Amyloid ␤ (A␤) damages neurons and triggers microglial inflammatory activation in the Alzheimer disease (AD) brain. BACE1 is the primary enzyme in A␤ generation. Neuroinflammation potentially up-regulates BACE1 expression and increases A␤ production. In Alzheimer amyloid precursor protein-transgenic mice and SH-SY5Y cell models, we specifically knocked out or knocked down gene expression of mapk14, which encodes p38␣ MAPK, a kinase sensitive to inflammatory and oxidative stimuli. Using immunological and biochemical methods, we observed that reduction of p38␣ MAPK expression facilitated the lysosomal degradation of BACE1, decreased BACE1 protein and activity, and subsequently attenuated A␤ generation in the AD mouse brain. Inhibition of p38␣ MAPK also enhanced autophagy. Blocking autophagy by treating cells with 3-methyladenine or overexpressing dominant-negative ATG5 abolished the deficiency of the p38␣ MAPK-induced BACE1 protein reduction in cultured cells. Thus, our study demonstrates that p38␣ MAPK plays a critical role in the regulation of BACE1 degradation and A␤ generation in AD pathogenesis. Alzheimer disease (AD)2 is pathologically characterized by the extracellular deposits of amyloid ␤ peptide (A␤). A␤ injures neurons in the neocortex and limbic system directly (1) and indirectly by triggering microglial release of various neurotoxic inflammatory mediators, including cytokines (tumor necrosis factor-␣ and interleukin-1␤ (IL-1␤)) and reactive oxygen species (2). A␤ is generated after serial digestion of Alzheimer amyloid precursor protein (APP) by the membrane-anchored ␤-site APP-cleaving enzyme (BACE1, ␤-secretase) and ␥-secretase (3). It has been observed that knock-out of BACE1 or administration of the BACE1 inhibitor dramatically decreases A␤ levels in the brain and attenuates behavioral and electrophysiological deficits in APP-transgenic mice (4 -6). Thus, extensive investigations have focused on the direct inhibition of BACE1 to reduce A␤ load in the AD brain; however, these studies have unfortunately not yet led to any efficacious therapy for AD patients due to the various physiological roles of BACE1 (7). Using alternative methods to inhibit BACE1 might be a preferable investigative approach.Inflammatory activation might lead to up-regulation of neuronal BACE1 expression in the AD brain, as NF-B signaling enhances (8), and PPAR␥ activation suppresses (9), the activity of bace1 gene promoter. Accumulating evidence has shown that posttranslational modification of BACE1 is extremely important for the activity, intracellular trafficking, and lysosomal degradation of BACE1. For example, phosphorylation of BACE1 at Thr-252 by p25/Cdk5 increases the secretase activity (10), and phosphorylation at Ser-498 facilitates retrograde transport of BACE1 from endosomes to the trans-Golgi network (11). Ubiquitination at Lys-501 targets BACE1 to late endosomes/lysosomes for degradation (12). Finally, bisecting N-acetylglucosamine modification blocks delivery of BACE1 to lysosomes (13).p38 mitogen-activated protei...

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In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), peripherally developed myelin-reactive T lymphocytes stimulate myeloid cells (ie, microglia and infiltrated macrophages) to trigger an inflammatory reaction in the central nervous system, resulting in demyelination and neurodegeneration. IκB kinase β (IKKβ) is a kinase that modulates transcription of inflammatory genes. To investigate the pathogenic role of IKKβ in MS, we developed strains in which IKKβ was conditionally ablated in myeloid cells and established active or passive EAE in these animals. Deficiency of IKKβ in myeloid cells ameliorated EAE symptoms and suppressed neuroinflammation, as shown by decreased infiltration of T lymphocytes and macrophages and reduced inflammatory gene transcription in the spinal cord at the peak or end stage of EAE. Myeloid deficiency of IKKβ also reduced the transcription of Rorc or Il17 genes in T lymphocytes isolated from lymph nodes, spleen, and spinal cord of EAE mice. Moreover, cultured splenocytes isolated from myeloid IKKβ-deficient EAE mice released less IL-17, interferon-γ, and granulocyte-macrophage colony-stimulating factor after treatment with myelin peptide than splenocytes from IKKβ wild-type EAE mice. Thus, deficiency of myeloid IKKβ attenuates the severity of EAE by inhibiting both the neuroinflammatory activity and the activation of encephalitogenic T lymphocytes. These results suggest IKKβ may be a potential target for MS patients, especially when neuroinflammation is the primary problem.

Alzheimer's disease (AD), the most common cause of dementia in the elderly, is pathologically characterized by extracellular deposition of amyloid‐β peptides (Aβ) and microglia‐dominated inflammatory activation in the brain. p38α‐MAPK is activated in both neurons and microglia. How p38α‐MAPK in microglia contributes to AD pathogenesis remains unclear. In this study, we conditionally knocked out p38α‐MAPK in all myeloid cells or specifically in microglia of APP‐transgenic mice, and examined animals for AD‐associated pathologies (i.e., cognitive deficits, Aβ pathology, and neuroinflammation) and individual microglia for their inflammatory activation and Aβ internalization at different disease stages (e.g., at 4 and 9 months of age). Our experiments showed that p38α‐MAPK‐deficient myeloid cells were more effective than p38α‐MAPK‐deficient microglia in reducing cerebral Aβ and neuronal impairment in APP‐transgenic mice. Deficiency of p38α‐MAPK in myeloid cells inhibited inflammatory activation of individual microglia at 4 months but enhanced it at 9 months. Inflammatory activation promoted microglial internalization of Aβ. Interestingly, p38α‐MAPK‐deficient myeloid cells reduced IL‐17a‐expressing CD4‐positive lymphocytes in 9 but not 4‐month‐old APP‐transgenic mice. By cross‐breeding APP‐transgenic mice with Il‐17a‐knockout mice, we observed that IL‐17a deficiency potentially activated microglia and reduced Aβ deposition in the brain as shown in 9‐month‐old myeloid p38α‐MAPK‐deficient AD mice. Thus, p38α‐MAPK deficiency in all myeloid cells, but not only in microglia, prevents AD progression. IL‐17a‐expressing lymphocytes may partially mediate the pathogenic role of p38α‐MAPK in peripheral myeloid cells. Our study supports p38α‐MAPK as a therapeutic target for AD patients.

