High particle uniformity, high photoluminescence quantum yields, narrow and symmetric emission spectral lineshapes and minimal single dot emission intermittency (known as blinking) have been recognized as universal requirements for the successful use of colloidal quantum dots (QDs) in nearly all optical applications. However, synthesizing samples that simultaneously meet all these four criteria has proven challenging. Here, we report the synthesis of such high-quality CdSe/CdS core/shell QDs in an optimized process which maintains a slow growth rate of the shell through the use of octanethiol and cadmium oleate as precursors. In contrast with previous observations, single-QD blinking is significantly suppressed with only a relatively thin shell. In addition, we demonstrate the elimination of the ensemble luminescence photodarkening that is an intrinsic consequence of QD blinking statistical aging. Furthermore, the small size and high photoluminescence quantum yields of these novel QDs render them superior in vivo imaging agents compared to conventional QDs. We anticipate that this new generation of QDs will also result in significant improvement in the performance of QDs in other applications such as solid-state lighting and illumination.

Gold nanorods excited at 830 nm on a far-field laser-scanning microscope produced strong two-photon luminescence (TPL) intensities, with a cos 4 dependence on the incident polarization. The TPL excitation spectrum can be superimposed onto the longitudinal plasmon band, indicating a plasmon-enhanced two-photon absorption cross section. The TPL signal from a single nanorod is 58 times that of the two-photon fluorescence signal from a single rhodamine molecule. The application of gold nanorods as TPL imaging agents is demonstrated by in vivo imaging of single nanorods flowing in mouse ear blood vessels.in vivo imaging ͉ plasmon resonance ͉ multiphoton ͉ nonlinear optics P hotoluminescence from noble metals was first reported in 1969 by Mooradian (1) and later observed as a broad background in surface-enhanced Raman scattering (2). Singlephoton luminescence from metals has been described as a three-step process as follows: (i) excitation of electrons from the d-to the sp-band to generate electron-hole pairs, (ii) scattering of electrons and holes on the picosecond timescale with partial energy transfer to the phonon lattice, and (iii) electron-hole recombination resulting in photon emission (1). Two-photon luminescence (TPL) was characterized by Boyd et al. (3) and is considered to be produced by a similar mechanism as singlephoton luminescence, but the relatively weak TPL signal can be amplified by several orders of magnitude when produced from roughened metal substrates. This amplification is due to a resonant coupling with localized surface plasmons, which are well known to enhance a variety of linear and nonlinear optical properties (4-9).Metal nanoparticles are also capable of photoluminescence, which has been shown to correlate strongly with their welldefined plasmon resonances (10-16). For example, Mohamed et al. (11) have observed that the quantum efficiency of singlephoton luminescence from gold nanorods is enhanced by a factor of Ͼ1 million under plasmon-resonant conditions. Plasmonresonant TPL is attractive for nonlinear optical imaging of biological samples with 3D spatial resolution (17). Gold nanorods are particularly appealing as TPL substrates because their longitudinal plasmon modes are resonant at near-infrared, where the absorption of water and biological molecules are minimized. Moreover, nanorods have larger local field enhancement factors than nanoparticles due to reduced plasmon damping (18). A scanning near-field optical microscopy study of TPL from single nanorods (diameter Ϸ 40 nm) has recently been reported by Imura et al. (16), who observed that the luminescence is greatest at their tips. However, further characterization of TPL from single gold nanorods is needed: the polarization dependence of TPL excitation and emission from nanorods has yet to be defined, as well as the relationship between TPL enhancement and the longitudinal and transverse plasmon modes. These studies can provide a deeper understanding of single-particle TPL and its potential application in nonlinear optical imag...

