Uniform Polarity Microtubule Assemblies Imaged in Native Brain Tissue by Second-harmonic Generation Microscopy

Proceedings of the National Academy of Sciences of the United States of America. Jun, 2003  |  Pubmed ID: 12766225

Microtubule (MT) ensemble polarity is a diagnostic determinant of the structure and function of neuronal processes. Here, polarized MT structures are selectively imaged with second-harmonic generation (SHG) microscopy in native brain tissue. This SHG is found to colocalize with axons in both brain slices and cultured neurons. Because SHG arises only from noninversion symmetric structures, the uniform polarity of axonal MTs leads to the observed signal, whereas the mixed polarity in dendrites leads to destructive interference. SHG imaging provides a tool to investigate the kinetics and function of MT ensemble polarity in dynamic native brain tissue structures and other subcellular motility structures based on polarized MTs.

In Vivo Multiphoton Microscopy of Deep Brain Tissue

Journal of Neurophysiology. Apr, 2004  |  Pubmed ID: 14668300

Although fluorescence microscopy has proven to be one of the most powerful tools in biology, its application to the intact animal has been limited to imaging several hundred micrometers below the surface. The rest of the animal has eluded investigation at the microscopic level without excising tissue or performing extensive surgery. However, the ability to image with subcellular resolution in the intact animal enables a contextual setting that may be critical for understanding proper function. Clinical applications such as disease diagnosis and optical biopsy may benefit from minimally invasive in vivo approaches. Gradient index (GRIN) lenses with needle-like dimensions can transfer high-quality images many centimeters from the object plane. Here, we show that multiphoton microscopy through GRIN lenses enables minimally invasive, subcellular resolution several millimeters in the anesthetized, intact animal, and we present in vivo images of cortical layer V and hippocampus in the anesthetized Thy1-YFP line H mouse. Microangiographies from deep capillaries and blood vessels containing fluorescein-dextran and quantum dot-labeled serum in wild-type mouse brain are also demonstrated.

Neural Activity Triggers Neuronal Oxidative Metabolism Followed by Astrocytic Glycolysis

Science (New York, N.Y.). Jul, 2004  |  Pubmed ID: 15232110

We have found that two-photon fluorescence imaging of nicotinamide adenine dinucleotide (NADH) provides the sensitivity and spatial three-dimensional resolution to resolve metabolic signatures in processes of astrocytes and neurons deep in highly scattering brain tissue slices. This functional imaging reveals spatiotemporal partitioning of glycolytic and oxidative metabolism between astrocytes and neurons during focal neural activity that establishes a unifying hypothesis for neurometabolic coupling in which early oxidative metabolism in neurons is eventually sustained by late activation of the astrocyte-neuron lactate shuttle. Our model integrates existing views of brain energy metabolism and is in accord with known macroscopic physiological changes in vivo.

Conformational Dependence of Intracellular NADH on Metabolic State Revealed by Associated Fluorescence Anisotropy

The Journal of Biological Chemistry. Jul, 2005  |  Pubmed ID: 15863500

Global analysis of fluorescence and associated anisotropy decays of intrinsic tissue fluorescence offers a sensitive and non-invasive probe of the metabolically critical free/enzyme-bound states of intracellular NADH in neural tissue. Using this technique, we demonstrate that the response of NADH to the metabolic transition from normoxia to hypoxia is more complex than a simple increase in NADH concentration. The concentration of free NADH, and that of an enzyme bound form with a relatively low lifetime, increases preferentially over that of other enzyme bound NADH species. Concomitantly, the intracellular viscosity is reduced, likely due to the osmotic swelling of mitochondria. These conformation and environmental changes effectively decrease the tissue fluorescence average lifetime, causing the usual total fluorescence increase measurements to significantly underestimate the calculated concentration increase. This new discrimination of changes in NADH concentration, conformation, and environment provides the foundation for quantitative functional imaging of neural energy metabolism.

Cortical Spreading Depression Causes and Coincides with Tissue Hypoxia

Nature Neuroscience. Jun, 2007  |  Pubmed ID: 17468748

Cortical spreading depression (CSD) is a self-propagating wave of cellular depolarization that has been implicated in migraine and in progressive neuronal injury after stroke and head trauma. Using two-photon microscopic NADH imaging and oxygen sensor microelectrodes in live mouse cortex, we find that CSD is linked to severe hypoxia and marked neuronal swelling that can last up to several minutes. Changes in dendritic structures and loss of spines during CSD are comparable to those during anoxic depolarization. Increasing O2 availability shortens the duration of CSD and improves local redox state. Our results indicate that tissue hypoxia associated with CSD is caused by a transient increase in O2 demand exceeding vascular O2 supply.