Amyloid β peptide (Aβ) is the major pathogenic molecule in Alzheimer's disease (AD). BACE1 enzyme is essential for the generation of Aβ. Deficiency of p38α‐MAPK in neurons increases lysosomal degradation of BACE1 and decreases Aβ deposition in the brain of APP‐transgenic mice. However, the mechanisms mediating effects of p38α‐MAPK are largely unknown. In this study, we used APP‐transgenic mice and cultured neurons and observed that deletion of p38α‐MAPK specifically in neurons decreased phosphorylation of Snapin at serine, increased retrograde transportation of BACE1 in axons and reduced BACE1 at synaptic terminals, which suggests that p38α‐MAPK deficiency promotes axonal transportation of BACE1 from its predominant locations, axonal terminals, to lysosomes in the cell body. In vitro kinase assay revealed that p38α‐MAPK directly phosphorylates Snapin. By further performing mass spectrometry analysis and site‐directed mutagenic experiments in SH‐SY5Y cell lines, we identified serine residue 112 as a p38α‐MAPK‐phosphorylating site on Snapin. Replacement of serine 112 with alanine did abolish p38α‐MAPK knockdown‐induced reduction of BACE1 activity and protein level, and transportation to lysosomes in SH‐SY5Y cells. Taken together, our study suggests that activation of p38α‐MAPK phosphorylates Snapin and inhibits the retrograde transportation of BACE1 in axons, which might exaggerate amyloid pathology in AD brain.

Microglial activation is a hall marker of Alzheimer disease (AD); its pathogenic role and regulating mechanisms are unclear. In APP-transgenic mice, we deleted p38α-MAPK in the myeloid cell lineage from birth or specifically in microglia from 9 months, and analysed the AD pathology at the age of 4, 9 and 12 months. In both experimental settings, p38α-MAPK deficiency decreased cerebral Aβ and improved cognitive function in AD mice; however, p38α-MAPK deficiency in whole myeloid cells was more effective than specifically in microglia in preventing AD pathogenesis. Deficiency of p38α-MAPK in whole myeloid cells inhibited the inflammatory activation of individual microglia by 4 months, but enhanced it by 9 months. Inflammatory activation was essential for p38α-MAPK deficiency to promote microglial internalization of Aβ in the brain. In the investigation of mechanisms mediating different effects of p38α-MAPK-deficient myeloid cells and p38α-MAPK-deficient microglia on the pathogenesis of AD mice, we observed that p38α-MAPK deficiency in peripheral myeloid cells reduced il-17a transcription in CD4-positive spleen cells. By cross-breeding APP-transgenic mice and IL-17a knockout mice, we further found that IL-17a deficiency activated microglia and decreased Aβ deposits in AD mouse brain. In summary, our study shows that p38α-MAPK deficiency in myeloid cells attenuates symptoms and pathology of APP-transgenic mice. As a potential mechanism, p38α-MAPK-deficient peripheral myeloid cells reduces IL-17a-expressing T lymphocytes, and subsequently regulates cerebral Aβ clearance in APP-transgenic mice. Together with our previous observations that a deficiency of p38α-MAPK in neurons prevents AD pathogenesis, our study supports p38α-MAPK as a novel target for AD therapy.

In Alzheimer's disease (AD) brain, inflammatory activation regulates protein levels of amyloid‐β‐peptide (Aβ) and phosphorylated tau (p‐tau), as well as neurodegeneration; however, the regulatory mechanisms remain unclear. We constructed APP‐ and tau‐transgenic AD mice with deletion of IKKβ specifically in neurons, and observed that IKKβ deficiency reduced cerebral Aβ and p‐tau, and modified inflammatory activation in both AD mice. However, neuronal deficiency of IKKβ decreased apoptosis and maintained synaptic proteins (e.g., PSD‐95 and Munc18‐1) in the brain and improved cognitive function only in APP‐transgenic mice, but not in tau‐transgenic mice. Additionally, IKKβ deficiency decreased BACE1 protein and activity in APP‐transgenic mouse brain and cultured SH‐SY5Y cells. IKKβ deficiency increased expression of PP2A catalytic subunit isoform A, an enzyme dephosphorylating cerebral p‐tau, in the brain of tau‐transgenic mice. Interestingly, deficiency of IKKβ in neurons enhanced autophagy as indicated by the increased ratio of LC3B‐II/I in brains of both APP‐ and tau‐transgenic mice. Thus, IKKβ deficiency in neurons ameliorates AD‐associated pathology in APP‐ and tau‐transgenic mice, perhaps by decreasing Aβ production, increasing p‐tau dephosphorylation, and promoting autophagy‐mediated degradation of BACE1 and p‐tau aggregates in the brain. However, IKKβ deficiency differently protects neurons in APP‐ and tau‐transgenic mice. Further studies are needed, particularly in the context of interaction between Aβ and p‐tau, before IKKβ/NF‐κB can be targeted for AD therapies.

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