PKN1 Is a Novel Regulator of Hippocampal GluA1 Levels

Alterations in the processes that control α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) expression, assembly and trafficking are closely linked to psychiatric and neurodegenerative disorders. We have recently shown that the serine/threonine kinase Protein kinase N1 (PKN1) is a developmentally active regulator of cerebellar synaptic maturation by inhibiting AKT and the neurogenic transcription factor neurogenic differentiation factor-2 (NeuroD2). NeuroD2 is involved in glutamatergic synaptic maturation by regulating expression levels of various synaptic proteins. Here we aimed to study the effect of Pkn1 knockout on AKT phosphorylation and NeuroD2 levels in the hippocampus and the subsequent expression levels of the NeuroD2 targets and AMPAR subunits: glutamate receptor 1 (GluA1) and GluA2/3. We show that PKN1 is expressed throughout the hippocampus. Interestingly, not only postnatal but also adult hippocampal phospho-AKT and NeuroD2 levels were significantly elevated upon Pkn1 knockout. Postnatal and adult Pkn1 -/- hippocampi showed enhanced expression of the AMPAR subunit GluA1, particularly in area CA1. Surprisingly, GluA2/3 levels were not different between both genotypes. In addition to higher protein levels, we also found an enhanced GluA1 content in the membrane fraction of postnatal and adult Pkn1 -/- animals, while GluA2/3 levels remained unchanged. This points toward a very specific regulation of GluA1 expression and/or trafficking by the novel PKN1-AKT-NeuroD2 axis. Considering the important role of GluA1 in hippocampal development as well as the pathophysiology of several disorders, ranging from Alzheimer's, to depression and schizophrenia, our results validate PKN1 for future studies into neurological disorders related to altered AMPAR subunit expression in the hippocampus.

Effects of Hypobaric Hypoxia in Various Modes on Expression of Neurogenesis Marker NeuroD2 in the Dentate Gyrus of Rats Hippocampus

The expression of neurogenesis marker--NeuroD2 transcription factor--in the hippocampal dentate gyrus was studied in rats exposed to severe destructive hypoxia, a single or three episodes of moderate hypobaric hypoxia, preconditioned severe hypoxia, and severe hypoxia followed by 3 sessions of postconditioning by moderate hypobaric hypoxia. All the studied hypoxic exposure modes led to an increase of NeuroD2 level. Three-fold moderate hypoxia per se and in the preconditioning mode (followed by exposure to severe hypoxia) produced most pronounced up-regulatory effect on NeuroD2 expression. The results indicated that stimulation of neurogenesis processes seemed to be one of the aspects of the neuroprotective effect of three-fold preconditioning moderate hypoxia, but not of hypoxic postconditioning.

Role of PKN1 in Retinal Cell Type Formation

We recently identified PKN1 as a developmentally active gatekeeper of the transcription factor neuronal differentiation-2 (NeuroD2) in several brain areas. Since NeuroD2 plays an important role in amacrine cell (AC) and retinal ganglion cell (RGC) type formation, we aimed to study the expression of NeuroD2 in the postnatal retina of WT and Pkn1-/- animals, with a particular focus on these two cell types. We show that PKN1 is broadly expressed in the retina and that the gross retinal structure is not different between both genotypes. Postnatal retinal NeuroD2 levels were elevated upon Pkn1 knockout, with Pkn1-/- retinae showing more NeuroD2+ cells in the lower portion of the inner nuclear layer. Accordingly, immunohistochemical analysis revealed an increased amount of AC in postnatal and adult Pkn1-/- retinae. There were no differences in horizontal cell, bipolar cell, glial cell and RGC numbers, nor defective axon guidance to the optic chiasm or tract upon Pkn1 knockout. Interestingly, we did, however, see a specific reduction in SMI-32+ α-RGC in Pkn1-/- retinae. These results suggest that PKN1 is important for retinal cell type formation and validate PKN1 for future studies focusing on AC and α-RGC specification and development.

Knockdown of NeuroD2 leads to seizure-like behavior, brain neuronal hyperactivity and a leaky blood-brain barrier in a Xenopus laevis tadpole model of DEE75

Developmental and Epileptic Encephalopathies (DEE) are a genetically diverse group of severe, early onset seizure disorders. DEE are normally identified clinically in the first six months of life by the presence of frequent, difficult to control seizures and accompanying stalling or regression of development. DEE75 results from de novo mutations of the NEUROD2 gene that result in loss of activity of the encoded transcription factor, and the seizure phenotype was shown to be recapitulated in Xenopus tropicalis tadpoles. We used CRISPR/Cas9 to make a DEE75 model in Xenopus laevis, to further investigate the developmental etiology. NeuroD2.S CRISPR/Cas9 edited tadpoles were more active, swam faster on average, and had more seizures (C-shaped contractions resembling unprovoked C-start escape responses) than their sibling controls. Live imaging of Ca2+ signaling revealed prolongued, strong signals sweeping through the brain, indicative of neuronal hyperactivity. While the resulting tadpole brain appeared grossly normal, the blood-brain barrier (BBB) was found to be leakier than that of controls. Additionally, the TGFβ antagonist Losartan was shown to have a short-term protective effect, reducing neuronal hyperactivity and reducing permeability of the BBB. Treatment of NeuroD2 CRISPant tadpoles with 5 mM Losartan decreased seizure events by more than 4-fold compared to the baseline. Our results support a model of DEE75 resulting from reduced NeuroD2 activity during vertebrate brain development, and indicate that a leaky BBB contributes to epileptogenesis.

Neurogenic differentiation 2 promotes inflammatory activation of macrophages in doxorubicin-induced myocarditis via regulating protein kinase D

BACKGROUND

Although it has been established that protein kinase D (PKD) plays a crucial role in various diseases, its precise role in myocarditis remains elusive.

METHODS

To investigate PKD's involvement in myocarditis, we established a mouse model of myocarditis using doxorubicin (DOX) to assess cardiac function, observe pathological changes, and quantify inflammatory cytokine levels in heart tissues. Additionally, macrophages were isolated from heart tissues of both control and DOX-treated groups to assess PKD expression and inflammatory cytokines in these macrophages. We explored the molecular mechanism of Neurogenic Differentiation 2 (NeuroD2) in myocarditis by utilizing NeuroD2 overexpression plasmids and NeuroD2 small interfering RNA (siRNA). Furthermore, we conducted dual-luciferase reporter and chromatin immunoprecipitation (ChIP) assays to investigate the interaction between NeuroD2 and PKD.

RESULTS

We observed significant upregulation of PKD in macrophages and heart tissues induced by DOX. The administration of a PKD inhibitor reduced inflammatory cytokine levels, improved cardiac function, and mitigated pathological changes in myocarditis-afflicted mice. Mechanistically, we found upregulated expression of NeuroD2 in both macrophages and heart tissues exposed to DOX. NeuroD2 could directly target PKD, enhancing the NLRP3/NF-κB signaling pathway and exacerbating macrophage inflammation.

CONCLUSIONS

Our study demonstrates that NeuroD2 can directly bind to the PKD promoter, potentially promoting inflammatory activation of macrophages in DOX-induced myocarditis via the NLRP3/NF-κB pathway. This suggests that the NeuroD2/PKD axis may hold promise as a potential therapeutic approach for treating DOX-induced myocarditis.