microfabrication-implantable-optics-integrated-microstructured

Microfabrication of Implantable Optics Integrated in a Microstructured Imaging Window for Advanced <em>In Vivo</em> Imaging

In the context of biomaterials and drug testing in animal models, this study presents a streamlined protocol for fabricating a novel implantable ...

Università di Milano-Bicocca

We have used the fluorescence anisotropy (FA) decay of retinol bound to bovine beta-lactoglobulin to monitor the time evolution of protein aggregation during the early stages of crystal growth. With this approach we have followed the formation of aggregates at different concentrations of ammonium sulfate, the precipitant used for crystallization. The average aggregation number is found to depend on precipitant concentration, and to be restricted to small numbers ranging from 2 to 5, also in the presence of visible growing crystals. The effect of particle distribution and of low probe-to-protein saturation on the FA response is also discussed in detail.

Probing protein aggregation by time-resolved fluorescence during beta-lactoglobulin crystal growth.

The fluorescence lifetime of ethidium bromide (EB), a widely used fluorescent dye, has been monitored in water solution versus trehalose concentration in order to learn how the presence of the disaccharide modifies the hydration of EB, which is used in this study as a model probe. The interest in trehalose, a naturally occurring bioprotector (osmolyte), stems from its ability to stabilize biomolecules under stress situations. The observed effects of trehalose on EB fluorescence properties have been compared with those induced by sucrose and glucose. Lifetime measurements have been performed by frequency domain fluorometry (2-40 MHz), and from the analysis of their changes versus sugar concentration, the EB-sugar interaction parameters have been obtained. The effect of trehalose on the EB decay in glasses has also been studied both by exploring the heterogeneity of lifetime decay and by single-molecule imaging. Trehalose appears to be more efficient in changing the EB fluorescence parameters, such as the emission lifetime, and it leads to a degree of heterogeneity higher than that induced by the other sugars. When EB is embedded in trehalose glasses, the heterogeneity of the emission and of the bleaching time is further enhanced.

Trehalose-induced changes of the ethidium hydration shell detected by time-resolved fluorescence.

Fluorescence anisotropy decay spectroscopy is a suitable tool for investigating the size and the shape of biological molecules. We coupled this technique to an optical microscope in order to reduce the excitation volume and to allow its application to spatially inhomogeneous samples. Phase modulated measurements of the fluorescence anisotropy decay were performed by feeding an intensity modulated linearly polarized laser beam to the epifluorescence port of a microscope. Here we report the test of the dynamic response of the microscope by comparing the lifetime and fluorescence polarization anisotropy decays obtained in cuvettes in a standard phase modulation fluorometer and on tiny drops on the microscope stage. We show that once a correction factor for the objective depolarization is introduced in the best-fit functions for the data analysis of the decays, the results obtained on the two setups are comparable. Some applications are reported here on long DNA tracts as well on short DNA fragments containing structural anomalies.

Fluorescence anisotropy in the frequency domain by an optical microscope.

Copolymers of camptothecin (CPT) and [N-(2-hydroxypropyl) methacrylamide] (HPMA) are novel anticancer drugs that show improved pharmacological profile in animal models as compared to the free drug CPT. We investigate here the aggregation properties of a HPMA-glycyl-6-aminohexanoyl-glycyl-CPT copolymer ( approximately 20,000 Da). The molecular size of HPMA-copolymer CPT is followed over 5 orders of magnitudes of concentration in isotonic buffer by measuring either the time resolved fluorescence anisotropy (FA) of CPT or the autocorrelation function of the light scattered by the copolymer. A detailed analysis of these data suggests the presence of elongated structures with axial ratio approximately 3 in the range 0.1-0.5 microg/ml and aggregates with association number higher than 2 in more concentrated solutions (up to 10 mg/ml). The binding affinity of HPMA-copolymer CPT for serum albumin is inversely dependent on the degree of aggregation of the copolymer. We also show that the copolymer concentration in plasma from mice treated with an active, non-toxic, dose of HPMA-copolymer CPT, decreases from 3 to 0.01 mg/ml in 72 h. In the same range of concentrations in vitro, we do not detect hydrophobic aggregates of polymers with high (>3) association number. Our study indicates that the circulating HPMA-copolymer CPT in mice should not undergo extensive aggregation and should interact with serum albumin more weakly than free CPT.

Aggregation properties of a HPMA-camptothecin copolymer in isotonic solutions.

