Mossy fiber mis-pathfinding and semaphorin reduction in the hippocampus of α-CaMKII hKO mice

BDNF and Lactate as Modulators of Hippocampal CA3 Network Physiology

Growing evidence supports the notion that brain-derived neurotrophic factor (BDNF) and lactate are potent modulators of mammalian brain function. The modulatory actions of those biomolecules influence a wide range of neuronal responses, from the shaping of neuronal excitability to the induction and expression of structural and synaptic plasticity. The biological actions of BDNF and lactate are mediated by their cognate receptors and specific transporters located in the neuronal membrane. Canonical functions of BDNF occur via the tropomyosin-related kinase B receptor (TrkB), whereas lactate acts via monocarboxylate transporters or the hydroxycarboxylic acid receptor 1 (HCAR1). Both receptors are highly expressed in the central nervous system, and some of their physiological actions are particularly well characterized in the hippocampus, a brain structure involved in the neurophysiology of learning and memory. The multifarious neuronal circuitry between the axons of the dentate gyrus granule cells, mossy fibers (MF), and pyramidal neurons of area CA3 is of great interest given its role in specific mnemonic processes and involvement in a growing number of brain disorders. Whereas the modulation exerted by BDNF via TrkB has been extensively studied, the influence of lactate via HCAR1 on the properties of the MF-CA3 circuit is an emerging field. In this review, we discuss the role of both systems in the modulation of brain physiology, with emphasis on the hippocampal CA3 network. We complement this review with original data that suggest cross-modulation is exerted by these two independent neuromodulatory systems.

Forebrain-specific conditional calcineurin deficiency induces dentate gyrus immaturity and hyper-dopaminergic signaling in mice

Calcineurin (Cn), a phosphatase important for synaptic plasticity and neuronal development, has been implicated in the etiology and pathophysiology of neuropsychiatric disorders, including schizophrenia, intellectual disability, autism spectrum disorders, epilepsy, and Alzheimer's disease. Forebrain-specific conditional Cn knockout mice have been known to exhibit multiple behavioral phenotypes related to these disorders. In this study, we investigated whether Cn mutant mice show pseudo-immaturity of the dentate gyrus (iDG) in the hippocampus, which we have proposed as an endophenotype shared by these disorders. Expression of calbindin and GluA1, typical markers for mature DG granule cells (GCs), was decreased and that of doublecortin, calretinin, phospho-CREB, and dopamine D1 receptor (Drd1), markers for immature GC, was increased in Cn mutants. Phosphorylation of cAMP-dependent protein kinase (PKA) substrates (GluA1, ERK2, DARPP-32, PDE4) was increased and showed higher sensitivity to SKF81297, a Drd1-like agonist, in Cn mutants than in controls. While cAMP/PKA signaling is increased in the iDG of Cn mutants, chronic treatment with rolipram, a selective PDE4 inhibitor that increases intracellular cAMP, ameliorated the iDG phenotype significantly and nesting behavior deficits with nominal significance. Chronic rolipram administration also decreased the phosphorylation of CREB, but not the other four PKA substrates examined, in Cn mutants. These results suggest that Cn deficiency induces pseudo-immaturity of GCs and that cAMP signaling increases to compensate for this maturation abnormality. This study further supports the idea that iDG is an endophenotype shared by certain neuropsychiatric disorders.

Role of Ca2+/Calmodulin-Dependent Protein Kinase Type II in Mediating Function and Dysfunction at Glutamatergic Synapses

Glutamatergic synapses harbor abundant amounts of the multifunctional Ca²⁺/calmodulin-dependent protein kinase type II (CaMKII). Both in the postsynaptic density as well as in the cytosolic compartment of postsynaptic terminals, CaMKII plays major roles. In addition to its Ca²⁺-stimulated kinase activity, it can also bind to a variety of membrane proteins at the synapse and thus exert spatially restricted activity. The abundance of CaMKII in glutamatergic synapse is akin to scaffolding proteins although its prominent function still appears to be that of a kinase. The multimeric structure of CaMKII also confers several functional capabilities on the enzyme. The versatility of the enzyme has prompted hypotheses proposing several roles for the enzyme such as Ca²⁺ signal transduction, memory molecule function and scaffolding. The article will review the multiple roles played by CaMKII in glutamatergic synapses and how they are affected in disease conditions.