HIV-1 Trans Activator of Transcription Protein Elicits Mitochondrial Hyperpolarization and Respiratory Deficit, with Dysregulation of Complex IV and Nicotinamide Adenine Dinucleotide Homeostasis in Cortical Neurons

Journal of Immunology (Baltimore, Md. : 1950). Jan, 2007  |  Pubmed ID: 17202348

HIV-1 causes a common, progressive neurological disorder known as HIV-associated dementia (HAD). The prevalence of this disorder has increased despite the use of highly active antiretroviral therapy, and its underlying pathogenesis remains poorly understood. However, evidence suggests that some aspects of HAD may be reversible. To model the reversible aspects of HAD, we have used the HIV-1 neurotoxin trans activator of transcription protein (Tat) to investigate nonlethal changes in cultured neurons. Exposure of rodent cortical neurons to sublethal concentrations of Tat elicits mitochondrial hyperpolarization. In this study, we used the cationic lipophilic dye rhodamine 123 to confirm this observation, and then performed follow-up studies to examine the mechanism involved. In intact neurons, we found Tat elicited a rapid drop in internal mitochondrial pH, and addition of Tat to purified mitochondrial extracts inhibited complex IV of the electron transport chain. To correlate enzyme activity in mitochondrial extracts with results in intact cells, we measured neuronal respiration following Tat exposure. Cortical neurons demonstrated decreased respiration upon Tat treatment, consistent with inhibition of complex IV. We examined mitochondrial Ca(2+) homeostasis using a mitochondrial targeted enhanced yellow fluorescent protein-calmodulin construct. We detected a decrease in mitochondrial calcium concentration following exposure to Tat. Finally, we measured the energy intermediate NAD(P)H after Tat treatment, and found a 20% decrease in the autofluorescence. Based on these findings, we suggest that decreased NADPH and calcium concentration contribute to subsequent respiratory decline after exposure to Tat, with detrimental effects on neuronal signaling.

A New Pathway for Lactate Production in the CNS

The Journal of Physiology. Mar, 2008  |  Pubmed ID: 18310130

Mitochondrial Superoxide Flashes: Metabolic Biomarkers of Skeletal Muscle Activity and Disease

FASEB Journal : Official Publication of the Federation of American Societies for Experimental Biology. Sep, 2011  |  Pubmed ID: 21646399

Mitochondrial superoxide flashes (mSOFs) are stochastic events of quantal mitochondrial superoxide generation. Here, we used flexor digitorum brevis muscle fibers from transgenic mice with muscle-specific expression of a novel mitochondrial-targeted superoxide biosensor (mt-cpYFP) to characterize mSOF activity in skeletal muscle at rest, following intense activity, and under pathological conditions. Results demonstrate that mSOF activity in muscle depended on electron transport chain and adenine nucleotide translocase functionality, but it was independent of cyclophilin-D-mediated mitochondrial permeability transition pore activity. The diverse spatial dimensions of individual mSOF events were found to reflect a complex underlying morphology of the mitochondrial network, as examined by electron microscopy. Muscle activity regulated mSOF activity in a biphasic manner. Specifically, mSOF frequency was significantly increased following brief tetanic stimulation (18.1 ± 1.6 to 22.3 ± 2.0 flashes/1000 μm²·100 s before and after 5 tetani) and markedly decreased (to 7.7 ± 1.6 flashes/1000 μm²·100 s) following prolonged tetanic stimulation (40 tetani). A significant temperature-dependent increase in mSOF frequency (11.9 ± 0.8 and 19.8 ± 2.6 flashes/1000 μm²·100 s at 23°C and 37°C) was observed in fibers from RYR1(Y522S/WT) mice, a mouse model of malignant hyperthermia and heat-induced hypermetabolism. Together, these results demonstrate that mSOF activity is a highly sensitive biomarker of mitochondrial respiration and the cellular metabolic state of muscle during physiological activity and pathological oxidative stress

Lactate Fuels the Neonatal Brain

Frontiers in Neuroenergetics. 2011  |  Pubmed ID: 21687795

Two-photon Microscopy of Cortical NADH Fluorescence Intensity Changes: Correcting Contamination from the Hemodynamic Response

