The Role of Oligomerization and Cooperative Regulation in Protein Function: The Case of Tryptophan Synthase

The oligomerization/co-localization of protein complexes and their cooperative regulation in protein function is a key feature in many biological systems. The synergistic regulation in different subunits often enhances the functional properties of the multi-enzyme complex. The present study used molecular dynamics and Brownian dynamics simulations to study the effects of allostery, oligomerization and intermediate channeling on enhancing the protein function of tryptophan synthase (TRPS). TRPS uses a set of α/β–dimeric units to catalyze the last two steps of L-tryptophan biosynthesis, and the rate is remarkably slower in the isolated monomers. Our work shows that without their binding partner, the isolated monomers are stable and more rigid. The substrates can form fairly stable interactions with the protein in both forms when the protein reaches the final ligand–bound conformations. Our simulations also revealed that the α/β–dimeric unit stabilizes the substrate–protein conformation in the ligand binding process, which lowers the conformation transition barrier and helps the protein conformations shift from an open/inactive form to a closed/active form. Brownian dynamics simulations with a coarse-grained model illustrate how protein conformations affect substrate channeling. The results highlight the complex roles of protein oligomerization and the fine balance between rigidity and dynamics in protein function

Real-time block flow tracking of atrial septal defect motion in 4D cardiac ultrasound

Real-time cardiac ultrasound allows monitoring the heart motion during intracardiac beating heart procedures. Our application assists atrial septal defect (ASD) closure techniques using real-time 3D ultrasound guidance. One major image processing challenge is the processing of information at high frame rate. We present an optimized block flow technique, which combines the probability-based velocity computation for an entire block with template matching. We propose adapted similarity constraints both from frame to frame, to conserve energy, and globally, to minimize errors. We show tracking results on eight in-vivo 4D datasets acquired from porcine beating-heart procedures. Computing velocity at the block level with an optimized scheme, our technique tracks ASD motion at 41 frames/s. We analyze the errors of motion estimation and retrieve the cardiac cycle in ungated images. © 2007 IEEE

Imaging artifacts of medical instruments in ultrasound-guided interventions.

OBJECTIVE: Real-time 3-dimensional (3D) ultrasound imaging has the potential to become a dominant imaging technique for minimally invasive surgery. One barrier to its widespread use is that surgical instruments generate imaging artifacts, which can obfuscate their location, orientation, and geometry and obscure nearby tissue. The purpose of this study was to identify and describe the types of artifacts which could be produced by metallic instruments during interventions guided by 3D ultrasound imaging. METHODS: Three imaging studies were performed. First, imaging artifacts from stainless steel rods were identified in vitro and acoustically characterized. Second, 3 typical minimally invasive instruments were imaged (in vitro and in vivo), and their artifacts were analyzed. The third study compared the intensity of imaging artifacts (in vitro and in vivo) from stainless steel rods with rods composed of 3 different materials and stainless steel rods with roughened and coated surfaces. RESULTS: For the stainless steel rods, all observed artifacts are described and illustrated, and their physical origins are explained. Artifacts from the 3 minimally invasive instruments are characterized with the use of the artifacts observed with the rods. Finally, it is shown that artifacts can be greatly reduced through the use of alternate materials or by surface modification. CONCLUSIONS: Instrument artifacts in 3D ultrasound images can be more confusing than those from the same instruments imaged in 2 dimensions. Real-time 3D ultrasound imaging can, however, be used effectively for in vivo imaging of minimally invasive instruments by using artifact mitigation techniques, including careful selection of probe and incision locations, as well as by instrument modification

