Discover the sophisticated communication network operating beneath the surface of neuronal activity
Imagine your brain as a bustling city, where billions of cells must coordinate their activities with split-second precision. While we often hear about neurons communicating through synaptic connections, there exists another sophisticated communication network operating beneath the surface—a system of channel proteins that enable direct cellular messaging. At the heart of this system are two remarkable protein families: connexins and pannexins.
Different connexin genes in humans
Pannexin family members
Connexin isoforms in CNS
These molecular channels form crucial pathways that allow brain cells to exchange information, nutrients, and signals in ways scientists are only beginning to understand. They're not just passive pores but dynamic structures that respond to the brain's changing needs, participating in everything from memory formation to the brain's defense systems. Recent research has revealed that these channels form an architectural blueprint for both normal brain function and the development of neurological diseases, making them promising targets for tomorrow's therapies 1 .
Connexins are tetraspan proteins—meaning they weave through the cell membrane four times—creating a distinctive structure with both ends inside the cell. Six connexin subunits join together to form what's called a "hemichannel" or "connexon." When two hemichannels on adjacent cells connect, they create a gap junction channel that serves as a direct tunnel between cells 1 2 .
These gap junctions allow the passage of ions, metabolic substrates, and signaling molecules up to about 1.5 kilodaltons in size, including critical compounds like calcium, potassium, glucose, lactate, and ATP 1 . This direct cell-to-cell communication enables coordinated responses across networks of cells, making them electrically and metabolically synchronized.
The human genome contains 21 different connexin genes, with over 14 isoforms present in the central nervous system. Each brain cell type maintains a unique "connexin portfolio" that defines its communication capabilities 1 .
Pannexins share a similar structural blueprint with connexins—they're also tetraspan proteins with comparable membrane topology. However, despite this structural resemblance, they show no significant sequence homology with connexins 3 . The pannexin family has three members: Panx1, Panx2, and Panx3.
Unlike connexins, pannexins typically function as single-membrane channels rather than forming intercellular bridges. They assemble into heptameric structures (seven subunits) that connect the cell's interior to the extracellular space, serving as important pathways for ATP release and ionic exchange 5 9 .
| Feature | Connexins | Pannexins |
|---|---|---|
| Sequence Homology | Unique family | Homologous to invertebrate innexins |
| Oligomeric State | Typically hexameric (6 subunits) | Typically heptameric (7 subunits) |
| Channel Type | Form both gap junctions and hemichannels | Primarily single-membrane channels |
| Glycosylation | Not glycosylated | Glycosylated extracellular loops |
| Primary Function | Direct cell-cell communication | Autocrine/paracrine signaling |
The sophisticated communication mediated by connexins and pannexins plays multiple critical roles in the central nervous system, contributing to what scientists call the neuro-glio-vascular unit (NGVU)—the functional integration of neurons, glial cells, and blood vessels 1 .
This integrated system ensures that brain activity is matched with adequate energy supply and waste removal. Connexins and pannexins contribute to:
Different brain cells employ distinct connexin and pannexin combinations to perform their specialized functions:
| Cell Type | Connexins Expressed | Pannexins Expressed |
|---|---|---|
| Neurons | Cx30.2, Cx36, Cx45 | Panx1, Panx2 |
| Astrocytes | Cx30, Cx43, Cx45 | Panx1, Panx2 |
| Oligodendrocytes | Cx29, Cx32, Cx47 | Panx1, Panx2 |
| Microglia | Cx32, Cx36, Cx43 (activated) | Panx1 |
| Endothelial Cells | Cx37, Cx40, Cx43 | Panx1, Panx2 |
Recent advances in cryo-electron microscopy (cryo-EM) have revolutionized our understanding of pannexin and connexin structures at the atomic level. These technological breakthroughs have revealed previously unknown details about how these channels are assembled and function.
In 2024, researchers determined the cryo-EM structure of pannexin 3 at 3.9 Å resolution, comparing it with other pannexin isoforms 5 . The study revealed that Panx3 possesses a wider pore radius and a vestibule with two distinct chambers, explaining its unique functional properties compared to Panx1 and Panx2.
Similarly, a 3.0 Å resolution cryo-EM structure of frog Panx1 published in eLife revealed the channel's heptameric architecture and identified a critical constriction point formed by amino acid Trp74 in the first extracellular loop 9 . This narrow region, along with nearby Arg75, helps determine the channel's ion selectivity, and mutating these residues disrupts normal channel function.
