Raman Spectroscopy Reveals Photobiomodulation-Induced α-Helix to ß-Sheet Transition in Tubulins: Potential Implications for Alzheimer's and Other Neurodegenerative Diseases.

In small clinical studies, the application of transcranial photobiomodulation (PBM), which typically delivers low-intensity near-infrared (NIR) to treat the brain, has led to some remarkable results in the treatment of dementia and several neurodegenerative diseases. However, despite the extensive literature detailing the mechanisms of action underlying PBM outcomes, the specific mechanisms affecting neurodegenerative diseases are not entirely clear. While large clinical trials are warranted to validate these findings, evidence of the mechanisms can explain and thus provide credible support for PBM as a potential treatment for these diseases. Tubulin and its polymerized state of microtubules have been known to play important roles in the pathology of Alzheimer's and other neurodegenerative diseases. Thus, we investigated the effects of PBM on these cellular structures in the quest for insights into the underlying therapeutic mechanisms. In this study, we employed a Raman spectroscopic analysis of the amide I band of polymerized samples of tubulin exposed to pulsed low-intensity NIR radiation (810 nm, 10 Hz, 22.5 J/cm2 dose). Peaks in the Raman fingerprint region (300-1900 cm-1)-in particular, in the amide I band (1600-1700 cm-1)-were used to quantify the percentage of protein secondary structures. Under this band, hidden signals of C=O stretching, belonging to different structures, are superimposed, producing a complex signal as a result. An accurate decomposition of the amide I band is therefore required for the reliable analysis of the conformation of proteins, which we achieved through a straightforward method employing a Voigt profile. This approach was validated through secondary structure analyses of unexposed control samples, for which comparisons with other values available in the literature could be conducted. Subsequently, using this validated method, we present novel findings of statistically significant alterations in the secondary structures of polymerized NIR-exposed tubulin, characterized by a notable decrease in α-helix content and a concurrent increase in ß-sheets compared to the control samples. This PBM-induced α-helix to ß-sheet transition connects to reduced microtubule stability and the introduction of dynamism to allow for the remodeling and, consequently, refreshing of microtubule structures. This newly discovered mechanism could have implications for reducing the risks associated with brain aging, including neurodegenerative diseases like Alzheimer's disease, through the introduction of an intervention following this transition.

Structure of the Single-lobe Myosin Light Chain C in Complex with the Light Chain-binding Domains of Myosin-1C Provides Insights into Divergent IQ Motif Recognition.

Myosin light chains are key regulators of class 1 myosins and typically comprise two domains, with calmodulin being the archetypal example. They bind IQ motifs within the myosin neck region and amplify conformational changes in the motor domain. A single lobe light chain, myosin light chain C (MlcC), was recently identified and shown to specifically bind to two sequentially divergent IQ motifs of the Dictyostelium myosin-1C. To provide a molecular basis of this interaction, the structures of apo-MlcC and a 2:1 MlcC·myosin-1C neck complex were determined. The two non-functional EF-hand motifs of MlcC pack together to form a globular four-helix bundle that opens up to expose a central hydrophobic groove, which interacts with the N-terminal portion of the divergent IQ1 and IQ2 motifs. The N- and C-terminal regions of MlcC make critical contacts that contribute to its specific interactions with the myosin-1C divergent IQ motifs, which are contacts that deviate from the traditional mode of calmodulin-IQ recognition.

Structure of the small Dictyostelium discoideum myosin light chain MlcB provides insights into MyoB IQ motif recognition.

Dictyostelium discoideum MyoB is a class I myosin involved in the formation and retraction of membrane projections, cortical tension generation, membrane recycling, and phagosome maturation. The MyoB-specific, single-lobe EF-hand light chain MlcB binds the sole IQ motif of MyoB with submicromolar affinity in the absence and presence of Ca(2+). However, the structural features of this novel myosin light chain and its interaction with its cognate IQ motif remain uncharacterized. Here, we describe the NMR-derived solution structure of apoMlcB, which displays a globular four-helix bundle. Helix 1 adopts a unique orientation when compared with the apo states of the EF-hand calcium-binding proteins calmodulin, S100B, and calbindin D9k. NMR-based chemical shift perturbation mapping identified a hydrophobic MyoB IQ binding surface that involves amino acid residues in helices I and IV and the functional N-terminal Ca(2+) binding loop, a site that appears to be maintained when MlcB adopts the holo state. Complementary mutagenesis and binding studies indicated that residues Ile-701, Phe-705, and Trp-708 of the MyoB IQ motif are critical for recognition of MlcB, which together allowed the generation of a structural model of the apoMlcB-MyoB IQ complex. We conclude that the mode of IQ motif recognition by the novel single-lobe MlcB differs considerably from that of stereotypical bilobal light chains such as calmodulin.

