Differential behaviors of calcium-induced calcium release in one dimensional dendrite by Nernst-Planck equation, cable model and pure diffusion model.

The source and dynamics of calcium is the key factor that regulates dendritic integration. Apart from the voltage-gated and ligand-gated calcium influx, an important source of calcium is from inner store of endoplasmic reticulum with a regenerative process of calcium-induced calcium release (CICR). To trigger this process, inositol 1,4,5-trisphosphate (IP3) and calcium are needed to satisfy certain requirements. The aim of our paper is to investigate how the CICR depends on the dynamics of membrane potential. We utilize one dimensional dendritic model to calculate membrane potential by Nernst-Planck Equation (NPE) and cable model and Pure Diffusion (PD) model, computational simulations are carried out to inject the calcium influx by synaptic stimulation and to predict subsequent CICR and calcium wave propagation. Our results demonstrate that CICR initiation and calcium wave propagation have much difference between electro-diffusion process of NPE and cable model. We find that cable model has lower threshold of IP3 stimulation to trigger CICR but is more difficult for calcium propagation than NPE, PD model requires even higher threshold of IP3 to initiate CICR process and calcium duration is shorter than NPE; the regenerative calcium wave propagates with faster speed in NPE than that in cable model and in PD model. Our work addresses the important role of electro-diffusion dynamics of charged ions in regulating CICR process in dendritic structure; and provides theoretical predictions for neurological process which requires sustaining calcium for downstream signaling processes.

The Mobility of Neurofilaments in Mature Myelinated Axons of Adult Mice.

Studies in cultured neurons have shown that neurofilaments are cargoes of axonal transport that move rapidly but intermittently along microtubule tracks. However, the extent to which axonal neurofilaments move in vivo has been controversial. Some researchers have proposed that most axonally transported neurofilaments are deposited into a persistently stationary network and that only a small proportion of axonal neurofilaments are transported in mature axons. Here we use the fluorescence photoactivation pulse-escape technique to test this hypothesis in intact peripheral nerves of adult male hThy1-paGFP-NFM mice, which express low levels of mouse neurofilament protein M tagged with photoactivatable GFP. Neurofilaments were photoactivated in short segments of large, myelinated axons, and the mobility of these fluorescently tagged polymers was determined by analyzing the kinetics of their departure. Our results show that >80% of the fluorescence departed the window within 3 h after activation, indicating a highly mobile neurofilament population. The movement was blocked by glycolytic inhibitors, confirming that it was an active transport process. Thus, we find no evidence for a substantial stationary neurofilament population. By extrapolation of the decay kinetics, we predict that 99% of the neurofilaments would have exited the activation window after 10 h. These data support a dynamic view of the neuronal cytoskeleton in which neurofilaments cycle repeatedly between moving and pausing states throughout their journey along the axon, even in mature myelinated axons. The filaments spend a large proportion of their time pausing, but on a timescale of hours, most of them move.

Deciphering functional roles of synaptic plasticity and intrinsic neural firing in developing mouse visual cortex layer IV microcircuit.

Between the onset of the critical period of mouse primary visual cortex and eye opening at postnatal day 14 is a complex process and that is vital for the cognitive function of vision. The onset of the critical period of mouse primary visual cortex involves changes of the intrinsic firing property of each neuron and short term plasticity of synapses. In order to investigate the functional role of each factor in regulating the circuit firing activity during the critical period plasticity, we adopted the Markram's model for short term plasticity and Wilson's model for intrinsic neuron firing activity, and construct a microcircuit for mouse visual cortex layer IV based on the connection probabilities from experimental results. Our results indicate that, during CP development, the most critical factors that regulate the firing pattern of microcircuit is the short term plasticity of the synapse from PC to PV and SST interneurons, which upregulates the PV interneuron firing and produces new balance between excitation and inhibition; the intrinsic firing activity of PC and PV during development downregulates the firing frequency of the circuits. In addition, we have investigated the function of feedforward excitatory thalamic-cortical projection to PC and PV interneuron during CP, and found that neural firing activity largely depends on the TC input and the results are similar to the local circuit with minor differences. We conclude that the short term plasticity development during critical period plays a crucial role in regulating the circuit behavior.

