Anatomic Gene Expression Atlas (AGEA)

What is AGEA?

The Allen Institute has built a data-driven three-dimensional atlas of the adult C57Bl/6J mouse brain based on the ISH gene expression images of the Allen Mouse Brain Atlas. Only the coronal gene expression data was used to construct this application. The project is called AGEA which stands for Anatomic Gene Expression Atlas. Essentially, AGEA characterizes the multi-scale spatial relationship in the mouse brain as derived from coronal gene expression data without a prior knowledge of classical anatomy. In the AGEA online application, you can:

• View and navigate 3-D spatial relationship maps (Correlation mode) and search for genes withing the coronal dataset with local regionality (Find Genes) and,
• Explore a transcriptome based spatial organization of the brain (Clusters mode).

The application is loaded by clicking on the AGEA tab from the Allen Mouse Brain Atlas and looks like the following image.

The upper panels of images are orthogonal views of a 3-D Nissl reference atlas volume. The reference space was divided into 200 μm volumes or voxels that demonstrate a unique gene expression profile. The cross hairs in the top panel of images select for a seed voxel from which to compare to gene expression profiles to all other voxels in the brain. For more detailed information on how AGEA was constructed please see the user guide or Ng L, et al.(2009) An anatomic gene expression atlas of the adult mouse brain. Nature Neuroscience 12(3): 356-362.

The AGEA Viewer

This section describes the controls of the Correlation, Cluster and Gene Finder modes of AGEA.

A. Seed Selector panel: Use this area to select the seed voxel as marked by the red crosshairs. Either click or click and drag to navigate to a different correlation map.
B. Map panel: This area displays the three orthogonal views of the currently selected correlation map in coronal, sagittal and horizontal planes. The green crosshair marks the currently selected voxel. Either click or click and drag to move to a new 3-D location. Note that this volume can be navigated in 3-D for any selected voxel location from A.

1. Permalink/Zoom: Clicking on “Permalink” creates an URL in the address bar that effectively save information about your current viewing state. This URL can be saved for latter access to directly take you back to the current view and color scale settings. Clicking the arrows will toggle between zooming the images to fit in the window and a higher resolution more zoomed mode. At a fixed zoom level, you may need to use the browser scroll bars to view all the images.

2. Mode Selector: Click on Correlation or Clusters to switch the viewer to different modes.

3. Position/ARA: The position of the crosshairs in the seed selector in millimeters from bregma. Click on the icon next to the position to view the closest coronal section of the Allen Reference Atlas.

4. Lock/Sync: The icon on the left locks the planes shown in the selected expression map B to the same position as the seed map A. Click to toggle the locking behavior. Conversely, click on the right icon to move the seed voxel to the current selected map voxel.

5. ARA Label: The structure or structural grouping from the Allen Reference Atlas to which the seed voxel (indicated by red crosshairs) belongs. Click on the name to get information about the structure.

6. ARA Blend: This icon toggles blending Allen Reference Atlas structural delineations on the Seed Selector images for direct anatomic comparison. A 1 mm grid is also included with the reference atlas colors. The darker lines indicate the origin of the coordinate system, which is at bregma.

7. Position/ARA: The position of the crosshairs in the correlation map in millimeters from bregma. Click on the icon next to the position to view the closest coronal section of the reference atlas.

8. Color scale control: Use to adjust the false color mapping of the correlation map to threshold the images for regions of higher significance. All voxels with correlation within the select range are rescaled to span the color scale.

9. ARA Label: The structure or structural grouping to which the selected voxel (indicated by green crosshairs) belongs. Click on the name to get information about the structure.

10. Correlation: Value of the correlation at the currently selected voxel with respect to the seed voxel selected in panel A. This shows numerically how well the target voxel is correlated with the seed.

11. Nissl Blend: This icon toggles blending the Nissl reference atlas volume on the correlation map. As with (6), a 1 mm grid is shown, and the brighter lines indicate the origin.

12. Download: Click to download the currently selected correlation map as raw flat file with numbers saved as floats.

Using Correlation Mode

In Correlation mode, the lower panels show orthogonal views of the selected spatial relationship map. The correlation values in the bottom figures can be interpreted as a measure of average co-expression between two voxels. The higher the correlation value between voxels, the more common it is for genes from the seed voxel to be co-expressed. Higher correlation between two voxels indicates more spatial correlation of expression and thus potentially higher possibility that the spatial regions spanned by the voxels are anatomically related. This may indicate that the voxels compared share common cell types or represent a coherent functional map. The correlation map can also be used to locate coexpressing areas in other brain regions. In the above figure, the map indicates that there is higher coexpression with layer 5 than other layers of the cerebral cortex.

When a seed voxel is selected using the crosshairs in the top panels, you can find correlations between the seed voxel and other voxels in the brain by selecting a distinct voxel in the lower panels using those crosshairs. The correlation between the regions will be displayed below the lower panels (section 10).

Clusters Mode

Selecting Clusters mode switches the lower panels to view a data-driven hierarchical binary-tree spatial organization of the brain computed from the AGEA correlation maps. To construct the decomposition all 51,533 voxels were assigned to the root node of the tree. As we descend the tree, a node is bifurcated into two nodes to achieve maximal dissimilarity between two groups of voxels based on correlation values. The final bi-tree consists of 103,065 nodes with a maximum depth of 53 levels and 51,533 leaf nodes (one for each voxel in the brain). Effective visualization of this large data structure is via an easy-to-use Tree Depth slider mechanism to navigate the bi-tree, providing 3-D context and visualizing the multi-scale partitioning.

In the above example, when the dentate gyrus is selected from the top panels you can see the relationship of the dentate gyrus to the rest of the hippocampal formation at a tree depth of 8.

The voxels of a node are visualized with a systematic color coding scheme. All voxels of a node are assigned a color based on the ‘jet’ color scheme where the leaf node with low order number is assigned shades of blues. The colors then run through green, yellow and orange. Finally, higher order voxels are assigned shades of reds.

Gene Finder Mode

The Gene Finder search facility is among the most powerful aspects of AGEA’s functionality. It enables users to search an anatomic region of interest for genes within the ABA coronal database that exhibit localized enrichment. With the Gene Finder tab selected, you can select a region of interest with the crosshairs in the upper panels. Voxels in the orange to red range (top third of the range) are considered the local region of interest while all voxels above threshold cutoff (all non dark blue voxels) forms the domain region. The top 200 image series are returned by rank order in a separate page. You will have to visually inspect the dataset to ensure the signal that AGEA detected was real and not an artifact of the image collection.