Characterization of specific GTP binding sites in C2C12 mouse skeletal muscle cells

Broadband Electrical Spectroscopy to Distinguish Single-Cell Ca2+ Changes Due to Ionomycin Treatment in a Skeletal Muscle Cell Line

Many skeletal muscle diseases such as muscular dystrophy, myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), and sarcopenia share the dysregulation of calcium (Ca²⁺) as a key mechanism of disease at a cellular level. Cytosolic concentrations of Ca²⁺ can signal dysregulation in organelles including the mitochondria, nucleus, and sarcoplasmic reticulum in skeletal muscle. In this work, a treatment is applied to mimic the Ca²⁺ increase associated with these atrophy-related disease states, and broadband impedance measurements are taken for single cells with and without this treatment using a microfluidic device. The resulting impedance measurements are fitted using a single-shell circuit simulation to show calculated electrical dielectric property contributions based on these Ca²⁺ changes. From this, similar distributions were seen in the Ca²⁺ from fluorescence measurements and the distribution of the S-parameter at a single frequency, identifying Ca²⁺ as the main contributor to the electrical differences being identified. Extracted dielectric parameters also showed different distribution patterns between the untreated and ionomycin-treated groups; however, the overall electrical parameters suggest the impact of Ca²⁺-induced changes at a wider range of frequencies.

Guanine inhibits the growth of human glioma and melanoma cell lines by interacting with GPR23

Guanine-based purines (GBPs) exert numerous biological effects at the central nervous system through putative membrane receptors, the existence of which is still elusive. To shed light on this question, we screened orphan and poorly characterized G protein-coupled receptors (GPRs), selecting those that showed a high purinoreceptor similarity and were expressed in glioma cells, where GBPs exerted a powerful antiproliferative effect. Of the GPRs chosen, only the silencing of GPR23, also known as lysophosphatidic acid (LPA) 4 receptor, counteracted GBP-induced growth inhibition in U87 cells. Guanine (GUA) was the most potent compound behind the GPR23-mediated effect, acting as the endpoint effector of GBP antiproliferative effects. Accordingly, cells stably expressing GPR23 showed increased sensitivity to GUA. Furthermore, while GPR23 expression was low in a hypoxanthine-guanine phosphoribosyl-transferase (HGPRT)-mutated melanoma cell line showing poor sensitivity to GBPs, and in HGPRT-silenced glioma cells, GPR23-induced expression in both cell types rescued GUA-mediated cell growth inhibition. Finally, binding experiments using [3H]-GUA and U87 cell membranes revealed the existence of a selective GUA binding (KD = 29.44 ± 4.07 nM; Bmax 1.007 ± 0.035 pmol/mg prot) likely to GPR23. Overall, these data suggest GPR23 involvement in modulating responses to GUA in tumor cell lines, although further research needs to verify whether this receptor mediates other GUA effects.

Guanosine-Based Nucleotides, the Sons of a Lesser God in the Purinergic Signal Scenario of Excitable Tissues

Purines are nitrogen compounds consisting mainly of a nitrogen base of adenine (ABP) or guanine (GBP) and their derivatives: nucleosides (nitrogen bases plus ribose) and nucleotides (nitrogen bases plus ribose and phosphate). These compounds are very common in nature, especially in a phosphorylated form. There is increasing evidence that purines are involved in the development of different organs such as the heart, skeletal muscle and brain. When brain development is complete, some purinergic mechanisms may be silenced, but may be reactivated in the adult brain/muscle, suggesting a role for purines in regeneration and self-repair. Thus, it is possible that guanosine-5′-triphosphate (GTP) also acts as regulator during the adult phase. However, regarding GBP, no specific receptor has been cloned for GTP or its metabolites, although specific binding sites with distinct GTP affinity characteristics have been found in both muscle and neural cell lines. Finally, even if the cross regulation mechanisms between the two different purines (ABP and GBP) are still largely unknown, it is now possible to hypothesize the existence of specific signal paths for guanosine-based nucleotides that are capable of modulating the intensity and duration of the intracellular signal, particularly in excitable tissues such as brain and muscle.

