Genetic modifiers of Drosophila palmitoyl-protein thioesterase 1-induced degeneration

Lost in traffic: consequences of altered palmitoylation in neurodegeneration

One of the first molecular events in neurodegenerative diseases, regardless of etiology, is protein mislocalization. Protein mislocalization in neurons is often linked to proteostasis deficiencies leading to the build-up of misfolded proteins and/or organelles that contributes to cellular toxicity and cell death. By understanding how proteins mislocalize in neurons, we can develop novel therapeutics that target the earliest stages of neurodegeneration. A critical mechanism regulating protein localization and proteostasis in neurons is the protein-lipid modification S-acylation, the reversible addition of fatty acids to cysteine residues. S-acylation is more commonly referred to as S-palmitoylation or simply palmitoylation, which is the addition of the 16-carbon fatty acid palmitate to proteins. Like phosphorylation, palmitoylation is highly dynamic and tightly regulated by writers (i.e., palmitoyl acyltransferases) and erasers (i.e., depalmitoylating enzymes). The hydrophobic fatty acid anchors proteins to membranes; thus, the reversibility allows proteins to be re-directed to and from membranes based on local signaling factors. This is particularly important in the nervous system, where axons (output projections) can be meters long. Any disturbance in protein trafficking can have dire consequences. Indeed, many proteins involved in neurodegenerative diseases are palmitoylated, and many more have been identified in palmitoyl-proteomic studies. It follows that palmitoyl acyl transferase enzymes have also been implicated in numerous diseases. In addition, palmitoylation can work in concert with cellular mechanisms, like autophagy, to affect cell health and protein modifications, such as acetylation, nitrosylation, and ubiquitination, to affect protein function and turnover. Limited studies have further revealed a sexually dimorphic pattern of protein palmitoylation. Therefore, palmitoylation can have wide-reaching consequences in neurodegenerative diseases.

Identification of substrates of palmitoyl protein thioesterase 1 highlights roles of depalmitoylation in disulfide bond formation and synaptic function

Loss-of-function mutations in the depalmitoylating enzyme palmitoyl protein thioesterase 1 (PPT1) cause neuronal ceroid lipofuscinosis (NCL), a devastating neurodegenerative disease. The substrates of PPT1 are largely undescribed, posing a limitation on molecular dissection of disease mechanisms and therapeutic development. Here, we provide a resource identifying >100 novel PPT1 substrates. We utilized Acyl Resin-Assisted Capture (Acyl RAC) and mass spectrometry to identify proteins with increased in vivo palmitoylation in PPT1 knockout (KO) mouse brains. We then validated putative substrates through direct depalmitoylation with recombinant PPT1. This stringent screen elucidated diverse PPT1 substrates at the synapse, including channels and transporters, G-protein–associated molecules, endo/exocytic components, synaptic adhesion molecules, and mitochondrial proteins. Cysteine depalmitoylation sites in transmembrane PPT1 substrates frequently participate in disulfide bonds in the mature protein. We confirmed that depalmitoylation plays a role in disulfide bond formation in a tertiary screen analyzing posttranslational modifications (PTMs). Collectively, these data highlight the role of PPT1 in mediating synapse functions, implicate molecular pathways in the etiology of NCL and other neurodegenerative diseases, and advance our basic understanding of the purpose of depalmitoylation.

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.

Depalmitoylation by Palmitoyl-Protein Thioesterase 1 in Neuronal Health and Degeneration

Protein palmitoylation is the post-translational, reversible addition of a 16-carbon fatty acid, palmitate, to proteins. Protein palmitoylation has recently garnered much attention, as it robustly modifies the localization and function of canonical signaling molecules and receptors. Protein depalmitoylation, on the other hand, is the process by which palmitic acid is removed from modified proteins and contributes, therefore, comparably to palmitoylated-protein dynamics. Palmitoylated proteins also require depalmitoylation prior to lysosomal degradation, demonstrating the significance of this process in protein sorting and turnover. Palmitoylation and depalmitoylation serve as particularly crucial regulators of protein function in neurons, where a specialized molecular architecture and cholesterol-rich membrane microdomains contribute to synaptic transmission. Three classes of depalmitoylating enzymes are currently recognized, the acyl protein thioesterases, α/β hydrolase domain-containing 17 proteins (ABHD17s), and the palmitoyl-protein thioesterases (PPTs). However, a clear picture of depalmitoylation has not yet emerged, in part because the enzyme-substrate relationships and specific functions of depalmitoylation are only beginning to be uncovered. Further, despite the finding that loss-of-function mutations affecting palmitoyl-protein thioesterase 1 (PPT1) function cause a severe pediatric neurodegenerative disease, the role of PPT1 as a depalmitoylase has attracted relatively little attention. Understanding the role of depalmitoylation by PPT1 in neuronal function is a fertile area for ongoing basic science and translational research that may have broader therapeutic implications for neurodegeneration. Here, we will briefly introduce the rapidly growing field surrounding protein palmitoylation and depalmitoylation, then will focus on the role of PPT1 in development, health, and neurological disease.

