Of course you're right; I'm using colloquial shorthand. I'm referring to apolipoproteins bound to lipids and cholesterol (at which point we can call them lipoproteins). What I really am talking about is VLDL-cholesterol, LDL-cholesterol, HDL-cholesterol, and triglycerides.
awolf's challenge is particularly relevant and on-point because extremely low levels of LDL-cholesterol from birth (hypobetalipoproteinemia or abetalipoproteinemia) is associated with neurological deficits. When caused by defective APOB, it is also associated with non-alcoholic fatty liver disease.
A simple and generally correct statement: LDL particles deliver cholesterol from the liver to the cells throughout your body. Cholesterol is water insoluble, so it needs a carrier; the apolipoproteins (ApoB, which is the key protein component of VLDL and LDL) are those carriers.
"Cholesterol" is an organic compound that is essential for life. What exactly do you mean you can nearly eliminate it in mice?