A functional role for intrinsic disorder in the tau-tubulin complex.

Tau is an intrinsically disordered protein with an important role in maintaining the dynamic instability of neuronal microtubules. Despite intensive study, a detailed understanding of the functional mechanism of tau is lacking. Here, we address this deficiency by using intramolecular single-molecule Förster Resonance Energy Transfer (smFRET) to characterize the conformational ensemble of tau bound to soluble tubulin heterodimers. Tau adopts an open conformation on binding tubulin, in which the long-range contacts between both termini and the microtubule binding region that characterize its compact solution structure are diminished. Moreover, the individual repeats within the microtubule binding region that directly interface with tubulin expand to accommodate tubulin binding, despite a lack of extension in the overall dimensions of this region. These results suggest that the disordered nature of tau provides the significant flexibility required to allow for local changes in conformation while preserving global features. The tubulin-associated conformational ensemble is distinct from its aggregation-prone one, highlighting differences between functional and dysfunctional states of tau. Using constraints derived from our measurements, we construct a model of tubulin-bound tau, which draws attention to the importance of the role of tau's conformational plasticity in function.

Association of Patient Characteristics and Tumor Genomics With Clinical Outcomes Among Patients With Non-Small Cell Lung Cancer Using a Clinicogenomic Database.

Importance: Data sets linking comprehensive genomic profiling (CGP) to clinical outcomes may accelerate precision medicine. Objective: To assess whether a database that combines EHR-derived clinical data with CGP can identify and extend associations in non-small cell lung cancer (NSCLC). Design, Setting, and Participants: Clinical data from EHRs were linked with CGP results for 28 998 patients from 275 US oncology practices. Among 4064 patients with NSCLC, exploratory associations between tumor genomics and patient characteristics with clinical outcomes were conducted, with data obtained between January 1, 2011, and January 1, 2018. Exposures: Tumor CGP, including presence of a driver alteration (a pathogenic or likely pathogenic alteration in a gene shown to drive tumor growth); tumor mutation burden (TMB), defined as the number of mutations per megabase; and clinical characteristics gathered from EHRs. Main Outcomes and Measures: Overall survival (OS), time receiving therapy, maximal therapy response (as documented by the treating physician in the EHR), and clinical benefit rate (fraction of patients with stable disease, partial response, or complete response) to therapy. Results: Among 4064 patients with NSCLC (median age, 66.0 years; 51.9% female), 3183 (78.3%) had a history of smoking, 3153 (77.6%) had nonsquamous cancer, and 871 (21.4%) had an alteration in EGFR, ALK, or ROS1 (701 [17.2%] with EGFR, 128 [3.1%] with ALK, and 42 [1.0%] with ROS1 alterations). There were 1946 deaths in 7 years. For patients with a driver alteration, improved OS was observed among those treated with (n = 575) vs not treated with (n = 560) targeted therapies (median, 18.6 months [95% CI, 15.2-21.7] vs 11.4 months [95% CI, 9.7-12.5] from advanced diagnosis; P < .001). TMB (in mutations/Mb) was significantly higher among smokers vs nonsmokers (8.7 [IQR, 4.4-14.8] vs 2.6 [IQR, 1.7-5.2]; P < .001) and significantly lower among patients with vs without an alteration in EGFR (3.5 [IQR, 1.76-6.1] vs 7.8 [IQR, 3.5-13.9]; P < .001), ALK (2.1 [IQR, 0.9-4.0] vs 7.0 [IQR, 3.5-13.0]; P < .001), RET (4.6 [IQR, 1.7-8.7] vs 7.0 [IQR, 2.6-13.0]; P = .004), or ROS1 (4.0 [IQR, 1.2-9.6] vs 7.0 [IQR, 2.6-13.0]; P = .03). In patients treated with anti-PD-1/PD-L1 therapies (n = 1290, 31.7%), TMB of 20 or more was significantly associated with improved OS from therapy initiation (16.8 months [95% CI, 11.6-24.9] vs 8.5 months [95% CI, 7.6-9.7]; P < .001), longer time receiving therapy (7.8 months [95% CI, 5.5-11.1] vs 3.3 months [95% CI, 2.8-3.7]; P < .001), and increased clinical benefit rate (80.7% vs 56.7%; P < .001) vs TMB less than 20. Conclusions and Relevance: Among patients with NSCLC included in a longitudinal database of clinical data linked to CGP results from routine care, exploratory analyses replicated previously described associations between clinical and genomic characteristics, between driver mutations and response to targeted therapy, and between TMB and response to immunotherapy. These findings demonstrate the feasibility of creating a clinicogenomic database derived from routine clinical experience and provide support for further research and discovery evaluating this approach in oncology.

