PROJET : "Polycystins, vascular mechanotransduction & cerebral aneurysms"
 
 Mutations in the genes PKD1 and PKD2, encoding the polycystins PC1 and PC2, provoke autosomal dominant polycystic kidney disease (ADPKD). This is one of the most frequent inherited kidney diseases with a prevalence of about 1/1000 leading ultimately to kidney failure. Arterial hypertension, cerebral aneurysms and prolapse of the mitral valve are also associated  with ADPKD. The molecular and cellular mechanisms linking polcystins with the cardio-vascular phenotype are still unknown.

ADPKD is characterized by the presence of numerous cysts leading to renal hyperplasia (the figure shows a healthy human kidney on the left and a polykystic kidney on the right).

In 85% of cases, the PKD1 gene is implicated, while PKD2 is mutated in 15%. The PKD2 form of the disease (life expectancy of 69 years) is less severe than the PKD1 form (life expectancy of 53 years).

Mutations are very variable and occur throughout the genes. The double inactivation (a germinal mutation on one allele and a somatic mutation on the other) of PKD1 or PKD2 is the initiating molecular event in the genesis of cysts. The only therapeutic recourse is dialysis or kidney transplant.

Polycystin1 (PKD1, PC1, TRPP1 : 4303 amino acids), the gene product of PKD1, posseses a large extracellular domain (around 3 000 amino acids), where several protein motifs are juxtaposed. These domains are involved in protein-protein or protein-sugar interactions and could link PC1 to one or several ligands of the extracellular matrix. These domains are also capable of homophilic association. PC1 has 11 transmembrane segments and the cytosolic carboxy terminal domain (225 amino acids) includes a coiled-coil domain which is implicated in protein-protein interactions. PC1 plays an important role in cell-extracellular matrix interactions at focal adhesions, in cell-cell interactions at adherins junctions/desmosomes and in cellular signalling.

PC2 (PKD2, TRP2 : 968 amino acids) includes two extremities which are cytosolic, with an EF hand calcium fixation domain in the C terminal domain and six transmembrane segments (the figure shows the membrane topology of PC1 and PC2). PC2 belongs to the TRP family of cationic channels. These channels are permeable to calcium ions and are implicated in sensory function (mechanotransduction, chemoreception and pain).

The coiled-coil domain of PC1 interacts with the carboxy terminal domain of PC2. PC1 is localized at the plasma membrane while endogenous PC2 seems able to localize both at the plasma membrane and at the endoplasmic reticulum. The PC1/PC2 interaction is required for expression of PC2 at the plasma membrane. This suggests that PC1 participates as a molecular chaperone or as a functional subunit of the channel. At the level of the plasma membrane, PC1 thus behaves like a membrane receptor coupled to the ion channel PC2. PC1 and PC2 are expressed at the surface of primary cilium of renal epithelial cells. In response to the flux of urine, the cilia bend and calcium influx leads to to a rise in intracellular calcium, this in turn leads to calcium release from intracellular stores. Thus, the PC1/PC2 complex plays a mechanosensory role in renal epithelia. In the endoplasmic reticulum, PC2 forms a channel which is implicated in the mechanism of intracellular calcium release. PC2 is directly activated by intracellular calcium (calcium-induced calcium release) and amplifies the effect of inositol tri-phosphate (InsP3).

Besides the renal disease, a large number of patients affected by ADPKD suffer from major cardio-vascular lesions. Intracranial aneurysms are associated with ADPKD (around 8 % versus 1.2 % of the general population)  (the figure shows an aneurysm on the cerebral basilar artery).

 These are hernias of the cerebral arterial wall which can achieve sizes in the centimeter range or greater. The sites of prediliction for the aneurysms is the circle of Willis. Flooding of blood into the subarachinoid space is seriously detrimental (50 % die within 6 months and have major handicap after effects). The formation of an aneurysm is linked to an anomaly in the arterial wall. Angiographic examination is actually systematically proposed to young subjects with a familial history of ADPKD. Other vascular anomalies such as prolapse of the mitral valve (in 20 to 30 % of cases) and aortic aneurysms have also been described in patients with ADPKD. Moreover, hypertension (50 to 70 % of patients) appears at an early stage of the disease, well before the appearence of renal lesions. Arterial hypertension leads to an increase in the size of aneurysms and represents a risk factor for cerebral hemorrhaging by aneurysm rupture.

The expression of PC1 and PC2 has been shown in vascular smooth muscle as well as in the endothelium in most blood vessels including the aorta and cerebral arteries (the figure shows in blue the expession of pkd1 in the cerebral vasculature ; from Boulter et al., 1997).

 

The objectives of this project are two-fold. Firstly, the understanding of the physiological role of polycystins at the level of the endothelium and arterial smooth muscle. Secondly, the identification of the molecular and cellular mechanisms which are implicated in the formation of cerebral aneurysms associated with ADPKD.

We are studying the role of polycystins in the mechanosensory function of vascular cells. Cellular and animal models of genetic inactivation or over-expression of polycystins have been developed in the laboratory. This project of functional genomics is dedicated to study the role of polycystins in vascular physiopathology with the aim of developing novel molecular markers and elaborate therapeutical strategies.