TDP-43-induced Death Is Associated with Altered Regulation of BIM and Bcl-xL and Attenuated by Caspase-mediated TDP-43 Cleavage

Abstract

FTLD-U is the most common variant of FTLD. FTLD, a heterogeneous syndrome characterized by progressive dementia, is caused by degeneration of the frontal and anterior temporal lobes, and is a leading cause of dementia in patients presenting before the age of 65 years (5). The pathogenesis of FTLD remains unknown. ALS is the most common motor neuron disease, characterized by progressive loss of motor neurons (6, 7). The pathogenesis of ALS remains undetermined although various hypotheses have been proposed: e.g. misfolded protein aggregates, mitochondrial dysfunction, glutamate toxicity, oxidative stress, disturbance of intracellular trafficking, and ER stress (8). About 10% of ALS cases occur in a genetically inherited manner.Post-translational modifications of TDP-43 including truncation, hyperphosphorylation, and ubiquitination are assumed to be linked to abnormal aggregation (1, 2). In particular, C-terminal fragments (CTFs) of TDP-43 are prone to form cytoplasmic aggregates (9 -11). Although it is apparent that cytoplasmic aggregates of TDP-43 are closely related to the pathogenesis of FTLD-U and ALS, it remains unknown how CTFs are generated and whether the aggregate formation of TDP-43 is a cause or a result of neuronal toxicity.Caspase activation has been observed in motor neurons of ALS (12-15) and neurons in FTLD patients (16). One possible mechanism underlying the generation of CTFs is the cleavage of TDP-43 by activated caspases. In reality, multiple studies have shown that TDP-43 is cleaved in a caspase-dependent manner and the resulting CTFs of TDP-43 are constituents in the inclusion bodies (17)(18)(19).Some clinical studies have suggested that the levels of TDP-43 are up-regulated in motor neurons of sporadic ALS patients, based on the finding that the levels of TDP-43 are up-regulated in cerebrospinal fluids (20)

The cleavage pattern of TDP-43 determines its rate of clearance and cytotoxicity

“…In contrast to the potent cytotoxicity that is induced by the overexpression of full-length TDP-43, it was reported that the overexpression of CTF35 or CTF25(D169) has very little or no ability, respectively, to induce cell death 15 . Furthermore, the expression of a CTF in the fly eye fails to induce neurodegeneration 12 .…”

“…Although the pharmacological induction of ER stress can cause TDP-43 to accumulate in the cytoplasm and promotes fragmentation, it has also been reported that overexpression of TDP-43 induces ER stress 15,[28][29][30] . Furthermore, it has been shown that caspase-4 and markers of ER stress are upregulated in the spinal cords of patients with sporadic ALS 31,32 .…”

“…Initially, Asp219 was suggested as the candidate cleavage site for CTF25; however, the estimated molecular weight of the fragment that comprises a.a. 220-414 is less than 25 kDa (refs 13,16,17). Subsequently, Asp169 was suggested as the cleavage site, but introduction of the mutation D169E does not inhibit the generation of CTF25 completely, which suggests the existence of a second CTF25 fragment 15 . Moreover, the knockout of caspase-3 does not prevent the generation of CTF25, which implies that at least a proportion of CTF25 is produced by a different mechanism from that by which CTF35 is generated 14 .…”

“…Although the associated N-terminal fragments (NTFs) of TDP-43 are degraded quickly, the CTFs form aggregates that can also contain fulllength TDP-43. CTF35 is generated by caspase-3/7-mediated cleavage after Asp89, which is embedded in the consensus caspase-recognition motif [13][14][15][16] . Initially, Asp219 was suggested as the candidate cleavage site for CTF25; however, the estimated molecular weight of the fragment that comprises a.a. 220-414 is less than 25 kDa (refs 13,16,17).…”

Addition of exogenous SOD1 aggregates causes TDP-43 mislocalisation and aggregation

“…There is a strong link between ALS pathology and ER stress (Walker and Atkin 2011), and it has been proposed that prolonged ER stress leads to motor neuron death in ALS (Walker and Atkin 2011). While generally the unfolded protein response activated by ER stress is a protective response able to rescue cells from proteotoxicity, in the ALS context this can lead to further aggregation of ALS-associated proteins such as TDP-43 (Suzuki et al 2011). Taken together, these data are consistent with the idea that ER stress caused by uptake and maintenance of SOD1 aggregates may trigger TDP-43 mislocalisation and accumulation.…”