Lemur tyrosine kinase-2 signalling regulates kinesin-1 light chain-2 phosphorylation and binding of Smad2 cargo. Oncogene. 2012;31:2773-2782. [PMID: 21996745 PMCID: PMC3272475 DOI: 10.1038/onc.2011.437]

LMTK2 and CRB1 are two novel risk genes for Alzheimer's disease in Han Chinese

BACKGROUND

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease with a substantial genetic background. However, its underlying genetic architecture remains to be elucidated.

METHODS

In this study, we performed whole-exome sequencing in 282 familial and/or early-onset AD patients and 1086 cognitively normal controls in the Han Chinse populations. According to minor allele frequency, variants were divided into common variants (MAF ≥ 0.01) and rare variants (MAF < 0.01). Common variant-based association analysis and gene-based association test aggregating rare variants were performed by PLINK 1.9 and Sequence Kernel Association Test-Optimal, respectively. We replicated the significant results by using the same AD samples and controls from whole genome sequencing (n = 1879). Furthermore, we determined the functions of the novel AD risk genes in vitro.

RESULTS

Common variants association analysis revealed that APOE rs429358 reached statistical whole-exome significance. Gene-level aggregation testing identified that rare damaging variants in LMTK2 and CRB1 conferred risk to AD. All variants are located in highly conserved amino acid regions and are predicted to be damaging. Furthermore, functional studies showed that LMTK2 rare damaging variants (R234P and S974G) enhanced tau phosphorylation levels, tau aggregates formation, and Aβ generation. Meanwhile, the CRB1 Y556X variant caused incomplete translation of CRB1 protein and increased the Aβ42 level and Aβ42/Aβ40 ratio.

CONCLUSION

Our findings indicated that LMTK2 and CRB1 are two novel AD risk genes in Han Chinese, which may provide promising targets for diagnosis and intervention.

The lemur tail kinase family in neuronal function and disfunction in neurodegenerative diseases

The complex neuronal architecture and the long distance of synapses from the cell body require precisely orchestrated axonal and dendritic transport processes to support key neuronal functions including synaptic signalling, learning and memory formation. Protein phosphorylation is a major regulator of both intracellular transport and synaptic functions. Some kinases and phosphatases such as cyclin dependent kinase-5 (cdk5)/p35, glycogen synthase kinase-3β (GSK3β) and protein phosphatase-1 (PP1) are strongly involved in these processes. A primary pathological hallmark of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis/frontotemporal dementia, is synaptic degeneration together with disrupted intracellular transport. One attractive possibility is that alterations to key kinases and phosphatases may underlie both synaptic and axonal transport damages. The brain enriched lemur tail kinases (LMTKs, formerly known as lemur tyrosine kinases) are involved in intracellular transport and synaptic functions, and are also centrally placed in cdk5/p35, GSK3β and PP1 signalling pathways. Loss of LMTKs is documented in major neurodegenerative diseases and thus can contribute to pathological defects in these disorders. However, whilst function of their signalling partners became clearer in modulating both synaptic signalling and axonal transport progress has only recently been made around LMTKs. In this review, we describe this progress with a special focus on intracellular transport, synaptic functions and neurodegenerative diseases.

LMTK2 switches on canonical TGF-β1 signaling in human bronchial epithelial cells

Transforming growth factor (TGF-β1) is a critical profibrotic mediator in chronic lung disease, and there are no specific strategies to mitigate its adverse effects. Activation of TGF-β1 signaling is a multipart process involving ligands, transmembrane receptors, and transcription factors. In addition, an intricate network of adaptor proteins fine-tunes the signaling strength, duration, and activity. Namely, Smad7 recruits growth arrest and DNA damage (GADD34) protein that then interacts with the catalytic subunit of phosphoprotein phosphatase 1 (PP1c) to inactivate TGF-β receptor (TβR)-I and downregulate TGF-β1 signaling. Little is known about how TGF-β1 releases TβR-I from the GADD34-PP1c inhibition to activate its signaling. Transmembrane lemur tyrosine kinase 2 (LMTK2) is a PP1c inhibitor, and our published data showed that TGF-β1 recruits LMTK2 to the cell surface. Here, we tested the hypothesis that TGF-β1 recruits LMTK2 to inhibit PP1c, allowing activation of TβR-I. First, LMTK2 interacted with the TGF-β1 pathway in the human bronchial epithelium at multiple checkpoints. Second, TGF-β1 inhibited PP1c by an LMTK2-dependent mechanism. Third, TGF-β1 used LMTK2 to activate canonical Smad3-mediated signaling. We propose a model whereby the LMTK2-PP1c and Smad7-GADD34-PP1c complexes serve as on-and-off switches in the TGF-β1 signaling in human bronchial epithelium.NEW & NOTEWORTHY Activation of the transforming growth factor (TGF)-β1 signaling pathway is complex, involving many ligands, transmembrane receptors, transcription factors, and modulating proteins. The mechanisms of TGF-β1 signaling activation/inactivation are not fully understood. We propose for the first time a model by which transmembrane lemur tyrosine kinase 2 (LMTK2) forms a complex with phosphoprotein phosphatase 1 (PP1c) to activate TGF-β1 signaling and Smad7, growth arrest and DNA damage (GADD34), and PP1C form a complex to inactivate TGF-β1 signaling in human bronchial epithelium.

Proximal protein landscapes of the type I interferon signaling cascade reveal negative regulation by PJA2

Deciphering the intricate dynamic events governing type I interferon (IFN) signaling is critical to unravel key regulatory mechanisms in host antiviral defense. Here, we leverage TurboID-based proximity labeling coupled with affinity purification-mass spectrometry to comprehensively map the proximal human proteomes of all seven canonical type I IFN signaling cascade members under basal and IFN-stimulated conditions. This uncovers a network of 103 high-confidence proteins in close proximity to the core members IFNAR1, IFNAR2, JAK1, TYK2, STAT1, STAT2, and IRF9, and validates several known constitutive protein assemblies, while also revealing novel stimulus-dependent and -independent associations between key signaling molecules. Functional screening further identifies PJA2 as a negative regulator of IFN signaling via its E3 ubiquitin ligase activity. Mechanistically, PJA2 interacts with TYK2 and JAK1, promotes their non-degradative ubiquitination, and limits the activating phosphorylation of TYK2 thereby restraining downstream STAT signaling. Our high-resolution proximal protein landscapes provide global insights into the type I IFN signaling network, and serve as a valuable resource for future exploration of its functional complexities.

A revised nomenclature for the lemur family of protein kinases

The lemur family of protein kinases has gained much interest in recent years as they are involved in a variety of cellular processes including regulation of axonal transport and endosomal trafficking, modulation of synaptic functions, memory and learning, and they are centrally placed in several intracellular signalling pathways. Numerous studies have also implicated role of the lemur kinases in the development and progression of a wide range of cancers, cystic fibrosis, and neurodegenerative diseases. However, parallel discoveries and inaccurate prediction of their kinase activity have resulted in a confusing and misleading nomenclature of these proteins. Herein, a group of international scientists with expertise in lemur family of protein kinases set forth a novel nomenclature to rectify this problem and ultimately help the scientific community by providing consistent information about these molecules.

