthesis of the mannose 6-phosphate signal that targets acid hydrolases to the lysosome and the product of a gene previously associated with stuttering. These findings implicate deficits in intracellular trafficking in persistent stuttering.

The endosomal-lysosomal system: from acidification and cargo sorting to neurodegeneration

The endosomal-lysosomal system is made up of a set of intracellular membranous compartments that dynamically interconvert, which is comprised of early endosomes, recycling endosomes, late endosomes, and the lysosome. In addition, autophagosomes execute autophagy, which delivers intracellular contents to the lysosome. Maturation of endosomes and/or autophagosomes into a lysosome creates an unique acidic environment within the cell for proteolysis and recycling of unneeded cellular components into usable amino acids and other biomolecular building blocks. In the endocytic pathway, gradual maturation of endosomes into a lysosome and acidification of the late endosome are accompanied by vesicle trafficking, protein sorting and targeted degradation of some sorted cargo. Two opposing sorting systems are operating in these processes: the endosomal sorting complex required for transport (ESCRT) supports targeted degradation and the retromer supports retrograde retrieval of certain cargo. The endosomal-lysosomal system is emerging as a central player in a host of neurodegenerative diseases, demonstrating potential roles which are likely to be revealed in pathogenesis and for viable therapeutic strategies. Here we focus on the physiological process of endosomal-lysosomal maturation, acidification and sorting systems along the endocytic pathway, and further discuss relationships between abnormalities in the endosomal-lysosomal system and neurodegenerative diseases, especially Alzheimer’s disease (AD).

Frontotemporal dementia-associated N279K tau mutant disrupts subcellular vesicle trafficking and induces cellular stress in iPSC-derived neural stem cells

Background

Pallido-ponto-nigral degeneration (PPND), a major subtype of frontotemporal dementia with parkinsonism related to chromosome 17 (FTDP-17), is a progressive and terminal neurodegenerative disease caused by c.837 T > G mutation in the MAPT gene encoding microtubule-associated protein tau (rs63750756; N279K). This MAPT mutation induces alternative splicing of exon 10, resulting in a modification of microtubule-binding region of tau. Although mutations in the MAPT gene have been linked to multiple tauopathies including Alzheimer’s disease, frontotemporal dementia and progressive supranuclear palsy, knowledge regarding how tau N279K mutation causes PPND/FTDP-17 is limited.

Results

We investigated the underlying disease mechanism associated with the N279K tau mutation using PPND/FTDP-17 patient-derived induced pluripotent stem cells (iPSCs) and autopsy brains. In iPSC-derived neural stem cells (NSCs), the N279K tau mutation induced an increased ratio of 4-repeat to 3-repeat tau and accumulation of stress granules indicating elevated cellular stress. More significant, NSCs derived from patients with the N279K tau mutation displayed impaired endocytic trafficking as evidenced by accumulation of endosomes and exosomes, and a reduction of lysosomes. Since there were no significant differences in cellular stress and distribution of subcellular organelles between control and N279K skin fibroblasts, N279K-related vesicle trafficking defects are likely specific to the neuronal lineage. Consistently, the levels of intracellular/luminal vesicle and exosome marker flotillin-1 were significantly increased in frontal and temporal cortices of PPND/FTDP-17 patients with the N279K tau mutation, events that were not seen in the occipital cortex which is the most spared cortical region in the patients.

Conclusion

Together, our results demonstrate that alterations of intracellular vesicle trafficking in NSCs/neurons likely contribute to neurodegeneration as an important disease mechanism underlying the N279K tau mutation in PPND/FTDP-17.

Motor and Sensory Deficits in the teetering Mice Result from Mutation of the ESCRT Component HGS

