The Ion Channel Polycystin-2 Is Required for Left-Right Axis Determination in Mice

Abstract

Generation of laterality depends on a pathway which involves the asymmetrically expressed genes nodal, Ebaf, Leftb, and Pitx2. In mouse, node monocilia are required upstream of the nodal cascade. In chick and frog, gap junctions are essential prior to node/organizer formation. It was hypothesized that differential activity of ion channels gives rise to unidirectional transfer through gap junctions, resulting in asymmetric gene expression. PKD2, which if mutated causes autosomal dominant polycystic kidney disease (ADPKD) in humans, encodes the calcium release channel polycystin-2. We have generated a knockout allele of Pkd2 in mouse. In addition to malformations described previously, homozygous mutant embryos showed right pulmonary isomerism, randomization of embryonic turning, heart looping, and abdominal situs. Leftb and nodal were not expressed in the left lateral plate mesoderm (LPM), and Ebaf was absent from floorplate. Pitx2 was bilaterally expressed in posterior LPM but absent anteriorly. Pkd2 was ubiquitously expressed at headfold and early somite stages, with higher levels in floorplate and notochord. The embryonic midline, however, was present, and normal levels of Foxa2 and shh were expressed, suggesting that polycystin-2 acts downstream or in parallel to shh and upstream of the nodal cascade.

Mutations in INVS encoding inversin cause nephronophthisis type 2, linking renal cystic disease to the function of primary cilia and left-right axis determination

“…The gene PKD2, mutations in which cause autosomal dominant PKD and which encodes the calcium release channel polycystin-2, had been shown in a Pkd2 −/− mouse model to represent a master gene regulating left-right axis determination, acting upstream of Nodal, Ebaf, Leftb and Pitx2 (ref. 40). In addition to the malformations described previously 41,42 , homozygous mutant embryos showed right pulmonary isomerism, randomization of embryonic turning, heart looping and abdominal situs inversus.…”

“…In mouse, node monocilia are required upstream of the nodal cascade 40 . These findings indicate a strong link between ciliary function and development of laterality.…”

Laterality defects are influenced by timing of treatments and animal model

“…A considerable number of studies have reported complete absence of nodal, lefty, or pitx2 expression (Field et al, 2011; Oki et al, 2010; Sakano et al, 2010; Gaio et al, 1999; Takeuchi et al, 2007; Zhang et al, 2001; Yan et al, 1999; Tsukui et al, 1999; Collignon et al, 1996; Kramer-Zucker et al, 2005; Krebs et al, 2003; Pennekamp et al, 2002), with some reporting 100% absent expression for all three genes. What does complete absence of gene expression mean?…”

Mice with mutations in Mahogunin ring finger‐1 (Mgrn1) exhibit abnormal patterning of the left–right axis

“…While this theory of nodal flow has been widely accepted and modified to include means for both chemical (Nonaka et al, 1998;Okada et al, 2005) and physical (McGrath et al, 2003) modes of generating a left-specific signaling event, other theories have been pre-sented, including a prolonged and stable up-regulation of Ca 2ϩ in the left side of the node, which has been proposed to be involved in regulating the left-sided signaling cascade (McGrath et al, 2003;Tanaka et al, 2005). Mice lacking the membrane cation channel Polycystin-2 (PKD2) have impaired left-right axis determination and do not show left-sided elevation of Ca 2ϩ in the node (Pennekamp et al, 2002;McGrath et al, 2003). Although the mechanism of Ca 2ϩ -dependent regulation of left-right asymmetry remains unclear, work in chick embryos has shown that transient accumulation of extracellular calcium leads to asymmetric activation of Notch (Raya et al, 2004).…”