Multicolor nonlinear microscopy of living tissue using two- and three-photon-excited intrinsic fluorescence combined with second harmonic generation by supermolecular structures produces images with the resolution and detail of standard histology without the use of exogenous stains. Imaging of intrinsic indicators within tissue, such as nicotinamide adenine dinucleotide, retinol, indoleamines, and collagen provides crucial information for physiology and pathology. The efficient application of multiphoton microscopy to intrinsic imaging requires knowledge of the nonlinear optical properties of specific cell and tissue components. Here we compile and demonstrate applications involving a range of intrinsic molecules and molecular assemblies that enable direct visualization of tissue morphology, cell metabolism, and disease states such as Alzheimer's disease and cancer.

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Live tissue intrinsic emission microscopy using multiphoton-excited native fluorescence and second harmonic generation

The key role of vitamin A in embryonal development is reviewed. Special emphasis is given to the physiological action of retinoids, as evident from the retinoid ligand knockout models. Retinoid metabolism in embryonic tissues and teratogenic consequences of retinoid administration at high doses are presented. Physiological and pharmacological actions of retinoids are outlined and explained on the basis of their interactions as ligands of the nuclear retinoid receptors. Immediate target genes and the retinoid response elements of their promoters are summarized. The fundamental role of homeobox genes in embryonal development and the actions of retinoids on their expression are discussed. The similarity of the effects of retinoid ligand knockouts to effects of compound retinoid receptor knockouts on embryogenesis is presented. Although much remains to be clarified, the emerging landscape offers exciting views for future research.

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.3.1021

Retinoids in Embryonal Development

Oxidative stress induced-neurodegenerative diseases: the need for antioxidants that penetrate the blood brain barrier.

Vitamin A has diverse biological functions. It is transported in the blood as a complex with retinol binding protein (RBP), but the molecular mechanism by which vitamin A is absorbed by cells from the vitamin A-RBP complex is not clearly understood. We identified in bovine retinal pigment epithelium cells STRA6, a multitransmembrane domain protein, as a specific membrane receptor for RBP. STRA6 binds to RBP with high affinity and has robust vitamin A uptake activity from the vitamin A-RBP complex. It is widely expressed in embryonic development and in adult organ systems. The RBP receptor represents a major physiological mediator of cellular vitamin A uptake.

https://science.sciencemag.org/content/sci/315/5813/820.full.pdf

A Membrane Receptor for Retinol Binding Protein Mediates Cellular Uptake of Vitamin A

https://www.cell.com/cell/pdf/0092-8674(81)90018-0.pdf

A direct repeat in the cellular retinol-binding protein type II gene confers differential regulation by RXR and RAR.

Evidence for a lecithin-retinol acyltransferase activity in the rat small intestine.

Acyl-CoA-independent esterification of retinol bound to cellular retinol-binding protein (type II) by microsomes from rat small intestine.

Esterification of retinol in rat liver. Possible participation by cellular retinol-binding protein and cellular retinol-binding protein II.

Binding specificities of cellular retinol-binding protein and cellular retinol-binding protein, type II.

Brain is not generally recognized as an organ that requires vitamin A, perhaps because no obvious histologic lesions have been observed in severely vitamin A-deficient animals. However, brain tissue does contain cellular vitamin A-binding proteins and a nuclear receptor protein for retinoic acid. In the present study, immunohistochemical techniques were used to determine the cell-specific location of cellular retinol-binding protein in human and rat brain tissue. Cellular retinol-binding protein was localized specifically within the endothelial cells of the brain microvasculature and within the cuboidal epithelial cells of the choroid plexus, two primary sites of the mammalian blood-brain barrier. In addition, autoradiographic procedures demonstrated binding sites for serum retinol-binding protein in the choroidal epithelium. These observations suggest that a significant movement of retinol across the blood-brain barrier may occur.

Paul N. MacDonald

Papers

Evidence for a lecithin-retinol acyltransferase activity in the rat small intestine.

Acyl-CoA-independent esterification of retinol bound to cellular retinol-binding protein (type II) by microsomes from rat small intestine.

Esterification of retinol in rat liver. Possible participation by cellular retinol-binding protein and cellular retinol-binding protein II.

Binding specificities of cellular retinol-binding protein and cellular retinol-binding protein, type II.

Localization of cellular retinol-binding protein and retinol-binding protein in cells comprising the blood-brain barrier of rat and human.