Cathepsin D (CathD) and B (CathB) reduce mHtt-induced toxicity in primary cortical neurons. A. Neurons were transfected with full length 23QHtt or 145QmHtt, either alone or co-transfected with CathD or CathB. Con = empty vector control. Cell death was increased by 145QmHtt and reduced by co-transfection with CathD or CathB. *p < 0.05 compared between 145QmHtt alone versus 145QmHtt co-transfection with CathD or CathB. †p < 0.05 compared between 145QmHtt and 23QHtt transfected cells. B. Blockade of autophagy reduces the effects of CathD and CathB in neuroprotection against 145QmHtt induced cortical neuron death. In 23QHtt transfected neurons, overexpression of CathD or CathB, or 3-MA inhibition alone did not cause neuron death. However, in these 23QHtt transfected neurons when autophagy is blocked by 3-MA, increasing CathD or CathB increased cell death. In 145QmHtt transfected neurons, CathD and CathB reduced 145QmHtt-induced neuron death. When autophagy is blocked by 3-MA, 145QmHtt is more toxic, and CathD or CathB enhancement could no longer reduce 145QmHtt-induced cell death. Con = empty vector control. †p < 0.05 compared between 23QHtt transfected versus 145QmHtt transfected cells. *p < 0.05 compared between 145Q and 145Q+CathD or CathB transfected cells. #p < 0.05 compared between transfected cells with 3-MA treatment and without 3-MA treatment. C. 145QmHtt increases LC3 II/LC3 I ratio. Primary neurons were transfected with 23QHtt, 23QHtt plus CathD, 23QHtt plus CathB, 145QmHtt, 145QmHtt plus CathD, and 145QmHtt plus CathB constructs. Western blot analyses were performed with an anti-LC3 antibody. β-actin western blots were used as loading controls. Immunoreactive bands were quantified and shown in the graphs. *p < 0.05 compared between 145QmHtt and 23QHtt transfection.