Vijayalakshmi Santhakumar

Developing conductive scaffolds for neural regeneration is often hindered by the cytotoxicity and mechanical mismatch inherent to conventional electronic fillers. Here, we report an ultrasoft ionically conductive hydrogel engineered by grafting choline-based bioionic liquids (BIL) onto fish gelatin (FiBGel). Systematic optimization yields brain-mimetic conductivity (0.25–0.68 S m⁻¹), elastic moduli (0.6–0.9 kPa), and physiological water content (72–81%). In 3D culture, the matrix drives spontaneous neurogenesis, achieving ∼90% MAP2+ neurons with robust calcium signaling, while concurrently supporting endothelial network formation. We report the first intracerebral administration of a BIL hydrogel, demonstrating remarkable biocompatibility and immunomodulation. In a traumatic brain injury model, a single FiBGel dose halves reactive astrogliosis, enhances cortical revascularization, and limits lesion cavity expansion. Leveraging an FDA-recognized micronutrient, FiBGel offers a translatable strategy that integrates neuromechanics, conductivity, immune modulation, and vascular support within one injectable scaffold for minimally invasive treatment of brain injuries.

Traumatic brain injury (TBI) is a major cause of long-term neurological disability, yet the mechanisms underlying human-specific injury responses remain poorly understood. Rodent models of TBI have yielded significant insights into mechanisms and pathophysiology of TBI but may not capture human-specific developmental and molecular responses. Here, we establish a human forebrain organoid (hFBO) injury paradigm to examine developmental stage and region-specific responses to concussive injury. Dorsal and ventral hFBOs, enriched in glutamatergic and GABAergic neurons, respectively, showed progressive physiological maturation with rudimentary firing characteristics at 80 days in vitro (DIV) and spontaneous synaptic activity by 150 DIV. Injury to dorsal and ventral hFBOs at 25 DIV, when neural progenitors dominate, caused cell death that scaled with severity and increased expression of neural proliferation markers and TLR4/NF-κB associated neuroimmune signaling. Injury to physiologically mature organoids at 150 DIV produced severity-dependent acute intracellular calcium dysregulation. Together, these findings establish a human organoid-based platform that captures developmental stage-dependent neurogenic, molecular, and cellular dysfunction in responses to concussive injury.

The dentate gyrus (DG) is a critical regulator of cortical input processing in the hippocampus, supporting spatial navigation and memory processing. In epilepsy, the DG undergoes extensive structural and functional reorganization that diminishes its ability to filter afferent inputs, resulting in enhanced hippocampal hyperexcitability, seizures, and cognitive impairment. Yet, how DG network dynamics evolve over time in the epileptic brain is not fully understood. Here, we investigated DG network excitability, feedback inhibition, and throughput in rats 1 week, 8 weeks, and 4 months after pilocarpine-induced status epilepticus (SE), and examined the role of presynaptic GABA _B receptor-mediated modulation of DG activity. Recording from urethane-anesthetized rats in vivo , we show a persistent increase in excitation-spike (E-S) coupling after SE. In contrast, feedback inhibition measured by short-duration paired-pulse ratios (PPR at 20ms, 40 ms and 60ms intervals) was significantly increased 1 and 8 weeks after SE, but was indistinguishable from controls by 4 months. Notably, we found that dentate spike (DS) frequency remained persistently elevated up to 10 months post-SE. Selective inhibition of presynaptic GABA _B receptors in the DG using CGP36216 further enhanced E-S coupling at 1 and 8 weeks in post-SE rats without affecting age-matched controls. CGP36216 did not alter E-S coupling in rats 4 months after SE, revealing a homeostatic role for basal presynaptic GABA _B signaling in restraining network excitability early after SE. CGP36216 robustly increased PPR at 20ms across all groups without altering DS frequency, suggesting that GABAB receptors boost DG feedback inhibition. Mechanistically, CGP36216 reduced single stimulus-evoked inhibitory postsynaptic currents (eIPSCs) in dentate granule cells without altering excitatory transmission after SE. Together, these findings support a disinhibitory role for presynaptic GABA _B receptors, which bolsters synaptic inhibition to transiently counter post-SE hyperexcitability, and offers a targetable mechanism for therapeutic intervention during epileptogenesis.