The possibility of predicting the overall shape of a macromolecule in solution from its diffusional properties has gained increasing importance in the structural genomic era. Here we explore and quantify the influence that unstructured and flexible regions have on the motions of a globular protein, a situation that can occur from the presence of such regions in the natural sequence or from additional tags. I27, an immunoglobulin-like module from the muscle protein titin, whose structure and properties are well characterized, was selected for our studies. The backbone dynamics and the overall tumbling of three different constructs of I27 were investigated using (15)N NMR relaxation collected at two (15)N frequencies (60.8 and 81.1 MHz) and fluorescence depolarization spectroscopy after labeling of a reactive cysteine with an extrinsic fluorophore. Our data show that the presence of disordered tags clearly exerts a frictional drag that increases with the length of the tags, thus affecting the module tumbling in solution. We discuss the use and the limitations of current approaches to hydrodynamic calculations, especially when having to take into account local flexibility.

The role of unstructured extensions in the rotational diffusion properties of a globular protein: the example of the titin i27 module.

Dimethyl-pepep (D-pepep), a newly developed and very efficient two-photon absorber, has been tested here for two-photon excitation (TPE) cellular imaging. The spectral characteristics of the dye following one-photon excitation (OPE) and TPE (excitation and emission spectra, fluorescence lifetime, molecular brightness, saturation intensity) are reported. In vitro interaction studies with biomolecules show that dimethyl-pepep has a large affinity for DNA. A comparison with a widely used DNA stainer, 4-6-diamidino-2-phenylindole (DAPI) bound to DNA shows that the D-pepep brightness is one order of magnitude higher than that of DAPI, making this dye suitable for microscopy and imaging applications. TPE images taken from double-stained yeast Saccharomyces cerevisiae cells have revealed that D-pepep localizes mainly in the nucleus, similarly to DAPI, and in mitochondria, although to a minor extent. Preliminary tests have shown that the dye cellular toxicity is negligible.

Dimethyl-pepep: a DNA probe in two-photon excitation cellular imaging.

We report investigations on the properties of nanoenvironments around single-GFP-mut2 proteins in trehalose-water matrixes. Single-GFPmut2 molecules embedded in thin trehalose-water films were characterized in terms of their fluorescence brightness, bleaching dynamics, excited state lifetime, and fluorescence polarization. For each property, sets of approximately 100-150 single molecules have been investigated as a function of trehalose content and hydration. Three distinct and interconverting families of proteins have been found which differ widely in terms of bleaching dynamics, brightness, and fluorescence polarization, whose relative populations sizably depend on sample hydration. The reported results evidence the simultaneous presence of different protein-trehalose-water nanostructures whose rigidity increases by lowering the sample hydration. Such spatial inhomogeneity is in line with the well-known heterogeneous dynamics in supercooled fluids and in nonsolid carbohydrate glasses and gives a pictorial representation of the sharp, sudden reorganization of the above structures after uptake <==>release of water molecules.

GFP-mut2 proteins in trehalose-water matrixes: spatially heterogeneous protein-water-sugar structures.

Non-linear excitation microscopy is considered an ideal spectroscopic method for imaging thick tissues in vivo due to the reduced scattering of infrared radiation. Although imaging has been reported on brain neocortex at 600-800 microm of depth, much less uniform tissues, such as lymphonodes, are characterized by highly anisotropic light scattering that limits the penetration length. We show that the most severe limitation for deep imaging of lymphonodes appears to be the tissue scattering and the diffuse fluorescence emission of labeled cell (lymphocytes) in layers above the focusing plane. We report a study of the penetration depth of the infrared radiation in a model system and in ex vivo lymphonodes and discuss the possibility to apply Fourier filtering to the images in order to improve the observation depth.

Image filtering for two-photon deep imaging of lymphonodes.

p53 is a tumor suppressor whose detection in the body is highly valuable as marker for early cancer diagnosis and prognosis. In this report we have studied constructs based on gold nanoparticles (NPs) functionalized with specific anti-p53 antibodies and with a fluoresceine derivative, FITC. The interaction of surface plasmons on gold NPs few nm in size, with fluorophores bound within few nanometers from the surface, induces changes in the fluorophore excited state lifetime. This parameter follows linearly the p53 concentration in solutions up to 200-400 pM, depending on the size of the NP, with an uncertainty approximatley =25 pM. We have evaluated the specificity of the nanosensor for p53 by testing it against bovine serum albumin, beta-lactoglobulin and lysozyme solutions. The titration of total cell extracts from p53 positive or p53-null cells with the anti-p53 antibody decorated gold NPs indicates that this construct is promising for possible applications to in vivo screening.

