Abundant expression of the microtubule-associated protein, ensconsin (E-MAP-115), alters the cellular response to Taxol

On the need for standardized reporting of photophysical parameters of in vitro photodynamic therapy studies

In vitro dose escalation experiments are one of the first gatekeepers in therapeutic evaluation and development. This also holds for evaluating novel photosensitizers (PS) and Photodynamic Therapy (PDT) co-therapies as needed to provide dose response guidelines before engaging in further pre-clinical studies. The dose needed to achieve 50% cell kill (LD₅₀) is a standard metric to report the potency of a therapeutic agents that is widely accepted for single-drug therapies. In reporting results of PDT experiments, which involve delivery of both drug and light, it is inherently more complicated to identify such a convenient dose response metric that actually captures the larger space of treatment parameters. In addition to ubiquitous sources of biological variability that apply broadly in biomedical research, PDT treatment efficacy is determined by multiple key parameters that may or may not have been documented, including PS concentration and light fluence, where the latter is itself a function of the spectral properties of the light source used (often not described), not to mention dose rate, fractionation and other parameters that potentially vary between individual studies. It is impossible to compare results between two study when, for example one reports LD₅₀ PS concentration without providing essential light dosimetry details. Motivated by this challenge in comparing outcomes and establishing reproducibility of in vitro PDT studies, we endeavored to perform a meta-analysis of the reporting of PDT results by converting, where possible, the disparately reported experimental details into a consistent metric that could be used to compare across studies. In this context we adopt here the number of photons absorbed by photosensitizers per unit volume to affect a 50% decline in cell survival as a standardized metric. By choosing this metric one can acknowledge the quantum-based generation of cytotoxins. While this metric does not cover every possible source of variability between any two studies, for a PS with known optical properties, this does encapsulate PS concentration as well as irradiance and spectral properties of light delivered. For the sake of focus we adopt this approach for study of reported results with two photosensitizers, Protoporphyrin IX, either synthesized in the cells by aminolevulinic acid or administered exogenously, and Chlorin e6. A literature search was performed to identify in vitro studies with these two photosensitizers and collect necessary information to calculate the absorbed photon LD₅₀ threshold for each study. Only approximately 1/10 of the manuscripts reporting on in vitro studies provide the minimum required information to calculate the threshold values. While the majority of the determined threshold values are within a factor of 10, the range of threshold values spanned close to 7 orders of magnitude for both photosensitizers. To contrast with single-agent therapies, a similar exercise was performed for chemotherapeutic drugs targeting cellular mitosis or tyrosine kinase inhibitors resulted in an LD₅₀ or IC₅₀ range of 1-2 orders of magnitude, with LD₅₀ or IC₅₀ values for a single cell line being within a factor of 5. This review underscores challenges in the reporting of in vitro PDT efficacy. In many cases it takes considerable effort to extract the necessary methodology information to make meaningful comparison between PDT studies. Only when results between studies can be compared is it possible to begin to assess reproducibility which, as shown here, can be a major issue. Hence, guidelines need to be developed and enforced through the peer review process for meaningful reporting of preclinical PDT results in order for the most promising sensitizers and co-therapies to be identified and translated into the clinic.

MAP7 interacts with RC3H1 and cooperatively regulate cell-cycle progression of cervical cancer cells via activating the NF-κB signaling

Ensconsin is encoded by the MAP7 gene and belongs to the microtubule-associated proteins. This study aimed to explore its functional roles and partners in cell-cycle progression in cervical cancer. Data from the Cancer Genome Atlas-Cervical & Endocervical Cancer (TCGA-CESC) and the Genotype-Tissue Expression project were used for bioinformatic analysis. SiHa cells were used for in-vitro and in-vivo analysis. Co-immunoprecipitation (Co-IP) assay was conducted to explore the proteins interacted with MAP7. Results showed that MAP7 mRNA expression might serve as an independent biomarker of shorter survival. MAP7 overexpression elevated cyclin D1/cyclin B1 expression, facilitated cell-cycle progression and promoted SiHa cell growth in a xenograft tumor model. Co-IP experiments confirmed a novel interaction between MAP7 and RC3H1. Knockdown of either RC3H1 or MAP7 significantly attenuated cyclin D1/cyclin B1 upregulation, and cell-cycle progression induced by the other partner. MAP7 overexpression led to increased expression of P-IKK (Ser176/177) and P-p65 (Ser536). RC3H1 inhibition abrogated MAP7 induced upregulation of P-IKK and P-p65. Data in TCGA-CESC showed that MAP7 expression was positively correlated with its copy number segments, but was negatively correlated with the methylation level of three CpG sites within the gene locus. Demethylation treatment by 5-Aza-dC elevated both MAP7 mRNA and protein expression in a dose-dependent manner. In conclusion, this study revealed a novel interaction between MAP7 and RC3H1 in cervical cancer cells, which cooperatively enhanced cyclin D1/cyclin B1 expression and facilitated cell-cycle progression. These effects were at least partly mediated by activated canonical IKK/NF-kB signaling.

