Wanxia He

Bace1 modulates myelination in the central and peripheral nervous system

Bace1 is an endopeptidase that cleaves the amyloid precursor protein at the β-secretase site. Apart from this cleavage, the functional importance of Bace1 in other physiological events is unknown. We show here that Bace1 regulates the process of myelination and myelin sheath thickness in the central and peripheral nerves. In Bace1-null mice, the process of myelination was delayed and myelin thickness was markedly reduced, indicating that genetic deletion of Bace1 causes hypomyelination. Bace1-null mice also showed altered neurological behaviors such as elevated pain sensitivity and reduced grip strength. Further mechanistic studies showed an altered neuregulin-Akt signaling pathway in Bace1-null mice. Full-length neuregulin-1 was increased and its cleavage product was decreased in the CNS of Bace1-null mice. Furthermore, phosphorylated Akt was also reduced. Based upon these and previous studies, we postulate that neuronally enriched Bace1 cleaves neuregulin-1 and that processed neuregulin-1 regulates myelination by means of phosphorylation of Akt in myelin-forming cells.

Reticulon family members modulate BACE1 activity and amyloid-β peptide generation

Inhibiting the activity of the β-amyloid converting enzyme 1 (BACE1) or reducing levels of BACE1 in vivo decreases the production of amyloid-β. The reticulon family of proteins has four members, RTN1, RTN2, RTN3 and RTN4 (also known as Nogo), the last of which is well known for its role in inhibiting neuritic outgrowth after injury. Here we show that reticulon family members are binding partners of BACE1. In brain, BACE1 mainly colocalizes with RTN3 in neurons, whereas RTN4 is more enriched in oligodendrocytes. An increase in the expression of any reticulon protein substantially reduces the production of Aβ. Conversely, lowering the expression of RTN3 by RNA interference increases the secretion of Aβ, suggesting that reticulon proteins are negative modulators of BACE1 in cells. Our data support a mechanism by which reticulon proteins block access of BACE1 to amyloid precursor protein and reduce the cleavage of this protein. Thus, changes in the expression of reticulon proteins in the human brain are likely to affect cellular amyloid-β and the formation of amyloid plaques.

Genetic deletion of BACE1 in mice affects remyelination of sciatic nerves

BACE1 is a promising therapeutic and preventive target for Alzheimers disease because it is essential for amyloid deposition. However, the recent demonstration of BACE1 in modulating developmental myelination in both peripheral and central nervous systems raises a concern of its effect on myelin maintenance or remyelination, and inhibition of these processes will potentially be detrimental to the BACE1 inhibitor users who are susceptible to myelination diseases such as adult peripheral nerve injury or multiple sclerosis. In this report, we investigated the role of BACE1 during peripheral nerve remyelination in wildtype (WT) and BACE1‐null mice. We show here that genetic deletion of BACE1 affects sciatic nerve remyelination. The impaired remyelination appears to stem from the loss of neuregulin‐1 cleavage by BACE1. To demonstrate a direct cleavage of neuregulin‐1 by BACE1, we have identified a BACE1 cleavage site that turns out be highly conserved among neuregulin‐1 paralogues. Moreover, we show that neuregulin‐1 family member neuregulin‐3 is also cleavable by BACE1. We hypothesize that the BACE1‐cleaved extracellular domain of axonal neuregulin‐1, perhaps neuregulin‐3 as well, binds to Schwann cell ErbB receptors, which in turn regulate remyelination. Pharmacological inhibition of BACE1 should be carefully monitored to avoid alteration of signaling pathway that regulates remyelination.—Hu, X., He, W., Diaconu, C., Tang, X., Kidd, G. J., Macklin, W. B., Trapp, B. D., Yan, R. Genetic deletion of BACE1 in mice affects remyelination of sciatic nerves. FASEB J. 22, 2970–2980 (2008)

VPS35 haploinsufficiency increases Alzheimer’s disease neuropathology

The retromer complex component VPS35 prevents activation of the BACE1 and Aβ production and thus plays an essential role in limiting Alzheimer’s disease neuropathology.

BACE1 deficiency causes altered neuronal activity and neurodegeneration.