Neurons are particularly vulnerable to perturbations in endo-lysosomal transport, as several neurological disorders are caused by a primary deficit in this pathway. In this report, we used positional cloning to show that the spontaneously occurring neurological mutation teetering (tn) is a single nucleotide substitution in hepatocyte growth factor-regulated tyrosine kinase substrate (Hgs/Hrs), a component of the endosomal sorting complex required for transport (ESCRT). The tn mice exhibit hypokenesis, muscle weakness, reduced muscle size and early perinatal lethality by 5-weeks of age. Although HGS has been suggested to be essential for the sorting of ubiquitinated membrane proteins to the lysosome, there were no alterations in receptor tyrosine kinase levels in the central nervous system, and only a modest decrease in tropomyosin receptor kinase B (TrkB) in the sciatic nerves of the tn mice. Instead, loss of HGS resulted in structural alterations at the neuromuscular junction (NMJ), including swellings and ultra-terminal sprouting at motor axon terminals and an increase in the number of endosomes and multivesicular bodies. These structural changes were accompanied by a reduction in spontaneous and evoked release of acetylcholine, indicating a deficit in neurotransmitter release at the NMJ. These deficits in synaptic transmission were associated with elevated levels of ubiquitinated proteins in the synaptosome fraction. In addition to the deficits in neuronal function, mutation of Hgs resulted in both hypermyelinated and dysmyelinated axons in the tn mice, which supports a growing body of evidence that ESCRTs are required for proper myelination of peripheral nerves. Our results indicate that HGS has multiple roles in the nervous system and demonstrate a previously unanticipated requirement for ESCRTs in the maintenance of synaptic transmission.

Endocytic trafficking involves the internalization, endosomal sorting and lysosomal degradation of cell surface cargo. Many factors involved in endosomal sorting in mammalian cells have been identified, and mutations in these components are associated with a variety of neurological disorders. While the function of endosomal sorting components has been intensely studied in immortalized cell lines, it is not known what role these factors play in endosomal sorting in the nervous system. In this study, we show that the teetering (tn) gene encodes the hepatocytegrowth factor regulated tyrosine kinasesubstrate (Hgs), a core component of the endosomal sorting pathway. The tn mice exhibit several signs of motor neuron disease, including reduced muscle mass, muscle weakness and motor abnormalities. Although HGS is predicted to be required for the lysosomal degradation of receptor tyrosine kinases, there was no change in the levels of receptor tyrosine kinases in the spinal cords of the tn mice. Instead, we found that HGS is required for synaptic transmission at the neuromuscular junction and for the proper myelination of the peripheral nervous system.

BK Channels Alleviate Lysosomal Storage Diseases by Providing Positive Feedback Regulation of Lysosomal Ca2+ Release

Promoting lysosomal trafficking represents a promising therapeutic approach for lysosome storage diseases. Efficient Ca(2+) mobilization from lysosomes is important for lysosomal trafficking. Ca(2+) release from lysosomes could generate a negative potential in the lumen to disturb subsequent Ca(2+) release in the absence of counter ion flux. Here we report that lysosomes express big-conductance Ca(2+)-activated potassium (BK) channels that form physical and functional coupling with the lysosomal Ca(2+) release channel, TRPML1. Ca(2+) release via TRPML1 causes BK activation, which in turn facilitates further lysosomal Ca(2+) release and membrane trafficking. Importantly, BK overexpression rescues the impaired TRPML1-mediated Ca(2+) release and abnormal lysosomal storage in cells from Niemann-Pick C1 patients. Therefore, we have identified a lysosomal K(+) channel that provides a positive feedback mechanism to facilitate TRPML1-mediated Ca(2+) release and membrane trafficking. Our findings suggest that upregulating BK may be a potential therapeutic strategy for certain lysosomal storage diseases and common neurodegenerative disorders.

Cellular functions of the amyloid precursor protein from development to dementia

Amyloid precursor protein (APP) is a key player in Alzheimer's disease (AD). The Aβ fragments of APP are the major constituent of AD-associated amyloid plaques, and mutations or duplications of the gene coding for APP can cause familial AD. Here we review the roles of APP in neuronal development, signaling, intracellular transport, and other aspects of neuronal homeostasis. We suggest that APP acts as a signaling nexus that transduces information about a range of extracellular conditions, including neuronal damage, to induction of intracellular signaling events. Subtle disruptions of APP signaling functions may be major contributors to AD-causing neuronal dysfunction.

Bridging the age spectrum of neurodegenerative storage diseases

For over a century, researchers have observed similar neurodegenerative hallmarks in brains of people affected by rare early-onset lysosomal storage diseases and late-onset neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Increasing evidence suggests these apparently disparate diseases share a common underlying feature, namely, a dysfunctional clearance of cellular cargo through the secretory-endosomal-autophagic-lysosomal-exocytic (SEALE) network. By providing examples of rare and common neurodegenerative diseases known to have pathologically altered cargo flux through the SEALE network, we explore the unifying hypothesis that impaired catabolism or exocytosis of SEALE cargo, places a burden of stress on neurons that initiates pathogenesis. We also describe how a growing understanding of genetic, epigenetic and age-related modifications of the SEALE network, has inspired a number of novel disease-modifying therapeutic approaches aimed at alleviating SEALE storage and providing therapeutic benefit to people affected by these devastating diseases across the age spectrum.

