Inhaled insulin forms toxic pulmonary amyloid aggregates

It is well known that interfaces, such as polar-nonpolar or liquid-air, play a key role in triggering protein aggregation in vitro, in particular the aggregation of peptides and proteins with the predisposition of misfolding and aggregation. Here we show that the interface present in the lungs predisposes the lungs to form aggregation of inhaled insulin. Insulin inhalers were introduced, and a large number of diabetic patients have used them. Although inhalers were safe and effective, decreases in pulmonary capacity have been reported in response to inhaled insulin. We hypothesize that the lung air-tissue interface provides a template for the aggregation of inhaled insulin. Our studies were designed to investigate the harmful potential that inhaled insulin has in pulmonary tissue in vivo, through an amyloid formation mechanism. Our data demonstrate that inhaled insulin rapidly forms amyloid in the lungs causing a significant reduction in pulmonary air flow. Our studies exemplify the importance that interfaces play in protein aggregation in vivo, illustrating the potential aggregation of inhaled proteins and the formation of amyloid deposits in the lungs. These insulin deposits resemble the amyloid structures implicated in protein misfolding disorders, such as Alzheimer's and Parkinson's diseases, and could as well be deleterious in nature.

Neurodegenerative models in Drosophila: polyglutamine disorders, Parkinson disease, and amyotrophic lateral sclerosis

Neurodegenerative diseases encompass a large group of neurological disorders. Clinical symptoms can include memory loss, cognitive impairment, loss of movement or loss of control of movement, and loss of sensation. Symptoms are typically adult onset (although severe cases can occur in adolescents) and are reflective of neuronal and glial cell loss in the central nervous system. Neurodegenerative diseases also are considered progressive, with increased severity of symptoms over time, also reflective of increased neuronal cell death. However, various neurodegenerative diseases differentially affect certain brain regions or neuronal or glial cell types. As an example, Alzheimer disease (AD) primarily affects the temporal lobe, whereas neuronal loss in Parkinson disease (PD) is largely (although not exclusively) confined to the nigrostriatal system. Neuronal loss is almost invariably accompanied by abnormal insoluble aggregates, either intra- or extracellular. Thus, neurodegenerative diseases are categorized by (a) the composite of clinical symptoms, (b) the brain regions or types of brain cells primarily affected, and (c) the types of protein aggregates found in the brain. Here we review the methods by which Drosophila melanogaster has been used to model aspects of polyglutamine diseases, Parkinson disease, and amyotrophic lateral sclerosis and key insights into that have been gained from these models; Alzheimer disease and the tauopathies are covered elsewhere in this special issue.

Association of GSK3B with Alzheimer disease and frontotemporal dementia

BACKGROUND

Deposits of abnormally hyperphosphorylated tau are a hallmark of several dementias, including Alzheimer disease (AD), and about 10% of familial frontotemporal dementia (FTD) cases are caused by mutations in the tau gene. As a known tau kinase, GSK3B is a promising candidate gene in the remaining cases of FTD and in AD, for which tau mutations have not been found.

OBJECTIVE

To examine the promoter of GSK3B and all 12 exons, including the surrounding intronic sequence, in patients with FTD, patients with AD, and aged healthy subjects to identify single-nucleotide polymorphisms associated with disease.

DESIGN, SETTING, AND PARTICIPANTS

Single-nucleotide polymorphism frequency was examined in a case-control cohort of 48 patients with probable AD, 102 patients with FTD, 38 patients with primary progressive aphasia, and 85 aged healthy subjects. Results were followed up in 2 independent AD family samples consisting of 437 multiplex families with AD (National Institute of Mental Health Genetics Initiative AD Study) or 150 sibships discordant for AD (Consortium on Alzheimer's Genetics Study).

RESULTS

Several rare sequence variants in GSK3B were identified in the case-control study. An intronic polymorphism (IVS2-68G>A) occurred at more than twice the frequency among patients with FTD (10.8%) and patients with AD (14.6%) than in aged healthy subjects (4.1%). The polymorphism showed association with disease in both follow-up samples independently, although only the Consortium on Alzheimer's Genetics sample showed the same direction of association as the case-control sample.

