Role of the Lysosomal Membrane Protein, CLN3, in the Regulation of Cathepsin D Activity

The role of lysosomal membrane proteins in autophagy and related diseases

As a self-degrading and highly conserved survival mechanism, autophagy plays an important role in maintaining cell survival and recycling. The discovery of autophagy-related (ATG) genes has revolutionized our understanding of autophagy. Lysosomal membrane proteins (LMPs) are important executors of lysosomal function, and increasing evidence has demonstrated their role in the induction and regulation of autophagy. In addition, the functional dysregulation of the process mediated by LMPs at all stages of autophagy is closely related to neurodegenerative diseases and cancer. Here, we review the role of LMPs in autophagy, focusing on their roles in vesicle nucleation, vesicle elongation and completion, the fusion of autophagosomes and lysosomes, and degradation, as well as their broad association with related diseases.

Integrative human and murine multi-omics: Highlighting shared biomarkers in the neuronal ceroid lipofuscinoses

Neuronal ceroid lipofuscinosis (NCL) is a group of neurodegenerative disorders whose molecular mechanisms remain largely unknown. Omics approaches are among the methods that generate new information on modifying factors and molecular signatures. Moreover, omics data integration can address the need to progressively expand knowledge around the disease and pinpoint specific proteins to promote as candidate biomarkers. In this work, we integrated a total of 62 proteomic and transcriptomic datasets originating from humans and mice, employing a new approach able to define dysregulated processes across species, stages and NCL forms. Moreover, we selected a pool of differentially expressed proteins and genes as species- and form-related biomarkers of disease status/progression and evaluated local and spatial differences in most affected brain regions. Our results offer promising targets for potential new therapeutic strategies and reinforce the hypothesis of a connection between NCLs and other forms of dementia, particularly Alzheimer's disease.

The conserved cellular roles of CLN proteins: Novel insights from Dictyostelium discoideum

The neuronal ceroid lipofuscinoses (NCLs), collectively referred to as Batten disease, are a group of fatal neurodegenerative disorders that primarily affect children. The etiology of Batten disease is linked to mutations in 13 genes that encode distinct CLN proteins, whose functions have yet to be fully elucidated. The social amoeba Dictyostelium discoideum has been adopted as an efficient and powerful model system for studying the diverse cellular roles of CLN proteins. The genome of D. discoideum encodes several homologs of human CLN proteins, and a growing body of literature supports the conserved roles and networking of CLN proteins in D. discoideum and humans. In humans, CLN proteins have diverse cellular roles related to autophagy, signal transduction, lipid homeostasis, lysosomal ion homeostasis, and intracellular trafficking. Recent work also indicates that CLN proteins play an important role in protein secretion. Remarkably, many of these findings have found parallels in studies with D. discoideum. Accordingly, this review will highlight the translatable value of novel work with D. discoideum in the field of NCL research and propose further avenues of research using this biomedical model organism for studying the NCLs.

A modified density gradient proteomic-based method to analyze endolysosomal proteins in cardiac tissue

The importance of lysosomes in cardiac physiology and pathology is well established, and evidence for roles in calcium signaling is emerging. We describe a label-free proteomics method suitable for small cardiac tissue biopsies based on density-separated fractionation, which allows study of endolysosomal (EL) proteins. Density gradient fractions corresponding to tissue lysate; sarcoplasmic reticulum (SR), mitochondria (Mito) (1.3 g/mL); and EL with negligible contamination from SR or Mito (1.04 g/mL) were analyzed using Western blot, enzyme activity assay, and liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis (adapted discontinuous Percoll and sucrose differential density gradient). Kyoto Encyclopedia of Genes and Genomes, Reactome, Panther, and Gene Ontology pathway analysis showed good coverage of RAB proteins and lysosomal cathepsins (including cardiac-specific cathepsin D) in the purified EL fraction. Significant EL proteins recovered included catalytic activity proteins. We thus present a comprehensive protocol and data set of guinea pig atrial EL organelle proteomics using techniques also applicable for non-cardiac tissue.

