PREHEAT oven to 350°F. Spray large nonstick skillet with cooking spray. Add mushrooms and garlic; cook 5 min. on medium heat, stirring frequently. Remove from heat. Beat egg whites lightly in medium bowl. Add mushroom mixture, spinach, ricotta cheese and half each of the mozzarella and Parmesan cheeses; mix well. Spoon evenly into manicotti shells.
PLACE in 13x9-inch baking dish sprayed with cooking spray. Cover with spaghetti sauce.
BAKE 40 min. or until heated through. Top with the remaining mozzarella and Parmesan cheeses; continue baking 5 min. or until mozzarella cheese is melted.