P53 detection by fluorescence lifetime on a hybrid fluorescein isothiocyanate gold nanosensor.

We have measured in human alveolar cells the cytoplasmic distribution of the fluorophore coumarin-6 carried by Solid Lipid Nanoparticles (SLNs) and observed a perinuclear accumulation of the fluorescence that can be described by a single exponential growth along an ideal line joining the plasma membrane to the nuclear border and by a sigmoidal relationship as a function of time. Intracellular distribution was affected by lowering the temperature from 37 to 4° C and by the disruption of cytoskeleton by cytochalasin D, but it was minimally perturbed by the inhibition of ATP dependent molecular motors. A biophysical model was developed for an accumulation of loaded particles against a diffusion gradient based on a mean field interaction energy, whose origin we ascribe to the actin structure of the cytoskeleton. The estimated value for the load diffusion coefficient was four and two orders of magnitude less than that of free coumarin-6 and of SLNs in aqueous solutions, respectively, suggesting that the load moves within the cell cytoplasm in a form still reminiscent of the nanocarrier structure.

A biophysical model of intracellular distribution and perinuclear accumulation of particulate matter.

In vivo studies of blood circulation pathologies have great medical relevance and need methods for the characterization of time varying flows at high spatial and time resolution in small animal models. We test here the efficacy of the combination of image correlation techniques and single plane illumination microscopy (SPIM) in characterizing time varying flows in vitro and in vivo. As indicated by numerical simulations and by in vitro experiments on straight capillaries, the complex analytical form of the cross-correlation function for SPIM detection can be simplified, in conditions of interest for hemodynamics, to a superposition of Gaussian components, easily amenable to the analysis of variable flows. The possibility to select a wide field of view with a good spatial resolution along the collection optical axis and to compute the cross-correlation between regions of interest at varying distances on a single time stack of images allows one to single out periodic flow components from spurious peaks on the cross-correlation functions and to infer the duration of each flow component. We apply this cross-correlation analysis to the blood flow in Zebrafish embryos at 4 days after fertilization, measuring the average speed and the duration of the systolic and diastolic phases.

Image Cross-Correlation Analysis of Time Varying Flows.

Ramification of blood circulation is relevant in a number of physiological and pathological conditions. The oxygen exchange occurs largely in the capillary bed, and the cancer progression is closely linked to the angiogenesis around the tumor mass. Optical microscopy has made impressive improvements in in vivo imaging and dynamic studies based on correlation analysis of time stacks of images. Here, we develop and test advanced methods that allow mapping the flow fields in branched vessel networks at the resolution of 10 to 20  μm. The methods, based on the application of spatiotemporal image correlation spectroscopy and its extension to cross-correlation analysis, are applied here to the case of early stage embryos of zebrafish.

Spatiotemporal image correlation analysis of blood flow in branched vessel networks of zebrafish embryos.

Surgical excision followed by histopathological examination is the gold standard for melanoma screening. However, the color-based inspection of hematoxylin-and-eosin-stained biopsies does not provide a space-resolved quantification of the melanin content in melanocytic lesions. We propose a non-destructive photo-thermal imaging method capable of characterizing the microscopic distribution and absolute concentration of melanin pigments in excised melanoma biopsies. By exploiting the photo-thermal effect primed by melanin absorption of visible laser light we obtain label-free super-resolution far-infrared thermal images of tissue sections where melanin is spatially mapped at sub-diffraction 40-μm resolution. Based on the finite-element simulation of the full 3D heat transfer model, we are able to convert temperature maps into quantitative images of the melanin molar concentration on B16 murine melanoma biopsies, with 4·10 M concentration sensitivity. Being readily applicable to human melanoma biopsies in combination with hematoxylin-and-eosin staining, the proposed approach could complement traditional histopathology in the characterization of pigmented lesions ex-vivo.

Melanin concentration maps by label-free super-resolution photo-thermal imaging on melanoma biopsies.

Prussian blue (PB) is a coordination polymer based on the FeCNFe sequence. It is an FDA-approved drug, intended for oral use at the acidic pH of the stomach and of most of the intestine track. However, based on FDA approval, a huge number of papers proposed the use of PB nanoparticles (PBnp) under "physiological conditions", meaning pH buffered at 7.4 and high saline concentration. While most of these papers report that PBnp are stable at this pH, a small number of papers describes instead PBnp degradation at the same or similar pH values, i.e. in the 7-8 range. Here we give a definitively clear picture: PBnp are intrinsically unstable at pH ≥ 7, degrading with the fast disappearance of their 700 nm absorption band, due to the formation of OH complexes from the labile Fe centers. However, we show also that the presence of a polymeric coating (PVP) can protect PBnp at pH 7.4 for over 24 h. Moreover, we demonstrate that when "physiological conditions" include serum, a protein corona is rapidly formed on PBnp, efficiently avoiding degradation. We also show that the viability of PBnp-treated EA.hy926, NCI-H1299, and A549 cells is not affected in a wide range of conditions that either prevent or promote PBnp degradation.

