Adaptor protein 4 (AP-4) is a heterotetrameric complex composed of ε, β4, μ4, and σ4 subunits that mediates export of a subset of transmembrane cargos, including autophagy protein 9A (ATG9A), from ...the
-Golgi network (TGN). AP-4 has received particular attention in recent years because mutations in any of its subunits cause a complicated form of hereditary spastic paraplegia referred to as "AP-4-deficiency syndrome." The identification of proteins that interact with AP-4 has shed light on the mechanisms of AP-4-dependent cargo sorting and distribution within the cell. However, the mechanisms by which the AP-4 complex itself is assembled have remained unknown. Here, we report that the alpha- and gamma-adaptin-binding protein (AAGAB, also known as p34) binds to and stabilizes the AP-4 ε and σ4 subunits, thus promoting complex assembly. The physiological importance of these interactions is underscored by the observation that AAGAB-knockout cells exhibit reduced levels of AP-4 subunits and accumulation of ATG9A at the TGN like those in cells with mutations in AP-4-subunit genes. These findings demonstrate that AP-4 assembly is not spontaneous but AAGAB-assisted, further contributing to the understanding of an adaptor protein complex that is critically involved in development of the central nervous system.
Targeting of soluble lysosomal enzymes requires mannose 6-phosphate (M6P) signals whose formation is initiated by the hexameric N-acetylglucosamine (GlcNAc)-1-phosphotransferase complex (α2β2γ2). ...Upon proteolytic cleavage by site-1 protease, the α/β-subunit precursor is catalytically activated but the functions of γ-subunits (Gnptg) in M6P modification of lysosomal enzymes are unknown. To investigate this, we analyzed the Gnptg expression in mouse tissues, primary cultured cells, and in Gnptg reporter mice in vivo, and found high amounts in the brain, eye, kidney, femur, vertebra and fibroblasts. Consecutively we performed comprehensive quantitative lysosomal proteome and M6P secretome analysis in fibroblasts of wild-type and Gnptgko mice mimicking the lysosomal storage disorder mucolipidosis III. Although the cleavage of the α/β-precursor was not affected by Gnptg deficiency, the GlcNAc-1-phosphotransferase activity was significantly reduced. We purified lysosomes and identified 29 soluble lysosomal proteins by SILAC-based mass spectrometry exhibiting differential abundance in Gnptgko fibroblasts which was confirmed by Western blotting and enzymatic activity analysis for selected proteins. A subset of these lysosomal enzymes show also reduced M6P modifications, fail to reach lysosomes and are secreted, among them α-l-fucosidase and arylsulfatase B. Low levels of these enzymes correlate with the accumulation of non-degraded fucose-containing glycostructures and sulfated glycosaminoglycans in Gnptgko lysosomes. Incubation of Gnptgko fibroblasts with arylsulfatase B partially rescued glycosaminoglycan storage. Combinatorial treatments with other here identified missorted enzymes of this degradation pathway might further correct glycosaminoglycan accumulation and will provide a useful basis to reveal mechanisms of selective, Gnptg-dependent formation of M6P residues on lysosomal proteins.
BORC is a multisubunit complex previously shown to promote coupling of mammalian lysosomes and C. elegans synaptic vesicle (SV) precursors (SVPs) to kinesins for anterograde transport of these ...organelles along microtubule tracks. We attempted to meld these observations into a unified model for axonal transport in mammalian neurons by testing two alternative hypotheses: (1) that SV and lysosomal proteins are co-transported within a single type of “lysosome-related vesicle” and (2) that SVPs and lysosomes are distinct organelles, but both depend on BORC for axonal transport. Analyses of various types of neurons from wild-type rats and mice, as well as from BORC-deficient mice, show that neither hypothesis is correct. We find that SVPs and lysosomes are transported separately, but only lysosomes depend on BORC for axonal transport in these neurons. These findings demonstrate that SVPs and lysosomes are distinct organelles that rely on different machineries for axonal transport in mammalian neurons.
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•Lysosomes and synaptic vesicle precursors (SVPs) move separately in mammalian axons•BORC is required for transport of lysosomes but not SVPs in mammalian axons•BORC-subunit knockout causes axon swellings and neonatal death in mice•Defective axonal lysosome transport may underlie pathology of human BORC deficiency
De Pace et al. show that lysosomes and synaptic vesicle precursors (SVPs) are distinct organelles that move separately from the soma to the axon in rat and mouse neurons. Moreover, they demonstrate that the BLOC-1-related complex (BORC) is required for the transport of lysosomes but not SVPs in mouse neurons.
