Molecular modeling of ARF6 dysregulation caused by mutations in IQSEC2

IQSEC2 gene mutations are associated with epilepsy, autism, and intellectual disability. The primary function IQSEC2, mediated via its Sec 7 domain, is to act as a guanine nucleotide exchange factor for ARF6. We sought to develop a molecular model, which may explain the aberrant Sec 7 activity on ARF6 of different human IQSEC2 mutations. We integrated experimental data of IQSEC2 mutants with protein structure prediction by the RaptorX server combined with molecular modeling and molecular dynamics simulations. Normally, apocalmodulin (apoCM) binds to IQSEC2 resulting in its N-terminal fragment inhibiting access of its Sec 7 domain to ARF6. An increase in Ca2+ concentration destabilizes the interaction of IQSEC2 with apoCM and removes steric hindrance of Sec 7 binding with ARF6. Mutations at amino acid residue 350 of IQSEC2 result in loss of steric hindrance of Sec 7 binding with ARF6 leading to constitutive activation of ARF6 by Sec 7. On the other hand, a mutation at amino acid residue 359 of IQSEC2 results in constitutive hindrance of Sec 7 binding to ARF6 leading to the loss of the ability of IQSEC2 to activate ARF6. These studies provide a model for dysregulation of IQSEC2 Sec 7 activity by mutant IQSEC2 proteins.Communicated by Ramaswamy H. Sarma.

Novel Missense Mutation A789V in IQSEC2 Underlies X-Linked Intellectual Disability in the MRX78 Family

Disease gene discovery in neurodevelopmental disorders, including X-linked intellectual disability (XLID) has recently been accelerated by next-generation DNA sequencing approaches. To date, more than 100 human X chromosome genes involved in neuronal signaling pathways and networks implicated in cognitive function have been identified. Despite these advances, the mutations underlying disease in a large number of XLID families remained unresolved. We report the resolution of MRX78, a large family with six affected males and seven affected females, showing X-linked inheritance. Although a previous linkage study had mapped the locus to the short arm of chromosome X (Xp11.4-p11.23), this region contained too many candidate genes to be analyzed using conventional approaches. However, our X-chromosome exome resequencing, bioinformatics analysis and inheritance testing revealed a missense mutation (c.C2366T, p.A789V) in IQSEC2, encoding a neuronal GDP-GTP exchange factor for Arf family GTPases (ArfGEF) previously implicated in XLID. Molecular modeling of IQSEC2 revealed that the A789V substitution results in the insertion of a larger side-chain into a hydrophobic pocket in the catalytic Sec7 domain of IQSEC2. The A789V change is predicted to result in numerous clashes with adjacent amino acids and disruption of local folding of the Sec7 domain. Consistent with this finding, functional assays revealed that recombinant IQSEC2^A789V was not able to catalyze GDP-GTP exchange on Arf6 as efficiently as wild-type IQSEC2. Taken together, these results strongly suggest that the A789V mutation in IQSEC2 is the underlying cause of XLID in the MRX78 family.

Septin 6 regulates the cytoarchitecture of neurons through localization at dendritic branch points and bases of protrusions

Septins, a conserved family of GTP-binding proteins with a conserved role in cytokinesis, are present in eukaryotes ranging from yeast to mammals. Septins are also highly expressed in neurons, which are post-mitotic cells. Septin6 (SEPT6) forms SEPT2/6/7 complexes in vivo. In this study, we produced a very specific SEPT6 antibody. Immunocytochemisty (ICC) of dissociated hippocampal cultures revealed that SEPT6 was highly expressed in neurons. Developmentally, the expression of SEPT6 was very low until stage 3 (axonal outgrowth). Significant expression of SEPT6 began at stage 4 (outgrowth of dendrites). At this stage, SEPT6 clusters were positioned at the branch points of developing dendrites. In maturing and mature neurons (stage 5), SEPT6 clusters were positioned at the base of filopodia and spines, and pre-synaptic boutons. Detergent extraction experiments also indicated that SEPT6 is not a post-synaptic density (PSD) protein. Throughout morphologic development of neurons, SEPT6 always formed tiny rings (external diameter, ∼0.5 μm), which appear to be clusters at low magnification. When a Sept6 RNAi vector was introduced at the early developmental stage (DIV 2), a significant reduction in dendritic length and branch number was evident. Taken together, our results indicate that SEPT6 begins to be expressed at the stage of dendritic outgrowth and regulates the cytoarchitecture.

