Prickle is phosphorylated by Nemo and targeted for degradation to maintain Prickle/Spiny-legs isoform balance during planar cell polarity establishment.
PLoS Genet 2018;
14:e1007391. [PMID:
29758044 PMCID:
PMC5967807 DOI:
10.1371/journal.pgen.1007391]
[Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2017] [Revised: 05/24/2018] [Accepted: 05/02/2018] [Indexed: 11/19/2022] Open
Abstract
Planar cell polarity (PCP) instructs tissue patterning in a wide range of organisms from fruit flies to humans. PCP signaling coordinates cell behavior across tissues and is integrated by cells to couple cell fate identity with position in a developing tissue. In the fly eye, PCP signaling is required for the specification of R3 and R4 photoreceptors based upon their positioning relative to the dorso-ventral axis. The ‘core’ PCP pathway involves the asymmetric localization of two distinct membrane-bound complexes, one containing Frizzled (Fz, required in R3) and the other Van Gogh (Vang, required in R4). Inhibitory interactions between the cytosolic components of each complex reinforce asymmetric localization. Prickle (Pk) and Spiny-legs (Pk-Sple) are two antagonistic isoforms of the prickle (pk) gene and are cytoplasmic components of the Vang complex. The balance between their levels is critical for tissue patterning, with Pk-Sple being the major functional isoform in the eye. Here we uncover a post-translational role for Nemo kinase in limiting the amount of the minor isoform Pk. We identified Pk as a Nemo substrate in a genome-wide in vitro band-shift screen. In vivo, nemo genetically interacts with pkpk but not pksple and enhances PCP defects in the eye and leg. Nemo phosphorylation limits Pk levels and is required specifically in the R4 photoreceptor like the major isoform, Pk-Sple. Genetic interaction and biochemical data suggest that Nemo phosphorylation of Pk leads to its proteasomal degradation via the Cullin1/SkpA/Slmb complex. dTAK and Homeodomain interacting protein kinase (Hipk) may also act together with Nemo to target Pk for degradation, consistent with similar observations in mammalian studies. Our results therefore demonstrate a mechanism to maintain low levels of the minor Pk isoform, allowing PCP complexes to form correctly and specify cell fate.
For functional tissues to form, individual cells must correctly orient themselves and function appropriately for their particular location in the body. The Planar Cell Polarity (PCP) complexes transmit one set of spatial cues by acting as signposts to mark direction across an epithelial layer. PCP signals can direct and coordinate cell differentiation, the behavior of groups of cells, or the orientation of individual cellular protrusions, depending on the tissue. PCP signals act as a polarization relay with two different complexes being positioned on opposite sides of each cell. This pattern of polarity is transmitted to neighboring cells and so extends across the tissue. In the fly eye, PCP signals control the differentiation of a pair of photoreceptors, R3 and R4, where the cell that is positioned closer to the dorso-ventral midline becomes R3. An excess of the PCP protein Prickle prevents the proper assembly of PCP complexes in the eye and so alters R3/R4 fate. Here we show that Nemo kinase is required in the R4 cell to phosphorylate Prickle and promote its degradation by the proteasome. Maintenance of low Prickle levels allows proper formation of PCP complexes, cell fate specification, and eye development.
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