Need Help To Protect my MVC PHP app folder using htaccess

Question

I have an issue with my .htaccess.

Let's say that my app folder are:

/APP/
/CONTENT/
/UPLOADS/
/ASSETS/

And I have a .htaccess file that point all requests to my index.php, and the index php there is an included file called router.php that manage my roots:

Options +FollowSymLinks
RewriteEngine On

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f

RewriteRule ^([\s\S]*)$ index.php?url=$0 [QSA,L]

What i would want to do right now is to point APP and CONTENT folder also to the .htaccess and like that, i can display my custom 404 error page.