A mitotic CDK5-PP4 phospho-signaling cascade primes 53BP1 for DNA repair in G1.

Mitotic cells attenuate the DNA damage response (DDR) by phosphorylating 53BP1, a critical DDR mediator, to prevent its localization to damaged chromatin. Timely dephosphorylation of 53BP1 is critical for genome integrity, as premature recruitment of 53BP1 to DNA lesions impairs mitotic fidelity. Protein phosphatase 4 (PP4) dephosphorylates 53BP1 in late mitosis to allow its recruitment to DNA lesions in G1. How cells appropriately dephosphorylate 53BP1, thereby restoring DDR, is unclear. Here, we elucidate the underlying mechanism of kinetic control of 53BP1 dephosphorylation in mitosis. We demonstrate that CDK5, a kinase primarily functional in post-mitotic neurons, is active in late mitotic phases in non-neuronal cells and directly phosphorylates PP4R3ß, the PP4 regulatory subunit that recognizes 53BP1. Specific inhibition of CDK5 in mitosis abrogates PP4R3ß phosphorylation and abolishes its recognition and dephosphorylation of 53BP1, ultimately preventing the localization of 53BP1 to damaged chromatin. Our results establish CDK5 as a regulator of 53BP1 recruitment.

Identification of LIMK2 as a therapeutic target in castration resistant prostate cancer.

This study identified LIMK2 kinase as a disease-specific target in castration resistant prostate cancer (CRPC) pathogenesis, which is upregulated in response to androgen deprivation therapy, the current standard of treatment for prostate cancer. Surgical castration increases LIMK2 expression in mouse prostates due to increased hypoxia. Similarly, human clinical specimens showed highest LIMK2 levels in CRPC tissues compared to other stages, while minimal LIMK2 was observed in normal prostates. Most notably, inducible knockdown of LIMK2 fully reverses CRPC tumorigenesis in castrated mice, underscoring its potential as a clinical target for CRPC. We also identified TWIST1 as a direct substrate of LIMK2, which uncovered the molecular mechanism of LIMK2-induced malignancy. TWIST1 is strongly associated with CRPC initiation, progression and poor prognosis. LIMK2 increases TWIST1 mRNA levels upon hypoxia; and stabilizes TWIST1 by direct phosphorylation. TWIST1 also stabilizes LIMK2 by inhibiting its ubiquitylation. Phosphorylation-dead TWIST1 acts as dominant negative and fully prevents EMT and tumor formation in vivo, thereby highlighting the significance of LIMK2-TWIST1 signaling axis in CRPC. As LIMK2 null mice are viable, targeting LIMK2 should have minimal collateral toxicity, thereby improving the overall survival of CRPC patients.

Multifaceted Regulation of ALDH1A1 by Cdk5 in Alzheimer's Disease Pathogenesis.

This study revealed multifaceted regulation of ALDH1A1 by Cdk5 in Alzheimer's disease (AD) pathogenesis. ALDH1A1 is a multifunctional enzyme with dehydrogenase, esterase, and anti-oxidant activities. ALDH1A1 is also a major regulator of retinoic acid (RA) signaling, which is critical for normal brain homeostasis. We identified ALDH1A1 as both physiological and pathological target of Cdk5. First, under neurotoxic conditions, Cdk5-induced oxidative stress upregulates ALDH1A1 transcription. Second, Cdk5 increases ALDH1A1 levels by preventing its ubiquitylation via direct phosphorylation. Third, ALDH1A1 phosphorylation increases its dehydrogenase activity by altering its tetrameric state to a highly active monomeric state. Fourth, persistent oxidative stress triggered by deregulated Cdk5 inactivates ALDH1A1. Thus, initially, the good Cdk5 attempts to mitigate ensuing oxidative stress by upregulating ALDH1A1 via phosphorylation and paradoxically by increasing oxidative stress. Later, sustained oxidative stress generated by Cdk5 inhibits ALDH1A1 activity, leading to neurotoxicity. ALDH1A1 upregulation is highly neuroprotective. In human AD tissues, ALDH1A1 levels increase with disease severity. However, ALDH1A1 activity was highest at mild and moderate stages, but declines significantly at severe stage. These findings confirm that during the initial stages, neurons attempt to upregulate and activate ALDH1A1 to protect from accruing oxidative stress-induced damage; however, persistently deleterious conditions inactivate ALDH1A1, further contributing to neurotoxicity. This study thus revealed two faces of Cdk5, good and bad in neuronal function and survival, with a single substrate, ALDH1A1. The bad Cdk5 prevails in the end, overriding the good Cdk5 act, suggesting that Cdk5 is an effective therapeutic target for AD.

Aß plaque-selective NIR fluorescence probe to differentiate Alzheimer's disease from tauopathies.

Selective detection and staining of toxic amyloid plaques, a potential biomarker present in the Alzheimer's disease (AD) brain is crucial for both clinical diagnosis and monitoring AD disease progression. Herein, we report a coumarin-quinoline (CQ) conjugate-based turn-on near-infrared (NIR) fluorescence probe for specific detection of ß-amyloid (Aß) aggregates. CQ probe is highly sensitive and exhibits ~100-fold fluorescence enhancement in vitro upon binding Aß aggregates with enhanced quantum yield. Furthermore, the probe has ~10-fold higher binding affinity towards Aß aggregates (86nM) compared to commonly used Thioflavin T. Most importantly, CQ probe displays unambiguous selectivity towards Aß aggregates compared to other toxic protein aggregates such as tau, α-synuclein (α-Syn) and islet amyloid polypeptide (IAPP). In addition, CQ is nontoxic to neuronal cells and shows significant blood brain barrier permeability. Remarkably, CQ stains Aß plaques in human brain tissue over co-existing tau aggregates and neurofibrillary tangles (NFTs), which are associated in AD and tauopathies. This is a highly desirable attribute to distinguish AD from tau pathology and mixed dementia.

