Molecular swarm robots: recent progress and future challenges

Peptide-mediated display of Tau-derived peptide for construction of microtubule superstructures

Microtubules are major cytoskeletons involved in various cellular functions, such as regulating cell shape and division and cargo transport via motor proteins. In addition to widely studied singlet microtubules, complex microtubule superstructures, including doublets and bundles, provide unique mechanical and functional properties in vivo. However, a method to construct such superstructures in vitro remains unresolved. This study presents a peptide-based approach for constructing microtubule superstructures by displaying Tau-derived peptides (TP) on the outer surface of microtubules using KA7 peptides as binding units. The KA7-connected TP (KA7-TP) bound to the C-terminal tail on the outer surface of microtubules and induced doublets and bundles by recruiting tubulin. Notably, the outer layers of the doublet microtubules generated by KA7-TP dissociated, highlighting the utility of this approach for studying the formation/dissociation mechanisms of microtubule superstructures. The simple peptide-based approach facilitates our understanding of microtubule superstructures and offers new opportunities for applying microtubule superstructures to nanotechnology.

Optical Control of Microtubule Accumulation and Dispersion by Tau-Derived Peptide-Fused Photoresponsive Protein

Microtubules, a major component of the cytoskeleton consisting of tubulin dimers, are involved in various cellular functions, including forming axons and dendrites of neurons and retaining cell shapes by forming various accumulated superstructures such as bundles and doublets. Moreover, microtubule-accumulated structures like swarming microtubule assemblies are attractive components for dynamic materials, such as active matter and molecular robots. Thus, dynamic control of microtubule superstructures is an important topic. However, implementing stimulus-dependent control of superstructures remains challenging. This challenge can be resolved by developing designer protein approaches. We have previously developed a Tau-derived peptide (TP), which binds to the inner or outer surface of microtubules depending on the timing of the incubation. In this report, we designed the TP-fused photoswitchable protein Dronpa (TP-Dronpa) that reversibly photoconverts between monomeric and tetrameric states to photocontrol microtubule assemblies. The formation of microtubule superstructures, including bundles and doublets, was induced by tetrameric TP-Dronpa, whereas monomeric TP-Dronpa ensured that microtubules remained dispersed. Tetrameric TP-Dronpa also induced motile aster-like structures and swarming movement of microtubules on a kinesin-coated substrate. The formation/dissociation of these microtubule superstructures can be controlled by light irradiation. This system can generate and photocontrol various microtubule superstructures and provides an approach to facilitate the assembly of dynamic materials for various applications.

Nonequilibrium Self-Assembly of Microtubules Through Stepwise Sequential Interactions of DNA

The assembly of biological systems forms nonequilibrium patterns with different functionalities through molecular-level communication via stepwise sequential interaction and activation. The mimicking of this molecular signaling offers extensive opportunities to design self-assemblies of bioinspired synthetic nonequilibrium systems to develop molecular robots with active, adaptive, and autonomous behavior. Herein, the design and construction of biomolecular motor system, microtubule (MT)-kinesin based molecular swarm system, are reported through stepwise sequential interactions of DNA. DNA signals are exchanged between three different DNA-tethered MTs, whereby the DNA signal from the first MT can activate the DNA strand on the second MT by communicating through physical contact, which facilitates assembly formation between the second and third DNA-tethered MTs. The DNA strands on the MTs can recognize the specific sequences of other DNA strands in the system and communicate with the complementary DNA on other MTs. This work will pave the way for developing autonomous molecular machines with advanced functionalities for complex nanotechnological applications.

Development of clobetasol-loaded biodegradable nanoparticles as an endodontic intracanal medicament

AIM

The aim of current study is the development and optimization of biodegradable polymeric nanoparticles (NPs) to be used in the field of Endodontics as intracanal medication in cases of avulsed teeth with extended extra-oral time, utilizing PLGA polymers loaded with the anti-inflammatory drug clobetasol propionate (CP).

METHODOLOGY

CP-loaded nanoparticles (CP-NPs) were prepared using the solvent displacement method. CP release profile from CP-NPs was assessed for 48 h against free CP. Using extracted human teeth, the degree of infiltration inside the dentinal tubules was studied for both CP-NPs and CP. The anti-inflammatory capacity of CP-NPs was evaluated in vitro measuring their response and reaction against inflammatory cells, in particular against macrophages. The enzyme-linked immunosorbent assay (ELISA) was used to examine the cytokine release of IL-1β and TNF-α.

RESULTS

Optimized CP-NPs displayed an average size below 200 nm and a monomodal population. Additionally, spherical morphology and non-aggregation of CP-NPs were confirmed by transmission electron microscopy. Interaction studies showed that CP was encapsulated inside the NPs and no covalent bonds were formed. Moreover, CP-NPs exhibited a prolonged and steady release with only 21% of the encapsulated CP released after 48 h. Using confocal laser scanning microscopy, it was observed that CP-NPs were able to display enhanced penetration into the dentinal tubules. Neither the release of TNF-α nor IL-1β increased in CP-NPs compared to the LPS control, displaying results similar and even less than the TCP after 48 h. Moreover, IL-1β release in LPS-stimulated cells, decreased when macrophages were treated with CP-NPs.

