Mutations in the gene encoding the amyloid protein precursor (APP) cause autosomal dominant Alzheimer's disease. Cleavage of APP by unidentified proteases, referred to as beta- and gamma-secretases, generates the amyloid beta-peptide, the main component of the amyloid plaques found in Alzheimer's disease patients. The disease-causing mutations flank the protease cleavage sites in APP and facilitate its cleavage. Here we identify a new membrane-bound aspartyl protease (Asp2) with beta-secretase activity. The Asp2 gene is expressed widely in brain and other tissues. Decreasing the expression of Asp2 in cells reduces amyloid beta-peptide production and blocks the accumulation of the carboxy-terminal APP fragment that is created by beta-secretase cleavage. Solubilized Asp2 protein cleaves a synthetic APP peptide substrate at the beta-secretase site, and the rate of cleavage is increased tenfold by a mutation associated with early-onset Alzheimer's disease in Sweden. Thus, Asp2 is a new protein target for drugs that are designed to block the production of amyloid beta-peptide peptide and the consequent formation of amyloid plaque in Alzheimer's disease.

We report, for the first time to the best of our knowledge, spectral phase characterization and line-by-line pulse shaping of an optical frequency comb generated by nonlinear wave mixing in a microring resonator. Through programmable pulse shaping the comb is compressed into a train of near-transform-limited pulses of ≈ 300 fs duration (intensity full width half maximum) at 595 GHz repetition rate. An additional, simple example of optical arbitrary waveform generation is presented. The ability to characterize and then stably compress the frequency comb provides new data on the stability of the spectral phase and

Release of A␤ peptides from ␤-amyloid precursor protein (APP) requires sequential cleavage by two endopeptidases, ␤-and ␥-secretases. ␤-Secretase was recently identified as a novel membrane-bound aspartyl protease, named BACE1, Asp2, or memapsin 2. Employing confocal microscopy and subcellular fractionation, we have found that BACE1 is largely situated in the distal Golgi membrane with a minor presence in the endoplasmic reticulum, endosomes, and plasma membrane in human neuroblastoma SHEP cells and in mouse Neuro-2a cell lines expressing either endogenous mouse BACE1 or additional exogenous human BACE1. The major cellular ␤-secretase activity is located in the late Golgi apparatus, consistent with its cellular localization. Furthermore, we demonstrate that the single transmembrane domain of BACE1 alone determines the retention of BACE1 to the Golgi compartments, through examination of recombinant proteins of various BACE1 fragments fused to a reporter green fluorescence protein. In addition, we show that the transmembrane domain of BACE1 is required for the access of BACE1 enzymatic activity to the cellular APP substrate and hence for the optimal generation of the C-terminal fragment of APP (CTF99). The results suggest a molecular and cell biological mechanism for the regulation of ␤-secretase activity in vivo.The pathological hallmarks of Alzheimer's disease are neuritic plaques and neurofibrillary tangles (see reviews in Refs. 1-3). The neuritic plaques, also known as senile plaques, are predominantly composed of A␤, a cluster of aggregated and heterogeneous peptides of 39 -43 amino acids (4). The most pathogenic A␤ peptide is the less soluble 42-amino acid peptide (A␤ 42 ), although the concentration of A␤ 40 is generally much higher than A␤ 42 . Excision of A␤ peptides from their amyloid precursor protein requires sequential proteolytic cleavages by the ␤-and ␥-secretases, respectively. ␤-secretase cleaves APP 1 to produce two major components: secreted ectodomain sAPP ␤ and the C-terminal fragment CTF99. The latter can be further cleaved by ␥-secretase to release A␤. Genetic linkage analysis suggests that increased total A␤ levels or an increase in the ratio of A␤ 42 /A␤ 40 is associated with the severity of AD dementia (reviewed in Refs.

Binding of the gp120 envelope glycoprotein to the CD4 receptor is the first step in the HIV-1 infectious cycle. Although the CD4-binding site has been extensively characterized, the initial receptor interaction has been difficult to study because of major CD4-induced structural rearrangements. Here, we use cryogenic electron microscopy to visualize the initial contact of CD4 with the HIV-1-envelope trimer at 6.8-Å resolution. A single CD4 molecule is embraced by a quaternary HIV-1-Env surface formed by coalescence of the previously defined CD4-contact region with a second CD4-binding site (CD4-BS2) in the inner domain of a neighboring gp120 protomer. Disruption of CD4-BS2 destabilized CD4-trimer interaction and abrogated HIV-1 infectivity by preventing acquisition of coreceptor-binding competence. A corresponding reduction in HIV-1 infectivity occurred upon mutation of CD4 residues interacting with CD4-BS2. These results document the critical role of quaternary interactions in the initial HIV-1 envelope-receptor contact, with implications for treatment and vaccine design.

A moiré pattern is created by superimposing two black-and-white or gray-scale patterns of regular geometry, such as two sets of evenly spaced lines. We observed an analogous effect between two transparent phase masks in a light beam which occurs at a distance. This phase moiré effect and the classic moiré effect are shown to be the two ends of a continuous spectrum. The phase moiré effect allows the detection of sub-resolution intensity or phase patterns with a transparent screen. When applied to x-ray imaging, it enables a polychromatic far-field interferometer (PFI) without absorption gratings. X-ray interferometry can non-invasively detect refractive index variations inside an object1–10. Current bench-top interferometers operate in the near field with limitations in sensitivity and x-ray dose efficiency2, 5, 7–10. The universal moiré effect helps overcome these limitations and obviates the need to make hard x-ray absorption gratings of sub-micron periods.