Sphingolipid lysosomal storage disorders

Lysosomal storage diseases are inborn errors of metabolism, the hallmark of which is the accumulation, or storage, of macromolecules in the late endocytic system. They are monogenic disorders that occur at a collective frequency of 1 in 5,000 live births and are caused by inherited defects in genes that mainly encode lysosomal proteins, most commonly lysosomal enzymes. A subgroup of these diseases involves the lysosomal storage of glycosphingolipids. Through our understanding of the genetics, biochemistry and, more recently, cellular aspects of sphingolipid storage disorders, we have gained insights into fundamental aspects of cell biology that would otherwise have remained opaque. In addition, study of these disorders has led to significant progress in the development of therapies, several of which are now in routine clinical use. Emerging mechanistic links with more common diseases suggest we need to rethink our current concept of disease boundaries.

Analysis of the Retromer complex-WASH complex interaction illuminates new avenues to explore in Parkinson disease

The retromer complex mediates endosomal protein sorting by concentrating membrane proteins (cargo) into nascent tubules formed through the action of sorting nexin (SNX) proteins. The WASH complex is recruited to endosomes by binding to the VPS35 subunit of retromer and facilitates cargo protein sorting by promoting formation of endosomally-localized F-actin. The VPS35 protein is mutated in Parkinson disease (PD) and a recent report has revealed that the PD-causing mutation impairs the association of retromer with the WASH complex leading to perturbed endosomal protein sorting. Another important player in endosomal protein sorting is the DNAJC13/RME-8 protein, which associates with SNX1 and has also recently been linked to PD. An additional recent report has now shown that RME-8 also interacts with the WASH complex thus establishing retromer and WASH complex-mediated endosomal protein sorting as a key pathway linked to the pathology of PD and providing new avenues to explore in the search for insights into the disease mechanism.

Lysosomal storage diseases and the heat shock response: convergences and therapeutic opportunities

Lysosomes play a vital role in the maintenance of cellular homeostasis through the recycling of cell constituents, a key metabolic function which is highly dependent on the correct function of the lysosomal hydrolases and membrane proteins, as well as correct membrane lipid stoichiometry and composition. The critical role of lysosomal functionality is evident from the severity of the diseases in which the primary lesion is a genetically defined loss-of-function of lysosomal hydrolases or membrane proteins. This group of diseases, known as lysosomal storage diseases (LSDs), number more than 50 and are associated with severe neurodegeneration, systemic disease, and early death, with only a handful of the diseases having a therapeutic option. Another key homeostatic system is the metabolic stress response or heat shock response (HSR), which is induced in response to a number of physiological and pathological stresses, such as protein misfolding and aggregation, endoplasmic reticulum stress, oxidative stress, nutrient deprivation, elevated temperature, viral infections, and various acute traumas. Importantly, the HSR and its cardinal members of the heat shock protein 70 family has been shown to protect against a number of degenerative diseases, including severe diseases of the nervous system. The cytoprotective actions of the HSR also include processes involving the lysosomal system, such as cell death, autophagy, and protection against lysosomal membrane permeabilization, and have shown promise in a number of LSDs. This review seeks to describe the emerging understanding of the interplay between these two essential metabolic systems, the lysosomes and the HSR, with a particular focus on their potential as a therapeutic target for LSDs.

Small regulators, major consequences - Ca²⁺ and cholesterol at the endosome-ER interface

The ER is the largest cellular compartment and a major storage site for lipids and ions. In recent years, much attention has focused on contacts between the ER and other organelles, and one particularly intimate relationship is that between the ER and the endosomal system. ER-endosome contacts intensify when endosomes mature, and the ER participates in endosomal processes, such as the termination of surface receptor signaling, multi-vesicular body formation, and transport and fusion events. Cholesterol and Ca(2+) are transferred between the ER and endosomes, possibly acting as messengers for ER-endosome crosstalk. Here, we summarize different types of ER-endosomal communication and discuss membrane contact sites that might facilitate this crosstalk. We review the protein pairs that interact at the ER-endosome interface and find that many of these have a role in cholesterol exchange. We also summarize Ca(2+) exchange between the ER and endosomes, and hypothesize that ER-endosome contacts integrate several cellular functions to guide endosomal maturation. We post the hypothesis that failure in ER-endosome contacts is an unrecognized but important contributor to diseases, such as Niemann-Pick type C disease, Alzheimer's disease and amyotrophic lateral sclerosis.