Proteolytically generated soluble Tweak Receptor Fn14 is a blood biomarker for γ-secretase activity.

Fn14 is a cell surface receptor with key functions in tissue homeostasis and injury but is also linked to chronic diseases. Despite its physiological and medical importance, the regulation of Fn14 signaling and turnover is only partly understood. Here, we demonstrate that Fn14 is cleaved within its transmembrane domain by the protease γ-secretase, resulting in secretion of the soluble Fn14 ectodomain (sFn14). Inhibition of γ-secretase in tumor cells reduced sFn14 secretion, increased full-length Fn14 at the cell surface, and enhanced TWEAK ligand-stimulated Fn14 signaling through the NFκB pathway, which led to enhanced release of the cytokine tumor necrosis factor. γ-Secretase-dependent sFn14 release was also detected ex vivo in primary tumor cells from glioblastoma patients, in mouse and human plasma and was strongly reduced in blood from human cancer patients dosed with a γ-secretase inhibitor prior to chimeric antigen receptor (CAR)-T-cell treatment. Taken together, our study demonstrates a novel function for γ-secretase in attenuating TWEAK/Fn14 signaling and suggests the use of sFn14 as an easily measurable pharmacodynamic biomarker to monitor γ-secretase activity in vivo.

Multi-omics profiling identifies a deregulated FUS-MAP1B axis in ALS/FTD-associated UBQLN2 mutants.

Ubiquilin-2 (UBQLN2) is a ubiquitin-binding protein that shuttles ubiquitinated proteins to proteasomal and autophagic degradation. UBQLN2 mutations are genetically linked to the neurodegenerative disorders amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD). However, it remains elusive how UBQLN2 mutations cause ALS/FTD. Here, we systematically examined proteomic and transcriptomic changes in patient-derived lymphoblasts and CRISPR/Cas9-engineered HeLa cells carrying ALS/FTD UBQLN2 mutations. This analysis revealed a strong up-regulation of the microtubule-associated protein 1B (MAP1B) which was also observed in UBQLN2 knockout cells and primary rodent neurons depleted of UBQLN2, suggesting that a UBQLN2 loss-of-function mechanism is responsible for the elevated MAP1B levels. Consistent with MAP1B's role in microtubule binding, we detected an increase in total and acetylated tubulin. Furthermore, we uncovered that UBQLN2 mutations result in decreased phosphorylation of MAP1B and of the ALS/FTD-linked fused in sarcoma (FUS) protein at S439 which is critical for regulating FUS-RNA binding and MAP1B protein abundance. Together, our findings point to a deregulated UBQLN2-FUS-MAP1B axis that may link protein homeostasis, RNA metabolism, and cytoskeleton dynamics, three molecular pathomechanisms of ALS/FTD.

Canonical and Noncanonical Autophagy Pathways in Microglia.

Besides the ubiquitin-proteasome system, autophagy is a major degradation pathway within cells. It delivers invading pathogens, damaged organelles, aggregated proteins, and other macromolecules from the cytosol to the lysosome for bulk degradation. This so-called canonical autophagy activity contributes to the maintenance of organelle, protein, and metabolite homeostasis as well as innate immunity. Over the past years, numerous studies rapidly deepened our knowledge on the autophagy machinery and its regulation, driven by the fact that impairment of autophagy is associated with several human pathologies, including cancer, immune diseases, and neurodegenerative disorders. Unexpectedly, components of the autophagic machinery were also found to participate in various processes that do not involve lysosomal delivery of cytosolic constituents. These functions are defined as noncanonical autophagy. Regarding neurodegenerative diseases, most research was performed in neurons, while for a long time, microglia received considerably less attention. Concomitant with the notion that microglia greatly contribute to brain health, the understanding of the role of autophagy in microglia expanded. To facilitate an overview of the current knowledge, here we present the fundamentals as well as the recent advances of canonical and noncanonical autophagy functions in microglia.

Glia-specific autophagy dysfunction in ALS.

Neuronal cell death is the main pathological feature of chronic neurodegenerative diseases (NDs) such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). As age is strongly linked to NDs, these diseases are one of the leading medical and societal challenges faced by the rapidly aging western societies. Despite the increasing prevalence, the causes and mechanisms behind most NDs are still vague. A common hallmark of several NDs is the accumulation and aggregation of proteins. Prominent examples are amyloid beta and tau in Alzheimer's disease, α-synuclein in Parkinson's disease and transactive response DNA binding protein 43 kDa (TDP-43) in ALS and FTD. Under physiological conditions, protein quality control systems, namely the ubiquitin proteasome system and the autophagy machinery, eliminate such aberrant protein forms and thereby prevent proteotoxic stress. However, as proteins must unfold to undergo proteasomal degradation, aggregated proteins are poor substrates for the proteasome. Such proteins are thought to be primarily turned over by autophagy. Therefore, autophagy is considered a critical ND-protective pathway, which opens up potential new therapeutic interventions. One drawback is that the majority of research in NDs has been focused on elucidating the underlying pathomechanisms in neurons. However, neurons make up only about half of the brain cells with neuroglia being the other major central nervous system (CNS) cell type. Due to the ubiquitous presence of disease-causing mutations in all cells of the CNS, it is likely that non-neuronal cells contribute to the disease onset and/or progression. While our understanding of the roles of autophagy and its contribution to neurodegeneration in neurons deepened considerably over the last years, still comparatively little is known about the functions and disease contribution of the autophagy machinery in glia cells.