Chapter 1 The Molecular Links Between TDP‐43 Dysfunction and Neurodegeneration
Introduction
Nuclear factor TDP-43 is a multifunctional RNA binding protein that has been described to play a role in a great variety of cellular processes such as transcription, pre-mRNA splicing, stability, transport, and translation. In a recent review we have tried to be as comprehensive as possible (Buratti and Baralle, 2008) but the fast pace of research makes it desirable to look at the field again especially with regards to its involvement in neurodegenerative diseases. From a human disease point of view, TDP‐43 was first described to participate in the development of monosymptomatic and full forms of cystic fibrosis through its inhibitory effects on the recognition of CFTR exon 9 (Buratti et al., 2001). More recently, its major claim to fame comes from the observation that TDP-43 is the main protein component of the intracellular inclusions found in the neuronal tissues of patients affected by a series of neurodegenerative diseases which include frontotemporal lobar degeneration ubiquitin (FTLD-U), amyotrophic lateral sclerosis (ALS) (Arai et al., 2006, Geser et al., 2009b, Neumann et al., 2006), and Alzheimer disease (Amador-Ortiz et al., 2007, Bigio, 2008, Rohn, 2008). The number of neurodegenerative diseases involved, the distribution/morphology of inclusions in different brain regions, and the associated clinical characteristics have already been reviewed in several recent publications (Bugiani, 2007, Cook et al., 2008, Dickson, 2008, Dickson et al., 2007, Elman et al., 2008, Forman et al., 2007, Kwong et al., 2008, Liscic et al., 2008, Mackenzie and Rademakers, 2007, Mackenzie and Rademakers, 2008, Neumann et al., 2007b, Tolnay and Frank, 2007, Wang et al., 2008b). Indeed, the impact of TDP-43 in the neurodegeneration field has been so pervasive that disease nomenclature consensus are currently being modified to better reflect the new clinical and pathological findings originating from recent research (Geser et al., 2009a, Mackenzie et al., 2009).
In this review we will rather concentrate on available data regarding the biochemical and functional changes of TDP-43 that lead to the human pathology. In particular we will focus on the potential pathological mechanisms mediated by TDP-43 mutations (both natural and artificial) and the results obtained so far in simple or existing animal models of disease.
From a classification point of view, TDP-43 is a member of the hnRNP protein family (Krecic and Swanson, 1999), a family that includes many of the most common and powerful splicing modulators known so far, such as hRNP A1/A2, PTB (hnRNP I), and hnRNP H (Martinez-Contreras et al., 2007). A characteristic of many of these factors, however, is the role they play in numerous diverse functions depending on their relative abundance, cellular localization, and the interactions with themselves or other components of the cellular machinery (Carpenter et al., 2006, Dreyfuss et al., 2002, Glisovic et al., 2008). TDP-43 is no exception and Fig. 1.1 shows a schematic diagram of the protein and the best characterized functions reported previously. These include the ability of TDP-43 to regulate splicing of several exons such as CFTR exon 9 (Buratti et al., 2001), ApoAII exon 3 (Arrisi-Mercado et al., 2004, Mercado et al., 2005), and SMN exon 7 (Bose et al., 2008) or its role in regulating transcription of the HIV-1 genome (Ou et al., 1995) and of the SP-10 mouse promoter (Abhyankar et al., 2007, Acharya et al., 2006).
In addition to its role in transcription/splicing TDP-43 may also possess other functions that are still at a very early stage of characterization. These include some very diverse properties such as acting as a neuronal response activity factor and an in vitro mRNA translational repressor (Wang et al., 2008a), or an mRNA stability factor for neurofilaments (Strong et al., 2007).
