Eyeing up Drosophila Models of Frontotemporal Dementia: Identifying Conserved Mechanisms in Disease Pathology

KEYWORDS:

• autophagy
• drosophila
• endosomal trafficking
• FTD
• frontotemporal dementia
• immunity

Frontotemporal dementia (FTD) is a comprehensive term referring to a group of disorders associated with frontotemporal lobar degeneration, a progressive atrophic degeneration primarily affecting the frontal and/or temporal lobes of the brain. It is, after Alzheimer's disease, commonly regarded as the most prevalent form of early onset dementia, with an average age of onset between 55 and 65 years of age. FTD is clinically, pathologically and genetically highly heterogeneous, presenting with a broad array of symptoms and pathologies and displaying significant overlap with other neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), progressive supranuclear palsy and corticobasal degeneration. In fact a growing body of evidence suggests that different forms of FTD and ALS may constitute a spectrum of a single disease, with FTD and ALS representing extremes of this continuum. FTD has been shown to display a strong genetic association and so Drosophila have proven a powerful model with which to dissect the genetic and molecular basis of disease pathology. Such studies have hinted at potential common mechanisms in pathology including perturbed endosomal-lysosomal trafficking, autophagy and innate immunity. Here I discuss how these themes may converge and how Drosophila are proving to be a useful tool for understanding the molecular and genetic basis of FTD.

One approach in which Drosophila have proven an invaluable tool for the dissection of signal transduction pathways and molecular pathology in FTD is through unbiased, dominant, genome-wide modifier screens [1]. These screens are typically conducted by expressing an FTD-associated mutant transgene within the Drosophila eye, using the UAS/Gal4 system, to generate a ‘sensitized’ background that presents with a perturbed eye phenotype. Expression in the eye as an isolated tissue allows analysis of the effect of dominant mutations that may be highly detrimental to the organism as a whole in an in vivo system. By crossing fly lines known as deficiencies, which contain large contiguous deletions within the Drosophila genome, to this sensitized background regions containing modifiers of the eye phenotype, and thus FTD toxicity, can be identified. Using readily available deficiency collections this can be done with multiple deficiency lines together representing more than 80% of the genome. Once genomic regions containing modifiers are identified these can be subsequently narrowed down to individual genes, allowing identification of novel modifiers of disease pathology. Using this approach a number of FTD loci have been screened, identifying new players in the disease. For example, Ahmad et al. (2009) established a Drosophila model of FTD associated with expression of the CHMP2B^Intron5-mutant transgene [2]. CHMP2B^Intron5 was previously implicated in FTD associated with chromosome 3 (FTD-3), in which a single G-to-C transition within the splice acceptor site of exon 6, in the CHMP2B gene, leads to the formation of the pathogenic, C-terminally truncated CHMP2B^Intron5 protein [3]. This truncated protein forms an aberrant, more avid association with one of its ESCRT-III counterparts, Shrub/Snf7, resulting in perturbed endosomal and autophagic dynamics [4]. Using this model, expressing CHMP2B^Intron5 in the fly eye, Ahmad et al. conducted genome-wide modifier screens, initially identifying 29 modifiers of CHMP2B^Intron5 toxicity. Subsequent characterization of these modifiers revealed many of these loci, including Rab8, Syntaxin 13, N-ethylmaleimide-sensitive factor 2 and soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) to be regulators of endosomal and autophagic processes [2,5,6]. Furthermore they revealed that CHMP2B^Intron5 mutants displayed endosomal perturbations that resulted in unregulated TGF-β, JNK and Notch signaling due to a disruption to the normal processing and regulation of cell signaling receptors [7,8].

Using Drosophila similar modifier screens have also been used to demonstrate genetic interactions between other FTD loci including VCP, TDP-43 and FUS [9,10]. In one study Ritson et al. revealed that pathogenic VCP mutations resulted in cytosolic accumulation of TDP-43, leading to cytotoxicity [10]. Accumulation of TDP-43 is reminiscent of aggregates seen in FTD patients and hints toward a failure in protein turnover, again pointing toward perturbed autophagy and endosomal trafficking, a process VCP has been implicated in. Azuma et al. revealed that VCP levels could also affect FUS toxicity, with overexpression of VCP alleviating toxicity associated with FUS knockdown while VCP depletion potentiated it [9]. Both of these papers suggest that at least some of the FTD loci represent a common genetic pathological pathway.

