Early death in a mouse model of Alzheimer’s disease exacerbated by microglial loss of TAM receptor signaling

Recurrent seizure is a common comorbidity in early-stage Alzheimer’s disease (AD) and may contribute to AD pathogenesis and cognitive decline. Similarly, many mouse models of Alzheimer’s disease that overproduce amyloid beta are prone to epileptiform seizures that may result in early sudden death. We studied one such model, designated APP/PS1, and found that mutation of the TAM receptor tyrosine kinase (RTK) Mer or its ligand Gas6 greatly exacerbated early death. Lethality was tied to violent seizures that appeared to initiate in the dentate gyrus (DG) of the hippocampus, where Mer plays an essential role in the microglial phagocytosis of both apoptotic and newborn cells normally generated during adult neurogenesis. We found that newborn DG neurons and excitatory synapses between the DG and the cornu ammonis field 3 (CA3) field of the hippocampus were increased in TAM-deficient mice, and that premature death and adult neurogenesis in these mice were coincident. In contrast, the incidence of lethal seizures and the deposition of dense-core amyloid plaques were strongly anticorrelated. Together, these results argue that TAM-mediated phagocytosis sculpts synaptic connectivity in the hippocampus, and that seizure-inducing amyloid beta polymers are present prior to the formation of dense-core plaques.

Alzheimer’s disease (AD), the most prevalent neurogenerative disease and most common cause of human dementia, is associated with seizures. Unprovoked seizures are substantially more frequent in sporadic AD patients compared to age-matched nondemented populations (1–6) and are exceptionally prominent in early stages of early-onset familial AD (4, 7, 8). In addition, subclinical epileptiform activity has been detected by long-term electroencephalography (EEG) in more than 40% of AD patients, where it is correlated with accelerated cognitive decline (9). This abnormal network activity often begins decades before the symptomatic onset of AD (10, 11). Correspondingly, many amyloidogenic mouse models of AD that rely on the elevated production of amyloid beta (Aβ) peptides—as a result of transgenic expression of mutant forms of the human amyloid precursor protein (APP) and/or the presenilin 1 (PSEN1) component of the APP gamma secretase—also display cortical and hippocampal hyperexcitability and experience unprovoked seizures early in disease before the appearance of Aβ plaques (12–19).We have recently published analyses of one such mouse—the APP/PS1 line that carries an amyloidogenic “Swedish” mutation in the human APP gene and a pathogenic mutation in the human PSEN1 gene (20–22). Although not routinely addressed in the literature, it is well documented that APP/PS1 populations on a C57BL/6 background exhibit 10 to 15% mortality due to sudden death early in life (12, 13). This sudden death, which is not associated with any gross postmortem neuropathology, has been found to be the result of lethal epileptic seizures (12). These seizures are in turn linked to markedly higher levels of interictal spiking of cortical pyramidal neurons, as detected by EEG, patch clamp recording in cortical slices, and implanted electrodes in vivo (12).In our previous studies (22), we examined the effects of germline loss-of-function mutations in the mouse genes encoding the TAM receptor tyrosine kinases (RTKs) Mer and Axl (23, 24), and their ligand Gas6 (25), when crossed into APP/PS1. In the current analysis, we found that mutation of Mer (gene name Mertk), Gas6, or Axl and Mer together all led to a marked exacerbation of early sudden death in both sexes. Importantly, this was also seen for APP/PS1 compound mutants that carried microglial-restricted mutations in Mertk. As for APP/PS1 alone, nearly all sudden death in the compound mutants occurred before postnatal day (P)150, after which time all populations were stable. Video recording in our colonies demonstrated that the compound mutants died from the same spontaneous epileptic seizures that have been shown to kill young APP/PS1 mice. Analyses of the activation marker Arc immediately following lethal seizures were consistent with these seizures being initiated in the dentate gyrus (DG) of the hippocampus.Together with the subventricular zone (SVZ) adjacent to the lateral ventricle, the DG harbors a prominent site of adult neurogenesis in the mouse (26, 27). At both sites, microglial Mer has been shown to play an essential role in the phagocytic clearance of the large number of apoptotic cells (ACs) that are generated during this process (28, 29). Importantly, the bulk of adult neurogenesis and AC generation in the mouse occurs over a delimited developmental window between birth and P150 (26), the same window in which APP/PS1 mice die from seizures. During this time, Mer-expressing microglia also phagocytose live newborn cells that express the “eat-me” signal phosphatidylserine (PtdSer) (29, 30). In the SVZ, TAM mutations have been shown to result in the survival of many of these newborn cells, which would otherwise have been “eaten alive,” and in their integration as viable differentiated neurons in the olfactory bulb (29). Consistent with these earlier findings, we observed that APP/PS1 mice lacking both Axl and Mer displayed 6-fold more ACs and 2.1-fold more viable 5-bromo-2-deoxyuridine (BrdU)-labeled cells in the DG at 4 wk after a BrdU pulse than did APP/PS1 mice alone. Moreover, these TAM-deficient APP/PS1 mice also displayed a 1.4-fold increase, relative to their APP/PS1 counterparts, in excitatory connections between DG granule cell neurons and their synaptic targets in the cornu ammonis field 3 (CA3) of the hippocampus. Taken together, these results indicate that the loss of TAM signaling exacerbates the preexisting hyperexcitability of APP/PS1 mice via increased excitatory connections between the DG and CA3, and that TAM-mediated microglial phagocytosis is normally required for the proper sculpting of these hippocampal circuits during adult neurogenesis.