Paracrine signaling through plasma membrane hemichannels.

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Plasma membrane hemichannels, composed of connexin (Cx) and pannexin (Panx) proteins, are increasingly recognized for functions beyond gap junction communication.
• Unapposed hemichannels can open in response to various signals, creating a conduit for molecule exchange between the cell interior and extracellular environment.

Purpose of the Study:

• To review the current evidence for hemichannel-mediated release of paracrine messengers.
• To highlight the roles of specific hemichannels (Cx26, Cx32, Cx36, Cx43, Panx1) in releasing molecules like ATP, glutamate, and NAD+.
• To identify gaps in knowledge regarding downstream signaling and functional effects of hemichannel activity.

Main Methods:

• Literature review of in vitro studies on connexin and pannexin hemichannels.
• Analysis of evidence for the release of specific signaling molecules through hemichannels.
• Discussion of challenges in differentiating hemichannel function from gap junction activity.

Main Results:

• Connexin and pannexin hemichannels facilitate the release of signaling molecules, including ATP, glutamate, prostaglandins, NAD+, and glutathione.
• Evidence strongly supports ATP as a key messenger released via hemichannels.
• In vitro studies provide a foundation, but in vivo validation and separation of hemichannel functions remain critical.

Conclusions:

• Hemichannels represent a significant pathway for paracrine signaling, particularly for molecules like ATP and glutamate.
• Further investigation into hemichannel-mediated signaling cascades is essential for understanding their contribution to physiological and pathological processes.
• Distinguishing hemichannel functions from gap junctions and pannexin channels is crucial for advancing the field.