Lunedi 15 Giugno 2009
Aula Francesco Crucitti
Center for Molecular and Vascular Biology
University Hospital Gasthuisberg
University of Leuven, Belgium
HUMAN PLATELET PATHOLOGY
RELATED TO DEFECTS
IN THE GS-PROTEIN SIGNALING CASCADE
Despite the increasing use of platelets as an easy model to study the physiological role of signal transduction molecules, of G Protein Coupled Receptors (GPCRs) and Gs proteins, the role of the platelet stimulatory G protein of adenylyl cyclase (Gs) has not been well studied in relation to human pathological conditions.
We developed a functional Gs test based on the inhibition of platelet aggregation by cAMP by stimulating human platelets after incubation with different Gs agonists.
Our studies show an important role for the Gs protein in regulating platelet cAMP levels and platelet reactivity leading to an increased bleeding tendency or a thrombotic risk in Gs hyper- or hypofunction, respectively.
Thanks to our Gs test, we could identify and characterize six Pseudohypoparathyrodism Type Ia (PHPIa) or Pseudo-pseudohypoparathyroidism (PPHP) patients with a Gs loss-of-function due to novel Gs gene mutations.
A platelet Gs gain-of-function defect was found in a group of nine patients due to a paternally insertion of the XL region of the GNAS cluster, where the Gs gene is located, and some of them showed a trauma-related increased bleeding tendency.
The Gs test is also useful to identify defects that involved rather than the platelet Gs subunit itself, one of its upstream activator, such as PACAP, that is able to bind the GPCR VCAP1, normally expressed in megakaryocytes and platelets. A group of patients with a partial trisomy 18p, resulting in three copies of the PACAP gene and elevated PACAP plasma levels, showed platelet Gs-hyperfunction, pronounced bleeding tendency and mild thrombocytopenia.
In conclusion, the platelet-based Gs test proved to be useful in the difficult clinical diagnosis of PHPIa and PPHP and more importantly to expand the broad spectrum of Gs defects with novel phenotypes.
Il messaggio originale del seminario è che le piastrine, cellule del sangue facilmente accessibili, possono essere utilizzate per studiare diverse patologie, che dipendono (nei casi che saranno illustrati) da difetti nella cascata di trasduzione del segnale delle proteine Gs, a livello della proteina stessa, o anche di altri fattori a monte, come nel caso del PACAP/VPAC.