Phosphorylation-dependent regulation of ALDH1A1 by Aurora kinase A: insights on their synergistic relationship in pancreatic cancer.

BACKGROUND: Epithelial-to-mesenchymal transition (EMT) and cancer stem cell (CSC) formation are key underlying causes that promote extensive metastasis, drug resistance, and tumor recurrence in highly lethal pancreatic cancer. The mechanisms leading to EMT and CSC phenotypes are not fully understood, which has hindered the development of effective targeted therapies capable of improving treatment outcomes in patients with pancreatic cancer. RESULTS: We show a central role of Aurora kinase A (AURKA) in promoting EMT and CSC phenotypes via ALDH1A1, which was discovered as its direct substrate using an innovative chemical genetic screen. AURKA phosphorylates ALDH1A1 at three critical residues which exert a multifaceted regulation over its level, enzymatic activity, and quaternary structure. While all three phosphorylation sites contribute to its increased stability, T267 phosphorylation primarily regulates ALDH1A1 activity. AURKA-mediated phosphorylation rapidly dissociates tetrameric ALDH1A1 into a highly active monomeric species. ALDH1A1 also reciprocates and prevents AURKA degradation, thereby triggering a positive feedback activation loop which drives highly aggressive phenotypes in cancer. Phospho-resistant ALDH1A1 fully reverses EMT and CSC phenotypes, thus serving as dominant negative, which underscores the clinical significance of the AURKA-ALDH1A1 signaling axis in pancreatic cancer. CONCLUSIONS: While increased levels and activity of ALDH1A1 are hallmarks of CSCs, the underlying molecular mechanism remains unclear. We show the first phosphorylation-dependent regulation of ALDH1A1, which increases its levels and activity via AURKA. Recent global phospho-proteomic screens have revealed increased phosphorylation of ALDH1A1 at the T267 site in human cancers and healthy liver tissues where ALDH1A1 is highly expressed and active, indicating that this regulation is likely crucial both in normal and diseased states. This is also the first study to demonstrate oligomer-dependent activity of ALDH1A1, signifying that targeting its oligomerization state may be an effective therapeutic approach for counteracting its protective functions in cancer. Finally, while AURKA inhibition provides a potent tool to reduce ALDH1A1 levels and activity, the reciprocal loop between them ensures that their concurrent inhibition will be highly synergistic when inhibiting tumorigenesis, chemoresistance, and metastasis in highly aggressive pancreatic cancer and beyond.

The Aurora-A-Twist1 axis promotes highly aggressive phenotypes in pancreatic carcinoma.

We uncovered a crucial role for the Aurora kinase A (AURKA)-Twist1 axis in promoting epithelial-to-mesenchymal transition (EMT) and chemoresistance in pancreatic cancer. Twist1 is the first EMT-specific target of AURKA that was identified using an innovative screen. AURKA phosphorylates Twist1 at three sites, which results in its multifaceted regulation - AURKA inhibits its ubiquitylation, increases its transcriptional activity and favors its homodimerization. Twist1 reciprocates and prevents AURKA degradation, thereby triggering a feedback loop. Ablation of either AURKA or Twist1 completely inhibits EMT, highlighting both proteins as central players in EMT progression. Phosphorylation-dead Twist1 serves as a dominant-negative and fully reverses the EMT phenotype induced by Twist1, underscoring the crucial role of AURKA-mediated phosphorylation in mediating Twist1-induced malignancy. Likewise, Twist1-overexpressing BxPC3 cells formed large tumors in vivo, whereas expression of phosphorylation-dead Twist1 fully abrogated this effect. Furthermore, immunohistochemical analysis of pancreatic cancer specimens revealed a 3-fold higher level of Twist1 compared to that seen in healthy normal tissues. This is the first study that links Twist1 in a feedback loop with its activating kinase, which indicates that concurrent inhibition of AURKA and Twist1 will be synergistic in inhibiting pancreatic tumorigenesis and metastasis.

Cdk5-Foxo3 axis: initially neuroprotective, eventually neurodegenerative in Alzheimer's disease models.

Deregulated Cdk5 causes neurotoxic amyloid beta peptide (Aß) processing and cell death, two hallmarks of Alzheimer's disease, through the Foxo3 transcriptional factor in hippocampal cells, primary neurons and an Alzheimer's disease mouse model. Using an innovative chemical genetic screen, we identified Foxo3 as a direct substrate of Cdk5 in brain lysates. Cdk5 directly phosphorylates Foxo3, which increased its levels and nuclear translocation. Nuclear Foxo3 initially rescued cells from ensuing oxidative stress by upregulating MnSOD (also known as SOD2). However, following prolonged exposure, Foxo3 upregulated Bim (also known as BCL2L11) and FasL (also known as FASLG) causing cell death. Active Foxo3 also increased Aß(1-42) levels in a phosphorylation-dependent manner. These events were completely inhibited either by expressing phosphorylation-resistant Foxo3 or by depleting Cdk5 or Foxo3, highlighting a key role for Cdk5 in regulating Foxo3. These results were confirmed in an Alzheimer's disease mouse model, which exhibited increased levels and nuclear localization of Foxo3 in hippocampal neurons, which preceded neurodegeneration and Aß plaque formation, indicating this phenomenon is an early event in Alzheimer's disease pathogenesis. Collectively, these results show that Cdk5-mediated phospho-regulation of Foxo3 can activate several genes that promote neuronal death and aberrant Aß processing, thereby contributing to the progression of neurodegenerative pathologies.