Frank Hirth

The clade Trilobita is recognized for its extraordinary morphological diversifications amidst a highly constrained body plan. This raises the question of whether trilobite nervous systems were conservative or comparably diverse. However, despite the identification of thousands of trilobite species and increasing evidence for neural preservation in other Paleozoic euarthropods, the recognition of a fossilized trilobite brain has remained elusive. Here we describe the identification of a trilobite brain and its spatially constrained position within its cranidium. An unusually preserved specimen of the lower Cambrian trilobite Yunnanocephalus yunnanensis reveals a bilaterally organized organ connected to its lateral eyes. Comparisons to the brain of a trilobitomorph xandarellid support the interpretation of this organ as a modest cerebrum comprising prosocerebral, protocerebral, and deutocerebral domains, an organization typifying the euarthropod cerebral ground pattern. These findings suggest that the cerebrum of Yunnanocephalus could have supported the multimodal sensory processing and behavioral repertoires of trilobites, while retaining the deep evolutionary conservation of brain organization across euarthropod lineages.

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.

Fossilized traces of neuropils, nerves and ganglia have demonstrated that cerebral organization in Cambrian arthropods conforms to a ground pattern defining one of today’s two existing euarthropod clades, Mandibulata and Chelicerata ^1–8 . Artiopoda - a third clade including trilobites and soft-bodied relatives - persisted until the late Carboniferous ^9,10 , but its cerebral organization has remained unknown. Here we identify and reconstruct fossilized neural traces of the artiopodan Xandarella spectaculum ¹⁰ , which reveal an expanded prosocerebrum associated with paired ocelli, a truncated protocerebrum supplied by substantial lateral eyes, and salient deutocerebral antennular lobes. This arrangement predicts reliance on chemosensory-guided foraging, with visual processing largely limited to dorsal orientational cues and simple local motion signals. The artiopodan brain thus reveals clade-specific modifications of homologous domains of the euarthropod cerebral ground pattern ^4,6–8 established in the early Cambrian.

Cambrian fossils from the Chengjiang biota demonstrate that over half a billion years ago early stem euarthropods existed coevally with representatives of already recognizable crown groups. Prominent stem taxa were Fuxianhuia protensa and Alalcomenaeus whose cerebral and ganglionic traits identify them as, respectively, stem mandibulates and stem chelicerates. Here we report on the visual systems and brain of the enigmatic lower Cambrian euarthropod Jianfengia multisegmentalis, which reveals neural traits suggestive of Pancrustacea despite its possession of ‘great appendages’. As occur in pancrustaceans, three nested optic neuropils are resolved in the eyestalks of Jianfengia, together with rostral ocelli and their associated nerves supplying a discrete forebrain region. Sutured eyestalks typifying crown Malacostraca provide compound eyes populated by ommatidia revealing structures suggesting cone-building cells. These and other neuroanatomical traits provide a powerful tool for resolving euarthropod relationships. Phylogenetic analyses deploying neural traits of Jianfengia, other Cambrian taxa, and extant Euarthropoda elucidate the status of Jianfengia as sister to total Mandibulata and reveal the short-bodied ‘great appendage’ Leanchoiliidae as sister to total Chelicerata. Together these data provide independent evidence for a 23 year-old proposition that ‘great appendage’ morphology defines the early stem from which derived the two branches of the euarthropod tree of life.

Cytoplasmic mislocalisation and nuclear depletion of TDP-43 are pathological hallmarks of amyotrophic lateral sclerosis (ALS), including mutations in the C9ORF72 gene that characterise the most common genetic form of ALS (C9ALS). Studies in human cells and animal models have associated cytoplasmic mislocalisation of TDP-43 with abnormalities in nuclear transport receptors, referred to as karyopherins, that mediate the nucleocytoplasmic shuttling of TDP-43. Yet the relationship between karyopherin abnormalities and TDP-43 pathology are unclear. Here we report karyopherin-α4 (KPNA4) pathology in the spinal cord of TDP-43-positive sporadic ALS and C9ALS patients. Structural analyses revealed the selective interaction between KPNA subtypes, especially KPNA4, with the nuclear localisation signal (NLS) of TDP-43. Targeted cytoplasmic mislocalisation and nuclear depletion of TDP-43 caused KPNA4 pathology in human cells. Similar phenotypes were observed in Drosophila whereby cytoplasmic accumulation of the TDP-43 homolog, TBPH, caused the nuclear decrease and cytosolic mislocalisation of the KPNA4 homolog, Importin-α3 (Impα3). In contrast, induced accumulation of Impα3 was not sufficient to cause TBPH mislocalisation. Instead, targeted gain of Impα3 in the presence of accumulating cytosolic TBPH, restored Impα3 localisation and partially rescued nuclear TBPH. These results demonstrate that cytoplasmic accumulation of TDP-43 causes karyopherin pathology that characterises ALS spinal cord. Together with earlier reports, our findings establish KPNA4 abnormalities as a molecular signature of TDP-43 proteinopathies and identify it as a potential therapeutic target to sustain nuclear TDP-43 essential for cellular homeostasis affected in ALS and frontotemporal dementia.