Decreased sensitivity of palmitoyl protein thioesterase 1-deficient neurons to chemical anoxia

Infantile CLN1 disease, also known as infantile neuronal ceroid lipofuscinosis, is a fatal childhood neurodegenerative disorder caused by mutations in the CLN1 gene. CLN1 encodes a soluble lysosomal enzyme, palmitoyl protein thioesterase 1 (PPT1), and it is still unclear why neurons are selectively vulnerable to the loss of PPT1 enzyme activity in infantile CLN1 disease. To examine the effects of PPT1 deficiency on several well-defined neuronal signaling and cell death pathways, different toxic insults were applied in cerebellar granule neuron cultures prepared from wild type (WT) and palmitoyl protein thioesterase 1-deficient (Ppt1 ^-/- ) mice, a model of infantile CLN1 disease. Glutamate uptake inhibition by t-PDC (L-trans-pyrrolidine-2,4-dicarboxylic acid) or Zn²⁺-induced general mitochondrial dysfunction caused similar toxicity in WT and Ppt1 ^-/- cultures. Ppt1 ^-/- neurons, however, were more sensitive to mitochondrial complex I inhibition by MPP⁺ (1-methyl-4-phenylpyridinium), and had significantly decreased sensitivity to chemical anoxia induced by the mitochondrial complex IV inhibitor, sodium azide. Our results indicate that PPT1 deficiency causes alterations in the mitochondrial respiratory chain.

The neuronal ceroid lipofuscinoses: Opportunities from model systems

The neuronal ceroid lipofuscinoses are a group of severe and progressive neurodegenerative disorders, generally with childhood onset. Despite the fact that these diseases remain fatal, significant breakthroughs have been made in our understanding of the genetics that underpin these conditions. This understanding has allowed the development of a broad range of models to study disease processes, and to develop new therapeutic approaches. Such models have contributed significantly to our knowledge of these conditions. In this review we will focus on the advantages of each individual model, describe some of the contributions the models have made to our understanding of the broader disease biology and highlight new techniques and approaches relevant to the study and potential treatment of the neuronal ceroid lipofuscinoses. This article is part of a Special Issue entitled: "Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease)".

An organellar nα-acetyltransferase, naa60, acetylates cytosolic N termini of transmembrane proteins and maintains Golgi integrity

N-terminal acetylation is a major and vital protein modification catalyzed by N-terminal acetyltransferases (NATs). NatF, or Nα-acetyltransferase 60 (Naa60), was recently identified as a NAT in multicellular eukaryotes. Here, we find that Naa60 differs from all other known NATs by its Golgi localization. A new membrane topology assay named PROMPT and a selective membrane permeabilization assay established that Naa60 faces the cytosolic side of intracellular membranes. An Nt-acetylome analysis of NAA60-knockdown cells revealed that Naa60, as opposed to other NATs, specifically acetylates transmembrane proteins and has a preference for N termini facing the cytosol. Moreover, NAA60 knockdown causes Golgi fragmentation, indicating an important role in the maintenance of the Golgi's structural integrity. This work identifies a NAT associated with membranous compartments and establishes N-terminal acetylation as a common modification among transmembrane proteins, a thus-far poorly characterized part of the N-terminal acetylome.

