Nuclear TAR DNA-binding protein 43: A new target for amyotrophic lateral sclerosis treatment

A role for SUMOylation in the Formation and Cellular Localization of TDP-43 Aggregates in Amyotrophic Lateral Sclerosis

In amyotrophic lateral sclerosis, motor neurons undergoing degeneration are characterized by the presence of cytoplasmic aggregates containing TDP-43 protein. SUMOylation, a posttranslational modification of proteins, has been previously implicated in the formation of aggregates positives for SOD1, another protein enriched in a subset of ALS patients. We show in this study that TDP-43 is also a target of SUMOylation. The inhibition of the first step of the SUMOylation process by anacardic acid significantly reduces the presence of TDP-43 aggregates and improves neuritogenesis and cell viability in vitro. Interestingly, the mutation of the unique SUMOylation site on TDP-43, using site-directed mutagenesis, modifies the intracellular localization of TDP-43 aggregates. Instead of being cytoplasmic where they are associated with toxic effects, they are located inside the nucleus. This change of localization results in improvement in cell viability and in global cellular functions. Our results implicate the SUMOylation site of TDP-43 in the formation of cytoplasmic TDP-43 aggregates, a hallmark of ALS, and thus identifies this region as a new target for novel therapeutic strategies.

Mercury Involvement in Neuronal Damage and in Neurodegenerative Diseases

Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis are characterized by a chronic and selective process of neuronal cell death. Although the causes of neurodegenerative diseases remain still unknown, it is now a well-established idea that more factors, such as genetic, endogenous, and environmental, are involved. Among environmental causes, the accumulation of mercury, a heavy metal considered a toxic agent, was largely studied as a probable factor involved in neurodegenerative disease course. Mercury exists in three main forms: elemental mercury, inorganic mercury, and organic mercury (methylmercury and ethylmercury). Sources of elemental mercury can be natural (volcanic emission) or anthropogenic (coal-fired electric utilities, waste combustion, hazardous-waste incinerators, and gold extraction). Moreover, mercury is still used as an antiseptic, as a medical preservative, and as a fungicide. Dental amalgam can emit mercury vapor. Mercury vapor, being highly volatile and lipid soluble, can cross the blood-brain barrier and the lipid cell membranes and can be accumulated into the cells in its inorganic forms. Also, methylmercury can pass through blood-brain and placental barriers, causing serious damage in the central nervous system. This review describes the toxic effects of mercury in cell cultures, in animal models, and in patients with neurodegenerative diseases. In vitro experiments showed that mercury exposure was principally involved in oxidative stress and apoptotic processes. Moreover, motor and cognitive impairment and neural loss have been confirmed in various studies performed in animal models. Finally, observational studies on patients with neurodegenerative diseases showed discordant data about a possible mercury involvement.

Neurotrophic effects of progranulin in vivo in reversing motor neuron defects caused by over or under expression of TDP-43 or FUS

Progranulin (PGRN) is a glycoprotein with multiple roles in normal and disease states. Mutations within the GRN gene cause frontotemporal lobar degeneration (FTLD). The affected neurons display distinctive TAR DNA binding protein 43 (TDP-43) inclusions. How partial loss of PGRN causes TDP-43 neuropathology is poorly understood. TDP-43 inclusions are also found in affected neurons of patients with other neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) and Alzheimer's disease. In ALS, TDP-43 inclusions are typically also immunoreactive for fused in sarcoma (FUS). Mutations within TDP-43 or FUS are themselves neuropathogenic in ALS and some cases of FTLD. We used the outgrowth of caudal primary motor neurons (MNs) in zebrafish embryos to investigate the interaction of PGRN with TDP-43 and FUS in vivo. As reported previously, depletion of zebrafish PGRN-A (zfPGRN-A) is associated with truncated primary MNs and impaired motor function. Here we found that depletion of zfPGRN-A results in primary MNs outgrowth stalling at the horizontal myoseptum, a line of demarcation separating the myotome into dorsal and ventral compartments that is where the final destination of primary motor is assigned. Successful axonal outgrowth beyond the horizontal myoseptum depends in part upon formation of acetylcholine receptor clusters and this was found to be disorganized upon depletion of zfPGRN-A. PGRN reversed the effects of zfPGRN-A knockdown, but a related gene, zfPGRN-1, was without effect. Both knockdown of TDP-43 or FUS, as well as expression of humanTDP-43 and FUS mutants results in MN abnormalities that are reversed by co-expression of hPGRN mRNA. Neither TDP-43 nor FUS reversed MN phenotypes caused by the depletion of PGRN. Thus TDP-43 and FUS lie upstream of PGRN in a gene complementation pathway. The ability of PGRN to override TDP-43 and FUS neurotoxicity due to partial loss of function or mutation in the corresponding genes may have therapeutic relevance.