SUMMARY: Early Cambrian fossils from the Chengjiang biota demonstrate that over half a billion years ago early stem euarthropods existed coevally with representatives of already recognizable crown groups. Prominent stem taxa were Fuxianhuia protensa and Alalcomenaeus whose cerebral and ganglionic traits identify them as, respectively, stem mandibulates and stem chelicerates. Here we report features of the visual systems and brain of the enigmatic lower Cambrian stem euarthropod Jianfengia multisegmentalis , which despite the absence of trunk tagmatization reveals neural traits that justify its inclusion within stem Pancrustacea also known as Tetraconata. The sutured eyestalks of Jianfengia, typifying crown Malacostraca, terminate as compound eyes populated by ommatidia that reveal tetradic structures indicative of cone-building Semper cells. Preserved neuropils and tracts in the eyestalks of Jianfengia resolve nested optic neuropils as occur in eucrustaceans. The nauplius-like eyes of Jianfengia and their associated nerves supply a discrete forebrain region, as they do in pancrustaceans. These attributes distinguish Pancrustacea from stem Chelicerata and extant Myriapoda as does the organization of the jianfengiid deutocerebrum and tritocerebrum. In the absence of external traits that differentiate identities amongst stem euarthropods, neuroanatomical traits provide a powerful tool for discerning characteristic distinctions and correspondences for elucidating euarthropod relationships. Here, we demonstrate that cerebral arrangements in Jianfengia correspond to those of pancrustaceans existing today.

Fossils from the lower Cambrian provide crucial insights into the diversification of arthropod lineages: Mandibulata, exemplified by centipedes, insects, and crustaceans; and Chelicerata, represented by sea spiders, horseshoe crabs, and arachnids, the last including spiders, scorpions, and ticks ¹ . Two mid-Cambrian genera claimed as stem chelicerates are Mollisonia and Sanctacaris, defined by a carapaced prosoma equipped with clustered limbs, followed by a segmented trunk opisthosoma equipped with appendages for swimming and respiration ^2–4 . Up to the present, the phyletic status of Mollisoniidae and Sanctacariidae has been that of a basal chelicerate ² , stemward of Leanchoiliidae, whose neuromorphology resembles that of extant Merostomata (horseshoe crabs) ⁵ . Here, we identify preserved traces of neuronal tissues in Mollisonia symmetrica that crucially depart from a merostome organization. Instead, a radiating organization of metameric neuropils occupying most of its prosoma is situated behind a pair of oval unsegmented neuropils directly connected to paired chelicerae extending from the front of the prosoma. The disposition and connections of these neuropils is the neuroanatomical signature that denotes a complete reversal of the three genetically distinct domains defining arthropod brains ⁶ . Thus, in Mollisonia the deutocerebrum is the most rostral domain with the proso-and protocerebral domains folded backwards such that tracts from the principal eyes extend caudally to reach their prosocerebral destination. These defining neuromorphological features illuminate a marine origin of Arachnida from which evolved the planet’s most successful arthropodan predators.

Lysosomal clearance of proteotoxic stressors through autophagy is an essential process underlying cellular homeostasis. Its failure can lead to cell death ^1,2 , by initiating apoptotic pathways ^3–11 . However, in aging and neurodegenerative diseases apoptosis is insufficient to account for all neuronal death, and different cell death types may also be involved ^12–14 . Here, we identify karyoptosis as a previously unknown form of non-apoptotic cell death. We report that karyoptosis is induced by lysosomal autophagic clearance impairment and proteotoxic stress; develops through nuclear degeneration; shares features with cellular senescence and is regulated by the p38a signalling pathway through direct phosphorylation of the nuclear lamina protein LaminB1. We further demonstrate that karyoptosis affects neurons in in-vitro and in-vivo 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, suggesting a common pathological signature. Together these findings establish a novel form of cell death directly linked to proteotoxic stress and cellular senescence that is associated with neurodegeneration. Finally, this mechanism provides new therapeutic opportunities in age-related neurodegenerative diseases.