Genetic studies in Drosophila and humans support a model for the concerted function of CISD2, PPT1 and CLN3 in disease

Wolfram syndrome (WFS) is a progressive neurodegenerative disease characterized by diabetes insipidus, diabetes mellitus, optic atrophy, and deafness. WFS1 and WFS2 are caused by recessive mutations in the genes Wolfram Syndrome 1 (WFS1) and CDGSH iron sulfur domain 2 (CISD2), respectively. To explore the function of CISD2, we performed genetic studies in flies with altered expression of its Drosophila orthologue, cisd2. Surprisingly, flies with strong ubiquitous RNAi-mediated knockdown of cisd2 had no obvious signs of altered life span, stress resistance, locomotor behavior or several other phenotypes. We subsequently found in a targeted genetic screen, however, that altered function of cisd2 modified the effects of overexpressing the fly orthologues of two lysosomal storage disease genes, palmitoyl-protein thioesterase 1 (PPT1 in humans, Ppt1 in flies) and ceroid-lipofuscinosis, neuronal 3 (CLN3 in humans, cln3 in flies), on eye morphology in flies. We also found that cln3 modified the effects of overexpressing Ppt1 in the eye and that overexpression of cln3 interacted with a loss of function mutation in cisd2 to disrupt locomotor ability in flies. Follow-up multi-species bioinformatic analyses suggested that a gene network centered on CISD2, PPT1 and CLN3 might impact disease through altered carbohydrate metabolism, protein folding and endopeptidase activity. Human genetic studies indicated that copy number variants (duplications and deletions) including CLN3, and possibly another gene in the CISD2/PPT1/CLN3 network, are over-represented in individuals with developmental delay. Our studies indicate that cisd2, Ppt1 and cln3 function in concert in flies, suggesting that CISD2, PPT1 and CLN3 might also function coordinately in humans. Further, our studies raise the possibility that WFS2 and some lysosomal storage disorders might be influenced by common mechanisms and that the underlying genes might have previously unappreciated effects on developmental delay.

Palmitoyl-protein thioesterase 1 (PPT1): an obesity-induced rat testicular marker of reduced fertility

Male obesity may lead to declines in testosterone levels, reproductive hormonal profile, and semen quantity. To assess the effects of obesity on spermatogenesis, Sprague-Dawley rats fed a high-fat diet served as a model of induced obesity. The litter sizes for females mated to obese males were significantly lower as compared to females mated with normal-diet-fed controls. Their serum high-density lipoprotein, low-density lipoprotein, cholesterol, and estradiol levels increased in obese males, but testosterone and follicle-stimulating hormone levels decreased. Testicular morphology disruptions included Sertoli-cell atrophy, disrupted tight junctions, and mitochondrial degeneration in spermatogenic cells. To further investigate the molecular mechanisms leading to high-fat-diet-induced changes, we employed testicular proteomic analysis on rats fed both types of diet. Three spots were up-regulated in rats fed a high-fat diet whereas two others were downregulated. One of the upregulated spots was palmitoyl-protein thioesterase 1 (PPT1), a lipoprotein metabolizing related enzyme localized to Sertoli cells. In a Sertoli-cell line cultured in a high-fat supplemented medium, PPT1 abundance was accompanied by increases in the endocytic vesicle-associated protein, clathrin, and decreases in the tight junctional proteins, ZO-1 and occludin. In conclusion, declines in rat male fertility induced by a high-fat diet are associated with an altered testicular protein expression pattern as well as disruption of testicular Sertoli-cell and spermatogenic-cell morphology. PPT1 expression may provide a testicular marker of reduced fertility in obese males, as increases in its expression may be detrimental to Sertoli-cell function during spermatogenesis.

A murine model of variant late infantile ceroid lipofuscinosis recapitulates behavioral and pathological phenotypes of human disease

Neuronal ceroid lipofuscinoses (NCLs; also known collectively as Batten Disease) are a family of autosomal recessive lysosomal storage disorders. Mutations in as many as 13 genes give rise to ∼10 variants of NCL, all with overlapping clinical symptomatology including visual impairment, motor and cognitive dysfunction, seizures, and premature death. Mutations in CLN6 result in both a variant late infantile onset neuronal ceroid lipofuscinosis (vLINCL) as well as an adult-onset form of the disease called Type A Kufs. CLN6 is a non-glycosylated membrane protein of unknown function localized to the endoplasmic reticulum (ER). In this study, we perform a detailed characterization of a naturally occurring Cln6 mutant (Cln6^nclf) mouse line to validate its utility for translational research. We demonstrate that this Cln6^nclf mutation leads to deficits in motor coordination, vision, memory, and learning. Pathologically, we demonstrate loss of neurons within specific subregions and lamina of the cortex that correlate to behavioral phenotypes. As in other NCL models, this model displays selective loss of GABAergic interneuron sub-populations in the cortex and the hippocampus with profound, early-onset glial activation. Finally, we demonstrate a novel deficit in memory and learning, including a dramatic reduction in dendritic spine density in the cerebral cortex, which suggests a reduction in synaptic strength following disruption in CLN6. Together, these findings highlight the behavioral and pathological similarities between the Cln6^nclf mouse model and human NCL patients, validating this model as a reliable format for screening potential therapeutics.