Kinesin-1 transports morphologically distinct intracellular virions during vaccinia infection

Intracellular mature viruses (IMVs) are the first and most abundant infectious form of vaccinia virus to assemble during its replication cycle. IMVs can undergo microtubule-based motility, but their directionality and the motor involved in their transport remain unknown. Here, we demonstrate that IMVs, like intracellular enveloped viruses (IEVs), the second form of vaccinia that are wrapped in Golgi-derived membranes, recruit kinesin-1 and undergo anterograde transport. In vitro reconstitution of virion transport in infected cell extracts revealed that IMVs and IEVs move toward microtubule plus ends with respective velocities of 0.66 and 0.56 µm/s. Quantitative imaging established that IMVs and IEVs recruit an average of 139 and 320 kinesin-1 motor complexes, respectively. In the absence of kinesin-1, there was a near-complete loss of in vitro motility and reduction in the intracellular spread of both types of virions. Our observations demonstrate that kinesin-1 transports two morphologically distinct forms of vaccinia. Reconstitution of vaccinia-based microtubule motility in vitro provides a new model to elucidate how motor number and regulation impacts transport of a bona fide kinesin-1 cargo.

PP1A prevents ROS-induced pyroptosis by inhibiting MAPK/caspase-3 in mouse adipose tissue

Pyroptosis is a type of programmed cell death triggered by a variety of exogenous stimuli, playing important roles in the development of cells. Recent studies have shown that pyroptosis also occurs in human and mouse adipocytes. The serine/threonine protein phosphatase 1 catalytic subunit α (PP1A) is located in the nucleus and functions in the regulation of cell development, glucose metabolism and protein synthesis. PP1A can also target important proteins in the process of apoptosis. However, it is still unclear whether PP1A participates in the regulation of pyroptosis in mouse adipocytes. In the present study, we investigated the function of PP1A in reactive oxygen species-induced pyroptosis and the related mechanism in mouse adipose tissue. Our results demonstrated that PP1A suppressed pyroptosis in adipocytes by inhibiting the reactive oxygen species/mitogen-activated protein kinase/caspase-3 signaling pathway and promoting M2 macrophage polarization. This experiment provides a theoretical basis for understanding the regulatory function and mechanism of PP1A in pyroptosis caused by oxidative stress in adipocytes.

Lemur Tyrosine Kinases and Prostate Cancer: A Literature Review

The members of the Lemur Tyrosine Kinases (LMTK1-3) subfamily constitute a group of three membrane-anchored kinases. They are known to influence a wide variety of key cellular events, often affecting cell proliferation and apoptosis. They have been discovered to be involved in cancer, in that they impact various signalling pathways that influence cell proliferation, migration, and invasiveness. Notably, in the context of genome-wide association studies, one member of the LMTK family has been identified as a candidate gene which could contribute to the development of prostate cancer. In this review, of published literature, we present evidence on the role of LMTKs in human prostate cancer and model systems, focusing on the complex network of interacting partners involved in signalling cascades that are frequently activated in prostate cancer malignancy. We speculate that the modulators of LMTK enzyme expression and activity would be of high clinical relevance for the design of innovative prostate cancer treatment.

Review of novel tissue-based biomarkers for prostate cancer: towards personalised and targeted medicine

Background:

Prostate cancer is the most commonly diagnosed cancer in men and responsible for about 10% of all cancer mortality in both Canadian and American men. Currently, serum PSA level is the most commonly used test for the detection of prostate cancer, though the levels can also be elevated in benign conditions, has limited specificity and has a high rate of overdiagnosis and treatment of indolent disease. Consequently, in recent years, several investigations have been conducted to identify novel cancer biomarkers capable of both effective screening and diagnosis, as well as assisting to shift the diagnostic and treatment paradigm of prostate cancer towards more patient-specific and targeted medicine. The goal of this narrative review paper is to describe eleven novel and promising tissue-based biomarkers for prostate cancer capable to account for individual patient variabilities and have the potential for risk assessment, early detection and diagnosis, identification of patients who will benefit from a particular treatment and monitoring patient response to treatment.

Materials and methods:

We searched several databases from August to December 2020 for relevant studies published in English between 2000 and 2020 and reporting on tissue-based biomarkers for screening and early diagnosis, treatment and monitoring of prostate cancer.

Conclusions:

Emerging prostate cancer biomarkers have the potential to guide clinical decision-making since they have the potential to detect the disease early, measure the risk of developing the disease and the risk of progression, provide accurate information of patient response to a specific treatment and are capable of informing clinicians about the likely outcome of a cancer diagnosis independent of the treatment received. Therefore, the future holds promise for personalised and targeted medicine from prevention to diagnosis and treatment that considers the individual patient’s variabilities in the management of prostate cancer.

Differential Gene Expression Analysis Reveals Global LMTK2 Regulatory Network and Its Role in TGF-β1 Signaling

Lemur tyrosine kinase 2 (LMTK2) is a transmembrane Ser/Thr kinase whose role has been increasingly recognized; however, when compared to other kinases, understanding of the LMTK2 networks and biological functions is still limited. Recent data have shown that transforming growth factor (TGF)-β1 plays a role in modulating LMTK2 function by controlling its endocytic trafficking in human bronchial epithelial cells. Here, we aimed to unveil the LMTK2 regulatory network and elucidate how it affects cellular functions and disease pathways in either TGF-β1 dependent or independent manner. To understand how the LMTK2 and TGF-β1 pathways interconnect, we knocked down (KD) LMTK2 using small(si)RNA-mediated silencing in human bronchial epithelial CFBE41o- cells, treated cells with TGF-β1 or vehicle control, and performed differential gene expression analysis by RNA sequencing (RNAseq). In vehicle-treated cells, LMTK2 KD affected expression of 2,506 genes while it affected 4,162 genes after TGF-β1 stimulation. Bioinformatics analysis shows that LMTK2 is involved in diverse cellular functions and disease pathways, such as cell death and survival, cellular development, and cancer susceptibility. In summary, our study increases current knowledge about the LMTK2 network and its intersection with the TGF-β1 signaling pathway. These findings will serve as basis for future exploration of the predicted LMTK2 interactions and signaling pathways.

Discovery of cyclic guanidine-linked sulfonamides as inhibitors of LMTK3 kinase

Lemur tyrosine kinase 3 (LMTK3) is oncogenic in various cancers. In breast cancer, LMTK3 phosphorylates and modulates the activity of estrogen receptor-α (ERα) and is essential for the growth of ER-positive cells. LMTK3 is highly expressed in ER-negative breast cancer cells, where it promotes invasion via integrin β1. LMTK3 abundance and/or high nuclear expression have been linked to shorter disease free and overall survival time in a variety of cancers, supporting LMTK3 as a potential target for anticancer drug development. We sought to identify small molecule inhibitors of LMTK3 with the ultimate goal to pharmacologically validate this kinase as a novel target in cancer. We used a homogeneous time resolve fluorescence (HTRF) assay to screen a collection of mixture-based combinatorial chemical libraries containing over 18 million compounds. We identified several cyclic guanidine-linked sulfonamides with sub-micromolar activity and evaluated their binding mode using a 3D homology model of the LMTK3 K_D.