Extracellular Guanosine 5'-Triphosphate Induces Human Muscle Satellite Cells to Release Exosomes Stuffed With Guanosine

The extracellular guanosine 5'-triphosphate, GTP, has been demonstrated to be an enhancer of myogenic cell differentiation in a murine cell line, not yet in human muscle cells. Our hypothesis was that GTP could influence also human skeletal muscle regeneration, specifically in the first phases. We tested GTP stimulus on human muscle precursor cells established in culture by human satellite cells derived from Vastus Lateralis of three young male. Our data show that extracellular GTP (a) up-regulated miRNA (specifically miR133a and miR133b) and myogenic regulator factor and (b) induces human myogenic precursor cells to release exosomes stuffed with guanosine based molecules (mainly guanosine) in the extracellular milieu. We think that probably these exosomes could be addressed to influence by means of their content (mainly guanosine) in paracrine or autocrine manner the surrounding cells and/or at distance other muscles or tissues.

Neuromuscular electrical stimulation improves skeletal muscle regeneration through satellite cell fusion with myofibers in healthy elderly subjects

The aim of this study was to determine whether neuromuscular electrical stimulation (NMES) affects skeletal muscle regeneration through a reduction of oxidative status in satellite cells of healthy elderly subjects. Satellite cells from the vastus lateralis skeletal muscle of 12 healthy elderly subjects before and after 8 wk of NMES were allowed to proliferate to provide myogenic populations of adult stem cells [myogenic precursor cells (MPCs)]. These MPCs were then investigated in terms of their proliferation, their basal cytoplasmic free Ca2+concentrations, and their expression of myogenic regulatory factors ( PAX3, PAX7, MYF5, MYOD, and MYOG) and micro-RNAs (miR-1, miR-133a/b, and miR-206). The oxidative status of these MPCs was evaluated through superoxide anion production and superoxide dismutase and glutathione peroxidase activities. On dissected single skeletal myofibers, the nuclei were counted to determine the myonuclear density, the fiber phenotype, cross-sectional area, and tension developed. The MPCs obtained after NMES showed increased proliferation rates along with increased cytoplasmic free Ca2+concentrations and gene expression of MYOD and MYOG on MPCs. Muscle-specific miR-1, miR-133a/b, and miR-206 were upregulated. This NMES significantly reduced superoxide anion production, along with a trend to reduction of superoxide dismutase activity. The NMES-dependent stimulation of muscle regeneration enhanced satellite cell fusion with mature skeletal fibers. NMES improved the regenerative capacity of skeletal muscle in elderly subjects. Accordingly, the skeletal muscle strength and mobility of NMES-stimulated elderly subjects significantly improved. NMES may thus be further considered for clinical or ageing populations.

NEW & NOTEWORTHY The neuromuscular electrical stimulation (NMES) effect on skeletal muscle regeneration was assessed in healthy elderly subjects for the first time. NMES improved the regenerative capacity of skeletal muscle through increased myogenic precursor cell proliferation and fusion with mature myofibers. The increased cytoplasmic free Ca2+concentration along with MYOD, MYOG, and micro-RNA upregulation could be related to reduced O2·−production, which, in turn, favors myogenic regeneration. Accordingly, the skeletal muscle strength of NMES-stimulated lower limbs of healthy elderly subjects improved along with their mobility.

Low Intensity Exercise Training Improves Skeletal Muscle Regeneration Potential

Purpose: The aim of this study was to determine whether 12 days of low-to-moderate exercise training at low altitude (598 m a.s.l.) improves skeletal muscle regeneration in sedentary adult women.

Methods: Satellite cells were obtained from the vastus lateralis skeletal muscle of seven women before and after this exercise training at low altitude. They were investigated for differentiation aspects, superoxide anion production, antioxidant enzymes, mitochondrial potential variation after a depolarizing insult, intracellular Ca²⁺ concentrations, and micro (mi)RNA expression (miR-1, miR-133, miR-206).

Results: In these myogenic populations of adult stem cells, those obtained after exercise training, showed increased Fusion Index and intracellular Ca²⁺ concentrations. This exercise training also generally reduced superoxide anion production in cells (by 12–67%), although not in two women, where there was an increase of ~15% along with a reduced superoxide dismutase activity. miRNA expression showed an exercise-induced epigenetic transcription profile that was specific according to the reduced or increased superoxide anion production of the cells.