Disrupted mossy fiber connections from defective embryonic neurogenesis contribute to SOX11-associated schizophrenia

Abnormal mossy fiber connections in the hippocampus have been implicated in schizophrenia. However, it remains unclear whether this abnormality in the patients is genetically determined and whether it contributes to the onset of schizophrenia. Here, we showed that iPSC-derived hippocampal NPCs from schizophrenia patients with the A/A allele at SNP rs16864067 exhibited abnormal NPC polarity, resulting from the downregulation of SOX11 by this high-risk allele. In the SOX11-deficient mouse brain, abnormal NPC polarity was also observed in the hippocampal dentate gyrus, and this abnormal NPC polarity led to defective hippocampal neurogenesis-specifically, irregular neuroblast distribution and disrupted granule cell morphology. As granule cell synapses, the mossy fiber pathway was disrupted, and this disruption was resistant to activity-induced mossy fiber remodeling in SOX11 mutant mice. Moreover, these mutant mice exhibited diminished PPI and schizophrenia-like behaviors. Activation of hippocampal neurogenesis in the embryonic brain, but not in the adult brain, partially alleviated disrupted mossy fiber connections and improved schizophrenia-related behaviors in mutant mice. We conclude that disrupted mossy fiber connections are genetically determined and strongly correlated with schizophrenia-like behaviors in SOX11-deficient mice. This disruption may reflect the pathological substrate of SOX11-associated schizophrenia.

Regulation of axon pruning of mossy fiber projection in hippocampus by CRMP2 and CRMP4

Axon pruning facilitates the removal of ectopic and misguided axons and plays an important role in neural circuit formation during brain development. Sema3F and its receptor neuropilin-2 (Nrp2) have been shown to be involved in the stereotyped pruning of the infrapyramidal bundle (IPB) of mossy fibers of the dentate gyrus (DG) in the developing hippocampus. Collapsin response mediator proteins (CRMPs) were originally identified as an intracellular mediator of semaphorin signaling, and the defective pruning of IPB was recently reported in CRMP2-/- and CRMP3-/- mice. CRMP1 and CRMP4 have high homology to CRMP2 and CRMP3, and their expression in the developing mouse brain overlaps; however, their role in IPB pruning has not yet been examined. In this study, we report that CRMP4, but not CRMP1, is involved in IPB pruning during neural circuit formation in the hippocampus. Our genetic interaction analyses indicated that CRMP2 and CRMP4 have distinct functions and that CRMP2 mediates IPB pruning via Nrp2. We also observed the altered synaptic terminals of mossy fibers in CRMP2 and CRMP4 mutant mice. These results suggest that CRMP family members have a distinct function in the axon pruning and targeting of mossy fibers of the hippocampal DG in the developing mouse brain.

Zinc modulates synaptic transmission by differentially regulating synaptic glutamate homeostasis in hippocampus

A subset of presynaptic glutamatergic vesicles in the brain co-releases zinc (Zn²⁺ ) with glutamate into the synapse. However, the role of synaptically released Zn²⁺ is still under investigation. Here, we studied the effect of Zn²⁺ on glutamate homeostasis by measuring the evoked extracellular glutamate level (EGL) and the probability of evoked action potential (P_EAP ) at the Zn²⁺ -containing or zincergic mossy fiber-CA3 synapses of the rat hippocampus. We found that the application of Zn²⁺ (ZnCl₂ ) exerted bidirectional effects on both EGL and P_EAP : facilitatory at low concentration (~1 µM) while repressive at high concentration (~50 µM). To determine the action of endogenous Zn²⁺ , we also used extracellular Zn²⁺ chelator to remove the synaptically released Zn²⁺ . Zn²⁺ chelation reduced both EGL and P_EAP , suggesting that endogenous Zn²⁺ has mainly a facilitative role in glutamate secretion on physiological condition. We revealed that calcium/calmodulin-dependent protein kinase II was integral to the mechanism by which Zn²⁺ facilitated the release of glutamate. Moreover, a glutamate transporter was the molecular entity for the action of Zn²⁺ on glutamate uptake by which Zn²⁺ decreases glutamate availability. Taken together, we show a novel action of Zn²⁺ , which is to biphasically regulate glutamate homeostasis via Zn²⁺ concentration-dependent synaptic facilitation and depression. Thus, co-released Zn²⁺ is physiologically important for enhancing weak stimulation, but potentially mitigates excessive stimulation to keep synaptic transmission within optimal physiological range.