TGF-β1 Augments the Apical Membrane Abundance of Lemur Tyrosine Kinase 2 to Inhibit CFTR-Mediated Chloride Transport in Human Bronchial Epithelia

The most common disease-causing mutation in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene, F508del, leads to cystic fibrosis (CF), by arresting CFTR processing and trafficking to the plasma membrane. The FDA-approved modulators partially restore CFTR function and slow down the progression of CF lung disease by increasing processing and delivery to the plasma membrane and improving activity of F508del-CFTR Cl^– channels. However, the modulators do not correct compromised membrane stability of rescued F508del-CFTR. Transforming growth factor (TGF)-β1 is a well-established gene modifier of CF associated with worse lung disease in F508del-homozygous patients, by inhibiting CFTR biogenesis and blocking the functional rescue of F508del-CFTR. Lemur tyrosine kinase 2 (LMTK2) is a transmembrane protein localized at the apical and basolateral membrane domain of human bronchial epithelial cells. Phosphorylation of the apical membrane CFTR by LMTK2 triggers its endocytosis and reduces the abundance of membrane-associated CFTR, impairing the CFTR-mediated Cl^– transport. We have previously shown that LMTK2 knockdown improves the pharmacologically rescued F508del-CFTR abundance and function. Thus, reducing the LMTK2 recruitment to the plasma membrane may provide a useful strategy to potentiate the pharmacological rescue of F508del-CFTR. Here, we elucidate the mechanism of LMTK2 recruitment to the apical plasma membrane in polarized CFBE41o- cells. TGF-β1 increased LMTK2 abundance selectively at the apical membrane by accelerating its recycling in Rab11-positive vesicles without affecting LMTK2 mRNA levels, protein biosynthesis, or endocytosis. Our data suggest that controlling TGF-β1 signaling may attenuate recruitment of LMTK2 to the apical membrane thereby improving stability of pharmacologically rescued F508del-CFTR.

Lemur Tyrosine Kinase 2 (LMTK2) Level Inversely Correlates with Phospho-Tau in Neuropathological Stages of Alzheimer's Disease

: Alzheimer's disease (AD) is the most common neurodegenerative dementia. Mapping the pathomechanism and providing novel therapeutic options have paramount significance. Recent studies have proposed the role of LMTK2 in AD. However, its expression pattern and association with the pathognomonic neurofibrillary tangles (NFTs) in different brain regions and neuropathological stages of AD is not clear. We performed chromogenic (CHR) LMTK2 and fluorescent phospho-tau/LMTK2 double-labelling (FDL) immunohistochemistry (IHC) on 10-10 postmortem middle frontal gyrus (MFG) and anterior hippocampus (aHPC) samples with early and late neuropathological Braak tau stages of AD. MFG in early stage was our 'endogenous control' region as it is not affected by NFTs. Semiquantitative CHR-IHC intensity scoring revealed significantly higher (p < 0.001) LMTK2 values in this group compared to NFT-affected regions. FDL-IHC demonstrated LMTK2 predominance in the endogenous control region, while phospho-tau overburden and decreased LMTK2 immunolabelling were detected in NFT-affected groups (aHPC in early and both regions in late stage). Spearman's correlation coefficient showed strong negative correlation between phospho-tau/LMTK2 signals within each group. According to our results, LMTK2 expression is inversely proportionate to the extent of NFT pathology, and decreased LMTK2 level is not a general feature in AD brain, rather it is characteristic of the NFT-affected regions.

Kinesin light chain-1 serine-460 phosphorylation is altered in Alzheimer's disease and regulates axonal transport and processing of the amyloid precursor protein

Damage to axonal transport is an early pathogenic event in Alzheimer’s disease. The amyloid precursor protein (APP) is a key axonal transport cargo since disruption to APP transport promotes amyloidogenic processing of APP. Moreover, altered APP processing itself disrupts axonal transport. The mechanisms that regulate axonal transport of APP are therefore directly relevant to Alzheimer’s disease pathogenesis. APP is transported anterogradely through axons on kinesin-1 motors and one route for this transport involves calsyntenin-1, a type-1 membrane spanning protein that acts as a direct ligand for kinesin-1 light chains (KLCs). Thus, loss of calsyntenin-1 disrupts APP axonal transport and promotes amyloidogenic processing of APP. Phosphorylation of KLC1 on serine-460 has been shown to reduce anterograde axonal transport of calsyntenin-1 by inhibiting the KLC1-calsyntenin-1 interaction. Here we demonstrate that in Alzheimer’s disease frontal cortex, KLC1 levels are reduced and the relative levels of KLC1 serine-460 phosphorylation are increased; these changes occur relatively early in the disease process. We also show that a KLC1 serine-460 phosphomimetic mutant inhibits axonal transport of APP in both mammalian neurons in culture and in Drosophila neurons in vivo. Finally, we demonstrate that expression of the KLC1 serine-460 phosphomimetic mutant promotes amyloidogenic processing of APP. Together, these results suggest that increased KLC1 serine-460 phosphorylation contributes to Alzheimer’s disease.

Neuropathological characterization of Lemur tyrosine kinase 2 (LMTK2) in Alzheimer's disease and neocortical Lewy body disease

Alzheimer's disease (AD) and neocortical Lewy body disease (LBD) are the most common neurodegenerative dementias, with no available curative treatment. Elucidating pathomechanism and identifying novel therapeutic targets are of paramount importance. Lemur tyrosine kinase 2 (LMTK2) is involved in several physiological and pathological cellular processes. Herewith a neuropathological characterization is presented in AD and neocortical LBD samples using chromogenic and fluorescent LMTK2 immunohistochemistry on post-mortem brain tissues and compared them to age-matched controls (CNTs). LMTK2 immunopositivity was limited to the neuronal cytoplasm. Neurons, including tau-positive tangle-bearing ones, showed decreased chromogenic and immunofluorescent labelling in AD in every cortical layer compared to CNT and neocortical LBD. Digital image analysis was performed to measure the average immunopositivity of groups. Mean grey values were calculated for each group after measuring the grey scale LMTK2 signal intensity of each individual neuron. There was significant difference between the mean grey values of CNT vs. AD and neocortical LBD vs. AD. The moderate decrease in neocortical LBD suggests the effect of coexisting AD pathology. We provide neuropathological evidence on decreased neuronal LMTK2 immunolabelling in AD, with implications for pathogenesis.

Overexpression of lemur tyrosine kinase-2 protects neurons from oxygen-glucose deprivation/reoxygenation-induced injury through reinforcement of Nrf2 signaling by modulating GSK-3β phosphorylation

Lemur tyrosine kinase-2 (LMTK2), a newly identified serine/threonine kinase, is a potential regulator of cell survival and apoptosis. However, little is known about its role in regulating neuronal survival during cerebral ischemia/reperfusion injury. The present study aimed to explore the potential function of LMTK2 in regulating neuronal survival using an in vitro model of oxygen-glucose deprivation/reoxygenation (OGD/R)-induced injury. Herein, we found that LMTK2 expression was markedly decreased in neurons following OGD/R exposure. Gain-of-function experiments demonstrated that LMTK2 overexpression significantly improved the viability and reduced apoptosis of neurons with OGD/R-induced injury. Moreover, LMTK2 overexpression reduced the production of reactive oxygen species (ROS) in OGD/R-exposed neurons. Notably, our results elucidated that LMTK2 overexpression reinforced the activation of nuclear factor erythroid 2-related factor (Nrf2)/antioxidant response element (ARE) antioxidant signaling associated with increased glycogen synthase kinase-3β (GSK-3β) phosphorylation. GSK-3β inhibition by its specific inhibitor significantly reversed LMTK2-inhibition-linked apoptosis and ROS production. Additionally, silencing Nrf2 partially reversed the LMTK2-overexpression-mediated neuroprotective effect in OGD/R-injured neurons. Taken together, our results demonstrated that LMTK2 overexpression alleviated OGD/R-induced neuronal apoptosis and oxidative damage by enhancing Nrf2/ARE antioxidant signaling via modulation of GSK-3β phosphorylation. Our study suggests LMTK2 is a potential target for neuroprotection during cerebral ischemia/reperfusion.

