82 research outputs found
Investigation of Membrane Curvature Dependency on Cytochrome c Binding to Cardiolipin
Cytochrome c (Cyt c), an efficient electron transport protein in cellular respiration that makes biochemical energy ATP, is recently found to take part in initiating apoptosis (programmed cell death) through first oxidizing a lipid called cardiolipin, and then dissociating from the inner membrane of mitochondria to trigger the apoptosis cascade. If cell apoptosis is inhibited, it can cause cancer. Regulation of Cyt c in cardiolipin binding on the mitochondrial membranes potentially enables regulation of the intrinsic pathway of apoptosis. Cardiolipin has four hydrocarbon chains and a negatively charged head group which can interact with anionic site A on Cyt c that contains positively charged lysine amino acids. It is believed that the electrostatic interactions between anionic site A and CL on the inner membrane of a mitochondria lead to protein binding and partial unfolding. In this experiment, we isolate anionic site A, and use cardiolipin liposomes, a spherical sac formed artificially that has a lipid bilayer, to trap Cyt c as a mimic of the concave curvature of the cristae of the mitochondrial inner membrane. Circular dichroism spectroscopy is used to monitor the amount of trapped Cyt c. Previous studies have examined Cyt c-CL binding but using a convex surface that is not physiologically relevant other than it is composed of lipid CL. By comparing to previous similar studies, we can find out whether lipid curvature affects Cyt c-CL binding affinity. The understanding of apoptosis can be used toward novel therapies that can be developed to specifically engage apoptosis in cancer treatments
Normalized Wolfe-Powell-type local minimax method for finding multiple unstable solutions of nonlinear elliptic PDEs
The local minimax method (LMM) proposed in [Y. Li and J. Zhou, SIAM J. Sci.
Comput., 23(3), 840--865 (2001)] and [Y. Li and J. Zhou, SIAM J. Sci. Comput.,
24(3), 865--885 (2002)] is an efficient method to solve nonlinear elliptic
partial differential equations (PDEs) with certain variational structures for
multiple solutions. The steepest descent direction and the Armijo-type
step-size search rules are adopted in [Y. Li and J. Zhou, SIAM J. Sci. Comput.,
24(3), 865--885 (2002)] and play a significant role in the performance and
convergence analysis of traditional LMMs. In this paper, a new algorithm
framework of the LMMs is established based on general descent directions and
two normalized (strong) Wolfe-Powell-type step-size search rules. The
corresponding algorithm framework named as the normalized Wolfe-Powell-type LMM
(NWP-LMM) is introduced with its feasibility and global convergence rigorously
justified for general descent directions. As a special case, the global
convergence of the NWP-LMM algorithm combined with the preconditioned steepest
descent (PSD) directions is also verified. Consequently, it extends the
framework of traditional LMMs. In addition, conjugate gradient-type (CG-type)
descent directions are utilized to speed up the NWP-LMM algorithm. Finally,
extensive numerical results for several semilinear elliptic PDEs are reported
to profile their multiple unstable solutions and compared for different
algorithms in the LMM's family to indicate the effectiveness and robustness of
our algorithms. In practice, the NWP-LMM combined with the CG-type direction
indeed performs much better than its known LMM companions.Comment: 27 pages, 9 figures; Accepted by SCIENCE CHINA Mathematics on January
17, 202
Nonmonotone local minimax methods for finding multiple saddle points
In this paper, by designing a normalized nonmonotone search strategy with the
Barzilai--Borwein-type step-size, a novel local minimax method (LMM), which is
a globally convergent iterative method, is proposed and analyzed to find
multiple (unstable) saddle points of nonconvex functionals in Hilbert spaces.
Compared to traditional LMMs with monotone search strategies, this approach,
which does not require strict decrease of the objective functional value at
each iterative step, is observed to converge faster with less computations.
Firstly, based on a normalized iterative scheme coupled with a local peak
selection that pulls the iterative point back onto the solution submanifold, by
generalizing the Zhang--Hager (ZH) search strategy in the optimization theory
to the LMM framework, a kind of normalized ZH-type nonmonotone step-size search
strategy is introduced, and then a novel nonmonotone LMM is constructed. Its
feasibility and global convergence results are rigorously carried out under the
relaxation of the monotonicity for the functional at the iterative sequences.
Secondly, in order to speed up the convergence of the nonmonotone LMM, a
globally convergent Barzilai--Borwein-type LMM (GBBLMM) is presented by
explicitly constructing the Barzilai--Borwein-type step-size as a trial
step-size of the normalized ZH-type nonmonotone step-size search strategy in
each iteration. Finally, the GBBLMM algorithm is implemented to find multiple
unstable solutions of two classes of semilinear elliptic boundary value
problems with variational structures: one is the semilinear elliptic equations
with the homogeneous Dirichlet boundary condition and another is the linear
elliptic equations with semilinear Neumann boundary conditions. Extensive
numerical results indicate that our approach is very effective and speeds up
the LMMs significantly.Comment: 32 pages, 7 figures; Accepted by Journal of Computational Mathematics
on January 3, 202
Convergence analysis of a spectral-Galerkin-type search extension method for finding multiple solutions to semilinear problems
In this paper, we develop an efficient spectral-Galerkin-type search
extension method (SGSEM) for finding multiple solutions to semilinear elliptic
boundary value problems. This method constructs effective initial data for
multiple solutions based on the linear combinations of some eigenfunctions of
the corresponding linear eigenvalue problem, and thus takes full advantage of
the traditional search extension method in constructing initials for multiple
solutions. Meanwhile, it possesses a low computational cost and high accuracy
due to the employment of an interpolated coefficient Legendre-Galerkin spectral
discretization. By applying the Schauder's fixed point theorem and other
technical strategies, the existence and spectral convergence of the numerical
solution corresponding to a specified true solution are rigorously proved. In
addition, the uniqueness of the numerical solution in a sufficiently small
neighborhood of each specified true solution is strictly verified. Numerical
results demonstrate the feasibility and efficiency of our algorithm and present
different types of multiple solutions.Comment: 23 pages, 7 figures; Chinese version of this paper is published in
SCIENTIA SINICA Mathematica, Vol. 51 (2021), pp. 1407-143
Improving Generalization in Visual Reinforcement Learning via Conflict-aware Gradient Agreement Augmentation
Learning a policy with great generalization to unseen environments remains
challenging but critical in visual reinforcement learning. Despite the success
of augmentation combination in the supervised learning generalization, naively
applying it to visual RL algorithms may damage the training efficiency,
suffering from serve performance degradation. In this paper, we first conduct
qualitative analysis and illuminate the main causes: (i) high-variance gradient
magnitudes and (ii) gradient conflicts existed in various augmentation methods.
To alleviate these issues, we propose a general policy gradient optimization
framework, named Conflict-aware Gradient Agreement Augmentation (CG2A), and
better integrate augmentation combination into visual RL algorithms to address
the generalization bias. In particular, CG2A develops a Gradient Agreement
Solver to adaptively balance the varying gradient magnitudes, and introduces a
Soft Gradient Surgery strategy to alleviate the gradient conflicts. Extensive
experiments demonstrate that CG2A significantly improves the generalization
performance and sample efficiency of visual RL algorithms.Comment: accepted by iccv202
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