Identification of calmodulin and MlcC as light chains for Dictyostelium myosin-I isozymes.

Dictyostelium discoideum express seven single-headed myosin-I isozymes (MyoA-MyoE and MyoK) that drive motile processes at the cell membrane. The light chains for MyoA and MyoE were identified by expressing Flag-tagged constructs consisting of the motor domain and the two IQ motifs in the neck region in Dictyostelium. The MyoA and MyoE constructs both copurified with calmodulin. Isothermal titration calorimetry (ITC) showed that apo-calmodulin bound to peptides corresponding to the MyoA and MyoE IQ motifs with micromolar affinity. In the presence of calcium, calmodulin cross-linked two IQ motif peptides, with one domain binding with nanomolar affinity and the other with micromolar affinity. The IQ motifs were required for the actin-activated MgATPase activity of MyoA but not MyoE; however, neither myosin exhibited calcium-dependent activity. A Flag-tagged construct consisting of the MyoC motor domain and the three IQ motifs in the adjacent neck region bound a novel 8.6 kDa two EF-hand protein named MlcC, for myosin light chain for MyoC. MlcC is most similar to the C-terminal domain of calmodulin but does not bind calcium. ITC studies showed that MlcC binds IQ1 and IQ2 but not IQ3 of MyoC. IQ3 contains a proline residue that may render it nonfunctional. Each long-tailed Dictyostelium myosin-I has now been shown to have a unique light chain (MyoB-MlcB, MyoC-MlcC, and MyoD-MlcD), whereas the short-tailed myosins-I, MyoA and MyoE, have the multifunctional calmodulin as a light chain. The diversity in light chain composition is likely to contribute to the distinct cellular functions of each myosin-I isozyme.

The nucleoid binding protein H-NS acts as an anti-channeling factor to favor intermolecular Tn10 transposition and dissemination.

Dissemination of the bacterial transposon Tn10 is limited by target site channeling, a process wherein the transposon ends are forced to interact with and insert into a target site located within the transposon. Integration host factor (IHF) promotes this self-destructive event by binding to the transpososome and forming a DNA loop close to one or both transposon ends; this loop imposes geometric and topological constraints that are responsible for channeling. We demonstrate that a second 'host' protein, histone-like nucleoid structuring protein (H-NS), acts as an anti-channeling factor to limit self-destructive intramolecular transposition events in vitro. Evidence that H-NS competes with IHF for binding to the Tn10 transpososome to block channeling and that this event is relatively insensitive to the level of DNA supercoiling present in the Tn10-containing substrate plasmid are presented. This latter observation is atypical for H-NS, as H-NS binding to other DNA sequences, such as promoters, is generally affected by subtle changes in DNA structure.

Tn10 transposase mutants with altered transpososome unfolding properties are defective in hairpin formation.

Transposition reactions take place in the context of higher-order protein-DNA complexes called transpososomes. In the Tn10 transpososome, IHF binding to an "outside end" creates a bend in the DNA that allows the transposase protein to contact the end at two different sites, the terminal and subterminal binding sites. Presumably this helps to stabilize the transposase-end interaction. However, the DNA loop that is formed must be unfolded at a later stage in order for the transposon to integrate into other DNA molecules. It has been proposed that transpososome unfolding also plays a role in transposon excision. To investigate this possibility further, we have isolated and characterized transposase mutants with altered transpososome unfolding properties. Two such mutants were identified, R182A and R184A. Both mutants fail to carry out hairpin formation, an intermediate step in transposon excision, specifically with outside end-containing substrates. These results support the idea that transpososome unfolding and excision are linked. Also, based on the importance of residues R182 and R184 in transpososome unfolding, we propose a new model for the Tn10 transpososome, wherein both DNA ends of the transpososome make subterminal contacts with transposase.