Neuronal function relies on careful coordination of organelle organization and transport. Kinesin-1 mediates transport of the endoplasmic reticulum (ER) and lysosomes into the axon and it is increasingly recognized that contacts between the ER and lysosomes influence organelle organization. However, it is unclear how organelle organization, inter-organelle communication and transport are linked and how this contributes to local organelle availability in neurons. Here, we show that somatic ER tubules are required for proper lysosome transport into the axon. Somatic ER tubule disruption causes accumulation of enlarged and less motile lysosomes at the soma. ER tubules regulate lysosome size and axonal translocation by promoting lysosome homo-fission. ER tubule – lysosome contacts often occur at a somatic pre-axonal region, where the kinesin-1-binding ER-protein P180 binds microtubules to promote kinesin-1-powered lysosome fission and subsequent axonal translocation. We propose that ER tubule – lysosome contacts at a pre-axonal region finely orchestrate axonal lysosome availability for proper neuronal function.

The mammalian circadian clock is well-known to be important for our sleep–wake cycles, as well as other daily rhythms such as temperature regulation, hormone release or feeding–fasting cycles. Under normal conditions, these daily cyclic events follow 24 h limit cycle oscillations, but under some circumstances, more complex nonlinear phenomena, such as the emergence of chaos, or the splitting of physiological dynamics into oscillations with two different periods, can be observed. These nonlinear events have been described at the organismic and tissue level, but whether they occur at the cellular level is still unknown. Our results show that period-doubling, chaos and splitting appear in different models of the mammalian circadian clock with interlocked feedback loops and in the absence of external forcing. We find that changes in the degradation of clock genes and proteins greatly alter the dynamics of the system and can induce complex nonlinear events. Our findings highlight the role of degradation rates in determining the oscillatory behaviour of clock components, and can contribute to the understanding of molecular mechanisms of circadian dysregulation.

Neuronal function relies on careful coordination of organelle organization and transport. Kinesin-1 mediates transport of the ER and lysosomes into the axon and it is increasingly recognized that contacts between the ER and lysosomes influence organelle organization. However, it is unclear how organelle organization, inter-organelle communication and transport are linked and how this contributes to local organelle availability in neurons. Here, we show that somatic ER tubules are required for proper lysosome transport into the axon. Somatic ER tubule disruption causes accumulation of enlarged and less motile lysosomes at the soma. ER tubules regulate lysosome size and axonal translocation by promoting lysosome homo-fission. ER tubule -lysosome contacts often occur at a somatic pre-axonal region, where the kinesin-1-binding ER-protein P180 binds microtubules to promote kinesin-1-powered lysosome fission and subsequent axonal translocation. We propose that ER tubule -lysosome contacts at a preaxonal region finely orchestrate axonal lysosome availability for proper neuronal function. KEYWORDSNeurons, axonal transport, lysosome, ER organization, ER tubules, kinesin-1, microtubules, pre-axonal region, lysosome motility and fission, P180. Özkan et al.Yet, it remains unclear how local ER organization regulates LE/ lysosome size and how this is linked to motor transfer and MT interaction at contact sites.Proper organization and transport of ER tubules and LEs/ lysosomes are crucial for neuronal development and function. ER tubules and LEs/ lysosomes are translocated from the soma into the axon by the kinesin-1 motor (Farías et al., 2017;Farías et al., 2019). Local availability of ER tubules instructs axon formation and regulates axonal synaptic vesicle cycling (Farías et al., 2019;Lindhout et al., 2019) and active transport of LEs/ lysosomes into the axon is required for proper clearance of faulty proteins and organelles located far away from the cell soma (Farías et al., 2017; Farfel-Becker et al., 2019). Interestingly, mutations in genes encoding ER-shaping proteins cause the neurodegenerative disease hereditary spastic paraplegia, in which aberrant lysosomes have been observed (Westrate et al., 2015; Allison et al., 2017;Lee and Blackstone, 2020). Therefore, it is important to understand how the organization of the ER and inter-organelle communication contribute to lysosome organization and local availability in neurons.Here, we show that ER shape regulates local lysosome availability in neurons, in which somatic ER tubules promote lysosome translocation into the axon. Disruption of somatic ER tubules causes accumulation of enlarged and less motile mature lysosomes in the soma due to impaired lysosome homofission. We find that ER tubule -lysosome contacts are enriched in a pre-axonal region. The MT-and kinesin-1-binding ER protein P180 is enriched and co-distributed with kinesin-1-decorated axonal MT tracks in the same pre-axonal region, where it promotes lysosome motility, fission and axonal translocation. Together, ...