MAP7 Regulates Axon Collateral Branch Development in Dorsal Root Ganglion Neurons

Collateral branches from axons are key components of functional neural circuits that allow neurons to connect with multiple synaptic targets. Like axon growth and guidance, formation of collateral branches depends on the regulation of microtubules, but how such regulation is coordinated to ensure proper circuit development is not known. Based on microarray analysis, we have identified a role for microtubule-associated protein 7 (MAP7) during collateral branch development of dorsal root ganglion (DRG) sensory neurons. We show that MAP7 is expressed at the onset of collateral branch formation. Perturbation of its expression by overexpression or shRNA knockdown alters axon branching in cultured DRG neurons. Localization and time-lapse imaging analysis reveals that MAP7 is enriched at branch points and colocalizes with stable microtubules, but enters the new branch with a delay, suggesting a role in branch maturation. We have also investigated a spontaneous mutant mouse that expresses a truncated MAP7 and found a gain-of-function phenotype both in vitro and in vivo Further domain analysis suggests that the amino half of MAP7 is responsible for branch formation, suggesting a mechanism that is independent of its known interaction with kinesin. Moreover, this mouse exhibits increased pain sensitivity, a phenotype that is consistent with increased collateral branch formation. Therefore, our study not only uncovers the first neuronal function of MAP7, but also demonstrates the importance of proper microtubule regulation in neural circuit development. Furthermore, our data provide new insights into microtubule regulation during axonal morphogenesis and may shed light on MAP7 function in neurological disorders.SIGNIFICANCE STATEMENT Neurons communicate with multiple targets by forming axonal branches. In search of intrinsic factors that control collateral branch development, we identified a role for microtubule-associated protein 7 (MAP7) in dorsal root ganglion sensory neurons. We show that MAP7 expression is developmentally regulated and perturbation of this expression alters branch formation. Cell biological analysis indicates that MAP7 promotes branch maturation. Analysis of a spontaneous mouse mutant suggests a molecular mechanism for branch regulation and the potential influence of collateral branches on pain sensitivity. Our studies thus establish the first neuronal function of MAP7 and demonstrate its role in branch morphogenesis and neural circuit function. These findings may help in our understanding of the contribution of MAP7 to neurological disorders and nerve regeneration.

Src family kinases and paclitaxel sensitivity

Src-family Kinases (SFKs) participate in the regulation of proliferation, differentiation, apoptosis, autophagy, adhesion, migration, invasion and angiogenesis in normal and cancer cells. Abnormal expression of SFKs has been documented in cancers that arise in breast, colon, ovary, melanocyte, gastric mucosa, head and neck, pancreas, lung, and brain. Targeting SFKs in cancer cells has been shown to be a promising therapeutic strategy in solid tumors, particularly in ovarian, colon and breast cancers. Paclitaxel is one of most widely used chemotherapeutic agents for the management of ovarian, breast, lung and head/neck cancers. As a microtubule-stabilizing agent, paclitaxel possesses both mitosis-dependent and mitosis-independent activities against cancer cells. A variety of mechanisms such as deregulation of P-glycoprotein, alteration of tubulin isotypes, alteration of microtubule-regulatory proteins, deregulation of apoptotic signaling pathways, mutation of tubulins and overexpression of copper transporters have been implicated in the development of primary or secondary resistance to paclitaxel. By affecting cancer cell survival, proliferation, autophagy, microtubule stability, motility, and/or angiogenesis, SFKs interact with mechanisms that regulate paclitaxel sensitivity. Inhibition of SFKs can potentiate the anti-tumor activity of paclitaxel by enhancing apoptosis, autophagy and microtubule stability. Based on pre-clinical observations, administration of SFK inhibitors in combination with paclitaxel could improve treatment for ovarian, breast, lung and head/neck cancers. Identification and validation of predictive biomarkers could also permit personalization of the therapy.

Mechanisms of Taxol resistance related to microtubules

Since its approval by the FDA in 1992 for the treatment of ovarian cancer, the use of Taxol has dramatically increased. Although treatment with Taxol has led to improvement in the duration and quality of life for some cancer patients, the majority eventually develop progressive disease after initially responding to Taxol treatment. Drug resistance represents a major obstacle to improving the overall response and survival of cancer patients. This review focuses on mechanisms of Taxol resistance that occur directly at the microtubule, such as mutations, tubulin isotype selection and post-translational modifications, and also at the level of regulatory proteins. A review of tubulin structure, microtubule dynamics, the mechanism of action of Taxol and its binding site on the microtubule are included, so that the reader can evaluate Taxol resistance in context.

