Endocytosis & Recycling 10

  1.  Introduction: The Importance of Endocytosis and Recycling

Endocytosis refers to the active process by which cells engulf molecules by wrapping their membrane around them to bring them into the cell. This process is crucial for the recycling of neurotransmitter-filled vesicles at the synapse, ensuring efficient neurotransmission.

Synaptic Transmission at the Neuromuscular Junction (NMJ):

  • An action potential (AP) arrives at the nerve terminal, causing voltage-gated calcium (Ca²⁺) channels to open.

  • Ca²⁺ influx triggers the release of acetylcholine (ACh) from synaptic vesicles, which are docked at the presynaptic membrane, through the process of exocytosis (as covered in a previous lecture).

  • ACh binds to receptors on the muscle membrane, opening non-selective cation channels that depolarize the muscle, creating an end plate potential (EPP). Once the threshold is reached (around -40mV), sodium-gated channels open, leading to the generation of an action potential in the muscle.

  • The high frequency of action potentials means that the vesicle pool must be continuously replenished; if ACh and vesicles are not recycled quickly, neurotransmitter release could cease, causing muscle function to fail.

Recycling of Acetylcholine and Vesicles:

  • After ACh is released and activates receptors, acetylcholinesterase (AChE) breaks down ACh into acetate and choline.

  • Choline is taken up into the nerve terminal via the choline transporter, while acetate remains in the muscle.

  • Choline combines with acetyl-CoA (sourced from mitochondria) through the action of choline acetyltransferase (ChAT) to form new ACh in the cytoplasm.

  • A vesicle transporter (the vesicular ACh transporter) then loads the newly synthesized ACh into synaptic vesicles for release in future transmission.

2. Mechanisms of Endocytosis and Recycling

The recycling of synaptic vesicles can occur through three distinct mechanisms, each serving a specific purpose in maintaining synaptic vesicle integrity and neurotransmission efficiency.

1) 'Kiss and Stay' Mechanism:

  • This mechanism involves the vesicle briefly fusing with the presynaptic membrane to release its neurotransmitter, then "kissing" the membrane before immediately sealing up and remaining docked.

  • The vesicle does not release any more neurotransmitter but is ready to be refilled with ACh and reused.

  • Advantage: Very fast recycling, ensuring rapid turnover of vesicles in the active zone, but no additional neurotransmitter is released until the vesicle is refilled.

2) 'Kiss and Run' Mechanism:

  • In this case, the vesicle forms a transient fusion pore with the presynaptic membrane, releasing neurotransmitter before quickly detaching and "running" away from the active zone.

  • The vesicle is replaced rapidly by a new one from the reserve pool, ensuring efficient synaptic transmission.

  • Advantage: Fast recycling, allowing the vesicle to be replaced and ready for reuse quickly.

3) 'Full Collapse' Mechanism (Clathrin-Mediated Endocytosis):

  • This mechanism involves complete fusion of the vesicle with the presynaptic membrane, after which clathrin-coated pits form around the membrane and bud off, creating a new vesicle.

  • Clathrin plays a critical role in selecting the vesicle membrane components, ensuring efficient recycling.

  • Advantage: This mechanism is slower but essential for sustaining long-term neurotransmitter release, especially during prolonged or repetitive stimulation.

Why Three Different Mechanisms?

  • 'Kiss and Stay' is used for rapid recycling but does not allow further neurotransmitter release from that vesicle.

  • 'Kiss and Run' also offers rapid recycling and allows for the vesicle to be replaced, but at a slightly slower pace.

  • 'Full Collapse' (clathrin-mediated) is crucial for sustained neurotransmitter release and the recycling of vesicles for longer periods.

3.  Molecular Machinery of Synaptic Vesicle Recycling

Vesicle recycling involves several key molecular components that coordinate the endocytosis and recycling processes:

  • Synaptojanin: This protein plays a significant role in the uncoating and sorting of vesicles during endocytosis.

  • Dephosphins: Proteins activated by calcineurin (a calcium-activated phosphatase), dephosphorylate key components, triggering vesicle internalization and membrane retrieval.

  • Clathrin: A critical protein in forming clathrin-coated vesicles, ensuring that synaptic vesicle membranes are efficiently recycled.

Process of Vesicle Endocytosis:

  1. Nucleation: The initial phase involves the recruitment of proteins such as Eps15 and AP180, which help form the clathrin-coated pit.

  2. Invagination: As clathrin accumulates around the membrane, it causes the membrane to invaginate, eventually forming a vesicle.

  3. Fission: Dynamin, a GTPase, wraps around the neck of the budding vesicle and uses energy from GTP hydrolysis to pinch off the vesicle from the membrane.

  4. Uncoating: Once the vesicle is budded off, the clathrin coat is removed to prepare the vesicle for reuse.


4. Vesicle Composition and Filling 

The composition of a synaptic vesicle is detailed, including the high concentration of ACh and other ions such as H+, ATP, and Mg2+. 

The mechanisms of filling a vesicle with neurotransmitter, driven by proton pumps and ATP transporters, are discussed. 