Finally, it is also important to mention the possibility of some additional properties, such as microRNA processing, that up to this moment are simply based on its association with both the human and mouse microprocessor complexes (Fukuda et al., 2007, Gregory et al., 2004). The list of potential functions is not likely to end here, as according to protein–protein association studies that use high-throughput methodologies several other candidates have been put forward (Wang et al., 2008b). In particular, two proteomics studies (Lehner and Sanderson, 2004, Stelzl et al., 2005) involving yeast two hybrid systems found XRN2 and PM/Scl100 (mRNA decay), ZHX1 (transcriptional repressor), SETDB1 (chromatin remodeling regulator), and NSFL1C and ARF6 (both involved in membrane trafficking) as potential TDP-43 binding partners. In this respect, it is important to note that TDP-43 has been described to be part of RNA granules responsible for trafficking, sequestering, and degrading RNA species (Elvira et al., 2006) and has been observed to colocalize strongly with Staufen (a protein involved in mRNA transport in dendrites), moderately with TIA-1 (a splicing factor) and weakly with XRN1, an exoribonuclease involved in mRNA decay (Moisse et al., 2009). TDP-43 has also been shown to colocalize with wild-type and mutant forms of valosin-containing protein (VCP) in culture cells (Gitcho et al., 2009) and with UBAP1 in the neuronal cytoplasmic inclusions of a familial FTD case (Rollinson et al., 2009). Partial colocalization has also been observed with GW182 and eIF4E (both markers of P-bodies) (Wang et al., 2008a). Finally, in sporadic ALS patient samples TDP-43 has also been found to colocalize with the phosphorylated Smad2/3 factors (pSmad2/3) (Nakamura et al., 2008), which are central mediators of the TGF-beta signal transduction pathway, essential for maintaining the survival of neurons. In this respect, it should also be noted that TDP-43 has been originally suggested to play a central role in organizing the higher order structures of eukaryotic nuclear bodies (Wang et al., 2002). In conclusion, although for a complete list of TDP-43 properties and functions we will certainly have to wait for a long time these observations are already sufficient to support the view that TDP-43 is involved in more than one cellular process, possibly through interactions of different parts of its molecule.
Interestingly, TDP-43 is not the only DNA/RNA binding protein recently found to be involved in ALS. In fact, the recent reports regarding the involvement of mutations in the FUS/TLS protein in a series of patients affected by familial forms of ALS suggests that alterations in DNA/RNA processing mechanisms may be a fertile area for research to further our understanding of neurodegenerative diseases (Kwiatkowski et al., 2009, Vance et al., 2009). Hopefully, this will place us in the best prospective position to design viable therapeutic strategies.
Before tackling this issue, however, it is best to summarize what happens to the predominantly nuclear, wild-type TDP-43 in a pathological setting.
Section snippets
Main Text
As clearly highlighted by the two pioneering studies in this field, the pathological TDP-43 protein analyzed in patients displays some very clear modifications with respect to the wild-type protein (Arai et al., 2006, Neumann et al., 2006). These include an increased cytoplasmic localization under the form of insoluble aggregates, ubiquitination, phosphorylation, and degradation to yield C-terminal fragments (schematically reported in Fig. 1.2). In the past 2 years, several advances have been
Early Lessons From Animal Models
The field of animal disease models dealing with TDP-43 is still in its infancy. Since the discovery of TDP-43 involvement in FTD/ALS too little time has passed to engineer TDP-43 knockout/knock-in models in higher animals. Nonetheless, some information is already available with regards to the analysis of TDP-43 expression in simpler organisms and in respect to already established animal models of ALS (Kato, 2008). The results of these studies are summarized in Table 1.2. For example, TDP-43
Gain- Versus Loss-of-Function Scenarios
The first question that was asked with regards to the possible role played by TDP-43 in FTD/ALS was whether TDP-43 simply represented an indicator of disease or it was also an active player. The results obtained so far are all conducive to the conclusion that TDP-43 (or rather the lack of it, see below) represents an active player in neurodegeneration.
Regarding the TDP‐43 role in the pathophysiological mechanisms of neurodegeneration, the issue that needs to be settled, briefly discussed above,
Conclusions
Despite recent advances, the mechanism(s) that link TDP-43 to this ever growing list of neurodegenerative pathologies still remain obscure. The discovery of disease-associated TDP-43 mutations and the observations that some TDP-43 modifications (degradation, aggregation, phosphorylation) closely correlate with disease tell us that a link does indeed exist. At present, the greatest impact of TDP-43 research has been on the clinical and diagnostic community. As far as the clinic is concerned,
Acknowledgments
This work was supported by Telethon Onlus Foundation (Italy) (grant no. GGP06147) and by a European community grant (EURASNET-LSHG-CT-2005-518238).
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