Spastic paraplegia 50 (SPG50) is an ultrarare childhood-onset neurological disorder caused by biallelic loss-of-function variants in the AP4M1 gene. SPG50 is characterized by progressive spastic ...paraplegia, global developmental delay, and subsequent intellectual disability, secondary microcephaly, and epilepsy. We preformed preclinical studies evaluating an adeno-associated virus (AAV)/AP4M1 gene therapy for SPG50 and describe in vitro studies that demonstrate transduction of patient-derived fibroblasts with AAV2/AP4M1, resulting in phenotypic rescue. To evaluate efficacy in vivo, Ap4m1-KO mice were intrathecally (i.t.) injected with 5 × 1011, 2.5 × 1011, or 1.25 × 1011 vector genome (vg) doses of AAV9/AP4M1 at P7-P10 or P90. Age- and dose-dependent effects were observed, with early intervention and higher doses achieving the best therapeutic benefits. In parallel, three toxicology studies in WT mice, rats, and nonhuman primates (NHPs) demonstrated that AAV9/AP4M1 had an acceptable safety profile up to a target human dose of 1 × 1015 vg. Of note, similar degrees of minimal-to-mild dorsal root ganglia (DRG) toxicity were observed in both rats and NHPs, supporting the use of rats to monitor DRG toxicity in future i.t. AAV studies. These preclinical results identify an acceptably safe and efficacious dose of i.t.-administered AAV9/AP4M1, supporting an investigational gene transfer clinical trial to treat SPG50.
The hereditary spastic paraplegias (HSP) are a clinically and genetically heterogeneous group of disorders characterized by progressive lower limb spasticity. Mutations in subunits of the ...heterotetrameric (ε-β4-μ4-σ4) adaptor protein 4 (AP-4) complex cause an autosomal recessive form of complicated HSP referred to as "AP-4 deficiency syndrome". In addition to lower limb spasticity, this syndrome features intellectual disability, microcephaly, seizures, thin corpus callosum and upper limb spasticity. The pathogenetic mechanism, however, remains poorly understood. Here we report the characterization of a knockout (KO) mouse for the AP4E1 gene encoding the ε subunit of AP-4. We find that AP-4 ε KO mice exhibit a range of neurological phenotypes, including hindlimb clasping, decreased motor coordination and weak grip strength. In addition, AP-4 ε KO mice display a thin corpus callosum and axonal swellings in various areas of the brain and spinal cord. Immunohistochemical analyses show that the transmembrane autophagy-related protein 9A (ATG9A) is more concentrated in the trans-Golgi network (TGN) and depleted from the peripheral cytoplasm both in skin fibroblasts from patients with mutations in the μ4 subunit of AP-4 and in various neuronal types in AP-4 ε KO mice. ATG9A mislocalization is associated with increased tendency to accumulate mutant huntingtin (HTT) aggregates in the axons of AP-4 ε KO neurons. These findings indicate that the AP-4 ε KO mouse is a suitable animal model for AP-4 deficiency syndrome, and that defective mobilization of ATG9A from the TGN and impaired autophagic degradation of protein aggregates might contribute to neuroaxonal dystrophy in this disorder.
ABSTRACT
Mucolipidosis (ML) II and MLIII alpha/beta are two pediatric lysosomal storage disorders caused by mutations in the GNPTAB gene, which encodes an α/β‐subunit precursor protein of ...GlcNAc‐1‐phosphotransferase. Considerable variations in the onset and severity of the clinical phenotype in these diseases are observed. We report here on expression studies of two missense mutations c.242G>T (p.Trp81Leu) and c.2956C>T (p.Arg986Cys) and two frameshift mutations c.3503_3504delTC (p.Leu1168GlnfsX5) and c.3145insC (p.Gly1049ArgfsX16) present in severely affected MLII patients, as well as two missense mutations c.1196C>T (p.Ser399Phe) and c.3707A>T (p.Lys1236Met) reported in more mild affected individuals. We generated a novel α‐subunit‐specific monoclonal antibody, allowing the analysis of the expression, subcellular localization, and proteolytic activation of wild‐type and mutant α/β‐subunit precursor proteins by Western blotting and immunofluorescence microscopy. In general, we found that both missense and frameshift mutations that are associated with a severe clinical phenotype cause retention of the encoded protein in the endoplasmic reticulum and failure to cleave the α/β‐subunit precursor protein are associated with a severe clinical phenotype with the exception of p.Ser399Phe found in MLIII alpha/beta. Our data provide new insights into structural requirements for localization and activity of GlcNAc‐1‐phosphotransferase that may help to explain the clinical phenotype of MLII patients.
