Cyclic GMP in the retinas of normal mice and those heterozygous for early-onset photoreceptor dystrophy

A hypothalamic circuit underlying the dynamic control of social homeostasis

Social grouping increases survival in many species, including humans1,2. By contrast, social isolation generates an aversive state ('loneliness') that motivates social seeking and heightens social interaction upon reunion3-5. The observed rebound in social interaction triggered by isolation suggests a homeostatic process underlying the control of social need, similar to physiological drives such as hunger, thirst or sleep3,6. In this study, we assessed social responses in several mouse strains, among which FVB/NJ mice emerged as highly, and C57BL/6J mice as moderately, sensitive to social isolation. Using both strains, we uncovered two previously uncharacterized neuronal populations in the hypothalamic preoptic nucleus that are activated during either social isolation or social rebound and orchestrate the behaviour display of social need and social satiety, respectively. We identified direct connectivity between these two populations and with brain areas associated with social behaviour, emotional state, reward and physiological needs and showed that mice require touch to assess the presence of others and fulfil their social need. These data show a brain-wide neural system underlying social homeostasis and provide significant mechanistic insights into the nature and function of circuits controlling instinctive social need and for the understanding of healthy and diseased brain states associated with social context.

A Hypothalamic Circuit Underlying the Dynamic Control of Social Homeostasis

Social grouping increases survival in many species, including humans1,2. By contrast, social isolation generates an aversive state (loneliness) that motivates social seeking and heightens social interaction upon reunion3-5. The observed rebound in social interaction triggered by isolation suggests a homeostatic process underlying the control of social drive, similar to that observed for physiological needs such as hunger, thirst or sleep3,6. In this study, we assessed social responses in multiple mouse strains and identified the FVB/NJ line as exquisitely sensitive to social isolation. Using FVB/NJ mice, we uncovered two previously uncharacterized neuronal populations in the hypothalamic preoptic nucleus that are activated during social isolation and social rebound and that orchestrate the behavior display of social need and social satiety, respectively. We identified direct connectivity between these two populations of opposite function and with brain areas associated with social behavior, emotional state, reward, and physiological needs, and showed that animals require touch to assess the presence of others and fulfill their social need, thus revealing a brain-wide neural system underlying social homeostasis. These findings offer mechanistic insight into the nature and function of circuits controlling instinctive social need and for the understanding of healthy and diseased brain states associated with social context.

Inhibition of mTOR signaling by rapamycin protects photoreceptors from degeneration in rd1 mice

Retinitis pigmentosa (RP) is an inherited retinal degenerative disease that begins with defective rod photoreceptor function, followed by impaired cone function, and complete blindness in its late stage. To date, however, there is no effective treatment for RP. By carrying a nonsense mutation in the Pde6b gene, rd1 mice display elevated cGMP in conjunction with higher intracellular Ca²⁺ in their rod photoreceptors, resulting in fast retinal degeneration. Ca²⁺ has been linked to activation of the mammalian target of rapamycin (mTOR) pathway. The mTOR pathway integrates extracellular and intracellular signals to sense the supply of nutrients and plays a central role in regulating protein and lipid synthesis as well as apoptosis and autophagy. In the present study, we showed that mTOR and phosphorylated mTOR (p-mTOR, activated form of mTOR) are up-regulated in rd1 photoreceptors at postnatal day 10 (P10), a pre-degenerative stage. Moreover, the downstream effectors of mTOR, such as pS6K and S6K, are also increased, suggesting activation of the mTOR signaling pathway. Intravitreal administration of rapamycin, a negative regulator of mTOR, inhibits the mTOR pathway in rd1 photoreceptors. Consequently, the progression of retinal degeneration is slower and retinal function is enhanced, possibly mediated by activation of autophagy in the photoreceptors. Taken together, these results highlight rapamycin as a potential therapeutic avenue for retinal degeneration.

New mouse models for recessive retinitis pigmentosa caused by mutations in the Pde6a gene

The heterotetrameric phosphodiesterase (PDE) 6 complex, made up of alpha, beta and two gamma subunits, regulates intracellular cGMP levels by hydrolyzing cGMP in response to light activation of G protein coupled receptors in cones and rods, making it an essential component of the visual phototransduction cascade [Zhang, X. and Cote, R.H. (2005) cGMP signaling in vertebrate retinal photoreceptor cells. Front. Biosci., 10, 1191-1204.]. Using a genetic positional candidate cloning strategy, we have identified missense mutations within the catalytic domain of the Pde6a gene in two mouse models from an ethyl nitrosourea chemical mutagenesis screen. In these first small rodent models of PDE6A, significantly different biochemical outcomes and rates of degeneration of murine photoreceptor cells were observed, indicating allelic variation and previously unrecognized structure-function relationships. In addition, these new models reveal that the mutations not only affect the function of the PDE6A protein itself, but also the level of PDE6B within the retina. Finally, we show that the variation of the disease phenotype by background modifier genes may be dependent upon the particular disease allele present.

