Juan Alcalde

Neurodegenerative diseases are frequently associated with proteotoxic stress linked to disease specific proteins. The autophagy-lysosome system provides essential control of proteotoxic stress and its failure can lead to initiation of apoptosis. However, in aging and neurodegenerative diseases apoptosis is insufficient to account for all neuronal death, and several different cell death types have been reported in these contexts. Here we show that karyoptosis, a distinct form of cell death, can be induced by proteotoxic stress and then develops through nuclear degeneration and cellular expulsion of nuclear material. We establish that karyoptosis is regulated by the p38 kinase signalling pathway, which controls stability of the nuclear lamina protein LaminB1 via direct phosphorylation. We demonstrate that karyoptosis affects neurons in models of amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD) pathology. Finally, we identify karyoptotic features in post-mortem frontal cortex of FTD and Alzheimer’s disease (AD) patients. Together these findings characterise a form of cell death directly linked to proteotoxic stress and nuclear lamina stability that is associated with neurodegeneration.

The RNA-binding protein Fused in Sarcoma (FUS) undergoes phase separation associated with RNA processing. However, the prion-like low complexity (LC) domain of FUS forms solid-like aggregates in neurodegenerative diseases. Whether the formation of β-sheet structure associated with pathology is also physiologically/functionally relevant is debated. Similarly, if mislocalization alone or concomitant aggregation is responsible for FUS gain-of-function toxicity remains to be probed. Here, we introduce β-sheet breaking proline residues into FUS LC with the goal of preventing cross-β-driven aggregation without disrupting essential functions and phase separation. β-sheet-deficient FUS variants maintain native-like global motions, disorder, and phase separation, but no longer show a liquid-to-solid transition (LST). Biochemical partitioning, cellular localization, and auto- and cross-regulatory functions of FUS all remain essentially unchanged. Conversely, FUS-induced neurodegeneration in several Drosophila models is drastically reduced. These findings suggest a strategy for mitigating disease-related toxicity through backbone structure modulation to prevent prion-like domain protein aggregation. GRAPHICAL SUMMARY: The RNA-binding protein Fused in Sarcoma (FUS) undergoes phase separation as part of its physiological function but can aberrantly aggregate into solid-like assemblies in amyotrophic lateral sclerosis and frontotemporal dementia. To dissect the role of β-sheets in both function and pathological transition, we engineered β-sheet-preventing FUS variants via targeted proline residue insertions in the prion-like disordered region. These variants retained native structure, motions, and phase behavior yet showed dramatically reduced aggregation, both as an isolated prion-like domain and in full-length FUS. Crucially, these variants maintained a panel of FUS cellular functions that depend on FUS condensation but prevented FUS toxicity in fly models of neurodegeneration. Our findings implicate β-sheets as key drivers of FUS condensate maturation and neuronal toxicity, highlighting β-sheet modulation as a therapeutic strategy against FUS-related neurodegeneration. HIGHLIGHTS: Targeted proline additions disrupt β-sheet formation in FUS without altering native conformations, dynamics, or phase separation behavior β-sheet-deficient FUS variants prevent aggregation and liquid-to-solid transitions while retaining key biological functions In vivo models reveal attenuated toxicity of β-sheet-deficient FUS in Drosophila β-sheets are identified as central drivers of condensate maturation and neuronal death, offering a therapeutic entry point for modulating prion-like domain pathology

Summary: RNA-binding proteins (RBPs) are key regulators of gene expression and often contain intrinsically disordered regions that drive biomolecular condensation. Yet, how condensation affects RBP specificity and function remains poorly defined. Here, we use strategically designed point mutations to selectively impair canonical RNA-binding and condensation of the amyotrophic lateral sclerosis (ALS)-linked RBP Fused in Sarcoma (FUS). Using automated high-content imaging, we show that both properties shape nuclear ribonucleoprotein condensates and govern distinct aspects of FUS function in the DNA damage response. Transcriptome-wide mapping of FUS-RNA interactions reveals that the canonical RNA-binding domains recognise G-rich and C-rich motifs, whereas condensation selectively enhances binding to G-rich and structured sequences, often acting in concert with canonical RNA-binding to control transcriptional programs and splicing decisions. These findings provide a mechanistic framework for how FUS integrates condensation with sequence-specific RNA recognition to orchestrate nuclear organisation, genome stability and RNA metabolism. Furthermore, our rigorously validated mutants offer a platform for future mechanistic dissection of ALS pathogenesis and the development of targeted therapeutic strategies. Highlights: FUS condensation and RNA-binding can be functionally uncoupled with targeted point mutations Condensation drives FUS recruitment and assembly of nuclear RNP condensates FUS condensation and RNA-binding play distinct roles in the DNA damage response The structured RNA-binding domains confer specificity to G-rich and C-rich motifs Condensation enhances FUS binding to G-rich, structured RNA elements FUS condensation and canonical RNA-binding synergistically regulate gene expression

Cannabinoids act as pleiotropic compounds exerting, among others, a broad-spectrum of neuroprotective effects. These effects have been investigated in the last years in different preclinical models of neurodegeneration, with the cannabinoid type-1 (CB₁) and type-2 (CB₂) receptors concentrating an important part of this research. However, the issue has also been extended to additional targets that are also active for cannabinoids, such as the orphan G-protein receptor 55 (GPR55). In the present study, we investigated the neuroprotective potential of VCE-006.1, a chromenopyrazole derivative with biased orthosteric and positive allosteric modulator activity at GPR55, in murine models of two neurodegenerative diseases. First, we proved that VCE-006.1 alone could induce ERK1/2 activation and calcium mobilization, as well as increase cAMP response but only in the presence of lysophosphatidyl inositol. Next, we investigated this compound administered chronically in two neurotoxin-based models of Parkinson’s disease (PD), as well as in some cell-based models. VCE-006.1 was active in reversing the motor defects caused by 6-hydroxydopamine (6-OHDA) in the pole and the cylinder rearing tests, as well as the losses in tyrosine hydroxylase-containing neurons and the elevated glial reactivity detected in the substantia nigra. Similar cytoprotective effects were found in vitro in SH-SY5Y cells exposed to 6-OHDA. We also investigated VCE-006.1 in LPS-lesioned mice with similar beneficial effects, except against glial reactivity and associated inflammatory events, which remained unaltered, a fact confirmed in BV2 cells treated with LPS and VCE-006.1. We also analyzed GPR55 in these in vivo models with no changes in its gene expression, although GPR55 was down-regulated in BV2 cells treated with LPS, which may explain the lack of efficacy of VCE-006.1 in such an assay. Furthermore, we investigated VCE-006.1 in two genetic models of amyotrophic lateral sclerosis (ALS), mutant SOD1, or TDP-43 transgenic mice. Neither the neurological decline nor the deteriorated rotarod performance were prevented with this compound, and the same happened with the elevated microglial and astroglial reactivities, albeit modest spinal motor neuron preservation was achieved in both models. We also analyzed GPR55 in these in vivo models and found no changes in both TDP-43 transgenic and mSOD1 mice. Therefore, our findings support the view that targeting the GPR55 may afford neuroprotection in experimental PD, but not in ALS, thus stressing the specificities for the development of cannabinoid-based therapies in the different neurodegenerative disorders.