Identification and characterization of ubiquitinylation sites in TAR DNA-binding protein of 43 kDa (TDP-43).

TAR DNA-binding protein of 43 kDa (TDP-43) forms pathological aggregates in neurodegenerative diseases, particularly in certain forms of frontotemporal dementia and amyotrophic lateral sclerosis. Pathological modifications of TDP-43 include proteolytic fragmentation, phosphorylation, and ubiquitinylation. A pathognomonic TDP-43 C-terminal fragment (CTF) spanning amino acids 193-414 contains only four lysine residues that could be potentially ubiquitinylated. Here, serial mutagenesis of these four lysines to arginine revealed that not a single residue is responsible for the ubiquitinylation of mCherry-tagged CTF. Removal of all four lysines was necessary to suppress ubiquitinylation. Interestingly, Lys-408 substitution enhanced the pathological phosphorylation of the immediately adjacent serine residues 409/410 in the context of mCherry-CTF. Thus, Lys-408 ubiquitinylation appears to hinder Ser-409/410 phosphorylation in TDP-43 CTF. However, we did not observe the same effect for full-length TDP-43. We extended the mutagenesis study to full-length TDP-43 and performed MS. Ubiquitinylated lysine residues were identified in the nuclear localization sequence (NLS; Lys-84 and Lys-95) and RNA-binding region (mostly Lys-160, Lys-181, and Lys-263). Mutagenesis of Lys-84 confirmed its importance as the major determinant for nuclear import, whereas Lys-95 mutagenesis did not significantly affect TDP-43's nucleo-cytoplasmic distribution, solubility, aggregation, and RNA-processing activities. Nevertheless, the K95A mutant had significantly reduced Ser-409/410 phosphorylation, emphasizing the suspected interplay between TDP-43 ubiquitinylation and phosphorylation. Collectively, our analysis of TDP-43 ubiquitinylation sites indicates that the NLS residues Lys-84 and Lys-95 have more prominent roles in TDP-43 function than the more C-terminal lysines and suggests a link between specific ubiquitinylation events and pathological TDP-43 phosphorylation.

Regulation of LRRK2: insights from structural and biochemical analysis.

Leucine-rich repeat kinase 2 (LRRK2) is a multi-domain protein and its mutations can lead to Parkinson's disease. Recent studies on LRRK2 and homologue proteins have advanced our mechanistic understanding of LRRK2 regulation. Here, we summarize the available data on the biochemistry and structure of LRRK2 and postulate three possible layers of regulation, translocation, monomer-dimer equilibrium and intramolecular activation of domains.

Cellular Effects of the Antiepileptic Drug Valproic Acid in Glioblastoma.

BACKGROUND/AIMS: Valproic acid (VPA), an anticonvulsant and mood-stabilizing drug is used to treat epileptic seizure of glioblastoma patients. Besides its antiepileptic activity, VPA has been attributed further functions that improve the clinical outcome of glioblastoma patients. Those comprise the inhibition of some histone deacetylase (HDAC) isoforms which reportedly may result in radiosensitization. Retrospective analysis of patient data, however, could not unequivocally confirm a prolonged survival of glioblastoma patients receiving VPA. The present study aimed to identify potential VPA targets at the cellular level. METHODS: To this end, the effect of VPA on metabolism, Ca2+-, biochemical and electro-signaling, cell-cycling, clonogenic survival and transfilter migration was analyzed in three human glioblastoma lines (T98G, U-87MG, U251) by MTT assay, Ca2+ imaging, immunoblotting, patch-clamp recording, flow cytometry, delayed plating colony formation and modified Boyden chamber assays, respectively. In addition, the effect of VPA on clonogenic survival of primary glioblastoma spheroid cultures treated with temozolomide and fractionated radiation was assessed by limited dilution assay. RESULTS: In 2 of 3 glioblastoma lines, clinical relevant concentrations of VPA slightly slowed down cell cycle progression and decreased clonogenic survival. Furthermore, VPA induced Ca2+ signaling which was accompanied by pronounced K+ channel activity and transfilter cell migration. VPA did not affect metabolic NAD(P)H formation or radioresistance of the glioblastoma lines. Finally, VPA did not impair clonogenic survival or radioresistance of temozolomide-treated primary spheroid cultures. CONCLUSIONS: Combined, our in vitro data do not propose a general use of VPA as a radiosensitizer in anti-glioblastoma therapy.

TRPM8 is required for survival and radioresistance of glioblastoma cells.

TRPM8 is a Ca2+-permeable nonselective cation channel belonging to the melastatin sub-group of the transient receptor potential (TRP) family. TRPM8 is aberrantly overexpressed in a variety of tumor entities including glioblastoma multiforme where it reportedly contributes to tumor invasion. The present study aimed to disclose further functions of TRPM8 in glioma biology in particular upon cell injury by ionizing radiation. To this end, TCGA data base was queried to expose the TRPM8 mRNA abundance in human glioblastoma specimens and immunoblotting was performed to analyze the TRPM8 protein abundance in primary cultures of human glioblastoma. Moreover, human glioblastoma cell lines were irradiated with 6 MV photons and TRPM8 channels were targeted pharmacologically or by RNA interference. TRPM8 abundance, Ca2+ signaling and resulting K+ channel activity, chemotaxis, cell migration, clonogenic survival, DNA repair, apoptotic cell death, and cell cycle control were determined by qRT-PCR, fura-2 Ca2+ imaging, patch-clamp recording, transfilter migration assay, wound healing assay, colony formation assay, immunohistology, flow cytometry, and immunoblotting. As a result, human glioblastoma upregulates TRPM8 channels to variable extent. TRPM8 inhibition or knockdown slowed down cell migration and chemotaxis, attenuated DNA repair and clonogenic survival, triggered apoptotic cell death, impaired cell cycle and radiosensitized glioblastoma cells. Mechanistically, ionizing radiation activated and upregulated TRPM8-mediated Ca2+ signaling that interfered with cell cycle control probably via CaMKII, cdc25C and cdc2. Combined, our data suggest that TRPM8 channels contribute to spreading, survival and radioresistance of human glioblastoma and, therefore, might represent a promising target in future anti-glioblastoma therapy.

K+ channel signaling in irradiated tumor cells.

K+ channels crosstalk with biochemical signaling cascades and regulate virtually all cellular processes by adjusting the intracellular K+ concentration, generating the membrane potential, mediating cell volume changes, contributing to Ca2+ signaling, and directly interacting within molecular complexes with membrane receptors and downstream effectors. Tumor cells exhibit aberrant expression and activity patterns of K+ channels. The upregulation of highly "oncogenic" K+ channels such as the Ca2+-activated IK channel may drive the neoplastic transformation, malignant progression, metastasis, or therapy resistance of tumor cells. In particular, ionizing radiation in doses used for fractionated radiotherapy in the clinic has been shown to activate K+ channels. Radiogenic K+ channel activity, in turn, contributes to the DNA damage response and promotes survival of the irradiated tumor cells. Tumor-specific overexpression of certain K+ channel types together with the fact that pharmacological K+ channel modulators are already in clinical use or well tolerated in clinical trials suggests that K+ channel targeting alone or in combination with radiotherapy might become a promising new strategy of anti-cancer therapy. The present article aims to review our current knowledge on K+ channel signaling in irradiated tumor cells. Moreover, it provides new data on molecular mechanisms of radiogenic K+ channel activation and downstream signaling events.