Regulated Secretion - Exocytosis

What is the fundamental difference between constitutive and regulated secretion?

Constitutive secretion continuously delivers vesicles directly to the plasma membrane upon synthesis, whereas regulated secretion clamps vesicles at the membrane until fusion is triggered by a specific stimulus like calcium.

Quantal hypothesis

The concept established by Bernard Katz stating that neurotransmitters are released in discrete, uniform multimolecular packets corresponding to the contents of individual synaptic vesicles.

What key role does calcium ion influx play in presynaptic neurotransmission?

It serves as the essential trigger that binds to synaptotagmin-1 to relieve vesicle clamping and induce rapid membrane fusion.

Which three neuronal SNARE proteins form the core fusion complex at the presynaptic active zone?

Syntaxin-1 and SNAP-25 on the plasma membrane, and Synaptobrevin-2 (VAMP2) on the synaptic vesicle membrane.

Synaptotagmin-1

A synaptic vesicle transmembrane protein containing C2A and C2B domains that acts as the primary calcium sensor for fast regulated exocytosis.

What are the six sequential structural steps of membrane fusion in exocytosis?

Tethering, docking/priming, fusion initiation, hemifusion, fusion-pore opening, and full collapse (or kiss-and-run).

Hemifusion

An intermediate state of membrane fusion where the outer leaflets of the vesicle and target plasma membrane fuse while the inner leaflets remain separate.

How did cryo-electron microscopy resolve the debate regarding SNARE involvement in vesicle docking?

It avoided chemical fixation artifacts to show that knocking out SNAP-25 or Syntaxin-1 directly eliminates morphologically docked vesicles at the presynaptic membrane.

What conformational state of Syntaxin-1 is recognized by Munc18-1 during initial vesicle docking?

The closed conformation, where the Habc domain folds back over its own SNARE domain.

Munc18-1 (STXBP1)

An essential Sec1/Munc18-like (SM) protein that binds Syntaxin-1 to facilitate vesicle docking and subsequently stabilizes the trans-SNARE complex to drive membrane fusion.

How do in vitro liposome fusion assays prove that Munc18-1 is a positive regulator of fusion?

Pre-incubating SNARE-containing liposomes with Munc18-1 dramatically accelerates the rate and overall efficiency of membrane fusion.

What happens to vesicle docking and fusion when SNAP-25 is overexpressed in Munc18-1 knockout cells?

SNAP-25 overexpression rescues morphological vesicle docking but fails to rescue membrane fusion, proving Munc18-1 has distinct roles in both steps.

Complexin

A small cytosolic protein that binds across assembling SNAREpins to clamp vesicles in a primed, hemifused state prior to calcium-triggered release.

STXBP1 encephalopathies

Severe early-onset neurodevelopmental disorders and epilepsies in humans caused by haploinsufficiency mutations in the STXBP1 gene encoding Munc18-1.