Simon Lam

Temporal and tissue specific regulation of RP-associated splicing factor genes PRPF3, PRPF31 and PRPC8--implications in the pathogenesis of RP.

Background Genetic mutations in several ubiquitously expressed RNA splicing genes such as PRPF3, PRP31 and PRPC8, have been found to cause retina-specific diseases in humans. To understand this intriguing phenomenon, most studies have been focused on testing two major hypotheses. One hypothesis assumes that these mutations interrupt retina-specific interactions that are important for RNA splicing, implying that there are specific components in the retina interacting with these splicing factors. The second hypothesis suggests that these mutations have only a mild effect on the protein function and thus affect only the metabolically highly active cells such as retinal photoreceptors. Methodology/Principal Findings We examined the second hypothesis using the PRPF3 gene as an example. We analyzed the spatial and temporal expression of the PRPF3 gene in mice and found that it is highly expressed in retinal cells relative to other tissues and its expression is developmentally regulated. In addition, we also found that PRP31 and PRPC8 as well as snRNAs are highly expressed in retinal cells. Conclusions/Significance Our data suggest that the retina requires a relatively high level of RNA splicing activity for optimal tissue-specific physiological function. Because the RP18 mutation has neither a debilitating nor acute effect on protein function, we suggest that retinal degeneration is the accumulative effect of decades of suboptimal RNA splicing due to the mildly impaired protein.

The real stabilizability radius of the multi-link inverted pendulum

A multi-link inverted pendulum with an arbitrary number of v links and controlled by a single torque input is considered in this paper. It is well known that as the number of pendulum links increases, an experimental pendulum system becomes more difficult to stabilize, and this is demonstrated in this paper for a nonlinear model of a multi-link inverted pendulum system. It is conjectured that the primary reason for such an increase of difficulty is due to the poor stabilizability robustness properties of the pendulums linearized model as additional links are added. Using the real stability and stabilizability radius, this conjecture is confirmed

rbm47, a novel RNA binding protein, regulates zebrafish head development

Background: Vertebrate trunk induction requires inhibition of bone morphogenetic protein (BMP) signaling, whereas vertebrate head induction requires concerted inhibition of both Wnt and BMP signaling. RNA binding proteins play diverse roles in embryonic development and their roles in vertebrate head development remain to be elucidated. Results: We first characterized the human RBM47 as an RNA binding protein that specifically binds RNA but not single‐stranded DNA. Next, we knocked down rbm47 gene function in zebrafish using morpholinos targeting the start codon and exon‐1/intron‐1 splice junction. Down‐regulation of rbm47 resulted in headless and small head phenotypes, which can be rescued by a wnt8a blocking morpholino. To further reveal the mechanism of rbm47s role in head development, microarrays were performed to screen genes differentially expressed in normal and knockdown embryos. epcam and a2ml were identified as the most significantly up‐ and down‐regulated genes, respectively. The microarrays also confirmed up‐regulation of several genes involved in head development, including gsk3a, otx2, and chordin, which are important regulators of Wnt signaling. Conclusions: Altogether, our findings reveal that Rbm47 is a novel RNA‐binding protein critical for head formation and embryonic patterning during zebrafish embryogenesis which may act through a Wnt8a signaling pathway. Developmental Dynamics 242:1395–1404, 2013.

Generalized real perturbation values with applications to the structured real controllability radius of LTI systems

This paper generalizes the notion of real perturbation values of a complex matrix to account for a more general perturbation structure. Formulas for computing the so-called generalized real perturbation values of a matrix are derived and presented. Using these results, we revisit the computation of the structured real controllability radius that was previously used to evaluate the robustness of the multi-link inverted pendulum system, and we also study a new normalized version of the transmission zero at s radius.

Subretinal gene delivery using helper-dependent adenoviral vectors

This study describes the successful delivery of helper-dependent adenoviral vectors to the mouse retina with long term and robust levels of reporter expression in the retina without apparent adverse effects. Since these vectors have a large cloning capacity, they have great potential to extend the success of gene therapy achieved using the adeno-associated viral vector.