A novel deletion variant in CLN3 with highly variable expressivity is responsible for juvenile neuronal ceroid lipofuscinoses

Mutations in CLN3 (OMIM: 607042) are associated with juvenile neuronal ceroid lipofuscinoses (JNCL)-a rare neurodegenerative disease with early retinal degeneration and progressive neurologic deterioration. The study aimed to determine the underlying genetic factors justifying the NCL phenotype in a large Iraqi consanguineous family. Four affected individuals with an initial diagnosis of NCL were recruited. By doing neuroimaging and also pertinent clinical examinations, e.g. fundus examination, due to heterogeneity of neurodevelopmental disorders, the proband was subjected to the paired-end whole-exome sequencing to identify underlying genetic factors. The candidate variant was also confirmed by Sanger sequencing. Various in silico predictions were used to show the pathogenicity of the variant. This study revealed a novel homozygous frameshift variant-NM_000086.2: c.1127del; p.(Leu376Argfs*15)-in the exon 14 of the CLN3 gene as the most likely disease-causing variant. Three out of 4 patients showed bilateral vision loss (< 7 years) and retinal degeneration with macular changes in both eyes. Electroencephalography demonstrated the loss of normal posterior alpha rhythm and also low amplitude multifocal slow waves. Brain magnetic resonance imaging of the patients with a high degree of deterioration showed mild cerebral and cerebellar cortical atrophy, mild ventriculomegaly, thinning of the corpus callosum and vermis, and non-specific periventricular white matter signal changes in the occipital area. The novel biallelic deletion variant of CLN3 was identified that most probably led to JNCL with variable expressivity of the phenotype. This study also expanded our understanding of the clinical and genetic spectrum of JNCL.

Differential accumulation of storage bodies with aging defines discrete subsets of microglia in the healthy brain

To date, microglia subsets in the healthy CNS have not been identified. Utilizing autofluorescence (AF) as a discriminating parameter, we identified two novel microglia subsets in both mice and non-human primates, termed autofluorescence-positive (AF+) and negative (AF-). While their proportion remained constant throughout most adult life, the AF signal linearly and specifically increased in AF+ microglia with age and correlated with a commensurate increase in size and complexity of lysosomal storage bodies, as detected by transmission electron microscopy and LAMP1 levels. Post-depletion repopulation kinetics revealed AF- cells as likely precursors of AF+ microglia. At the molecular level, the proteome of AF+ microglia showed overrepresentation of endolysosomal, autophagic, catabolic, and mTOR-related proteins. Mimicking the effect of advanced aging, genetic disruption of lysosomal function accelerated the accumulation of storage bodies in AF+ cells and led to impaired microglia physiology and cell death, suggestive of a mechanistic convergence between aging and lysosomal storage disorders.

Molecular networking in the neuronal ceroid lipofuscinoses: insights from mammalian models and the social amoeba Dictyostelium discoideum

The neuronal ceroid lipofuscinoses (NCLs), commonly known as Batten disease, belong to a family of neurological disorders that cause blindness, seizures, loss of motor function and cognitive ability, and premature death. There are 13 different subtypes of NCL that are associated with mutations in 13 genetically distinct genes (CLN1-CLN8, CLN10-CLN14). Similar clinical and pathological profiles of the different NCL subtypes suggest that common disease mechanisms may be involved. As a result, there have been many efforts to determine how NCL proteins are connected at the cellular level. A main driving force for NCL research has been the utilization of mammalian and non-mammalian cellular models to study the mechanisms underlying the disease. One non-mammalian model that has provided significant insight into NCL protein function is the social amoeba Dictyostelium discoideum. Accumulated data from Dictyostelium and mammalian cells show that NCL proteins display similar localizations, have common binding partners, and regulate the expression and activities of one another. In addition, genetic models of NCL display similar phenotypes. This review integrates findings from Dictyostelium and mammalian models of NCL to highlight our understanding of the molecular networking of NCL proteins. The goal here is to help set the stage for future work to reveal the cellular mechanisms underlying the NCLs.