Functions of Plexins/Neuropilins and Their Ligands during Hippocampal Development and Neurodegeneration

There is emerging evidence that molecules, receptors, and signaling mechanisms involved in vascular development also play crucial roles during the development of the nervous system. Among others, specific semaphorins and their receptors (neuropilins and plexins) have, in recent years, attracted the attention of researchers due to their pleiotropy of functions. Their functions, mainly associated with control of the cellular cytoskeleton, include control of cell migration, cell morphology, and synapse remodeling. Here, we will focus on their roles in the hippocampal formation that plays a crucial role in memory and learning as it is a prime target during neurodegeneration.

Multi-level genomic analyses suggest new genetic variants involved in human memory

Development of high-throughput genotyping platforms provides an opportunity to identify new genetic elements related to complex cognitive functions. Taking advantage of multi-level genomic analysis, here we studied the genetic basis of human short-term (STM, n = 1623) and long-term (LTM, n = 1522) memory functions. Heritability estimation based on single nucleotide polymorphism showed moderate (61%, standard error 35%) heritability of short-term memory but almost zero heritability of long-term memory. We further performed a two-step genome-wide association study, but failed to find any SNPs that could pass genome-wide significance and survive replication at the same time. However, suggestive significance for rs7011450 was found in the shared component of the two STM tasks. Further inspections on its nearby gene zinc finger and at-hook domain containing and SNPs around this gene showed suggestive association with STM. In LTM, a polymorphism within branched chain amino acid transaminase 2 showed suggestive significance in the discovery cohort and has been replicated in another independent population of 1862. Furthermore, we performed a pathway analysis based on the current genomic data and found pathways including mTOR signaling and axon guidance significantly associated with STM capacity. These findings warrant further replication in other larger populations.

Hippocampal Pathophysiology: Commonality Shared by Temporal Lobe Epilepsy and Psychiatric Disorders

Accumulating evidence points to the association of epilepsy, particularly, temporal lobe epilepsy (TLE), with psychiatric disorders, such as schizophrenia. Among these illnesses, the hippocampus is considered the regional focal point of the brain, playing an important role in cognition, psychosis, and seizure activity and potentially suggesting common etiologies and pathophysiology of TLE and schizophrenia. In the present review, we overview abnormal network connectivity between the dentate gyrus (DG) and the Cornus Ammonis area 3 (CA3) subregions of the hippocampus relative to the induction of epilepsy and schizophrenia. In light of our recent finding on the misguidance of hippocampal mossy fiber projection in the rodent model of schizophrenia, we discuss whether ectopic mossy fiber projection is a commonality in order to evoke TLE as well as symptoms related to schizophrenia.

Ectopic Mossy Fiber Pathfinding in the Hippocampus Caused the Abnormal Neuronal Transmission in the Mouse Models of Psychiatric Disease

Appropriate axonal pathfinding is a necessary step for the function of neuronal circuits. The mossy fibers (MFs) in the hippocampus of CaMKIIα heterozygous knockout (CaMKIIα-hKO) psychiatric model mice project onto not only the stratum lucidum but also the stratum oriens region in the CA3, which is a projection pattern distinct from that in normal mice. Thus, we examined the electrophysiological properties of the MF-CA3 connection in this mutant mouse on field recordings and found a lower synaptic connection. This study suggested that the phenotype of abnormal MF pathfindings could induce aberrant neuronal functions, which may link to cognition and memory.