Tau mutants bind tubulin heterodimers with enhanced affinity.

Tau is a microtubule binding protein that forms pathological aggregates in the brain in Alzheimer's disease and other tauopathies. Disease etiology is thought to arise from loss of native interactions between tau and microtubules, as well as from gain of toxicity tied to tau aggregation, although neither mechanism is well understood. Here we investigate the link between function and disease using disease-associated and disease-motivated mutants of tau. We find that mutations to highly conserved proline residues in repeats 2 and 3 of the microtubule binding domain have differential effects on tau binding to tubulin and the capacity of tau to enhance tubulin polymerization. Notably, mutations to these residues result in an increased affinity for tubulin dimers while having a negligible effect on binding to stabilized microtubules. We measure conformational changes in tau on binding to tubulin that provide a structural framework for the observed altered affinity and function. Additionally, we find that these mutations do not necessarily enhance aggregation, which could have important implications for tau therapeutic strategies that focus solely on searching for tau aggregation inhibitors. We propose a model that describes tau binding to tubulin dimers and a mechanism by which disease-relevant alterations to tau impact its function. Together, these results draw attention to the interaction between tau and free tubulin as playing an important role in mechanisms of tau pathology.

Fluorescence correlation spectroscopy reveals highly efficient cytosolic delivery of certain penta-arg proteins and stapled peptides.

We used fluorescence correlation spectroscopy (FCS) to accurately and precisely determine the relative efficiencies with which three families of "cell-penetrating peptides" traffic to the cytosol of mammalian cells. We find that certain molecules containing a "penta-arg" motif reach the cytosol, intact, with efficiencies greater than 50%. This value is at least 10-fold higher than that observed for the widely studied cationic sequence derived from HIV Tat or polyarginine Arg8, and equals that of hydrocarbon-stapled peptides that are active in cells and animals. Moreover, we show that the efficiency with which stapled peptides reach the cytosol, as determined by FCS, correlates directly with their efficacy in cell-based assays. We expect that these findings and the associated technology will aid the design of peptides, proteins, and peptide mimetics that predictably and efficiently reach the interior of mammalian cells.

Inhibition of calpain but not caspase activity by spectrin fragments.

Calpains and caspases are ubiquitous cysteine proteases that are associated with a variety of cellular pathways. Calpains are involved in processes such as long term potentiation, cell motility and apoptosis, and have been shown to cleave non-erythroid (brain) alpha- and beta-spectrin and erythroid beta-spectrin. The cleavage of erythroid alpha-spectrin by calpain has not been reported. Caspases play an important role in the initiation and execution of apoptosis, and have been shown to cleave non-erythroid but not erythroid spectrin. We have studied the effect of spectrin fragments on calpain and caspase activities. The erythroid and non-erythroid spectrin fragments used were from the N-terminal region of alpha-spectrin, and C-terminal region of beta-spectrin, both consisting of regions involved in spectrin tetramer formation. We observed that the all spectrin fragments exhibited a concentration-dependent inhibitory effect on calpain, but not caspase activity. It is clear that additional studies are warranted to determine the physiological significance of calpain inhibition by spectrin fragments. Our findings suggest that calpain activity is modulated by the presence of spectrin partial domains at the tetramerization site. It is not clear whether the inhibitory effect is substrate specific or is a general effect. Further studies of this inhibitory effect may lead to the identification and development of new therapeutic agents specifically for calpains, but not for caspases. Proteins/peptides with a coiled coil helical conformation should be studied for potential inhibitory effects on calpain activity.

Investigating the mechanism of disease in the RP10 form of retinitis pigmentosa.