Mutations in palmitoyl-protein thioesterase 1 alter exocytosis and endocytosis at synapses in Drosophila larvae

Infantile-onset neuronal ceroid lipofuscinosis (INCL) is a severe pediatric neurodegenerative disorder produced by mutations in the gene encoding palmitoyl-protein thioesterase 1 (Ppt1). This enzyme is responsible for the removal of a palmitate group from its substrate proteins, which may include presynaptic proteins like SNAP-25, cysteine string protein (CSP), dynamin, and synaptotagmin. The fruit fly, Drosophila melanogaster, has been a powerful model system for studying the functions of these proteins and the molecular basis of neurological disorders like the NCLs. Genetic modifier screens and tracer uptake studies in Ppt1 mutant larval garland cells have suggested that Ppt1 plays a role in endocytic trafficking. We have extended this analysis to examine the involvement of Ppt1 in synaptic function at the Drosophila larval neuromuscular junction (NMJ). Mutations in Ppt1 genetically interact with temperature sensitive mutations in the Drosophila dynamin gene shibire, accelerating the paralytic behavior of shibire mutants at 27 °C. Electrophysiological work in NMJs of Ppt1-deficient larvae has revealed an increase in miniature excitatory junctional potentials (EJPs) and a significant depression of evoked EJPs in response to repetitive (10 hz) stimulation. Endocytosis was further examined in Ppt1-mutant larvae using FM1-43 uptake assays, demonstrating a significant decrease in FM1-43 uptake at the mutant NMJs. Finally, Ppt1-deficient and Ppt1 point mutant larvae display defects in locomotion that are consistent with alterations in synaptic function. Taken together, our genetic, cellular, and electrophysiological analyses suggest a direct role for Ppt1 in synaptic vesicle exo- and endocytosis at motor nerve terminals of the Drosophila NMJ.

Cell biology and function of neuronal ceroid lipofuscinosis-related proteins

Neuronal ceroid lipofuscinoses (NCL) comprise a group of inherited lysosomal disorders with variable age of onset, characterized by lysosomal accumulation of autofluorescent ceroid lipopigments, neuroinflammation, photoreceptor- and neurodegeneration. Most of the NCL-related genes encode soluble and transmembrane proteins which localize to the endoplasmic reticulum or to the endosomal/lysosomal compartment and directly or indirectly regulate lysosomal function. Recently, exome sequencing led to the identification of four novel gene defects in NCL patients and a new NCL nomenclature currently comprising CLN1 through CLN14. Although the precise function of most of the NCL proteins remains elusive, comprehensive analyses of model organisms, particularly mouse models, provided new insight into pathogenic mechanisms of NCL diseases and roles of mutant NCL proteins in cellular/subcellular protein and lipid homeostasis, as well as their adaptive/compensatorial regulation at the transcriptional level. This review summarizes the current knowledge on the expression, function and regulation of NCL proteins and their impact on lysosomal integrity. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

Use of model organisms for the study of neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses are a group of fatal progressive neurodegenerative diseases predominantly affecting children. Identification of mutations that cause neuronal ceroid lipofuscinosis, and subsequent functional and pathological studies of the affected genes, underpins efforts to investigate disease mechanisms and identify and test potential therapeutic strategies. These functional studies and pre-clinical trials necessitate the use of model organisms in addition to cell and tissue culture models as they enable the study of protein function within a complex organ such as the brain and the testing of therapies on a whole organism. To this end, a large number of disease models and genetic tools have been identified or created in a variety of model organisms. In this review, we will discuss the ethical issues associated with experiments using model organisms, the factors underlying the choice of model organism, the disease models and genetic tools available, and the contributions of those disease models and tools to neuronal ceroid lipofuscinosis research. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

Invertebrate models of lysosomal storage disease: what have we learned so far?

