Electroretinographic oscillatory potentials are reduced in adenylyl cyclase type I deficient mice

Adenylyl Cyclase in Ocular Health and Disease: A Comprehensive Review

Adenylyl cyclases (ACs) are a group of enzymes that convert adenosine-5'-triphosphate (ATP) to cyclic adenosine 3',5' monophosphate (cAMP), a vital and ubiquitous signalling molecule in cellular responses to hormones and neurotransmitters. There are nine transmembrane (tmAC) forms, which have been widely studied; however, the tenth, soluble AC (sAC) is less extensively characterised. The eye is one of the most metabolically active sites in the body, where sAC has been found in abundance, making it a target for novel therapeutics and biomarking. In the cornea, AC plays a role in endothelial cell function, which is vital in maintaining stromal dehydration, and therefore, clarity. In the retina, AC has been implicated in axon cell growth and survival. As these cells are irreversibly damaged in glaucoma and injury, this molecule may provide focus for future therapies. Another potential area for glaucoma management is the source of aqueous humour production, the ciliary body, where AC has also been identified. Furthering the understanding of lacrimal gland function is vital in managing dry eye disease, a common and debilitating condition. sAC has been linked to tear production and could serve as a therapeutic target. Overall, ACs are an exciting area of study in ocular health, offering multiple avenues for future medical therapies and diagnostics. This review paper explores the diverse roles of ACs in the eye and their potential as targets for innovative treatments.

Cone ERG Changes During Light Adaptation in Two All-Cone Mutant Mice: Implications for Rod-Cone Pathway Interactions

Purpose

The b-wave of the cone ERG increases in amplitude and speed during the first few minutes of adaptation to a rod-suppressing background light. Earlier studies implicate rod pathway input to the cone pathway in these changes.

Methods

The timing and amplitude of the cone b-wave and isolated oscillatory potentials (OP) during the first 10 minutes of light adaptation in wild-type (WT) mice and two mutant lines without functional rods was examined: rhodopsin knockout (Rho-/-), lacking rod outer segments, and NRL knockout (Nrl-/-), in which rods are replaced by S-cones. Expression of the immediate-early gene c-fos, which is increased in the inner retina by light-induced activity, was evaluated by immunohistochemistry in dark- and light-adapted retinas.

Results

WT b-wave and OP amplitudes increased, and implicit times decreased during light adaptation. Subtracting OP did not alter b-wave changes. Rho-/- b-wave and OP amplitudes did not increase during adaptation. B-wave timing and amplitude and the timing of the major OP at 1 minute of adaptation were equivalent to WT at 10 minutes. The light-adapted ERG b-wave in Nrl-/- mice, which originates in both the rod and cone pathways, changed in absolute amplitude and timing similar to WT. C-fos expression was present in the inner retinas of dark-adapted Rho-/- but not WT or Nrl-/- mice.

Conclusions

Activity in the distal rod pathway produces changes in the cone ERG during light adaptation. Rods in Rho-/- mice constitutively activate this rod-cone pathway interaction. The rod pathway S-cones in Nrl-/- mice may maintain the WT interaction.

Adenylate cyclase 1 (ADCY1) mutations cause recessive hearing impairment in humans and defects in hair cell function and hearing in zebrafish

Cyclic AMP (cAMP) production, which is important for mechanotransduction within the inner ear, is catalyzed by adenylate cyclases (AC). However, knowledge of the role of ACs in hearing is limited. Previously, a novel autosomal recessive non-syndromic hearing impairment locus DFNB44 was mapped to chromosome 7p14.1-q11.22 in a consanguineous family from Pakistan. Through whole-exome sequencing of DNA samples from hearing-impaired family members, a nonsense mutation c.3112C>T (p.Arg1038*) within adenylate cyclase 1 (ADCY1) was identified. This stop-gained mutation segregated with hearing impairment within the family and was not identified in ethnically matched controls or within variant databases. This mutation is predicted to cause the loss of 82 amino acids from the carboxyl tail, including highly conserved residues within the catalytic domain, plus a calmodulin-stimulation defect, both of which are expected to decrease enzymatic efficiency. Individuals who are homozygous for this mutation had symmetric, mild-to-moderate mixed hearing impairment. Zebrafish adcy1b morphants had no FM1-43 dye uptake and lacked startle response, indicating hair cell dysfunction and gross hearing impairment. In the mouse, Adcy1 expression was observed throughout inner ear development and maturation. ADCY1 was localized to the cytoplasm of supporting cells and hair cells of the cochlea and vestibule and also to cochlear hair cell nuclei and stereocilia. Ex vivo studies in COS-7 cells suggest that the carboxyl tail of ADCY1 is essential for localization to actin-based microvilli. These results demonstrate that ADCY1 has an evolutionarily conserved role in hearing and that cAMP signaling is important to hair cell function within the inner ear.

Biology and therapeutic potential of adult retinal stem cells

Retinal degeneration encompasses a constellation of common pathologies for which there is no regenerative treatment. Vision loss has a devastating impact on quality of life and activities of daily living. Pharmacologic treatments serve to stave off disease progression but do not represent a restorative approach. Cellular transplantation is considered to be a promising approach for future therapy for retinal degeneration. There are, however, significant barriers that must be overcome if cell transplantation is to become a clinical reality. In this review, we focus on the need for a cellular replacement therapy for retinal disease and the promise of stem cells as candidate cellular therapeutics. In particular, we discuss the origins of stem cells in the retina, the discovery and characterization of retinal stem cells isolated from adult humans, and their transplantation potential and clinical implications.