Prussian Blue nanoparticles: An FDA-approved substance that may quickly degrade at physiological pH.

Tissue histopathology, based on hematoxylin and eosin (H&E) staining of thin tissue slices, is the gold standard for the evaluation of the immune reaction to the implant of a biomaterial. It is based on lengthy and costly procedures that do not allow longitudinal studies. The use of non-linear excitation microscopy , largely label-free, has the potential to overcome these limitations. With this purpose, we develop and validate an implantable microstructured device for the non-linear excitation microscopy assessment of the immune reaction to an implanted biomaterial label-free. The microstructured device, shaped as a matrix of regular 3D lattices, is obtained by two-photon laser polymerization. It is subsequently implanted in the chorioallantoic membrane (CAM) of embryonated chicken eggs for 7 days to act as an intrinsic 3D reference frame for cell counting and identification. The histological analysis based on H&E images of the tissue sections sampled around the implanted microstructures is compared to non-linear excitation and confocal images to build a cell atlas that correlates the histological observations to the label-free images. In this way, we can quantify the number of cells recruited in the tissue reconstituted in the microstructures and identify granulocytes on label-free images within and outside the microstructures. Collagen and microvessels are also identified by means of second-harmonic generation and autofluorescence imaging. The analysis indicates that the tissue reaction to implanted microstructures is like the one typical of CAM healing after injury, without a massive foreign body reaction. This opens the path to the use of similar microstructures coupled to a biomaterial, to image  the regenerating interface between a tissue and a biomaterial with label-free non-linear excitation microscopy. This promises to be a transformative approach, alternative to conventional histopathology, for the bioengineering and the validation of biomaterials in i longitudinal studies.

Article	Year
Probing protein aggregation by time-resolved fluorescence during beta-lactoglobulin crystal growth. European biophysics journal : EBJ\| PubMed ID: 12012114	2002
Trehalose-induced changes of the ethidium hydration shell detected by time-resolved fluorescence. Photochemistry and photobiology\| PubMed ID: 12733649	2003
Fluorescence anisotropy in the frequency domain by an optical microscope. Applied spectroscopy\| PubMed ID: 15000709	2004
Aggregation properties of a HPMA-camptothecin copolymer in isotonic solutions. Biophysical chemistry\| PubMed ID: 15228964	2004
The role of unstructured extensions in the rotational diffusion properties of a globular protein: the example of the titin i27 module. Biophysical journal\| PubMed ID: 15298925	2004
Dimethyl-pepep: a DNA probe in two-photon excitation cellular imaging. Biophysical chemistry\| PubMed ID: 15792859	2005
GFP-mut2 proteins in trehalose-water matrixes: spatially heterogeneous protein-water-sugar structures. Biophysical journal\| PubMed ID: 17416616	2007
Image filtering for two-photon deep imaging of lymphonodes. European biophysics journal : EBJ\| PubMed ID: 18389230	2008
P53 detection by fluorescence lifetime on a hybrid fluorescein isothiocyanate gold nanosensor. Journal of biomedical nanotechnology\| PubMed ID: 20201230	2009
A biophysical model of intracellular distribution and perinuclear accumulation of particulate matter. Biophysical chemistry\| PubMed ID: 21741753	2011
Image Cross-Correlation Analysis of Time Varying Flows. Analytical chemistry\| PubMed ID: 27348197	2016
Spatiotemporal image correlation analysis of blood flow in branched vessel networks of zebrafish embryos. Journal of biomedical optics\| PubMed ID: 29030941	2017
Melanin concentration maps by label-free super-resolution photo-thermal imaging on melanoma biopsies. Biomedical optics express\| PubMed ID: 35414966	2022
Prussian Blue nanoparticles: An FDA-approved substance that may quickly degrade at physiological pH. Colloids and surfaces. B, Biointerfaces\| PubMed ID: 37257303	2023
label-free tissue histology through a microstructured imaging window. APL bioengineering\| PubMed ID: 38222895	2024

Maddalena Collini

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