CONCLUSIONS

In the present work, CP-NPs were prepared, optimized and characterized displaying significant increase in the degree of infiltration inside the dentinal tubules against CP and were able to significantly reduce TNF-α release. Therefore, CP-NPs constitute a promising therapy for the treatment of avulsed teeth with extended extra-oral time.

Autonomous assembly and disassembly of gliding molecular robots regulated by a DNA-based molecular controller

In recent years, there has been a growing interest in engineering dynamic and autonomous systems with robotic functionalities using biomolecules. Specifically, the ability of molecular motors to convert chemical energy to mechanical forces and the programmability of DNA are regarded as promising components for these systems. However, current systems rely on the manual addition of external stimuli, limiting the potential for autonomous molecular systems. Here, we show that DNA-based cascade reactions can act as a molecular controller that drives the autonomous assembly and disassembly of DNA-functionalized microtubules propelled by kinesins. The DNA controller is designed to produce two different DNA strands that program the interaction between the microtubules. The gliding microtubules integrated with the controller autonomously assemble to bundle-like structures and disassemble into discrete filaments without external stimuli, which is observable by fluorescence microscopy. We believe this approach to be a starting point toward more autonomous behavior of motor protein-based multicomponent systems with robotic functionalities.

Collective Molecular Machines: Multidimensionality and Reconfigurability

Molecular machines are key to cellular activity where they are involved in converting chemical and light energy into efficient mechanical work. During the last 60 years, designing molecular structures capable of generating unidirectional mechanical motion at the nanoscale has been the topic of intense research. Effective progress has been made, attributed to advances in various fields such as supramolecular chemistry, biology and nanotechnology, and informatics. However, individual molecular machines are only capable of producing nanometer work and generally have only a single functionality. In order to address these problems, collective behaviors realized by integrating several or more of these individual mechanical units in space and time have become a new paradigm. In this review, we comprehensively discuss recent developments in the collective behaviors of molecular machines. In particular, collective behavior is divided into two paradigms. One is the appropriate integration of molecular machines to efficiently amplify molecular motions and deformations to construct novel functional materials. The other is the construction of swarming modes at the supramolecular level to perform nanoscale or microscale operations. We discuss design strategies for both modes and focus on the modulation of features and properties. Subsequently, in order to address existing challenges, the idea of transferring experience gained in the field of micro/nano robotics is presented, offering prospects for future developments in the collective behavior of molecular machines.

Generation of stable microtubule superstructures by binding of peptide-fused tetrameric proteins to inside and outside

Microtubules play important roles in biological functions by forming superstructures, such as doublets and branched structures, in vivo. Despite the importance, it is challenging to construct these superstructures in vitro. Here, we designed a tetrameric fluorescent protein Azami-Green (AG) fused with His-tag and Tau-derived peptide (TP), TP-AG, to generate the superstructures. Main binding sites of TP-AG can be controlled to the inside and outside of microtubules by changing the polymerization conditions. The binding of TP-AG to the inside promoted microtubule formation and generated rigid and stable microtubules. The binding of TP-AG to the outside induced various microtubule superstructures, including doublets, multiplets, branched structures, and extremely long microtubules by recruiting tubulins to microtubules. Motile microtubule aster structures were also constructed by TP-AG. The generation of various microtubule superstructures by a single type of exogenous protein is a new concept for understanding the functions of microtubules and constructing microtubule-based nanomaterials.

Nanobots-based advancement in targeted drug delivery and imaging: An update

Manipulation and targeted navigation of nanobots in complex biological conditions can be achieved by chemical reactions, by applying external forces, and via motile cells. Several studies have applied fuel-based and fuel-free propulsion mechanisms for nanobots movements in environmental sciences and robotics. However, their applications in biomedical sciences are still in the budding phase. Therefore, the current review introduces the fundamentals of different propulsion strategies based on the advantageous features of applied nanomaterials or cellular components. Furthermore, the recent developments reported in various literatures on next-generation nanobots, such as Xenobots with applications of in-vitro and in-vivo drug delivery and imaging were also explored in detail. The challenges and the future prospects are also highlighted with corresponding advantages and limitations of nanobots in biomedical applications. This review concludes that with ever booming research enthusiasm in this field and increasing multidisciplinary cooperation, micro-/nanorobots with intelligence and multifunctions will emerge in the near future, which would have a profound impact on the treatment of diseases.

Functionalization of Tubulin: Approaches to Modify Tubulin with Biotin and DNA

The filamentous cytoskeletal protein microtubule, a polymer of α and β heterodimers of tubulin, plays major roles in intracellular transport as well as in vitro molecular actuation and transportation. Functionalization of tubulin dimers through covalent linkage facilitates utilization of microtubule in the nanobioengineering. Here we present a detailed description of the methodologies used to modify tubulin dimers with DNA strand and biotin through covalent interaction.

