Adenosine inhibition of the regeneration in vitro of adult frog sciatic sensory axons

Purinergic signaling in oligodendrocyte development and function

Myelin, an insulating membrane that enables rapid action potential propagation, is an essential component of an efficient, functional vertebrate nervous system. Oligodendrocytes, the myelinating glia of the central nervous system (CNS), produce myelin throughout the CNS, which requires continuous proliferation, migration, and differentiation of oligodendrocyte progenitor cells. Because myelination is essential for efficient neurotransmission, researchers hypothesize that neuronal signals may regulate the cascade of events necessary for this process. The ability of oligodendrocytes and oligodendrocyte progenitor cells to detect and respond to neuronal activity is becoming increasingly appreciated, although the specific signals involved are still a matter of debate. Recent evidence from multiple studies points to purinergic signaling as a potential regulator of oligodendrocyte development and differentiation. Adenosine triphosphate (ATP) and its derivatives are potent signaling ligands with receptors expressed on many populations of cells in the nervous system, including cells of the oligodendrocyte lineage. Release of ATP into the extracellular space can initiate a multitude of signaling events, and these downstream signals are specific to the particular purinergic receptor (or receptors) expressed, and whether enzymes are present to hydrolyze ATP to its derivatives adenosine diphosphate and adenosine, each of which can activate their own unique downstream signaling cascades. This review will introduce purinergic signaling in the CNS and discuss evidence for its effects on oligodendrocyte proliferation, differentiation, and myelination. We will review sources of extracellular purines in the nervous system and how changes in purinergic receptor expression may be coupled to oligodendrocyte differentiation. We will also briefly discuss purinergic signaling in injury and diseases of the CNS.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Axon outgrowth is regulated by an intracellular purine-sensitive mechanism in retinal ganglion cells

Although purinergic compounds are widely involved in the intra- and intercellular communication of the nervous system, little is known of their involvement in the growth and regeneration of neuronal connections. In dissociated cultures, the addition of adenosine or guanosine in the low micromolar range induced goldfish retinal ganglion cells to extend lengthy neurites and express the growth-associated protein GAP-43. These effects were highly specific and did not reflect conversion of the nucleosides to their nucleotide derivatives; pyrimidines, purine nucleotides, and membrane-permeable, nonhydrolyzable cyclic nucleotide analogs were all inactive. The activity of adenosine required its conversion to inosine, because inhibitors of adenosine deaminase rendered adenosine inactive. Exogenously applied inosine and guanosine act directly upon an intracellular target, which may coincide with a kinase described in PC12 cells. In support of this, the effects of the purine nucleosides were blocked with purine transport inhibitors and were inhibited competitively with the purine analog 6-thioguanine (6-TG). In PC12 cells, others have shown that 6-TG blocks nerve growth factor-induced neurite outgrowth and selectively inhibits the activity of protein kinase N, a partially characterized, nerve growth factor-inducible serine-threonine kinase. In both goldfish and rat retinal ganglion cells, 6-TG completely blocked outgrowth induced by other growth factors, and this inhibition was reversed with inosine. These results suggest that axon outgrowth in central nervous system neurons critically involves an intracellular purine-sensitive mechanism.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Adenosine receptor-mediated inhibition of neurite outgrowth from cultured sensory neurons is via an A1 receptor and is reduced by nerve growth factor

Adult dorsal root ganglion (DRG) cells are capable of neurite outgrowth in vitro as well as in vivo. We have investigated the influence of adenosine and analogs on the potential of cultured adult mouse DRG neurons to produce neurites in the presence and absence of nerve growth factor (NGF) which is a well-established trophic factor of sympathetic and sensory neurons during development. It is also believed to be essential for the maintenance or regulation of differentiated phenotypes of mature peripheral neurons. The results demonstrate that DRG neurons are modulated by purines in the absence of exogenous NGF. The addition of 100 microM adenosine to neurite-bearing DRG neurons inhibited neurite growth by 47% after 2-day exposures in vitro and by 50% after 5 days whereas in the presence of NGF this inhibition was reduced to 28% and 32%, respectively. 100 microM CHA (N(6)-cyclohexyl adenosine) alone reduced neurite total length by 47% after 2 days and by 48% after 5 days. 100 microM CGS21680 (2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride) alone also reduced neurite total length by 46% after 2 days and by 58% after 5 days which was reduced to 21% and 37%, respectively, in the presence of 100 ng/ml NGF. The antagonist studies revealed that activation of A1 adenosine receptors is primarily responsible for the effect on neuritogenesis since the inclusion of 1 or 10 microM CPX (8-cyclopentyl-1,3-dipropyl xanthine) fully prevented the inhibitory activity of adenosine or CHA whereas DMPX (3,7-dimethyl-1-propargyl xanthine) did not prevent inhibition by CHA. The converse experiment yielded the consistent result that inhibition by the A2 receptor agonist CGS21680 could be prevented by CPX, but not DMPX.

Detection of a purine analogue-sensitive kinase in frog sciatic nerves--possible involvement in nerve regeneration

This study reports the existence of a purine analogue-sensitive protein kinase in adult frog sciatic nerves. Cell-free supernatants of homogenized regenerating sciatic nerves were found to contain a phosphoprotein (MW 90 kDa, referred to as PP90), that was phosphorylated to a much higher degree than in normal, uninjured nerves. The spatial and temporal characteristics of PP90 phosphorylation suggested a relationship with the injury-induced proliferation of support cells of the regenerating nerve, i.e. its appearance and increment over time correlated with that of [3H]thymidine incorporation in the nerve. PP90 was phosphorylated under conditions that excluded enzyme activities due to Ca2+/calmodulin kinases, cyclic nucleotide-dependent kinases or protein kinase C. On the other hand, the phosphorylation could be selectively inhibited by the purine analogues adenosine, 2-aminopurine and 6-thioguanine (6-TG). The latter was the most potent and gave complete inhibition at 50 microM. Addition of histone H1 to the cell-free assay stimulated the phosphorylation of several proteins in both normal and regenerating nerves. The stimulation could be blocked by 6-TG, indicating the presence of a purine-sensitive kinase also in uninjured nerves. Separate experiments showed that in vitro regeneration of the frog sciatic sensory axons, as well as the proliferation of the support cells, was inhibited by 100 microM 6-TG.