Silencing of lemur tyrosine kinase 2 restricts the proliferation and invasion of hepatocellular carcinoma through modulation of GSK-3β/Wnt/β-catenin signaling

Lemur tyrosine kinase 2 (LMTK2) was recently identified as a novel cancer-related gene in several human cancers. However, little is known of its function in hepatocellular carcinoma (HCC). Here we aim to investigate the expression pattern, biological function, and regulatory mechanism of LMTK2 in HCC. We found that LMTK2 was highly expressed in HCC tissues, and patients with high expression of LMTK2 in tumor tissues had shorter survival times. LMTK2 expression was also elevated in HCC cell lines, and LMTK2 silencing markedly repressed the proliferation and invasion of HCC cells. By contrast, LMTK2 overexpression exerted promotion effects on HCC cell proliferation and invasion. Our results demonstrate that LMTK2 silencing decreases the phosphorylation of glycogen synthase kinase-3β (GSK-3β) and the expression of an active β-catenin protein, leading to inhibition of Wnt/β-catenin signaling. Notably, GSK-3β inhibition significantly reversed the LMTK2 silencing-mediated antitumor effect on proliferation, invasion, and Wnt/β-catenin signaling in HCC cells. LMTK2 silencing retarded the tumor growth of HCC cells in an in vivo xenograft tumor model, associated with downregulation of Wnt/β-catenin signaling. In conclusion, our findings suggest that silencing of LMTK2 suppresses the proliferation and invasion of HCC cells through the inhibition of Wnt/β-catenin signaling, via GSK-3β, highlighting the importance of LMTK2/GSK-3β/Wnt/β-catenin signaling in HCC progression.

LMTK2 binds to kinesin light chains to mediate anterograde axonal transport of cdk5/p35 and LMTK2 levels are reduced in Alzheimer's disease brains

Cyclin dependent kinase-5 (cdk5)/p35 is a neuronal kinase that regulates key axonal and synaptic functions but the mechanisms by which it is transported to these locations are unknown. Lemur tyrosine kinase-2 (LMTK2) is a binding partner for p35 and here we show that LMTK2 also interacts with kinesin-1 light chains (KLC1/2). Binding to KLC1/2 involves a C-terminal tryptophan/aspartate (WD) motif in LMTK2 and the tetratricopeptide repeat (TPR) domains in KLC1/2, and this interaction facilitates axonal transport of LMTK2. Thus, siRNA loss of KLC1 or mutation of the WD motif disrupts axonal transport of LMTK2. We also show that LMTK2 facilitates the formation of a complex containing KLC1 and p35 and that siRNA loss of LMTK2 disrupts axonal transport of both p35 and cdk5. Finally, we show that LMTK2 levels are reduced in Alzheimer's disease brains. Damage to axonal transport and altered cdk5/p35 are pathogenic features of Alzheimer's disease. Thus, LMTK2 binds to KLC1 to direct axonal transport of p35 and its loss may contribute to Alzheimer's disease.

Lemur tyrosine kinase 2 acts as a positive regulator of NF-κB activation and colon cancer cell proliferation

Lemur tyrosine kinase 2 (LMTK2) belongs to both protein kinase and tyrosine kinase families. LMTK2 is less studied and little is known about its function. Here we demonstrate that LMTK2 modulates NF-κB activity and functions to promote colonic tumorigenesis. We found that LMTK2 protein was abundant in colon cancer cells and LMTK2 knockdown (LMTK2-KD) inhibited proliferation of colon cancer cells through inactivating NF-κB. In unstimulated condition, LMTK2 modulated NF-κB through inhibiting phosphorylation of p65 at Ser468. Mechanistically, LMTK2 phosphorylated protein phosphatase 1A (PP1A) to prevent PP1A from dephosphorylating p-GSK3β(Ser9). The p-GSK3β(Ser9) could not phosphorylate p65 at Ser468, which maintained the basal NF-κB activity. LMTK2 also modulated TNFα-activated NF-κB. LMTK2-KD repressed TNFα-induced IKKβ phosphorylation, IκBα degradation and NF-κB activation, implying that LMTK2 modulates TNFα-activated NF-κB via IKK. These results suggest that LMTK2 modulates basal and TNFα-induced NF-κB activities in different mechanisms. Animal studies show that LMTK2-KD suppressed colon cancer cell xenograft growth, decreased PP1A phosphorylation and increased p-p65(Ser468). Our results reveal the role and underlying mechanism of LMTK2 in colonic tumorigenesis and suggest that LMTK2 may serve as a potential target for chemotherapy of colon cancer.

Unraveling the Function of Lemur Tyrosine Kinase 2 Network

Lemur Tyrosine Kinase 2 (LMTK2) is a recently cloned transmembrane protein, actually a serine/threonine kinase named after the Madagascar primate lemur due to the long intracellular C-terminal tail. LMTK2 is relatively little known, compared to other kinases but its role has been increasingly recognized. Published data show that LMTK2 regulates key cellular events, including endocytic trafficking, nerve growth factor signaling, apoptosis, and Cl- transport. Abnormalities in the expression and function of LMTK2 are associated with human disease, such as neurodegeneration, cancer and infertility. We summarized the current state of knowledge on LMTK2 structure, regulation, interactome, intracellular localization, and tissue expression and point out future research directions to better understand the role of LMTK2.

Physiology of the Gut: Experimental Models for Investigating Intestinal Fluid and Electrolyte Transport

Once thought to be exclusively an absorptive tissue, the intestine is now recognized as an important secretory tissue, playing a key role in body ion and fluid homeostasis. Given the intestine's role in fluid homeostasis, it is not surprising that important clinical pathologies arise from imbalances in fluid absorption and secretion. Perhaps the most important examples of this can be seen in enterotoxigenic secretory diarrheas with extreme fluid secretion, and Cystic Fibrosis with little or no fluid secretion. A mechanistic understanding of the cellular pathways regulating ion and fluid transport has been obtained from a variety of approaches and model systems. These have ranged from the intact intestine to a single intestinal epithelial cell type. Although for many years a reductionist approach has held sway for investigating intestinal transport, the growing realization that physiologic processes should really be examined within a physiological context has seen a marked increase in studies using models that are essentially mini-intestines in a dish. The aim of this chapter is to provide a historical context for our understanding of intestinal ion and fluid transport, and to highlight the model systems that have been used to acquire this knowledge.