Many of the genetic loci associated with FTD, including CHMP2B, VCP, C9ORF72, TANK-binding kinase 1, Progranulin, TDP43 and MAPT, have been implicated in the regulation of normal endosomal trafficking and autophagy [3,11–14]. These pathways are fundamental in the regulation of cellular homeostasis, targeting cellular constituents either for recycling or for degradation via the lysosome. Studies in Drosophila have demonstrated the importance of endosomal–lysosomal and autophagic pathways in regulating normal neuronal homeostasis, with mutants in these processes displaying perturbed neuronal growth and function. In these studies it has been shown that a failure to correctly traffic cell signaling receptors through the endosome for either recycling via the recycling endosome or lysosomal degradation results in accumulation of active receptors in the endosome [15–17]. This, in turn, leads to aberrant, ectopic growth signals emanating from the endosome, leading to neuronal dysfunction [15–17]. In a recent study such perturbations were observed in a Drosophila model of FTD [6]. In this study it was shown that mutations in Rab8, a regulator of recycling endosomes, significantly potentiated toxicity in a Drosophila model of FTD. In addition, endosomal perturbation in CHMP2B mutant flies resulted in upregulation of TGF-β and JNK signaling leading to unregulated neuronal growth, all of which could be alleviated through overexpression of Rab8 [6]. Similarly VCP, another FTD-associated protein, has been implicated both in endosomal-lysosomal trafficking and as a regulator of TGF-β signaling [18,19]. Taken together these findings implicate endosomal trafficking, and its role in the regulation of cell signaling, as a potentially central mechanism in the regulation of neuronal homeostasis and in FTD pathology. These studies also provide an insight into the power of Drosophila modifier screens as an unbiased, genome-wide method for identifying novel loci implicated in FTD. Subsequent characterization of identified loci has proven successful in dissecting genetic interactions and the molecular mechanisms involved in disease pathology. By dissecting the molecular mechanisms implicated in FTD, and in normal neuronal homeostasis, we may identify novel therapeutic targets. Models in which pathogenic mutations are expressed in the Drosophila eye may then provide an attractive tool for screening such therapeutic compounds, with a clear phenotype that can be observed for signs of amelioration.

In addition to an apparently central role for endosomal–lysosomal trafficking and autophagy in FTD, studies of Drosophila have also alluded to an interesting and novel role for innate immune signaling in the pathology of the disease. First, it is important to consider that the melanization phenotype commonly observed in Drosophila eyes during modifier screens is, essentially, an immune response. Melanization is used by Drosophila as a mechanism of encapsulating pathogens and is activated in response to immune signaling pathways conserved between flies and humans. In addition to this phenotype a number of novel loci and molecular signaling pathways identified as regulators of FTD disease pathology have a role in regulating immunity and immune signaling. For example, both POSH and TAK1, regulators of innate immune responses, have been identified as modifiers of FTD toxicity as well as regulators of normal neuronal growth [6]. Similarly Serpin5, a regulator of toll signaling, was identified as a modifier of CHMP2B^Intron5 toxicity [2]. In addition it was recently shown that expression of TDP-43 in Drosophila neurons elicited activation of innate immune responses, while suppression of innate immune signaling could extend the lifespan of TDP-43 transgenic flies [20]. It is also of interest that one of the latest loci implicated in FTD, TANK-binding kinase 1, has previously been implicated in autophagosome maturation and autophagy-mediated antimicrobial defense in a downstream role of its interacting partner Rab8 [13]. These observations strongly hint toward a conserved mechanism of pathology in FTD, in which the endosomal and autophagic pathways and regulation of innate immune responses may play a fundamental and interconnected role.

Acknowledgement

The author thanks S Sweeney (The University of York, UK) for his comments on this article.

Financial & competing interests disclosure

The author thanks the Alzheimer's Society UK for their support (Alzheimer's Society UK grant AS-PG-2013–005 awarded to S Sweeney, York, UK). The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Additional information

Funding

The author thanks the Alzheimer's Society UK for their support (Alzheimer's Society UK grant AS-PG-2013–005 awarded to S Sweeney, York, UK). The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.