TDP-43 and FUS: a nuclear affair

Misfolded TAR DNA binding protein 43 (TDP-43) and Fused-In-Sarcoma (FUS) protein have recently been identified as pathological hallmarks of the neurodegenerative disorders amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) characterized by the presence of ubiquitin-positive inclusions (FTLD-U). Although TDP-43 and FUS are normally located predominantly in the nucleus, pathological TDP-43 and FUS inclusions are mostly found in the cytosol. Cytosolic deposition is paralleled by a striking nuclear depletion of either protein. Based on a number of recent findings, we postulate that defects in nuclear import are an important step towards TDP-43 and FUS dysfunction. Failure of nuclear transport can arise from mutations within a nuclear localization signal or from age-related decline of nuclear import mechanisms. We propose that nuclear import defects in combination with additional hits, for example cellular stress and genetic risk factors, may be a central underlying cause of ALS and FTLD-U pathology.

Introduction

Since the first description of amyotrophic lateral sclerosis (ALS) (see Glossary) and frontotemporal lobar degeneration (FTLD) in the second half of the 19th century, research progress on these devastating neurodegenerative diseases has been painstakingly slow. In the past two decades, however, research on ALS and FTLD has gained new momentum through the discovery of genes involved in familial forms of these disorders and the identification of the major aggregating protein species 1, 2, 3, 4, 5, 6, 7, 8, 9.

ALS is the most frequent motor neuron disease and is characterized by a degeneration of motor neurons, which leads to progressive muscle weakening and eventually fatal paralysis, typically within 1 to 5 years after disease onset [3]. FTLD is a clinically diverse dementia syndrome, with phenotypes including behavioral changes, semantic dementia and progressive non-fluent aphasia [4]. The majority of FTLD cases are pathologically characterized by the presence of ubiquitin-positive inclusions (FTLD-U), which, in recent years, were found to contain as major components the TAR DNA binding protein 43 (TDP-43) or, less frequently, the Fused-In-Sarcoma (FUS) protein 5, 6, 7, 8, 9 and hence are referred to as FTLD-TDP and FTLD-FUS, respectively [10] (Figure 1). In only a few cases of FTLD-U, the ubiquitinated protein is still unknown and these cases are classified as FTLD-UPS, to indicate that deposits are labeled by markers of the ubiquitin-proteasome system (UPS) only [10]. Interestingly, TDP-43 or FUS inclusions were not only identified as pathological hallmarks in the vast majority of FTLD-U cases, but were also found in ALS patients 5, 7 (referred to as ALS-TDP and ALS-FUS, respectively) (Figure 1). The only exception are ALS patients with superoxide dismutase 1 (SOD1) mutations, which have TDP-43- and FUS-negative, SOD1-positive inclusions (ALS-SOD) [11], as discussed extensively in other reviews (e.g. 2, 3). This pathological overlap of ALS and FTLD-U confirmed the long-standing thought that the two diseases are related and gave an explanation for the observation that a substantial number of ALS patients develop cognitive deficits with frontotemporal features and that FTLD-U patients can present with symptoms of motor neuron disease 12, 13.

Approximately 10% of ALS cases and 50% of FTLD-U cases are familial (reviewed in 1, 2). Mutations in TDP-43 and FUS have been identified as a genetic cause in approximately 5% of familial ALS and in rare cases of FTLD-U (reviewed in [4]). Even though a significant proportion of ALS and FTLD-U cases are sporadic and TDP-43 and FUS mutations are only present in a subset of the familial cases, nevertheless the identification of TDP-43 and FUS mutations was an extremely important finding because it unequivocally demonstrated the pivotal importance of these proteins for neurodegeneration. Furthermore, the identification of such mutations has been critical for understanding disease mechanisms in both familial and sporadic ALS/FTLD-U, similar to the situation in Alzheimer's disease (AD), where presenilin mutations have been of fundamental importance in unraveling the amyloid cascade in sporadic and familial AD [14].

Even though the discovery of TDP-43 and FUS as key aggregating proteins in ALS and FTLD-U was a major breakthrough in ALS/FTLD research, it has provoked many new questions, and we are still far from understanding the disease mechanisms or even therapeutic approaches. For example, one puzzle is posed by the peculiar pathology of TDP-43 and FUS (Figure 1). In the brain and spinal cord of affected patients, TDP-43 or FUS are mostly found in cytoplasmic inclusions of neurons and sometimes glia cells 5, 6, 7, 9, whereas in healthy cells both proteins are predominantly located in the nucleus 6, 7, 8, 9. The nuclear localization certainly reflects the physiological function of these DNA/RNA binding proteins, which appear to regulate transcription and pre-mRNA splicing (reviewed in 15, 16). Strikingly, inclusion-bearing cells not only show an aberrant accumulation of TDP-43 and FUS in the cytoplasm, but also a complete or partial loss of nuclear TDP-43 or FUS 5, 6, 7, 8, 9, 17 (Figure 1). In some cases, the proteins are also found within intranuclear inclusions, again leaving the remaining nucleoplasm largely devoid of TDP-43 or FUS, respectively 7, 17, 18. This peculiar pathology has raised a number of interesting questions. First, is toxicity mediated by the loss of TDP-43 and FUS from the nucleus (‘loss-of-function’) or by their aberrant accumulation/aggregation in the cytoplasm (‘gain-of-function’) or even by both mechanisms? This important question has been discussed elsewhere 16, 19, 20 and has been experimentally addressed by various animal models that overexpress 21, 22, 23, 24, 25 or downregulate 26, 27, 28, 29, 30 TDP-43 or FUS. However, to date, model systems that faithfully reproduce the human pathology have not been generated, thus a definitive answer to this question has remained elusive. Second, how does the pathological redistribution of the TDP-43 and FUS arise? Despite the fact that we are still far from completely answering this pivotal question, based on a number of recent findings, we suggest that defects in nuclear import are an important step in the initiation of TDP-43/FUS pathology.