Journal of Biomedical Optics. Oct, 2011  |  Pubmed ID: 22029350

Quantification of nicotinamide adenine dinucleotide (NADH) changes during functional brain activation and pathological conditions provides critical insight into brain metabolism. Of the different imaging modalities, two-photon laser scanning microscopy (TPLSM) is becoming an important tool for cellular-resolution measurements of NADH changes associated with cellular metabolic changes. However, NADH fluorescence emission is strongly absorbed by hemoglobin. As a result, in vivo measurements are significantly affected by the hemodynamics associated with physiological and pathophysiological manipulations. We model NADH fluorescence excitation and emission in TPLSM imaging based on precise maps of cerebral microvasculature. The effects of hemoglobin optical absorption and optical scattering from red blood cells, changes in blood volume and hemoglobin oxygen saturation, vessel size, and location with respect to imaging location are explored. A simple technique for correcting the measured NADH fluorescence intensity changes is provided, with the utilization of a parallel measurement of a physiologically inert fluorophore. The model is applied to TPLSM measurements of NADH fluorescence intensity changes in rat somatosensory cortex during mild hypoxia and hyperoxia. The general approach of the correction algorithm can be extended to other TPLSM measurements, where changes in the optical properties of the tissue confound physiological measurements, such as the detection of calcium dynamics.

Two-photon NADH Imaging Exposes Boundaries of Oxygen Diffusion in Cortical Vascular Supply Regions

Journal of Cerebral Blood Flow and Metabolism : Official Journal of the International Society of Cerebral Blood Flow and Metabolism. Jan, 2011  |  Pubmed ID: 20859293

Oxygen transport imposes a possible constraint on the brain's ability to sustain variable metabolic demands, but oxygen diffusion in the cerebral cortex has not yet been observed directly. We show that concurrent two-photon fluorescence imaging of endogenous nicotinamide adenine dinucleotide (NADH) and the cortical microcirculation exposes well-defined boundaries of tissue oxygen diffusion in the mouse cortex. The NADH fluorescence increases rapidly over a narrow, very low pO(2) range with a p(50) of 3.4 ± 0.6 mm Hg, thereby establishing a nearly binary reporter of significant, metabolically limiting hypoxia. The transient cortical tissue boundaries of NADH fluorescence exhibit remarkably delineated geometrical patterns, which define the limits of tissue oxygen diffusion from the cortical microcirculation and bear a striking resemblance to the ideal Krogh tissue cylinder. The visualization of microvessels and their regional contribution to oxygen delivery establishes penetrating arterioles as major oxygen sources in addition to the capillary network and confirms the existence of cortical oxygen fields with steep microregional oxygen gradients. Thus, two-photon NADH imaging can be applied to expose vascular supply regions and to localize functionally relevant microregional cortical hypoxia with micrometer spatial resolution.

Methamphetamine Causes Sustained Depression in Cerebral Blood Flow

Brain Research. Feb, 2011  |  Pubmed ID: 21156163

The use prevalence of the highly addictive psychostimulant methamphetamine (MA) has been steadily increasing over the past decade. MA abuse has been associated with both transient and permanent alterations in cerebral blood flow (CBF), hemorrhage, cerebrovascular accidents and death. To understand MA-induced changes in CBF, we exposed C56BL/6 mice to an acute bolus of MA (5mg/kg MA, delivered IP). This elicited a biphasic CBF response, characterized by an initial transient increase (~ 5 minutes) followed by a prolonged decrease (~ 30 minutes) of approximately 25% relative to baseline CBF--as measured by laser Doppler flowmetry over the somatosensory cortex. To assess if this was due to catecholamine derived vasoconstriction, phentolamine, an α-adrenergic antagonist was administered prior to MA treatment. This reduced the initial increase in CBF but failed to prevent the subsequent, sustained decrease in CBF. Consistent with prior reports, MA caused a transient increase in mean arterial blood pressure, body temperature and respiratory rate. Elevated respiratory rate resulted in hypocapnia. When respiratory rate was controlled by artificially ventilating mice, blood PaCO(2) levels after MA exposure remained unchanged from physiologic levels, and the MA-induced decrease in CBF was abolished. In vivo two-photon imaging of cerebral blood vessels revealed sustained MA-induced vasoconstriction of pial arterioles, consistent with laser Doppler flowmetry data. These findings show that even a single, acute exposure to MA can result in profound changes in CBF, with potentially deleterious consequences for brain function.