These structural insights are more than just molecular blueprints—they provide the foundation for designing targeted therapies that can modulate channel activity in specific diseases.
To understand how connexin hemichannels control molecular movement, researchers have developed an innovative experimental approach that combines two-electrode voltage clamp (TEVC) with dye uptake assays 8 .
Connexin genes are expressed in translucent Xenopus laevis oocytes, which naturally lack significant large-pore channel proteins.
Cell-impermeable fluorescent dyes like ethidium or YO-PRO are introduced. When these dyes enter the oocyte and bind to DNA, their fluorescence dramatically increases.
The technique allows researchers to simultaneously measure ionic currents (via TEVC) and molecular flux (via fluorescence), providing a comprehensive view of transport mechanisms.
By testing different dye concentrations, researchers can determine the kinetic parameters of transport, described by Michaelis-Menten kinetics with apparent KM and Vmax values 8 .
This sophisticated methodology revealed that connexin hemichannels don't behave as simple open pores. Instead, molecular transport through these channels displays saturation kinetics—a characteristic typically associated with transporter proteins rather than open channels 8 .
Even more surprisingly, researchers discovered that molecular transport and ion conduction can be uncoupled—meaning an increase in ionic flow doesn't necessarily correlate with increased molecular permeation, and vice versa 8 .
These findings have profound implications for understanding how connexin hemichannels function in the brain and suggest they're far more sophisticated than previously imagined.
Studying connexin and pannexin channels requires specialized reagents and tools designed to probe their unique properties:
| Tool/Reagent | Function/Application | Examples/Specifics |
|---|---|---|
| Cryo-EM | High-resolution structure determination | Revealed heptameric architecture of pannexins 5 9 |
| Two-Electrode Voltage Clamp | Measures ionic currents while controlling membrane potential | Used in Xenopus oocyte system 8 |
| Fluorescent Dyes | Track molecular permeation through channels | Ethidium, YO-PRO; bind DNA for signal amplification 8 |
| Channel Blockers | Pharmacological inhibition to study function | Carbenoxolone, flufenamates, probenecid 2 |
| Mimetic Peptides | Selective inhibition of specific channel functions | Gap19 (targets Cx43), 10panx1 (targets Panx1) 7 |
| Genetically Modified Mice | Study channel function in physiological contexts | Panx1 knockout mice show reduced seizure activity 5 |
The growing understanding of connexin and pannexin biology has opened exciting avenues for therapeutic intervention in neurological disorders.
Research has linked connexin and pannexin dysfunction to multiple brain conditions:
In mouse models of Alzheimer's, Cx43 hemichannels are chronically activated in astrocytes, leading to excessive release of ATP and glutamate that contributes to neuronal damage .
Dysfunction of astroglial gap junctions impairs potassium clearance, promoting seizure activity. Blocking connexin hemichannels can normalize brain activity in epileptic mice .
Both Cx43 hemichannels and Panx1 channels contribute to stroke damage. Blocking these channels provides neuroprotection in animal models .
Connexin and pannexin hemichannels act as upstream amplifiers of neuroinflammation, releasing ATP and other danger signals that activate inflammatory pathways 7 .
The translational potential of targeting these channels is demonstrated by several innovative strategies:
Short peptides like Gap19 that mimic specific connexin domains can selectively block hemichannels without affecting gap junction communication 7 .
Compounds like D4 and the pleiotropic alkaloid boldine show promise in curbing epileptiform activity, neurodegeneration, and depressive-like behavior in animal models 7 .
Research initiatives like the PANACHE project are working to develop a new generation of selective Panx1, Cx43, and Cx32 hemichannel inhibitors for treating inflammatory diseases 4 .
The study of connexin and pannexin signaling pathways has revealed an elegant architectural blueprint that underpins both normal brain function and pathological conditions. What began as basic research into cellular structures has evolved into a rich understanding of how brain cells coordinate their activities through multiple communication modalities.
As research continues, scientists are working to address key questions about the long-term safety of chronic channel inhibition and developing more specific modulators that can target particular channel subtypes without disrupting essential physiological functions 7 .
The journey from characterizing fundamental channel biology to developing targeted therapies exemplifies how basic scientific discovery can transform into promising medical applications. As we continue to decipher the complex language of intercellular communication in the brain, connexins and pannexins will undoubtedly remain central characters in this unfolding story, potentially holding keys to future treatments for some of our most challenging neurological disorders.
This article was based on current scientific literature up to November 2024.