SummaryCentrioles are key structural elements of centrosomes and primary cilia. In mammals, only a few proteins including PLK4, CPAP (CENPJ), SAS6, CEP192, CEP152 and CEP135 have thus far been identified to be required for centriole duplication. STIL (SCL/TAL1 interrupting locus, also known as SIL) is a centrosomal protein that is essential for mouse and zebrafish embryonic development and mutated in primary microcephaly. Here, we show that STIL localizes to the pericentriolar material surrounding parental centrioles. Its overexpression results in excess centriole formation. siRNA-mediated depletion of STIL leads to loss of centrioles and abrogates PLK4-induced centriole overduplication. Additionally, we show that STIL is necessary for SAS6 recruitment to centrioles, suggesting that it is essential for daughter centriole formation, interacts with the centromere protein CPAP and rapidly shuttles between the cytoplasm and centrioles. Consistent with the requirement of centrioles for cilia formation, Stil -/-mouse embryonic fibroblasts lack primary cilia -a phenotype that can be reverted by restoration of STIL expression. These findings demonstrate that STIL is an essential component of the centriole replication machinery in mammalian cells.

Primary microcephaly, Seckel syndrome, and microcephalic osteodysplastic primordial dwarfism type II (MOPD II) are disorders exhibiting marked microcephaly, with small brain sizes reflecting reduced neuron production during fetal life. Although primary microcephaly can be caused by mutations in microcephalin (MCPH1), cells from patients with Seckel syndrome and MOPD II harbor mutations in ataxia telangiectasia and Rad3 related (ATR) or pericentrin (PCNT), leading to disturbed ATR signaling. In this study, we show that a lack of MCPH1 or PCNT results in a loss of Chk1 from centrosomes with subsequently deregulated activation of centrosomal cyclin B–Cdk1.

Although most animal cells contain centrosomes, consisting of a pair of centrioles, their precise contribution to cell division and embryonic development is unclear. Genetic ablation of STIL, an essential component of the centriole replication machinery in mammalian cells, causes embryonic lethality in mice around mid gestation associated with defective Hedgehog signaling. Here, we describe, by focused ion beam scanning electron microscopy, that STIL(-/-) mouse embryos do not contain centrioles or primary cilia, suggesting that these organelles are not essential for mammalian development until mid gestation. We further show that the lack of primary cilia explains the absence of Hedgehog signaling in STIL(-/-) cells. Exogenous re-expression of STIL or STIL microcephaly mutants compatible with human survival, induced non-templated, de novo generation of centrioles in STIL(-/-) cells. Thus, while the abscence of centrioles is compatible with mammalian gastrulation, lack of centrioles and primary cilia impairs Hedgehog signaling and further embryonic development.