Shootin1a-mediated actin-adhesion coupling generates force to trigger structural plasticity of dendritic spines

Shootin1 Regulates Retinal Ganglion Cell Neurite Development: Insights From an RGC Direct Somatic Cell Reprogramming Model

Purpose

Retinal ganglion cells (RGCs) connect the retina to the brain. Proper development of the axons and dendrites of RGCs is the basis for these cells to function as projection neurons to deliver visual information to the brain. The purpose of this study was to investigate the function of Shtn1 (which encodes shootin1) in RGC neurite development.

Methods

Immunofluorescence (IF) was used to characterize the expression pattern of marker genes. An in vitro direct somatic cell reprogramming system was used to generate RGC-like neurons (iRGCs), which was subsequently used to study the function of Shtn1. Short-hairpin RNAs (shRNAs) were used to knock down Shtn1, and the coding sequence (CDS) of Shtn1 was used to overexpress the gene. Lentiviruses were used to deliver shRNAs or CDSs into iRGCs. The patch clamp technique was used to measure the electrophysiological properties of the iRGCs. RNA sequencing (RNA-seq) was used to examine transcriptome expression.

Results

Using IF, we demonstrated that shootin1 is distinctively expressed in RGCs during the period in which RGCs actively develop and adjust the connections of their neurites with upstream and downstream neurons. Using the iRGC system, we demonstrated that Shtn1 promotes the growth and complexity of neurites and thus the electrophysiological maturation, of iRGCs. RNA-seq analyses showed that Shtn1 may also regulate gene expression and neurogenesis in iRGCs.

Conclusions

Shtn1 promotes RGC neurite development. These findings improve our understanding of the molecular machinery governing RGC neurite development and may help to optimize future RGC regeneration methods.

Biophysical Modeling of Actin-Mediated Structural Plasticity Reveals Mechanical Adaptation in Dendritic Spines

Synaptic plasticity is important for learning and memory formation; it describes the strengthening or weakening of connections between synapses. The postsynaptic part of excitatory synapses resides in dendritic spines, which are small protrusions on the dendrites. One of the key features of synaptic plasticity is its correlation with the size of these spines. A long-lasting synaptic strength increase [long-term potentiation (LTP)] is only possible through the reconfiguration of the actin spine cytoskeleton. Here, we develop an experimentally informed three-dimensional computational model in a moving boundary framework to investigate this reconfiguration. Our model describes the reactions between actin and actin-binding proteins leading to the cytoskeleton remodeling and their effect on the spine membrane shape to examine the spine enlargement upon LTP. Moreover, we find that the incorporation of perisynaptic elements enhances spine enlargement upon LTP, exhibiting the importance of accounting for these elements when studying structural LTP. Our model shows adaptation to repeated stimuli resulting from the interactions between spine proteins and mechanical forces.

Mechanics in the nervous system: From development to disease

Physical forces are ubiquitous in biological processes across scales and diverse contexts. This review highlights the significance of mechanical forces in nervous system development, homeostasis, and disease. We provide an overview of mechanical signals present in the nervous system and delve into mechanotransduction mechanisms translating these mechanical cues into biochemical signals. During development, mechanical cues regulate a plethora of processes, including cell proliferation, differentiation, migration, network formation, and cortex folding. Forces then continue exerting their influence on physiological processes, such as neuronal activity, glial cell function, and the interplay between these different cell types. Notably, changes in tissue mechanics manifest in neurodegenerative diseases and brain tumors, potentially offering new diagnostic and therapeutic target opportunities. Understanding the role of cellular forces and tissue mechanics in nervous system physiology and pathology adds a new facet to neurobiology, shedding new light on many processes that remain incompletely understood.

Adhesion-clutch between DCC and netrin-1 mediates netrin-1-induced axonal haptotaxis

The growth cone, a motile structure located at the tip of growing axons, senses extracellular guidance cues and translates them into directional forces that drive axon outgrowth and guidance. Axon guidance directed by chemical cues on the extracellular adhesive substrate is termed haptotaxis. Recent studies reported that netrin-1 on the substrate functions as a haptotactic axon guidance cue. However, the mechanism mediating netrin-1-induced axonal haptotaxis remains unclear. Here, we demonstrate that substrate-bound netrin-1 induces axonal haptotaxis by facilitating physical interactions between the netrin-1 receptor, DCC, and the adhesive substrates. DCC serves as an adhesion receptor for netrin-1. The clutch-linker molecule shootin1a interacted with DCC, linking it to actin filament retrograde flow at the growth cone. Speckle imaging analyses showed that DCC underwent either grip (stop) or retrograde slip on the adhesive substrate. The grip state was more prevalent on netrin-1-coated substrate compared to the control substrate polylysine, thereby transmitting larger traction force on the netrin-1-coated substrate. Furthermore, disruption of the linkage between actin filament retrograde flow and DCC by shootin1 knockout impaired netrin-1-induced axonal haptotaxis. These results suggest that the directional force for netrin-1-induced haptotaxis is exerted on the substrates through the adhesion-clutch between DCC and netrin-1 which occurs asymmetrically within the growth cone.

