Running title: BICD2 ablation in muscle causes motor neuron loss Characters (excluding spaces): 18,564 Summary Missense mutations in the cargo adaptor protein BICD2 cause SMALED2, a developmental disease of motor neurons. In this study, the authors show that BICD2 mutations cause motor neuron loss by a non-cell autonomous mechanism determining a disabling impairment of muscle function.3 Abstract BICD2 is a key component of the dynein/dynactin motor complex. Autosomal dominant mutations in BICD2 cause Spinal Muscular Atrophy Lower Extremity Predominant 2 (SMALED2), a developmental disease of motor neurons. In this study we sought to examine the motor neuron phenotype of conditional Bicd2 -/mice. Bicd2 -/mice show a significant reduction in the number of motor axons of the L4 ventral root compared to wild type mice. Muscle-specific knockout of Bicd2, but not motor neuronspecific Bicd2 loss, results in a reduction in L4 ventral axons comparable to global Bicd2 -/mice. Rab6, a small GTPase required for the sorting of secretory vesicles from the TGN to the plasma membrane is a major binding partner of BICD2. We therefore examined the secretory pathway in SMALED2 patient fibroblasts and demonstrated impaired flow of constitutive secretory cargoes. Together, these data indicate that BICD2 loss from muscles is a major driver of non-cell autonomous pathology with important implications for future therapeutic approaches to SMALED2.

Aggregation of the Tau protein defines progression of neurodegenerative diseases, including Alzheimer's Disease. Tau assembles into oligomers and fibrils. The molecular basis of their toxicity is poorly understood. Here we show that p-stacking by Arginine side chains rewires the interactome of Tau upon aggregation. Oligomeric nano-aggregates scavenge the COPI complex, fibrils attract proteins involved in microtubule binding, RNA binding and phosphorylation. The aberrant interactors have disordered regions with unusual sequence features. Arginines are crucial to initiate such aberrant interactions. Remarkably, substitution of Arginines by Lysines abolishes scavenging, which indicates a key role for the pi-stacking of the Arginine side chain. The molecular chaperone Hsp90 tames such re-arrangements, which suggests that the natural protein quality control system can suppress aberrant interactions. Together, our data present a molecular mode of action for derailment of protein-protein interaction in neurodegeneration.

Neuronal development is a multistep process with different regulatory programs that shapes neurons to form dendrites, axons and synapses. To date, knowledge on neuronal development is largely based on murine data and largely restricted to the genomic and transcriptomic level. Advances in stem cell differentiation now enable the study of human neuronal development, and here we provide a mass spectrometry-based quantitative proteomic signature, at high temporal resolution, of human stem cellderived neurons. To reveal proteomic changes during neuronal development we make use of two differentiation approaches, either by expression of neurogenin-2 (Ngn2) leading to glutamatergic induced neurons (iN) or via small molecule manipulations, leading to patterned motor neurons. Our analysis revealed key proteins that show significant expression changes (FDR <0.001) during neuronal differentiation. We overlay our proteomics data with available transcriptomic data during neuronal differentiation and show distinct, datatype-specific, signatures. Overall, we provide a rich resource of information on proteins associated with human neuronal development, and moreover, highlight several signaling pathways involved, such as Wnt and Notch. 2 KEYWORDS iPSC; human; neuron differentiation; quantitative mass spectrometry; TMT-10plex