A possible mechanism for neurofilament slowing down in myelinated axon: Phosphorylation-induced variation of NF kinetics.

Neurofilaments(NFs) are the most abundant intermediate filaments that make up the inner volume of axon, with possible phosphorylation on their side arms, and their slow axonal transport by molecular motors along microtubule tracks in a "stop-and-go" manner with rapid, intermittent and bidirectional motion. The kinetics of NFs and morphology of axon are dramatically different between myelinate internode and unmyelinated node of Ranvier. The NFs in the node transport as 7.6 times faster as in the internode, and the distribution of NFs population in the internode is 7.6 folds as much as in the node of Ranvier. We hypothesize that the phosphorylation of NFs could reduce the on-track rate and slow down their transport velocity in the internode. By modifying the '6-state' model with (a) an extra phosphorylation kinetics to each six state and (b) construction a new '8-state' model in which NFs at off-track can be phosphorylated and have smaller on-track rate, our model and simulation demonstrate that the phosphorylation-induced decrease of on-track rate could slow down the NFs average velocity and increase the axonal caliber. The degree of phosphorylation may indicate the extent of velocity reduction. The Continuity equation used in our paper predicts that the ratio of NFs population is inverse proportional to the ratios of average velocity of NFs between node of Ranvier and internode. We speculate that the myelination of axon could increase the level of phosphorylation of NF side arms, and decrease the possibility of NFs to get on-track of microtubules, therefore slow down their transport velocity. In summary, our work provides a potential mechanism for understanding the phosphorylation kinetics of NFs in regulating their transport and morphology of axon in myelinated axons, and the different kinetics of NFs between node and internode.

Anterograde Viral Tracer Herpes Simplex Virus 1 Strain H129 Transports Primarily as Capsids in Cortical Neuron Axons.

The features of herpes simplex virus 1 (HSV-1) strain 129 (H129), including natural neurotropism and anterograde transneuronal trafficking, make it a potential tool for anterograde neural circuitry tracing. Recently anterograde polysynaptic and monosynaptic tracers were developed from H129 and have been applied for the identification of novel connections and functions of different neural circuitries. However, how H129 viral particles are transported in neurons, especially those of the central nervous system, remains unclear. In this study, we constructed recombinant H129 variants with mCherry-labeled capsids and/or green fluorescent protein (GFP)-labeled envelopes and infected the cortical neurons to study axonal transport of H129 viral particles. We found that different types of viral particles were unevenly distributed in the nucleus, cytoplasm of the cell body, and axon. Most H129 progeny particles were unenveloped capsids and were transported as capsids rather than virions in the axon. Notably, capsids acquired envelopes at axonal varicosities and terminals where the sites forming synapses are connected with other neurons. Moreover, viral capsids moved more frequently in the anterograde direction in axons, with an average velocity of 0.62 ± 0.18 µm/s and maximal velocity of 1.80 ± 0.15 µm/s. We also provided evidence that axonal transport of capsids requires the kinesin-1 molecular motor. These findings support that H129-derived tracers map the neural circuit anterogradely and possibly transsynaptically. These data will guide future modifications and improvements of H129-based anterograde viral tracers.IMPORTANCE Anterograde transneuronal tracers derived from herpes simplex virus 1 (HSV-1) strain 129 (H129) are important tools for mapping neural circuit anatomic and functional connections. It is, therefore, critical to elucidate the transport pattern of H129 within neurons and between neurons. We constructed recombinant H129 variants with genetically encoded fluorescence-labeled capsid protein and/or glycoprotein to visualize viral particle movement in neurons. Both electron microscopy and light microscopy data show that H129 capsids and envelopes move separately, and notably, capsids are enveloped at axonal varicosity and terminals, which are the sites forming synapses to connect with other neurons. Superresolution microscopy-based colocalization analysis and inhibition of H129 particle movement by inhibitors of molecular motors support that kinesin-1 contributes to the anterograde transport of capsids. These results shed light into the mechanisms for anterograde transport of H129-derived tracer in axons and transmission between neurons via synapses, explaining the anterograde labeling of neural circuits by H129-derived tracers.