AIPL1, the protein that is defective in Leber congenital amaurosis, is essential for the biosynthesis of retinal rod cGMP phosphodiesterase

Aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) is a member of the FK-506-binding protein family expressed specifically in retinal photoreceptors. Mutations in AIPL1 cause Leber congenital amaurosis, a severe early-onset retinopathy that leads to visual impairment in infants. Here we show that knockdown of AIPL1 expression in mice also produces a retinopathy but over a more extended time course. Before any noticeable pathology, there was a reduction in the level of rod cGMP phosphodiesterase (PDE) proportional to the decrease in AIPL1 expression, whereas other photoreceptor proteins were unaffected. Consistent with less PDE in rods, flash responses had a delayed onset, a reduced gain, and a slower recovery of flash responses. We suggest that AIPL1 is a specialized chaperone required for rod PDE biosynthesis. Thus loss of AIPL1 would result in a condition that phenocopies retinal degenerations in the rd mouse and in a subgroup of human patients.

Anti-Ig-induced calcium influx in rat B lymphocytes mediated by cGMP through a dihydropyridine-sensitive channel

In contrast to excitable tissues where calcium channels are well characterized, the nature of the B lymphocyte calcium channel is unresolved. Here, we demonstrate by single cell analysis of freshly isolated rat B cells that the anti-immunoglobulin (Ig)-induced calcium influx takes place through a channel which shares pharmacologic and serologic properties with the L-type calcium channel found in excitable tissues. It is sensitive to the dihydropyridines nicardipine and Bay K 8644, to calciseptine, and to an anti-peptide antibody raised against the alpha1 subunit of the L-type calcium channel, but is voltage-insensitive. Anti-alpha1 and anti-alpha2 antibodies stain B but not T lymphocytes. Application of a cGMP agonist, measurement of cGMP levels in anti-Ig-stimulated B cells, and examining the effect of a guanylyl cyclase inhibitor on the anti-Ig response show that cGMP mediates the influx. This possibly involves a cGMP-dependent protein kinase. The anti-Ig-induced response is not abolished by prior treatment of B cells with a high dose of thapsigargin. These findings undermine the widely held belief of a categorical divide between excitable and non-excitable tissue calcium channels, demonstrate the limitations of the capacitative calcium influx theory, and point to a distinction between the calcium response mechanisms utilized by B and T lymphocytes.

Nitric oxide-induced mobilization of intracellular calcium via the cyclic ADP-ribose signaling pathway

Cyclic adenosine diphosphate ribose (cADPR) is a potent endogenous calcium-mobilizing agent synthesized from beta-NAD+ by ADP-ribosyl cyclases in sea urchin eggs and in several mammalian cells (Galione, A., and White, A. (1994) Trends Cell Biol. 4, 431 436). Pharmacological studies suggest that cADPR is an endogenous modulator of Ca2+-induced Ca2+ release mediated by ryanodine-sensitive Ca2+ release channels. An unresolved question is whether cADPR can act as a Ca2+-mobilizing intracellular messenger. We show that exogenous application of nitric oxide (NO) mobilizes Ca2+ from intracellular stores in intact sea urchin eggs and that it releases Ca2+ and elevates cADPR levels in egg homogenates. 8-Amino-cADPR, a selective competitive antagonist of cADPR-mediated Ca2+ release, and nicotinamide, an inhibitor of ADP-ribosyl cyclase, inhibit the Ca2+-mobilizing actions of NO, while, heparin, a competitive antagonist of the inositol 1,4,5-trisphosphate receptor, did not affect NO-induced Ca2+ release. Since the Ca2+-mobilizing effects of NO can be mimicked by cGMP, are inhibited by the cGMP-dependent-protein kinase inhibitor, Rp-8-pCPT-cGMPS, and in egg homogenates show a requirement for the guanylyl cyclase substrate, GTP, we suggest a novel action of NO in mobilizing intracellular calcium from microsomal stores via a signaling pathway involving cGMP and cADPR. These results suggest that cADPR has the capacity to act as a Ca2+-mobilizing intracellular messenger.

Recessive mutations in the gene encoding the beta-subunit of rod phosphodiesterase in patients with retinitis pigmentosa

We have found four mutations in the human gene encoding the beta-subunit of rod cGMP phosphodiesterase (PDE beta) that cosegregate with autosomal recessive retinitis pigmentosa, a degenerative disease of photoreceptors. In one family two affected siblings both carry allelic nonsense mutations at codons 298 and 531. Affected individuals have abnormal rod and cone electroretinograms. PDE beta is the second member of the phototransduction cascade besides rhodopsin that is absent or altered as a cause of retinitis pigmentosa, suggesting that other members of this pathway may be defective in other forms of this disease.