The lysosomal storage diseases (LSDs) collectively account for death in 1 in 8,000 children. Although some forms are treatable, they are essentially incurable and usually are lethal in the first decade of life. The most intractable forms of LSD are those with neuronal involvement. In an effort to identify the pathological signaling driving pathology in the LSDs, invertebrate models have been developed. In this review, we outline our current understanding of LSDs and recent findings using invertebrate models. We outline strategies and pitfalls for the development of such models. Available models of LSD in Drosophila and Caenorhabditis elegans are uncovering roles for LSD-related proteins with previously unknown function using both gain-of-function and loss-of-function strategies. These models of LSD in Drosophila and C. elegans have identified potential pathogenic signaling cascades that are proving critical to our understanding of these lethal diseases.

The Batten disease Palmitoyl Protein Thioesterase 1 gene regulates neural specification and axon connectivity during Drosophila embryonic development

Palmitoyl Protein Thioesterase 1 (PPT1) is an essential lysosomal protein in the mammalian nervous system whereby defects result in a fatal pediatric disease called Infantile Neuronal Ceroids Lipofuscinosis (INCL). Flies bearing mutations in the Drosophila ortholog Ppt1 exhibit phenotypes similar to the human disease: accumulation of autofluorescence deposits and shortened adult lifespan. Since INCL patients die as young children, early developmental neural defects due to the loss of PPT1 are postulated but have yet to be elucidated. Here we show that Drosophila Ppt1 is required during embryonic neural development. Ppt1 embryos display numerous neural defects ranging from abnormal cell fate specification in a number of identified precursor lineages in the CNS, missing and disorganized neurons, faulty motoneuronal axon trajectory, and discontinuous, misaligned, and incorrect midline crossings of the longitudinal axon bundles of the ventral nerve cord. Defects in the PNS include a decreased number of sensory neurons, disorganized chordotonal neural clusters, and abnormally shaped neurons with aberrant dendritic projections. These results indicate that Ppt1 is essential for proper neuronal cell fates and organization; and to establish the local environment for proper axon guidance and fasciculation. Ppt1 function is well conserved from humans to flies; thus the INCL pathologies may be due, in part, to the accumulation of various embryonic neural defects similar to that of Drosophila. These findings may be relevant for understanding the developmental origin of neural deficiencies in INCL.

The Drosophila protein palmitoylome: characterizing palmitoyl-thioesterases and DHHC palmitoyl-transferases

Palmitoylation is the post-translational addition of a palmitate moiety to a cysteine residue through a covalent thioester bond. The addition and removal of this modification is controlled by both palmitoyl acyl-transferases and thioesterases. Using bioinformatic analysis, we identified 22 DHHC family palmitoyl acyl-transferase homologs in the Drosophila genome. We used in situ hybridization,RT-PCR, and published FlyAtlas microarray data to characterize the expression patterns of all 22 fly homologs. Our results indicate that all are expressed genes, but several, including CG1407, CG4676, CG5620, CG6017/dHIP14, CG6618, CG6627 and CG17257 appear to be enriched in neural tissues suggesting that they are important for neural function. Furthermore, we have found that several may be expressed in a sex-specific manner with adult male specific expression of CG4483 and CG17195. Using tagged versions of the DHHC genes, we demonstrate that fly DHHC proteins are primarily located in either the Golgi Apparatus or Endoplasmic Reticulum in S2 cells, except for CG1407, which was found on the plasma membrane. We also characterized the subcellular localization and expression of the three known thioesterases: Palmitoyl-protein Thioesterase 1 (Ppt1), Palmitoyl-protein Thioesterase 2 (Ppt2)and Acyl-protein Thioesterase 1 (APT1). Our results indicate that Ppt1 and Ppt2 are the major lysosomal thioesterases while APT1 is the likely cytoplasmic thioesterase. Finally, in vivo rescue experiments show that Ppt2 expression cannot rescue the neural inclusion phenotypes associated with loss of Ppt1, further supporting distinct functions and substrates for these two thioesterases. These results will serve as the basis for a more complete understanding of the protein palmitoylome's normal cellular functions in the fly and will lead to further insights into the molecular etiology of diseases associated with the mis-regulation of palmitoylation.