Maximizing functional photoreceptor differentiation from adult human retinal stem cells

Retinal stem cells (RSCs) are present in the ciliary margin of the adult human eye and can give rise to all retinal cell types. Here we show that modulation of retinal transcription factor gene expression in human RSCs greatly enriches photoreceptor progeny, and that strong enrichment was obtained with the combined transduction of OTX2 and CRX together with the modulation of CHX10. When these genetically modified human RSC progeny are transplanted into mouse eyes, their retinal integration and differentiation is superior to unmodified RSC progeny. Moreover, electrophysiologic and behavioral tests show that these transplanted cells promote functional recovery in transducin mutant mice. This study suggests that gene modulation in human RSCs may provide a source of photoreceptor cells for the treatment of photoreceptor disease.

Isoflurane is an effective alternative to ketamine/xylazine/acepromazine as an anesthetic agent for the mouse electroretinogram

The electroretinogram (ERG) is an essential measure of retinal function for studying mouse models of retinal disease. Ketamine, in combination with xylazine and/or acepromazine, is the most commonly used anesthetic agent. Although it works well in most situations, some fragile mouse strains have high mortality rates with this ketamine cocktail. We compared isoflurane with the ketamine cocktail in a longitudinal study of light-adapted and dark-adapted ERGs in C57BL/6J mice. Waveforms were averaged, oscillatory potentials (OPs) were extracted by digital filtration, and key ERG parameters were analyzed. The ERG waveforms were qualitatively similar with both anesthetics, and the male and female ERG parameters did not show significant differences. For light-adapted ERGs, b-wave amplitude and implicit time, and wavelet index were decreased under isoflurane anesthesia, whereas for dark-adapted ERGs, a- and b-wave implicit times were decreased and wavelet index was increased. The dark-adapted b-wave amplitude showed a significant inverse correlation with animal weight and age. Rod phototransduction gain and the Naka-Rushton n and R (max) parameters were the same for both anesthetics, and only the Naka-Rushton log k parameter was significantly elevated for isoflurane anesthesia. We propose that isoflurane is a satisfactory alternative to the ketamine cocktail for anesthesia in the mouse ERG. Precise quantitative comparisons, however, should only employ study designs using isoflurane versus isoflurane, or ketamine versus ketamine. Moreover, in light of the effects of both isoflurane and the ketamine cocktail on blood glucose levels, it would be prudent to control the fasting state of the animals in quantitative ERG studies.

Role of the calcium modulated cyclases in the development of the retinal projections

Transmembrane isoforms of adenylate cyclases (AC) integrate a wide variety of extracellular signals from neurotransmitters to morphogens and can also regulate cAMP production in response to calcium entry. Based on observations in the barrelless mouse strain, the Adcy1 gene (AC1) was involved in the segregation of binocular retinal inputs. To determine the potential role of other AC isoforms we localized the Adcy genes in the visual centres during development, using in situ hybridization. Six different AC subtypes were found in the developing retinal ganglion cell layer (RGC; AC1, AC2, AC3, AC5, AC8, and AC9), and three AC subtypes were expressed in the central brain targets, the dorsal lateral geniculate nucleus (AC1 and AC8), the ventral lateral geniculate nucleus (AC2 and AC8) and the superior colliculus (AC1, AC2, AC8). Using a genetic approach we tested the role of the calcium modulated cyclases AC1, AC5 and AC8 for the segregation retinal fibres. Ipsilateral retinal axons remained exuberant in the AC1(-/-) mice, with overlapping retinal projections from both eyes in the superior colliculus and the visual thalamus. These abnormalities were similar to those of barrelless mouse mutants. No abnormalities were detectable in the AC5(-/-) or the AC8(-/-) mice. Similar abnormalities were noted in the single AC1(-/-) and the AC1/AC8 double-knockout mice (DKO). Thus, only AC1 is required for the maturation of the retinal axon terminals whereas AC5 and AC8 are not needed. The specificity of AC1's action is linked to its cellular localization in the RGCs and to its distinctive functional profile, compared with the other cyclases expressed in the same cells.

Importance of K+-dependent Na+/Ca2+-exchanger 2, NCKX2, in motor learning and memory

Plasma membrane Na+/Ca2+-exchangers play a predominant role in Ca2+ extrusion in brain. Neurons express several different Na+/Ca2+-exchangers belonging to both the K+-independent NCX family and the K+-dependent NCKX family. The unique contributions of each of these proteins to neuronal Ca2+ homeostasis and/or physiology remain largely unexplored. To address this question, we generated mice in which the gene encoding the abundant neuronal K+ -dependent Na+/Ca2+-exchanger protein, NCKX2, was knocked out. Analysis of these animals revealed a significant reduction in Ca2+ flux in cortical neurons, a profound loss of long term potentiation and an increase in long term depression at hippocampal Schaffer/CA1 synapses, and clear deficits in specific tests of motor learning and spatial working memory. Surprisingly, there was no obvious loss of photoreceptor function in cones, where expression of the NCKX2 protein had been reported previously. These data emphasize the critical and non-redundant role of NCKX2 in the local control of neuronal [Ca2+] that is essential for the development of synaptic plasticity associated with learning and memory.