Adenine Nucleotides Control Proliferation In Vivo of Rat Retinal Progenitors by P2Y1 Receptor

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Bankable human iPSC-derived retinal progenitors represent a valuable source of multipotent cells

Retinal progenitor cells (RPCs) are the source of all retinal cell types during retinogenesis. Until now, the isolation and expansion of RPCs has been at the expense of their multipotency. Here, we report simple methods and media for the generation, expansion, and cryopreservation of human induced pluripotent stem-cell derived-RPCs (hiRPCs). Thawed and passed hiRPCs maintained biochemical and transcriptional RPC phenotypes and their ability to differentiate into all retinal cell types. Specific conditions allowed the generation of large cultures of photoreceptor precursors enriched up to 90% within a few weeks and without a purification step. Combined RNA-seq analysis between hiRPCs and retinal organoids identified genes involved in developmental or degenerative retinal diseases. Thus, hiRPC lines could provide a valuable source of retinal cells for cell-based therapies or drug discovery and could be an advanced cellular tool to better understand retinal dystrophies.

P2X7 and A2A receptor endogenous activation protects against neuronal death caused by CoCl2 -induced photoreceptor toxicity in the zebrafish retina

Injured retinas in mammals do not regenerate and heal with loss of function. The adult retina of zebrafish self-repairs after damage by activating cell-intrinsic mechanisms, which are regulated by extrinsic signal interactions. Among relevant regulatory extrinsic systems, purinergic signaling regulates progenitor proliferation during retinogenesis and regeneration and glia proliferation in proliferative retinopathies. ATP-activated P2X7 (P2RX7) and adenosine (P1R) receptors are involved in the progression of almost all retinopathies leading to blindness. Here, we examined P2RX7 and P1R participation in the retina regenerative response induced by photoreceptor damage caused by a specific dose of CoCl₂ . First, we found that treatment of uninjured retinas with a potent agonist of P2RX7 (BzATP) provoked photoreceptor damage and mitotic activation of multipotent progenitors. In CoCl₂ -injured retinas, blockade of endogenous extracellular ATP activity on P2RX7 caused further neurodegeneration, Müller cell gliosis, progenitor proliferation, and microglia reactivity. P2RX7 inhibition in injured retinas also increased the expression of lin28a and tnfα genes, which are related to multipotent progenitor proliferation. Levels of hif1α, vegf3r, and vegfaa mRNA were enhanced by blockade of P2RX7 immediately after injury, indicating hypoxic like damage and endothelial cell growth and proliferation. Complete depletion of extracellular nucleotides with an apyrase treatment strongly potentiated cell death and progenitor proliferation induced with CoCl₂ . Blockade of adenosine P1 and A_2A receptors (A_2A R) had deleterious effects and deregulated normal timing for progenitor and precursor cell proliferation following photoreceptor damage. ATP via P2RX7 and adenosine via A_2A R are survival extracellular signals key for retina regeneration in zebrafish.

Purinergic signaling in the retina: From development to disease

Retinal injuries and diseases are major causes of human disability involving vision impairment by the progressive and permanent loss of retinal neurons. During development, assembly of this tissue entails a successive and overlapping, signal-regulated engagement of complex events that include proliferation of progenitors, neurogenesis, cell death, neurochemical differentiation and synaptogenesis. During retinal damage, several of these events are re-activated with both protective and detrimental consequences. Purines and pyrimidines, along with their metabolites are emerging as important molecules regulating both retinal development and the tissue's responses to damage. The present review provides an overview of the purinergic signaling in the developing and injured retina. Recent findings on the presence of vesicular and channel-mediated ATP release by retinal and retinal pigment epithelial cells, adenosine synthesis and release, expression of receptors and intracellular signaling pathways activated by purinergic signaling in retinal cells are reported. The pathways by which purinergic receptors modulate retinal cell proliferation, migration and death of retinal cells during development and injury are summarized. The contribution of nucleotides to the self-repair of the injured zebrafish retina is also discussed.

