Presenilin 2 influences miR146 level and activity in microglia.

Identification of Alzheimer Disease (AD) causing mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) genes has provided valuable insights into the mechanisms involved in AD neurodegeneration. However, the pathophysiology of AD remains enigmatic despite decades of rigorous study. The clinical and neuropathological similarities between early onset familial and late onset AD suggest that APP, PSEN1, and PSEN2 may also play pivotal roles in sporadic AD. Newly identified variants in these genes are associated with late onset familial AD cases (Cruchaga et al. 2012), thus supporting the possibility that altered function of APP, PSEN1, or PSEN2 contributes to AD pathogenesis in both familial and sporadic forms. We have recently described an early onset AD patient with a novel PSEN2 mutation predicted to lead to a premature termination codon causing either haploinsufficiency or a dramatically truncated protein (Jayadev et al. 2010b). Our previous work has demonstrated that the deficiency of presenilin 2 (PS2) protein function is associated with an exaggerated pro-inflammatory state in microglia (Jayadev et al. 2010a). Therefore, we propose that loss of PS2 function through mutation or cumulative effects of aging, may contribute to the neurotoxic inflammatory milieu of AD. Neuroinflammation is a common pathological feature of neurodegenerative disease and a core characteristic of AD. Numerous epidemiological, mechanistic and discovery driven studies strongly suggest a functional role for neuroinflammation in promoting or exacerbating neurodegeneration (McGeer et al. 1996; Hensley 2010). During neuroinflammation, microglia execute functions with both neurotoxic and neuroprotective consequences in the CNS (Ransohoff and Cardona 2010; Aguzzi et al. 2013). For instance, unchecked anti-microbial cytokine release may lead to a CNS environment as inhospitable to neurons as it is to invading pathogens, potentially contributing to neurodegeneration in the setting of AD associated chronic inflammation. By understanding the molecular mechanisms behind the regulation of microglial inflammatory pathways, we may identify more specific targets for neuroimmunomodulatory interventions to ameliorate the resultant neurodegeneration. In vivo murine models first suggested a position for presenilin proteins at the functional intersection between CNS inflammation and neurodegeneration. Presenilins are the catalytic subunit of the multi-protein γ-secretase complex, which cleaves type 1 membrane proteins involved in a panoply of regulatory pathways including apoptosis, cell differentiation, mitochondrial integrity, calcium regulation and inflammation (Haapasalo and Kovacs 2011; Ho and Shen 2011). PS2 knockout mice in which PS1 is deleted in adult forebrain neurons show progressive neurodegeneration, cognitive deficits and marked neuroinflammation. Similar findings were not observed in wild-type mice with a similar neuronal PSEN1 conditional deletion (Beglopoulos et al. 2004; Shen and Kelleher 2007). It seems possible therefore, that PS2 dysfunction has impacts on the developed CNS and can promote neuroinflammation. However, the mechanism by which PS2 influences microglia inflammatory behavior has not been determined. MicroRNAs (miRNAs) are a class of small non-coding 22 nucleotide RNAs that regulate gene expression through post-transcriptional regulation. MiRNAs bind the 3′untranslated region of target mRNAs to promote mRNA degradation or interfere with translation (Bartel 2004). Recent reports demonstrate that miRNAs are key regulators of the intensity of the innate immune response (O’Connell et al. 2012). Experimental data have demonstrated a role for several specific miRNAs, for example, miR155, miR146a/b, and miR132, in regulating the expression of key innate immunity signaling proteins (O’Neill et al. 2011). MiR-146a is a potent negative regulator of innate immunity and responsive to inflammatory cytokines and viral infection (Taganov et al. 2006; Hou et al. 2009; Zhao et al. 2011). It acts as a pivotal molecule in the negative feedback regulation of the powerful pro-inflammatory pathway mediated by nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB), a transcription factor regulating inflammation, immunity and cell survival. NFκB activation induces transcription of pro-inflammatory cytokines and is thus a critical factor in downstream innate immunity signaling (Newton and Dixit 2012). As a fast-acting inflammatory signal NFκB is subject to complex regulation and miRNAs are a significant component to the ‘fine-tuning’ of NFκB activity (Kondo et al. 2012). By suppressing expression of proteins that promote NFκB activity such as interleukin-1 receptor-associated kinase-1 (IRAK-1), IRAK-2 and tumor necrosis factor receptor-associated factor-6, miR146a attenuates proinflammatory responses, functioning as a brake in the potentially harmful proinflammatory response. Our previous work had suggested that PS2 may play a role in the control of the microglial response to classical stimulation, and therefore we set out to identify important molecular factors in that putative regulation. We found that PS2 appears to influence levels of the negative regulator of innate immunity, miR146 with concomitant effects on protein levels of the miR146 target, IRAK-1. In addition to increased IRAK-1, PS2KO microglia demonstrate elevated NFκB activity. Taken together our findings support our hypothesis that PS2 is an important modulator of neuroinflammation and that dysfunction of PS2 may contribute to the aggravated inflammatory reaction in neurodegeneration.