Neuronal activation increases the density of eukaryotic translation initiation factor 4E mRNA clusters in dendrites of cultured hippocampal neurons

Activity-dependent dendritic translation in CNS neurons is important for the synapse-specific provision of proteins that may be necessary for strengthening of synaptic connections. A major rate-limiting factor during protein synthesis is the availability of eukaryotic translation initiation factor 4E (eIF4E), an mRNA 5-cap-binding protein. In this study we show by fluorescence in situ hybridization (FISH) that the mRNA for eIF4E is present in the dendrites of cultured rat hippocampal neurons. Under basal culture conditions, 58.7 +/-11.6% of the eIF4E mRNA clusters localize with or immediately adjacent to PSD-95 clusters. Neuronal activation with KCl (60 mM, 10 min) very significantly increases the number of eIF4E mRNA clusters in dendrites by 50.1 and 74.5% at 2 and 6 h after treatment, respectively. In addition, the proportion of eIF4E mRNA clusters that localize with PSD-95 increases to 74.4+/-7.7% and 77.8+/-7.6% of the eIF4E clusters at 2 and 6 h after KCl treatment, respectively. Our results demonstrate the presence of eIF4E mRNA in dendrites and an activity-dependent increase of these clusters at synaptic sites. This provides a potential mechanism by which protein translation at synapses may be enhanced in response to synaptic stimulation.

Upregulation by KCl treatment of eukaryotic translation elongation factor 1A (eEF1A) mRNA in the dendrites of cultured rat hippocampal neurons

Activity-dependent local translation in the dendrites of brain neurons plays an important role in the synapse-specific provision of proteins necessary for strengthening synaptic connections. In this study we carried out combined fluorescence in situ hybridization (FISH) and immunocytochemistry (IC) and showed that more than half of the eukaryotic elongation factor 1A (eEF1A) mRNA clusters overlapped with or were immediately adjacent to clusters of PSD-95, a postsynaptic marker, in the dendrites of cultured rat hippocampal neurons. Treatment of the neurons with KCl increased the density of the dendritic eEF1A mRNA clusters more than two-fold. FISH combined with IC revealed that the KCl treatment increased the density of eEF1A mRNA clusters that overlapped with or were immediately adjacent to PSD-95 clusters. These results indicate that KCl treatment increases both the density of eEF1A mRNA clusters and their synaptic association in dendrites of cultured neurons.

A simple method for combined fluorescence in situ hybridization and immunocytochemistry

By combining in situ hybridization (ISH) and immunocytochemistry (IC), microscopic topological localization of mRNAs and proteins can be determined. Although this technique can be applied to a variety of tissues, it is particularly important for use on neuronal cells which are morphologically complex and in which specific mRNAs and proteins are located in distinct subcellular domains such as dendrites and dendritic spines. One common technical problem for combined ISH and IC is that the signal for immunocytochemical localization of proteins often becomes much weaker after conducting ISH. In this manuscript, we report a simplified but robust protocol that allows immunocytochemical localization of proteins after ISH. In this protocol, we fix cultured cortical or hippocampal neurons with 4% paraformaldehyde (PFA), rinse briefly in PBS, and then further fix the cells with C methanol. Our method has several major advantages over previously described ones in that (1) it is simple, as it is just consecutive routine fixation procedures, (2) it does not require any special alteration to the fixation procedures such as changes in salt concentration, and (3) it can be used with antibodies that are compatible with either methanol (MeOH-) or PFA-fixed target proteins. To our best knowledge, we are the first to employ this fixation method for fluorescence ISH + IC.

A four PDZ domain-containing splice variant form of GRIP1 is localized in GABAergic and glutamatergic synapses in the brain

