Purinergic P 2 Y Receptors Are Involved in Xenopus Head Formation

P2Y receptors belong to the family of G protein-coupled receptors and are activated by nucleotides in the extracellular space. We showed that Xenopus P2Y1 and P2Y11 were expressed in the dorsal marginal zone from early gastrula stage and enriched in the central nervous system from neurula stages. They were expressed in the prospective head region during early development. Knockdown of P2Y1 and P2Y11 caused head malformation, such as small eyes, brain atrophy, and defect in cartilage tissues, as well as reduced expression of neural, placode, and neural crest markers. Furthermore, the expression of neural plate and epidermal markers was affected by P2Y1 or P2Y11 depletion at early neurula stage, suggesting that P2Y1 or P2Y11 might be required for the neural induction. Our findings suggested that P2Y receptors might be involved in distinguishing between neural and non-neural fates. The results also suggested that P2Y1 or P2Y11 could play a role in neural induction and/or maintenance of neural tissues in the head formation processes.


Introduction
Purinergic signaling plays a crucial role in the nervous system, and its functions have been physiologically and pathophysiologically investigated [1] [2] [3].The receptors of purine and pyrimidine are divided into adenosine (P1) receptors and adenosine triphosphate (ATP)/adenosine diphosphate (ADP) (P2) receptors.P2 receptors are composed of ionotropic P2X and G protein-coupled P2Y families.In mammals, eight P2Y receptors, namely, P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13 and P2Y14, have been proven to be nucleotide receptors, and they have different molecular structures and selectivities to antagonist and agonist [3].P2Y receptors are stimulated by nucleotides released in the extracellular space, after which they activate or inhibit phospholipase C (PLC) and adenylyl cyclase [2] [4].In Xenopus laevis, six subtypes: P2Y1, P2Y4, P2Y10, P2Y11, P2Y12 and P2Y13, have been identified.Phylogenetic analysis showed that these subtypes could be divided into two subgroups: one group of P2Y1, P2Y4 and P2Y11, the other group of P2Y10, P2Y12 and P2Y13 (data not shown).Comparative analysis of amino acid sequences indicated that X. laevis P2Y receptors were highly conserved among vertebrates, but not found in invertebrates.We expected that the emergence of P2Y receptors might be related to the acquisition of new head of vertebrate.
P2Y receptors participate in neuromodulation and neurotransmitter release, and are abundant in the central nervous system and the peripheral nervous system [5].In X. laevis, P2Y1 receptor and ectonucleoside triphosphate diphosphohydrolase (E-NTPD2) are synergistically involved in eye development [6].
P2Y11 receptor is expressed during early development and enriched in the central nervous system [7], and is known to contribute to convergent extension during gastrulation [8].Furthermore, intracellular Ca 2+ increase is induced in supporting cells (SCs) of the main olfactory epithelium (MOE) through P2Y4 receptors [9] [10].
We were interested in the role of P2Y receptors in early development and evolution.P2Y1, P2Y4 and P2Y11 belong to same subgroup and originate from the same ancestral gene; thus, they are expected to exhibit the same functions.In this study, we uncovered the expression patterns of P2Y1 and P2Y11 and their involvement in head formation in X. laevis embryos.P2Y1 or P2Y11 was observed in the prospective neuroectoderm and the dorsal marginal zone from early gastrula stage, and its expression was mainly enriched in the central nervous system and notochord.We found that P2Y1 or P2Y11 knockdown by morpholino antisense oligonucleotide (MO) led to head malformation.The disruption of P2Y1 or P2Y11 led to decreases in the expression of some neural, placode, and neural crest markers.Further, the morphant showed reduction of neural plate and enlargement of epidermis area at early neurula stage, suggesting that P2Y1 or P2Y11 might be required for cell fate specification in neural tissue.

Cartilage Staining
Embryos at stage 45 were fixed in MEMFA and stained overnight in 0.2% alcian blue/30% acetic acid in EtOH.After that, the embryos were washed with 80% glycerol/2% KOH.

TUNEL Assay
Fixed embryos were rehydrated in PBS containing 0.1% Tween 20 and washed with terminal deoxynucleotidyltransferase (TdT) buffer (Takara) for 30 min.End labeling was carried out at room temperature overnight in TdT buffer containing 2 uM digoxigenin-11-dUTP (Roche) and 150 U/ml TdT (Takara).Embryos were washed with 1 mM EDTA in PBS for 1 hour and detected with NBT/BCIP.

Histological Analysis
Embryos were fixed with MEMFA overnight and dehydrated with 100% MeOH.The samples were embedded in paraffin (Pathoprep 568, Wako) and sectioned to 10 um thickness.The sections were deparaffinized and stained with hematoxylin and eosin.