Conclusions: The present study shows that low-to-moderate exercise training at low altitude improves the regenerative capacity of skeletal muscle in adult women. The differentiation of cells was favored by increased intracellular calcium concentration and increased the fusion index. This low-to-moderate training at low altitude also depicted the epigenetic signature of cells.

New insights into the relationship between mIGF-1-induced hypertrophy and Ca2+ handling in differentiated satellite cells

Muscle regeneration involves the activation of satellite cells, is regulated at the genetic and epigenetic levels, and is strongly influenced by gene activation and environmental conditions. The aim of this study was to determine whether the overexpression of mIGF-1 can modify functional features of satellite cells during the differentiation process, particularly in relation to modifications of intracellular Ca²⁺ handling.

Satellite cells were isolated from wild-type and MLC/mIGF-1 transgenic mice. The cells were differentiated in vitro, and morphological analyses, intracellular Ca²⁺ measurements, and ionic current recordings were performed.

mIGF-1 overexpression accelerates satellite cell differentiation and promotes myotube hypertrophy. In addition, mIGF-1 overexpression-induced potentiation of myogenesis triggers both quantitative and qualitative changes to the control of intracellular Ca²⁺ handling. In particular, the differentiated MLC/mIGF-1 transgenic myotubes have reduced velocity and amplitude of intracellular Ca²⁺ increases after stimulation with caffeine, KCl and acetylcholine. This appears to be due, at least in part, to changes in the physico-chemical state of the sarcolemma (increased membrane lipid oxidation, increased output currents) and to increased expression of dihydropyridine voltage-operated Ca²⁺ channels. Interestingly, extracellular ATP and GTP evoke intracellular Ca²⁺ mobilization to greater extents in the MLC/mIGF-1 transgenic satellite cells, compared to the wild-type cells.

These data suggest that these MLC/mIGF-1 transgenic satellite cells are more sensitive to trophic stimuli, which can potentiate the effects of mIGF-1 on the myogenic programme.

Wnt7a-Fzd7 signalling directly activates the Akt/mTOR anabolic growth pathway in skeletal muscle

Wnt7a signals through its receptor Fzd7 to activate the planar-cell-polarity pathway and drive the symmetric expansion of satellite stem cells resulting in enhanced repair of skeletal muscle. In differentiated myofibres, we observed that Wnt7a binding to Fzd7 directly activates the Akt/mTOR growth pathway thereby inducing myofibre hypertrophy. Notably, the Fzd7 receptor complex was associated with Gαs and PI3kinase and these components were required for Wnt7a to activate the Akt/mTOR growth pathway in myotubes. Wnt7a/Fzd7 activation of this pathway was completely independent of IGF-receptor activation. Together, these experiments demonstrate that Wnt7a/Fzd7 activates distinct pathways at different developmental stages during myogenic lineage progression, and together identify a novel non-canonical anabolic signalling pathway for Wnt7a and its receptor Fzd7 in skeletal muscle.

Transcriptional profile of denervated vastus lateralis muscle derived from a patient 8 months after spinal cord injury: a case-report

A lack of motor neurons abolishes both neurotrophic factor secretion and contractile activity in muscle, which impairs mass, contractile properties, and fibre-type characteristics of the muscle. However, the molecular pathways that can be stimulated or repressed in the scenario of spinal cord injury remain unknown. We investigated for the first time the transcriptional profile of a young male patient 8 months after spinal cord injury. Adaptive metabolic changes of complete denervated skeletal muscle were revealed. In particular, the main molecular pathways involved include metabolic and proteolitic pathways, mitochondrial and synaptic function, calcium homeostasis, sarcomere and anchorage structures. Our data depict the molecular signalling still present in complete denervated skeletal muscle fibres a few months after spinal cord injury. These data could be of interest also to design a specific therapeutic approach aimed at the electrical-stimulation of severe atrophied skeletal muscle.