Proteomic analysis of intracellular protein corona of nanoparticles elucidates nano-trafficking network and nano-bio interactions

The merits of nanomedicines are significantly impacted by the surrounding biological environment. Similar to the protein corona generated on the surface of nanoparticles in the circulation system, the intracellular protein corona (IPC) might be formed on nanoparticles when transported inside the cells. However, little is known currently about the formation of IPC and its possible biological influence.

Methods: Caco-2 cells, a classical epithelial cell line, were cultured in Transwell plates to form a monolayer. Gold nanoparticles (AuNPs) were prepared as the model nanomedicine due to their excellent stability. Here we focused on identifying IPC formed on the surface of AuNPs during cell transport. The nanoparticles in the basolateral side of the Caco-2 monolayer were collected and analyzed by multiple techniques to verify IPC formation. High-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based proteomics was utilized to analyze the composition of IPC proteins. In particular, we established a dual-filtration strategy to exclude various interference in IPC identification. Based on the subcellular localization of specific IPC proteins, we elicited the nano-trafficking network of AuNPs. The transport pathways of AuNPs identified by proteomic analysis were also verified by various conventional technologies. Finally, we explored the influence of IPC on the uptake and stress response of endothelium.

Results: The existence of IPC was demonstrated on the surface of AuNPs, in which 227 proteins were identified. Among them, 40 proteins were finally ascertained as the specific IPC proteins. The subcellular location analysis indicated that these “specific” IPC proteins could back-track the transport pathways of nanoparticles in the epithelial cell monolayer. According to the subcellular distribution of IPC proteins and co-localization, we discovered a new pathway of nanoparticles from endosomes to secretory vesicles which was dominant during the transcytosis. After employing conventional imageology and pharmacology strategies to verify the result of proteomic analysis, we mapped a comprehensive intracellular transport network. Our study also revealed the merits of IPC analysis, which could readily elucidate the molecular mechanisms of transcytosis. Besides, the IPC proteins increased the uptake and stress response of endothelium, which was likely mediated by extracellular matrix and mitochondrion-related IPC proteins.

Conclusion: The comprehensive proteomic analysis of IPC enabled tracing of transport pathways in epithelial cells as well as revealing the biological impact of nanoparticles on endothelium.

Synapse alterations precede neuronal damage and storage pathology in a human cerebral organoid model of CLN3-juvenile neuronal ceroid lipofuscinosis

The juvenile form of neuronal ceroid Lipofuscinosis (JNCL) is the most common form within this group of rare lysosomal storage disorders, causing pediatric neurodegeneration. The genetic disorder, which is caused by recessive mutations affecting the CLN3 gene, features progressive vision loss, cognitive and motor decline and other psychiatric conditions, seizure episodes, leading to premature death. Animal models have traditionally aid the understanding of the disease mechanisms and pathology and are very relevant for biomarker research and therapeutic testing. Nevertheless, there is a need for establishing reliable and predictive human cellular models to study the disease. Since patient material, particularly from children, is scarce and difficult to obtain, we generated an engineered a CLN3-mutant isogenic human induced pluripotent stem cell (hiPSC) line carrying the c.1054C → T pathologic variant, using state of the art CRISPR/Cas9 technology. To prove the suitability of the isogenic pair to model JNCL, we screened for disease-specific phenotypes in non-neuronal two-dimensional cell culture models as well as in cerebral brain organoids. Our data demonstrates that the sole introduction of the pathogenic variant gives rise to classical hallmarks of JNCL in vitro. Additionally, we discovered an alteration of the splicing caused by this particular mutation. Next, we derived cerebral organoids and used them as a neurodevelopmental model to study the particular effects of the CLN3^Q352X mutation during brain formation in the disease context. About half of the mutation -carrying cerebral organoids completely failed to develop normally. The other half, which escaped this severe defect were used for the analysis of more subtle alterations. In these escapers, whole-transcriptome analysis demonstrated early disease signatures, affecting pathways related to development, corticogenesis and synapses. Complementary metabolomics analysis confirmed decreased levels of cerebral tissue metabolites, some particularly relevant for synapse formation and neurotransmission, such as gamma-amino butyric acid (GABA). Our data suggests that a mutation in CLN3 severely affects brain development. Furthermore, before disease onset, disease -associated neurodevelopmental changes, particular concerning synapse formation and function, occur.