Sensitive transduction of the motion of a microscale cantilever is central to many applications in mass, force, magnetic resonance, and displacement sensing. Reducing cantilever size to nanoscale dimensions can improve the bandwidth and sensitivity of techniques like atomic force microscopy, but current optical transduction methods suffer when the cantilever is small compared to the achievable spot size. Here, we demonstrate sensitive optical transduction in a monolithic cavity-optomechanical system in which a sub-picogram silicon cantilever with a sharp probe tip is separated from a microdisk optical resonator by a nanoscale gap. High quality factor (Q ≈ 10 5 ) microdisk optical modes transduce the cantilever's MHz frequency thermally-driven vibrations with a displacement sensitivity of ≈ 4.4×10 −16 m/ √ Hz and bandwidth > 1 GHz, and a dynamic range > 10 6 is estimated for a 1 s measurement. Optically-induced stiffening due to the strong optomechanical interaction is observed, and engineering of probe dynamics through cantilever design and electrostatic actuation is illustrated.Micro-and nanoscale cantilevers are at the heart of many applications in mass, force, magnetic resonance, and displacement sensing 1,2,3,4 . In atomic force microscopy (AFM) 5 , the push towards smaller cantilevers 6,7 is motivated by the ability to increase mechanical frequencies while maintaining a desired level of stiffness. This influences the force sensitivity and measurement bandwidth, in turn determining the image acquisition rate and ability to resolve time-dependent forces and acquire additional information about the tip-sample interaction potential 8 . Standard optical methods for transducing cantilever motion include beam deflection 9 and laser interferometry 10 , and in macroscopic devices that are 1 mm × 1 mm × 60 µm (length, width, and height), quantumlimited displacement sensitivity of 4×10 −19 m/ √ Hz has been achieved 11 . Interferometric approaches using a high numerical aperture objective have also been used in micro-scale devices, resulting in displacement sensitivities of 3×10 −14 m/ √ Hz for cantilevers that are 20 µm × 4 µm× 0.2 µm and 1×10−15 m/ √ Hz for larger conventional cantilevers (223 µm × 31 µm× 6.7 µm) 12 . However, as the cantilever dimensions are pushed below the detection wavelength, diffraction effects limit the sensitivity of these approaches 13 , and near-field optics and/or integrated on-chip detection methods can be of significant benefit.To that end, researchers have recently used evanescently coupled on-chip waveguides 14 acting as doublyclamped cantilevers 15,16 to demonstrate displacement sensitivities of 3.5×10 −14 m/ √ Hz, while end-to-end waveguides acting as singly-clamped devices 17 have achieved similar performance 18 . Although these waveguide-based approaches are optically broadband, the strong, multi-pass interaction provided by optical cavities can be of considerable advantage. Cavity optomechanics 19,20,21 has seen substantial recent progress, where in many cases the optical resonato...

The natural history of HIV-1 infection is highly variable in different individuals, spanning from a rapidly progressive course to a longterm asymptomatic infection. A major determinant of the pace of disease progression is the in vivo level of HIV-1 replication, which is regulated by a complex network of cytokines and chemokines expressed by immune and inflammatory cells. The chemokine system is critically involved in the control of HIV-1 replication by virtue of the role played by specific chemokine receptors, most notably CCR5 and CXCR4, as cell-surface coreceptors for HIV-1 entry; hence, the chemokines that naturally bind such coreceptors act as endogenous inhibitors of HIV-1. Here, we show that the CXC chemokine CXCL4 (PF-4), the most abundant protein contained within the α-granules of platelets, is a broad-spectrum inhibitor of HIV-1 infection. Unlike other known HIV-suppressive chemokines, CXCL4 inhibits infection by the majority of primary HIV-1 isolates regardless of their coreceptor-usage phenotype or genetic subtype. Consistent with the lack of viral phenotype specificity, blockade of HIV- 1 infection occurs at the level of virus attachment and entry via a unique mechanism that involves direct interaction of CXCL4 with the major viral envelope glycoprotein, gp120. The binding site for CXCL4 was mapped to a region of the gp120 outer domain proximal to the CD4-binding site. The identification of a platelet-derived chemokine as an endogenous antiviral factor may have relevance for the pathogenesis and treatment of HIV-1 infection

The intestinal mucosa is a key anatomical site for HIV-1 replication and CD4+ T-cell depletion. Accordingly, in vivo treatment with an antibody to the gut-homing integrin α4β7 was shown to reduce viral transmission, delay disease progression, and induce persistent virus control in macaques challenged with SIV. Here, we show that integrin α4β7 is efficiently incorporated into the envelope of HIV-1 virions. Incorporated α4β7 is functionally active as it binds MAdCAM-1, promoting HIV-1 capture by and infection of MAdCAM-expressing cells, which in turn mediate trans-infection of bystander cells. Functional α4β7 is present in circulating virions from HIV-infected patients and SIV-infected macaques, with peak levels during the early stages of infection. In vivo homing experiments documented selective and specific uptake of α4β7+ HIV-1 virions by high endothelial venules in the intestinal mucosa. These results extend the paradigm of tissue homing to a retrovirus and are relevant for the pathogenesis, treatment and prevention of HIV-1 infection.