We have isolated, from a rat brain cDNA library, a clone corresponding to a 2779-bp cDNA encoding a novel splice form of the glutamate receptor interacting protein-1 (GRIP1). We call this 696-amino acid splice form GRIP1c 4-7 to differentiate it from longer splice forms of GRIP1a/b containing seven PDZ domains. The four PDZ domains of GRIP1c 4-7 are identical to PDZ domains 4-7 of GRIP1a/b. GRIP1c 4-7 also contains 35 amino acids at the N terminus and 12 amino acids at the C terminus that are different from GRIP1a/b. In transfected HEK293 cells, a majority of GRIP1c 4-7 was associated with the plasma membrane. GRIP1c 4-7 interacted with GluR2/3 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor. In low density hippocampal cultures, GRIP1c 4-7 clusters colocalized with GABAergic (where GABA is gamma-aminobutyric acid) and glutamatergic synapses, although a higher percentage of GRIP1c 4-7 clusters colocalized with gamma-aminobutyric acid, type A, receptor (GABA(A)R) clusters than with alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor clusters. Transfection of hippocampal neurons with hemagglutinin-tagged GRIP1c 4-7 showed that it could target to the postsynaptic complex of GABAergic synapses colocalizing with GABA(A)R clusters. GRIP1c 4-7-specific antibodies, which did not recognize previously described splice forms of GRIP1, recognized a 75-kDa protein that is enriched in a postsynaptic density fraction isolated from rat brain. EM immunocytochemistry experiments showed that in intact brain GRIP1c 4-7 concentrates at postsynaptic complexes of both type I glutamatergic and type II GABAergic synapses although it is also presynaptically localized. These results indicate that GRIP1c 4-7 plays a role not only in glutamatergic synapses but also in GABAergic synapses.

Excretion of catecholamines and metabolites in response to increased dietary phosphate intake

The urinary excretion of free dopamine, norepinephrine, and epinephrine could reflect the contribution of the neural release and filtration of these catecholamines as well as the intrarenal tubular synthesis and metabolism of dopamine. Because these catecholamines are rapidly metabolized, the excretion of the free amines represents only a fraction of the total release and synthesis by the kidney. The present study determined the effect of increasing dietary phosphate intake on the excretion of free dopamine, norepinephrine, and epinephrine and their primary stable metabolites. Seven male rats were placed in metabolic balance cages and fed 12 gm/day of normal phosphate diet (NPD) (0.7% inorganic phosphorus [Pi]) for 4 days and then fed a high phosphate diet (HPD) (1.8% Pi) for 4 days. Twenty-four-hour urine samples were collected for determination of free catecholamines, their major stable metabolites, and electrolyte excretions. The urinary excretion data for the seven rats was combined for all 4 days of each dietary regimen. Increasing phosphate intake from 0.7% to 1.8% significantly increased free dopamine excretion by 23%, from 5.6 +/- 0.2 to 6.8 +/- 0.1 micrograms/day (n = 7, p < 0.05). This increase in free dopamine excretion was associated with similar increases in urinary excretion of dopamine glucuronide, 21.6 +/- 1.3 to 27.9 +/- 1.8 micrograms/day (32%) and the dopamine metabolite DOPAC, 9.4 +/- 0.5 to 12.1 +/- 0.6 micrograms/day (30%) and total dopamine excretion from 32.9 +/- 1.7 to 41.0 +/- 1.9 micrograms/day (27%). Plasma DOPA levels were unchanged by increased dietary phosphate intake; however, plasma norepinephrine levels decreased significantly. Excretion of free or sulfated norepinephrine was not changed by increased phosphate intake. However, excretion of MHPG, a metabolite of norepinephrine and epinephrine, decreased significantly, from 33.7 +/- 2.1 to 23.9 +/- 0.8 micrograms/day, n = 7, p < 0.05.(ABSTRACT TRUNCATED AT 250 WORDS)

Attractiveness of the male Acheta domesticus calling song to females. III. The relation of age-correlated changes in syllable period recognition and phonotactic threshold to juvenile hormone III biosynthesis

1. Most crickets first demonstrated positive phonotaxis to 65 dB CSs having a 53-62 ms SP by day 3 following the imaginal molt (Fig. 3B). The onset of copulatory readiness occurred on average at 3.2 days. 2. The attractive range of SPs for most females became progressively broader as they aged (Fig. 4). Three to 4-day-old females were attracted to a smaller number of CS SPs than were 20-21 day old females (Fig. 4). 3. Older, less selective females did not typically respond to the same range of CS SPs (Fig. 6). However, they were more likely to respond to some SPs (especially 50 ms) than to others (Fig. 7). 4. The phonotactic threshold decreased from 95 dB or greater on day 0 to a mean of 55 dB by day 3, during a period of increasing JHIII biosynthesis, and thereafter remained at that level (Fig. 8). 5. During a period of maximal JHIII production, 3-5 day-old females usually responded to 4 of the 7 SPs presented (Fig. 8). Females older than 12 days were unselective for CS SP, and JHIII synthesis remained at a level below the peak production on day 4 (Fig. 8). 6. Older females, that were unselective for CS SP, became as selective as 3 to 5-day-old females within 4 days of topical application of JHIII (Figs. 9-11).