Local modulation of Neurofilament transport at Nodes of Ranvier.

Neurofilaments (NFs) are the most abundant cytoskeletal filaments undergoing 'slow axonal transport' in axons, and the population of NFs determines the axonal morphology. Both in vitro and ex-vivo experimental evidences show that the caliber of node is much thinner and the number of NFs in the node is much lower than the internode. Based on the Continuity equation, lower population of NFs indicates faster transport velocity. We propose that the local acceleration of NFs transport at node may result from the higher on-track rate [Formula: see text] or higher transition rate [Formula: see text] from pausing to running. We construct a segment of axon including both node and internode, and inject NFs by a fixed flux into it continuously. By upregulating transition rate of either [Formula: see text] or [Formula: see text] locally at the Node of Ranvier in the '6-state'model, we successfully accelerate NFs velocity and reproduce constriction of nodes. Our work demonstrates that local modulation of NF kinetics can change NFs distribution and shape the morphology of Node of Ranvier.

Local Acceleration of Neurofilament Transport at Nodes of Ranvier.

Myelinated axons are constricted at nodes of Ranvier. These constrictions are important physiologically because they increase the speed of saltatory nerve conduction, but they also represent potential bottlenecks for the movement of axonally transported cargoes. One type of cargo are neurofilaments, which are abundant space-filling cytoskeletal polymers that function to increase axon caliber. Neurofilaments move bidirectionally along axons, alternating between rapid movements and prolonged pauses. Strikingly, axon constriction at nodes is accompanied by a reduction in neurofilament number that can be as much as 10-fold in the largest axons. To investigate how neurofilaments navigate these constrictions, we developed a transgenic mouse strain that expresses a photoactivatable fluorescent neurofilament protein in neurons. We used the pulse-escape fluorescence photoactivation technique to analyze neurofilament transport in mature myelinated axons of tibial nerves from male and female mice of this strain ex vivo Fluorescent neurofilaments departed the activated region more rapidly in nodes than in flanking internodes, indicating that neurofilament transport is faster in nodes. By computational modeling, we showed that this nodal acceleration can be explained largely by a local increase in the duty cycle of neurofilament transport (i.e., the proportion of the time that the neurofilaments spend moving). We propose that this transient acceleration functions to maintain a constant neurofilament flux across nodal constrictions, much as the current increases where a river narrows its banks. In this way, neurofilaments are prevented from piling up in the flanking internodes, ensuring a stable neurofilament distribution and uniform axonal morphology across these physiologically important axonal domains.SIGNIFICANCE STATEMENT Myelinated axons are constricted at nodes of Ranvier, resulting in a marked local decrease in neurofilament number. These constrictions are important physiologically because they increase the efficiency of saltatory nerve conduction, but they also represent potential bottlenecks for the axonal transport of neurofilaments, which move along axons in a rapid intermittent manner. Imaging of neurofilament transport in mature myelinated axons ex vivo reveals that neurofilament polymers navigate these nodal axonal constrictions by accelerating transiently, much as the current increases where a river narrows its banks. This local acceleration is necessary to ensure a stable axonal morphology across nodal constrictions, which may explain the vulnerability of nodes of Ranvier to neurofilament accumulations in animal models of neurotoxic neuropathies and neurodegenerative diseases.

Induction and Consolidation of Calcium-Based Homo- and Heterosynaptic Potentiation and Depression.

