Intracellular expression of purinoceptors

Abstract

Purinoceptors, receptors for nucleosides and nucleotides, have been identified on the plasma membranes of many cell types [1]. The early hints and recent evidence for localization of purinoceptors on intracellular sites, including lysosomes [2][3][4], mitochondria [5,6] and in nuclei where they open ion channels and appear to influence mRNA activity [7,8], offers up a whole new aspect of purinergic signalling.Purinergic signalling, ATP acting as an extracellular signalling molecule, was proposed in 1972 [9]. Separate families of purinergic receptors were recognised, named P1 receptors for adenosine and P2 receptors for ATP and ADP [10]. Two subtypes of P2 receptors were shown in 1985, based on pharmacology [11] and in the early 1990s P1, P2X and P2Y receptor subtypes were cloned and characterised: four subtypes of P1 receptors (A 1 , A 2A , A 2B and A 3 ), seven subtypes of P2X ion channel receptors (P2X1-7) and eight subtypes of P2Y G protein-coupled receptors (P2Y 1 , P2Y 2 , P2Y 4 , P2Y 6 , P2Y 11 , P2Y 12 , P2Y 13 and P2Y 14 ) [1,12]. Cloning of receptors made it possible to generate polyclonal antisera for immunohistochemical studies of their expression and distribution [13]. Many papers using this technique were published [14][15][16][17]. As expected, the receptors were located on the plasma membranes of cells, but sometimes there was also intracellular immunostaining. This was often dismissed by referees as artefacts. However, later studies have revealed that intracellular localization of purinoceptors is genuine. For example, it was suggested that uptake of P2X1 receptors into smooth muscle cells of the rat vas deferens was responsible for desensitization ([18] and see [19]). P2Y 2 receptor internalization via the clathrin-mediated pathway was observed in HEK293 cells using receptors tagged with green fluorescent protein (GFP), to be colocalized with endosomes and lysosomes [2]. GFP was also used to show internalization of P2Y 1 receptors in HEK-293 cells [20,21]. Ser352 and Ser354 in the carboxyl terminus of human P2Y 1 receptors were shown to be needed for internalization in MDCK cells [22]. P2X3 receptors transfected into HEK-293 cells and expressed endogenously in dorsal root ganglion sensory neurons undergo rapid constitutive endocytosis, targeting the late endosomal/lysosomal system [23]. The role of internalised P2X7 receptors on lysosomes in macrophages in the killing of mycobacteria is discussed in a review [24]. A Rab5-dependent pathway was described for internalisation of P2X4 receptors in HEK-293 cells [25]. The internalised P2X4 receptors are located on lamellar bodies, lysosomes, vesicles and vacuoles in HEK-293 cells, hippocampal neurons and alveolar type II cells [3,4]. P2X4 receptor channel activity was directly measured in intact lysosomes in HEK-239 cells [26]. Both ATP and P2X4 receptors were present in lysosomes and the lysosomal P2X4 receptors were activated by ATP at the luminal side in a pH-dependent manner. The lysosomal P2X4-mediated responses were potentiated by ivermectin, b...

Incomplete Healing as a Cause of Aging: The Role of Mitochondria and the Cell Danger Response

“…In addition, an unknown fraction of nuclear mitochondrial genes is regulated transcriptionally and post-transcriptionally by purinergic signaling via the 12 G-protein coupled receptors (GPCRs; 4 P1R and 8 P2YR) and seven ionotropic receptors (P2XR) that are widely distributed in all tissues [49]. Some purinergic receptors are also expressed in intracellular compartments such as mitochondria, lysosomes, and the nucleus [50]. For example, P2X6 receptors are translocated to the nucleus in an age-dependent manner, interact with mRNA splicing factor 3A1, decrease mRNA processing, and contribute to aging [51].…”

Structural and Functional Basis for Understanding the Biological Significance of P2X7 Receptor

“…Additionally, heterologously expressed P2X7R seems to accumulate on the basolateral membranes in animal epithelial cells [ 91 ]. Finally, the P2X7R protein has been localized inside the cells [ 92 ]—intracellularly (leukocytes, platelets, neurons [ 93 , 94 , 95 ]), in the nuclei (epithelial and smooth muscle cells [ 96 , 97 , 98 ]), and in phagosomes (macrophages [ 99 ]). The specific role of intracellular P2X7R remains unclear, although some studies have pointed to the functions of differentiation and autophagy [ 93 , 100 ].…”

Inseparable tandem: evolution chooses ATP and Ca2+to control life, death and cellular signalling

“…Incidentally, P2X receptors may also serve as intracellular Ca 2þ -release channels, as has been shown in D. discoideum [233], further corroborating an intimate relation between ATP and Ca 2þ from the very early evolution of cellular signalling. The P2X receptors seem to operate in the membranes of other organelles, including mitochondria, lysosomes and nuclei [234], although whether activation of these intracellular receptors is linked to Ca 2þ movements remains to be found. rstb.royalsocietypublishing.org Phil.…”