The lysosomal disorder mucolipidosis II (MLII) is caused by mutations in GNPTAB encoding an α/β‐subunit precursor protein of the oligomeric Golgi‐resident GlcNAc‐1‐phosphotransferase. This enzyme forms mannose 6‐phosphate targeting signals on lysosomal enzymes prerequisite for lysosomal function. Here, we analyzed the expression of six missense and frameshift mutations identified in patients and found that they either affect ER‐Golgi transport, site‐1 protease mediated proteolytic activation or mRNA stability of the α/β‐subunit precursor protein, and associate with different clinical courses of the disease
We report the development and characterization of a method, named reversible association with motor proteins (RAMP), for manipulation of organelle positioning within the cytoplasm. RAMP consists of ...coexpressing in cultured cells (i) an organellar protein fused to the streptavidin-binding peptide (SBP) and (ii) motor, neck, and coiled-coil domains from a plus-end-directed or minus-end-directed kinesin fused to streptavidin. The SBP-streptavidin interaction drives accumulation of organelles at the plus or minus end of microtubules, respectively. Importantly, competition of the streptavidin-SBP interaction by the addition of biotin to the culture medium rapidly dissociates the motor construct from the organelle, allowing restoration of normal patterns of organelle transport and distribution. A distinctive feature of this method is that organelles initially accumulate at either end of the microtubule network in the initial state and are subsequently released from this accumulation, allowing analyses of the movement of a synchronized population of organelles by endogenous motors.
Human polyomaviruses (HPyVs) cause persistent/latent infections in a large fraction of the population. HPyV infections may cause severe diseases in immunocompromised patients. Malawi polyomavirus ...(MWPyV) is the 10th discovered human polyomavirus (HPyV 10). MWPyV was found in stool samples of healthy children. So far, the few investigations carried out on HPyV 10 did not find an association with human disease.
In this study, to verify the putative association between MWPyV and human diseases, MWPyV seroprevalence was investigated in patients affected by i) lymphoproliferative disorders (LPDs) and ii) immune system disorders, i.e., autoimmune diseases (ADs), and in iii) healthy subjects. An indirect ELISA, employing virus-like particles (VLPs) to detect serum IgG antibodies against MWPyV/HPyV 10, was carried out. The study also revealed the prevalence of another polyomavirus, Merkel cell polyomavirus (MCPyV).
Sera from patients with distinct autoimmune diseases (
= 44; mean age 20 years) had a prevalence of MWPyV antibodies of 68%, while in patients with lymphoproliferative disorders (
= 15; mean age 14 years), subjected to bone marrow transplantation, the prevalence was 47%. In healthy subjects (
= 66; mean age 13 years), the prevalence of MWPyV antibodies was 67%. Our immunological investigation indicates that MWPyV/HPyV 10 seroconversion occurs early in life and MWPyV/HPyV 10 appears to be another polyomavirus ubiquitous in the human population. A significantly lower MWPyV antibody reactivity together with a lower immunological profile was detected in the sera of LPD patients compared with HS2 (*
< 0.05) (Fisher's exact test). LPD and AD patients have a similar MCPyV seroprevalence compared with healthy subjects.
MWPyV seroprevalence indicates that this HPyV is not associated with lymphoproliferative and autoimmune diseases. However, the ability to produce high levels of antibodies against MWPyV appears to be impaired in patients with lymphoproliferative disorders. Immunological investigations indicate that MWPyV seroconversion occurs early in life. MCPyV appears to be a ubiquitous polyomavirus, like other HPyVs, in the human population.
Niemann-Pick disease, type C (NPC) is a childhood-onset, lethal, neurodegenerative disorder caused by autosomal recessive mutations in the genes NPC1 or NPC2 and characterized by impaired cholesterol ...homeostasis, a lipid essential for cellular function. Cellular cholesterol levels are tightly regulated, and mutations in either NPC1 or NPC2 lead to deficient transport and accumulation of unesterified cholesterol in the late endosome/lysosome compartment, and progressive neurodegeneration in affected individuals. Previous cell-based studies to understand the NPC cellular pathophysiology and screen for therapeutic agents have mainly used patient fibroblasts. However, these do not allow modeling the neurodegenerative aspect of NPC disease, highlighting the need for an in vitro system that permits understanding the cellular mechanisms underlying neuronal loss and identifying appropriate therapies. This study reports the development of a novel human iPSC-derived, inducible neuronal model of Niemann-Pick disease, type C1 (NPC1).
We generated a null i3Neuron (inducible × integrated × isogenic) (NPC1
i
Neuron) iPSC-derived neuron model of NPC1. The NPC1
and the corresponding isogenic NPC1
i
Neuron cell lines were used to efficiently generate homogenous, synchronized neurons that can be used in high-throughput screens. NPC1
i
Neurons recapitulate cardinal cellular NPC1 pathological features including perinuclear endolysosomal storage of unesterified cholesterol, accumulation of GM2 and GM3 gangliosides, mitochondrial dysfunction, and impaired axonal lysosomal transport. Cholesterol storage, mitochondrial dysfunction, and axonal trafficking defects can be ameliorated by treatment with 2-hydroxypropyl-β-cyclodextrin, a drug that has shown efficacy in NPC1 preclinical models and in a phase 1/2a trial.
Our data demonstrate the utility of this new cell line in high-throughput drug/chemical screens to identify potential therapeutic agents. The NPC1
i
Neuron line will also be a valuable tool for the NPC1 research community to explore the pathological mechanisms contributing to neuronal degeneration.
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