Cooperative cargo transportation by a swarm of molecular machines

Cooperation is a strategy that has been adopted by groups of organisms to execute complex tasks more efficiently than single entities. Cooperation increases the robustness and flexibility of the working groups and permits sharing of the workload among individuals. However, the utilization of this strategy in artificial systems at the molecular level, which could enable substantial advances in microrobotics and nanotechnology, remains highly challenging. Here, we demonstrate molecular transportation through the cooperative action of a large number of artificial molecular machines, photoresponsive DNA-conjugated microtubules driven by kinesin motor proteins. Mechanical communication via conjugated photoresponsive DNA enables these microtubules to organize into groups upon photoirradiation. The groups of transporters load and transport cargo, and cargo unloading is achieved by dissociating the groups into single microtubules. The group formation permits the loading and transport of cargoes with larger sizes and in larger numbers over long distances compared with single transporters. We also demonstrate that cargo can be collected at user-determined locations defined by ultraviolet light exposure. This work demonstrates cooperative task performance by molecular machines, which will help to construct molecular robots with advanced functionalities in the future.

Controlling the Rigidity of Kinesin-Propelled Microtubules in an In Vitro Gliding Assay Using the Deep-Sea Osmolyte Trimethylamine N-Oxide

The biomolecular motor protein kinesin and its associated filamentous protein microtubule have been finding important nanotechnological applications in the recent years. Rigidity of the microtubules, which are propelled by kinesin motors in an in vitro gliding assay, is an important metric that determines the success of utilization of microtubules and kinesins in various applications, such as transportation, sensing, sorting, molecular robotics, etc. Therefore, regulating the rigidity of kinesin-propelled microtubules has been critical. In this work, we report a simple strategy to regulate the rigidity of kinesin-propelled microtubules in an in vitro gliding assay. We demonstrate that rigidity of the microtubules, propelled by kinesins in an in vitro gliding assay, can be modulated simply by using the natural osmolyte trimethylamine N-oxide (TMAO). By varying the concentration of TMAO in the gliding assay, the rigidity of microtubules can be modulated over a wide range. Based on this strategy, we are able to reduce the persistence length of microtubules, a measure of microtubule rigidity, ∼8 fold by using TMAO at the concentration of 1.5 M. Furthermore, we found that the decreased rigidity of the kinesin-propelled microtubules can be restored upon elimination of TMAO from the in vitro gliding assay. Alteration in the rigidity of microtubules is accounted for by the non-uniformity of the force applied by kinesins along the microtubules in the presence of TMAO. This work offers a facile strategy to reversibly regulate the rigidity of kinesin-propelled microtubules in situ, which would widen the applications of the biomolecular motor kinesin and its associated protein microtubule in various fields.

Encapsulation of Nanomaterials Inside Microtubules by Using a Tau-Derived Peptide

Microtubules (MTs) are tubular cytoskeletons, which are used for the various applications such as active matters and therapeutic targets. Although modification of the exterior surface of MTs is frequently used for functionalization of MTs, there was no approach to introduce molecules inside MTs. We previously developed a unique peptide binding to the inner surface of MT, which is derived from a MT-associated protein, Tau. The Tau-derived peptide (TP) can be used to introduce various nanomaterials inside MTs. Here we describe the TP-based encapsulation of fluorescent dye, gold nanoparticle, green fluorescent protein, and magnetic CoPt nanoparticles inside MTs.

An Overview of Micronanoswarms for Biomedical Applications

Micronanoswarms have attracted extensive attention worldwide due to their great promise in biomedical applications. The collective behaviors among thousands, or even millions, of tiny active agents indicate immense potential for benefiting the progress of clinical therapeutic and diagnostic methods. In recent years, with the development of smart materials, remote actuation modalities, and automatic control strategies, the motion dexterity, environmental adaptability, and functionality versatility of micronanoswarms are improved. Swarms can thus be designed as dexterous platforms inside living bodies to perform a multitude of tasks related to healthcare. Existing surveys summarize the design, functionalization, and biomedical applications of micronanorobots and the actuation and motion control strategies of micronanoswarms. This review presents the recent progress of micronanoswarms, aiming for biomedical applications. The recent advances on structural design of artificial, living, and hybrid micronanoswarms are summarized, and the biomedical applications that could be tackled using micronanoswarms are introduced, such as targeted drug delivery, hyperthermia, imaging and sensing, and thrombolysis. Moreover, potential challenges and promising trends of future developments are discussed. It is envisioned that the future success of these promising tools will have a significant impact on clinical treatment.

Engineering control circuits for molecular robots using synthetic biology

The integration of molecular robots and synthetic biology allows for the creation of sophisticated behaviors at the molecular level. Similar to the synergy between bioelectronics and soft robotics, synthetic biology provides control circuitry for molecular robots. By encoding perception-action modules within synthetic circuits, molecular machines can advance beyond repeating tasks to the incorporation of complex behaviors. In particular, cell-free synthetic biology provides biomolecular circuitry independent of living cells. This research update reviews the current progress in using synthetic biology as perception-action control modules in robots from molecular robots to macroscale robots. Additionally, it highlights recent developments in molecular robotics and cell-free synthetic biology and suggests their combined use as a necessity for future molecular robot development.