Sensitivity of neural-hemodynamic coupling to alterations in cerebral blood flow during hypercapnia.

The relationship between measurements of cerebral blood oxygenation and neuronal activity is highly complex and depends on both neurovascular and neurometabolic biological coupling. While measurements of blood oxygenation changes via optical and MRI techniques have been developed to map functional brain activity, there is evidence that the specific characteristics of these signals are sensitive to the underlying vascular physiology and structure of the brain. Since baseline blood flow and oxygen saturation may vary between sessions and across subjects, functional blood oxygenation changes may be a less reliable indicator of brain activity in comparison to blood flow and metabolic changes. In this work, we use a biomechanical model to examine the relationships between neural, vascular, metabolic, and hemodynamic responses to parametric whisker stimulation under both normal and hypercapnic conditions in a rat model. We find that the relationship between neural activity and oxy- and deoxyhemoglobin changes is sensitive to hypercapnia-induced changes in baseline cerebral blood flow. In contrast, the underlying relationships between evoked neural activity, blood flow, and model-estimated oxygen metabolism changes are unchanged by the hypercapnic challenge. We conclude that evoked changes in blood flow and cerebral oxygen metabolism are more closely associated with underlying evoked neuronal responses.

Mutations in amyloid precursor protein affect its interactions with presenilin/gamma-secretase.

Alzheimer's disease is characterized by accumulation of toxic beta-amyloid (Abeta) in the brain and neuronal death. Several mutations in presenilin (PS1) and beta-amyloid precursor protein (APP) associate with an increased Abeta(42/40) ratio. Abeta(42), a highly fibrillogenic species, is believed to drive Abeta aggregation. Factors shifting gamma-secretase cleavage of APP to produce Abeta(42) are unclear. We investigate the molecular mechanism underlying altered Abeta(42/40) ratios associated with APP mutations at codon 716 and 717. Using FRET-based fluorescence lifetime imaging to monitor APP-PS1 interactions, we show that I716F and V717I APP mutations increase the proportion of interacting molecules earlier in the secretory pathway, resulting in an increase in Abeta generation. A PS1 conformation assay reveals that, in the presence of mutant APP, PS1 adopts a conformation reminiscent of FAD-associated PS1 mutations, thus influencing APP binding to PS1/gamma-secretase. Mutant APP affects both intracellular location and efficiency of APP-PS1 interactions, thereby changing the Abeta(42/40) ratio.

Simultaneous multispectral reflectance imaging and laser speckle flowmetry of cerebral blood flow and oxygen metabolism in focal cerebral ischemia.

Real-time investigation of cerebral blood flow (CBF), and oxy- and deoxyhemoglobin concentration (HbO, HbR) dynamics has been difficult until recently due to limited spatial and temporal resolution of techniques like laser Doppler flowmetry and magnetic resonance imaging (MRI). The combination of laser speckle flowmetry (LSF) and multispectral reflectance imaging (MSRI) yields high-resolution spatiotemporal maps of hemodynamic and metabolic changes in response to functional cortical activation. During acute focal cerebral ischemia, changes in HbO and HbR are much larger than in functional activation, resulting in the failure of the Beer-Lambert approximation to yield accurate results. We describe the use of simultaneous LSF and MSRI, using a nonlinear Monte Carlo fitting technique, to record rapid changes in CBF, HbO, HbR, and cerebral metabolic rate of oxygen (CMRO(2)) during acute focal cerebral ischemia induced by distal middle cerebral artery occlusion (dMCAO) and reperfusion. This technique captures CBF and CMRO(2) changes during hemodynamic and metabolic events with high temporal and spatial resolution through the intact skull and demonstrates the utility of simultaneous LSF and MSRI in mouse models of cerebrovascular disease.

A multicompartment vascular model for inferring baseline and functional changes in cerebral oxygen metabolism and arterial dilation.

Functional hemodynamic responses are the composite results of underlying variations in cerebral oxygen consumption and the dilation of arterial vessels after neuronal activity. The development of biophysically based models of the cerebral vasculature allows the separation of the neuro-metabolic and neuro-vascular influences on measurable hemodynamic signals such as functional magnetic resonance imaging or optical imaging. We describe a multicompartment model of the vascular and oxygen transport dynamics associated with stimulus-driven neuronal activation. Our model offers several unique features compared with previous formulations such as the ability to estimate baseline blood flow, volume, and oxygen consumption from functional data. In addition, we introduce a capillary compliance model, arterial and venous oxygen permeability, and model the dynamics of extravascular tissue oxygenation. We apply this model to multimodal optical spectroscopic and laser speckle imaging of the rat somato-sensory cortex during nine conditions of whisker stimulation. By fitting the model using a psuedo-Bayesian framework to incorporate multimodal observations, we estimate baseline blood flow to be 94 (+/-15) mL/100 g min and baseline oxygen consumption to be 6.7 (+/-1.3) mL O(2)/100 g min. We calculate parametric, linear increases in arterial dilation (R(2)=0.96) and CMRO(2) (R(2)=0.87) responses over the nine conditions. Other parameters estimated by the model include vascular transit time and volume reserve, oxygen content, saturation, diffusivity rate constants, and partial pressure of oxygen in the vascular compartments and in the extravascular tissue. Finally, we compare this model to earlier work and find that the multicompartment model more accurately describes the observed oxygenation changes when compared with a single compartment version.