CONCLUSIONS

To our knowledge, this is the first evidence that a gene known to be involved in tau phosphorylation, GSK3B, is associated with risk for primary neurodegenerative dementias. This supports previous work in animal models suggesting that such genes are therapeutic targets.

Dissociation of tau toxicity and phosphorylation: role of GSK-3beta, MARK and Cdk5 in a Drosophila model

Hyperphosphorylation of tau at multiple sites has been implicated in the formation of neurofibrillary tangles in Alzheimer’s disease; however, the relationship between toxicity and phosphorylation of tau has not been clearly elucidated. Putative tau kinases that play a role in such phosphorylation events include the proline-directed kinases glycogen synthase kinase-3β (GSK-3β) and cyclin-dependent kinase 5 (Cdk5), as well as nonproline-directed kinases such as microtubule affinity-regulating kinase (MARK)/PAR-1; however, whether the cascade of events linking tau phosphorylation and neurodegeneration involves sequential action of kinases as opposed to parallel pathways is still a matter of controversy. Here, we employed a well-characterized Drosophila model of tauopathy to investigate the interdependence of tau kinases in regulating the phosphorylation and toxicity of tau in vivo. We found that tau mutants resistant to phosphorylation by MARK/PAR-1 were indeed less toxic than wild-type tau; however, this was not due to their resistance to phosphorylation by GSK-3β/Shaggy. On the contrary, a tau mutant resistant to phosphorylation by GSK-3β/Shaggy retained substantial toxicity and was found to have increased affinity for microtubules compared with wild-type tau. The fly homologs of Cdk5/p35 did not have major effects on tau toxicity or phosphorylation in this model. These data suggest that, in addition to tau phosphorylation, microtubule binding plays a crucial role in the regulation of tau toxicity when misexpressed. These data have important implications for the understanding and interpretation of animal models of tauopathy.

Guide to understanding Drosophila models of neurodegenerative diseases

Demystifying how genetic studies inDrosophila inform human disease conditions, this article highlights two studies that identify genetic modifiers of neurodegeneration.

A Drosophila model of ALS: human ALS-associated mutation in VAP33A suggests a dominant negative mechanism

ALS8 is caused by a dominant mutation in an evolutionarily conserved protein, VAPB (vesicle-associated membrane protein (VAMP)-associated membrane protein B)/ALS8). We have established a fly model of ALS8 using the corresponding mutation in Drosophila VAPB (dVAP33A) and examined the effects of this mutation on VAP function using genetic and morphological analyses. By simultaneously assessing the effects of VAP^wt and VAP^P58S on synaptic morphology and structure, we demonstrate that the phenotypes produced by neuronal expression of VAP^P58S resemble VAP loss of function mutants and are opposite those of VAP overexpression, suggesting that VAP^P58S may function as a dominant negative. This is brought about by aggregation of VAP^P58S and recruitment of wild type VAP into these aggregates. Importantly, we also demonstrate that the ALS8 mutation in dVAP33A interferes with BMP signaling pathways at the neuromuscular junction, identifying a new mechanism underlying pathogenesis of ALS8. Furthermore, we show that mutant dVAP33A can serve as a powerful tool to identify genetic modifiers of VAPB. This new fly model of ALS, with its robust pathological phenotypes, should for the first time allow the power of unbiased screens in Drosophila to be applied to study of motor neuron diseases.

A Drosophila model of mutant human parkin-induced toxicity demonstrates selective loss of dopaminergic neurons and dependence on cellular dopamine

Mutations in human parkin have been identified in familial Parkinson's disease and in some sporadic cases. Here, we report that expression of mutant but not wild-type human parkin in Drosophila causes age-dependent, selective degeneration of dopaminergic (DA) neurons accompanied by a progressive motor impairment. Overexpression or knockdown of the Drosophila vesicular monoamine transporter, which regulates cytosolic DA homeostasis, partially rescues or exacerbates, respectively, the degenerative phenotypes caused by mutant human parkin. These results support a model in which the vulnerability of DA neurons to parkin-induced neurotoxicity results from the interaction of mutant parkin with cytoplasmic dopamine.

