Pannexin1 and Pannexin2 expression in the developing and mature rat brain

Abstract

Recent studies have identified a new family of gap junction-forming proteins in vertebrates, called pannexins. Although their function in vivo is still not known, studies in Xenopus oocytes have indicated that pannexin1 (Px1) and pannexin2 (Px2) can form functional gap junction channels and can contribute to functional hemichannels. In this study, we have utilized a combination of radioactive and non-radioactive in situ hybridization experiments to characterize the expression pattern of the two pannexin genes during development and maturation of the rat brain. Expression analysis revealed a widespread and similar mRNA distribution for both genes, but indicated that Px1 and Px2 are inversely regulated during the development of the rat brain. Px1 is expressed at a high level in the embryonic and young postnatal brain and declines considerably in the adult, whereas Px2 mRNA is low in the prenatal brain but increases substantially during subsequent postnatal development. Immunohistochemical studies using different antibodies confirm the neuronal origin of pannexin-expressing cells and ascertain the presence of both pannexins in the majority of pyramidal cells and in GABAergic interneurons. The abundant presence of both pannexins in most neurons suggests that they may play a role in intercellular communication in many neuronal circuits. Furthermore, the temporal difference in the expression of the two genes indicates that the relative contribution of the two pannexins in immature and mature neuronal circuits may vary.

Introduction

Gap junctions are specialized structures containing accumulations of intercellular channels, each of which is formed by the apposition of a hexameric assembly of connexin proteins from two adjacent cells [4], [8], [18]. Gap junctions are intercellular channels that permit the passage of ions and small molecules, thus coordinating biochemical and electrical activity of cellular networks. In the central nervous system, gap junctions are the substrate of electrical synapses and can synchronize neural activity in the developing and mature brain and can thus contribute to the formation of defined networks [4], [8], [15].

In addition to the large multigene family of connexins [28], there is evidence that the recently discovered pannexins may also comprise a family of gap junction-forming proteins [22]. Functional studies in Xenopus oocytes demonstrated that Px1 is able to form hemichannels and intercellular channels alone and in combination with Px2 [5]. Furthermore, a recent study has indicated that Px1 hemichannels serve as a conduit for the release of ATP to the extracellular space upon mechanical stress [1]. Since Px1 and Px2 are expressed in the central nervous system [2], [5], [27], it is possible that intercellular channels composed of these proteins may also contribute to oscillatory activity in the brain [10].

To provide a basis for future in vivo studies on the function of pannexin channels, we have conducted in situ hybridization experiments to evaluate the distribution of Px1 and Px2 mRNA in the brain. This expression analysis focuses on several brain regions such as hippocampus, cerebral cortex, olfactory bulb and cerebellum, and demonstrates that multiple classes of neurons express Px1 and Px2 implicating an extensive role for pannexins in central nervous system function.