Transcriptional profile of GTP-mediated differentiation of C2C12 skeletal muscle cells

Several purine receptors have been localised on skeletal muscle membranes. Previous data support the hypothesis that extracellular guanosine 5'-triphosphate (GTP) is an important regulatory factor in the development and function of muscle tissue. We have previously described specific extracellular binding sites for GTP on the plasma membrane of mouse skeletal muscle (C2C12) cells. Extracellular GTP induces an increase in intracellular Ca(2+) concentrations that results in membrane hyperpolarisation through Ca(2+)-activated K(+) channels, as has been demonstrated by patch-clamp experiments. This GTP-evoked increase in intracellular Ca(2+) is due to release of Ca(2+) from intracellular inositol-1,4,5-trisphosphate-sensitive stores. This enhances the expression of the myosin heavy chain in these C2C12 myoblasts and commits them to fuse into multinucleated myotubes, probably via a phosphoinositide-3-kinase-dependent signal-transduction mechanism. To define the signalling of extracellular GTP as an enhancer or modulator of myogenesis, we investigated whether the gene-expression profile of differentiated C2C12 cells (4 and 24 h in culture) is affected by extracellular GTP. To investigate the nuclear activity and target genes modulated by GTP, transcriptional profile analysis and real-time PCR were used. We demonstrate that in the early stages of differentiation, GTP up-regulates genes involved in different pathways associated with myogenic processes, including cytoskeleton structure, the respiratory chain, myogenesis, chromatin reorganisation, cell adhesion, and the Jak/Stat pathway, and down-regulates the mitogen-activated protein kinase pathway. GTP also increases the expression of three genes involved in myogenesis, Pp3ca, Gsk3b, and Pax7. Our data suggests that in the myogenic C2C12 cell line, extracellular GTP acts as a differentiative factor in the induction and sustaining of myogenesis.

Extracellular guanosine and GTP promote expression of differentiation markers and induce S-phase cell-cycle arrest in human SH-SY5Y neuroblastoma cells

SH-SY5Y neuroblastoma cells, a model for studying neuronal differentiation, are able to differentiate into either cholinergic or dopaminergic/adrenergic phenotypes depending on media conditions. Using this system, we asked whether guanosine (Guo) or guanosine-5'-triphosphate (GTP) are able to drive differentiation towards one particular phenotype. Differentiation was determined by evaluating the frequency of cells bearing neurites and assessing neurite length after exposure to different concentrations of Guo or GTP for different durations. After 6 days, 0.3 mM Guo or GTP induced a significant increase in the number of cells bearing neurites and increased neurite length. Western blot analyses confirmed that purines induced differentiation; cells exposed to purines showed increases in the levels of GAP43, MAP2, and tyrosine hydroxylase. Proliferation assays and cytofluorimetric analyses indicated a significant anti-proliferative effect of purines, and a concentration-dependent accumulation of cells in S-phase, starting after 24 h of purine exposure and extending for up to 6 days. A transcriptional profile analysis using gene arrays showed that an up-regulation of cyclin E2/cdk2 evident after 24 h was responsible for S-phase entry, and a concurrent down-regulation of cell-cycle progression-promoting cyclin B1/B2 prevented S-phase exit. In addition, patch-clamp recordings revealed that 0.3 mM Guo or GTP, after 6 day incubation, significantly decreased Na(+) currents. In conclusion, we showed Guo- and GTP-induced cell-cycle arrest in neuroblastoma cells and suggest that this makes these cells more responsive to differentiation processes that favor the dopaminergic/adrenergic phenotype.

Neurotrophic effects of extracellular guanosine

Central nervous system (CNS) astrocytes release guanosine extracellularly, that exerts trophic effects. In CNS, extracellular guanosine (GUO) stimulates mitosis, synthesis of trophic factors, and cell differentiation, including neuritogenesis, is neuroprotective, and reduces apoptosis due to several stimuli. Specific receptor-like binding sites for eGUO in the nervous system may mediate its effects through both MAP kinase and PI3-kinase signalling pathways. Extracellular guanine (eGUA) also exerts several effects; the trophic effects of eGUO are likely regulated by conversion of eGUO to eGUA by a membrane located purine nucleoside phosphorylase (ecto-PNP) and by conversion of eGUA to xanthine by guanine deaminase.

Proposal of a guanine-based purinergic system in the mammalian central nervous system

Guanine-based purines have been traditionally studied as modulators of intracellular processes, mainly G-protein activity. However, they also exert several extracellular effects not related to G proteins, including modulation of glutamatergic activity, trophic effects on neural cells, and behavioral effects. In this article, the putative roles of guanine-based purines on the nervous system are reviewed, and we propose a specific guanine-based purinergic system in addition to the well-characterized adenine-based purinergic system. Current evidence suggest that guanine-based purines modulate glutamatergic parameters, such as glutamate uptake by astrocytes and synaptic vesicles, seizures induced by glutamatergic agents, response to ischemia and excitotoxicity, and are able to affect learning, memory and anxiety. Additionally, guanine-based purines have important trophic functions affecting the development, structure, or maintenance of neural cells. Although studies addressing the mechanism of action (receptors and second messenger systems) of guanine-based purines are still insufficient, these findings point to the guanine-based purines (nucleotides and guanosine) as potential new targets for neuroprotection and neuromodulation.