Mechanical regulation of synapse formation and plasticity

Dendritic spines are small protrusions arising from dendrites and constitute the major compartment of excitatory post-synapses. They change in number, shape, and size throughout life; these changes are thought to be associated with formation and reorganization of neuronal networks underlying learning and memory. As spines in the brain are surrounded by the microenvironment including neighboring cells and the extracellular matrix, their protrusion requires generation of force to push against these structures. In turn, neighboring cells receive force from protruding spines. Recent studies have identified BAR-domain proteins as being involved in membrane deformation to initiate spine formation. In addition, forces for dendritic filopodium extension and activity-induced spine expansion are generated through cooperation between actin polymerization and clutch coupling. On the other hand, force from expanding spines affects neurotransmitter release from presynaptic terminals. Here, we review recent advances in our understanding of the physical aspects of synapse formation and plasticity, mainly focusing on spine dynamics.

Altered Amantadine Effects after Repetitive Treatment for l-dopa-induced Involuntary Movements in a Rat Model of Parkinson's Disease

BACKGROUND

l-3,4-dihydroxyphenylalanine (l-dopa) is the most effective drug for Parkinson's disease (PD); however, most PD patients develop motor fluctuations including wearing-off and l-dopa-induced dyskinesia (LID). Amantadine is beneficial for improving the motor symptoms, reducing "off" time, and ameliorating LID, although its long-term efficacy remains unknown.

OBJECTIVES

To investigate the effects of amantadine on PD and LID using a rat model with repetitive drug treatment.

METHOD

We utilized 6-hydroxydopamine injections to develop a hemiparkinsonian rat model. The rats were assigned to four groups: five rats received l-dopa and benserazide for 31 days, six rats received l-dopa and benserazide plus amantadine for 31 days, five rats received l-dopa and benserazide for 15 days followed by l-dopa and benserazide plus amantadine for 16 days, and five rats received l-dopa and benserazide plus amantadine for 15 days followed by l-dopa and benserazide treatment for 16 days. We evaluated the l-dopa-induced abnormal involuntary movements on treatment days 1, 7, 14, 16, 22, and 29. Subsequently, immunohistochemistry for drebrin was performed.

RESULTS

l-dopa-induced abnormal movements were reduced on the first day of amantadine treatment, and these effects disappeared with repetitive treatment. In contrast, the extension of l-dopa "on" time was observed after repetitive amantadine treatment. All groups showed enlarged drebrin immunoreactive dots in the dopamine-denervated striatum, indicating that amantadine did not prevent priming effects of repetitive l-dopa treatment.

CONCLUSION

Anti-LID effect of amantadine diminished after repetitive treatment, and the effect of amantadine on wearing-off emerged after repetitive treatment in a hemiparkinsonian rat model. Fluctuations in amantadine effects should be considered when using it in clinical settings.

Dephosphorylation of neural wiring protein shootin1 by PP1 phosphatase regulates netrin-1-induced axon guidance

Axon pathfinding is an essential step in neuronal network formation. Shootin1a is a clutch-linker molecule that is mechanically involved in axon outgrowth and guidance. It was previously shown that concentration gradients of axon guidance molecule netrin-1 in the extracellular environment elicit asymmetrically localized Pak1 kinase-mediated phosphorylation of shootin1a within axonal growth cones, which is higher on the netrin-1 source side. This asymmetric phosphorylation promotes shootin1a-mediated local actin-adhesion coupling within growth cones, thereby generating directional forces for turning the growth cone toward the netrin-1 source. However, how the spatial differences in netrin-1 concentration are transduced into the asymmetrically localized signaling within growth cones remains unclear. Moreover, the protein phosphatases that dephosphorylate shootin1a remain unidentified. Here, we report that protein phosphatase-1 (PP1) dephosphorylates shootin1a in growth cones. We found that PP1 overexpression abolished the netrin-1-induced asymmetric localization of phosphorylated-shootin1a as well as axon turning. In addition, we show PP1 inhibition reversed the asymmetrically localized shootin1a phosphorylation within growth cones under netrin-1 gradient, thereby changing the netrin-1-induced growth cone turning from attraction to repulsion. These data indicate that PP1-mediated shootin1a dephosphorylation plays a key role in organizing asymmetrically-localized phosphorylated shootin1a within growth cones, which regulates netrin-1-induced axon guidance.