This "Correspondence" is a "Response" to the "Comment" by Dr. Aihua Yuan and Dr. Bo Yuan, titled "Re-examining interneuron-specific Nrp2 deletion: overlooked striatal and cortical contributions", in relation to our published article "Dysregulation of neuropilin-2 expression in inhibitory neurons impairs hippocampal circuit development and enhances risk for autism-related behaviors and seizures". While the limitations expressed in the comment by Drs Yuan and Yuan regarding the potential extrahippocampal contributions to the behavioral deficits in mice with interneuron-specific Neuropilin 2 deletion are valid, we would like to emphasize that these issues were extensively and were thoroughly discussed in our publication and not overlooked.

For decades, electrical neuromodulation has been used as a therapeutic mechanism to disrupt and desynchronize pathological neural activity in various neurological disorders. Despite notable progress, however, patient outcomes remain highly variable, particularly in medically intractable epilepsy where surgery still provides the greatest chance of seizure freedom. Here we propose passive neuromodulation (PNM) as a radical alternative to conventional neurostimulation, whereby analogue feedback is used to drain energy from an epileptic circuit and thus suppress the initiation or spread of electrographic seizures. We provide pilot evidence on the efficacy and robustness of PNM using two computational models of epileptic dynamics: a detailed biophysical network model of dentate gyrus, and the Epileptor neural mass model of seizure dynamics. Despite the vast differences between these models, our results show the robust ability of PNM to suppress seizures in both models. We further demonstrate the efficacy and robustness of responsive PNM , whereby brief (50ms) windows of PNM are triggered by a simultaneously-running seizure detection algorithm, as well as the safe and tunable nature of PNM, where more robust seizure suppression can be achieved by parametrically titrating the amount of power drained from the tissue, without inducing any seizures even if applied interictally. Overall, our results provide strong evidence on the promise of PNM for the closed-loop control of epileptic seizures and other neurological disorders where damping pathological network activity can restore healthy dynamics.

The dentate gyrus is a major locus for structural and synaptic reorganization in temporal lobe epilepsy. While physiological changes during epileptogensis are well characterized in the principal dentate projection neuron, granule cells (GCs), epilepsy-related changes in semilunar granule cells (SGCs), a distinct subset of dentate projection neurons, remain unknown. Using a mouse pilocarpine model of epilepsy, we show that, unlike GCs, SGCs exhibit an increase in intrinsic excitability 1 week after status epilepticus (SE). GCs, not SGCs. have a lower threshold for action potential firing in mice 1-month post-SE. Both GCs and SGCs display increased frequency of spontaneous excitatory postsynaptic currents (EPSCs) 1-week and 1-month after SE. Uniquely, SGCs received more frequent spontaneous inhibitory postsynaptic currents (sIPSCs) and exhibited smaller afferent-evoked IPSCs early after SE. Additionally, evoked EPSC amplitude in SGCs was reduced 1-month post-SE. Behaviorally, mice 1-month post-SE showed impairments in their ability to use spatial search strategies in a Barnes maze paradigm. Post-SE TRAP2::tdT mice showed reduced activity dependent neuronal ensemble labeling, with fewer tdT-labeled neurons in both task-naïve and trained conditions and reduced c-Fos co-expression following task re-acquisition compared to controls. Notably, the proportion of SGCs within labeled ensembles was reduced in task-naïve epileptic mice but not in trained animals. Collectively, our findings identify selective changes in SGC intrinsic excitability during epileptogenesis that could contribute to enhanced network excitability. The cell-specific alterations in SGC circuit connectivity during epileptogenesis, alongside the apparent reduction in neuronal recruitment to behavioral ensembles likely contribute to spatial navigation deficits observed in epilepsy.