Extracellular guanosine-5'-triphosphate modulates myogenesis via intermediate Ca(2+)-activated K+ currents in C2C12 mouse cells

In this study we investigated the role of extracellular 5'-guanosine-triphosphate (GTP) on early phases of skeletal muscle differentiation using the widely used C2C12 mouse cells as a myogenic model. We show that extracellular GTP binding to specific sites activates a metabotropic cascade that leads to a transient intracellular Ca2+ mobilization, consequent activation of the intermediate Ca(2+)-activated K+ channels (IK(Ca)), and hyperpolarization of the plasma membrane. We further show that in differentiating C2C12 myoblasts GTP induces a proliferative boost, and increases the number of cells positive for the myosin heavy chain (MyHC) proteins. These effects were shown to be mediated by the IK(Ca) channel-dependent hyperpolarization, as evidenced by their disappearance when myoblasts were incubated with the IK(Ca) channel inhibitor charybdotoxin. These data give new insights into nucleotide purinergic signalling pathways, and address the role of the GTP-dependent IK(Ca) channel activation and hyperpolarization in myogenesis.

Signal transduction events induced by extracellular guanosine 5' triphosphate in excitable cells

A better understanding of the physiological effects of guanosine-based purines should help clarify the complex subject of purinergic signalling. We studied the effect of extracellular guanosine 5′triphosphate (GTP) on the differentiation of two excitable cell lines that both have specific binding sites for GTP: PC12 rat pheochromocytoma cells and C2C12 mouse skeletal muscle cells. PC12 cells can be differentiated into fully functional sympathetic-like neurons with 50′00 ng ml⁻¹ of nerve growth factor, whereas serum starvation causes C2C12 cells to differentiate into myotubes showing functional excitation–contraction coupling, with the expression of myosin heavy chain proteins. Our results show that GTP enhances the differentiation of both of these excitable cell lines. The early events in guanosine-based purine signal transduction appear to involve an increase in intracellular Ca²⁺ levels and membrane hyperpolarization. We further investigated the early activation of extracellular-regulated kinases and phosphoinositide 3-kinase in GTP-stimulated PC12 and C2C12 cells, respectively. We found that GTP promotes the activation of both kinases. Together, our results suggest that, even if there are some differences in the signalling pathways, GTP-induced differentiation in both cell lines is dependent on an increase in intracellular Ca²⁺.

Responses of the ciliates Tetrahymena and Paramecium to external ATP and GTP

The unicellular ciliates Paramecium and Tetrahymena are the simplest eukaryotic cells to show reliable depolarizing responses to micromolar concentrations of external ATP and GTP. Their simplicity allows for combined analysis of swimming behavior, electrophysiology, receptor binding, behavioral mutant and drug screens as well as molecular genetic approaches such as RNAi and gene knockouts experiments. ATP and GTP are depolarizing chemorepellents in both ciliates, producing measurable receptor potentials and Ca(2+)-based action potentials that are correlated with jerking behaviors called avoiding reactions (AR). GTP also causes repetitive continuous ciliary reversals (CCR) and oscillating plateau depolarizations in Paramecium. Both ciliates show high affinity, saturable external binding of (32)P-GTP and (32)P-ATP but GTP does not compete for ATP binding and vice versa. Chemosensory adaptation occurs after continued exposure (15 min) to these ligands, producing a loss of external binding and forward swimming. However, cells adapted to ATP still bind and respond to GTP and GTP-adapted cells still bind and respond to ATP. This, combined with pharmacological analyses, suggests that there are two separate receptor systems: A metabotropic ATP receptor pathway and a different, novel GTP receptor pathway. A Paramecium mutant (ginA) lacks the GTP-induced oscillating depolarizations but does show AR in GTP, unveiling isolated GTP-receptor potentials for study. An ecto-ATPase is also present that may be involved in inactivation of ATP and GTP signals. Gene knockout experiments are currently underway to determine the roles of the ecto-ATPase and a putative 7-transmembrane spanning receptor in these responses.

