Identifying gene expression profiles associated with neurogenesis and inflammation in the human subependymal zone from development through aging

The generation of new neurons within the mammalian forebrain continues throughout life within two main neurogenic niches, the subgranular zone (SGZ) of the hippocampal dentate gyrus, and the subependymal zone (SEZ) lining the lateral ventricles. Though the SEZ is the largest neurogenic niche in the adult human forebrain, our understanding of the mechanisms regulating neurogenesis from development through aging within this region remains limited. This is especially pertinent given that neurogenesis declines dramatically over the postnatal lifespan. Here, we performed transcriptomic profiling on the SEZ from human post-mortem tissue from eight different life-stages ranging from neonates (average age ~ 2 months old) to aged adults (average age ~ 86 years old). We identified transcripts with concomitant profiles across these decades of life and focused on three of the most distinct profiles, namely (1) genes whose expression declined sharply after birth, (2) genes whose expression increased steadily with age, and (3) genes whose expression increased sharply in old age in the SEZ. Critically, these profiles identified neuroinflammation as becoming more prevalent with advancing age within the SEZ and occurring with time courses, one gradual (starting in mid-life) and one sharper (starting in old age).

Why do hippocampal mossy cells matter? It depends on the frequency and context

The discrimination of similar episodes and places, and their representation as distinct memories, depend on a process called pattern separation that relies on the circuitry of the hippocampal dentate gyrus (DG). Mossy cells (MCs) are key neurons in the circuitry, but how they influence DG network dynamics, function, and seizure risk has not been fully elucidated. We found the net impact of MCs was inhibitory at physiological frequencies connected with learning and behaviour, and their absence associated with deficits in pattern separation and spatial memory; at higher frequencies, their net impact was excitatory, and their absence protected against seizures. Thus, MCs influence DG outputs in a highly dynamic manner that varies with frequency and context.One-Sentence SummaryHippocampal mossy cells are required for learning and memory; but their absence protects against seizures.

Oligodendrocytes are a lifelong source of nuclear and ribosomal material for neurons in the mouse brain

Nuclear and ribosomal components define cell identity and function by regulating chromatin dynamics, gene expression, and protein turnover. Here we report that in the mouse central nervous system (CNS) under normal conditions, neurons accumulate nuclear and ribosomal material of oligodendrocyte (OL) origin. We show that neuronal accumulation of OL-derived nuclear and ribosomal material is brain area-specific, and in the cortex and hippocampal dentate gyrus gradually propagates during postnatal brain maturation. We further demonstrate that OL-to-neuron material transfer persists throughout adulthood and responds to neuroinflammation. We found that satellite OL of the gray matter form internuclear contacts with receiving neurons in the mouse brain. Similar close internuclear associations between satellite OL and neurons are present in the adult human cortex. Our findings provide the first evidence of wide-spread dynamic and selective OL-to-neuron nuclear and ribosomal material transfer in the mouse CNS and indicate that satellite OL serve as powerful mediators of neuronal function. Equivalent processes may occur in the human CNS and cause neurological disorders when dysregulated.

Neuroprotective effects of thymoquinone against MDMA-induced neurotoxicity

MDMA (3, 4-methylenedioxymethamphetamine) is a psychoactive substance that is associated with neurotoxicity. MDMA exposure to human results in the degeneration of neuronal cells in the hippocampus. Hence, the purpose of this study was to examine the potential of a natural compound known as thymoquinone (TQ) to protect against neuronal damage and memory impairment in rats stimulated by MDMA. The administration of TQ into MDMA-induced neuronal damage rats was carried out in male Sprague Dawley via a 1-week treatment dividing into four groups (n=36, 7-9 per group). The studied groups involved with the treatments comprise i) Control (1 mL/kg saline), ii) MDMA (10 mg/kg MDMA), iii) MDMA+TQ (10 mg/kg MDMA + 40 mg/kg TQ) and iv) TQ control (40 mg/kg TQ). A novel object recognition test (NORT) was carried out to evaluate the memory performance of the rats, followed by a histopathological assessment of the hippocampal dentate gyrus. The histopathology analysis revealed a significant increase in numbers of positive cells by Fluoro-Jade C following the effect of MDMA on neuronal damage (MDMA induced group) compared to control (P<0.05). Next, the TQ treatments observed in MDMA+TQ exhibited a decline in positive cells from Fluoro-Jade C. The index of recognition memory was found to be increased in MDMA+TQ compared to the MDMA alone (P<0.05). This study suggests that the neuronal damage inflicted by MDMA in a rat model has the potential to be treated by TQ.

Disynaptic Effect of Hilar Cells on Pattern Separation in A Spiking Neural Network of Hippocampal Dentate Gyrus

We investigate the disynaptic effect of the hilar cells on pattern separation in a spiking neural network of the hippocampal dentate gyrus (DG). The principal granule cells (GCs) in the DG perform pattern separation, transforming similar input patterns into less-similar output patterns. In our DG network, the hilus consists of excitatory mossy cells (MCs) and inhibitory HIPP (hilar perforant path-associated) cells. Here, we consider the disynaptic effects of the MCs and the HIPP cells on the GCs, mediated by the inhibitory basket cells (BCs) in the granular layer; MC → BC → GC and HIPP → BC → GC. Disynaptic inhibition from the MCs tends to decrease the firing activity of the GCs. On the other hand, the HIPP cells disinhibit the intermediate BCs, which leads to increasing the activity of the GCs. By changing the synaptic strength K(BC, X) [from the presynaptic X (= MC or HIPP) to the postsynaptic BC] from the default value K(BC, X)*, we study the change in the pattern separation degree Sd. When decreasing K(BC, MC) or independently increasing K(BC, HIPP) from their default values, Sd is found to decrease (i.e., pattern separation is reduced). On the other hand, as K(BC, MC) is increased or independently K(BC, HIPP) is decreased from their default values, pattern separation becomes enhanced (i.e., Sd increases). In this way, the disynaptic effects of the MCs and the HIPP cells on the pattern separation are opposite ones. Thus, when simultaneously varying both K(BC, MC) and K(BC, HIPP), as a result of balance between the two competing disynaptic effects of the MCs and the HIPP cells, Sd forms a bell-shaped curve with an optimal maximum at their default values. Moreover, we also investigate population and individual behaviors of the sparsely synchronized rhythm of the GCs, and find that the amplitude measure Ma (representing population synchronization degree) and the random-phase-locking degree Ld (denoting individual activity degree) are strongly correlated with the pattern separation degree Sd. Consequently, the larger the synchronization and the random phase-locking degrees of the sparsely synchronized rhythm is, the more the pattern separation becomes enhanced.