Regulation of spine density and morphology by IQGAP1 protein domains

Abstract

IQGAP1 is a scaffolding protein that regulates spine number. We now show a differential role for IQGAP1 domains in spine morphogenesis, in which a region of the N-terminus that promotes Arp2/3-mediated actin polymerization and branching stimulates spine head formation while a region that binds to Cdc42 and Rac is required for stalk extension. Conversely, IQGAP1 rescues spine deficiency induced by expression of dominant negative Cdc42 by stimulating formation of stubby spines. Together, our observations place IQGAP1 as a crucial regulator of spine number and shape acting through the N-Wasp Arp2/3 complex, as well as upstream and downstream of Cdc42.

Figure 1. Domain organization of IQGAP1 and representation of the deletion mutants.

CHD: Calponin-homology domain. IRS: WW: protein domain containing two highly conserved triptophans that bind proline- rich peptide motifs; responsible for interaction with ERKs. IQ: calmodulin-binding motif; the term refers to the first two amino acids of the motif: isoleucine and glutamine. GRD: Ras GTP related activating protein domain; responsible for interactions with Cdc42 and Rac. CT: C-terminus; responsible for interactions with cadherin, CLIP-170, APC, etc.

Figure 2. IQGAP1 stimulates spine formation and increases spine head size.

(A) Confocal image showing an example of a 17 DIV cultured hippocampal neuron transfected with mock plasmid plus GFP (green) and double stained with MAP2 (red). (B) A high magnification view of a dendritic segment from another neuron of the same culture (C) A 3-D reconstruction of dendritic shafts from a sister culture; note the morphology and density of dendritic spines. (D–G) Confocal images showing a dendritic segment from a neuron co-transfected with myc-tagged-mock plasmid (blue) plus GFP-PSD95 (green) and stained for synaptophysin (red). (G) Merge image; note that GFP-PSD95 (+) spines colocalize with synaptophysin puncta (arrowheads). (H) A Confocal image showing an example of a 17 DIV cultured hippocampal neuron transfected with myc-tagged IQGAP1 WT+GFP (green) and double stained with MAP2 (red). (I) A high magnification view of a dendritic segment from another neuron of the same culture. (J) A 3-D reconstruction of dendritic shafts from a sister culture; note the increase in the number and size of spines. (K–N) Confocal images showing a segment of a dendritic shaft of a neuron co-transfected with myc-tagged-IQGAP1 WT (blue) plus GFP-PSD95 (green) and stained for synaptophysin (red). (N) Merge image. (O–Q) Graphs showing effects of ectopic expression of myc-tagged IQGAP1 WT on spine number, spine type and spine head size. Bars represent mean ± standard deviation. *p<0.0001. The effect of IQGAP1 mutants on spine number is also shown (O).

Figure 3. IQGAP1 CHD domain is required for spine head formation.

(A) Confocal image showing an example of a 17 DIV cultured hippocampal neuron transfected with myc-tagged Δ-CHD IQGAP1 plus GFP (green) and double stained with MAP2 (red). (B) A high magnification view of dendritic segments from another neuron of the same culture; note the long filopodial-like protrusions merging from dendritic shafts (arrowheads). (C) A 3-D reconstruction of dendritic shafts from a sister culture; note the morphology and density of filopodial-like protrusions (arrowheads). (D–E) Confocal images showing a dendritic segment from a neuron co-transfected with myc-tagged-Δ-CHD-IQGAP1 (blue) plus GFP-PSD95 (green) and stained for synaptophysin (red). (G) Merge image; note that GFP-PSD95 (+) protrusions colocalize with synaptophysin puncta (arrowheads). (H) Graphs showing effects of the ectopic expression of myc-tagged Δ-CHD-IQGAP1 on the number of different types of dendritic spines; note that Δ-CHD-IQGAP1 significantly increases the number of thin spines and filopodial extensions, while decreases the number of mushroom-shaped spines. (I) Graphs showing the effect of scrambled-sh-Arp3, sh-Arp3, and sh-WASP on the number of dendritic protrusions. For this experiment, cultures were transfected with the corresponding GFP-or HcRed-sh plasmids at 17 DIV and fixed 24 h later. Note the significant decrease in the total number of dendritic protrusions in the sh-Arp3 and sh-WASP-treated groups. (J) Graphs showing the number of dendritic protrusions contacting synaptophysin puncta in neurons transfected with IQGAP1 WT, Δ-CHD-IQGAP1 and sh-Arp3 plus myc-tagged-IQGAP1 WT. Note the dramatic decrease in the number of dendritic protrusions contacting synaptophysin puncta in the cells treated with sh-Arp3 plus IQGAP1 WT; most of these protrusion resemble filopodial extensions. (K) Confocal image showing an example of a 17 DIV cultured hippocampal neuron transfected with HcRed-sh-Arp3 (red) plus myc-tagged IQGAP1 WT (green). (L) A high magnification view of a dendritic segment from another neuron of the same culture. (M) A 3-D reconstruction of dendritic shafts from a sister culture; note the presence of many filopodial-like protrusions. (N-P) Confocal images showing a segment of a dendritic shaft of a neuron co-transfected with myc-tagged-IQGAP1 WT (blue) plus GFP-sh-Arp3 (green) and stained for synaptophysin (red). (Q) Merge image. Note that many filopodial protrusions are not contacted by synaptophysin puncta. Bars represent mean ± standard deviation. * p<0.0001.