 

  • Needs to be done in a sequential process through proton pumps 

  • Use of H+ ATPase to drive hydrogen from the cytoplasm and drives it into the vesicle  

  • Creates a highly acidic environment inside the vesicle (pH 5.5) 

  • This is because we want to exchange those protons to a counter transporter of vesicular ACh to allow H+ out whilst ACh comes into the vesicle 

  • Use of a transporter (vesicular Ach transporter) -> creates a high concentration of ACh (850mM) 

  • Other components – ATP (through ATP transporter) (200mM) , Ca2+ (usually binded with ATP) (200mM), Mg2+ (60mM) 

  • Most vesicles only have one or two proton pumps – they are highly dependent on them  

 



5. Vesicle Pools and Recycling Dynamics

There are different pools of synaptic vesicles based on their proximity to the active zone and their readiness for release:

  • Docked Vesicles: These vesicles are already attached to the presynaptic membrane, ready for immediate release.

  • Readily Releasable Pool (RRP): These vesicles are close to the active zone and can be released quickly upon stimulation.

  • Reserve Pool: These vesicles are stored further from the active zone and are mobilized to replace released vesicles during intense or prolonged activity.

Experimental Methods for Measuring Recycling Time:

  • FM1-43 Dye Labeling: This method involves using a fluorescent dye that is incorporated into vesicle membranes during endocytosis, allowing researchers to track vesicle recycling.

  • Sulforhodamine 101 (SOL) Destaining: This technique uses a fluorescent dye to monitor the kinetics of vesicle release and recycling.

  • Electrophysiological Recordings: Changes in fluorescence or electrophysiological signals (such as EPP and MEPP) are used to quantify vesicle depletion and release rates.

6. Correlation Between Dye Loss and Vesicle Loss 

By correlating the cumulative dye loss with the cumulative loss of vesicles (quantified through quantal content), researchers can estimate the number of synaptic vesicles represented by a certain percentage of dye loss. 

 

  • Cumulative curve generated as you lose more dye overtime  

  • We lose 100000 vesicles overtime  

  • Cumulative dye loss to slow down compared to transmitter release (cumulative QC) 

    • This is because the vesicles at the beginning loses dye and transmitter at the same time  

    • The vesicle then gets recycled and comes back and gets refilled but it's lost its dye therefore this is no longer extra dye loss – hence explaining the discrepency  

  • Can allow us to plot the number of synaptic vesicles as a % of dye loss 

    •  I.e. 40% dye loss = 100000 vesicles lost  

    • 100% dye loss = total synaptic vesicles in the terminal (250000 vesicles?) 

Recycle Time Calculation The methodology for calculating vesicle recycle time is explained, taking into account initial quantal content, vesicle pool size, and the point of deviation in dye loss experiments. 

Takes about 100 seconds to recycle a single vesicle! 

 

  • EDL vesicles don’t last for very long so vesicle pool is not very big; whereas soleus vesicles last longer  

  • When comparing the recycle time of EDL and soleus, they are very similar  

    • So EDL doesn't recycle its vesicles any faster hence why it runs out of vesicles when it can't recycle fast enough 

    • The soleus contains enough vesicles so that given this recycle time, it can always have a constant supply of vesicles coming back – enough vesicles to keep going as it is releasing fewer vesicles per stimulus  

    • THEREFORE, you need to either have a larger or smaller vesicle pool size or you need to release fewer of all vesicles in order to adjust your neurotransmitter at least 

 

  • How long to endocytose a vesicle ? 

    • Add FM1-43 and track – will get brighter as more dye accumulates 

    • Measure how much endocytosis is left when you wait 30 seconds before adding in dye  

  • Less and less uptake the more you delay adding the dye 

  • Takes about 60s to internalise the membrane for your vesicle (endocytosis) 

  • Key stats:  

    • Total recycle time = 100s 

    • Endocytosis time = 60s 

    • Vesicle refill and priming = 30-40s 

    • Can change the amount of vesicle being released in response to stimulus and external environment, how it is affected in disease 


7. Phasic vs. Tonic Motor Neuron Properties

Phasic and tonic motor neurons exhibit different properties related to their vesicle pools and recycling times:

  • Phasic Motor Neurons: These neurons are involved in rapid bursts of activity and require a large readily releasable vesicle pool, but they are less reliant on sustained recycling.

  • Tonic Motor Neurons: These neurons are fatigue-resistant, with sustained firing patterns. They have a small readily releasable pool, a large reserve pool, and numerous mitochondria, allowing for sustained neurotransmitter release over longer periods.

VI. Summary and Key Findings

  • The overall vesicle recycling time is approximately 100 seconds.

  • Endocytosis takes about 60 seconds, while vesicle refilling and priming takes an additional 30-40 seconds.

  • The dynamics of vesicle recycling are influenced by the size of the vesicle pool and the frequency of neurotransmitter release.

  • Phasic and tonic motor neurons differ in their vesicle pool sizes, recycling speeds, and fatigue resistance, adjusting their recycling processes to suit their functional roles.

These processes are fundamental in maintaining efficient neurotransmission at the NMJ, ensuring that synaptic transmission remains reliable and sustainable during repetitive activity.