Journal Club | Ucar et al. (Mechanical actions of dendritic-spine enlargement on presynaptic exocytosis)

What is the paper about?

• Research question

  • Can mechanical force between 2 sides of a synapse affect neurotransmitter release?

• Approach

  • They (1) mechanically pushed presynaptic boutons, (2) tested osmotic pressure, (3) induced dendritic-spine enlargement

• Main finding

  • Fine mechanical pushing enhances evoked glutamate release and increases SNARE-associated FRET

• Conclusion

  • Authors proposed PREST (pressure sensation and transduction), through which localized pressure can produce lasting effect on presynaptic function

What was the knowledge gap?

• Scientists already knew that dendritic spines enlarge during activities associated with learning and synaptic plasticity.

• It remains unestablished whether this postsynaptic structural change could mechanically influence the neighboring presynaptic bouton and alter neurotransmitter release

Why did the researchers perform 3 different types of experiments?

Direct manipulation → molecular mechanism → alternative mechanical manipulation → physiological relevance

• Figure 1: Test whether directly pushing presynaptic bouton affects neurotransmitter release

• Figure 2: Test whether mechanical pushing changes SNARE-associated molecular interactions

• Figure 3: Test whether osmotic pressure can reproduce the same effect

• Figure 4: Test whether spine enlargement can naturally produce the same effect by physically pushing the bouton

What is a synapse? What does it have to do with this paper?

Synapse is a specialized junction where one neuron can communicate with another

• In a chemical synapse, (upon an action potential/stimulus like Ca+ influx) a presynaptic neuron would release neurotransmitters that would cross the synaptic cleft and activate receptors on the postsynaptic cell

• In this paper, researchers studied excitatory glutamatergic synapses in rat hippocampal slice cultures  containing connections between two neuronal populations

  • CA3 neurons: The presynaptic neurons whose axons carry signals toward CA1.

  • CA1 neurons: The region where the researchers examined individual presynaptic boutons of CA3 axons.

  • Hippocampus = region in brain that regulates emotions, memories, and drives

What is a presynaptic bouton and a postsynaptic dendritic spine? Apply to paper

Presynaptic bouton = swollen end of an axon containing machinery for neurotransmitter release

Postsynaptic dendritic spine = small protrusion on dendrite that receives the excitatory synaptic input

• Axons = carry signals away from cell body to other neurons, etc.

• Dendrites = receive signals from other neurons to cell body

In this paper, researchers investigated whether an enlarging spine could physically push the bouton and influence its neurotransmitter release function

What is glutamate? Apply in this paper

Glutamate = major excitatory neurotransmitter in the CNS; activates postsynaptic glutamate receptors and helps transmit signals between neurons

• In this paper, glutamate release was monitored using the fluorescent biosensor iGluSnFR to investigate whether mechanical force on 2 sides of synapse affects neurotransmitter release

What is exocytosis? Apply to paper

Exocytosis = a form of active transport, where cell moves large molecules or wastes out of cytoplasm and into extracellular space by fusing membrane-bound vesicles with plasma membrane

• In this paper, synaptic vesicles release glutamate through exocytosis by fusing with plasma membrane of presynaptic bouton

What is evoked neurotransmitter release? Apply to paper

Neurotransmitter release is triggered by a stimulus like an action potential arriving at presynaptic terminal.

• In this paper, researchers stimulated neurons using optogenetics and measured consequent glutamate release.

• In particular, they expressed red-light-sensitive opsin CsChrimsonR in CA3 neurons bc with brief red light pulses, this would stimulate neurons, triggering Ca+ influx that promotes glutamate release via exocytosis (synaptic vesicle fusion w plasma membrane)

What is a synaptic vesicle? Apply to paper

Synaptic vesicle = small membrane-bound compartment that stores neurotransmitters inside presynaptic terminal

• In this paper, SNARE proteins help synaptic vesicles fuse with the presynaptic plasma membrane so glutamate can be released.

What are SNARE proteins? Apply to paper

SNARE proteins are molecular components of the machinery that facilitates fusion of synaptic vesicles and presynaptic plasma membrane to allow neurotransmitter release

• The researchers investigated whether mechanical force increased SNARE-associated interactions, potentially explaining the enhancement of glutamate release

What are syntaxin-1A, VAMP2, and SNAP-25? Apply to paper

They are proteins involved in the SNARE machinery:

• Syntaxin-1A: Associated with the presynaptic plasma membrane.