Purinergic Signaling Modulates Survival/Proliferation of Human Dental Pulp Stem Cells

Human dental pulp stem cells (hDPSCs) reside in postnatal dental pulp and exhibit the potential to differentiate into odontoblasts as well as neurons. However, the intercellular signaling niches necessary for hDPSC survival and self-renewal remain largely unknown. The objective of this study is to demonstrate the existence of intercellular purinergic signaling in hDPSCs and to assess the impact of purinergic signaling on hDPSC survival and proliferation. hDPSCs were isolated from extracted third molars and cultured in minimum essential medium. To demonstrate responsiveness to ATP application and inhibitions by purinergic receptor antagonists, whole cell patch-clamp recordings of ATP-induced currents were recorded from cultured hDPSCs. Immunofluorescence and enzymatic histochemistry staining were performed to assess purinergic receptor expression and ectonucleotidase activity in hDPSCs, respectively. To determine the effects of purinergic signaling on hDPSC, purinergic receptor antagonists and an ectonucleotidase inhibitor were applied in culture medium, and hDPSC survival and proliferation were assessed with DAPI staining and Ki67 immunofluorescence staining, respectively. We demonstrated that ATP application induced inward currents in hDPSCs. P2X and P2Y receptors are involved in the generation of ATP-induced inward currents. We also detected expression of NTPDase3 and ectonucleotidase activity in hDPSCs. We further demonstrated that purinergic receptors were tonically activated in hDPSCs and that inhibition of ectonucleotidase activity enhanced ATP-induced inward currents. Furthermore, we found that blocking P2Y and P2X receptors reduced-and inhibition of ecto-ATPase activity enhanced-the survival and proliferation of hDPSCs, while blocking P2X receptors alone affected only hDPSC proliferation. Autocrine/paracrine purinergic signaling is essential for hDPSC survival and proliferation. These results reveal potential targets to manipulate hDPSCs to promote tooth/dental pulp repair and regeneration.

P2Y12 but not P2Y13 Purinergic Receptor Controls Postnatal Rat Retinogenesis In Vivo

Adenine nucleotides through P2Y₁ receptor stimulation are known to control retinal progenitor cell (RPC) proliferation by modulating expression of the p57^KIP2, a cell cycle regulator. However, the role of Gi protein-coupled P2Y₁₂ and P2Y₁₃ receptors also activated by adenine nucleotides in RPC proliferation is still unknown. Gene expression of the purinergic P2Y₁₂ subtype was detected in rat retina during early postnatal days (P0 to P5), while expression levels of P2Y₁₃ were low. Immunohistochemistry assays performed with rat retina on P3 revealed P2Y₁₂ receptor expression in both Ki-67-positive cells in the neuroblastic layer and Ki-67-negative cells in the ganglion cell layer and inner nuclear layer. Nonetheless, P2Y₁₃ receptor expression could not be detected in any stratum of rat retina. Intravitreal injection of PSB 0739 or clopidogrel, both selective P2Y₁₂ receptor antagonists, increased by 20 and 15%, respectively, the number of Ki-67-positive cells following 24 h of exposure. Moreover, the P2Y₁₂ receptor inhibition increased cyclin D1 and decreased p57^KIP2 expression. However, there were no changes in the S phase of the cell cycle (BrdU-positive cells) or in mitosis (phospho-histone-H3-positive cells). Interestingly, an increase in the number of cyclin D1/TUNEL-positive cells after treatment with PSB 0739 was observed. These data suggest that activation of P2Y₁₂ receptors is required for the successful exit of RPCs from cell cycle in the postnatal rat retina.

Injury-induced purinergic signalling molecules upregulate pluripotency gene expression and mitotic activity of progenitor cells in the zebrafish retina

Damage in fish activates retina repair that restores sight. The purinergic signalling system serves multiple homeostatic functions and has been implicated in cell cycle control of progenitor cells in the developing retina. We examined whether changes in the expression of purinergic molecules were instrumental in the proliferative phase after injury of adult zebrafish retinas with ouabain. P2RY1 messenger RNA (mRNA) increased early after injury and showed maximal levels at the time of peak progenitor cell proliferation. Extracellular nucleotides, mainly ADP, regulate P2RY1 transcriptional and protein expression. The injury-induced upregulation of P2RY1 is mediated by an autoregulated mechanism. After injury, the transcriptional expression of ecto-nucleotidases and ecto-ATPases also increased and ecto-ATPase activity inhibitors decreased Müller glia-derived progenitor cell amplification. Inhibition of P2RY1 endogenous activation prevented progenitor cell proliferation at two intervals after injury: one in which progenitor Müller glia mitotically activates and the second one in which Müller glia-derived progenitor cells amplify. ADPβS induced the expression of lin28a and ascl1a genes in mature regions of uninjured retinas. The expression of these genes, which regulate multipotent Müller glia reprogramming, was significantly inhibited by blocking the endogenous activation of P2RY1 early after injury. We consistently observed that the number of glial fibrillary acidic protein-BrdU-positive Müller cells after injury was larger in the absence than in the presence of the P2RY1 antagonist. Ecto-ATPase activity inhibitors or P2RY1-specific antagonists did not modify apoptotic cell death at the time of peak progenitor cell proliferation. The results suggested that ouabain injury upregulates specific purinergic signals which stimulates multipotent progenitor cell response.