Time-domain fluorescent plate reader for cell based protein-protein interaction and protein conformation assays.

Fluorescence lifetime measurement is widely used in the biological sciences due to its inherent sensitivity and concentration independence. Frequency domain high-throughput plate readers and time-resolved energy transfer (TRET) plate readers are in common use and have been successful in a variety of applications ranging from basic biochemistry to drug discovery. Time-domain systems would have advantages due to their ability to distinguish both FRETing and non-FRETing populations, but have been difficult to develop due to inherent difficulties with background autofluorescence and lifetime component separation. Using a modified commercial lifetime plate reader, we demonstrate a method for removal of the complex auto-fluorescent background decay, described using a stretched exponential function (StrEF). We develop a generalized multi-exponential fitting algorithm (GeMEF), which progressively accounts for confounding lifetime components in FRET-based assays using a series of control experiments. We demonstrate the separability of FRET strength and efficiency and apply the technique to protein-protein interactions and protein conformational assays in a cell-based format. Presenilin 1 (PS1) is known to be important in Amyloid Precursor Protein (APP) processing in Alzheimer's disease. Using transfected cells, we demonstrate APP-PS1 interactions by FRET in a cell-based, 96-well plate format.

Ubiquilin 1 modulates amyloid precursor protein trafficking and Abeta secretion.

Ubiquilin 1 (UBQLN1) is a ubiquitin-like protein, which has been shown to play a central role in regulating the proteasomal degradation of various proteins, including the presenilins. We recently reported that DNA variants in UBQLN1 increase the risk for Alzheimer disease, by influencing expression of this gene in brain. Here we present the first assessment of the effects of UBQLN1 on the metabolism of the amyloid precursor protein (APP). For this purpose, we employed RNA interference to down-regulate UBQLN1 in a variety of neuronal and non-neuronal cell lines. We demonstrate that down-regulation of UBQLN1 accelerates the maturation and intracellular trafficking of APP, while not interfering with alpha-, beta-, or gamma-secretase levels or activity. UBQLN1 knockdown increased the ratio of APP mature/immature, increased levels of full-length APP on the cell surface, and enhanced the secretion of sAPP (alpha- and beta-forms). Moreover, UBQLN1 knockdown increased levels of secreted Abeta40 and Abeta42. Finally, employing a fluorescence resonance energy transfer-based assay, we show that UBQLN1 and APP come into close proximity in intact cells, independently of the presence of the presenilins. Collectively, our findings suggest that UBQLN1 may normally serve as a cytoplasmic "gatekeeper" that may control APP trafficking from intracellular compartments to the cell surface. These findings suggest that changes in UBQLN1 steady-state levels affect APP trafficking and processing, thereby influencing the generation of Abeta.

Interaction between presenilin 1 and ubiquilin 1 as detected by fluorescence lifetime imaging microscopy and a high-throughput fluorescent plate reader.

Presenilin 1 (PS1) in its active heterodimeric form is the catalytic center of the gamma-secretase complex, an enzymatic activity that cleaves amyloid precursor protein (APP) to produce amyloid beta (Abeta). Ubiquilin 1 is a recently described PS1 interacting protein, the overexpression of which increases PS1 holoprotein levels and leads to reduced levels of functionally active PS1 heterodimer. In addition, it has been suggested that splice variants of the UBQLN1 gene are associated with an increased risk of developing Alzheimer disease (AD). However, it is still unclear whether PS1 and ubiquilin 1 interact when expressed at endogenous levels under normal physiological conditions. Here, we employ three novel fluorescence resonance energy transfer-based techniques to investigate the interaction between PS1 and ubiquilin 1 in intact cells. We consistently find that the ubiquilin 1 N terminus is in close proximity to several epitopes on PS1. We show that ubiquilin 1 interacts both with PS1 holoprotein and heterodimer and that the interaction between PS1 and ubiquilin 1 takes place near the cell surface. Furthermore, we show that the PS1-ubiquilin 1 interaction can be detected between endogenous proteins in primary neurons in vitro as well as in brain tissue of healthy controls and Alzheimer disease patients, providing evidence of its physiological relevance.