• VAMP2: Located on synaptic vesicles.

• SNAP-25: A presynaptic plasma-membrane-associated SNARE protein.

Together, these proteins participate in the molecular machinery that enables synaptic vesicle fusion.

What is SNARE complex assembly? Apply to paper

The formation of molecular interactions among SNARE proteins that help bring vesicle and plasma membranes together.

• In this paper, authors used FRET–FLIM to investigate SNARE-associated interactions and tested whether mechanical force increased the corresponding signal

Critical nuance: Increased FRET supports increased SNARE-associated interactions; it does not directly count fully assembled, fusion-ready complexes.

What is actin polymerization? Apply to paper

Actin polymerization is the assembly of individual actin molecules into actin filaments, which contribute to cellular structure and organization.

• In this paper, the authors used latrunculin A (latA) to inhibit actin polymerization.

• This blocked the pushing-induced FRET increase, supporting a role for actin in the mechanical response.

Why might actin matter at a presynaptic bouton?

Actin contributes to the organization of presynaptic structures and synaptic vesicle pools.

• The authors suggest that actin polymerization helps support vesicle organization and facilitates the SNARE-associated response to mechanical force.

What is optogenetics? Apply to paper

Optogenetics uses light-sensitive proteins to control the activity of selected cells.

• The researchers expressed the red-light-sensitive opsin CsChrimsonR in CA3 neurons.

• Red-light stimulation evoked neuronal activity, allowing them to measure glutamate release from individual boutons in the CA1 region.

  
  

• Why it mattered: It provided a controlled stimulus for comparing release before and after mechanical manipulation.

What is CsChrimsonR? Apply to paper

CsChrimsonR is a red-light-sensitive opsin used to stimulate neurons.

• In this paper, it enabled red-light stimulation of CA3 neurons to evoke neurotransmitter release at their presynaptic boutons.

What is iGluSnFR? Apply to paper

iGluSnFR is a genetically encoded fluorescent biosensor that changes its fluorescence when it binds extracellular glutamate.

• In this paper, researchers monitored its fluorescence near individual boutons to estimate glutamate release following neuronal stimulation.
  
  

Remember: iGluSnFR reports extracellular glutamate; it does not directly image vesicle fusion.

How did the researchers measure release probability?

Release probability refers to the proportion of stimulation trials that produced a detectable glutamate-release event

• In this paper, they repeatedly stimulated a bouton and classified each trial as a detectable release success or failure.

• A trial counted as a success when the iGluSnFR signal exceeded three standard deviations above baseline within 20 milliseconds of stimulation.
  
  In this paper: Fine pushing increased the proportion of trials with detectable glutamate-release responses.

What is FWHM? Apply to paper

Full width at half-maximum (FWHM) is the width of a fluorescence intensity profile measured at half of its maximum intensity.

• In this paper: The researchers used FWHM to quantify changes in bouton width during mechanical pushing.

• Why it mattered: It verified and quantified the physical deformation rather than relying only on visual impressions.

What is FRET? Apply to paper

Förster resonance energy transfer (FRET) is the transfer of energy from an excited donor fluorophore to a nearby, appropriately oriented acceptor fluorophore.

• In this paper, FRET between labelled syntaxin-1A and VAMP2 was used to investigate changes in SNARE-associated molecular interactions.

• Why it mattered: It provided a way to examine a potential molecular explanation for the increased glutamate release.

What is FLIM? Apply to paper

Fluorescence lifetime imaging microscopy (FLIM) measures how long a fluorophore remains in its excited state before emitting light

• In this paper: FLIM was combined with FRET to estimate changes in interactions between labelled SNARE proteins.
  
  

Why combine them?
FRET can be assessed through changes in donor fluorescence lifetime, helping researchers investigate molecular proximity without relying only on fluorescence intensity.

• When FRET occurs, energy transfer from the donor to the acceptor can shorten the donor’s fluorescence lifetime.

• Their analysis included a parameter called A1%, which they used as an indicator of SNARE-associated molecular interactions.

What is iSLIM? Apply to paper

iSLIM stands for intermolecular SNARE FRET–FLIM.