The Real DFM Radius and Minimum Norm Plant Perturbation for General Control Information Flow Constraints

Abstract The real decentralized fixed mode radius measures how “near” a decentralized LTI system is from having a decentralized fixed mode (DFM) present. In this paper, some properties of the real DFM radius are discussed, a procedure for computing the actual system parametric perturbations that achieve the real DFM radius is presented, and the real DFM radius is extended to deal with structured perturbations and general information flow constraints. A study of applying the radius to determine how to pair system inputs and outputs to obtain a robust decentralized control system structure with respect to parametric perturbations is also presented.

The Transmission Zero at S Radius and the Minimum Phase Radius of LTI Systems

Abstract In this paper, the transmission zero at s radius and the minimum phase radius of a linear time-invariant (LTI) system are introduced. The former radius gives a measure of how “near” a LTI system is from having a transmission zero at a specified point s ∈ ℂ in the complex plane, and the latter radius measures how “near” a minimum phase system is to a nonminimum phase system. Formulas for computing both radii are presented, along with the procedures for constructing the minimum norm perturbations that achieve the respective radius. Some properties of the two radii and numerical examples are also given.

Computation of the Real Controllability Radius and Minimum-Norm Perturbations of Higher-Order, Descriptor, and Time-Delay LTI Systems

A linear time-invariant (LTI) systems controllability radius measures the norm of the smallest parametric perturbation such that the perturbed system is uncontrollable, and is of practical importance. In this note, we study the real controllability radii of i) higher-order systems; ii) descriptor systems; and iii) time-delay systems, where the perturbations are restricted to the set of real values, and the spectral norm is considered. Formulas for these radii are presented using a framework involving generalized real perturbation values, which has certain computational advantages over other formulations found in the literature. In particular, the formulas are readily more computable, especially for higher-dimensional systems, and a minimum-norm perturbation can also easily be obtained. Numerical examples are presented.

Highly efficient retinal gene delivery with helper-dependent adenoviral vectors

There have been significant advancements in the field of retinal gene therapy in the past several years. In particular, therapeutic efficacy has been achieved in three separate human clinical trials conducted to assess the ability of adeno-associated viruses (AAV) to treat of a type of Lebers congenital amaurosis caused by RPE65 mutations. However, despite the success of retinal gene therapy with AAV, challenges remain for delivering large therapeutic genes or genes requiring long DNA regulatory elements for controlling their expression. For example, Stargardts disease, a form of juvenile macular degeneration, is caused by defects in ABCA4, a gene that is too large to be packaged in AAV. Therefore, we investigated the ability of helper dependent adenovirus (HD-Ad) to deliver genes to the retina as it has a much larger transgene capacity. Using an EGFP reporter, our results showed that HD-Ad can transduce the entire retinal epithelium of a mouse using a dose of only 1 × 105 infectious units and maintain transgene expression for at least 4 months. The results demonstrate that HD-Ad has the potential to be an effective vector for the gene therapy of the retina.

Multivariable three-term optimal controller design for large-scale systems

This paper deals with the design of controllers for large-scale linear time-invariant multi-input multi-output systems, such as those that often arise in process control. We focus on two standard controller objectives: (i) asymptotic regulation subject to unmeasurable constant disturbances, and/or (ii) asymptotic tracking of constant set-points. In either case, standard output-feedback controller design methodologies typically result in controllers that have order at least as large as that of the plant; for large-scale systems, the controller order can consequently be impractically large. The purpose of this paper is to introduce, for the subclass of plants that are open-loop stable, a low-order three-term (i.e., PID) multivariable control design approach that is practical to compute numerically, even for large-scale systems. A design algorithm and existence results to construct such a controller are given, and the approach is applied to several examples. Remarkably, at least for the examples considered, the three-term controllers performance is quite similar to that achieved by the standard (much higher order) controller that solves the servomechanism problem