Phenotypic and Biochemical Analyses of BACE1- and BACE2-deficient Mice

Abstract

␤-Secretase (BACE1) is the rate-limiting protease for the generation of the amyloid ␤-peptide (A␤) in Alzheimer disease. Mice in which the bace1 gene is inactivated are reported to be healthy. However, the presence of a homologous gene encoding BACE2 raises the possibility of compensatory mechanisms. Therefore, we have generated bace1, bace2, and double knockout mice. We report here that BACE1 mice display a complex phenotype. A variable but significant number of BACE1 offspring died in the first weeks after birth. The surviving mice remained smaller than their littermate controls and presented a hyperactive behavior. Electrophysiologically, subtle alterations in the steady-state inactivation of voltage-gated sodium channels in BACE1-deficient neurons were observed. In contrast, bace2 knockout mice displayed an overall healthy phenotype. However, a combined deficiency of BACE2 and BACE1 enhanced the bace1 ؊/؊ lethality phenotype. At the biochemical level, we have confirmed that BACE1 deficiency results in an almost complete block of A␤ generation in neurons, but not in glia. As glia are 10 times more abundant in brain compared with neurons, our data indicate that BACE2 could indeed contribute to A␤ generation in the brains of Alzheimer disease and, in particular, Down syndrome patients. In conclusion, our data challenge the general idea of BACE1 as a safe drug target and call for some caution when claiming that no major side effects should be expected from blocking BACE1 activity. Alzheimer disease (AD)1 is the most common cause of dementia for which neither a good diagnostic test nor an effective treatment is available yet. The most widely accepted hypothesis states that AD is initially triggered by the abnormal accumulation and possibly deposition of the small amyloid ␤-peptide (A␤) in different brain regions, which in turn initiates a pathogenic cascade that ultimately leads to neuronal death, AD pathology, and dementia. A␤ is cleaved from a long membranebound precursor, the amyloid precursor protein (APP), by two consecutive cleavages. ␤-and ␥-secretases are the enzymes that liberate the N and C termini of A␤, respectively, and are the subject of intense investigation because of their relevance as candidate therapeutic targets to treat AD.BACE1 and BACE2 are two highly homologous membranebound aspartyl proteases that can process APP at the ␤-secretase site (1-8). Although both enzymes exhibit many of the characteristics expected for ␤-secretase, it has been quite convincingly demonstrated that BACE1 is in fact the major ␤-secretase responsible for A␤ generation in brain (9 -11). Contrary to BACE1, BACE2 is more highly expressed in peripheral tissues, but also to some extent in brain (2,8,12,13), raising the question of whether BACE2 could contribute to the generation of the brain A␤ pool. Both BACE1 and BACE2 can cleave APP in vitro not only at Asp 1 (numbering considering the first amino acid of A␤ as position 1), but also at internal sites within the A␤ region. BACE1 cleaves between amino acids 10 and 11 o...

β-Site Amyloid Precursor Protein (APP)-cleaving Enzyme 1 (BACE1)-deficient Mice Exhibit a Close Homolog of L1 (CHL1) Loss-of-function Phenotype Involving Axon Guidance Defects

“…Only when mice are backcrossed onto an inbred genetic background (e.g. C57BL/6) do phenotypes become noticeable (21). This suggests that modifier genes in specific genetic backgrounds contribute to BACE1 null phenotypes.…”

“…Initial reports indicated that BACE1 Ϫ/Ϫ mice were viable, fertile, and devoid of abnormalities. However, upon closer examination, BACE1 Ϫ/Ϫ mice were found to have several subtle phenotypes that involved memory deficits (16 -18), hypomyelination (19,20), reduced survival and growth retardation (21), irregularities in neuronal excitability and electrophysiology (17,21,22), seizure (22,23), reduced dendritic spine density (24), schizophrenia endophenotypes (24), and axon guidance defects (1,25). Although these BACE1 null abnormalities are relatively mild, they are complex neurological phenotypes that raise a concern that BACE1 inhibitors may not be completely free of mechanism-based side effects.…”

Ion channel regulation by β-secretase BACE1 – enzymatic and non-enzymatic effects beyond Alzheimer's disease

“…Subsequent studies on BACE1-deficient mice, however, revealed significant postnatal lethality, reduced body weight, decreased anxiety-related behavior, and hyperactivity. 37 Further investigations found decreased thermal pain threshold, impaired motor coordination, myelination deficits, impaired spatial learning and memory, and altered synaptic function and neurotransmitter metabolism. [38][39][40] As a putative correlate of their learning and memory deficits, Wang et al found that activitydependent potentiation at the mossy-fiber -CA3 synapse in the hippocampus of BACE1-deficient mice is substantially impaired.…”

IFN‐γ‐induced BACE1 expression is mediated by activation of JAK2 and ERK1/2 signaling pathways and direct binding of STAT1 to BACE1 promoter in astrocytes

“…For example, knock out of the presenilin 1 (PS1) gene, which is a core component of g-secretase complex, resulted in lethality in a mouse model at the embryonic stage (Geling et al, 2002;Shen et al, 1997). On the other hand, b-secretase knock out mice have relatively normal phenotypes and were fully capable of growing up to become normal adults without Ab production (Luo et al, 2001;Roberds et al, 2001), though other studies reported abnormalities in b-secretase knock out mice (Dominguez et al, 2005). Thus, b-secretase is considered a good target for reducing Ab burden in the AD brains.…”