Is there a redundancy of β3 and other platelet receptors in the brain and central nervous system?

To the Editor

Structural similarities between blood platelets and cells of the central nervous system (CNS) are intriguing. For example, receptors and transport pathways involved in serotonin function are also present in platelets; the latter take up serotonin from the blood and store it in dense granules [1], [2]. Whereas, the brain serotonin is involved in memory and learning, anxiety and sleep, its release from platelets at sites of vessel injury induces vasoconstriction, and promotes wound repair. Platelets also bind and respond to other neurotransmitters including dopamine [3]. They are in addition rich in signaling molecules such as the SNARE/Rab secretory pathways and the calmodulin-binding protein neurogranin prominent in the nervous tissue [4].

The Gi-coupled platelet receptor, P2Y₁₂, assures the formation of macroscopic platelet aggregates following the onset of platelet aggregation through the interaction of ADP with P2Y₁ [5]. Remarkably, studies on mice lacking P2Y₁₂ led to the discovery that it is also responsible for nucleotide-induced microglial chemotaxis in response to CNS injury [6]. Microglia are primary immune sentinels of the CNS. Despite these findings, patients with a rare inherited bleeding disorder linked to mutations and absent P2Y₁₂ expression do not show outward signs of brain or CNS dysfunction [5], [7].

In primary hemostasis, platelet aggregation is assured by the αIIbβ3 integrin that binds adhesive proteins on activation and links platelets together [8]. Intriguingly, the ITGB3 gene that encodes β3 was identified both as a quantitative trait locus for serotonin blood levels and as a candidate gene for autism spectrum disorder (ASD) either alone or in interaction with allelic variants of the serotonin transporter (SERT) gene (SLC6A4) [9]. ITGB3 and SLC6A4 expression is correlated in platelets and brain both in humans and in mice [10]. Specific SNPs within ITGB3 have also been linked to a high frequency of spontaneous abortions, preterm delivery, and obstetric complications both in autistic individuals and in the general population [9–12]. Deletion of β3 in mice resulted in diminished SERT activity in platelets and increased fetal mortality [13]. Strikingly, adult mice lacking β3 also show altered social behavior implying that β3 is indeed involved in brain systems relevant to ASD [14].

Glutamate is an excitory neurotransmitter that binds both to the kainate N-methyl-D-aspartate-receptor and to the α-amino-3-hydroxy-5-methyl-4-isoxazoleprionic acid receptor (AMPAR) of the CNS. Activated platelets release glutamate; like serotonin it is a weak agonist acting through AMPAR and other glutamate receptors to increase platelet activation in the presence of low amount of thrombin and ADP [15]. Integrin-mediated signaling is essential for the orchestration of central excitory responses including glutamate signaling at the hippocampal synapse [16], [17]. As well as regulating excitory synaptic strength, β3 is required for synaptic scaling, a form of synaptic homeostasis [17]. Pozo et al. [18] have now shown that the surface level of β3 integrin in postsynaptic neurons correlates with synaptic strength and with the abundance of the GluA2 AMPAR subunit. In fact, β3 and GluA2 AMPAR form a complex in mouse brain by the way of direct binding through their cytoplasmic domains. While synaptic AMPAR currents and surface GluA2 levels were not differentially affected upon persistently altering β3 activation state, the effect of β3 deletion was not reported.

Glanzmann thrombasthenia (GT) is a rare autosomal recessive disease manifested by life-long mucocutaneous bleeding that often requires blood transfusion or other therapy [19], [20]. Platelet aggregation is defective due to quantitative or qualitative abnormalities of αIIbβ3. Mutations causing GT occur along both of the ITGA2B and ITGB3 genes [20]. While αIIbβ3 is basically restricted to megakaryocytes and platelets; β3 also links with αv to form the vitronectin receptor, αvβ3 [21]. Although a minor component in platelets, αvβ3 is present in a variety of blood and vascular cells and has a role in many physiologic processes in both health and disease. It is highly probable although not proven that β3 participation in brain and CNS function is through αvβ3. Whereas, in patients with ITGA2B defects the absent β3 expression is restricted to platelets, β3 deletion will extend to αvβ3 and affect many types of cell [20]. Despite this, an enigma in GT is the absence of clear-cut phenotypic differences between patients with ITGA2B or ITGB3 mutations.

By producing a mouse model in which the thymidine kinase gene was introduced at the Itga2b locus, Roux et al. were able to confirm the GT phenotype and also show that transcription of Itga2b began prior to commitment of stem cells to the MK lineage [22]. β3-/- mice have increased fetal mortality and in addition to the GT platelet phenotype they show osteoclast dysfunction due to the absence of αvβ3 [22–25]. Unexpectedly, in high-fat-fed hyperlipidemic mice, β3 deficiency promoted atherosclerosis and high mortality through pulmonary inflammation suggesting that αvβ3 has a suppressive effect [26]. Enhanced wound healing and accelerated re-epithelialization in β3-integrin deficient mice also suggested that αvβ3 was a natural suppressor of TGF-β1 signaling [27]. Finally, accelerated angiogenesis in β3 null mice was linked to elevated levels of VEGF receptor-2 (Flk-1) in endothelial cells, a finding associated with augmented increased responses to hypoxia and VEGF [28].

Despite the fact that several hundred GT patients have been described worldwide, no reports of autism or abnormal social behavior have been forthcoming. Although epidemiologic studies suggest an increase in fetal mortality in GT [data reviewed in 19, 20]; mouse models showing that αvβ3 deficiency can modify angiogenesis, bone metabolism, reproduction (and fetal development), or wound repair are not paralleled by a non-hemostatic phenotype for β3 deficiency in man. In line with this enigma, the question of what αIIbβ3 or αvβ3 or indeed other platelet receptors such as P2Y₁₂ or AMPAR are really doing in human brain and other tissues remains to be answered. Perhaps there is receptor redundancy or is it conceivable that non-hemostatic defects in GT have been overlooked, masked by the predominant bleeding phenotype? Although both αvβ3 and αIIbβ3 (through platelets) have been implicated in metastasis and tumor growth in mice [29], the consequences of β3-deficiency on tumorigenesis in GT also remains unclear. A concerted international study on the effects of β3 deficiency in GT on behavior and the risk of major illnesses is now feasible given the progress in genotyping and would appear to be worthwhile.

Acknowledgments

The author's work is financed by INSERM (ANR-08-GENO-028-03) and supported by GIS-Maladies Rares.