Photoreceptor-specific mRNAs in mice carrying different allelic combinations at the rd and rds loci

Several retinal mRNAs encoding photoreceptor-specific proteins have been examined in congenic lines of mice carrying different allelic combinations at the rd and rds loci, to determine how mRNA expression is affected by the presence of both the rd and rds genes together or by the presence of one or two rd and rds alleles in the visual cells. Slot blots with retinal RNA from 9 to 30 days old C3H +/+, +/+; rd/+, +/+; rd/rd, +/+; +/+, +/rds; +/+, rds/rds; and the rd/rd, rds/rds double homozygous mutant mice were hybridized successively to several [32P]cDNA probes encoding proteins involved in phototransduction. The increases and decreases in the levels of mRNA coding for opsin, the alpha-subunit of transducin, 48 kDa protein and the beta-subunit of cGMP-phosphodiesterase in the heterozygous and homozygous rds and rd mouse retinas reflected the growth and degeneration of the photoreceptor cells. The heterozygous rd mouse (rd/+, +/+) retina expressed cGMP-phosphodiesterase beta mRNA levels halfway between those in the control (+/+, +/+) and homozygous (rd/rd, +/+) mice, indicating a possible dosage effect. The double homozygous mutant (rd/rd, rds/rds) showed intermediate levels between those observed in the two homozygotes for all mRNAs studied, suggesting a possible interaction between the rd and rds genes.

Species differences in the response of mammalian photoreceptor cyclic GMP and PIII to a reduction in calcium

Changes in the content of cyclic GMP (cGMP), induced by exposure of isolated, dark-adapted mouse, cat and dog retinas to media depleted of calcium, have been compared with the amplitude of the trans-retinal PIII. Major differences exist in the time-course and magnitude of effects between the species and, in the cat and dog, changes in PIII (potentially a reflection of free cGMP in photoreceptor outer segments) do not correlate with those occurring in total cGMP. The observations imply species variation, not only in the enzymes maintaining cGMP homeostasis in photoreceptors, but also in phototransduction and allied processes.

Carriers of the mouse rd gene have reduced levels of the beta subunit of the retinal cyclic GMP phosphodiesterase

Polyclonal antipeptide antisera have been utilized to quantitate the amount of retinal rod outer segment cGMP phosphodiesterase alpha and beta catalytic subunits present in retinas from C57BL/6J mice which are normal or carriers for the rd gene defect. Results suggest that the quantity of PDE-beta subunit is reduced in carrier mice while PDE-alpha and PDE-gamma are not affected. In 21-day-old mice, the PDE-beta was reduced by about one-half while adult carrier mice had even more reduced levels of PDE-beta. Since PDE alpha was not reduced, this suggests that synthesis of PDE alpha and PDE beta may not be coordinately controlled.

Cyclic GMP, calcium and photoreceptor sensitivity in mice heterozygous for the rod dysplasia gene designated "rd"

The rise in photoreceptor cGMP, induced by less than 1.0 nM extracellular calcium, is delayed in retinas of mice heterozygous for the rod dysplasia gene (+/rd). The calcium ionophore A23187 reduces the delay, suggesting that +/rd outer segments contain more calcium than normal. In turn, this might explain the increased photosensitivity of the +/rd retina. During the response to low calcium there is no correlation in +/rd retinas between the total concentration of cGMP and the photoresponse amplitude and its time to peak. The observations imply that either free cGMP is abnormally independent of the bound pool in the +/rd photoreceptor outer segment or that factors other than cGMP and its phosphodiesterase are modulating the rising phase of the response. The time-to-peak of PIII in a +/rd retina, incubated in a standard medium and stimulated with dim light, is abnormally delayed. Reduction of extracellular calcium induces an abnormal delay as well in responses to higher light levels. In addition to this, a second delay manifests slowly in both the normal and the +/rd retina. More studies are needed to explain these observations.