BACE1 is required for the release of β-amyloid (Aβ) in vivo, and inhibition of BACE1 activity is targeted for reducing Aβ generation in Alzheimers patients. To further our understanding of the safe use of BACE1 inhibitors in human patients, we aimed to study the physiological functions of BACE1 by characterizing BACE1-null mice. Here, we report the finding of spontaneous behavioral seizures in BACE1-null mice. Electroencephalographic recordings revealed abnormal spike-wave discharges in BACE1-null mice, and kainic acid-induced seizures also occurred more frequently in BACE1-null mice compared with their wild-type littermates. Biochemical and morphological studies showed that axonal and surface levels of Nav1.2 were significantly elevated in BACE1-null mice, consistent with the increased fast sodium channel current recorded from BACE1-null hippocampal neurons. Patch-clamp recording also showed altered intrinsic firing properties of isolated BACE1-null hippocampal neurons. Furthermore, population spikes were significantly increased in BACE1-null brain slices, indicating hyperexcitability of BACE1-null neurons. Together, our results suggest that increased sodium channel activity contributes to the epileptic behaviors observed in BACE1-null mice. The knowledge from this study is crucial for the development of BACE1 inhibitors for Alzheimers therapy and to the applicative study of epilepsy.

Cleavage of neuregulin-1 by BACE1 or ADAM10 protein produces differential effects on myelination.

Neuregulin-1 (Nrg1) is encoded by a single gene and exists in naturally secreted and transmembrane isoforms. Nrg1 exerts its signaling activity through interaction with its cognate ErbB receptors. Multiple membrane-anchored Nrg1 isoforms, present in six different membrane topologies, must be processed by a protease to initiate a signaling cascade. Here, we demonstrate that BACE1 and ADAM10 can process type I and III Nrg1 at two adjacent sites. Our cleavage site mapping experiments showed that the BACE1 cleavage site is located eight amino acids downstream of the ADAM10 cleavage site, and this order of cleavage is the opposite of amyloid precursor protein cleavage by these two enzymes. Cleavages were further confirmed via optimized electrophoresis. Cleavage of type I or III Nrg1 by ADAM10 and BACE1 released a signaling-capable N-terminal fragment (ntf), either Nrg1-ntfα or Nrg1-ntfβ, which could similarly activate an ErbB receptor as evidenced by increased phosphorylation of Akt and ERK, two downstream signaling molecules. Although both Nrg1-ntfα and Nrg1-ntfβ could initiate a common signaling cascade, inhibition or down-regulation of ADAM10 alone in a co-culture system did not affect normal myelination, whereas specific inhibition of BACE1 impaired normal myelination. Thus, processing of Nrg1 by BACE1 appears to be more critical for regulating myelination. Our results imply that a significant inhibition of BACE1 could potentially impair Nrg1 signaling activity in vivo.

Reduced Amyloid Deposition in Mice Overexpressing RTN3 Is Adversely Affected by Preformed Dystrophic Neurites

Reticulon 3 (RTN3) was initially identified as a negative modulator of BACE1, an enzyme that cleaves amyloid precursor protein (APP) to release β-amyloid peptide. Interestingly, RTN3 can also form aggregates after accumulation, and increased RTN3 aggregation correlates with the formation of RTN3 immunoreactive dystrophic neurites (RIDNs) in brains of Alzheimers cases. Transgenic mice expressing RTN3 alone develop RIDNs in their hippocampus but not in their cortex. To determine the in vivo effects of RTN3 and preformed RIDNs on amyloid deposition, we crossed bitransgenic mice expressing APP and presenilin 1 (PS1) mutations with mice overexpressing RTN3. We found that amyloid deposition in cortex, the hippocampal CA3 region, and dentate gyrus was significantly reduced in triple transgenic mice compared with bitransgenic controls. However, reduction of amyloid deposition in the hippocampal CA1 region, where RIDNs predominantly formed before amyloid deposition, was less significant. Hence, preformed RTN3 aggregates in RIDNs clearly offset the negative modulation of BACE1 activity by RTN3. Furthermore, our study indicates that the increased expression of RTN3 could result in an alteration of BACE1 intracellular trafficking by retaining more BACE1 in the endoplasmic reticulum compartment where cleavage of APP by BACE1 is less favored. Our results suggest that inhibition of RTN3 aggregation is likely to be beneficial by reducing both amyloid deposition and the formation RIDNs.

Reversible overexpression of bace1-cleaved neuregulin-1 N-terminal fragment induces schizophrenia-like phenotypes in mice.

