Drosophila Pkd2 Is Haploid-insufficient for Mediating Optimal Smooth Muscle Contractility

Abstract

Humans heterozygous for PKD1 or PKD2 develop autosomal dominant polycystic kidney disease, a common genetic disorder characterized by renal cyst formation and extrarenal complications such as hypertension and vascular aneurysms. Cyst formation requires the somatic inactivation of the wild type allele. However, it is unknown whether this recessive mechanism applies to life-threatening vascular aneurysms, which could involve weakening of the endothelial lining or surrounding vascular smooth muscle cells (SMCs). Drosophila Pkd2 at 33E3 (Pkd2) encodes a PKD2 family of Ca 2؉ -activated Ca 2؉ -permeable cation channels. We show here that loss-offunction Pkd2 mutations severely reduced the contractility of the visceral SMCs, which was restored by expressing wild type Pkd2 cDNA via a muscle-specific Gal4 driver. The effect of Pkd2 mutations alone on the skeletal muscle was minimal but was exacerbated by ryanodine-induced perturbation of intracellular Ca 2؉ stores. Consistent with this, Pkd2 interacted strongly with a ryanodine receptor mutation, causing a synergistic reduction of larval body wall contraction rate that is normally regulated through Ca 2؉ oscillation during excitation-contraction coupling in the skeletal muscle. These results suggest that PKD2 cooperates with the ryanodine receptor to promote optimal muscle contractility through intracellular Ca 2؉ homeostasis. Further genetic analysis indicated that Pkd2 is strongly haploinsufficient for normal SMC contractility. Since Ca 2؉ homeostasis is a conserved mechanism for optimal muscle performance, our results raise the possibility that inactivation of just one PKD2 copy is sufficient to compromise vascular SMC contractility and function in PKD2 heterozygous patients, thus explaining their increased susceptibility to hypertension and vascular aneurysms.

Pkd2+/− Vascular Smooth Muscles Develop Exaggerated Vasocontraction in Response to Phenylephrine Stimulation

“…VSMC. In this regard, impaired visceral SM function that was observed in Pkd2 mutant drosophila (although cAMP level was not given) could be relevant (48). In our study, however, such considerations likely are overwhelmed by the dominant ␣1-AR signaling that was provoked by acute PE stimulation.…”

Regulation of ryanodine receptor-dependent calcium signaling by polycystin-2

“…The reduced SR Ca 2ϩ content is likely a consequence of the relief of inhibition of the RyR2, a mechanism that would allow an increased Ca 2ϩ leak from the SR stores, thereby reducing the total Ca 2ϩ present in the SR. These results also have implications in muscle contractility; Drosophila PC2 has been hypothesized to participate in vascular smooth muscle contractility through intracellular Ca 2ϩ homeostasis with the help of RyR (40).…”

Site Specificity of Aneurysmal Disease

“…6 The associated genes, PKD1 and PKD2, encode proteins that appear to regulate SMC contractility through calcium-regulatory mechanisms. 30 Susceptibility genes, rather than causal gene mutations, are important in aneurysms, particularly AAAs, which are genetically complex. 22 Recently, genome-wide association studies have identified loci on chromosome 9p21 that increase the risk of both AAAs and cerebral aneurysms.…”