Trouble changing 2006 Mariner Hybrid tire
 

My 2006 Mercury Mariner Hybrid recently got a flat and I was trying to replace the tire with the spare. I had no troubles getting the spare out, and lugnuts loosened and off but when I try to take the tire off it doesn't budge at all. It looks like there are some sort of teeth holding on to the threads. Anyone have any idea on how to get the tire off?