Figure 4. The IQGAP1 GRD domain is required for stalk spine extension.

(A) Confocal image showing an example of a 17 DIV cultured hippocampal neuron transfected with myc-tagged Δ-GRD-IQGAP1 plus GFP (green) and double stained with MAP2 (red). (B) A high magnification view of dendritic segments from another neuron of the same culture; note that many of the protrusions lack discernable stalks and are apposed to dendritic shafts. (C) A 3-D reconstruction of dendritic shafts from a sister culture; note the morphology and density of dendritic protrusions that resemble stubby spines lacking stalks or displaying very short ones. (D–F) Confocal images showing a dendritic segment from a neuron co-transfected with myc-tagged-Δ-GRD-IQGAP1 (blue) plus GFP-PSD95 (green) and stained for synaptophysin (red). (G) Merge image; note that patches of GFP-PSD95 colocalize with synaptophysin puncta (arrowheads). (H) Confocal image showing an example of a 17 DIV cultured hippocampal neuron transfected with myc-tagged IQGAP1 WT plus GFP (green) plus HA-tagged DN-Cdc42 (T17N; red). (I) A high magnification view of dendritic segments from another neuron of the same culture. (J) A 3-D reconstruction of dendritic shafts from a sister culture. Note that IQGAP1 WT failed to stimulate the formation of mushroom-shaped spines; most of them resemble stubby spines. (K) Graphs showing effects of the ectopic expression of myc-tagged Δ-GRD-IQGAP1 on the number of different types of dendritic spines; Δ-GRD-IQGAP1 significantly increases the number of stubby spines. (L) Graphs showing effects of the ectopic expression of myc-tagged IQGAP1 WT+HA-tagged T17N on the number of different types of dendritic spines; note the increase in the number of stubby spines. Bars represent mean ± standard deviation. * p<0.0001.

Figure 5. Δ-CT IQGAP1 stimulates spine formation.

(A) Confocal image showing an example of a 17 DIV cultured hippocampal neuron transfected with myc-tagged Δ-CT-IQGAP1 plus GFP (green) and double stained with MAP2 (red). (B) A high magnification view of dendritic segments from another neuron of the same culture; (C) A 3-D reconstruction of dendritic shafts from a sister culture. (D–F) Confocal images showing a dendritic segment from a neuron co-transfected with myc-tagged-Δ-CT-IQGAP1 (blue) plus GFP-PSD95 (green) and stained for synaptophysin (red). (G) Merge image. (H–J) Graphs showing the effect of expressing myc-tagged-Δ-CT-IQGAP1 on spine number/type and spine head size and length. Bars represent mean ± standard deviation. *p<0.0001.

Figure 6. Distribution of NR2A in hippocampal neurons expressing IQGAP1 WT or deletion mutants.

(A–C) High magnification views of a dendritic segment from a 17 DIV cultured hippocampal neuron transfected with myc-tagged-mock plasmid (blue in merge) plus GFP-NR2A (green) and double stained with anti-GFP for surface NR2A (red). (D–F) A similar set of images but from a culture transfected with myc-tagged IQGAP1 WT (blue in merge) plus GFP-NR2A (green) and double stained with anti-GFP for surface NR2A (red). (G–I) A similar set of images but from a culture transfected with myc-tagged Δ-GRD IQGAP1 (blue in merge) plus GFP-NR2A (green) and double stained with anti-GFP for surface NR2A (red). Note that in all cases dendritic spines contain NR2A labeling at their tips. (J–L) Quantification of GFP-NR2A total fluorescemce intensity (J), GFP-NR2A surface fluorescence intensity (K), and the ratio of surface vs. total GFP-NR2A fluorescence intensities (L) in neurons expressing myc-tagged mock plasmid (control), myc-tagged IQGAP1 WT and myc-tagged Δ-GRD IQGAP1. No significnt differences were detected between control neurons and those expressing IQGAP1 WT or Δ-GRD IQGAP1. Similar results were observed after ectopic expresson of Δ-GRD IQGAP1or Δ-CT IQGAP1 (not shown). Intensity values (8-bit images) are expressed in pixels. Black = 0/White = 256. For further details see Ref. –. Values represent the mean ± S.E.M. A least 20 dendritic segments (50 µm in length) per cell were measured for each experimental condition.