The precise mechanism by which mutant huntingtin elicits its toxicity remains unknown. However, synaptic alterations and increased susceptibility to neuronal death are known contributors to Huntington's disease (HD) symptomatology. While decreased metabolism has long been associated with HD, recent findings have surprisingly demonstrated reduced neuronal apoptosis in Caenorhabditis elegans and Drosophila models of HD by drugs that diminish ATP production. Interestingly, extracellular ATP has been recently reported to elicit neuronal death through stimulation of P2X7 receptors. These are ATP-gated cation channels known to modulate neurotransmitter release from neuronal presynaptic terminals and to regulate cytokine production and release from microglia. We hypothesized that alteration in P2X7-mediated calcium permeability may contribute to HD synaptic dysfunction and increased neuronal apoptosis. Using mouse and cellular models of HD, we demonstrate increased P2X7-receptor level and altered P2X7-mediated calcium permeability in somata and terminals of HD neurons. Furthermore, cultured neurons expressing mutant huntingtin showed increased susceptibility to apoptosis triggered by P2X7-receptor stimulation. Finally, in vivo administration of the P2X7-antagonist Brilliant Blue-G (BBG) to HD mice prevented neuronal apoptosis and attenuated body weight loss and motor-coordination deficits. These in vivo data strongly suggest that altered P2X7-receptor level and function contribute to HD pathogenesis and highlight the therapeutic potential of P2X7 receptor antagonists.

Previously, we reported that purinergic ionotropic P2X7 receptors negatively regulate neurite formation in Neuro-2a (N2a) mouse neuroblastoma cells through a Ca2+/calmodulin-dependent kinase II-related mechanism. In the present study we used this cell line to investigate a parallel though faster P2X7 receptor-mediated signaling pathway, namely Ca2+-regulated exocytosis. Selective activation of P2X7 receptors evoked exocytosis as assayed by high resolution membrane capacitance measurements. Using dual-wavelength total internal reflection microscopy, we have observed both the increase in near-membrane Ca2+ concentration and the exocytosis of fluorescently labeled vesicles in response to P2X7 receptor stimulation. Moreover, activation of P2X7 receptors also affects vesicle motion in the vertical and horizontal directions, thus, involving this receptor type in the control of early steps (docking and priming) of the secretory pathway. Immunocytochemical and RT-PCR experiments evidenced that N2a cells express the three neuronal SNAREs as well as vesicular nucleotide and monoamine (VMAT-1 and VMAT-2) transporters. Biochemical measurements indicated that ionomycin induced a significant release of ATP from N2a cells. Finally, P2X7 receptor stimulation and ionomycin increased the incidence of small transient inward currents, reminiscent of postsynaptic quantal events observed at synapses. Small transient inward currents were dependent on extracellular Ca2+ and were abolished by Brilliant Blue G, suggesting they were mediated by P2X7 receptors. Altogether, these results suggest the existence of a positive feedback mechanism mediated by P2X7 receptor-stimulated exocytotic release of ATP that would act on P2X7 receptors on the same or neighbor cells to further stimulate its own release and negatively control N2a cell differentiation.

The presence of ionotropic P2X 7 receptor has been studied in mice brain from wild type and P2X 7 receptor knockout animals. Western blot and immunocytochemical assays show the presence of a protein containing the P2X 7 immunogenic epitopes in the brain of knockout model. Reverse transcriptase polymerase chain reaction experiments demonstrate the absence of the disrupted sequence, but other sequences of P2X 7 specific mRNA expression have been detected. Functional calcium imaging experiments in cultured granule neurons from P2X 7 knockout cerebella show the existence of a functional P2X 7 -like receptor that keeps some of the properties of the genuine receptor.

Nucleotides present an important role in ocular physiology which has been demonstrated by recent works that indicate their involvement in many ocular processes. P2Y are important among P2 receptors since they can control tear production, corneal wound healing, aqueous humour dynamics and retinal physiology. Commercial antibodies have allowed us to investigate the distribution of P2Y receptors in the cornea, anterior and posterior chamber of the eye and retina. The P2Y1 receptor was present mainly in cornea, ciliary processes, and trabecular meshwork. The P2Y2 receptors were present in cornea, ciliary processes and retinal pigmented epithelium. P2Y4 was present in cornea, ciliary processes, photoreceptors, outer plexiform layer and ganglion cell layer. The P2Y6 presented almost an identical distribution as the P2Y4 receptor. The P2Y11 was also detectable in the retinal pigmented epithelium. The detailed distribution of the receptors clearly supports the recent findings indicating the relevant role of nucleotides in the ocular function.

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It is currently believed that in each vertebrate species Müller cells in the central retina constitutes a fairly homogeneous population from the morphologic point of view and that particularly the chick Müller cell attains full shape differentiation at prenatal stages. However, in this study of the chick retina, from day 1 to day 55 of life, we show that there is a large variety of Müller cell shapes and that many of them complete shape differentiation postnatally. We used a cell dissociation method that preserves the whole shape of the Müller cells. Unstained living and unstained fixed cells were studied by phase-contrast microscopy, and fixed cells immunostained for intermediate filaments of the cytoskeleton were studied by fluorescence microscopy. Our results show that (1) Müller cell shapes vary in the origination of the hair of vitread processes, in the shape of the ventricular (outer or apical) process, in the presence or absence of an accessory process, as well as in the number and shape of processes leaving from the ventricular process at the level of the outer nuclear and outer plexiform layers (ONL/OPL); (2) during the first month of life, many Müller cells differentiate the portion of the ventricular process that traverses the ONL, most Müller cells differentiate the ONL/OPL processes, and all Müller cells differentiate the thin short lateral processes leaving from the vitread hair processes at the level of the inner plexiform layer (IPL). The number of cells differing in the shape of the ventricular process and that of cells with and without accessory process were estimated. The spatial relationship between the outer portion of the ventricular process of the Müller cell and the photoreceptor cells was also studied. Our results show that the branching of the ventricular process and the refinement of Müller cell shape is achieved without apparent participation of growth cones. We give a schematic view of how the branching of the ventricular process might take place and propose the size increase of photoreceptor soma as a factor responsible for this branching.

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