mysql - referencing one foreign key to multiple possible primary keys
===

I'd like to set up the following database scenario:

> CREATE TABLE IF NOT EXISTS `points` (<br />
> &nbsp;&nbsp;&nbsp;&nbsp;`po_id` INT UNSIGNED NOT NULL AUTO_INCREMENT, <br />
> &nbsp;&nbsp;&nbsp;&nbsp;`po_north` INT, <br />
> &nbsp;&nbsp;&nbsp;&nbsp;`po_east` INT, <br />
> &nbsp;&nbsp;&nbsp;&nbsp;PRIMARY KEY (`po_id`),<br />
> ) ENGINE=InnoDB;<br />

> CREATE TABLE IF NOT EXISTS `lines`(<br/>
> &nbsp;&nbsp;&nbsp;&nbsp;`li_id` INT UNSIGNED NOT NULL AUTO_INCREMENT, <br/>
> &nbsp;&nbsp;&nbsp;&nbsp;`li_from` INT NOT NULL, <br />
> &nbsp;&nbsp;&nbsp;&nbsp;`li_to` INT NOT NULL, <br />
> &nbsp;&nbsp;&nbsp;&nbsp;PRIMARY KEY (`li_id`), <br />
> &nbsp;&nbsp;&nbsp;&nbsp;FOREIGN KEY (`li_from`) REFERENCES points(`po_id`), <br />
> &nbsp;&nbsp;&nbsp;&nbsp;FOREIGN KEY (`li_to`) REFERENCES points(`po_id`), <br />
>) ENGINE=InnoDB;

Now I want to set up a third table, that sores some metadata like who created or altered a point or a line:

> CREATE TABLE IF NOT EXISTS `metadata` (<br />
> &nbsp;&nbsp;&nbsp;&nbsp;`me_type` ENUM('point','line') NOT NULL,<br />
> &nbsp;&nbsp;&nbsp;&nbsp;`me_type_id` INT UNSIGNED NOT NULL,<br />
> &nbsp;&nbsp;&nbsp;&nbsp;`me_permissions` VARCHAR(255) NOT NULL,<br />
> &nbsp;&nbsp;&nbsp;&nbsp;`me_created_by` INT UNSIGNED NOT NULL,<br />
> &nbsp;&nbsp;&nbsp;&nbsp;`me_created_on` DATETIME NOT NULL,<br />
> &nbsp;&nbsp;&nbsp;&nbsp;`me_last_modified_by` INT UNSIGNED NOT NULL,<br />
> &nbsp;&nbsp;&nbsp;&nbsp;`me_last_modified_by` DATETIME NOT NULL,<br />
) ENGINE=InnoDB;

My first approach was to set an ENUM with two types (points and lines). But the problem is still, that I cannot properly reference a foreign key to one of the tables. Is there any recommended solution for such problem in MySQL?

BTW: 
The fields for `me_created_by` and `me_last_modified_by` shall reference to a table storing some user data.

Thanks for help

mgr