Kinesin light chain-4 depletion induces apoptosis of radioresistant cancer cells by mitochondrial dysfunction via calcium ion influx

Kinesins act as molecular microtubule-dependent motor proteins and have various important cellular functions related to cell division, intracellular transport, and membrane trafficking. However, the function of kinesin light chain 4 (KLC4) in cancer, especially radioresistance, has not been previously described. Thus, we investigated KLC4 function in lung cancer cells and radioresistant R-H460 cells by analyzing alterations in radiosensitivity after gene knockdown with siRNA and by evaluating cellular phenotypes and xenograft tumor growth. KLC4 was upregulated in human lung cancer cell lines. Moreover, in paired clinical specimens of lung cancer patients, KLC4 expression was significantly higher in tumor tissues than in paired adjacent normal tissues. Fluorescence-activated cell sorting (FACS) analysis showed that apoptosis rates and cleaved poly (ADP-ribose) polymerase (PARP) and cleaved caspase-3 levels in KLC4-knockdown lung cancer cells were significantly increased compared with those in control cells. Colony formation decreased as the radiation dose increased in KLC4-knockdown lung cancer cells, demonstrating an essential role for KLC4 in radioresistance. Importantly, KLC4 silencing suppressed tumor growth in an in vivo xenograft model, accompanied by increased apoptosis. Finally, KLC4-knockdown cells exhibited impaired mitochondrial respiration, increased mitochondrial reactive oxygen species production, and enhanced mitochondrial calcium uptake, resulting in mitochondrial dysfunction. Thus, KLC4 as a kinesin superfamily-targeted therapy may represent a novel, effective anticancer strategy, particularly for patients showing radioresistance.

Biological function of Lemur tyrosine kinase 2 (LMTK2): implications in neurodegeneration

Neurodegenerative disorders are frequent, incurable diseases characterised by abnormal protein accumulation and progressive neuronal loss. Despite their growing prevalence, the underlying pathomechanism remains unclear. Lemur tyrosine kinase 2 (LMTK2) is a member of a transmembrane serine/threonine-protein kinase family. Although it was described more than a decade ago, our knowledge on LMTK2’s biological functions is still insufficient. Recent evidence has suggested that LMTK2 is implicated in neurodegeneration. After reviewing the literature, we identified three LMTK2-mediated mechanisms which may contribute to neurodegenerative processes: disrupted axonal transport, tau hyperphosphorylation and enhanced apoptosis. Moreover, LMTK2 gene expression is decreased in an Alzheimer’s disease mouse model. According to these features, LMTK2 might be a promising therapeutic target in near future. However, further investigations are required to clarify the exact biological functions of this unique protein.

Three Cdk1 sites in the kinesin-5 Cin8 catalytic domain coordinate motor localization and activity during anaphase

The bipolar kinesin-5 motors perform essential functions in mitotic spindle dynamics. We previously demonstrated that phosphorylation of at least one of the Cdk1 sites in the catalytic domain of the Saccharomyces cerevisiae kinesin-5 Cin8 (S277, T285, S493) regulates its localization to the anaphase spindle. The contribution of these three sites to phospho-regulation of Cin8, as well as the timing of such contributions, remains unknown. Here, we examined the function and spindle localization of phospho-deficient (serine/threonine to alanine) and phospho-mimic (serine/threonine to aspartic acid) Cin8 mutants. In vitro, the three Cdk1 sites undergo phosphorylation by Clb2-Cdk1. In cells, phosphorylation of Cin8 affects two aspects of its localization to the anaphase spindle, translocation from the spindle-pole bodies (SPBs) region to spindle microtubules (MTs) and the midzone, and detachment from the mitotic spindle. We found that phosphorylation of S277 is essential for the translocation of Cin8 from SPBs to spindle MTs and the subsequent detachment from the spindle. Phosphorylation of T285 mainly affects the detachment of Cin8 from spindle MTs during anaphase, while phosphorylation at S493 affects both the translocation of Cin8 from SPBs to the spindle and detachment from the spindle. Only S493 phosphorylation affected the anaphase spindle elongation rate. We conclude that each phosphorylation site plays a unique role in regulating Cin8 functions and postulate a model in which the timing and extent of phosphorylation of the three sites orchestrates the anaphase function of Cin8.

Neurobiology of axonal transport defects in motor neuron diseases: Opportunities for translational research?

Intracellular trafficking of cargoes is an essential process to maintain the structure and function of all mammalian cell types, but especially of neurons because of their extreme axon/dendrite polarisation. Axonal transport mediates the movement of cargoes such as proteins, mRNA, lipids, membrane-bound vesicles and organelles that are mostly synthesised in the cell body and in doing so is responsible for their correct spatiotemporal distribution in the axon, for example at specialised sites such as nodes of Ranvier and synaptic terminals. In addition, axonal transport maintains the essential long-distance communication between the cell body and synaptic terminals that allows neurons to react to their surroundings via trafficking of for example signalling endosomes. Axonal transport defects are a common observation in a variety of neurodegenerative diseases, and mutations in components of the axonal transport machinery have unequivocally shown that impaired axonal transport can cause neurodegeneration (reviewed in El-Kadi et al., 2007, De Vos et al., 2008; Millecamps and Julien, 2013). Here we review our current understanding of axonal transport defects and the role they play in motor neuron diseases (MNDs) with a specific focus on the most common form of MND, amyotrophic lateral sclerosis (ALS).

Vaccinia virus proteins A36 and F12/E2 show strong preferences for different kinesin light chain isoforms

Vaccinia virus (VACV) utilizes microtubule‐mediated trafficking at several stages of its life cycle, of which virus egress is the most intensely studied. During egress VACV proteins A36, F12 and E2 are involved in kinesin‐1 interactions; however, the roles of these proteins remain poorly understood. A36 forms a direct link between virions and kinesin‐1, yet in its absence VACV egress still occurs on microtubules. During a co‐immunoprecipitation screen to seek an alternative link between virions and kinesin, A36 was found to bind isoform KLC1 rather than KLC2. The F12/E2 complex associates preferentially with the C‐terminal tail of KLC2, to a region that overlaps the binding site of cellular 14‐3‐3 proteins. F12/E2 displaces 14‐3‐3 from KLC and, unlike 14‐3‐3, does not require phosphorylation of KLC for its binding. The region determining the KLC1 specificity of A36 was mapped to the KLC N‐terminal heptad repeat region that is responsible for its association with kinesin heavy chain. Despite these differing binding properties F12/E2 can co‐operatively enhance A36 association with KLC, particularly when using a KLC1‐KLC2 chimaera that resembles several KLC1 spliceforms and can bind A36 and F12/E2 efficiently. This is the first example of a pathogen encoding multiple proteins that co‐operatively associate with kinesin‐1.

Lemur tyrosine kinase 2 (LMTK2) is a determinant of cell sensitivity to apoptosis by regulating the levels of the BCL2 family members

Using a high-throughput approach, we identified lemur tyrosine kinase 2 (LMTK2) as a novel determinant of cell sensitivity to TRAIL. LMTK2 is a poorly characterized serine/threonine kinase believed to play a role in endosomal membrane trafficking and neuronal physiology, and recently found to be mutated in diverse tumor types. We show that LMTK2 silencing sensitizes immortalized epithelial cells and cancer cells to TRAIL, and this phenomenon is accompanied by changes in the expression of BCL2 family members. In epithelial cells, LMTK2 targeting causes the down-regulation of the BCL2 and BCL-xL anti-apoptotic proteins and the reciprocal up-regulation of the pro-apoptotic protein BIM, while, in cancer cells, LMTK2 knock-down reduces BCL2 without increasing BIM levels. We provide evidence that both BIM and BCL2 proteins are regulated by LMTK2 in a GSK3β- and PP1A-dependent manner and that their perturbation, together with BCL-xL reduction, determines an increased sensitivity not only to TRAIL, but also to other compounds. Overall, our findings suggest a broad function of LMTK2 in the regulation of the apoptotic pathway and highlight LMTK2 as a novel candidate target to increase the cytotoxic activity of chemotherapeutic compounds.

