Variants of TREM2 are associated with Alzheimer's disease (AD). To study whether increasing TREM2 gene dosage could modify the disease pathogenesis, we developed BAC transgenic mice expressing human TREM2 (BAC-TREM2) in microglia. We found that elevated TREM2 expression reduced amyloid burden in the 5xFAD mouse model. Transcriptomic profiling demonstrated that increasing TREM2 levels conferred a rescuing effect, which includes dampening the expression of multiple disease-associated microglial genes and augmenting downregulated neuronal genes. Interestingly, 5xFAD/BAC-TREM2 mice showed further upregulation of several reactive microglial genes linked to phagocytosis and negative regulation of immune cell activation. Moreover, these mice showed enhanced process ramification and phagocytic marker expression in plaque-associated microglia and reduced neuritic dystrophy. Finally, elevated TREM2 gene dosage led to improved memory performance in AD models. In summary, our study shows that a genomic transgene-driven increase in TREM2 expression reprograms microglia responsivity and ameliorates neuropathological and behavioral deficits in AD mouse models.

Alterations in GABA A receptor (GABA A R) expression and function, similar to those we described previously during pregnancy in the mouse dentate gyrus, may also occur in other brain regions. Here we show, using immunohistochemical techniques, a decreased ␦ subunit-containing GABA A R (␦GABA A R) expression in the dentate gyrus, hippocampal CA1 region, thalamus, and striatum but not in the cerebral cortex. In the face of the highly elevated neurosteroid levels during pregnancy, which can act on ␦GABA A Rs, it may be beneficial to decrease the number of neurosteroid-sensitive receptors to maintain a steady-state level of neuronal excitability throughout pregnancy. Consistent with this hypothesis, the synaptic input/output (I/O) relationship in the dentate gyrus molecular layer in response to lateral perforant path stimulation was shifted to the left in hippocampal slices from pregnant compared with virgin mice. The addition of allopregnanolone, at levels comparable with those found during pregnancy (100 nM), shifted the I/O curves in pregnant mice back to virgin levels. There was a decreased threshold to induce epileptiform local field potentials in slices from pregnant mice compared with virgin, but allopregnanolone reverted the threshold for inducing epileptiform activity to virgin levels. According to these data, neuronal excitability is increased in pregnant mice in the absence of allopregnanolone attributable to brain region-specific downregulation of ␦GABA A R expression. In brain regions, such as the cortex, that do not exhibit alterations in ␦GABA A R expression, there were no changes in the I/O relationship during pregnancy. Similarly, no changes in network excitability were detected in pregnant Gabrd Ϫ/Ϫ mice that lack ␦GABA A Rs, suggesting that changes in neuronal excitability during pregnancy are attributable to alterations in the expression of these receptors. Our findings indicate that alterations in ␦GABA A R expression during pregnancy result in brain region-specific increases in neuronal excitability that are restored by the high levels of allopregnanolone under normal conditions but under pathological conditions may result in neurological and psychiatric disorders associated with pregnancy and postpartum.

Brain organoids represent a powerful tool for studying human neurological diseases, particularly those impacting brain growth and structure. However, many diseases manifest with clear evidence of physiological and network abnormality in the absence of anatomical changes, raising the question of whether organoids possess sufficient neural network complexity to model these conditions. Here, we explore the network level functions of brain organoids using calcium sensor imaging and extracellular recording approaches that together reveal the existence of complex network dynamics reminiscent of intact brain preparations. We demonstrate highly abnormal and epileptiform-like activity in organoids derived from Rett syndrome patient induced pluripotent stem cells accompanied by transcriptomic differences revealed by single-cell analyses. We also rescue key physiological activities with an unconventional neuroregulatory drug, Pifithrin-a.Together, these findings provide an essential foundation for the utilization of brain organoids to study intact and disordered human brain network formation and illustrate their utility in therapeutic discovery.

