The packaging and budding of Gag polyprotein and viral RNA is a critical step in the HIV-1 life cycle. High-resolution structures of the Gag polyprotein have revealed that the capsid (CA) and spacer peptide 1 (SP1) domains contain important interfaces for Gag self-assembly. However, the molecular details of the multimerization process, especially in the presence of RNA and the cell membrane, have remained unclear. In this work, we investigate the mechanisms that work in concert between the polyproteins, RNA, and membrane to promote immature lattice growth. We develop a coarse-grained (CG) computational model that is derived from subnanometer resolution structural data. Our simulations recapitulate contiguous and hexameric lattice assembly driven only by weak anisotropic attractions at the helical CA-SP1 junction. Importantly, analysis from CG and single-particle tracking photoactivated localization (spt-PALM) trajectories indicates that viral RNA and the membrane are critical constituents that actively promote Gag multimerization through scaffolding, while overexpression of short competitor RNA can suppress assembly. We also find that the CA amino-terminal domain imparts intrinsic curvature to the Gag lattice. As a consequence, immature lattice growth appears to be coupled to the dynamics of spontaneous membrane deformation. Our findings elucidate a simple network of interactions that regulate the early stages of HIV-1 assembly and budding.

The termination of the proliferation of Drosophila neural stem cells, also known as neuroblasts (NBs), requires a "decommissioning" phase that is controlled in a lineage-specific manner. Most NBs, with the exception of those of the Mushroom body (MB), are decommissioned by the ecdysone receptor and mediator complex causing them to shrink during metamorphosis, followed by nuclear accumulation of Prospero and cell cycle exit. Here, we demonstrate that the levels of Imp and Syp RNA-binding proteins regulate NB decommissioning. Descending Imp and ascending Syp expression have been shown to regulate neuronal temporal fate. We show that Imp levels decline slower in the MB than other central brain NBs. MB NBs continue to express Imp into pupation, and the presence of Imp prevents decommissioning partly by inhibiting the mediator complex. Late-larval induction of transgenic Imp prevents many non-MB NBs from decommissioning in early pupae. Moreover, the presence of abundant Syp in aged NBs permits Prospero accumulation that, in turn, promotes cell cycle exit. Together our results reveal that progeny temporal fate and progenitor decommissioning are co-regulated in protracted neuronal lineages.

In vivo intracellular recordings of hippocampal neurons reveal the occurrence of fast events of small amplitude called spikelets or fast prepotentials. Because intracellular recordings have been restricted to anesthetized or head-fixed animals, it is not known how spikelet activity contributes to hippocampal spatial representations. We addressed this question in CA1 pyramidal cells by using in vivo whole-cell recording in freely moving rats. We observed a high incidence of spikelets that occurred either in isolation or in bursts and could drive spiking as fast prepotentials of action potentials. Spikelets strongly contributed to spiking activity, driving approximately 30% of all action potentials. CA1 pyramidal cell firing and spikelet activity were comodulated as a function of the animal’s location in the environment. We conclude that spikelets have a major impact on hippocampal activity during spatial exploration.

Neurons of the central nervous system (CNS) exhibit a variety of forms of synaptic plasticity, including associative long-term potentiation and depression (LTP/D), homeostatic activity-dependent scaling and distance-dependent scaling. Regulation of synaptic neurotransmitter receptors is currently thought to be a common mechanism amongst many of these forms of plasticity. In fact, glutamate receptor 1 (GluR1 or GluRA) subunits containing L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors have been shown to be required for several forms of hippocampal LTP and a particular hippocampal-dependent learning task. Because of this importance in associative plasticity, we sought to examine the role of these receptors in other forms of synaptic plasticity in the hippocampus. To do so, we recorded from the apical dendrites of hippocampal CA1 pyramidal neurons in mice lacking the GluR1 subunit (GluR1 -/-). Here we report data from outside-out patches that indicate GluR1-containing receptors are essential to the extrasynaptic population of AMPA receptors, as this pool was nearly empty in the GluR1 -/- mice. Additionally, these receptors appear to be a significant component of the synaptic glutamate receptor pool because the amplitude of spontaneous synaptic currents recorded at the site of input and synaptic AMPA receptor currents evoked by focal glutamate uncaging were both substantially reduced in these mice. Interestingly, the impact on synaptic weight was greatest at distant synapses such that the normal distance-dependent synaptic scaling used by these cells to counter dendritic attenuation was lacking in GluR1 -/- mice. Together the data suggest that the highly regulated movement of GluR1-containing AMPA receptors between extrasynaptic and synaptic receptor pools is critically involved in establishing two functionally diverse forms of synaptic plasticity: LTP and distance-dependent scaling.