Degradation of tau protein by puromycin-sensitive aminopeptidase in vitro

Tau, a microtubule associated protein, aggregates into intracellular paired helical filaments (PHFs) by an unknown mechanism in Alzheimer's disease (AD) and other tauopathies. A contributing factor may be a failure to metabolize free cytosolic tau within the neuron. The buildup of tau may then drive the aggregation process through mass action. Therefore, proteases that normally degrade tau are of great interest. A recent genetic screen identified puromycin-sensitive aminopeptidase (PSA) as a potent modifier of tau-induced pathology and suggested PSA as a possible tau-degrading enzyme. Here we have extended these observations using human recombinant PSA purified from Escherichia coli. The enzymatic activity and characteristics of the purified PSA were verified using chromogenic substrates, metal ions, and several specific and nonspecific protease inhibitors, including puromycin. PSA was shown to digest recombinant human full-length tau in vitro, and this activity was hindered by puromycin. The mechanism of amino terminal degradation of tau was confirmed using a novel N-terminal cleavage-specific tau antibody (Tau-C6g, specific for cleavage between residues 13-14) and a C-terminal cleavage-specific tau antibody (Tau-C3). Additionally, PSA was able to digest soluble tau purified from normal human brain to a greater extent than either soluble or PHF tau purified from AD brain, indicating that post-translational modifications and/or polymerization of tau may affect its digestion by PSA. These results are consistent with observations that PSA modulates tau levels in vivo and suggest that this enzyme may be involved in tau degradation in human brain.

A genomic screen for modifiers of tauopathy identifies puromycin-sensitive aminopeptidase as an inhibitor of tau-induced neurodegeneration

Neurofibrillary tangles (NFT) containing tau are a hallmark of neurodegenerative diseases, including Alzheimer's disease (AD). NFT burden correlates with cognitive decline and neurodegeneration in AD. However, little is known about mechanisms that protect against tau-induced neurodegeneration. We used a cross species functional genomic approach to analyze gene expression in multiple brain regions in mouse, in parallel with validation in Drosophila, to identify tau modifiers, including the highly conserved protein puromycin-sensitive aminopeptidase (PSA/Npepps). PSA protected against tau-induced neurodegeneration in vivo, whereas PSA loss of function exacerbated neurodegeneration. We further show that human PSA directly proteolyzes tau in vitro. These data highlight the utility of using both evolutionarily distant species for genetic screening and functional assessment to identify modifiers of neurodegeneration. Further investigation is warranted in defining the role of PSA and other genes identified here as potential therapeutic targets in tauopathy.

Normal-repeat-length polyglutamine peptides accelerate aggregation nucleation and cytotoxicity of expanded polyglutamine proteins

The dependence of disease risk and age-of-onset on expanded CAG repeat length in diseases like Huntington's disease (HD) is well established and correlates with the repeat-length-dependent nucleation kinetics of polyglutamine (polyGln) aggregation. The wide variation in ages of onset among patients with the same repeat length, however, suggests a role for modifying factors. Here we describe the ability of normal-length polyGln repeat sequences to greatly accelerate the nucleation kinetics of an expanded polyGln peptide. We find that normal-length polyGln peptides enhance the in vitro nucleation kinetics of a Q(47) peptide in a concentration-dependent and repeat-length-dependent manner. In vivo, we show that coexpression of a Q(20) sequence in a Drosophila model of HD expressing Htt exon 1 protein with an Q(93) repeat accelerates both aggregate formation and neurotoxicity. The accelerating effect of short polyGln peptides is attributable to the promiscuity of polyGln aggregate elongation and reflects the intimate relationship between nucleus formation and early elongation events in establishing nucleation kinetics. The results suggest that the overall state of the polyGln protein network in a cellular environment may have a profound effect on the toxic consequences of polyGln expansion and thus may serve as a genetic modifier of age of onset in HD.

