Biology subjects

Lituma, P. J.

Publications and source records attributed to Lituma, P. J..

6 recordsLinked to original sources

bioRxiv2021

Real-time imaging of Arc/Arg3.1 transcription ex vivo reveals input-specific immediate early gene dynamics

The ability of neurons to process and store salient environmental features underlies information processing in the brain. Long-term information storage requires synaptic plasticity and regulation of gene expression. While distinct patterns of activity have been linked to synaptic plasticity, their impact on immediate early gene (IEG) expression remains poorly understood. The activity regulated cytoskeleton associated (Arc) gene has received wide attention as an IEG implicated in synaptic plasticity and memory. Yet, to date, the transcriptional dynamics of Arc in response to compartment and input-specific activity is unclear. By developing a knock-in mouse to fluorescently tag Arc alleles, we studied real-time transcription dynamics after stimulation of dentate granule cells (GCs) in acute hippocampal slices. To our surprise, we found that Arc transcription displayed distinct temporal kinetics depending on the activation of excitatory inputs that convey functionally distinct information, i.e. medial and lateral perforant paths (MPP and LPP, respectively). Moreover, the transcriptional dynamics of Arc after synaptic stimulation was similar to direct activation of GCs, although the contribution of ionotropic glutamate receptors, L-type voltage gated calcium channel, and the endoplasmic reticulum (ER) differed. Specifically, we observed an ER-mediated synapse-to-nucleus signal that supported elevations in nuclear calcium, and rapid induction of Arc transcription following MPP stimulation. However, activation of LPP inputs displayed lower nuclear calcium rise, which could underlie the delayed transcriptional onset of Arc. Our findings highlight how input-specific activity distinctly impacts transcriptional dynamics of an IEG linked to learning and memory. Significance statementEnvironmental experiences trigger neuronal activity that elicits gene expression in the nervous system. Rapid induction of specific genes known as immediate early genes (IEGs) supports activity-dependent changes of neuronal circuits to ultimately influence animal behavior. However, the cellular and molecular mechanisms controlling how distinct forms of neuronal activity modulate IEG expression remains unclear. The activity regulated cytoskeleton associated (Arc) gene is a critical IEG linked to memory. By imaging Arc transcription in real-time after neuronal activity, we identified how different receptors and signaling pathways influence transcriptional induction and dynamics of an IEG. Our findings provide insights into how information received by distinct synaptic inputs could be encoded by modulating IEG dynamics.

neuroscience↗

bioRxiv2021

Cycles of transcription and local translation consolidate Arc protein at dendritic sites

Long-term memory requires transcription and translation of activity-regulated genes. Many of these are immediate early genes (IEGs) with short-lived mRNAs and proteins, decaying rapidly after stimulation. It remains unknown how an IEG with rapid mRNA and protein turnover can impact long-lasting changes at the synapses. Using fluorescently tagged endogenous Arc, an IEG important for memory consolidation, we performed high-resolution imaging of transcription and translation in individual neurons to identify the long-term gene dynamics after stimulation. Unexpectedly, once induced, Arc underwent transcriptional reactivation often at the same allele. Cycles of transcription were coordinated with localized translation. This cyclical regulation of an IEG, dependent on protein synthesis, reactivates subsequent transcription for maintaining mRNA supply to dendrites. The ensuing Arc mRNAs were preferentially localized at sites marked by previous Arc protein, thereby consolidating local "hubs" of dendritic Arc. These findings revealed the spatio-temporal dynamics of transcription-translation coupling of an IEG and provide a mechanism by which short-lived synaptic proteins can be sustained over the long-time scales of memory consolidation.

neuroscience↗

bioRxiv2021

Altered synaptic connectivity and brain function in mice lacking microglial adapter protein Iba1

Growing evidence indicates that microglia impact brain function by regulating synaptic pruning and formation, as well as synaptic transmission and plasticity. Iba1 (Ionized Ca+2-binding adapter protein 1), encoded by the Allograft inflammatory factor 1 (Aif1) gene, is an actin-interacting protein in microglia. Although Iba1 has long been used as a cellular marker for microglia, its functional role remains unknown. Here, we used global Iba1-deficient (Aif1-/-) mice to characterize microglial activity, synaptic function and behavior. Microglial imaging in acute hippocampal slices and fixed tissues from juvenile mice revealed that Aif1-/- microglia display reductions in ATP-induced motility and ramification, respectively. Biochemical assays further demonstrated that Aif1-/- brain tissues exhibit an altered expression of microglial-enriched proteins associated with synaptic pruning. Consistent with these changes, juvenile Aif1-/- mice displayed deficits in excitatory synapse number and synaptic transmission assessed by neuronal labeling and whole-cell patch-clamp recording in acute hippocampal slices. Unexpectedly, microglial synaptic engulfment capacity was diminished in juvenile Aif1-/- mice. During early postnatal development when synapse formation is a predominant event in the hippocampus, excitatory synapse number was still reduced in Aif1-/- mice. Together these findings support an overall role of Iba1 in excitatory synaptic growth in juvenile mice. Lastly, postnatal synaptic deficits persisted in the adulthood and correlated with significant behavioral changes in adult Aif1-/- mice, which exhibited impairments in object recognition memory and social interaction. These results suggest that Iba1 critically contributes to microglial activity underlying essential neuro-glia developmental processes that may deeply influence behavior. SignificanceAbnormal microglia-neuron interaction is increasingly implicated in neurodevelopmental and neuropsychiatric conditions such as autism spectrum disorders and schizophrenia, as well as in neurodegenerative disorders such as Alzheimers disease. This study demonstrates that deletion of the microglia-specific protein Iba1, which has long been utilized as a selective microglial marker but whose role has remained unidentified, results in microglial structural and functional impairments that significantly impact synaptic development and behavior. These findings not only highlight the importance of microglia in brain function but may also suggest that modifying microglial function could provide a therapeutic strategy for treatment of neurodevelopmental, neuropsychiatric and neurodegenerative disorders.

