Pannexin 2 Is Expressed by Postnatal Hippocampal Neural Progenitors and Modulates Neuronal Commitment

Abstract

The pannexins (Panx1, -2, and -3) are a mammalian family of putative single membrane channels discovered through homology to invertebrate gap junction-forming proteins, the innexins. Because connexin gap junction proteins are known regulators of neural stem and progenitor cell proliferation, migration, and specification, we asked whether pannexins, specifically Panx2, play a similar role in the postnatal hippocampus. We show that Panx2 protein is differentially expressed by multipotential progenitor cells and mature neurons. Both in vivo and in vitro, Type I and IIa stem-like neural progenitor cells express an S-palmitoylated Panx2 species localizing to Golgi and endoplasmic reticulum membranes. Protein expression is down-regulated during neurogenesis in neuronally committed Type IIb and III progenitor cells and immature neurons. Panx2 is re-expressed by neurons following maturation. Protein expressed by mature neurons is not palmitoylated and localizes to the plasma membrane. To assess the impact of Panx2 on neuronal differentiation, we used short hairpin RNA to suppress Panx2 expression in Neuro2a cells. Knockdown significantly accelerated the rate of neuronal differentiation. Neuritic extension and the expression of antigenic markers of mature neurons occurred earlier in stable lines expressing Panx2 short hairpin RNA than in controls. Together, these findings describe an endogenous post-translational regulation of Panx2, specific to early neural progenitor cells, and demonstrate that this expression plays a role in modulating the timing of their commitment to a neuronal lineage. Fig. 4B). The physiological production (and deletion) of these new neurons is implicated in the consolidation of spatial learning and memory (reviewed in Ref. Neural stem and progenitor cells (NPCs2), whereas aberrant or impaired neurogenesis is associated with multiple neurological and cognitive disorders, including epilepsy, mood disorders, neurodegenerative diseases, and ischemia (reviewed in Refs. 3 and 4).The signaling systems that maintain multipotential NPC populations in their "stem-like" state within the adult hippocampal network have only begun to be elucidated (2). One mechanism, the expression of ion-permeable single membrane (ion channels and gap junction hemichannels) as well as double-membrane (gap junction intercellular) channels, is implicated in the control of embryonic and postnatal NPC migration, proliferation, and specification (5-11). Channel proteins, in part, regulate metabolic coupling, adhesion, and calcium wave propagation between NPCs and adjacent terminally differentiated cells (5-11). Pannexins are a family of membrane channel proteins discovered through their homology to the invertebrate family of gap junction proteins, the innexins. Panx1 is expressed in multiple tissues. Panx2 is enriched in the central nervous system. Panx3 is found in skin and cartilage (12)(13)(14). Given their sequence homology with innexins and their structural homology with connexins, pannexins were originally proposed to form ...

Pannexin channels are not gap junction hemichannels

“…In addition, Panx1 has been shown in some cells and tissues to also be concentrated within intracellular compartments. Whether Panx2 localizes well to the plasma membrane remains an area of intense investigation [27][28][29] as well as whether oligomers of different pannexin isoforms form and function in heterotypic combinations. 2,6,9 Vanden Abeele et al presented evidence for Panx1 forming intercellular channels in HEK293 and human prostate cancer epithelial LNCaP cells, 30 but the Panx1 staining in the plasma membrane is more similar to the dispersed pattern observed by several groups than to a canonical gap junction appearance.…”

Ion channels in postnatal neurogenesis

“…68,69 Recent work also detected expression of a member of the recently discovered pannexin family, Pannexin 2 in postnatal hippocampal neural stem cells with a role in modulating neuronal development. 70 Aquaporin-4, responsible for water and ion homeostasis in the central nervous system, is also expressed in NSCs and plays important roles in the proliferation, survival, migration and neuronal differentiation of adult VZ NSCs via modulation of intracellular Ca 2+ dynamics. 71 Highly selective pharmacological tools are still lacking for these particular large pore-type channels.…”

The cellular life of pannexins

“…8 are transported to the Golgi apparatus for further processing, whereas the fate of Panx2 remains less understood as many studies suggest that Panx2 is often retained in intracellular compartments that likely include the ER. 9,19,38 We have shown that Panx1 and Panx3 follow a classical ER-Golgi secretory pathway in a Sar1-mediated COPII-dependent manner. 39 Once in the Golgi apparatus pannexins can undergo further editing and be modified into complex glycoproteins that are preferentially targeted to the plasma membrane as was evidenced by cell surface biotinylation assays 9 (Figure 4).…”

“…One additional post-translational modification has been reported for Panx2 in a study where an ∼80 kDa species expressed in neural progenitor cells was found to be sensitive to hydroxylamine suggesting that Panx2 could be palmitoylated. 19 Further sitedirected mutagenesis studies will be necessary to confirm the palmitoylated status of Panx2, and additional studies are needed to see if either Panx1 or Panx3 is also palmitoylated.…”

“…9 In the developing dentate gyrus, Panx2 was found to be expressed in intracellular punctatelike structures of neural progenitor cells. 19 Although the cell surface distribution of pannexins supports their functional role in establishing cellular communication with the extracellular environment, 26,48 the intracellular localization of pannexins suggests that they may serve alternate roles. We previously showed that Panx1 dysregulated keratinocyte differentiation, whereas Panx3 maintained the epidermal integrity of murine skin.…”