Structure, Distribution, and Function of Neuronal/Synaptic Spinules and Related Invaginating Projections.

Area of Science:

• Neuroscience
• Cell Biology
• Evolutionary Biology

Background:

• Neurons and synapses feature thin, projecting processes that invaginate neighboring cells.
• These invaginating projections facilitate highly localized communication and material exchange between neurons or glial cells.
• The best-known examples are postsynaptic spinules that invaginate presynaptic terminals, particularly at active synapses.

Purpose of the Study:

• To provide a comprehensive review of various types of invaginating projections in neurons and synapses across the animal kingdom.
• To explore the evolutionary trajectory of these structures, from simple metazoans to higher animals with enhanced synaptic plasticity.
• To discuss the functional roles and potential changes in aging and disease.

Main Methods:

• Literature review and synthesis of existing research on neuronal and synaptic invaginating structures.
• Comparative analysis of invaginating projections across different animal phyla.
• Discussion of functional implications based on current scientific understanding.

Main Results:

• Identified and described diverse invaginating projections, including spinules, filopodial invaginations, amphid microvilli, trophospongium, capitate projections, and fly gnarls.
• Documented the presence of these structures in various animal groups, from basal metazoans to complex vertebrates.
• Highlighted examples of entire presynaptic or postsynaptic processes being invaginated.

Conclusions:

• Invaginating projections are a conserved mechanism for specialized intercellular communication and material transfer at synapses.
• These structures exhibit significant diversity and likely evolved to fine-tune synaptic function and plasticity.
• Further research is needed to fully understand their roles in membrane recycling, signaling, aging, and disease.