Gamma (γ) oscillations (30–120 Hz), an emergent property of neuronal networks, correlate with memory, cognition and encoding. In the hippocampal CA3 region, locally generated γ oscillations emerge through feedback between inhibitory parvalbumin-positive basket cells (PV+BCs) and the principal (pyramidal) cells. PV+BCs express δ-subunit-containing GABAARs (δ-GABAARs) and NMDA receptors (NMDA-Rs) that balance the frequency of γ oscillations. Neuroactive steroids (NS), such as the progesterone-derived (3α,5α)-3-hydroxy-pregnan-20-one (allopregnanolone; ALLO), modulate the expression of δ-GABAARs and the tonic conductance they mediate. Pregnancy produces large increases in ALLO and brain-region-specific homeostatic changes in δ-GABAARs expression. Here we show that in CA3, where most PV+ interneurons (INs) express δ-GABAARs, expression of δ-GABAARs on INs diminishes during pregnancy, but reverts to control levels within 48 h postpartum. These anatomical findings were corroborated by a pregnancy-related increase in the frequency of kainate-induced CA3 γ oscillations in vitro that could be countered by the NMDA-R antagonists D-AP5 and PPDA. Mimicking the typical hormonal conditions during pregnancy by supplementing 100 nM ALLO lowered the γ frequencies to levels found in virgin or postpartum mice. Our findings show that states of altered NS levels (e.g., pregnancy) may provoke perturbations in γ oscillatory activity through direct effects on the GABAergic system, and underscore the importance of δ-GABAARs homeostatic plasticity in maintaining constant network output despite large hormonal changes. Inaccurate coupling of NS levels to δ-GABAAR expression may facilitate abnormal neurological and psychiatric conditions such as epilepsy, post-partum depression, and post-partum psychosis, thus providing insights into potential new treatments.

Perisynaptic and extrasynaptic δ subunit-containing GABAA receptors (δ-GABAARs) mediate tonic conductances in many principal cells and interneurons (INs) of the central nervous system. δ-GABAARs comprise α4 subunits on principal cells of the neocortex and hippocampus, whereas they usually contain α1 on various INs. δ-GABAARs are highly plastic and exhibit a unique pharmacology, being insensitive to benzodiazepines, but sensitive to low concentrations of neurosteroids (NS). The neuroactive forms of several steroid hormones, NS act on δ-GABAARs as potent positive allosteric modulators leading to changes in neuronal excitability and network synchrony. In fact, δ-GABAARs on INs have been shown to control the dynamics of γ oscillations. During times of altered NS production, including stress, puberty, ovarian cycle and pregnancy, δ-GABAAR expression varies in different neurons regardless of the α subunits they contain. This plasticity has direct consequences for network functioning. In particular, plasticity of these receptors in pregnancy and the postpartum period affects CA3 γ oscillation frequencies (recorded in vitro), and during the ovarian cycle they underlie fluctuations in the amplitude of CA1 γ oscillations (recorded in vivo). Most δ-GABAAR-expressing INs in CA3 stratum pyramidale (SP) are parvalbumin (PV)+INs, whose fundamental role in γ oscillation generation and control has been extensively investigated. In this study we reduced or deleted δ-subunits in PV+INs, with the use of a PV/Cre-Gabrd/floxed genetic system. By means of immunohistochemistry we show that PV expression in PV-Gabrd−/− and PV-Gabrd+/− mice remains unaltered. Additionally, we confirm the absence of δ-GABAARs specifically from PV+INs. We find that in both PV-Gabrd−/− and PV-Gabrd+/− mice, CA3 γ oscillation frequencies measured in vitro were increased compared to Cre−/− controls. The increased frequencies could be lowered to control levels in PV-Gabrd+/− by the NS allopregnanolone (3α,5α-tetrahydroprogesterone, ALLO, 100 nM) but not by the synthetic δ-GABAAR positive allosteric modulator 4-Chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl] benzamide (DS-2, 10 µM). This is consistent with the idea that DS-2, in contrast to ALLO, selectively targets α4/δ-GABAARs but not the α1/δ-GABAARs found on INs. Therefore, development of drugs selective for IN-specific α1/δ-GABAARs may be useful in neurological and psychiatric conditions correlated with altered PV+IN function and aberrant γ oscillations.