P2Y1 and P2Y11 Expression Patterns
To analyze the spatial expression patterns of P2Y1 and P2Y11 during early development in Xenopus embryos, we performed whole-mount in situ hybridization.P2Y1 was first detected in the animal hemisphere and the dorsal blastopore lip at early gastrula stage (Figure 1 P2Y1 localized to various head regions, including eye, otic vesicle, anterior neural tissues and migrating cranial neural crest cells (Figure 1(M)).Thereafter, P2Y1 was detected in the pallium, the dorsal domain of telencephalon, dorsal diencephalon, mesencephalon and metencephalon at tadpole stage (Figure 1(N), Figure 1(O)).The earliest P2Y11 expression was also observed in the dorsal blastopore lip at early gastrula stage (Figure 2(A)), and a sagittal hemisection showed an expression pattern similar to P2Y1 expression (Figure 2(B), Figure 2(C)).The P2Y11 expression observed in the prospective neuroectoderm, the involuting mesoderm, and the central nervous system from gastrula to neurula stage had been reported previously [7].P2Y11 was expressed in the neural fold (Figure 2  P2Y11 localized to restricted parts of the brain, the telencephalon, the dorsal di- encephalon, the mesencephalon, and the anterior metencephalon (Figure 2(N)).
Thus, the expression of P2Y1 and P2Y11 during early development showed similar patterns, namely, they were predominantly detected in the head region and the notochord.Although little is known about the role of these genes in early development in X. laevis, it had been shown that E-NTDP2 and P2Y1 synergistically regulated eye development [6] [26].Moreover, P2Y11 was expressed in the notochord and was important for convergent extension, coordinated cell polarization, and elevation of intracellular calcium in chordamesoderm cells [8].
These expression patterns indicated that P2Y1 and P2Y11 were expressed in vital areas and the head formation stages, suggesting that P2Y1 and P2Y11 might have the same functions, assisting each other in the same tissues, particularly in the head region.

Loss of P2Y1 or P2Y11 Function Affects Neural Gene Expression Patterns
To analyze the effects of P2Y1 or P2Y11 depletion on Xenopus head formation in detail, we examined the expression of anterior neural marker genes (Figure 4).Embryos injected with P2Y1 MO or P2Y11 MO exhibited a significant decrease in the expression of fore to midbrain marker otx2 at mid-neurula stage (reduction in P2Y1 MO: 60%, n = 20, P2Y11 MO: 66.7%, n = 18, Figure 4(B) .It is known that otx2 is expressed in the prospective dorsal diencephalon and pax6 is expressed in the ventral diencephalon at neurula stage [27].
Therefore, the results may be reasonable in that the depletion of P2Y1 or P2Y11 causes loss of otx2 expression but has no effect on pax6 expression at mid-neurula stage.In the anterior neuroectoderm, anf is the earliest transcriptional repressor gene and plays a key role in rostral forebrain development [14] [28].It is essential to inhibit the expression of two key regulators, otx2 and pax6, in the posterior forebrain [27].Based on these reports, we attempted to investigate the effects of P2Y1 or P2Y11 MO on anf1 expression.anf1 was normally expressed in the prospective forebrain and the lens placode region at early neurula stage, and gradually localized to the prospective anterior pituitary at the end of neurulation [29].anf1 expression in embryos injected with P2Y1 or P2Y11 MO at early neurula stage did not show any significant difference (P2Y1 MO: n = 9, P2Y11 MO: n = 12, Figures 5(A)-(C)), but anf1 expression increased laterally in the injected side at late neurula stage (P2Y1 MO: 87.5%, n = 16, P2Y11 MO: 76.5%, n = 17, Figure 5(E) and Figure 5(F), arrowheads).At early tailbud stage, anf1 localized to the anterior pituitary upon P2Y1 knockdown (n = 45, Figure 5(H)), although it did not localize to the pituitary (93.3%, n = 15, Figure 5(I), arrowhead) in the P2Y11 MO injected side.In Xenopus embryo, anf overexpression can lead to enlargement of the rostral forebrain and inhibit genes that normally express in more posterior regions [14].Moreover, anf functions as a suppressor of posterior forebrain fate within the presumptive rostral forebrain and directly inhibits the expression of otx2 and pax6 in this area [27].Thus, we speculated that ectopic anf1 expression would repress otx2 and pax6 expression during neurulation.At early tailbud stage, anf1 localized to the pituitary in P2Y1 morphants, whereas it did not localize to the pituitary in P2Y11-depleted embryos (Figure 5(H), Figure 5(I)).Although P2Y1 and P2Y11 may play a similar role in head formation, it is likely that they have different regulation mechanisms.The expression domains of en2 and krox20 do not overlap with the anf1 expression area, and anf1 cannot directly regulate the expression of bf1, which is co-expressed in a more anterior region of the neural plate [27].Thus, we considered that the decrease in en2, krox20, and bf1 expression might be independent of ectopic anf1 expression.Together, our results demonstrated that the depletion of P2Y1 or P2Y11 led to loss of neural development as shown by the down-regulation of some neural marker genes.