Intermediate-conductance Ca2+-activated K+ channel is expressed in C2C12 myoblasts and is downregulated during myogenesis

We report here the expression in C2C12 myoblasts of the intermediate-conductance Ca2+-activated K+ (IK(Ca)) channel. The IK(Ca) current, recorded under perforated-patch configuration, had a transient time course when activated by ionomycin (0.5 microM; peak current density 26.2 +/- 3.7 pA/pF; n = 10), but ionomycin (0.5 microM) + 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (100 microM) evoked a stable outward current (28.4 +/- 8.2 pA/pF; n = 11). The current was fully inhibited by charybdotoxin (200 nM), clotrimazole (2 microM), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (300 microM), but not by tetraethylammonium (1 mM) or D-tubocurarine (300 microM). Congruent with the IK(Ca) channel, elevation of intracellular Ca2+ in inside-out patches resulted in the activation of a voltage-insensitive K+ channel with weak inward rectification, a unitary conductance of 38 +/- 6 pS (at negative voltages), and an IC50 for Ca2+ of 530 nM. The IK(Ca) channel was activated metabotropically by external application of ATP (100 microM), an intracellular Ca2+ mobilizer. Under current-clamp conditions, ATP application resulted in a membrane hyperpolarization of approximately 35 mV. The IK(Ca) current downregulated during myogenesis, ceasing to be detectable 4 days after the myoblasts were placed in differentiating medium. Downregulation was prevented by the myogenic suppressor agent basic FGF (bFGF). We also found that block of the IK(Ca) channel by charybdotoxin did not inhibit bFGF-sustained myoblast proliferation. These observations show that in C2C12 myoblasts the IK(Ca) channel expression correlates inversely with differentiation, yet it does not appear to have a role in myoblast proliferation.

Cooperation in signal transduction of extracellular guanosine 5' triphosphate and nerve growth factor in neuronal differentiation of PC12 cells

Guanosine 5' triphosphate (GTP), acting synergistically with the nerve growth factor (NGF), enhances the proportion of neurite-bearing cells in cultures of PC12 rat pheochromocytoma cells. We studied the transduction mechanisms activated by GTP in PC12 cells and found that addition of GTP (100 microM) increased intracellular calcium concentration ([Ca(2+)](i)) in cells that were between 60 and 70% confluent. Addition of GTP also enhanced activation of NGF-induced extracellular regulated kinases (ERKs) and induced Ca(2+) mobilization. This mobilization, due to the activation of voltage-sensitive and ryanodine-sensitive calcium channels, as well as pertussis toxin-sensitive purinoceptors, modulates Ca(2+)-activated K(+) channels not involved in activation of ERKs. The results presented here indicate that GTP-triggered [Ca(2+)](i) increase may be a key event in GTP signal transduction, which can modulate activity of ERKs. The physiological importance of the GTP effect lies in its capacity to interact with the NGF-activated pathway to enhance neurite outgrowth from PC12 cells.

Neuroprotective effect of adenine on purkinje cell survival in rat cerebellar primary cultures

Although adenosine or ATP is known to control various physiological functions in the brain, including synaptic transmission, neuronal cell death, and neurite outgrowth via P1 or P2 purinergic receptors in the nervous system, little is known about the functions of many other purine derivatives. We examined the effects of various purines on survival in the cerebellar cortex of Purkinje cells with large cell bodies and highly branched dendrites, and it was found that some purine and pyrimidine derivatives influence Purkinje cell survival. Treatment with adenine, guanine, guanosine, guanine nucleotides, and uracil nucleotides protected Purkinje cells from cell death in the cerebellar primary cultures. Among the effective compounds, adenine had the most potent survival activities on Purkinje cells. Other adenine-based purines such as adenosine, AMP, ADP, and ATP did not promote Purkinje cell survival. Furthermore, metabolic inhibitors of adenine had no effect on the protective ability of adenine for Purkinje cells, suggesting that adenine itself, not adenine metabolites, maintains Purkinje cell survival. These results suggest that adenine is involved in the control of Purkinje cell survival in cerebellar primary cultures via a novel adenine-dependent mechanism.