Identification of a nonsense mutation in the rod photoreceptor cGMP phosphodiesterase beta-subunit gene of the rd mouse

Retinal degeneration in the mouse mutant, rd, was previously shown to be a disorder of cyclic nucleotide metabolism involving a deficiency in the activity of the rod photoreceptor cGMP phosphodiesterase (PDE). We have characterized the normal and rd PDE beta-subunit gene, and their respective transcripts, by PCR and direct sequence analysis. We show that the gene consists of at least 22 exons ranging in size from 48 base pairs to several hundred base pairs, covering greater than 25 kilobases. Within a 67-base-pair exon of the rd PDE beta-subunit gene, we identified a nonsense ochre mutation (a C----A transversion in codon 347) that truncates the normal gene product, eliminating more than one-half of the peptide chain, including the putative catalytic domain. The consequences of the truncation are consistent with the observed phenotypes in rd mice heterozygous and homozygous for the disorder. The nonsense mutation was also found in another related and in six unrelated strains displaying the rd phenotype, indicating that the rd allele arose from a single genetic event. The results strongly argue for the nonsense mutation being responsible for retinal degeneration in the rd mouse.

Electrophysiologic characteristics of human and rat retinas in vitro

To promote studies on the human retina, we investigated the survival of function in postmortem specimens. Visual pigment has been regenerated in normal human retinas, 5 to 58 hours postmortem, by exposure to retinal isomers in the dark. Levels from 0.1 to 0.41 nmol/mg protein were reached. Photoresponses were obtained in 9 of 13 retinas: P III maximum amplitudes ranged from 20-398 microV and thresholds, taking the criterion amplitude as 3 microV, ranged from 8.8-1340 quanta/micros2. In three cases, the b-wave was also seen. The P III amplitude vs. log intensity curves gave values of n between 0.6 and 1.0, and sigma (the stimulus intensity for a half maximal response) between 132-3700 quanta/microns2. Recovery of sensitivity did not always correspond to that of maximum response.

Evidence for reduced binding of cyclic GMP to cyclic GMP phosphodiesterase in photoreceptors of mice heterozygous for the rd gene

The binding of radiolabelled cGMP to rod outer segment proteins has been investigated in mice, heterozygous for the recessive rd gene that leads to rod dysplasia. Two binding sites were detected, by Scatchard analysis, in a crude cGMP phosphodiesterase fraction, extracted with an EDTA wash from outer segments. Affinities were normal but the capacity of both was reduced 25-35%. Photoaffinity labelling with 3H-cGMP, followed by SDS PAGE and fluorography, suggested that cGMP PDE was the principal binding component in the extracts. If the finding reflects cGMP binding in situ, it might explain the 30-40% lower than normal level of cGMP found in the +/rd retina. Visual pigment has been regenerated in isolated normal and heterozygotic retinas by the application of active isomers of cis-retinal, and the time course of cGMP recovery to 'dark-adapted' levels monitored. The increase in the concentration of cGMP was significantly delayed as compared to that of rhodopsin. No differences in time course or kinetics of recovery were discerned between the two genotypes.

Survival of structure and function in postmortem rat and human retinas: rhodopsin regeneration, cGMP and the ERG

Procedures for regenerating visual pigment and restoring phototransduction have been established with freshly isolated, bleached rat retinas. Phosphatidyl choline liposomes containing a 500 microM mixture of retinal isomers, including the 9-cis and 11-cis forms, were employed and the results compared with dark-adapted retinas, incubated similarly but without retinal. The following were recovered in a 60 min incubation, rhodopsin (plus isorhodopsin) to 91% of the original rhodopsin concentration, 87% of cGMP and 89% of PIII amplitude at saturation. PIII amplitude vs. log intensity curves gave values of n between 0.6 and 1/2.0 and sigma between 85 and 439 quanta/micron 2. Human retinas, ranging from 18 to 58 hours postmortem and treated as above, also produced photoresponses. Of the 7 retinas studied so far, rhodopsin has been regenerated to 0.1-0.35 nmol/mg protein, cGMP to 23.5-49.2 pmol/mg protein, and PIII to 20-50 microV: in some cases a b-wave was also seen. Values of n varied between 0.6 and 1.0, and sigma between 132 and 3700 quanta/micron 2. PIII responses were also seen after retinas, approximately 30 hours postmortem, were incubated for a further 24 hours in fortified medium. After incubation, retinal vacuolation was reduced.

The corneal ERG of the heterozygous retinal degeneration mouse

In the retina of the mouse heterozygous for retinal degeneration (+ rd) cGMP levels are reported to be considerably reduced from normal. The sensitivity and timing of the a- and b-wave components of the electroretinogram of anaesthetized, dark-adapted, heterozygous (+ rd) mice were measured and compared with those of normals (+ +). The (+ rd) mouse was found to be 0.43 log units more sensitive to flashes of green light than the normal for both a- and b-waves; the Vmax amplitude of the b-wave was larger; and variations in time-to-peak were of small magnitude except at low a-wave intensities, where the + rd responses were slowed compared with those of + +. Reduced phosphodiesterase activity has been suggested previously as a mechanism for altered cGMP levels in the rd mouse. The findings reported are interpreted in terms of reduced rates of cGMP metabolism.