A Signal for Building Cone-to-OFF-Bipolar-Cell Synapses
Andrew H. Miller, Hollis B. Howe, Bryan M. Krause, Scott A. Friedle, Matthew I. Banks, et al.
(see pages 5220–5236)
For synapses to function properly, presynaptic release sites must be positioned near postsynaptic receptors. Synapse formation seems particularly challenging in the retina, where cone photoreceptors contact multiple postsynaptic cells. Horizontal- and ON-bipolar-cell dendrites extend deeply into invaginations at the base of cones and form synapses near presynaptic release sites that have synaptic ribbons. Dendrites of OFF bipolar cells, in contrast, extend less deeply into the invaginations and form flat contacts farther from synaptic ribbons. Little is known about the molecules that guide the formation of cone synapses, particularly those with OFF bipolar cells. But Miller et al. provide evidence that one contributor is pregnancy-associated plasma protein aa (Pappaa), a secreted metalloprotease that increases local availability of insulin-like growth factor 1 (IGF1).
A screen of mutant zebrafish revealed that those lacking Pappaa (pappaap170 mutants) often failed to make 180° turns (O-bends) when lights were suddenly, transiently dimmed. Nevertheless, these fish turned normally when bright lights flashed. These and other behaviors suggested that pappaap170 mutants were poor at detecting light decrements. Indeed, electroretinographic responses to light decrements were reduced or inverted in some pappaap170 larvae.
Treating pappaap170 mutants with IGF1 rescued O-bend behavior, whereas expressing dominant-negative IGF1 receptors (IGF1Rs) in wild-type fish replicated the deficit. Notably, both treatments were most effective when given 3–5 d postfertilization, a time when synapses form between cones and bipolar cells and when Pappaa is normally expressed in the vicinity of bipolar cell somata.
Although postsynaptic scaffolding proteins and AMPA receptors were similarly distributed in pappaap170 and wild-type retinas, the distribution of the presynaptic vesicle protein SV2 was abnormal in mutants. At the ultrastructural level, the number of flat contacts in pappaap170 cones was greatly reduced, but the number of synaptic invaginations was normal. Furthermore, more floating ribbons (those not anchored to the presynaptic membrane) were present in mutants. Notably, all these abnormalities were rescued by treating pappaap170 larvae with IGF1.
These data suggest that Pappaa promotes the formation of synapses between cones and OFF bipolar cells by enhancing IGF1-dependent signaling, which particularly affects presynaptic assembly. Identifying the downstream effectors of IGF1Rs should provide more insight into the molecular mechanisms that guide formation of these complex synapses.
Linking GAD65 Genotype, GABA Levels, and Anxiety
Lejla Colic, Meng Li, Liliana Ramona Demenescu, Shija Li, Iris Müller, et al.
(see pages 5067–5077)
Most neurological and psychiatric conditions have a hereditary component, but in most cases, the contribution of any single gene to disease risk is small. This makes identification of genetic risk factors challenging. One strategy for overcoming this difficulty is to examine intermediate phenotypes, such as physiological factors that might predispose one to a particular disease. For example, identifying genetic variants that upset the balance between excitation and inhibition in the brain might uncover risk factors for the many diseases in which this balance is disrupted. The same genetic variations might also affect brain function in healthy people, thus contributing to differences in personality traits and cognitive functioning. Linking genetic variations that alter physiology to disease-related behavioral phenotypes in healthy individuals could help elucidate a gene's role in disease.
Colic et al. used this approach to investigate a possible genetic influence on anxiety disorders. Because disruption of GABA signaling is thought to contribute to such disorders, the authors asked whether a single-nucleotide polymorphism (rs2236418) in the gene encoding the GABA-synthesizing enzyme GAD65 influences GABA levels in the brain. They focused on pregenual anterior cingulate cortex (pgACC), because this area influences fear and anxiety by regulating activity in the amygdala. High-field multivoxel 1H magnetic resonance spectroscopy revealed that women possessing at least one glycine allele of rs2236418 (G carriers) had a higher GABA–glutamate ratio in the pgACC than people with two copies of the alanine allele (AA homozygotes). At the same time, resting-state activity in pgACC was lower for G carriers than for AA homozygotes and lower for women than for men. Further analyses revealed that in women, GABA–glutamate ratio in the pgACC was negatively correlated with scores on a harm-avoidance assessment that measures propensity for anxiety. Finally, mediation analyses indicated that the influence of pgACC GABA–glutamate ratio on harm avoidance depended on genotype, and this effect was present only in women.
These results suggest that rs2236418 genotype influences anxiety phenotypes only in women and only when it increases the GABA–glutamate ratio in the pgACC. This illustrates the complex relationship between common genetic variants and psychiatric conditions and demonstrates how examining intermediate phenotypes might help to elucidate possible links between genes and disease.
Footnotes
This Week in The Journal was written by Teresa Esch, Ph.D.