Opposing roles for JNK and Aurora A in regulating the association of WDR62 with spindle microtubules.

WD40-repeat protein 62 (WDR62) is a spindle pole protein required for normal cell division and neuroprogenitor differentiation during brain development. Microcephaly-associated mutations in WDR62 lead to mitotic mislocalization, highlighting a crucial requirement for precise WDR62 spatiotemporal distribution, although the regulatory mechanisms are unknown. Here, we demonstrate that the WD40-repeat region of WDR62 is required for microtubule association, whereas the disordered C-terminal region regulates cell-cycle-dependent compartmentalization. In agreement with a functional requirement for the WDR62­JNK1 complex during neurogenesis, WDR62 specifically recruits JNK1 (also known as MAPK8), but not JNK2 (also known as MAPK9), to the spindle pole. However, JNK-mediated phosphorylation of WDR62 T1053 negatively regulated microtubule association, and loss of JNK signaling resulted in constitutive WDR62 localization to microtubules irrespective of cell cycle stage. In contrast, we identified that Aurora A kinase (AURKA) and WDR62 were in complex and that AURKA-mediated phosphorylation was required for the spindle localization of WDR62 during mitosis. Our studies highlight complex regulation of WDR62 localization, with opposing roles for JNK and AURKA in determining its spindle association.

WD40-repeat protein 62 is a JNK-phosphorylated spindle pole protein required for spindle maintenance and timely mitotic progression.

The impact of aberrant centrosomes and/or spindles on asymmetric cell division in embryonic development indicates the tight regulation of bipolar spindle formation and positioning that is required for mitotic progression and cell fate determination. WD40-repeat protein 62 (WDR62) was recently identified as a spindle pole protein linked to the neurodevelopmental defect of microcephaly but its roles in mitosis have not been defined. We report here that the in utero electroporation of neuroprogenitor cells with WDR62 siRNAs induced their cell cycle exit and reduced their proliferative capacity. In cultured cells, we demonstrated cell-cycle-dependent accumulation of WDR62 at the spindle pole during mitotic entry that persisted until metaphase-anaphase transition. Utilizing siRNA depletion, we revealed WDR62 function in stabilizing the mitotic spindle specifically during metaphase. WDR62 loss resulted in spindle orientation defects, decreased the integrity of centrosomes displaced from the spindle pole and delayed mitotic progression. Additionally, we revealed JNK phosphorylation of WDR62 is required for maintaining metaphase spindle organization during mitosis. Our study provides the first functional characterization of WDR62 and has revealed requirements for JNK/WDR62 signaling in mitotic spindle regulation that may be involved in coordinating neurogenesis.

c-Jun N-terminal kinase phosphorylation of stathmin confers protection against cellular stress.

The cell stress response encompasses the range of intracellular events required for adaptation to stimuli detrimental to cell survival. Although the c-Jun N-terminal kinase (JNK) is a stress-activated kinase that can promote either cell survival or death in response to detrimental stimuli, the JNK-regulated mechanisms involved in survival are not fully characterized. Here we show that in response to hyperosmotic stress, JNK phosphorylates a key cytoplasmic microtubule regulatory protein, stathmin (STMN), on conserved Ser-25 and Ser-38 residues. In in vitro biochemical studies, we identified STMN Ser-38 as the critical residue required for efficient phosphorylation by JNK and identified a novel kinase interaction domain in STMN required for recognition by JNK. We revealed that JNK was required for microtubule stabilization in response to hyperosmotic stress. Importantly, we also demonstrated a novel cytoprotective function for STMN, as the knockdown of STMN levels by siRNA was sufficient to augment viability in response to hyperosmotic stress. Our findings show that JNK targeting of STMN represents a novel stress-activated cytoprotective mechanism involving microtubule network changes.

c-Jun N-terminal kinase (JNK) signaling: recent advances and challenges.

c-Jun N-terminal kinases (JNKs), first characterized as stress-activated members of the mitogen-activated protein kinase (MAPK) family, have become a focus of inhibitor screening strategies following studies that have shown their critical roles in the development of a number of diseases, such as diabetes, neurodegeneration and liver disease. We discuss recent advances in the discovery and development of ATP-competitive and ATP-noncompetitive JNK inhibitors. Because understanding the modes of actions of these inhibitors and improving their properties will rely on a better understanding of JNK structure, JNK catalytic mechanisms and substrates, recent advances in these areas of JNK biochemistry are also considered. In addition, the use of JNK gene knockout animals is continuing to reveal in vivo functions for these kinases, with tissue-specific roles now being dissected with tissue-specific knockouts. These latest advances highlight the many challenges now faced, particularly in the directed targeting of the JNK isoforms in specific tissues.