Design and fabrication of a soft robotic direct cardiac compression device

A direct cardiac compression (DCC) device is an active sleeve that is surgically placed around the heart to help the failing heart to pump without contacting blood. Soft robotic techniques enable fabrication of a conformable DCC device containing modular actuators oriented in a biomimetic manner that can restore the natural motion of the heart and provide tunable active assistance. In this paper we describe the fabrication of a DCC device; the optimization of pneumatic actuators, their integration into a matrix with a modulus in the range of cardiac tissue and methods to affix this device to the heart wall. Pneumatic air muscles (PAMs) were fabricated using a modified McKibben technique and four types of internal bladders; low durometer silicone tubes molded inhouse, polyester terephthalate (PET) heat shrink tubing, nylon medical balloons and thermoplastic urethane (TPU) balloons thermally formed in-house. Balloons were bonded to air supply lines, placed inside a braided nylon mesh with a 6.35mm resting diameter and bonded at one end. When pressurized to 145kPa silicone tubes failed and PET, nylon and TPU actuators generated isometric axial forces of 14.28, 19.65 and 19.05N respectively, with axial contractions of 33.11, 28.69 and 37.54%. Circumferential actuators placed around the heart reduced the cross-sectional area by 33.34% and 50.63% for silicone and TPU actuators respectively. PAMs were integrated into a soft matrix in a biomimetic orientation using three techniques; casting, thermal forming and layering. Designs were compared on an in vitro cardiac simulator and generated a volumetric displacement of up to 96ml when actuated for 200ms at 1Hz. Layering produced the lowest profile device that successfully conformed to the heart and this design is currently undergoing in vivo testing

Tough adhesives for diverse wet surfaces

Adhesion to wet and dynamic surfaces, including biological tissues, is important in many fields but has proven to be extremely challenging. Existing adhesives are cytotoxic, adhere weakly to tissues, or cannot be used in wet environments. We report a bioinspired design for adhesives consisting of two layers: an adhesive surface and a dissipative matrix. The former adheres to the substrate by electrostatic interactions, covalent bonds, and physical interpenetration. The latter amplifies energy dissipation through hysteresis. The two layers synergistically lead to higher adhesion energies on wet surfaces as compared with those of existing adhesives. Adhesion occurs within minutes, independent of blood exposure and compatible with in vivo dynamic movements. This family of adhesives may be useful in many areas of application, including tissue adhesives, wound dressings, and tissue repair

Soft robotic sleeve supports heart function

There is much interest in form-fitting, low-modulus, implantable devices or soft robots that can mimic or assist in complex biological functions such as the contraction of heart muscle. We present a soft robotic sleeve that is implanted around the heart and actively compresses and twists to act as a cardiac ventricular assist device. The sleeve does not contact blood, obviating the need for anticoagulation therapy or blood thinners, and reduces complications with current ventricular assist devices, such as clotting and infection. Our approach used a biologically inspired design to orient individual contracting elements or actuators in a layered helical and circumferential fashion, mimicking the orientation of the outer two muscle layers of the mammalian heart. The resulting implantable soft robot mimicked the form and function of the native heart, with a stiffness value of the same order of magnitude as that of the heart tissue. We demonstrated feasibility of this soft sleeve device for supporting heart function in a porcine model of acute heart failure. The soft robotic sleeve can be customized to patient-specific needs and may have the potential to act as a bridge to transplant for patients with heart failure

A Blood-Resistant Surgical Glue for Minimally Invasive Repair of Vessels and Heart Defects

Currently, there are no clinically approved surgical glues that are nontoxic, bind strongly to tissue, and work well withinwet and highly dynamic environments within the body. This is especially relevant tominimally invasive surgery that is increasingly performed to reduce postoperative complications, recovery times, and patient discomfort. We describe the engineering of a bioinspired elastic and biocompatible hydrophobic light-activated adhesive (HLAA) that achieves a strong level of adhesion to wet tissue and is not compromised by preexposure to blood. The HLAA provided an on-demand hemostatic seal, within seconds of light application, when applied to high-pressure large blood vessels and cardiac wall defects in pigs. HLAA-coated patches attached to the interventricular septum in a beating porcine heart and resisted supraphysiologic pressures by remaining attached for 24 hours, which is relevant to intracardiac interventions in humans. The HLAA could be used for many cardiovascular and surgical applications, with immediate application in repair of vascular defects and surgical hemostasis