The aim of this study was to investigate effects of dietary levels of histidine (His) and iron (Fe) on cataract development in two strains of Atlantic salmon monitored through parr-smolt transformation. Three experimental diets were fed: (i) a control diet (CD) with 110 mg kg )1 Fe and 11.7 g kg )1 His; (ii) CD supplemented with crystalline His to a level of 18 g kg )1 (HD); and (iii) HD with added iron up to 220 mg kg )1 (HID). A cross-over design, with two feeding periods was used. A 6-week freshwater (FW) period was followed by a 20-week period, of which the first three were in FW and the following 17 weeks in sea water (SW). Fish were sampled for weighing, cataract assessment and tissue analysis at five time points. Cataracts developed in all groups in SW, but scores were lower in those fed high His diets (P < 0.05). This effect was most pronounced when HD or HID was given in SW, but was also observed when these diets were given in FW only. Histidine supplementation had a positive effect on growth performance and feed conversion ratio (P < 0.05), whereas this did not occur when iron was added. Groups fed HD or HID had higher lens levels of His and N-acetyl histidine (NAH), the latter showing a marked increase post-smoltification (P < 0.05). The HD or HID groups also showed higher muscle concentrations of the His dipeptide anserine (P < 0.05). There was a strong genetic influence on cataract development in the CD groups (P < 0.001), not associated with tissue levels of His or NAH. The role of His and Hisrelated compounds in cataractogenesis is discussed in relation to tissue buffering, osmoregulation and antioxidation.

This review highlights recent findings that describe how purines modulate the physiological and pathophysiological responses of ocular tissues. For example, in lacrimal glands the cross-talk between P2X7 receptors and both M3 muscarinic receptors and α1D-adrenergic receptors can influence tear secretion. In the cornea, purines lead to post-translational modification of EGFR and structural proteins that participate in wound repair in the epithelium and influence the expression of matrix proteins in the stroma. Purines act at receptors on both the trabecular meshwork and ciliary epithelium to modulate intraocular pressure (IOP); ATP-release pathways of inflow and outflow cells differ, possibly permitting differential modulation of adenosine delivery. Modulators of trabecular meshwork cell ATP release include cell volume, stretch, extracellular Ca2+ concentration, oxidation state, actin remodeling and possibly endogenous cardiotonic steroids. In the lens, osmotic stress leads to ATP release following TRPV4 activation upstream of hemichannel opening. In the anterior eye, diadenosine polyphosphates such as Ap4A act at P2 receptors to modulate the rate and composition of tear secretion, impact corneal wound healing and lower IOP. The Gq11-coupled P2Y1-receptor contributes to volume control in Müller cells and thus the retina. P2X receptors are expressed in neurons in the inner and outer retina and contribute to visual processing as well as the demise of retinal ganglion cells. In RPE cells, the balance between extracellular ATP and adenosine may modulate lysosomal pH and the rate of lipofuscin formation. In optic nerve head astrocytes, mechanosensitive ATP release via pannexin hemichannels, coupled with stretch-dependent upregulation of pannexins, provides a mechanism for ATP signaling in chronic glaucoma. With so many receptors linked to divergent functions throughout the eye, ensuring the transmitters remain local and stimulation is restricted to the intended target may be a key issue in understanding how physiological signaling becomes pathological in ocular disease.

a b s t r a c tExcessive Ca 2þ can be detrimental to cells and raised levels of Ca 2þ in human lenses with cortical cataract have been found to play a major role in the opacification process. Ca 2þ homeostasis is therefore, recognised as having fundamental importance in lens pathophysiology. Furthermore, Ca 2þ plays a central role as a second messenger in cell signalling and mechanisms have evolved which give cells exquisite control over intracellular Ca 2þ ([Ca 2þ ] i ) via an array of specialised regulatory and signalling proteins. In this review we discuss these mechanisms as they apply to the lens. Ca 2þ levels in human aqueous humour are approximately 1 mM and there is a large, 10,000 fold, inwardly directed gradient across the plasma membrane. In the face of such a large gradient highly efficient mechanisms are needed to maintain low [Ca 2þ ] i . The Na þ /Ca 2þ exchanger (NCX) and plasma membrane Ca 2þ -ATPase (PMCA) actively remove Ca 2þ from the cells, whereas the sarco(endo)plasmic reticulum Ca 2þ -ATPase (SERCA) sequesters Ca 2þ in the endoplasmic reticulum (ER) Ca 2þ store. In lens epithelial cells the dominant role is played by the ATPases, whilst in the fibre cells NCX activity appears to be more important. Usually, [Ca 2þ ] i can be increased in a number of ways. Ca 2þ influx through the plasma membrane, for example, is mediated by an array of channels with evidence in the lens for the presence of voltage-operated Ca 2þ channels (VOCCs), receptor-operated Ca 2þ channels (ROCCs) and channels mediating store-operated Ca 2þ entry (SOCE). Ca 2þ signalling is initiated via activation of G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTK) of which the lens expresses a surprisingly diverse array responding to various neurotransmitters, hormones, growth factors, autocoids and proteases. Downstream of plasma membrane receptors are IP 3 -gated channels (IP 3 Rs) and ryanodine receptors (RYRs) located in the ER, which when activated cause a rapid increase in [Ca 2þ ] i and these have also been identified in the lens. Through an appreciation of the diversity and complexity of the mechanisms involved in Ca 2þ homeostasis in normal lens cells we move closer to an understanding of the mechanisms which mediate pathological Ca 2þ overload as occurs in the process of cataract formation.

Optic neuropathies such as glaucoma occur when retinal ganglion cells (RGCs) in the eye are injured. Strong evidence suggests mesenchymal stem cells (MSCs) could be a potential therapy to protect RGCs; however, little is known regarding their effect on the human retina. We, therefore, investigated if human MSCs (hMSCs), or platelet‐derived growth factor (PDGF) as produced by hMSC, could delay RGC death in a human retinal explant model of optic nerve injury. Our results showed hMSCs and the secreted growth factor PDGF‐AB could substantially reduce human RGC loss and apoptosis following axotomy. The neuroprotective pathways AKT, ERK, and STAT3 were activated in the retina shortly after treatments with labeling seen in the RGC layer. A dose dependent protective effect of PDGF‐AB was observed in human retinal explants but protection was not as substantial as that achieved by culturing hMSCs on the retina surface which resulted in RGC cell counts similar to those immediately post dissection. These results demonstrate that hMSCs and PDGF have strong neuroprotective action on human RGCs and may offer a translatable, therapeutic strategy to reduce degenerative visual loss. Stem Cells 2018;36:65–78

These studies demonstrated that stimulation of the P2X7R can mediate RGC death and that this mechanism plays a role in ischemia-induced neurodegeneration in the human retina.