The contribution of multicellular model organisms to neuronal ceroid lipofuscinosis research

The NCLs (neuronal ceroid lipofuscinosis) are forms of neurodegenerative disease that affect people of all ages and ethnicities but are most prevalent in children. Commonly known as Batten disease, this debilitating neurological disorder is comprised of 13 different subtypes that are categorized based on the particular gene that is mutated (CLN1-8, CLN10-14). The pathological mechanisms underlying the NCLs are not well understood due to our poor understanding of the functions of NCL proteins. Only one specific treatment (enzyme replacement therapy) is approved, which is for the treating the brain in CLN2 disease. Hence there remains a desperate need for further research into disease-modifying treatments. In this review, we present and evaluate the genes, proteins and studies performed in the social amoeba, nematode, fruit fly, zebrafish, mouse and large animals pertinent to NCL. In particular, we highlight the use of multicellular model organisms to study NCL protein function, pathology and pathomechanisms. Their use in testing novel therapeutic approaches is also presented. With this information, we highlight how future research in these systems may be able to provide new insight into NCL protein functions in human cells and aid in the development of new therapies.

Molecular determinants as therapeutic targets in cancer chemotherapy: An update

It is well known that cancer cells are heterogeneous in nature and very distinct from their normal counterparts. Commonly these cancer cells possess different and complementary metabolic profile, microenvironment and adopting behaviors to generate more ATPs to fulfill the requirement of high energy that is further utilized in the production of proteins and other essentials required for cell survival, growth, and proliferation. These differences create many challenges in cancer treatments. On the contrary, such situations of metabolic differences between cancer and normal cells may be expected a promising strategy for treatment purpose. In this article, we focus on the molecular determinants of oncogene-specific sub-organelles such as potential metabolites of mitochondria (reactive oxygen species, apoptotic proteins, cytochrome c, caspase 9, caspase 3, etc.), endoplasmic reticulum (unfolded protein response, PKR-like ER kinase, C/EBP homologous protein, etc.), nucleus (nucleolar phosphoprotein, nuclear pore complex, nuclear localization signal), lysosome (microenvironment, etc.) and plasma membrane phospholipids, etc. that might be exploited for the targeted delivery of anti-cancer drugs for therapeutic benefits. This review will help to understand the various targets of subcellular organelles at molecular levels. In the future, this molecular level understanding may be combined with the genomic profile of cancer for the development of the molecularly guided or personalized therapeutics for complete eradication of cancer.

Comparative transcriptomics reveals mechanisms underlying cln3-deficiency phenotypes in Dictyostelium

Mutations in CLN3 cause a juvenile form of neuronal ceroid lipofuscinosis (NCL). This devastating neurological disorder, commonly known as Batten disease, is currently untreatable due to a lack of understanding of the physiological role of the protein. Recently, work in the social amoeba Dictyostelium discoideum has provided valuable new insight into the function of CLN3 in the cell. More specifically, research has linked the Dictyostelium homolog (gene: cln3, protein: Cln3) to protein secretion, adhesion, and aggregation during starvation, which initiates multicellular development. In this study, we used comparative transcriptomics to explore the mechanisms underlying the aberrant response of cln3^- cells to starvation. During starvation, 1153 genes were differentially expressed in cln3^- cells compared to WT. Among the differentially expressed genes were homologs of other human NCL genes including TPP1/CLN2, CLN5, CTSD/CLN10, PGRN/CLN11, and CTSF/CLN13. STRING and GO term analyses revealed an enrichment of genes linked to metabolic, biosynthetic, and catalytic processes. We then coupled the findings from the RNA-seq analysis to biochemical assays, specifically showing that loss of cln3 affects the expression and activity of lysosomal enzymes, increases endo-lysosomal pH, and alters nitric oxide homeostasis. Finally, we show that cln3^- cells accumulate autofluorescent storage bodies during starvation and provide evidence linking the function of Cln3 to Tpp1 and CtsD activity. In total, this study enhances our knowledge of the molecular mechanisms underlying Cln3 function in Dictyostelium.

