Edit in JSFiddle

Ext.define('demo.UserModel', {
    extend: 'Ext.data.Model',
    fields: ['id', 'name', 'profile_image_url']
});


var userTreeStore = Ext.create('Ext.data.TreeStore', {
    
    model: 'demo.UserModel',
    
    proxy: {
        type: 'jsonp', // Because it's a cross-domain request
        
        url : 'https://api.twitter.com/1/lists/members.json?owner_screen_name=Sencha&slug=sencha-team&skip_status=true',
        
        reader: {
            type: 'json',
            root: 'users' // The returned JSON will have array 
                          // of users under a "users" property
        },
        
        // Don't want proxy to include these params in request
        pageParam: undefined,
        startParam: undefined,
        pageParam: undefined,
        pageParam: undefined
    },
    
    listeners: {
        
        // Each demo.UserModel instance will be automatically 
        // decorated with methods/properties of Ext.data.NodeInterface 
        // (i.e., a "node"). Whenever a UserModel node is appended
        // to the tree, this TreeStore will fire an "append" event.
        append: function( thisNode, newChildNode, index, eOpts ) {
            
            // If the node that's being appended isn't a root node, then we can 
            // assume it's one of our UserModel instances that's been "dressed 
            // up" as a node
            if( !newChildNode.isRoot() ) {
                
                // The node is a UserModel instance with NodeInterface
                // properties and methods added. We want to customize those 
                // node properties  to control how it appears in the TreePanel.
                
                // A user "item" shouldn't be expandable in the tree
                newChildNode.set('leaf', true);
                
                // Use the model's "name" value as the text for each tree item
                newChildNode.set('text', newChildNode.get('name'));
                
                // Use the model's profile url as the icon for each tree item
                newChildNode.set('icon', newChildNode.get('profile_image_url'));
                newChildNode.set('cls', 'demo-userNode');
                newChildNode.set('iconCls', 'demo-userNodeIcon');
            }
        }
    }
});

userTreeStore.setRootNode({
    text: 'Users',
    leaf: false,
    expanded: false // If this were true, the store would load itself 
                    // immediately; we do NOT want that to happen
});

var settingsTreeStore = Ext.create('Ext.data.TreeStore', {
    root: {
        expanded: true,
        children: [
            {
                text: 'Settings',
                leaf: false,
                expanded: true,
                children: [
                    {
                        text: 'System Settings',
                        leaf: true
                    },
                    {
                        text: 'Appearance',
                        leaf: true
                    } 
                ]
            }
        ]
    }
});

// Graft our userTreeStore into the settingsTreeStore. Note that the call
// to .expand() is what triggers the userTreeStore to load its data.
settingsTreeStore.getRootNode().appendChild(userTreeStore.getRootNode()).expand();

Ext.create('Ext.tree.Panel', {
    id: 'usersTreePanel',
    title: 'Admin Control Panel',
    renderTo: Ext.getBody(),
    height: 300,
    width: 300,
    store: settingsTreeStore,
    rootVisible: false
});

#usersTreePanel .demo-userNode .x-grid-cell-inner {
    height: 30px;
    font-size: 1.2em;
}

.demo-userNodeIcon {
    height: 25px;
    width: 25px;
}