Regulation of Axonal Transport by Protein Kinases

The intracellular transport of organelles, proteins, lipids, and RNA along the axon is essential for neuronal function and survival. This process, called axonal transport, is mediated by two classes of ATP-dependent motors, kinesins, and cytoplasmic dynein, which carry their cargoes along microtubule tracks. Protein kinases regulate axonal transport through direct phosphorylation of motors, adapter proteins, and cargoes, and indirectly through modification of the microtubule network. The misregulation of axonal transport by protein kinases has been implicated in the pathogenesis of several nervous system disorders. Here, we review the role of protein kinases acting directly on axonal transport and discuss how their deregulation affects neuronal function, paving the way for the exploitation of these enzymes as novel drug targets.

Regulatory Crosstalk by Protein Kinases on CFTR Trafficking and Activity

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a member of the ATP binding cassette (ABC) transporter superfamily that functions as a cAMP-activated chloride ion channel in fluid-transporting epithelia. There is abundant evidence that CFTR activity (i.e., channel opening and closing) is regulated by protein kinases and phosphatases via phosphorylation and dephosphorylation. Here, we review recent evidence for the role of protein kinases in regulation of CFTR delivery to and retention in the plasma membrane. We review this information in a broader context of regulation of other transporters by protein kinases because the overall functional output of transporters involves the integrated control of both their number at the plasma membrane and their specific activity. While many details of the regulation of intracellular distribution of CFTR and other transporters remain to be elucidated, we hope that this review will motivate research providing new insights into how protein kinases control membrane transport to impact health and disease.

Signal dependent ER export of lemur tyrosine kinase 2

BACKGROUND

The membrane anchored kinase, LMTK2, is a serine/threonine kinase predominantly localized to endosomal compartments. LMTK2 has been shown to be involved in the trafficking of the CFTR ion channel, the androgen receptor, as well as modulating neurodegeneration. As a membrane anchored protein, LMTK2 must be exported from the ER, yet the mechanisms whereby LMTK2 is sequestered within the ER for efficient export are unknown.

METHODS

Sequence analysis of the carboxyl tail of LMTK2 revealed a putative di-acidic ER export motif. Site-directed mutagenesis was utilized to ablate this potential motif. Subcellular fractionation, immunofluorescence microscopy, and transferrin recycling assays were used to determine the consequence of mutating LMTK2's export motif.

RESULTS

Mutation of the di-acidic export motif led to ER retention of LMTK2, and an increase in protein half-life and a concomitant loss of LMTK2 from its appropriate terminal destination. Loss of LMTK2 from endosomal compartments by preventing its release from the ER is linked to a reduction in transferrin recycling.

CONCLUSIONS

We have identified a di-acidic ER export motif within the carboxyl tail of the membrane anchored kinase LMTK2. This sequence is used by LMTK2 for its efficient export from the ER.

Reduced number of axonal mitochondria and tau hypophosphorylation in mouse P301L tau knockin neurons

Expression of the frontotemporal dementia-related tau mutation, P301L, at physiological levels in adult mouse brain (KI-P301L mice) results in overt hypophosphorylation of tau and age-dependent alterations in axonal mitochondrial transport in peripheral nerves. To determine the effects of P301L tau expression in the central nervous system, we examined the kinetics of mitochondrial axonal transport and tau phosphorylation in primary cortical neurons from P301L knock-in (KI-P301L) mice. We observed a significant 50% reduction in the number of mitochondria in the axons of cortical neurons cultured from KI-P301L mice compared to wild-type neurons. Expression of murine P301L tau did not change the speed, direction of travel or likelihood of movement of mitochondria. Notably, the angle that defines the orientation of the mitochondria in the axon, and the volume of individual moving mitochondria, were significantly increased in neurons expressing P301L tau. We found that murine tau phosphorylation in KI-P301L mouse neurons was diminished and the ability of P301L tau to bind to microtubules was also reduced compared to tau in wild-type neurons. The P301L mutation did not influence the ability of murine tau to associate with membranes in cortical neurons or in adult mouse brain. We conclude that P301L tau is associated with mitochondrial changes and causes an early reduction in murine tau phosphorylation in neurons coupled with impaired microtubule binding of tau. These results support the association of mutant tau with detrimental effects on mitochondria and will be of significance for the pathogenesis of tauopathies.

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Expression of P301L tau reduces the number of axonal mitochondria in mice.

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Motile mitochondria exhibit increased volume in axons of neurons with P301L tau.

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P301L tau expressed in knockin mice is hypophosphorylated.

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The P301L tau mutation impairs microtubule binding but does not affect tau membrane localization.

Vaccinia virus protein complex F12/E2 interacts with kinesin light chain isoform 2 to engage the kinesin-1 motor complex

During vaccinia virus morphogenesis, intracellular mature virus (IMV) particles are wrapped by a double lipid bilayer to form triple enveloped virions called intracellular enveloped virus (IEV). IEV are then transported to the cell surface where the outer IEV membrane fuses with the cell membrane to expose a double enveloped virion outside the cell. The F12, E2 and A36 proteins are involved in transport of IEVs to the cell surface. Deletion of the F12L or E2L genes causes a severe inhibition of IEV transport and a tiny plaque size. Deletion of the A36R gene leads to a smaller reduction in plaque size and less severe inhibition of IEV egress. The A36 protein is present in the outer membrane of IEVs, and over-expressed fragments of this protein interact with kinesin light chain (KLC). However, no interaction of F12 or E2 with the kinesin complex has been reported hitherto. Here the F12/E2 complex is shown to associate with kinesin-1 through an interaction of E2 with the C-terminal tail of KLC isoform 2, which varies considerably between different KLC isoforms. siRNA-mediated knockdown of KLC isoform 1 increased IEV transport to the cell surface and virus plaque size, suggesting interaction with KLC isoform 1 is somehow inhibitory of IEV transport. In contrast, knockdown of KLC isoform 2 did not affect IEV egress or plaque formation, indicating redundancy in virion egress pathways. Lastly, the enhancement of plaque size resulting from loss of KLC isoform 1 was abrogated by removal of KLC isoforms 1 and 2 simultaneously. These observations suggest redundancy in the mechanisms used for IEV egress, with involvement of KLC isoforms 1 and 2, and provide evidence of interaction of F12/E2 complex with the kinesin-1 complex.

Viruses often hijack the cellular transport systems to facilitate their movement within and between cells. Vaccinia virus (VACV), the smallpox vaccine, is very adept at this and exploits cellular transport machinery at several stages during its life cycle. For instance, during transport of new virus particles to the cell surface VACV interacts with a protein motor complex called kinesin-1 that moves cargo on microtubules. However, details of the cellular and viral components needed and the molecular mechanisms involved remain poorly understood. Hitherto, only the VACV protein A36 has been shown to interact with kinesin-1, however viruses lacking A36 still reach the cell surface, albeit at reduced efficiency, indicating other factors are involved. Here we describe an interaction between kinesin-1 and a complex of VACV proteins F12 and E2, which are both needed for virus transport. The F12/E2 complex associates with a subset of kinesin-1 molecules (kinesin light chain isoform 2) with a region thought to be involved in modulation of cargo binding and kinesin-1 motor activity. Further study of this interaction will enhance understanding of the VACV life cycle and of the roles of different kinesin-1 subtypes in cellular processes and the mechanisms that regulate them.