Recent Insights into NCL Protein Function Using the Model Organism Dictyostelium discoideum

The neuronal ceroid lipofuscinoses (NCLs) are a group of devastating neurological disorders that have a global distribution and affect people of all ages. Commonly known as Batten disease, this form of neurodegeneration is linked to mutations in 13 genetically distinct genes. The precise mechanisms underlying the disease are unknown, in large part due to our poor understanding of the functions of NCL proteins. The social amoeba Dictyostelium discoideum has proven to be an exceptional model organism for studying a wide range of neurological disorders, including the NCLs. The Dictyostelium genome contains homologs of 11 of the 13 NCL genes. Its life cycle, comprised of both single-cell and multicellular phases, provides an excellent system for studying the effects of NCL gene deficiency on conserved cellular and developmental processes. In this review, we highlight recent advances in NCL research using Dictyostelium as a biomedical model.

Multi-hierarchical profiling the structure-activity relationships of engineered nanomaterials at nano-bio interfaces

Increasing concerns over the possible risks of nanotechnology necessitates breakthroughs in structure-activity relationship (SAR) analyses of engineered nanomaterials (ENMs) at nano-bio interfaces. However, current nano-SARs are often based on univariate assessments and fail to provide tiered views on ENM-induced bio-effects. Here we report a multi-hierarchical nano-SAR assessment for a representative ENM, Fe2O3, by metabolomics and proteomics analyses. The established nano-SAR profile allows the visualizing of the contributions of seven basic properties of Fe2O3 to its diverse bio-effects. For instance, although surface reactivity is responsible for Fe2O3-induced cell migration, the inflammatory effects of Fe2O3 are determined by aspect ratio (nanorods) or surface reactivity (nanoplates). These nano-SARs are examined in THP-1 cells and animal lungs, which allow us to decipher the detailed mechanisms including NLRP3 inflammasome pathway and monocyte chemoattractant protein-1-dependent signaling. This study provides more insights for nano-SARs, and may facilitate the tailored design of ENMs to render them desired bio-effects.

Immunomodulation with minocycline rescues retinal degeneration in juvenile neuronal ceroid lipofuscinosis mice highly susceptible to light damage

Juvenile neuronal ceroid lipofuscinosis (jNCL) is a rare but fatal inherited lysosomal storage disorder mainly affecting children. The disease is caused by mutations in the CLN3 gene that lead to the accumulation of storage material in many tissues, prominent immune responses and neuronal degeneration. One of the first symptoms is vision loss followed by motor dysfunction and mental decline. The established Cln3^Δex7/8 mouse model mimics many pathological features of the human disease except the retinal phenotype, which is very mild and occurs only very late in these mice. Here, we first carefully analyzed the retinal structure and microglia responses in these animals. While prominent autofluorescent spots were present in the fundus, only a moderate reduction of retinal thickness and no prominent microgliosis was seen in young CLN3-deficient mice. We next genetically introduced a light-sensitive RPE65 variant and established a light-damage paradigm that showed a high susceptibility of young Cln3^Δex7/8 mice after exposure to 10,000 lux bright light for 30 min. Under these ‘low light’ conditions, CLN3-deficient mice showed a strong retinal degeneration, microglial activation, deposition of autofluorescent material and transcriptomic changes compared to wild-type animals. Finally, we treated the light-exposed Cln3^Δex7/8 animals with the immunomodulatory compound minocycline, and thereby rescued the retinal phenotype and diminished microgliosis. Our findings indicate that exposure to specific light conditions accelerates CLN3-dependent retinal degeneration, and that immunomodulation by minocycline could be a possible treatment option to delay vision loss in jNCL patients.