Autophagy-dependent rhodopsin degradation prevents retinal degeneration in Drosophila

Recent studies have demonstrated protective roles for autophagy in various neurodegenerative disorders, including the polyglutamine diseases; however, the role of autophagy in retinal degeneration has remained unclear. Accumulation of activated rhodopsin in some Drosophila mutants leads to retinal degeneration, and although it is known that activated rhodopsin is degraded in endosomal pathways in normal photoreceptor cells, the contribution of autophagy to rhodopsin regulation has remained elusive. This study reveals that activated rhodopsin is degraded by autophagy in collaboration with endosomal pathways to prevent retinal degeneration. Light-dependent retinal degeneration in the Drosophila visual system is caused by the knockdown or mutation of autophagy-essential components, such as autophagy-related protein 7 and 8 (atg-7/atg-8), or genes essential for PE (phosphatidylethanolamine) biogenesis and autophagosome formation, including Phosphatidylserine decarboxylase (Psd) and CDP-ethanolamine:diacylglycerol ethanolaminephosphotransferase (Ept). The knockdown of atg-7/8 or Psd/Ept produced an increase in the amount of rhodopsin localized to Rab7-positive late endosomes. This rhodopsin accumulation, followed by retinal degeneration, was suppressed by overexpression of Rab7, which accelerated the endosomal degradation pathway. These results indicate a degree of cross talk between the autophagic and endosomal/lysosomal pathways. Importantly, a reduction in rhodopsin levels rescued Psd knockdown-induced retinal degeneration. Additionally, the Psd knockdown-induced retinal degeneration phenotype was enhanced by Ppt1 inactivation, which causes infantile neuronal ceroid lipofuscinosis, implying that autophagy plays a significant role in its pathogenesis. Collectively, the current data reveal that autophagy suppresses light-dependent retinal degeneration in collaboration with the endosomal degradation pathway and that rhodopsin is a key substrate for autophagic degradation in this context.

Greasing their way: lipid modifications determine protein association with membrane rafts

Increasing evidence suggests that biological membranes can be laterally subdivided into domains enriched in specific lipid and protein components and that these domains may be involved in the regulation of a number of vital cellular processes. An example is membrane rafts, which are lipid-mediated domains dependent on preferential association between sterols and sphingolipids and inclusive of a specific subset of membrane proteins. While the lipid and protein composition of rafts has been extensively characterized, the structural details determining protein partitioning to these domains remain unresolved. Here, we review evidence suggesting that post-translation modification by saturated lipids recruits both peripheral and transmembrane proteins to rafts, while short, unsaturated, and/or branched hydrocarbon chains prevent raft association. The most widely studied group of raft-associated proteins are glycophosphatidylinositol-anchored proteins (GPI-AP), and we review a variety of evidence supporting raft-association of these saturated lipid-anchored extracellular peripheral proteins. For transmembrane and intracellular peripheral proteins, S-acylation with saturated fatty acids mediates raft partitioning, and the dynamic nature of this modification presents an exciting possibility of enzymatically regulated raft association. The other common lipid modifications, that is, prenylation and myristoylation, are discussed in light of their likely role in targeting proteins to nonraft membrane regions. Finally, although the association between raft affinity and lipid modification is well-characterized, we discuss several open questions regarding regulation and remodeling of these post-translational modifications as well as their role in transbilayer coupling of membrane domains.

Interactions of the proteins of neuronal ceroid lipofuscinosis: clues to function

Neuronal ceroid lipofuscinoses (NCL) are caused by mutations in eight different genes, are characterized by lysosomal accumulation of autofluorescent storage material, and result in a disease that causes degeneration of the central nervous system (CNS). Although functions are defined for some of the soluble proteins that are defective in NCL (cathepsin D, PPT1, and TPP1), the primary function of the other proteins defective in NCLs (CLN3, CLN5, CLN6, CLN7, and CLN8) remain poorly defined. Understanding the localization and network of interactions for these proteins can offer clues as to the function of the NCL proteins and also the pathways that will be disrupted in their absence. Here, we present a review of the current understanding of the localization, interactions, and function of the proteins associated with NCL.

