Περίληψη
Οι πρωτεΐνες σχετιζόμενες με τις λιπιδικές φωσφατάσες (PLPPRs) αποτελούν μία οικογένεια διαμεμβρανικών πρωτεϊνών που αποτελείται από 5 μέλη και εντοπίζεται κυρίως στους νευρώνες κατά την ανάπτυξη. Συμμετέχουν στη ρύθμιση των γλουταμινεργικών συνάψεων, στο σχηματισμό φιλοποδίων και αξονικών κλάδων, καθώς και στην πλοήγηση του αυξητικού κώνου. Παρόλο που εμφανίζουν ομολογία και παρόμοια τοπολογία με τις λιπιδικές φωσφατάσες, είναι γνωστό πως δεν έχουν δραστικότητα φωσφατάσης του Λυσοφωσφατιδικού Οξέος (LPA). Το Λυσοφωσφατιδικό οξύ (LPA) είναι ένα βιοδραστικό φωσφολιπίδιο που συμμετέχει σε βασικές διεργασίες της νευρικής ανάπτυξης και της λειτουργίας του ενήλικου εγκεφάλου και εμπλέκεται σε διάφορες παθοφυσιολογικές καταστάσεις. Εκτός από τους έξι καλά χαρακτηρισμένους, τυπικούς υποδοχείς του, έχουν επίσης προταθεί άτυποι ρυθμιστές της σηματοδότησης τοte glutamatergic synapses, filopodia and branch formation, as well as growth cone navigation. While displaying homology and similar topology to Lipid Phosphate Phosphatases, they lack Lysophosphatidic Acid (LPA) phosphatase activity. Lysophosphatidic acid (LPA) is a bioactive phospholipid that participates in critical processes in neural development and adult brain function and is implicated in various pathophysiological conditions. Alongside its six well characterized receptors, atypical regulators of LPA signaling have also been suggested, including Phospholipid phosphatase-related proteins (PLPPRs). Previous studies have suggested the involvement of PLPPR4 in neuronal LPA signaling via an LPA transporter or scavenger function, but the role of other PLPPRs, including PLPPR3, a close PLPPR4 relative, in LPA signaling, remains elusive. The objectives of this dissertation were to explore the expression of all Plppr family members in distinct brain regions and in different types of neurons throughout development and in adulthood, to study their ability to form complexes, to investigate the functional and structural interactions of PLPPR3 with LPA, and to identify if LPA effects in neuronal morphology are mediated by PLPPR3.Plpprs were found to be differentially expressed in the striatum, the Nucleus accumbens, the Prefrontal cortex and the dorsal and ventral hippocampus of developing animals and their expression is maintained at significant levels during adulthood. They are also expressed in specific developing and adult GABAergic and glutamatergic neuronal types and subcellular fractionation experiments showed that both PLPPR4 and PLPPR3 are co-expressed in adult synaptosomal membranes. Moreover, specific Plpprs may compensate to some extent for Plppr3 in specific brain regions. Additionally, PLPPRs form homomeric and heteromeric complexes in vitro mostly through their transmembrane domains and these complexes are not controlled by LPA. The presence of PLPPR3, PLPPR4 and PLPPR5, increases the ability of HEK293 cells to take up LPA, however a different mechanism of action is suggested for PLPPR3 in neuroblastoma cells that depends on concentration and duration of treatment, as well as on the transcriptomic profile of the cell line. Furthermore, PLPPR3 was found to bind LPA with two different binding affinity constants: one in the low nanomolar range when assessing the full-length PLPPR3 and one in the low micromolar range when assessing PLPPR3-ICD.Finally, it was shown that PLPPR3 mediates complex neuronal LPA responses in early polarized but not in fully polarized neurons, suggesting its involvement as a sensor of LPA in early stages of development, and it influences LPA-related F-actin changes in fully polarized neurons. Overall, our data show that Plpprs are also present in the adult brain and also in the form of complexes and highlight PLPPR3 as a synaptic regulator of LPA signal transduction.
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