TGF-␤ can signal by means of Smad transcription factors, which are quintessential tumor suppressors that inhibit cell proliferation, and by means of Smad-independent mechanisms, which have been implicated in tumor progression. Although Smad mutations disable this tumor-suppressive pathway in certain cancers, breast cancer cells frequently evade the cytostatic action of TGF-␤ while retaining Smad function. Through immunohistochemical analysis of human breast cancer bone metastases and functional imaging of the Smad pathway in a mouse xenograft model, we provide evidence for active Smad signaling in human and mouse bonemetastatic lesions. Genetic depletion experiments further demonstrate that Smad4 contributes to the formation of osteolytic bone metastases and is essential for the induction of IL-11, a gene implicated in bone metastasis in this mouse model system. Activator protein-1 is a key participant in Smad-dependent transcriptional activation of IL-11 and its overexpression in bone-metastatic cells. Our findings provide functional evidence for a switch of the Smad pathway, from tumor-suppressor to prometastatic, in the development of breast cancer bone metastasis.IL-11 ͉ Smad4 ͉ TGF-␤ T GF-␤ plays a crucial role as a growth-inhibitory cytokine in many tissues (1, 2). The cytostatic effect of TGF-␤ is mediated by a serine͞threonine kinase receptor complex that phosphorylates Smad2 and Smad3, which then translocate into the nucleus and bind Smad4 to generate transcriptional regulatory complexes (3). SMAD4 (also known as Deleted in Pancreatic Carcinoma locus 4 or DPC4) and, to a lesser extent, SMAD2 suffer mutational inactivation in a proportion of pancreatic and colon cancers (1, 2). However, tumor cells that evade this antiproliferative control by other mechanisms may display an altered sensitivity to TGF-␤ and undergo tumorigenic progression in response to this cytokine (1, 2). Patients whose pancreatic or colon tumors express TGF-␤ receptors fare less well than those with low or absent TGF-␤ receptor expression in the tumor (4). In mouse models of breast cancer, TGF-␤ signaling promotes lung (5, 6) and bone metastasis (7). In the case of osteolytic bone metastasis by breast cancer cells, it has been proposed that TGF-␤ released from the decaying bone matrix stimulates neighboring tumor cells, establishing a vicious cycle that exacerbates the growth of the metastatic lesion (8).The TGF-␤ signaling mechanisms that foster metastasis in human cancer are an important open question and a subject of debate. Because Smad factors are quintessential tumor suppressors, the basis for the protumorigenic effects of TGF-␤ has been sought in the Smad-independent signaling pathways that may be triggered by TGF-␤. Results obtained by means of overexpression of dominant negative mutant components of the Rho pathway (9, 10) or pharmacologic inhibitors of p38 mitogen-activated protein kinase (11, 12) have implicated these pathways in the proinvasive and metastatic effects of TGF-␤ in transformed cells. In contrast, results obta...

Tissue homeostasis in mammals relies on powerful cytostatic and differentiation signals delivered by the cytokine TGFbeta and relayed within the cell via the activation of Smad transcription factors. Formation of transcription regulatory complexes by the association of Smad4 with receptor-phosphorylated Smads 2 and 3 is a central event in the canonical TGFbeta pathway. Here we provide evidence for a branching of this pathway. The ubiquitious nuclear protein Transcriptional Intermediary Factor 1gamma (TIF1gamma) selectively binds receptor-phosphorylated Smad2/3 in competition with Smad4. Rapid and robust binding of TIF1gamma to Smad2/3 occurs in hematopoietic, mesenchymal, and epithelial cell types in response to TGFbeta. In human hematopoietic stem/progenitor cells, where TGFbeta inhibits proliferation and stimulates erythroid differentiation, TIF1gamma mediates the differentiation response while Smad4 mediates the antiproliferative response with Smad2/3 participating in both responses. Thus, Smad2/3-TIF1gamma and Smad2/3-Smad4 function as complementary effector arms in the control of hematopoietic cell fate by the TGFbeta/Smad pathway.