Multiscale Mechanobiology in Brain Physiology and Diseases

Increasing evidence suggests that mechanics play a critical role in regulating brain function at different scales. Downstream integration of mechanical inputs into biochemical signals and genomic pathways causes observable and measurable effects on brain cell fate and can also lead to important pathological consequences. Despite recent advances, the mechanical forces that influence neuronal processes remain largely unexplored, and how endogenous mechanical forces are detected and transduced by brain cells into biochemical and genetic programs have received less attention. In this review, we described the composition of brain tissues and their pronounced microstructural heterogeneity. We discuss the individual role of neuronal and glial cell mechanics in brain homeostasis and diseases. We highlight how changes in the composition and mechanical properties of the extracellular matrix can modulate brain cell functions and describe key mechanisms of the mechanosensing process. We then consider the contribution of mechanobiology in the emergence of brain diseases by providing a critical review on traumatic brain injury, neurodegenerative diseases, and neuroblastoma. We show that a better understanding of the mechanobiology of brain tissues will require to manipulate the physico-chemical parameters of the cell microenvironment, and to develop three-dimensional models that can recapitulate the complexity and spatial diversity of brain tissues in a reproducible and predictable manner. Collectively, these emerging insights shed new light on the importance of mechanobiology and its implication in brain and nerve diseases.

Simultaneous analyses of clutch coupling and actin polymerization in dendritic spines of rodent hippocampal neurons during chemical LTP

Dendritic spine enlargement by synaptic activation is thought to increase synaptic efficacy underlying learning and memory. This process requires forces generated by actin polymerization and actin-adhesion coupling (clutch coupling). Here, we describe a protocol to monitor actin filament retrograde flow and actin polymerization within spines using a standard epi-fluorescence microscope. In combination with chemical long-term potentiation, this protocol allows us to quantify clutch coupling efficiency and actin polymerization rate, which are essential variables for generating forces for activity-dependent spine enlargement.

For complete details on the use and execution of this protocol, please refer to Kastian et al. (2021).

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Analysis of F-actin retrograde flow using a standard epi-fluorescence microscope

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F-actin flow velocity reflects the efficiency of clutch coupling

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Analysis of actin polymerization rate using a standard epi-fluorescence microscope

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Analysis of these variables in dendritic spines during chemical LTP induction

Actin Cytoskeleton Role in the Maintenance of Neuronal Morphology and Long-Term Memory

Evidence indicates that long-term memory formation creates long-lasting changes in neuronal morphology within a specific neuronal network that forms the memory trace. Dendritic spines, which include most of the excitatory synapses in excitatory neurons, are formed or eliminated by learning. These changes may be long-lasting and correlate with memory strength. Moreover, learning-induced changes in the morphology of existing spines can also contribute to the formation of the neuronal network that underlies memory. Altering spines morphology after memory consolidation can erase memory. These observations strongly suggest that learning-induced spines modifications can constitute the changes in synaptic connectivity within the neuronal network that form memory and that stabilization of this network maintains long-term memory. The formation and elimination of spines and other finer morphological changes in spines are mediated by the actin cytoskeleton. The actin cytoskeleton forms networks within the spine that support its structure. Therefore, it is believed that the actin cytoskeleton mediates spine morphogenesis induced by learning. Any long-lasting changes in the spine morphology induced by learning require the preservation of the spine actin cytoskeleton network to support and stabilize the spine new structure. However, the actin cytoskeleton is highly dynamic, and the turnover of actin and its regulatory proteins that determine and support the actin cytoskeleton network structure is relatively fast. Molecular models, suggested here, describe ways to overcome the dynamic nature of the actin cytoskeleton and the fast protein turnover and to support an enduring actin cytoskeleton network within the spines, spines stability and long-term memory. These models are based on long-lasting changes in actin regulatory proteins concentrations within the spine or the formation of a long-lasting scaffold and the ability for its recurring rebuilding within the spine. The persistence of the actin cytoskeleton network within the spine is suggested to support long-lasting spine structure and the maintenance of long-term memory.