Interactome analysis reveals that FAM161A, deficient in recessive retinitis pigmentosa, is a component of the Golgi-centrosomal network

Defects in FAM161A, a protein of unknown function localized at the cilium of retinal photoreceptor cells, cause retinitis pigmentosa, a form of hereditary blindness. By using different fragments of this protein as baits to screen cDNA libraries of human and bovine retinas, we defined a yeast two-hybrid-based FAM161A interactome, identifying 53 bona fide partners. In addition to statistically significant enrichment in ciliary proteins, as expected, this interactome revealed a substantial bias towards proteins from the Golgi apparatus, the centrosome and the microtubule network. Validation of interaction with key partners by co-immunoprecipitation and proximity ligation assay confirmed that FAM161A is a member of the recently recognized Golgi-centrosomal interactome, a network of proteins interconnecting Golgi maintenance, intracellular transport and centrosome organization. Notable FAM161A interactors included AKAP9, FIP3, GOLGA3, KIFC3, KLC2, PDE4DIP, NIN and TRIP11. Furthermore, analysis of FAM161A localization during the cell cycle revealed that this protein followed the centrosome during all stages of mitosis, likely reflecting a specific compartmentalization related to its role at the ciliary basal body during the G0 phase. Altogether, these findings suggest that FAM161A's activities are probably not limited to ciliary tasks but also extend to more general cellular functions, highlighting possible novel mechanisms for the molecular pathology of retinal disease.

High expression of kinesin light chain-2, a novel target of miR-125b, is associated with poor clinical outcome of elderly non-small-cell lung cancer patients

Background:

MiR-125b has critical role in non-small-cell lung cancer (NSCLC) cell migration, and its target genes have not been elucidated. Kinesin-1 light chain (KLC)-2 was predicted as one of miR-125b's targets by bioinformatics analysis. This study is to identify the function of KLC2 and its interaction with miR-125b in NSCLC.

Methods:

Kinesin-1 light chain-2 protein expression and its clinical relevance were analysed in 140 matched NSCLC and adjacent non-neoplastic lung tissues. Both KLC2 gain- and loss-of-function analyses were performed in NSCLC cell lines by transient transfection. The direct interaction between KLC2 and miR-125b was confirmed by a luciferase reporter assay and a transient co-transfection assay as well as an analysis of eight matched clinical samples.

Results:

KLC2 protein was upregulated in NSCLC cell lines and tissues, and was an independent predictor of poor prognosis for elderly NSCLC patients. Kinesin-1 light chain-2 remarkably enhanced the invasive and migratory ability of NSCLC cells. MiR-125b inhibited KLC2 3′-untranslated region luciferase activity and protein expression, and inversely correlated with KLC2 expression in clinical samples. Kinesin-1 light chain-2 almost completely reversed miR-125b-induced inhibition on migration and invasion.

Conclusions:

Kinesin-1 light chain-2 protein overexpression predicts poor survival in elderly NSCLC patients. Kinesin-1 light chain-2 acts as a proto-oncogene and a functional target of miR-125b in NSCLC cells.

In silico functional pathway annotation of 86 established prostate cancer risk variants

Heritability is one of the strongest risk factors of prostate cancer, emphasizing the importance of the genetic contribution towards prostate cancer risk. To date, 86 established prostate cancer risk variants have been identified by genome-wide association studies (GWAS). To determine if these risk variants are located near genes that interact together in biological networks or pathways contributing to prostate cancer initiation or progression, we generated gene sets based on proximity to the 86 prostate cancer risk variants. We took two approaches to generate gene lists. The first strategy included all immediate flanking genes, up- and downstream of the risk variant, regardless of distance from the index variant, and the second strategy included genes closest to the index GWAS marker and to variants in high LD (r2 ≥0.8 in Europeans) with the index variant, within a 100 kb window up- and downstream. Pathway mapping of the two gene sets supported the importance of the androgen receptor-mediated signaling in prostate cancer biology. In addition, the hedgehog and Wnt/β-catenin signaling pathways were identified in pathway mapping for the flanking gene set. We also used the HaploReg resource to examine the 86 risk loci and variants high LD (r2 ≥0.8) for functional elements. We found that there was a 12.8 fold (p = 2.9 x 10-4) enrichment for enhancer motifs in a stem cell line and a 4.4 fold (p = 1.1 x 10-3) enrichment of DNase hypersensitivity in a prostate adenocarcinoma cell line, indicating that the risk and correlated variants are enriched for transcriptional regulatory motifs. Our pathway-based functional annotation of the prostate cancer risk variants highlights the potential regulatory function that GWAS risk markers, and their highly correlated variants, exert on genes. Our study also shows that these genes may function cooperatively in key signaling pathways in prostate cancer biology.

Phosphoprotein phosphatase 1-interacting proteins as therapeutic targets in prostate cancer

Prostate cancer is a major public health concern worldwide, being one of the most prevalent cancers in men. Great improvements have been made both in terms of early diagnosis and therapeutics. However, there is still an urgent need for reliable biomarkers that could overcome the lack of cancer-specificity of prostate-specific antigen, as well as alternative therapeutic targets for advanced metastatic cases. Reversible phosphorylation of proteins is a post-translational modification critical to the regulation of numerous cellular processes. Phosphoprotein phosphatase 1 (PPP1) is a major serine/threonine phosphatase, whose specificity is determined by its interacting proteins. These interactors can be PPP1 substrates, regulators, or even both. Deregulation of this protein-protein interaction network alters cell dynamics and underlies the development of several cancer hallmarks. Therefore, the identification of PPP1 interactome in specific cellular context is of crucial importance. The knowledge on PPP1 complexes in prostate cancer remains scarce, with only 4 holoenzymes characterized in human prostate cancer models. However, an increasing number of PPP1 interactors have been identified as expressed in human prostate tissue, including the tumor suppressors TP53 and RB1. Efforts should be made in order to identify the role of such proteins in prostate carcinogenesis, since only 26 have yet well-recognized roles. Here, we revise literature and human protein databases to provide an in-depth knowledge on the biological significance of PPP1 complexes in human prostate carcinogenesis and their potential use as therapeutic targets for the development of new therapies for prostate cancer.

LMTK2-mediated phosphorylation regulates CFTR endocytosis in human airway epithelial cells

Cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl⁻-selective ion channel expressed in fluid-transporting epithelia. Lemur tyrosine kinase 2 (LMTK2) is a transmembrane protein with serine and threonine but not tyrosine kinase activity. Previous work identified CFTR as an in vitro substrate of LMTK2, suggesting a functional link. Here we demonstrate that LMTK2 co-immunoprecipitates with CFTR and phosphorylates CFTR-Ser⁷³⁷ in human airway epithelial cells. LMTK2 knockdown or expression of inactive LMTK2 kinase domain increases cell surface density of CFTR by attenuating its endocytosis in human airway epithelial cells. Moreover, LMTK2 knockdown increases Cl⁻ secretion mediated by the wild-type and rescued ΔF508-CFTR. Compared with the wild-type CFTR, the phosphorylation-deficient mutant CFTR-S737A shows increased cell surface density and decreased endocytosis. These results demonstrate a novel mechanism of the phospho-dependent inhibitory effect of CFTR-Ser⁷³⁷ mediated by LMTK2 via endocytosis and inhibition of the cell surface density of CFTR Cl⁻ channels. These data indicate that targeting LMTK2 may increase the cell surface density of CFTR Cl⁻ channels and improve stability of pharmacologically rescued ΔF508-CFTR in patients with cystic fibrosis.

