Microglia, the immune cells of the central nervous system, are attracted to sites of injury. The injury releases adenosine triphosphate (ATP) into the extracellular space, activating the microglia, but the full mechanism of release is not known. In glial cells, a family of physiologically regulated unpaired gap junction channels called innexons (invertebrates) or pannexons (vertebrates) located in the cell membrane is permeable to ATP. Innexons, but not pannexons, also pair to make gap junctions. Glial calcium waves, triggered by injury or mechanical stimulation, open pannexon/innexon channels and cause the release of ATP. It has been hypothesized that a glial calcium wave that triggers the release of ATP causes rapid microglial migration to distant lesions. In the present study in the leech, in which a single giant glial cell ensheathes each connective, hydrolysis of ATP with 10 U/ml apyrase or block of innexons with 10 µM carbenoxolone (CBX), which decreased injury-induced ATP release, reduced both movement of microglia and their accumulation at lesions. Directed movement and accumulation were restored in CBX by adding ATP, consistent with separate actions of ATP and nitric oxide, which is required for directed movement but does not activate glia. Injection of glia with innexin2 (Hminx2) RNAi inhibited release of carboxyfluorescein dye and microglial migration, whereas injection of innexin1 (Hminx1) RNAi did not when measured 2 days after injection, indicating that glial cells’ ATP release through innexons was required for microglial migration after nerve injury. Focal stimulation either mechanically or with ATP generated a calcium wave in the glial cell; injury caused a large, persistent intracellular calcium response. Neither the calcium wave nor the persistent response required ATP or its release. Thus, in the leech, innexin membrane channels releasing ATP from glia are required for migration and accumulation of microglia after nerve injury.

Background/Objectives: Cutaneous squamous cell carcinoma (cSCC) is the second most common skin cancer, with a lifetime risk of 14–20% that is rising every year. Although prognosis for cSCC is generally good, certain high-risk features of cSCC portend increased rates of nodal and distant metastasis, recurrence, and disease-specific mortality. One such high-risk factor is perineural invasion (PNI), which is broadly defined as the invasion of cancer into and around nerves. Compared to other high-risk factors, PNI presence is associated with the highest risk for locoregional and distant metastasis. Still, the mechanisms underlying the pathogenesis of PNI remain poorly understood. Recent studies suggest the migration and invasion of tumors into nerves is a result of complex molecular crosstalk within the tumor-nerve microenvironment, wherein the milieu of signaling molecules simultaneously promote neuronal growth and tumor cell invasion. Methods: Understanding the molecular and cellular mechanisms that promote PNI will lead to future developments of targeted therapies that may improve locoregional control and survival. Results/Conclusions: In our article, we aim to provide a comprehensive review of recent findings about the pathogenesis of PNI, clinical implications of PNI-positive disease in cSCC, available treatment modalities, and potential future therapeutic targets.