Molecular Mechanisms of Synapse Development and Plasticity

National Institute Of Mental Health

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Abstract

1. Dendritic distribution of autophagosomes underlies pathway-selective induction of LTD Alteration of synaptic strength via synaptic plasticity is a cellular model for learning, memory, and adaptive behavior. Dysfunctional synaptic plasticity is associated with many psychiatric and neurological disorders. Synaptic plasticity can last for hours and up to days and months. This long-term synaptic plasticity can be further subdivided into long-term potentiation (LTP) and long-term depression (LTD), the strengthening and weakening of synapses respectively. The mechanism through which the strength of excitatory synapses changes is largely through the alteration of surface AMPA receptors, either inserted into the cell membrane in the case of LTP or removed from the membrane in LTD. LTD, while comparatively less studied than LTP, is an equally important component of long-term synaptic plasticity. LTD has been shown to be involved in hippocampus-dependent functions such as spatial navigation, behavioral flexibility, and fear memory. The hippocampal formation is a group of structures consisting of Dentate Gyrus (DG), Cornu Ammonis (CA1-3), subiculum, and entorhinal cortex (EC). The connectivity of the hippocampus is extensively defined, with the major input to the hippocampus being layer II neurons of the EC projecting to DG. DG sends mossy fibers to CA3, and CA3 projects Schaffer collaterals (SC) to the Stratum Radiatum (SR) and Stratum Oriens (SO) of CA1 which serves as the primary output structure of the hippocampus. Layer III neurons of the EC also send a direct projection to the distal apical dendrites in the Stratum Lacunosum-Moleculare (SLM) of CA1 via the temporoammonic pathway (TAP). The TAP carries projections from the Reuniens Nucleus of the Thalamus (NeR) in addition to the direct cortical input from EC. Hence, different dendritic segments in the same CA1 neuron receive synaptic inputs from different neural circuits. Synaptic transmission from different neural circuits is not necessarily modulated concurrently even in the same dendrite. Circuit-selective synaptic plasticity can enhance the capacity and flexibility of the nervous system. Dendritic segment-specific synaptic properties in CA1 neurons such as differences in synapse number, AMPAR abundance, and synaptic structure have already been noted. However, the mechanism for circuit-selective control of synaptic plasticity in the same dendrite is largely unclear. The induction of LTD by low-frequency stimulation (LFS) at the SC synapse requires caspase-3 and the inhibition of autophagy. LFS leads to NMDAR activation and calcium influx which activates protein phosphatases including calcineurin and PP1. The phosphatases dephosphorylate the pro-apoptotic BCL-2 family protein BAD to activate caspase-3, which cleaves autophagy related proteins resulting in autophagy inhibition. Autophagy inhibition leads to a reduction in AMPA receptor recycling to the plasma membrane thereby decreasing surface AMPA receptor levels. This NMDAR-dependent LTD (NMDAR-LTD) is readily inducible in juvenile mice (postnatal day 16-19; P16-19), yet is difficult to induce in adult mice (>P56). This developmental shift in LTD inducibility is in part attributable to autophagy upregulation, as autophagy in CA1 is increased in adults as compared to juvenile mice and knocking out autophagy related proteins restores SC LTD inducibility in adult mice. LTD at the TAP synapse remains poorly understood with inconsistent data in the literature. Some studies indicate that LTD in the TAP can be induced in adult rats in an NMDAR-dependent fashion. Other work is contradictory, showing that TAP LTD is NMDAR independent. Little is known about the intracellular signaling cascade for TAP LTD. This stands in stark contrast to the extensively characterized SC LTD. In this study, we investigate the mechanism for TAP LTD. We demonstrate that LFS can induce TAP LTD. Additionally, the mechanism of LTD in the two synapses is shared, as both are reliant on NMDAR, caspase-3, and the inhibition of autophagy. While examining autophagosomes in dendrites, we noted that there is a decrease in autophagosomes from proximal to distal CA1 apical dendrites. Proximal apical dendrites have a density of autophagosomes four times greater than distal apical dendrites. Given that autophagy has to be inhibited to induce LTD, the relatively low abundance of autophagosomes in distal dendrites is more favorable for LTD induction than the comparatively high abundance of autophagosomes in proximal dendrites. Hence, we propose an autophagosome-based model for the differential LTD inducibility of distinct dendritic segments. In sum, this study demonstrates a potential link between the subcellular distribution of autophagosomes in hippocampus CA1 dendrites and the differential inducibility of LTD in the SC and TAP synapses, further contributing to the growing relationship between autophagy and long-term depression. 2. A new behavioral paradigm for frustrative non-reward reveals a global change in brain networks by frustration Irritability, defined as proneness to anger, can reach a pathological extent and is one of the most common reasons for psychiatric evaluation and treatment in youth. It is a hallmark symptom of Disruptive Mood Dysregulation Disorder (DMDD), a diagnosis introduced into the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) in 2013. Irritability is associated with long-lasting adverse outcomes including high rates of school suspensions, hospitalizations, suicidality, and diagnoses of anxiety and depression. Current treatments for irritability are limited and not specific. Increased understanding of its neurobiological mechanisms is warranted to facilitate the development of novel, specific interventions. However, despite increased irritability research over the past decade, the neuroscience of irritability is still in its infancy. Progress has been slow partly due to the lack of behavioral paradigms for studying irritability in model organisms. Youth with severe irritability have elevated responses to frustrative non-reward (FNR) i.e., the emotional and behavioral response to the omission of an expected reward. FNR is a translational, cross-species construct that can be leveraged to uncover pathophysiological mechanisms of irritability. In contrast to the much-studied rewarding and punishing events, little research has focused on the neural substrates of frustrating events. The previously used rodent behavioral paradigms to induce frustration work better for adult animals with well-developed muscles than for juveniles and require long training periods which are unsuited to study irritability-like behavior in mice, as DMDD is a disorder diagnosed in children aged 618 years, which corresponds to 37 weeks of ages in mice. To study FNR in young mice, we developed a new behavioral paradigm named Alternate Poking Reward Omission (APRO). We show that after exposing juvenile mice to FNR using APRO, mice increase locomotion, aggression, and resistance to extinction of instrumental behavior. Whole brain mapping of neural activation identifies brain regions activated by FNR and shows that the processing mode of brain network is changed by FNR. This work lays a foundation for further research on the neural mechanism of FNR and irritability by precisely manipulating and measuring neural activity with advanced genetic, optogenetic, and electrophysiological tools.