The adaptive mechanisms of homo- and heterosynaptic plasticity play an important role in learning and memory. In order to maintain plasticity-induced changes for longer time scales (up to several days), they have to be consolidated by transferring them from a short-lasting early-phase to a long-lasting late-phase state. The underlying processes of this synaptic consolidation are already well-known for homosynaptic plasticity, however, it is not clear whether the same processes also enable the induction and consolidation of heterosynaptic plasticity. In this study, by extending a generic calcium-based plasticity model with the processes of synaptic consolidation, we show in simulations that indeed heterosynaptic plasticity can be induced and, furthermore, consolidated by the same underlying processes as for homosynaptic plasticity. Furthermore, we show that by local diffusion processes the heterosynaptic effect can be restricted to a few synapses neighboring the homosynaptically changed ones. Taken together, this generic model reproduces many experimental results of synaptic tagging and consolidation, provides several predictions for heterosynaptic induction and consolidation, and yields insights into the complex interactions between homo- and heterosynaptic plasticity over a broad variety of time (minutes to days) and spatial scales (several micrometers).

Deciphering the axonal transport kinetics of neurofilaments using the fluorescence photoactivation pulse-escape method.

Neurofilaments are transported along axons stochastically in a stop-and-go manner, cycling between brief bouts of rapid movement and pauses that can vary from seconds to hours in length. Presently the only way to analyze neurofilament pausing experimentally on both long and short time scales is the pulse-escape method. In this method, fluorescence photoactivation is used to mark a population of axonal neurofilaments and then the loss of fluorescence from the activated region due to neurofilament movement is monitored by time-lapse imaging. Here we develop a mathematical description of the pulse-escape kinetics in terms of the rate constants of a tested mathematical model and we show how this model can be used to characterize neurofilament transport kinetics from fluorescence photoactivation pulse-escape experiments. This combined experimental and computational approach is a powerful tool for the analysis of the moving and pausing behavior of neurofilaments in axons.

Local regulation of neurofilament transport by myelinating cells.

Axons in the vertebrate nervous system only expand beyond ∼ 1 µm in diameter if they become myelinated. This expansion is due in large part to the accumulation of space-filling cytoskeletal polymers called neurofilaments, which are cargoes of axonal transport. One possible mechanism for this accumulation is a decrease in the rate of neurofilament transport. To test this hypothesis, we used a fluorescence photoactivation pulse-escape technique to compare the kinetics of neurofilament transport in contiguous myelinated and unmyelinated segments of axons in long-term myelinating cocultures established from the dorsal root ganglia of embryonic rats. The myelinated segments contained more neurofilaments and had a larger cross-sectional area than the contiguous unmyelinated segments, and this correlated with a local slowing of neurofilament transport. By computational modeling of the pulse-escape kinetics, we found that this slowing of neurofilament transport could be explained by an increase in the proportion of the time that the neurofilaments spent pausing and that this increase in pausing was sufficient to explain the observed neurofilament accumulation. Thus we propose that myelinating cells can regulate the neurofilament content and morphology of axons locally by modulating the kinetics of neurofilament transport.

Axonal transport of neurofilaments: a single population of intermittently moving polymers.

Studies on mouse optic nerve have led to the controversial proposal that only a small proportion of neurofilaments are transported in axons and that the majority are deposited into a persistently stationary and extensively cross-linked cytoskeletal network that remains fixed in place for months without movement. We have used computational modeling to address this issue, taking advantage of the wealth of published kinetic and morphometric data available for neurofilaments in the mouse visual system. We show that the transport kinetics and distribution of neurofilaments in mouse optic nerve can all be explained fully by a "stop-and-go" model of neurofilament transport, in which axons contain a single population of neurofilaments that all move stochastically in a rapid, intermittent, and bidirectional manner. Importantly, we find that the transport kinetics are not consistent with deposition of neurofilaments into a persistently stationary phase, and that deposition models cannot account for the observed distribution of neurofilaments along mouse optic nerve axons. Finally, we show that the apparent existence of a stationary neurofilament network in mouse optic nerve is most likely an experimental artifact due to contamination of the neurofilament transport kinetics with cytosolic proteins that move at faster rates. Thus, there is no evidence for the deposition of axonally transported neurofilaments into a persistently stationary neurofilament network in optic nerve axons. We conclude that all of the neurofilaments move and that they do so with a single broad and continuous distribution of average rates that is dictated by their intermittent and stochastic motile behavior.