Role of Ectonucleotidases in Synapse Formation During Brain Development: Physiological and Pathological Implications

Abstract

Background: Extracellular adenine nucleotides and nucleosides, such as ATP and adenosine, are among the most recently identified and least investigated diffusible signaling factors that contribute to the structural and functional remodeling of the brain, both during embryonic and postnatal development. Their levels in the extracellular milieu are tightly controlled by various ectonucleotidases: ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPP), alkaline phosphatases (AP), ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) and ecto-5'- nucleotidase (eN).

Methods: Studies related to the expression patterns of ectonucleotidases and their known features during brain development are reviewed, highlighting involvement of these enzymes in synapse formation and maturation in physiological as well as in pathological states.

Results: During brain development and in adulthood all ectonucleotidases have diverse expression pattern, cell specific localization and function. NPPs are expressed at early embryonic days, but the expression of NPP3 is reduced and restricted to ependymal area in adult brain. NTPDase2 is dominant ectonucleotidase existing in the progenitor cells as well as main astrocytic NTPDase in the adult brain, while NTPDase3 is fully expressed after third postnatal week, almost exclusively on varicose fibers. Specific brain AP is functionally associated with synapse formation and this enzyme is sufficient for adenosine production during neurite growth and peak of synaptogenesis. eN is transiently associated with synapses during synaptogenesis, however in adult brain it is more glial than neuronal enzyme.

Conclusion: Control of extracellular adenine nucleotide levels by ectonucleotidases are important for understanding the role of purinergic signaling in developing tissues and potential targets in developmental disorders such as autism.

Keywords: Brain development; NPP; NTPDase; TNAP; autism; ecto-5'-nucleotidase; ectonucleotidases; synaptogenesis..

Fig. (1)

Regional distribution of ectonucleotidases in the adult rodent brain. OB, olfactory bulb; RMS, rostral migratory stream; Ctx, cortex; Hip, hippocampus; CC, corpus callosum; CP, caudoputamen; TH, thalamus; HY, hypothalamus; SC, superior colliculus; IC, inferior colliculus; TG, tegmentum; Cer, cerebellum; MO, medulla oblongata. (The color version of the figure is available in the electronic copy of the article).

Fig. (2)

Role of ectonucleotidases during axonal growth, in immature and mature synapses. A) ATP negatively controls axonal growth and branching. TNAP hydrolyzes ATP directly to adenosine which induces axonal elongation, while eN acts as adhesion protein. B) During synapse maturation, synaptic TNAP abundance is decreased and replaced with NTPDases and eN. C) NTPDases and eN are main enzymes responsible for ATP/ADP dephosphorylation and adenosine formation in the synaptic cleft of mature synapses. (The color version of the figure is available in the electronic copy of the article).

Fig. (3)

Putative role of purinergic signaling in the pathophysiological mechanisms of autism. A) In physiological conditions, ATP is released as co-transmitter at GABAergic and glutamatergic synapse and binds to P2 receptors on astrocytes, microglia and postsynaptic neuron to exert numerous effects on cellular function. Astrocytes also release ATP into synaptic cleft, controlling synaptic function. Microglial cells monitor synaptic activity and respond to ATP to either stabilize or inhibit synapse formation. NTPDases, TNAP and eN rapidly terminate ATP signaling, producing adenosine, which acts through its pre- and postsynaptic P1 receptors, fine-tuning synaptic transmission and promoting synaptogenesis. Adenosine may also act on neighboring microglia, where inhibits their conversion to a reactive phenotype, preventing microglial activation. B) If increased efflux of purines occurs, whichc results in pathological concentrations of ATP in the extracellular space during vulnerable periods of brain development, ATP acts as a damage-associated molecular pattern, triggering innate immunity and inflammation, alters synapse formation and maturation, and contributes to excitotoxicity. Excessive release of ATP over activates microglial P2 receptors, activating microglia and promoting neuroinflammation. Since in different studies is found excess synapses in autistic brain, it is also possible that NTPDases are abnormally expressed and/or activated, leading to accumulation of ADP and/or AMP in the synaptic cleft of immature synapses. During synapse maturation, the main adenosine producing enzyme eN, is not fully expressed or does not have enzymatic role, thus generating lover adenosine levels in the synaptic cleft, inducing aberrant synaptic function. (The color version of the figure is available in the electronic copy of the article).