Quantitative studies of RET activation, deactivation and trafficking kinetics upon stimulation by its natural ligand Artemin

Receptor tyrosine kinases (RTKs) are key regulators of critical cellular processes, such as cell cycle, differentiation, proliferation, apoptosis and survival. Mutations, hyperactivity and loss of function of RTKs are responsible for numerous diseases. Because of the therapeutic importance of RTK signaling, intensive studies have been devoted to understanding the signaling mechanisms of RTKs, and the key components in their signaling networks. However, studying the cellular responses to RTK stimulation in a native cellular context is technically challenging. Consequently, many details of RTK signaling kinetics, and the underlying molecular mechanisms of action, remain unclear. The RET receptor tyrosine kinase is important for neuronal cell survival and function, and for the development of the kidneys and nervous system. Gain of function of RET leads to tumor formation, while loss of function in RET’s kinase activity is associated with the developmental kidney defect Hirschsprung’s disease. RET is activated by ligands of glial cell line-derived neurotrophic factor (GDNF) family, which consist of four homologs—GDNF, Neuturin, Artemin (ART) and Persephin. GDNF family ligands activate RET only in the presence of GPI-linked co-receptors (GFRα1–4). Formation of the pentameric ligand/co-receptor2/RET2 complex leads to dimerization of RET and autophosphrylation of its cytoplasmic kinase domain. RET phosphorylation results in the activation of multiple downstream signaling pathways, including the Ras-Raf-MEK-ERK and PI3K-Akt pathways. The ERK and Akt signaling pathways participate in a variety of cellular activities, including cell survival, proliferation, and differentiation. My project addresses the following questions: (1) What are the kinetics of RET activation and deactivation processes after ART stimulation? (2) How is RET activation coupled to the phosphorylation of ERK1/2 and Akt? (3) How does ligand-induced internalization of RET affect RET signaling? (4) How does each step in the RET-Ras-Raf-MEK-ERK cascade quantitatively regulate ERK phosphorylation levels? The results will elucidate the spatial and temporal dynamics of RET signaling upon stimulation by ART, and to determine how downstream signaling is regulated by the amplitude and timing of RET activation. Overall, the thesis aims to advance our understanding of RTK signaling, by establishing methods and principles that can potentially be applied to other RTK systems

Quantitative studies of RET activation, deactivation and trafficking kinetics upon stimulation by its natural ligand Artemin

Receptor tyrosine kinases (RTKs) are key regulators of critical cellular processes, such as cell cycle, differentiation, proliferation, apoptosis and survival. Mutations, hyperactivity and loss of function of RTKs are responsible for numerous diseases. Because of the therapeutic importance of RTK signaling, intensive studies have been devoted to understanding the signaling mechanisms of RTKs, and the key components in their signaling networks. However, studying the cellular responses to RTK stimulation in a native cellular context is technically challenging. Consequently, many details of RTK signaling kinetics, and the underlying molecular mechanisms of action, remain unclear. The RET receptor tyrosine kinase is important for neuronal cell survival and function, and for the development of the kidneys and nervous system. Gain of function of RET leads to tumor formation, while loss of function in RET’s kinase activity is associated with the developmental kidney defect Hirschsprung’s disease. RET is activated by ligands of glial cell line-derived neurotrophic factor (GDNF) family, which consist of four homologs—GDNF, Neuturin, Artemin (ART) and Persephin. GDNF family ligands activate RET only in the presence of GPI-linked co-receptors (GFRα1–4). Formation of the pentameric ligand/co-receptor2/RET2 complex leads to dimerization of RET and autophosphrylation of its cytoplasmic kinase domain. RET phosphorylation results in the activation of multiple downstream signaling pathways, including the Ras-Raf-MEK-ERK and PI3K-Akt pathways. The ERK and Akt signaling pathways participate in a variety of cellular activities, including cell survival, proliferation, and differentiation. My project addresses the following questions: (1) What are the kinetics of RET activation and deactivation processes after ART stimulation? (2) How is RET activation coupled to the phosphorylation of ERK1/2 and Akt? (3) How does ligand-induced internalization of RET affect RET signaling? (4) How does each step in the RET-Ras-Raf-MEK-ERK cascade quantitatively regulate ERK phosphorylation levels? The results will elucidate the spatial and temporal dynamics of RET signaling upon stimulation by ART, and to determine how downstream signaling is regulated by the amplitude and timing of RET activation. Overall, the thesis aims to advance our understanding of RTK signaling, by establishing methods and principles that can potentially be applied to other RTK systems