Retinitis pigmentosa (RP) is a disease characterized by its vast heterogeneity. Many genes are associated with RP, and the disease causing mutations identified in these genes are even more numerous. To date there are 15 genes that cause autosomal dominant RP (adRP) alone. The role of some of these genes, while complex and not completely understood, is somewhat intuitive in that they are involved in pathways such as phototransduction. However, the role of other genes in retinal disease is not as predictable due to their ubiquitous function and/or expression. One such gene is inosine monophosphate dehydrogenase 1 (IMPDH1) IMPDH1 is a gene involved in de novo purine synthesis and is ubiquitously expressed. IMPDH1 mutations account for 2% of all adRP cases and are a rare cause of Leiber Congenital Amaurosis. Despite its ubiquitous expression missense mutations in this gene cause only retinal degeneration. This paradox of tissue specific disease in the presence of ubiquitous expression has only recently begun to be explained. We have shown in a recent study that novel retinal isoforms of IMPDH1 exist and may account for the tissue specificity of disease. We have gone on to characterize these retinal isoforms both in our laboratory and in collaboration with Dr. Lizbeth Hedstrom's laboratory at Brandeis University (Waltham, MA) in order to understand more about them. We believe that through clarifying the mechanism of disease in RP10 we will be equipped to consider treatment options for this disease.

Quantification of protein delivery in live cells using fluorescence correlation spectroscopy.

Fluorescence correlation spectroscopy (FCS) is a quantitative single-molecule method that measures the concentration and rate of diffusion of fluorophore-tagged molecules, both large and small, in vitro and within live cells, and even within discrete cellular compartments. FCS is exceptionally well-suited to directly quantify the efficiency of intracellular protein delivery-specifically, how well different "cell-penetrating" proteins and peptides guide proteinaceous materials into the cytosol and nuclei of live mammalian cells. This article provides an overview of the procedures necessary to execute robust FCS experiments and evaluate endosomal escape efficiencies: preparation of fluorophore-tagged proteins, incubation with mammalian cells and preparation of FCS samples, setup and execution of an FCS experiment, and a detailed discussion of and custom MATLAB® script for analyzing the resulting autocorrelation curves in the context of appropriate diffusion models.

Retinal isoforms of inosine 5'-monophosphate dehydrogenase type 1 are poor nucleic acid binding proteins.

The RP 10 form of autosomal dominant retinitis pigmentosa (adRP) is caused by mutations in the widely expressed protein inosine 5'-monophosphate dehydrogenase type 1 (IMPDH1). These mutations have no effect on the enzymatic activity of IMPDH1, but do perturb the association of IMPDH1 with nucleic acids. Two newly discovered retinal-specific isoforms, IMPDH1(546) and IMPDH1(595), may provide the key to the photoreceptor specificity of disease [S.J. Bowne, Q. Liu, L.S. Sullivan, J. Zhu, C.J. Spellicy, C.B. Rickman, E.A. Pierce, S.P. Daiger, Invest. Ophthalmol. Vis. Sci. 47 (2006) 3754-3765]. Here we express and characterize the normal IMPDH1(546) and IMPDH1(595), together with their adRP-linked variants, D226N. The enzymatic activity of the purified IMPDH1(546), IMPDH1(595) and the D226N variants is indistinguishable from the canonical form. The intracellular distribution of IMPDH1(546) and IMPDH1(595) is also similar to the canonical IMPDH1 and unaffected by the D226N mutation. However, unlike the canonical IMPDH1, the retinal specific isoforms do not bind significant fractions of a random pool of oligonucleotides. This observation indicates that the C-terminal extension unique to the retinal isoforms blocks the nucleic acid binding site of IMPDH1, and thus uniquely regulates protein function within photoreceptors.

Cross-Scale Integrin Regulation Organizes ECM and Tissue Topology.

The diverse morphologies of animal tissues are underlain by different configurations of adherent cells and extracellular matrix (ECM). Here, we elucidate a cross-scale mechanism for tissue assembly and ECM remodeling involving Cadherin 2, the ECM protein Fibronectin, and its receptor Integrin α5. Fluorescence cross-correlation spectroscopy within the zebrafish paraxial mesoderm mesenchyme reveals a physical association between Integrin α5 on adjacent cell membranes. This Integrin-Integrin complex correlates with conformationally inactive Integrin. Cadherin 2 stabilizes both the Integrin association and inactive Integrin conformation. Thus, Integrin repression within the adherent mesenchymal interior of the tissue biases Fibronectin fibrillogenesis to the tissue surface lacking cell-cell adhesions. Along nascent somite boundaries, Cadherin 2 levels decrease, becoming anti-correlated with levels of Integrin α5. Simultaneously, Integrin α5 clusters and adopts the active conformation and then commences ECM assembly. This cross-scale regulation of Integrin activation organizes a stereotypic pattern of ECM necessary for vertebrate body elongation and segmentation.