• SNARE = molecular components of machinery that facilitate fusion of synaptic vesicles with presynaptic plasma membrane, allowing neurotransmitter release

• FRET = energy transfer between excited donor fluorophore and nearby approppriately oriented acceptor fluorophore

• FLIM = measures how long fluorophore remains in excited state before emitting lite (donor’s fluorescence lifetime typically shortens when FRET occurs)
  
  

In this paper, it was the authors' approach for investigating interactions between syntaxin-1A and VAMP2 to assess whether SNARE-associated interactions have to do w/ increased evoke glutamate release

• The FRET-associated fraction, A1%, served as an indicator of SNARE-associated molecular interactions.

What is two-photon glutamate uncaging? Apply to paper

2-photon glutamate uncaging = a technique where a focused 2-photon laser is used to release an active glutamate from a chemically protected, light-sensitive compound called caged glutamate

• In this paper, researchers used 2-photon glutamate uncaging to activate a selected dendritic spine and induce its enlargement while avoiding direct stimulation of nearby bouton

• Why it mattered: It allowed them to investigate whether postsynaptic structural changes could mechanically influence presynaptic function.

What is spike-timing dependent plasticity (STDP)? Apply to paper

Spike-timing-dependent plasticity (STDP) is a form of synaptic plasticity in which the relative timing of presynaptic and postsynaptic activity influences changes in synaptic strength.

In this paper: A protocol combining two-photon glutamate uncaging with postsynaptic spikes was used to induce dendritic-spine enlargement.

Why it mattered: It provided a controlled way to induce a plasticity-associated structural change and test its mechanical consequences.

What is osmotic pressure? Apply to paper

Osmotic pressure is the pressure associated with a difference in solute concentration across a selectively permeable membrane. Changes in extracellular osmolarity can cause water movement and alter cell volume or shape.

In this paper: A 20 mM (millimolar) sucrose solution produced a modest hyperosmotic challenge that reproduced aspects of the pushing-induced response.

Why it mattered: It tested whether a different way of mechanically deforming the bouton could produce similar effects.

What is BoNTE, and why was it used?

Botulinum neurotoxin E cleaves SNAP-25, disrupting SNARE-dependent exocytosis.

• The researchers used it to test whether the pushing-induced FRET response depended on functional SNARE machinery.

What is latrunculin A, and why was it used?

Latrunculin A inhibits actin polymerization.

• Because actin helps organize presynaptic structures and vesicle pools, the researchers used it to test whether actin polymerization contributes to the mechanical response.

• whether the pushing-induced FRET response depended on actin polymerization

Why did the researchers use calcium-free conditions?

To test whether extracellular calcium was required for the pushing-induced FRET increase.

• The response persisted without extracellular calcium, suggesting that this molecular response did not require extracellular calcium under the tested conditions.

It suggests that the mechanical force can promote SNARE assembly through a pathway that does not require extracellular Ca²⁺ influx, potentially involving actin-dependent changes in the presynaptic terminal rather than conventional calcium-triggered signaling. However, this applies to the pushing-induced FRET increase, not to ordinary action-potential-evoked neurotransmitter release, which normally requires Ca²⁺.

What is K252a, and why was it used?

K252a is a kinase inhibitor with activity against several kinases.

• The researchers used it to test whether kinase activity was necessary for the pushing-induced FRET response; the response persisted in its presence.

Does increased FRET prove that more glutamate was released?

No.

• FRET–FLIM investigates molecular interactions consistent with SNARE assembly, whereas iGluSnFR reports extracellular glutamate.

• The authors used these separate measurements together to connect molecular changes with enhanced release

Why is it important that the effect persists (enhanced evoke glutamate release + SNARE-associated mol interactions) after the bouton returns to its original shape?

It suggests that the functional effect cannot be explained simply by the bouton remaining physically compressed.

• A brief mechanical stimulus may trigger a longer-lasting molecular or structural change within the presynaptic terminal.

What is PREST?

PREST stands for Pressure Sensation and Transduction.

• It is the authors' proposed mechanism in which localized mechanical pressure enhances evoked neurotransmitter release for more than 20 minutes.

Why did stronger pushing suppress release?

The authors suggest that stronger deformation may disrupt the molecular arrangements, membrane structures, or interactions needed for efficient SNARE assembly and vesicle replenishment.

Why might PREST be described as a form of longer-lasting “working memory”?

It’s because a brief mechanical event—spine enlargement pushing the bouton—can enhance neurotransmitter release for over 20 minutes, even after the bouton returns to its original shape.

• In other words, the presynaptic terminal may temporarily retain a functional trace of an earlier mechanical event, although the authors propose this as a memory-like interpretation rather than demonstrating actual cognitive memory.