Translational profiles of medullary myofibroblasts during kidney fibrosis

Myofibroblasts secrete matrix during chronic injury, and their ablation ameliorates fibrosis. Development of new biomarkers and therapies for CKD will be aided by a detailed analysis of myofibroblast gene expression during the early stages of fibrosis. However, dissociating myofibroblasts from fibrotic kidney is challenging. We therefore adapted translational ribosome affinity purification (TRAP) to isolate and profile mRNA from myofibroblasts and their precursors during kidney fibrosis. We generated and characterized a transgenic mouse expressing an enhanced green fluorescent protein (eGFP)-tagged L10a ribosomal subunit protein under control of the collagen1α1 promoter. We developed a one-step procedure for isolation of polysomal RNA from collagen1α1-eGFPL10a mice subject to unilateral ureteral obstruction and analyzed and validated the resulting transcriptional profiles. Pathway analysis revealed strong gene signatures for cell proliferation, migration, and shape change. Numerous novel genes and candidate biomarkers were upregulated during fibrosis, specifically in myofibroblasts, and we validated these results by quantitative PCR, in situ, and Western blot analysis. This study provides a comprehensive analysis of early myofibroblast gene expression during kidney fibrosis and introduces a new technique for cell-specific polysomal mRNA isolation in kidney injury models that is suited for RNA-sequencing technologies.

Releasing the brake: restoring fast axonal transport in neurodegenerative disorders

Emerging evidence suggests that the dysregulation of fast axonal transport (FAT) plays a crucial role in several neurodegenerative disorders. Some of these diseases are caused by mutations affecting the molecular motors or adaptors that mediate FAT, and transport defects in organelles such as mitochondria and vesicles are observed in most, if not all neurodegenerative disorders. The relationship between neurodegenerative disorders and FAT is probably due to the extreme polarization of neurons, which extend long processes such as axons and dendrites. These characteristics render neurons particularly sensitive to transport alterations. Here we review the impact of such alterations on neuronal survival. We also discuss various strategies that might restore FAT, potentially slowing disease progression.

The importance of tau phosphorylation for neurodegenerative diseases

Fibrillar deposits of highly phosphorylated tau are a key pathological feature of several neurodegenerative tauopathies including Alzheimer's disease (AD) and some frontotemporal dementias. Increasing evidence suggests that the presence of these end-stage neurofibrillary lesions do not cause neuronal loss, but rather that alterations to soluble tau proteins induce neurodegeneration. In particular, aberrant tau phosphorylation is acknowledged to be a key disease process, influencing tau structure, distribution, and function in neurons. Although typically described as a cytosolic protein that associates with microtubules and regulates axonal transport, several additional functions of tau have recently been demonstrated, including roles in DNA stabilization, and synaptic function. Most recently, studies examining the trans-synaptic spread of tau pathology in disease models have suggested a potential role for extracellular tau in cell signaling pathways intrinsic to neurodegeneration. Here we review the evidence showing that tau phosphorylation plays a key role in neurodegenerative tauopathies. We also comment on the tractability of altering phosphorylation-dependent tau functions for therapeutic intervention in AD and related disorders.

A dynein motor attachment complex regulates TGFß/Smad3 signaling

Our previous results have demonstrated that km23-2 has functions in TGFß signaling that are distinct from those for km23-1. In the current report, we demonstrate that blockade of km23-2 decreased TGFß activation of the human Smad7 promoter Smad7-Luc, an endogenous Smad3-target promoter. Luminescence-based mammalian interaction mapping (LUMIER) analyses showed that TGFß stimulated the interaction of km23-2 preferentially with Smad3, relative to that with Smad2. Size exclusion chromatography experiments revealed that km23-2 and Smad3 were recruited into the same complex after TGFß treatment. Moreover, in the presence of TGFß, but not in the absence, km23-2 was present in early endosomes with the TGFß receptors (TßRs) and Smad3. Collectively, our data indicate that km23-2 is a critical signaling intermediate in a Smad3-dependent TGFß signaling pathway. We also provide evidence of the novel finding that TGFß stimulates the rapid recruitment of the km23-2 dimer to the dynein intermediate chain (DIC) of the dynein complex, whereas a kinase-deficient form of TßRII prevented this interaction. Finally, we demonstrate for the first time that TGFß stimulated not only assembly of the dynein motor attachment complex, but also triggered the tethering of the km23-2-Smad3 cargo to the other dynein components. Thus, our data demonstrate a novel function for km23-2 as a motor receptor to recruit Smad3 to the dynein complex for intracellular transport, thereby mediating Smad3-dependent TGFß signaling.

Targeting of CRMP-2 to the primary cilium is modulated by GSK-3β

CRMP-2 plays a pivotal role in promoting axon formation, neurite outgrowth and elongation in neuronal cells. CRMP-2′s role in other cells is unknown. Our preliminary results showed CRMP-2 expression in cilia of fibroblasts. To localize CRMP-2, define its role and study the regulation of CRMP-2′s expression in cilia we carried out the following experiments. We find that in fibroblasts CRMP-2 localizes to the centrosome and is associated with the basal body and -at a low level- is present in primary cilia. Phosphorylated pCRMP-2 can only be detected at the basal body. RNAi knockdown of CRMP-2 interfered with primary cilium assembly demonstrating a critical requirement for CRMP-2. Deletion analysis of CRMP-2 identified a 51 amino acid sequence in the C-terminus that is required for targeting to the basal body and primary cilium. This domain contains GSK-3β phosphorylation sites as well as two repeats of the VxPx motif, previously identified as a cilium targeting signal in other primary cilium proteins. To our surprise, mutation of the CRMP-2 VxPx motifs did not eliminate primary cilium targeting. Instead, mutation of the GSK-3β phosphorylation sites abolished CRMP-2 targeting to the primary cilium without affecting basal body localization. Treatment of cells with lithium, a potent GSK-3β inhibitor, or with two specific GSK-3β inhibitors (the L803-mts peptide inhibitor and CHIR99021) resulted in cilium elongation and decreased basal body levels of pCRMP-2 as well as increased levels of total CRMP-2 at the primary cilium. In summary, we identified CRMP-2 as a protein critically involved in primary cilia formation. To our knowledge this is the first demonstration of modulation of primary cilium targeting by GSK-3β.

Determination of the membrane topology of lemur tyrosine kinase 2 (LMTK2) by fluorescence protease protection

Lemur tyrosine kinase 2 (LMTK2) is a novel membrane-anchored kinase reported to be involved in several normal and pathophysiological conditions, including endosomal membrane recycling, prostate cancer, and neurodegeneration. In this study, we have investigated the topology and orientation of LMTK2 within cellular membranes utilizing fluorescence protease protection. Appending the green fluorescent protein to either the amino or carboxyl terminus of LMTK2, we were able to determine which side of intracellular membrane these regions were located. Our results indicate that LMTK2 is an integral membrane protein in which both the amino and carboxyl termini are exposed to the cytoplasm. Moreover, this topology places the kinase active site within the cytoplasm.