Chemokine Signal Regulates Embryonic Left-Right Asymmetry

Vertebrates exhibit striking left-right(L-R)asymmetries in the structure and position of their cardiovascular and gastrointestinal systems,for example,heart is located in left,but liver is located in right.Initially,early embryos develop symmetrically along the prospective body midline.This embryonic symmetry is broken during somite stages when an asymmetric fluid flow is generated by motile cilia within the L-R organizer(LRO),a transient structure located at the posterior end of the notochord.Specifically,in zebrafish,the ciliated LRO is referred to as Kupffer's vesicle(KV).It has been well established that the architecture of KV cells and asymmetric KV cilia generate a counter-clockwise nodal flow.This leads to the asymmetrical expression of early laterality genes,including nodal-related southpaw(spaw)and pitx2c in the left lateral plate mesoderm(LPM),and ultimately the establishment of L-R asymmetric patterning.The origin of L-R asymmetry is conserved across many vertebrates,and defects in the establishment of these asymmetries can result in a broad spectrum of birth defects,often including congenital heart malformations.Kupffer's vesicle(KV)forms from dorsal forerunner cells(DFCs),a group of superficial cells in the organizer region of the gastrula.Zebrafish dorsal forerunner cells(DFCs)undergo vigorous proliferation during epiboly and then exit from cell cycle to generate Kupffer's vesicle(KV),a ciliated organ necessary for establishing left-right(L-R)asymmetry.DFC proliferation defects are often accompanied by impaired cilia elongation in KV,but the functional and molecular interaction between cell-cycle progression and cilia formation remains unknown.Using zebrafish as the model organism,this study shows that chemokine receptor Cxcr4a is required for L-R laterality by controlling DFC proliferation and KV ciliogenesis.Functional analysis revealed that Cxcr4a stimulates cyclin D1 expression through ERK1/2 signaling,Cyclin D1-CDK4/6 drives G1/S transition during DFC proliferation,promotes DFC proliferation and phosphorylates Foxj1a,thereby disrupting its association with Psmd4b,a 19S regulatory subunit.This prevents the ubiquitin-independent proteasomal degradation of Foxjla.The increase of Foxj1a induces formation and extension of cilia in KV which is very important for KV's normal function.The rapid cell cycle progression of DFCs during epiboly stages is not only required for the generation of enough cells to construct KV,but also plays a critical role in reserving sufficient levels of zFoxj1a protein to support subsequent cilia formation.In a word,this study uncovers a role for Cxcr4 signaling in L-R patterning and provides fundamental insights into the molecular linkage between cell-cycle progression and ciliogenesis,this study also provides new ideas and methods for the following research on left-right asymmetry.