Cutting of cortical bone tissue: analysis of deformation and fracture process

Cortical bone tissue – one of the most intriguing materials found in nature – demonstrate some fascinating behaviours that have attracted great attention of many researchers from all over the world. In contrast to engineering materials, bone has its unique characters: it is a material that has both sufficient stiffness and toughness to provide physical support and protection to internal organs and yet adaptively balanced for its weight and functional requirements. Its structure and mechanical properties are of great importance to the physiological functioning of the body. Still, our understanding on the mechanical deformation processes of cortical bone tissue is rather limited. Penetration into a bone tissue is an intrinsic part of many clinical procedures, such as orthopaedic surgery, bone implant and repair operations. The success of bone-cutting surgery depends largely on precision of the operation and the extent of damage it causes to the surrounding tissues. The anisotropic behaviour of cortical bone acts as a distinctive protective mechanism and increases the difficulty during cutting process. A comprehensive understanding of deformation and damage mechanisms during the cutting process is necessary for improving the operational accuracy and postoperative recovery of patients. However, the current literature on experimental results provides limited information about processes in the vicinity of the cutting tool-bone interaction zone; while; numerical models cannot fully describe the material anisotropy and the effect of damage mechanisms of cortical bone tissue. In addition, a conventional finite-element scheme faces numerical challenges due to large deformation and highly localised distortion in the process zone. This PhD project is aimed at bridging the gap in current lack of understanding on cutting-induced deformation and fracture processes in the cortical bone tissue through experimental and numerical approaches. A number of experimental studies were accomplished to characterise the mechanical behaviour of bovine cortical bone tissue and to analyse deformation and damage mechanisms associated with the cutting process II along different bone axes in four anatomic cortices, namely, anterior, posterior, medial and lateral. These experiments included: (1) a Vickers hardness test to provide initial assessments on deformation and damage processes in the cortical bone tissue under a concentrated compressive load; (2) uniaxial tension and compression tests, performed to understand the effect of orientation and local variability of microstructure constituents on the macroscopic material properties of cortical bone; (3) fracture toughness tests, aimed at elucidating the anisotropic character of fracture toughness of cortical bone and its various fracture toughness mechanisms in relation to different orientations; (4) penetration tests, conducted to evaluate and validate mechanisms involved in bone cutting as well as orientation associated anisotropic deformation and damage processes at various different cortex positions. Information obtained in these experimental studies was used to assist the development of advanced finite-element models: (1) the effective homogenised XFEM models developed in conjunction with three-point bending test to represent a macroscopically, anisotropic elasticplastic fracture behaviour of cortical bone tissue; (2) three microstructured XFEM models to further investigate the effect of the randomly distributed microstructural constituents on the local fracture process and the variability of fracture toughness of cortical bone; (3) a novel finite-element modelling approach encompassing both conventional and SPH elements, incorporating anisotropic elastic-plastic material properties and progressive damage criteria to simulate large deformation and damage processes of cortical bone under penetration. The established models can adequately and accurately reflect large deformations and damage processes during the penetration in bone cutting. The results of this study made valuable contributions to our existing understanding of the mechanics of cortical bone tissue and most importantly to the understanding of its mechanical behaviours during the cutting process

Mechanical Stimuli in Prediction of Trabecular Bone Adaptation: Numerical Comparison

Adaptation is the process, with which bone responds to changes in loading environment and modifies its properties and organisation to meet the mechanical demands. Trabecular bone undergoes significant adaptation when subjected to external forces, accomplished through resorption of old and fractured bone and formation of a new bone material. These processes are assumed to be driven by mechanical stimuli of bone-matrix deformation sensed by bone mechanosensory cells. Although numerous in vivo and in vitro experimental evidence of trabecular bone morphology adaptation was obtained, the exact nature of mechanical stimuli triggering biological responses (i.e., osteoclastic resorption and osteoblastic formation) is still debated. This study aims to compare different mechanical stimuli with regard to their ability to initiate the load-induced adaptation in trabecular bone. For this purpose, a 2D model of two trabeculae, connected at their basement, with bone marrow in the intertrabecular space was developed. The finite-element method was implemented for the model loaded in compression to calculate magnitudes of several candidates of the bone-adaptation stimuli. A user material subroutine was developed to relate a magnitude of each candidate to changes in the shape of trabeculae

Theoretical Design and FPGA-Based Implementation of Higher-Dimensional Digital Chaotic Systems

Traditionally, chaotic systems are built on the domain of infinite precision in mathematics. However, the quantization is inevitable for any digital devices, which causes dynamical degradation. To cope with this problem, many methods were proposed, such as perturbing chaotic states and cascading multiple chaotic systems. This paper aims at developing a novel methodology to design the higher-dimensional digital chaotic systems (HDDCS) in the domain of finite precision. The proposed system is based on the chaos generation strategy controlled by random sequences. It is proven to satisfy the Devaney's definition of chaos. Also, we calculate the Lyapunov exponents for HDDCS. The application of HDDCS in image encryption is demonstrated via FPGA platform. As each operation of HDDCS is executed in the same fixed precision, no quantization loss occurs. Therefore, it provides a perfect solution to the dynamical degradation of digital chaos.Comment: 12 page

High channel count and high precision channel spacing multi-wavelength laser array for future PICs

Multi-wavelength semiconductor laser arrays (MLAs) have wide applications in wavelength multiplexing division (WDM) networks. In spite of their tremendous potential, adoption of the MLA has been hampered by a number of issues, particularly wavelength precision and fabrication cost. In this paper, we report high channel count MLAs in which the wavelengths of each channel can be determined precisely through low-cost standard μm-level photolithography/holographic lithography and the reconstruction-equivalent-chirp (REC) technique. 60-wavelength MLAs with good wavelength spacing uniformity have been demonstrated experimentally, in which nearly 83% lasers are within a wavelength deviation of ±0.20 nm, corresponding to a tolerance of ±0.032 nm in the period pitch. As a result of employing the equivalent phase shift technique, the single longitudinal mode (SLM) yield is nearly 100%, while the theoretical yield of standard DFB lasers is only around 33.3%

catena-Poly[[[dibromidocadmium]-μ2-1,1′-(butane-1,4-di­yl)bis­(pyridinium-4-carboxyl­ate)] monohydrate]

In the title compound, {[CdBr2(C16H16N2O4)]·H2O}n, the CdII ion is six-coordinated by a Br2O4 donor set, with four O atoms from two bridging 1,1′-(butane-1,4-di­yl)bis­(pyridinium-4-carboxyl­ate) ligands. The ligands link the CdII ions into a zigzag chain extending along [01]. O—H⋯O and O—H⋯Br hydrogen bonds involving the uncoordinated water mol­ecules connect the chains