Microtubules are critical for radial glial morphology: possible regulation by MAPs and MARKs

Radial glia are a polarized cell type that in most neural regions appear only transiently during development. They have long been recognized as glia or glial progenitors that support neuronal migration. Recent evidence indicates that radial glia also give rise to neurons and appear to be a major population of dividing precursor cells in the embryonic cortical ventricular zone. Radial glia extend long processes from the ventricular zone to the pial surface that provide guides for neuronal migration. We reasoned that the unique morphology of radial glia is due to the composition and organization of their cytoskeleton. In this present study, we have used C6-R, a radial glial-like cell line and isolated perinatal cerebellar radial glia to ask what are the critical cytoskeletal elements in radial glial cells and how they are regulated. Treatments with nocodazole and cytochalasin D showed that microtubules, but not microfilaments, are critical for the polarized morphology of radial glia. In addition, quantitative real-time PCR indicated that certain mRNAs specific for microtubule-associated proteins (MAPs) are selectively expressed in radial glia. These results together with the known ability of microtubule affinity-regulating kinases to regulate microtubule organization suggest that microtubules and MAPs are critical for the morphology of radial glia.

Alteration of the C-terminal amino acid of tubulin specifically inhibits myogenic differentiation

Detyrosination is an evolutionarily conserved post-translational modification of microtubule polymers that is known to be enhanced during early morphological differentiation of cultured myogenic cells (Gundersen, G. G., Khawaja, S., and Bulinski, J. C. (1989) J. Cell Biol. 109, 2275-2288). We proposed that altering the C terminus of alpha-tubulin by detyrosination plays a role in morphological differentiation. To test our hypothesis, we treated L6 myoblasts with 3-nitrotyrosine (Eiserich, J. P., Estevez, A. G., Bamberg, T. V., Ye, Y. Z., Chumley, P. H., Beckman, J. S., and Freeman, B. A. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6365-6375), a nontoxic inhibitor that resulted in high level inhibition of microtubule detyrosination and low level incorporation of nitrotyrosine into microtubules. Even though microtubule stabilization or modification by acetylation still occurred normally, morphological differentiation was blocked; myoblasts neither elongated significantly nor fused. Nitrotyrosine treatment prevented synthesis or activation of markers of myogenic differentiation, including muscle-specific myosin, alpha-actin, integrin alpha(7), and myogenin. Consistent with this, myoblast integrin beta(1A) remained highly expressed. In contrast, the increase in beta-catenin level characteristic of early myogenesis was unaffected by treatment. These results show that the identity of the C-terminal residue of alpha-tubulin modulates microtubule activity, possibly because binding to or signaling from modified microtubules is required for the myogenic program.

Rapid dynamics of the microtubule binding of ensconsin in vivo

Microtubule-associated proteins (MAPs) are proteins that reversibly bind to and regulate microtubule dynamics and functions in vivo. We examined the dynamics of binding of a MAP called ensconsin (E-MAP-115) to microtubules in vivo. We used 5×GFP-EMTB, a construct in which the microtubule-binding domain of ensconsin (EMTB) is fused to five copies of green fluorescent protein (GFP), as a reporter molecule amenable to the use of fluorescent speckle microscopy. Fluorescent speckle microscopy (FSM) sequences and kymograph analyses showed rapid dynamics of speckles comprised of 5×GFP-EMTB in untreated cells. By contrast, in detergent-lysed cytoskeletons, speckles were not dynamic. Since detergent-lysed cytoskeletons differ from living cells in that they lack both ATP and dynamic microtubules, we used azide treatment to substantially reduce the level of ATP in living cells and we used Taxol to halt microtubule dynamics. Both treatments slowed the dynamics of 5×GFP-EMTB speckles observed by FSM. We also used fluorescence recovery after photobleaching (FRAP) to quantify the half-time of binding and dissociation of the 5×GFP-EMTB chimera and to compare this half-time to that of the full-length MAP molecule. In untreated cells, the tg of either 5×GFP-EMTB or full-length GFP-ensconsin was similarly rapid (∼4 seconds), while in ATP-reduced and Taxol-treated cells, tg was increased to 210 seconds and 40 seconds, respectively. In detergent-extracted cells no recovery was seen. Consistent with the rapid dynamics of 5×GFP-EMTB measured with fluorescent speckle microscopy and FRAP, we estimated that the affinity of the MAP for microtubules is ∼40 μM in untreated living cells, compared with ∼1 μM in vitro. However, KD,app was not significantly changed in the presence of azide and was increased to 110 μM in the presence of Taxol. To test whether changes in the phosphorylation state of cellular proteins might be responsible for altering the dynamics of ensconsin binding, we used FSM to monitor staurosporine-treated cells. Staurosporine treatment substantially halted dynamics of 5×GFP-EMTB speckles along MTs. Our results show that ensconsin is highly dynamic in its association with microtubules, and its microtubule association can be altered by in vivo phosphorylation events.