Biology subjects

Halpain, S.

Publications and source records attributed to Halpain, S..

4 recordsLinked to original sources

bioRxiv2022

Growth cone macropinocytosis of neurotrophin receptor and neuritogenesis are regulated by Neuron Navigator 1

Neuron Navigator 1 (Nav1) is a cytoskeleton-associated protein expressed during brain development that is necessary for proper neuritogenesis, but the underlying mechanisms are poorly understood. Here we show that Nav1 is present in elongating axon tracts during mouse brain embryogenesis. We found that depletion of Nav1 in cultured neurons disrupts growth cone morphology and neurotrophin-stimulated neuritogenesis. In addition to regulating both F-actin and microtubule properties, Nav1 promotes actin-rich membrane ruffles in the growth cone, and promotes macropinocytosis at those membrane ruffles, including internalization of the TrkB receptor for the neurotrophin BDNF (brain-derived neurotropic factor). Growth cone macropinocytosis is important for downstream signaling, neurite targeting, and membrane recycling, implicating Nav1 in one or more of these processes. Depletion of Nav1 also induces transient membrane blebbing via disruption of signaling in the Rho GTPase signaling pathway, supporting the novel role of Nav1 in dynamic actin-based membrane regulation at the cell periphery. These data demonstrate that Nav1 works at the interface of microtubules, actin, and plasma membrane to organize the cell periphery and promote uptake of growth and guidance cues to facilitate neural morphogenesis during development.

neuroscience↗

bioRxiv2021

INF2-mediated actin filament reorganization confers intrinsic resilience to neuronal ischemic injury

During early stages of ischemic brain injury, glutamate receptor hyperactivation mediates neuronal death via osmotic cell swelling. Here we show that ischemia and excess NMDA receptor activation - conditions that trigger neuronal swelling -- cause actin filaments to undergo a rapid and extensive reorganization within the somatodendritic compartment. Normally, F-actin is concentrated within dendritic spines, with relatively little F-actin in the dendrite shaft. However, beginning <5 min after incubation of neurons with NMDA, F-actin depolymerizes within dendritic spines and polymerizes into long, stable filament bundles within the dendrite shaft and soma. A similar "actinification" of the somatodendritic compartment occurs after oxygen/glucose deprivation in vitro, and in mouse brain after photothrombotic stroke in vivo. Following transient, sub-lethal NMDA exposure these actin changes spontaneously reverse within 1-2 hours. A combination of Na+, Cl-, water, and Ca2+ entry are all necessary, but not individually sufficient, for induction of actinification. Spine F-actin depolymerization is also required. Actinification is driven by activation of the F-actin polymerization factor inverted formin-2 (INF2). Silencing of INF2 renders neurons more vulnerable to NMDA-induced membrane leakage and cell death, and formin inhibition markedly increases ischemic infarct severity in vivo. These results show that ischemia-induced actin filament reorganization within the dendritic compartment is an intrinsic pro-survival response that protects neurons from death induced by swelling.

cell biology↗

bioRxiv2020

Mechanical principles governing the shapes of dendritic spines

Dendritic spines are small, bulbous protrusions along the dendrites of neurons and are sites of excitatory postsynaptic activity. The morphology of spines has been implicated in their function in synaptic plasticity and their shapes have been well-characterized, but the potential mechanics underlying their shape development and maintenance have not yet been fully understood. In this work, we explore the mechanical principles that could underlie specific shapes using a minimal biophysical model of membrane-actin interactions. Using this model, we first identify the possible force regimes that give rise to the classic spine shapes - stubby, filopodia, thin, and mushroom-shaped spines. We also use this model to investigate how the spine neck might be stabilized using periodic rings of actin or associated proteins. Finally, we use this model to predict that the cooperation between force generation and ring structures can regulate the energy landscape of spine shapes across a wide range of tensions. Thus, our study provides insights into how mechanical aspects of actin-mediated force generation and tension can play critical roles in spine shape maintenance.

biophysics↗

bioRxiv2019

Computational modeling reveals frequency modulation of calcium-cAMP/PKA pathway in dendritic spines

Dendritic spines are the primary excitatory postsynaptic sites that act as subcompartments of signaling. Ca2+ is often the first and most rapid signal in spines. Downstream of calcium, the cAMP/PKA pathway plays a critical role in the regulation of spine formation, morphological modifications, and ultimately, learning and memory. While the dynamics of calcium are reasonably well-studied, calcium-induced cAMP/PKA dynamics, particularly with respect to frequency modulation, are not fully explored. In this study, we present a well-mixed model for the dynamics of calcium-induced cAMP/PKA dynamics in dendritic spines. The model is constrained using experimental observations in the literature. Further, we measured the calcium oscillation frequency in dendritic spines of cultured hippocampal CA1 neurons and used these dynamics as model inputs. Our model predicts that the various steps in this pathway act as frequency modulators for calcium and the high frequency of calcium input is filtered by AC1 and PDEs in this pathway such that cAMP/PKA only responds to lower frequencies. This prediction has important implications for noise filtering and long-timescale signal transduction in dendritic spines. A companion manuscript presents a three-dimensional spatial model for the same pathway.\n\nStatement of SignificancecAMP/PKA activity triggered by calcium is an essential biochemical pathway for synaptic plasticity, regulating spine structure, and long-term potentiation. In the current study, we predicted that for a given calcium input, AC1, and PDE1 kinetics reflect both the high and the low frequencies with different amplitudes and cAMP/PKA acts as a leaky integrator of calcium because of frequency attenuation by the intermediary steps. These findings have implications for cAMP/PKA signaling in dendritic spines in particular and neuronal signal transduction in general.

biophysics↗