RETRACTED: γ-Cleavage-Independent Functions of Presenilin, Nicastrin, and Aph-1 Regulate Cell-Junction Organization and Prevent Tau Toxicity In Vivo

Genetic analysis of familial Alzheimer's disease has revealed that mutations in the gamma-secretase enzyme presenilin promote toxic Abeta secretion; however, presenilin mutations might also influence tau hyperphosphorylation and neurodegeneration through gamma-secretase-independent mechanisms. To address this possibility and determine whether other components of the gamma-secretase complex possess similar regulatory functions, we analyzed the roles of presenilin, nicastrin, and aph-1 in a Drosophila model for tau-induced neurodegeneration. Here, we show that presenilin and nicastrin prevent tau toxicity by modulating the PI3K/Akt/GSK3beta phosphorylation pathway, whereas aph-1 regulates aPKC/PAR-1 activities. Moreover, we found that these transmembrane proteins differentially regulate the intracellular localization of GSK3beta and aPKC at cell junctions. Inhibition of gamma-secretase activity neither interfered with these kinase pathways nor induced aberrant tau phosphorylation. These results establish new in vivo molecular functions for the three components of the gamma-secretase complex and reveal a different mechanism that might contribute to neuronal degeneration in Alzheimer's disease.

Drosophila models of neurodegenerative disease

Over the last two decades, a number of mutations have been identified that give rise to neurodegenerative disorders, including familial forms of Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Although in most cases sporadic cases vastly outnumber familial forms of such diseases, study of such inherited forms has the potential to provide powerful clues regarding the pathophysiological basis of neurodegeneration. One powerful approach to analyzing disease mechanisms is the development of transgenic animal models, most notably in the mouse. However, development and analysis of such models can be costly and time consuming. Development of improved transgenic technologies have contributed to the development of Drosophila models of a number of neurodegenerative disorders that have shown striking similarities to the human diseases. Moreover, genetic screens using such models have begun to unravel aspects of the pathophysiological basis of neurodegenerative disorders. Here, we provide a general overview of fly models pertinent to trinucleotide repeat expansion disorders, Alzheimer's, and Parkinson's diseases, and highlight key genetic modifiers that have been identified to date using such models.

Hydrolethalus syndrome is caused by a missense mutation in a novel gene HYLS1

Hydrolethalus syndrome (HLS) is an autosomal recessive lethal malformation syndrome characterized by multiple developmental defects of fetus. We have earlier mapped and restricted the HLS region to a critical 1 cM interval on 11q23-25. The linkage disequilibrium (LD) and haplotype analyses of single nucleotide polymorphism (SNP) markers helped to further restrict the HLS locus to 476 kb between genes PKNOX2 and DDX25. An HLS associated mutation was identified in a novel regional transcript (GenBank accession no. FLJ32915), referred to here as the HYLS1 gene. The identified A to G transition results in a D211G change in the 299 amino acid polypeptide with unknown function. The HYLS1 gene shows alternative splicing and the transcript is found in multiple tissues during fetal development. In situ hybridization shows spatial and temporal distributions of transcripts in good agreement with the tissue phenotype of HLS patients. Immunostaining of in vitro expressed polypeptides from wild-type (WT) cDNA revealed cytoplasmic staining, whereas mutant polypeptides became localized in distinct nuclear structures, implying a disturbed cellular localization of the mutant protein. The Drosophila melanogaster model confirmed these findings and provides evidence for the significance of the mutation both in vitro and in vivo.