Stoned

The stoned proteins, stoned A (STNA) and stoned B (STNB), are essential for normal vesicle trafficking in Drosophila melanogaster neurons, and deletion of the stoned locus is lethal. Although there is a growing body of research aimed at defining the roles of these proteins, particularly for STNB where homologues have now been identified in all multicellular species, their functions and mechanisms of action are not yet established. The two proteins are structurally unrelated, consistent with two distinct cellular functions. The evidence suggests a critical requirement for stoned proteins in recycling/regulation or specification of a competent synaptic vesicle pool. As stoned proteins may be specific to a particular pathway of endocytosis, studies of their function are likely to be valuable in distinguishing between the different mechanisms of membrane retrieval and their respective contributions to synaptic vesicle recycling, a subject of considerable scientific debate. In this review, we examine the published literature on stoned and comment on the available data, conclusions from these analyses and how they may relate to alternative models of vesicle cycling.

Identifying cellular pathways modulated by Drosophila palmitoyl-protein thioesterase 1 function

Infantile-onset Neuronal Ceroid Lipofuscinosis (INCL) is a severe pediatric neurodegenerative disorder produced by mutations in the gene encoding palmitoyl-protein thioesterase 1 (Ppt1). This enzyme is responsible for the removal of a palmitate post-translational modification from an unknown set of substrate proteins. To better understand the function of Ppt1 in neurons, we performed an unbiased dominant loss-of-function genetic modifier screen in Drosophila using a previously characterized Ppt1 gain-of-function system. The enhancers and suppressors identified in our screen make novel connections between Ppt1 and genes involved in cellular trafficking and the modulation of synaptic growth. We further support the relevance of our screen by demonstrating that Garland cells from Ppt1 loss-of-function mutants have defects in endocytic trafficking. Endocytic tracer uptake and ultrastructural analysis of these non-neuronal cells points to Ppt1 playing a role in modulating the early stages of vesicle formation. This work lays the groundwork for further experimental exploration of these processes to better understand their contributions to the INCL disease process.

Genetic modifiers of degeneration in the cathepsin D deficient Drosophila model for neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses (NCLs) are pediatric, neurodegenerative, lysosomal storage disorders. Mutations in cathepsin D result in the most severe, congenital form of NCLs. We have previously generated a cathepsin D deficient Drosophila model, which exhibits the key features of NCLs: progressive intracellular accumulation of autofluorescent storage material and modest neurodegeneration in the brain areas related to visual functions. Here we extend the phenotypic characterization of cathepsin D deficient Drosophila and report that modest degenerative changes are also present in their retinae. Furthermore, by utilizing this phenotype, we examined the possible effect of 17 candidate modifiers, selected based on the results from other cathepsin D deficiency models. We found enhancers of this phenotype that support the involvement of endocytosis-, lipid metabolism- and oxidation-related factors in the cathepsin D deficiency induced degeneration. Our results warrant further investigation of these mechanisms in the pathogenesis of cathepsin D deficiency.

Drosophila melanogaster as a model organism of brain diseases

Drosophila melanogaster has been utilized to model human brain diseases. In most of these invertebrate transgenic models, some aspects of human disease are reproduced. Although investigation of rodent models has been of significant impact, invertebrate models offer a wide variety of experimental tools that can potentially address some of the outstanding questions underlying neurological disease. This review considers what has been gleaned from invertebrate models of neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, metabolic diseases such as Leigh disease, Niemann-Pick disease and ceroid lipofuscinoses, tumor syndromes such as neurofibromatosis and tuberous sclerosis, epilepsy as well as CNS injury. It is to be expected that genetic tools in Drosophila will reveal new pathways and interactions, which hopefully will result in molecular based therapy approaches.

Interactions between the juvenile Batten disease gene, CLN3, and the Notch and JNK signalling pathways

Mutations in the gene CLN3 are responsible for the neurodegenerative disorder juvenile neuronal ceroid lipofuscinosis or Batten disease. CLN3 encodes a multi-spanning and hydrophobic transmembrane protein whose function is unclear. As a consequence, the cell biology that underlies the pathology of the disease is not well understood. We have developed a genetic gain-of-function system in Drosophila to identify functional pathways and interactions for CLN3. We have identified previously unknown interactions between CLN3 and the Notch and Jun N-terminal kinase signalling pathways and have uncovered a potential role for the RNA splicing and localization machinery in regulating CLN3 function.

