Summary: A new study from the Picower Institute at MIT reveals how newly formed synapses gradually mature to transmit chemical signals correctly. Using time-stamped fluorescent markers in fruit flies, researchers tracked the sequential assembly of proteins at presynaptic active zones and showed that full functional maturation takes days and requires neural activity.
When neural activity was blocked in individual neurons, existing synapses became abnormally large but the formation of new active zones was halted. These results show that ongoing synaptic communication guides proper development of connections. The findings help explain how synaptic dysfunction contributes to disorders such as autism, epilepsy, and intellectual disability and point toward molecular strategies for adjusting synaptic strength in disease.
Key facts:
- Stepwise maturation: New active zones assemble in a defined order—early scaffold proteins arrive first, enabling spontaneous release, followed days later by additional components and calcium channels that permit evoked neurotransmitter release.
- Activity-dependent growth: Preventing neurotransmitter release stopped the creation of new active zones and caused neurons to enlarge existing sites, indicating that synaptic activity directs when and where new zones form.
- Relevance to disease: Proteins and mechanisms involved in active zone maturation are linked to neurodevelopmental disorders, suggesting potential therapeutic targets for correcting synaptic output.
Source: Picower Institute at MIT
Overview
Nervous system functions—ranging from movement to perception and cognition—depend on presynaptic active zones releasing the right amount of neurotransmitter at the right time. The Picower Institute team studied how those active zones form and mature at the neuromuscular junction of Drosophila larvae, revealing a basic model for how neuronal activity during development sculpts effective synaptic connections.
Senior author Troy Littleton, Menicon Professor at the Picower Institute and MIT’s Department of Biology, emphasized that understanding active zone development is critical both for basic neuroscience and for addressing conditions that arise from faulty synaptic transmission. The study, partly funded by a 2021 National Institutes of Health grant, shows that active zone maturation is not instantaneous or predetermined—it unfolds over days and is shaped by neural activity.
“If we can map the molecular levers that strengthen or weaken synapses, we may be able to design interventions to restore proper synaptic output in disease,” Littleton said.
Experimental approach: time-stamped synapses
To follow the age and development of individual active zones, the team engineered glutamate receptor subunits to include the photoconvertible fluorescent protein mMaple. After brief ultraviolet exposure, existing receptors converted from green to red fluorescence, while receptors inserted afterward remained green. This time-stamping allowed direct comparison between newly formed and older active zones in the same neuron over hours and days.
Using this method, the researchers tracked eight different active zone proteins. They observed that newly assembled active zones initially lacked the machinery for evoked release: early scaffold proteins permitted spontaneous vesicle fusion, but voltage-gated calcium channels and later scaffolds arrived later, enabling calcium-triggered, evoked neurotransmitter release.
Neural activity shapes when and where active zones form
To test whether synaptic activity influences active zone completion and the subsequent formation of new sites, the team blocked neurotransmitter release selectively in single neurons. One approach disrupted Synaptotagmin 1, a calcium sensor whose human mutations are associated with intellectual disability and autism. Because silencing all neurons is lethal, these manipulations were limited to individual cells.
When release was prevented, neurons stopped forming new active zones and instead accumulated more scaffold material at existing ones, making them larger. This enlargement appeared to be a compensatory response: the neuron seemed to attempt to restore output from an inactive site rather than start new construction. Tests indicated those enlarged active zones would function normally if not for the artificial block on release.
Further experiments showed the compensatory enlargement depended on feedback from the postsynaptic muscle. Knocking out a glutamate receptor component in muscle prevented the neuron from enlarging its active zones, implicating a retrograde signal that reports failed transmission back to the presynaptic cell.
Implications and next questions
These results demonstrate that active zone maturation controls both the mode of transmitter release and overall presynaptic output, while neuronal activity sets the balance between creating new active zones and amplifying existing ones. The Littleton Lab is now probing which molecular pathways initiate active zone seeding and which signals terminate growth once a site is mature.
Answering those questions will advance understanding of synapse development and may reveal strategies to correct synaptic defects in neurological disorders.
Study lead author: Yuliya Akbergenova. Additional authors: Jessica Matthias and Sofya Makeyeva.
Funding: National Institutes of Health and The Freedom Together Foundation.
Key questions answered
A: Active zones mature over several days in a stepwise process; early scaffolds enable spontaneous release, while later arrival of additional scaffolds and calcium channels allow evoked release. Maturation depends on neural activity.
A: Blocking neurotransmitter release in a neuron halts formation of new active zones and triggers hyperaccumulation of proteins at existing sites, an apparent compensatory response that prevents normal circuit expansion.
A: The molecules and pathways that regulate active zone maturation overlap with factors implicated in autism, epilepsy, and intellectual disability, making them promising targets for therapies that adjust synaptic strength.
About this neuroscience research news
Author: David Orenstein
Source: Picower Institute at MIT
Contact: David Orenstein, Picower Institute at MIT
Image credit: Neuroscience News
Original research: “Active zone maturation controls presynaptic output and release mode and is regulated by neuronal activity,” Troy Littleton et al., Journal of Neuroscience. (Closed access)
Abstract (condensed)
Synapse formation requires sequential accumulation of cytomatrix scaffold proteins and voltage-gated Ca2+ channels at presynaptic active zones. At Drosophila larval neuromuscular junctions, early scaffolds appear first, followed by late scaffolds and VGCCs. Time-stamped imaging using photoconvertible tags showed older active zones sustain greater evoked release, while immature sites lacking VGCCs support only spontaneous fusion. Cell-autonomous reductions in neurotransmitter release decreased seeding of new active zones and caused material to hyperaccumulate at existing sites. Postsynaptic glutamate receptor signaling mediates the retrograde feedback that drives this compensatory enlargement. Overall, active zone maturation determines presynaptic release mode and strength, and neuronal activity controls the number and size of active zones during development.