Ataxia-telangiectasia (A-T) is a neurodegenerative disease caused by mutation of the Atm gene. Here we report that ATM-deficiency causes nuclear accumulation of histone deacetylase 4 (HDAC4) in neurons and promotes neurodegeneration. Nuclear HDAC4 binds to chromatin as well as to MEF2A and CREB, leading to histone de-acetylation and altered neuronal gene expression. Blocking either HDAC4 activity or its nuclear accumulation blunts the neurodegenerative changes and rescues several behavioral abnormalities of Atm mutants. Full rescue, however, also requires HDAC4 in the cytoplasm, suggesting that the A-T phenotype results both from a loss of cytoplasmic HDAC4 and its nuclear accumulation. To remain cytoplasmic, HDAC4 must be phosphorylated. The HDAC4 phosphatase, PP2A, is down regulated by ATM-mediated phosphorylation. In ATM deficiency, enhanced PP2A activity leads to HDAC4 dephosphorylation and nuclear accumulation. Our results define a crucial role of nuclear accumulation and cytoplasmic depletion of HDAC4 in the events leading to A-T neurodegeneration.

The symptoms of ataxia-telangiectasia (A-T) include a progressive neurodegeneration caused by ATM protein deficiency. We previously found that nuclear accumulation of histone deacetylase-4, HDAC4, contributes to this degeneration; we now report that increased histone H3K27 trimethylation (H3K27me3) mediated by polycomb repressive complex 2 (PRC2) also plays an important role in the A-T phenotype. Enhancer of zeste homolog 2 (EZH2), a core catalytic component of PRC2, is a new ATM kinase target, and ATM-mediated S734 phosphorylation of EZH2 reduces protein stability. Thus, PRC2 formation is elevated along with H3K27me3in ATM deficiency. ChIP-sequencing shows a significant increase in H3K27me3 ‘marks’ and a dramatic shift in their location. The change of H3K27me3 chromatin-binding pattern is directly related to cell cycle re-entry and cell death of ATM-deficient neurons. Lentiviral knockdown of EZH2 rescues Purkinje cell degeneration and behavioral abnormalities in Atm−/− mice, demonstrating that EZH2 hyperactivity is another key factor in A-T neurodegeneration.

Amyloid-␤ (A␤) peptides play a key role in synaptic damage and memory deficits in the early pathogenesis of Alzheimer's disease (AD).Abnormal accumulation of A␤ at nerve terminals leads to synaptic pathology and ultimately to neurodegeneration. ␤-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the major neuronal ␤-secretase for A␤ generation. However, the mechanisms regulating BACE1 distribution in axons and ␤ cleavage of APP at synapses remain largely unknown. Here, we reveal that dynein-Snapin-mediated retrograde transport regulates BACE1 trafficking in axons and APP processing at presynaptic terminals. BACE1 is predominantly accumulated within late endosomes at the synapses of AD-related mutant human APP (hAPP) transgenic (Tg) mice and patient brains. Defective retrograde transport by genetic ablation of snapin in mice recapitulates late endocytic retention of BACE1 and increased APP processing at presynaptic sites. Conversely, overexpressing Snapin facilitates BACE1 trafficking and reduces synaptic BACE1 accumulation by enhancing the removal of BACE1 from distal AD axons and presynaptic terminals. Moreover, elevated Snapin expression via stereotactic hippocampal injections of adeno-associated virus particles in mutant hAPP Tg mouse brains decreases synaptic A␤ levels and ameliorates synapse loss, thus rescuing cognitive impairments associated with hAPP mice. Altogether, our study provides new mechanistic insights into the complex regulation of BACE1 trafficking and presynaptic localization through Snapin-mediated dynein-driven retrograde axonal transport, thereby suggesting a potential approach of modulating A␤ levels and attenuating synaptic deficits in AD.

Staphylococcal enterotoxin A (SEA) is a microbial superantigen that activates T-lymphocytes and induces production of various cytokines, including tumor necrosis factor-␣ (TNF␣). Previously, it was shown that SEA activates the hypothalamic-pituitary-adrenal axis and augments gustatory neophobic behaviors. In the present study, it was hypothesized that these effects involve neuronal activation in forebrain regions mediating fear and/or anxiety and are dependent on the production of TNF␣. Male C57BL/6J mice were given intraperitoneal injections of 10 g of SEA and 5 g of lipopolysaccharide (LPS) or saline and perfused 2 h later for histochemical determination of brain c-Fos immunoreactivity (IR). The results showed increased c-Fos IR in the paraventricular nucleus, arcuate nucleus, central nucleus of the amygdala, bed nucleus of the stria terminalis, and lateral septum. Challenge of TNF Ϫ/Ϫ mice with SEA did not produce a significant increase in brain c-Fos IR, although c-Fos was increased after exposure to a psychogenic stressor (i.e., open field). In additional experiments, the elevated corticosterone response to SEA was abrogated in TNF Ϫ/Ϫ mice and was shown to be corticotropin-releasing hormone dependent. Finally, the augmented reduction in novel food intake after SEA challenge was attenuated in TNF Ϫ/Ϫ mice as well as in wild-type mice administered antibody to TNF␣. In conclusion, challenge with SEA recruits brain regions mediating stress and anxiety responses, an effect that requires endogenous TNF␣. Whether this is indicative of all T-cell superantigens remains to be determined, although it stands in contrast to other models of neuroimmunomodulation (e.g., LPS) that involve multiple cytokine influences.

Stimulation of T-cells with staphylococcal enterotoxin B (SEB) significantly elevates interleukin-2 (IL-2) and contemporaneous activation of the hypothalamic-pituitary-adrenal (HPA) axis and c-fos in the paraventricular nucleus (PVN) of BALB/cByJ mice. Such neural signaling may promote cognitive and emotional adaptation before or during infectious illness. Because corticotropin-releasing hormone (CRH) is an anxiogenic neuropeptide that may mediate the stressor-like effects of immunological stimuli, we measured neuronal CRH mRNA alterations in mice challenged with SEB. Increased CRH mRNA levels were observed in the PVN and central nucleus of the amygdala (ceA) 4-6 hr after SEB administration. This was associated with plasma ACTH increases, which could be abrogated by the systemic administration of anti-CRH antiserum. Additional experiments did not support a role for IL-2 or prostaglandin synthesis in activating the HPA axis. Behavioral experiments testing for conditioned taste aversion did not confirm that SEB challenge promotes malaise. However, consistent with the notion that central CRH alterations induced by SEB may affect emotionality (e.g., fear), SEB challenge augmented appetitive neophobia in a context-dependent manner, being marked in a novel and stressful environment. It is hypothesized that immunological stimuli generate a cascade of events that solicit integrative neural processes involved in emotional behavior. As such, these data support the contention that affective illness may be influenced by immunological processes and the production of cytokines and are consistent with other evidence demonstrating that autoimmune reactivity is associated with enhanced emotionality.