A multitude of heptahelical receptors use heterotrimeric G proteins to transduce signals to specific effector target molecules. The G protein transducin, Gt, couples photon-activated rhodopsin with the effector cyclic GMP phosophodiesterase (PDE) in the vertebrate phototransduction cascade. The interactions of the Gt alpha-subunit (alpha(t)) with the inhibitory PDE gamma-subunit (PDEgamma) are central to effector activation, and also enhance visual recovery in cooperation with the GTPase-activating protein regulator of G-protein signalling (RGS)-9 (refs 1-3). Here we describe the crystal structure at 2.0 A of rod transducin alpha x GDP x AlF4- in complex with the effector molecule PDEgamma and the GTPase-activating protein RGS9. In addition, we present the independently solved crystal structures of the RGS9 RGS domain both alone and in complex with alpha(t/i1) x GDP x AlF4-. These structures reveal insights into effector activation, synergistic GTPase acceleration, RGS9 specificity and RGS activity. Effector binding to a nucleotide-dependent site on alpha(t) sequesters PDEgamma residues implicated in PDE inhibition, and potentiates recruitment of RGS9 for hydrolytic transition state stabilization and concomitant signal termination.

The rod outer segment phototransduction GAP (GTPase-accelerating protein) has been identified as RGS9, a member of the RGS family of G alpha GAPs. RGS9 mRNA expression is specific for photoreceptor cells, and RGS9 protein colocalizes with other phototransduction components to photoreceptor outer segment membranes. The RGS domain of RGS9 accelerates GTP hydrolysis by the visual G protein transducin (G alpha(t)), and this acceleration is enhanced by the gamma subunit of the phototransduction effector cGMP phosphodiesterase (PDEgamma). These unique properties of RGS9 match those of the rod outer segment GAP and implicate it as a key element in the recovery phase of visual transduction.

Medical imaging is routine in the diagnosis and staging of a wide range of medical conditions. In particular, magnetic resonance imaging (MRI) is critical for visualizing soft tissue and organs, with over 60 million MRI procedures performed each year worldwide. About one-third of these procedures are contrast-enhanced MRI, and gadolinium-based contrast agents (GBCAs) are the mainstream MRI contrast agents used in the clinic. GBCAs have shown efficacy and are safe to use with most patients; however, some GBCAs have a small risk of adverse effects, including nephrogenic systemic fibrosis (NSF), the untreatable condition recently linked to gadolinium (Gd) exposure during MRI with contrast. In addition, Gd deposition in the human brain has been reported following contrast, and this is now under investigation by the US Food and Drug Administration (FDA). To address a perceived need for a Gdfree contrast agent with pharmacokinetic and imaging properties comparable to GBCAs, we have designed and developed zwitterioncoated exceedingly small superparamagnetic iron oxide nanoparticles (ZES-SPIONs) consisting of ∼3-nm inorganic cores and ∼1-nm ultrathin hydrophilic shell. These ZES-SPIONs are free of Gd and show a high T 1 contrast power. We demonstrate the potential of ZES-SPIONs in preclinical MRI and magnetic resonance angiography. exceedingly small iron oxide nanoparticles | renal clearance | gadoliniumfree positive MR contrast agent | preclinical magnetic resonance imaging M RI signal arises from the excitation of low-energy nuclear spins, which are formed in a permanent magnetic field, by applying radiofrequency pulses followed by the measurement of the spin relaxation processes (i.e

Understanding the in vivo behavior of nanoparticles is critical for the translation of nanomedicine from laboratory research to clinical trials. In this work, in vivo Forster resonance energy transfer (FRET) imaging was employed to monitor the release of hydrophobic molecules from circulating poly(ethylene glycol)-poly( D, L-lactic acid) (PEG-PDLLA) micelles. A lipophilic FRET pair (DiIC(18) and DiOC(18)) was physically entrapped into micelle cores by mimicking the loading of hydrophobic drugs. The FRET efficiency was found significantly reduced within 15 min after intravenous injection, implying that DiIC(18) and DiOC(18) quickly escaped from the circulating micelles. FRET spectroscopy studies further demonstrated that alpha- and beta-globulins were major factors for the observed fast release, while gamma-globulins, albumin, and red blood cells played minor roles. These results provide useful information for developing blood-stable micelles to deliver hydrophobic drugs to the target site via prolonged circulation and extravasation from the vascular system.