Neuroinflammatory pathways activated by traumatic brain injury (TBI) are critical mediators of long-term neurological dysfunction and represent promising therapeutic targets. Toll-like receptor 4 (TLR4), an innate immune receptor, was previously shown to contribute to increased seizure susceptibility and cognitive deficits in rats after lateral fluid percussion injury (FPI). However, the cellular and molecular mechanisms underlying TLR4-mediated circuit dysfunction early after brain injury are not fully understood. In this study, we define a cell- and circuit-specific neuroimmune-enzyme effector signaling axis that mediates early post-TBI circuit dysfunction in the hippocampal Dentate Gyrus (DG). Using ex vivo electrophysiology in rat and mouse models one-week after brain injury, we demonstrate that neuronal TLR4 signaling regulates both excitatory and inhibitory synaptic inputs to dentate granule cells (DGC). Collectively, pharmacological inhibition of TLR4 in rats and cell-type-specific deletion of TLR4 in mice show that neuronal TLR4 mediates injury-driven increase in DGC excitatory input frequency and relies on downstream activation of Matrix Metalloproteinase-9 (MMP-9). In contrast, TLR4 signaling contributed to decrease in inhibitory current frequency after injury, but independent of MMP-9, revealing a mechanistic divergence. Systemic inhibition of either TLR4 signaling or MMP-9 activity in rats within 24 hours after injury reduced network hyperexcitability and improved long-term potentiation (LTP) in the DG measured in vivo one-week after injury. Either TLR4 or MMP-9 inhibition early after injury effectively attenuated spatial memory deficits in a Barnes maze task one-month post-injury. Paradoxically, in sham controls, inhibition of TLR4 increased frequency of both excitatory and inhibitory inputs to DGCs and augmented network excitability, without altering MMP-9 levels, identifying context-dependent roles for TLR4 signaling. Together, these results identify a novel TLR4-MMP-9 axis as a key driver of early post-TBI dentate gyrus circuit dysfunction and behavioral deficits.

The dentate gyrus is critical for spatial memory formation and shows task-related activation of cellular ensembles considered as memory engrams. Semilunar granule cells (SGCs), a sparse dentate projection neuron subtype, were reported to be enriched among behaviorally activated neurons. By examining SGCs and granule cells (GCs) labeled during contextual memory formation in TRAP2 mice, we empirically tested competing hypotheses for GC and SGC recruitment into memory ensembles. Consistent with more excitable neurons being recruited into memory ensembles, SGCs showed greater sustained firing than GCs. Additionally, labeled SGCs showed less adapting firing than unlabeled SGCs. The lack of glutamatergic connections between behaviorally labeled SGCs and GCs in our recordings is inconsistent with SGC-driven local circuit feedforward excitation underlying ensemble recruitment. Moreover, there was little evidence for individual SGCs or labeled neuronal ensembles supporting lateral inhibition of unlabeled neurons. Instead, labeled GCs and SGCs received more spontaneous excitatory synaptic inputs than their unlabeled counterparts. Labeled neuronal pairs received more temporally correlated spontaneous excitatory synaptic inputs than labeled-unlabeled neuronal pairs. These findings challenge the proposal that SGCs drive dentate GC ensemble refinement, while supporting a role for intrinsic excitability and correlated inputs in preferential SGC recruitment to contextual memory engrams.

The dentate gyrus is critical for spatial memory formation and shows task related activation of cellular ensembles considered as memory engrams. Semilunar granule cells (SGCs), a sparse dentate projection neuron subtype distinct from granule cells (GCs), were recently reported to be enriched among behaviorally activated neurons. However, the mechanisms governing SGC recruitment during memory formation and their role in engram refinement remains unresolved. By examining neurons labeled during contextual memory formation in TRAP2 mice, we empirically tested competing hypotheses for GC and SGC recruitment into memory ensembles. In support of the proposal that more excitable neurons are preferentially recruited into memory ensembles, SGCs showed greater sustained firing than GCs. Additionally, SGCs labeled during memory formation showed less adapting firing than unlabeled SGCs. Our recordings did not reveal glutamatergic connections between behaviorally labeled SGCs and GCs, providing evidence against SGC driven local circuit feedforward excitation in ensemble recruitment. Contrary to a leading hypothesis, there was little evidence for individual SGCs or labeled neuronal ensembles supporting lateral inhibition of unlabeled neurons. Instead, labeled GCs and SGCs received more spontaneous excitatory synaptic inputs than their unlabeled counterparts. Moreover, pairs of GCs and SGCs within labeled neuronal cohorts received more temporally correlated spontaneous excitatory synaptic inputs than labeled-unlabeled neuronal pairs, validating a role for correlated afferent inputs in neuronal ensemble selection. These findings challenge the proposal that SGCs drive dentate GC ensemble refinement, while supporting a role for intrinsic active properties and correlated inputs in preferential SGC recruitment to contextual memory engrams.