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A comprehensive understanding of the SARS-CoV-2 infection dynamics and the ensuing host immune responses is needed to explain the pathogenesis as it relates to viral transmission. Here, we report results from an ongoing, longitudinal, workplace clinical surveillance study that address knowledge gaps surrounding SARS-CoV-2 in vivo kinetics, particularly in the earliest stages after exposure. Nine study participants who developed COVID-19 between November, 2020 and March, 2021 were monitored at high temporal resolution for three months in terms of viral loads as well as associated inflammatory biomarker and antibody responses. Analysis of the resulting datasets, supported by Bayesian modeling, allowed the underlying kinetic processes to be described, yielding a number of unexpected findings. Early viral replication is more rapid (median doubling time, 3.1 hours) than previously thought, shrinking the window between exposure and viral shedding. Results from our small study give a rare insight into the life-cycle of COVID-19 infection and hold a number of important biological, clinical, and public health implications.

Neurons use multiple modes of endocytosis, including clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE), during mild and intense neuronal activity, respectively, to maintain stable neurotransmission. While molecular players modulating CME are well characterized, factors regulating ADBE and mechanisms coordinating CME and ADBE activations remain poorly understood. Here we report that Minibrain/DYRK1A (Mnb), a kinase mutated in autism and up-regulated in Down’s syndrome, plays a novel role in suppressing ADBE. We demonstrate that Mnb, together with calcineurin, delicately coordinates CME and ADBE by controlling the phosphoinositol phosphatase activity of synaptojanin (Synj) during varying synaptic demands. Functional domain analyses reveal that Synj’s 5′-phosphoinositol phosphatase activity suppresses ADBE, while SAC1 activity is required for efficient ADBE. Consequently, Parkinson’s disease mutation in Synj’s SAC1 domain impairs ADBE. These data identify Mnb and Synj as novel regulators of ADBE and further indicate that CME and ADBE are differentially governed by Synj’s dual phosphatase domains.

Synapses exhibit an astonishing degree of adaptive plasticity in healthy and disease states. We have investigated whether synapses also adjust to life stages imposed by novel developmental programs for which they were never molded by evolution. Under conditions where Drosophila larvae are terminally arrested, we have characterized synaptic growth, structure and function at the neuromuscular junction (NMJ). While wild-type larvae transition to pupae after 5 days, arrested third instar (ATI) larvae persist for 35 days, during which NMJs exhibit extensive overgrowth in muscle size, presynaptic release sites, and postsynaptic glutamate receptors. Remarkably, despite this exuberant growth, stable neurotransmission is maintained throughout the ATI lifespan through a potent homeostatic reduction in presynaptic neurotransmitter release. Arrest of the larval stage in stathmin mutants also reveals a degree of progressive instability and neurodegeneration that was not apparent during the typical larval period. Hence, an adaptive form of presynaptic depression stabilizes neurotransmission during an extended developmental period of unconstrained synaptic growth. More generally, the ATI manipulation provides a powerful system for studying neurodegeneration and plasticity across prolonged developmental timescales.

Synapses exhibit an astonishing degree of adaptive plasticity in healthy and disease states. We have investigated whether synapses also adjust to life stages imposed by novel developmental programs for which they were never molded by evolution. Under conditions in which Drosophila larvae are terminally arrested, we have characterized synaptic growth, structure and function at the neuromuscular junction (NMJ). Although wild-type larvae transition to pupae after 5 days, arrested third instar (ATI) larvae persist for 35 days, during which time NMJs exhibit extensive overgrowth in muscle size, presynaptic release sites and postsynaptic glutamate receptors. Remarkably, despite this exuberant growth, stable neurotransmission is maintained throughout the ATI lifespan through a potent homeostatic reduction in presynaptic neurotransmitter release. Arrest of the larval stage in stathmin mutants also reveals a degree of progressive instability and neurodegeneration that was not apparent during the typical larval period. Hence, an adaptive form of presynaptic depression stabilizes neurotransmission during an extended developmental period of unconstrained synaptic growth. More generally, the ATI manipulation provides a powerful system for studying neurodegeneration and plasticity across prolonged developmental timescales.