Natural Compounds from Saffron and Bear Bile Prevent Vision Loss and Retinal Degeneration

Abstract

All retinal disorders, regardless of their aetiology, involve the activation of oxidative stress and apoptosis pathways. The administration of neuroprotective factors is crucial in all phases of the pathology, even when vision has been completely lost. The retina is one of the most susceptible tissues to reactive oxygen species damage. On the other hand, proper development and functioning of the retina requires a precise balance between the processes of proliferation, differentiation and programmed cell death. The life-or-death decision seems to be the result of a complex balance between pro- and anti-apoptotic signals. It has been recently shown the efficacy of natural products to slow retinal degenerative process through different pathways. In this review, we assess the neuroprotective effect of two compounds used in the ancient pharmacopoeia. On one hand, it has been demonstrated that administration of the saffron constituent safranal to P23H rats, an animal model of retinitis pigmentosa, preserves photoreceptor morphology and number, the capillary network and the visual response. On the other hand, it has been shown that systemic administration of tauroursodeoxycholic acid (TUDCA), the major component of bear bile, to P23H rats preserves cone and rod structure and function, together with their contact with postsynaptic neurons. The neuroprotective effects of safranal and TUDCA make these compounds potentially useful for therapeutic applications in retinal degenerative diseases.

Keywords: Crocus sativus; P23H; TUDCA; apoptosis; oxidative stress; retina; safranal; tauroursodeoxycholic acid.

Figure 1

Neuroprotective effects of TUDCA and safranal on the morphological and functional changes associated to retinal degeneration. (a–c) Immunolabeling of retinal vertical sections for γ-transducin (cones, green) and recoverin (rods, cones, and two bipolar cell subtypes, red) in P120 P23H rats treated with vehicle (a) TUDCA (b) or safranal (c). Nuclei stained with TO-PRO 3 (blue). Images were collected from the central area of the retina, close to the optic nerve; (d–f) Representative scotopic full-field ERG waveforms from P120 P23H rats treated with vehicle (d) TUDCA (e) or safranal (f). Units on the left indicate input flash intensities in log cd·s/m². Note that ERG amplitudes in the P23H rat treated with TUDCA of safranal are higher than those recorded in the vehicle-treated animal; (g–h) Stimulus intensity curves for mixed scotopic b-waves from rats administered with TUDCA (g, squares), safranal (h, squares) or vehicle (g–h, circles). OS: outer segments, IS: inner segments, ONL: outer nuclear layer, OPL: outer plexiform layer, INL: inner nuclear layer. Scale bars: 10 μm.

Figure 1

Neuroprotective effects of TUDCA and safranal on the morphological and functional changes associated to retinal degeneration. (a–c) Immunolabeling of retinal vertical sections for γ-transducin (cones, green) and recoverin (rods, cones, and two bipolar cell subtypes, red) in P120 P23H rats treated with vehicle (a) TUDCA (b) or safranal (c). Nuclei stained with TO-PRO 3 (blue). Images were collected from the central area of the retina, close to the optic nerve; (d–f) Representative scotopic full-field ERG waveforms from P120 P23H rats treated with vehicle (d) TUDCA (e) or safranal (f). Units on the left indicate input flash intensities in log cd·s/m². Note that ERG amplitudes in the P23H rat treated with TUDCA of safranal are higher than those recorded in the vehicle-treated animal; (g–h) Stimulus intensity curves for mixed scotopic b-waves from rats administered with TUDCA (g, squares), safranal (h, squares) or vehicle (g–h, circles). OS: outer segments, IS: inner segments, ONL: outer nuclear layer, OPL: outer plexiform layer, INL: inner nuclear layer. Scale bars: 10 μm.

Figure 2

Activation of microglial cells. Vertical sections of retinas from a SD (a–c), untreated P23H (d–f) and TUDCA-treated P23H (g–i) rat at P120 stained for Iba1 (green; a, d, g) MHC-II RT 1B (red; b, e, h) or both (c, f, i). Nuclei stained with TO-PRO 3 (blue). All images were collected from the central area of the retina, close to the optic nerve. Note that microglia density in P23H rats treated with TUDCA is similar to the one observed in the SD rats and smaller than that shown in untreated P23H rats. The relative number of MHC-II-positive cells in TUDCA-treated P23H rats is also low. ONL: outer nuclear layer, OPL: outer plexiform layer, INL: inner nuclear layer, IPL: inner plexiform layer, GCL: ganglion cell layer. Scale bars: 20 μm.

Figure 3

Apoptotic pathways in the retina. Schematic representation of the most relevant pathways involved in programmed cell death (PCD) in the P23H rat retina and likely targets for safranal and TUDCA. Most retinal cells die as a result of caspase-dependent pathways, although caspase-independent pathways involving calpains and/or cathepsins are also present. TUDCA activity may be exerted by decreasing ER stress, stabilizing the outer mitochondrial membrane and blocking calpain-driven apoptosis. Among the major causes of stress and cell death in the retina is the accumulation of reactive oxygen species (ROS) associated with pathological conditions and damage to both mitochondria and lysosomes. Safranal could be decreasing oxidative stress due to ROS elevation, thereby ameliorating cell death. Akt/PKB: Protein kinase B; Bad: Bcl-2-associated agonist of cell death; Bax: Bcl-2-associated X protein; Bcl-2: apoptosis regulator Bcl-2 (B-cell lymphoma-2); ER: endoplasmic reticulum; MOMP: mitochondrial outer membrane permeabilization; PTPC: permeability transition pore complex; ROS: reactive oxygen species.