This article has an associated First Person interview with the first author of the paper.

Summary: Here, we established a light-damage paradigm to model retinal degeneration in the juvenile neuronal ceroid lipofuscinosis mouse and showed the beneficial effects of minocycline on retinal pathology.

Caspase 1 activity influences juvenile Batten disease (CLN3) pathogenesis

Juvenile Neuronal Ceroid Lipofuscinosis (JNCL) is an autosomal recessive lysosomal storage disease caused by loss-of-function mutations in CLN3. Symptoms appear between 5 and 10 years of age, beginning with blindness and seizures, followed by progressive cognitive and motor decline, and premature death. Glial activation and impaired neuronal activity are early signs of pathology in the Cln3^Δex7/8 mouse model of JNCL, whereas neuron death occurs much later in the disease process. We previously reported that Cln3^Δex7/8 microglia are primed toward a pro-inflammatory phenotype typified by exaggerated caspase 1 inflammasome activation and here we extend those findings to demonstrate heightened caspase activity in the Cln3^Δex7/8 mouse brain. Based on the ability of caspase 1 to cleave a large number of substrates that have been implicated in JNCL pathology, we examined the functional implications of caspase 1 inflammasome activity by crossing Cln3^Δex7/8 and caspase 1-deficient mice to create Cln3^Δex7/8 /Casp-1^-/- animals. Caspase 1 deletion influenced motor behavior deficits and astrocyte activation in the context of CLN3 mutation, since both were significantly reversed in Cln3^Δex7/8 /Casp-1^-/- mice, with phenotypes approaching that of wild-type animals. We also report a progressive age-dependent reduction in whisker length in Cln3^Δex7/8 mice that was partially caspase 1-dependent. However, not all CLN3 phenotypes were reversed following caspase 1 deletion, since no significant differences in lysosomal accumulation or microglial activation were observed between Cln3^Δex7/8 and Cln3^Δex7/8 /Casp-1^-/- mice. Although the molecular targets of aberrant caspase 1 activity in the context of CLN3 mutation remain to be identified, our studies suggest that caspase 1 may represent a potential therapeutic target to mitigate some attributes of CLN3 disease. This article is part of the Special Issue "Lysosomal Storage Disorders".

Multifaceted effects of astragaloside IV on promotion of random pattern skin flap survival in rats

Random pattern skin flap transplantation is frequently applied in plastic and reconstructive surgery, but the distal part of skin flaps often suffers necrosis due to ischemia. Astragaloside IV (AS-IV), a natural saponin purified from Astragalus membranaceus, may have beneficial functions for flap survival. In this study, rats were divided into a control group and an AS-IV treatment group, and underwent surgery using a modified "McFarlane flap" model. After intragastric administration of vehicle control or AS-IV for their respective groups, flap survival area and water content were measured 7 days after surgery. Flap tissue was separated to test protein expressions related to angiogenesis, inflammation, oxidative stress and autophagy via western blot, immunohistochemistry, and immunofluorescence. Results showed that AS-IV improved flap survival area and reduced tissue edema. AS-IV also increased mean vessel densities and upregulated levels of VEGF protein, both of which indicate increased angiogenesis. Furthermore, AS-IV depressed leukocyte infiltration, decreased expressions of inflammatory proteins TNF-α, IL1β and IL6, increased SOD activity, decreased MDA content, and stimulated autophagy. Overall, our results suggest that AS-IV promotes skin flap survival via inducing angiogenesis, depressing inflammation and dampening oxidative stress; it also activates autophagy, which may be an underlying mechanism for oxidative stress depression.