G protein-coupled estrogen receptor (GPER) in the dorsal hippocampus regulates memory consolidation in gonadectomized male mice, likely via different signaling mechanisms than in female mice.

Studies in ovariectomized (OVX) female rodents suggest that G protein-coupled estrogen receptor (GPER) is a key regulator of memory, yet little is known about its importance to memory in males or the cellular mechanisms underlying its mnemonic effects in either sex. In OVX mice, bilateral infusion of the GPER agonist G-1 into the dorsal hippocampus (DH) enhances object recognition and spatial memory consolidation in a manner dependent on rapid activation of c-Jun N-terminal kinase (JNK) signaling, cofilin phosphorylation, and actin polymerization in the DH. However, the effects of GPER on memory consolidation and DH cell signaling in males are unknown. Thus, the present study first assessed effects of DH infusion of G-1 or the GPER antagonist G-15 on object recognition and spatial memory consolidation in gonadectomized (GDX) male mice. As in OVX mice, immediate post-training bilateral DH infusion of G-1 enhanced, whereas G-15 impaired, memory consolidation in the object recognition and object placement tasks. However, G-1 did not increase levels of phosphorylated JNK (p46, p54) or cofilin in the DH 5, 15, or 30 min after infusion, nor did it affect phosphorylation of ERK (p42, p44), PI3K, or Akt. Levels of phospho-cAMP-responsive element binding protein (CREB) were elevated in the DH 30 min following G-1 infusion, indicating that GPER in males activates a yet unknown signaling mechanism that triggers CREB-mediated gene transcription. Our findings show for the first time that GPER in the DH regulates memory consolidation in males and suggests sex differences in underlying signaling mechanisms.

Sex differences in training-induced activity of the ubiquitin proteasome system in the dorsal hippocampus and medial prefrontal cortex of male and female mice.

The ubiquitin proteasome system (UPS) is a primary mechanism through which proteins are degraded in cells. UPS activity in the dorsal hippocampus (DH) is necessary for multiple types of memory, including object memory, in male rodents. However, sex differences in DH UPS activation after fear conditioning suggest that other forms of learning may also differentially regulate DH UPS activity in males and females. Here, we examined markers of UPS activity in the synaptic and cytoplasmic fractions of DH and medial prefrontal cortex (mPFC) tissue collected 1 h following object training. In males, training increased phosphorylation of proteasomal subunit Rpt6, 20S proteasome activity, and the amount of PSD-95 in the DH synaptic fraction, as well as proteasome activity in the mPFC synaptic fraction. In females, training did not affect measures of UPS or synaptic activity in the DH synaptic fraction or in either mPFC fraction but increased Rpt6 phosphorylation in the DH cytoplasmic fraction. Overall, training-induced UPS activity was greater in males than in females, greater in the DH than in the mPFC, and greater in synaptic fractions than in cytosol. These data suggest that object training drives sex-specific alterations in UPS activity across brain regions and subcellular compartments important for memory.

The effects of early-life immune activation on microglia-mediated neuronal remodeling and the associated ontogeny of hippocampal-dependent learning in juvenile rats.

Many neurodevelopmental disorders and associated learning deficits have been linked to early-life immune activation or ongoing immune dysregulation (Laskaris et al., 2016; O'Connor et al., 2014; Frick et al., 2013). Neuroscientists have begun to understand how the maturation of neural circuits allows for the emergence of cognitive and learning behaviors; yet we know very little about how these developing neural circuits are perturbed by certain events, including risk-factors such as early-life immune activation and immune dysregulation. To answer these questions, we examined the impact of early-life immune activation on the emergence of hippocampal-dependent learning in juvenile male and female rats using a well-characterized hippocampal-dependent learning task and we investigated the corresponding, dynamic multicellular interactions in the hippocampus that may contribute to these learning deficits. We found that even low levels of immune activation can result in hippocampal-depedent learning deficits days later, but only when this activation occurs during a sensitive period of development. The initial immune response and associated cytokine production in the hippocampus resolved within 24 h, several days prior to the observed learning deficit, but notably the initial immune response was followed by altered microglial-neuronal communication and synapse remodeling that changed the structure of hippocampal neurons during this period of juvenile brain development. We conclude that immune activation or dysregulation during a sensitive period of hippocampal development can precipitate the emergence of learning deficits via a multi-cellular process that may be initiated by, but not the direct result of the initial cytokine response. SIGNIFICANCE STATEMENT: Many neurodevelopmental disorders have been linked to early-life immune activation or immune dysregulation; however, very little is known about how dynamic changes in neuroimmune cells mediate the transition from normal brain function to the early stages of cognitive disorders, or how changes in immune signaling are subsequently integrated into developing neuronal networks. The current experiments examined the consequences of immune activation on the cellular and molecular changes that accompany the emergence of learning deficits during a sensitive period of hippocampal development. These findings have the potential to significantly advance our understanding of how early-life immune activation or dysregulation can result in the emergence of cognitive and learning deficits that are the largest source of years lived with disability in humans.

A Putative Role for Ubiquitin-Proteasome Signaling in Estrogenic Memory Regulation.

Sex steroid hormones such as 17ß-estradiol (E2) are critical neuromodulators of hippocampal synaptic plasticity and hippocampus-dependent memory in both males and females. However, the mechanisms through which E2 regulates memory formation in both sexes remain unclear. Research to date suggests that E2 regulates hippocampus-dependent memory by activating numerous cell-signaling cascades to promote the synthesis of proteins that support structural changes at hippocampal synapses. However, this work has largely overlooked the equally important contributions of protein degradation mediated by the ubiquitin proteasome system (UPS) in remodeling the synapse. Despite being critically implicated in synaptic plasticity and successful formation of long-term memories, it remains unclear whether protein degradation mediated by the UPS is necessary for E2 to exert its beneficial effects on hippocampal plasticity and memory formation. The present article provides an overview of the receptor and signaling mechanisms so far identified as critical for regulating hippocampal E2 and UPS function in males and females, with a particular emphasis on the ways in which these mechanisms overlap to support structural integrity and protein composition of hippocampal synapses. We argue that the high degree of correspondence between E2 and UPS activity warrants additional study to examine the contributions of ubiquitin-mediated protein degradation in regulating the effects of sex steroid hormones on cognition.