P2Y1 or P2Y11 Is Required for Placode and Neural Crest Development
As the depletion of P2Y1 or P2Y11 resulted in lens malformation (Figure 3(E), Figure 3(F)), we examined the expression of lens placode marker genes, foxe3 and l-maf, at early tailbud stage (Figures 6(A)-(F)).The morphants showed reduced expression of foxe3 (P2Y1 MO: 71.9%, n = 32, P2Y11 MO: 65%, n = 13, Figure 6(B) and Figure 6(C), arrowheads) and l-maf (P2Y1 MO: 7.1%, n = 14, P2Y11 MO: 73.3%, n = 15, Figure 6(E) and Figure 6(F), arrowheads).Embryos injected with P2Y1 or P2Y11 MO showed significant reduction of pax6 expression in the lens placode region at late neurula stage (Figure 4(K), Figure 4(L), arrowheads).The lens field is lens-biased ectoderm within preplacodal ectoderm and is formed at mid-neurula stage in response to signals emitted from the adjacent anterior neural plate [30].The reduction of pax6 and rx1 expression suggested that insufficient induction from retina might bring about a defect in lens placode at neurula stage.In addition, we examined the expression of dlx5, which marked the most cranial placode but not the lens placode, and detected a slight decrease in dlx5 expression in P2Y1 or P2Y11 knockdown embryos (P2Y1 MO: 55.6%, n = 18, P2Y11 MO: 42.9%, n = 14, Figure 6(H) and Figure 6(I), arrowheads).Furthermore, we examined the effect of P2Y1 or P2Y11 depletion on the expression of neural crest marker genes, snail1 (Figures 7(A by early hypoplasia of the neural crest region.Together, these suggested that P2Y1 or P2Y11 might also be required for the entire head components, such as the placode and the neural crest cells.

Loss of P2Y1 or P2Y11 Affects Induction or Maintenance of Neural Fate
P2Y1 or P2Y11 was detected in the prospective neuroectoderm and the dorsal marginal zone from early gastrula stage (Figure 1, Figure 2), and snail1 expression was affected by the depletion of P2Y1 or P2Y11 at early neurula stage (Figure 7(B), Figure 7(C)).Therefore, we speculated that morphants affected developmental events occurring earlier than neurula stage.To identify the effects of P2Y1 or P2Y11 knockdown on neural fate at early neurula stage, we examined the expression of epidermal marker epi-keratin (Figures 8(A Taken together, these data suggest that the depletion of P2Y1 or P2Y11 may affect the induction and/or maintenance of neural tissue.Bone morphogenetic protein (BMP) signaling plays a role in epidermal fate establishment at the presumptive ectoderm, and BMP antagonists, such as Noggin, Chordin and Follistatin, induce neural tissue by antagonizing BMP activities [31] [32] [33].P2Y1 and P2Y11 were expressed in the neural ectoderm, the endomesoderm and the involuting mesoderm (Figure 1, Figure 2).Thus, we speculated that P2Y1 and P2Y11 might be required for the establishment of the organizer and/or the competence of the ectoderm for neural induction.
Furthermore, we examined whether the observed effects of P2Y1 or P2Y11 knockdown might be attributed to an increase in the number of apoptotic cells.
We performed TUNEL assay and found significant enhancement of apoptosis at not sufficient to allow interpretation of the observed phenotypes, and minimal apoptosis was detected at early neurulastage (data not shown).P2Y receptors increase intracellular Ca 2+ concentration and modulate the intracellular environment as second messengers [34].The increase in intracellular Ca 2+ concentration is known to regulate cell proliferation, migration and differentiation in early development [35].In fact, intracellular Ca 2+ increase is induced in SCs of MOE through P2Y4 receptors in X. laevis, and is involved in cell turnover [9] [10] [36].It is also known that cell proliferation is essential for the induction and maintenance of neural crest cells [37], and P2Y1 and P2Y11 may maintain neural crest cells in proliferating and undifferentiated states.Although we could not entirely exclude the possibility that the depletion of P2Y1 or P2Y11 might di-rectly induce apoptosis, we considered that the significant increase in apoptosis at mid and late neurula stages might be due to reduced cell survival as development progressed.
In the present study, we revealed that P2Y1 and P2Y11 receptors were involved in X. laevis head formation.The knockdown approach revealed loss of neural development, suggesting that these receptors would be required for the induction and/or maintenance of neural tissues.As P2Y receptors in X. laevis have already been classified into six subtypes, the cellular and molecular functions of these receptors, including P2Y1 or P2Y11, should be studied further to fully understand the roles they play.The fact that P2Y receptors exist in only vertebrates and are highly conserved, and that P2Y1 or P2Y11 is involved in head formation has led us to expect that P2Y receptors participate in the acquisition of vertebrate head during evolution.As only the physiological and pathophysiological functions of P2Y receptors in the nervous system have been investigated, it is important to study further the roles of these receptors and their related molecules in early development.