Palmitoyl protein thioesterase-1 deficiency impairs synaptic vesicle recycling at nerve terminals, contributing to neuropathology in humans and mice

Neuronal ceroid lipofuscinoses represent the most common childhood neurodegenerative storage disorders. Infantile neuronal ceroid lipofuscinosis (INCL) is caused by palmitoyl protein thioesterase-1 (PPT1) deficiency. Although INCL patients show signs of abnormal neurotransmission, manifested by myoclonus and seizures, the molecular mechanisms by which PPT1 deficiency causes this abnormality remain obscure. Neurotransmission relies on repeated cycles of exo- and endocytosis of the synaptic vesicles (SVs), in which several palmitoylated proteins play critical roles. These proteins facilitate membrane fusion, which is required for neurotransmitter exocytosis, recycling of the fused SV membrane components, and regeneration of fresh vesicles. However, palmitoylated proteins require depalmitoylation for recycling. Using postmortem brain tissues from an INCL patient and tissue from the PPT1-knockout (PPT1-KO) mice that mimic INCL, we report here that PPT1 deficiency caused persistent membrane anchorage of the palmitoylated SV proteins, which hindered the recycling of the vesicle components that normally fuse with the presynaptic plasma membrane during SV exocytosis. Thus, the regeneration of fresh SVs, essential for maintaining the SV pool size at the synapses, was impaired, leading to a progressive loss of readily releasable SVs and abnormal neurotransmission. This abnormality may contribute to INCL neuropathology.

Huntingtin-interacting protein 14, a palmitoyl transferase required for exocytosis and targeting of CSP to synaptic vesicles

Posttranslational modification through palmitoylation regulates protein localization and function. In this study, we identify a role for the Drosophila melanogaster palmitoyl transferase Huntingtin-interacting protein 14 (HIP14) in neurotransmitter release. hip14 mutants show exocytic defects at low frequency stimulation and a nearly complete loss of synaptic transmission at higher temperature. Interestingly, two exocytic components known to be palmitoylated, cysteine string protein (CSP) and SNAP25, are severely mislocalized at hip14 mutant synapses. Complementary DNA rescue and localization experiments indicate that HIP14 is required solely in the nervous system and is essential for presynaptic function. Biochemical studies indicate that HIP14 palmitoylates CSP and that CSP is not palmitoylated in hip14 mutants. Furthermore, the hip14 exocytic defects can be suppressed by targeting CSP to synaptic vesicles using a chimeric protein approach. Our data indicate that HIP14 controls neurotransmitter release by regulating the trafficking of CSP to synapses.

Gene expression profiling in a mouse model of infantile neuronal ceroid lipofuscinosis reveals upregulation of immediate early genes and mediators of the inflammatory response

BACKGROUND

The infantile form of neuronal ceroid lipofuscinosis (also known as infantile Batten disease) is caused by hereditary deficiency of a lysosomal enzyme, palmitoyl-protein thioesterase-1 (PPT1), and is characterized by severe cortical degeneration with blindness and cognitive and motor dysfunction. The PPT1-deficient knockout mouse recapitulates the key features of the disorder, including seizures and death by 7-9 months of age. In the current study, we compared gene expression profiles of whole brain from PPT1 knockout and normal mice at 3, 5 and 8 months of age to identify temporal changes in molecular pathways implicated in disease pathogenesis.

RESULTS

A total of 267 genes were significantly (approximately 2-fold) up- or downregulated over the course of the disease. Immediate early genes (Arc, Cyr61, c-fos, jun-b, btg2, NR4A1) were among the first genes upregulated during the presymptomatic period whereas immune response genes dominated at later time points. Chemokine ligands and protease inhibitors were among the most transcriptionally responsive genes. Neuronal survival factors (IGF-1 and CNTF) and a negative regulator of neuronal apoptosis (DAP kinase-1) were upregulated late in the course of the disease. Few genes were downregulated; these included the alpha2 subunit of the GABA-A receptor, a component of cortical and hippocampal neurons, and Hes5, a transcription factor important in neuronal differentiation.

CONCLUSION

A molecular description of gene expression changes occurring in the brain throughout the course of neuronal ceroid lipofuscinosis suggests distinct phases of disease progression, provides clues to potential markers of disease activity, and points to new targets for therapy.