Inactivation of Drosophila Apaf-1 related killer suppresses formation of polyglutamine aggregates and blocks polyglutamine pathogenesis

Huntington's disease (HD) is caused by expansion of a polyglutamine tract near the N-terminal of huntingtin. Mutant huntingtin forms aggregates in striatum and cortex, where extensive cell death occurs. We used a Drosophila polyglutamine peptide model to assess the role of specific cell death regulators in polyglutamine-induced cell death. Here, we report that polyglutamine-induced cell death was dramatically suppressed in flies lacking Dark, the fly homolog of human Apaf-1, a key regulator of apoptosis. Dark appeared to play a role in the accumulation of polyglutamine-containing aggregates. Suppression of cell death, caspase activation and aggregate formation were also observed when mutant huntingtin exon 1 was expressed in homozygous dark mutant animals. Expanded polyglutamine induced a marked increase in expression of Dark, and Dark was observed to colocalize with ubiquitinated protein aggregates. Apaf-1 also was found to colocalize with huntingtin-containing aggregates in a murine model and HD brain, suggesting a common role for Dark/Apaf-1 in polyglutamine pathogenesis in invertebrates, mice and man. These findings suggest that limiting Apaf-1 activity may alleviate both pathological protein aggregation and neuronal cell death in HD.

Characterization of Na+-permeable cation channels in LLC-PK1 renal epithelial cells

In this study, the presence of Na(+)-permeable cation channels was determined and characterized in LLC-PK1 cells, a renal tubular epithelial cell line with proximal tubule characteristics derived from pig kidney. Patch-clamp analysis under cell-attached conditions indicated the presence of spontaneously active Na(+)-permeable cation channels. The channels displayed nonrectifying single channel conductance of 11 pS, substates, and an approximately 3:1 Na(+)/K(+) permeability-selectivity ratio. The Na(+)-permeable cation channels were inhibited by pertussis toxin and reactivated by G protein agonists. Cation channel activity was observed in quiescent cell-attached patches after vasopressin stimulation. The addition of protein kinase A and ATP to excised patches also induced Na(+) channel activity. Spontaneous and vasopressin-induced Na(+) channel activity were inhibited by extracellular amiloride. To begin assessing potential molecular candidates for this cation channel, both reverse transcription-PCR and immunocytochemical analyses were conducted in LLC-PK1 cells. Expression of porcine orthologs of the alphaENaC and ApxL genes were found in LLC-PK1 cells. The expression of both gene products was confirmed by immunocytochemical analysis. Although alphaENaC labeling was mostly intracellular, ApxL labeled to both the apical membrane and cytoplasmic compartments of subconfluent LLC-PK1 cells. Vasopressin stimulation had no effect on alphaENaC immunolabeling but modified the cellular distribution of ApxL, consistent with an increased membrane-associated ApxL. The data indicate that proximal tubular LLC-PK1 renal epithelial cells express amiloride-sensitive, Na(+)-permeable cation channels, which are regulated by the cAMP pathway, and G proteins. This channel activity may implicate previously reported epithelial channel proteins, although this will require further experimentation. The evidence provides new clues as to potentially relevant Na(+) transport mechanisms in the mammalian proximal nephron.

Human wild-type tau interacts with wingless pathway components and produces neurofibrillary pathology in Drosophila

Pathologic alterations in the microtubule-associated protein tau have been implicated in a number of neurodegenerative disorders, including Alzheimer's disease (AD), progressive supranuclear palsy (PSP), and frontotemporal dementia (FTD). Here, we show that tau overexpression, in combination with phosphorylation by the Drosophila glycogen synthase kinase-3 (GSK-3) homolog and wingless pathway component (Shaggy), exacerbated neurodegeneration induced by tau overexpression alone, leading to neurofibrillary pathology in the fly. Furthermore, manipulation of other wingless signaling molecules downstream from shaggy demonstrated that components of the Wnt signaling pathway modulate neurodegeneration induced by tau pathology in vivo but suggested that tau phosphorylation by GSK-3beta differs from canonical Wnt effects on beta-catenin stability and TCF activity. The genetic system we have established provides a powerful reagent for identification of novel modifiers of tau-induced neurodegeneration that may serve as future therapeutic targets.