neuroscience↗

bioRxiv2021

Donor-derived vasculature is required to support neocortical cell grafts after stroke

Neural precursor cells (NPCs) transplanted into the adult neocortex generate neurons that synaptically integrate with host neurons, supporting the possibility of achieving functional tissue repair. However, poor survival of transplanted NPCs greatly limits efficient engraftment. Here, we test the hypothesis that combining blood vessel-forming vascular cells with neuronal precursors improves engraftment. By transplanting mixed embryonic neocortical cells into adult mice with neocortical strokes, we show that transplant-derived neurons synapse with appropriate targets while donor vascular cells form vessels that fuse with the host vasculature to perfuse blood within the graft. Although all grafts became vascularized, larger grafts had greater contributions of donor-derived vessels that increased as a function of their distance from the host-graft border. Moreover, excluding vascular cells from the donor cell population strictly limited graft size. Thus, inclusion of vessel-forming vascular cells with NPCs is required for more efficient engraftment and ultimately for tissue repair.

neuroscience↗

bioRxiv2021

Presynaptic NMDA receptors facilitate short-term plasticity and BDNF release at hippocampal mossy fiber synapses

Neurotransmitter release is a highly controlled process by which synapses can critically regulate information transfer within neural circuits. While presynaptic receptors -typically activated by neurotransmitters and modulated by neuromodulators- provide a powerful way of fine-tuning synaptic function, their contribution to activity-dependent changes in transmitter release remains poorly understood. Here, we report that presynaptic NMDA receptors (preNMDARs) at mossy fiber boutons in the rodent hippocampus can be activated by physiologically relevant patterns of activity and selectively enhance short-term synaptic plasticity at mossy fiber inputs onto CA3 pyramidal cells and mossy cells, but not onto inhibitory interneurons. Moreover, preNMDARs facilitate brain-derived neurotrophic factor (BDNF) release and contribute to presynaptic calcium rise. Taken together, our results indicate that by increasing presynaptic calcium, preNMDARs fine tune mossy fiber neurotransmission and can control information transfer during dentate granule cell burst activity that normally occur in vivo.

neuroscience↗

bioRxiv2020

Retrograde suppression of post-tetanic potentiation at the mossy fiber-CA3 pyramidal cell synapse

In the hippocampus, the excitatory synapse between dentate granule cell axons - or mossy fibers (MF) - and CA3 pyramidal cells (MF-CA3) expresses robust forms of short-term plasticity, such as frequency facilitation and post-tetanic potentiation (PTP). These forms of plasticity are due to increases in neurotransmitter release, and can be engaged when dentate granule cells fire in bursts (e.g. during exploratory behaviors) and bring CA3 pyramidal neurons above threshold. While frequency facilitation at this synapse is limited by endogenous activation of presynaptic metabotropic glutamate receptors, whether MF-PTP can be regulated in an activity-dependent manner is unknown. Here, using physiologically relevant patterns of mossy fiber stimulation in acute mouse hippocampal slices, we found that disrupting postsynaptic Ca2+ dynamics increases MF-PTP, strongly suggesting a form of Ca2+-dependent retrograde suppression of this form of plasticity. PTP suppression requires a few seconds of MF bursting activity and Ca2+ release from internal stores. Our findings raise the possibility that the powerful MF-CA3 synapse can negatively regulate its own strength not only during PTP-inducing activity typical of normal exploratory behaviors, but also during epileptic activity. SIGNIFICANCE STATEMENTThe powerful mossy fiber-CA3 synapse exhibits strong forms of plasticity that are engaged during location-specific exploration, when dentate granule cells fire in bursts. While this synapse is well-known for its presynaptically-expressed LTP and LTD, much less is known about the robust changes that occur on a shorter time scale. How such short-term plasticity is regulated, in particular, remains poorly understood. Unexpectedly, an in vivo-like pattern of presynaptic activity induced robust post-tetanic potentiation (PTP) only when the postsynaptic cell was loaded with a high concentration of Ca2+ buffer, indicating a form of Ca2+-dependent retrograde suppression of PTP. Such suppression may have profound implications for how environmental cues are encoded into neural assemblies, and for limiting network hyperexcitability during seizures.

neuroscience↗