Defects in mitochondrial DNA (mtDNA) are associated with several different human diseases, including the mitochondrial encephalomyopathies. The mutations include deletions but also duplications and point mutations. Individuals with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) carry a common A-to-G substitution in a highly conserved portion of the gene for transfer RNA(Leu(UUR)). Although the MELAS mutation may be comparable to the defect in the tRNA(Lys) gene associated with MERRF (myoclonus epilepsy associated with ragged-red fibres), it is also embedded in the middle of a tridecamer sequence necessary for the formation of the 3’ ends of 16S ribosomal RNA in vitro. We found that the MELAS mutation results in severe impairment of 16S rRNA transcription termination, which correlates with a reduced affinity of the partially purified termination protein for the MELAS template. This suggests that the molecular defect in MELAS is the inability to produce the correct type and quantity of rRNA relative to other mitochondrial gene products.

The mammalian hippocampus is crucial for episodic memory formation and transiently retains information for about 3-4 weeks in adult mice and longer in humans. Although neuroscientists widely believe that neural synapses are elemental sites of information storage, there has been no direct evidence that hippocampal synapses persist for time intervals commensurate with the duration of hippocampal-dependent memory. Here we tested the prediction that the lifetimes of hippocampal synapses match the longevity of hippocampal memory. By using time-lapse two-photon microendoscopy in the CA1 hippocampal area of live mice, we monitored the turnover dynamics of the pyramidal neurons' basal dendritic spines, postsynaptic structures whose turnover dynamics are thought to reflect those of excitatory synaptic connections. Strikingly, CA1 spine turnover dynamics differed sharply from those seen previously in the neocortex. Mathematical modelling revealed that the data best matched kinetic models with a single population of spines with a mean lifetime of approximately 1-2 weeks. This implies ∼100% turnover in ∼2-3 times this interval, a near full erasure of the synaptic connectivity pattern. Although N-methyl-d-aspartate (NMDA) receptor blockade stabilizes spines in the neocortex, in CA1 it transiently increased the rate of spine loss and thus lowered spine density. These results reveal that adult neocortical and hippocampal pyramidal neurons have divergent patterns of spine regulation and quantitatively support the idea that the transience of hippocampal-dependent memory directly reflects the turnover dynamics of hippocampal synapses.

Continuous monitoring of blood pressure in laboratory animals is necessary to understand the effect of treatments for cardiovascular related conditions, such as hypertension. Current methods to measure laboratory rat blood pressure require the animal to be constrained. Our proposed method is a small implantable device which fits around the carotid artery of the rat. Initial data from a mock rat artery setup, with equivalent artery pressure as found in the rat, show that the cuff design effectively detects the pressure change inside the mock artery.

The Importance Weighted Auto Encoder (IWAE) objective has been shown to improve the training of generative models over the standard Variational Auto Encoder (VAE) objective. Here, we derive importance weighted extensions to Adversarial Variational Bayes (AVB) and Adversarial Autoencoder (AAE). These latent variable models use implicitly defined inference networks whose approximate posterior density qφ(z|x) cannot be directly evaluated, an essential ingredient for importance weighting. We show improved training and inference in latent variable models with our adversarially trained importance weighting method, and derive new theoretical connections between adversarial generative model training criteria and marginal likelihood based methods. We apply these methods to the important problem of inferring spiking neural activity from calcium imaging data, a challenging posterior inference problem in neuroscience, and show that posterior samples from the adversarial methods outperform factorized posteriors used in VAEs.

Inducible chemical-genetic fluorescent markers are promising tools for live cell imaging requiring high spatiotemporal resolution and low background fluorescence. The fluorescence-activating and absorption shifting tag (FAST) was recently developed to form fluorescent molecular complexes with a family of small, synthetic fluorogenic chromophores (so-called fluorogens). Here, we use rational design to modify the binding pocket of the protein and screen for improved fluorescence performances with four different fluorogens. The introduction of a single mutation results in improvements in both quantum yield and dissociation constant with nearly all fluorogens tested. Our improved FAST (iFAST) allowed the generation of a tandem iFAST (td-iFAST) that forms green and red fluorescent reporters 1.6-fold and 2-fold brighter than EGFP and mCherry, respectively, while having a comparable size.