BACKGROUND Neuregulin-1 (Nrg1) is a pleiotropic signaling molecule that regulates neural development, and mutation of Nrg1 is a risk factor for schizophrenia. Cleavage of type I β1 Nrg1 isoform by Bace1 releases a secreted N-terminal fragment (Nrg1-ntfβ), which can bind to a cognate ErbB receptor to activate the specific signaling cascade. This study aimed to determine whether increased expression of Nrg1 is beneficial for brain development and functions. METHODS We generated transgenic mice overexpressing this fragment under the control of a tetracycline-inducible promoter and examined functional and behavioral changes in mice upon reversible expression of the transgene. RESULTS Increased expression of full-length Nrg1 in mouse neurons has been previously shown to enhance myelination in the central nervous system. Overexpressing Nrg1-ntfβ enhanced the expression of myelin proteins, consistent with the expected activation of the Nrg1 signaling pathway by Nrg1-ntfβ. Contrary to expectations, overexpressing Nrg1-ntfβ transgene caused schizophrenia-like behaviors in transgenic mice, and these abnormal behaviors were reversible if the expression of the Nrg1-ntfβ transgene was turned off. Our molecular assay suggests that protein levels of N-methyl-D-aspartate receptors are reduced in this transgenic mouse model, which might underlie the observed social and cognitive behavioral impairments. CONCLUSIONS Our results indicate that overexpressing the secreted form of Nrg1 is sufficient to cause schizophrenia-like behaviors in a mouse model, meaning the effect is independent of the transmembrane and C-terminal domains of Nrg1. Hence, genetic gain-of-function mutations of Nrg1 are also risk factors for schizophrenia.

The Membrane Topology of RTN3 and Its Effect on Binding of RTN3 to BACE1

Reticulon 3 (RTN3) has recently been shown to modulate Alzheimer BACE1 activity and to play a role in the formation of dystrophic neurites present in Alzheimer brains. Despite the functional importance of this protein in Alzheimer disease pathogenesis, the functional correlation to the structural domain of RTN3 remained unclear. RTN3 has two long transmembrane domains, but its membrane topology was not known. We report here that the first transmembrane domain dictates membrane integration and its membrane topology. RTN3 adopts a ω-shape structure with two ends facing the cytosolic side. Subtle changes in RTN3 membrane topology can disrupt its binding to BACE1 and its inhibitory effects on BACE1 activity. Thus, the determination of RTN3 membrane topology may provide an important structural basis for our understanding of its cellular functions.

Increased Expression of Reticulon 3 in Neurons Leads to Reduced Axonal Transport of β Site Amyloid Precursor Protein-cleaving Enzyme 1

Background: Axonal transport of BACE1 and the regulation of BACE1 synaptic localization remain to be fully characterized. Results: Colocalization of BACE1 with synaptophysin was reduced by overexpression of RTN3. This reduction was due to reduced BACE1 axonal transport. Conclusion: Increased interaction of RTN3 with BACE1 in the soma impacts axonal transport of BACE1. Significance: Changes of BACE1 synaptic localization potentially alter synaptic Aβ generation and amyloid deposition. BACE1 is the sole enzyme responsible for cleaving amyloid precursor protein at the β-secretase site, and this cleavage initiates the generation of β-amyloid peptide (Aβ). Because amyloid precursor protein is predominantly expressed by neurons and deposition of Aβ aggregates in the human brain is highly correlated with the Aβ released at axonal terminals, we focused our investigation of BACE1 localization on the neuritic region. We show that BACE1 was not only enriched in the late Golgi, trans-Golgi network, and early endosomes but also in both axons and dendrites. BACE1 was colocalized with the presynaptic vesicle marker synaptophysin, indicating the presence of BACE1 in synapses. Because the excessive release of Aβ from synapses is attributable to an increase in amyloid deposition, we further explored whether the presence of BACE1 in synapses was regulated by reticulon 3 (RTN3), a protein identified previously as a negative regulator of BACE1. We found that RTN3 is not only localized in the endoplasmic reticulum but also in neuritic regions where no endoplasmic reticulum-shaping proteins are detected, implicating additional functions of RTN3 in neurons. Coexpression of RTN3 with BACE1 in cultured neurons was sufficient to reduce colocalization of BACE1 with synaptophysin. This reduction correlated with decreased anterograde transport of BACE1 in axons in response to overexpressed RTN3. Our results in this study suggest that altered RTN3 levels can impact the axonal transport of BACE1 and demonstrate that reducing axonal transport of BACE1 in axons is a viable strategy for decreasing BACE1 in axonal terminals and, perhaps, reducing amyloid deposition.