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Dendritic spines are sites of synaptic plasticity and their head size correlates with the strength of the corresponding synapse. We recently showed that the distribution of spine head sizes follows a lognormal-like distribution even after blockage of activity or plasticity induction. As the cytokine tumor necrosis factor (TNF) influences synaptic transmission and constitutive TNF and receptor (TNF-R)-deficiencies cause changes in spine head size distributions, we tested whether these genetic alterations disrupt the lognormality of spine head sizes. Furthermore, we distinguished between spines containing the actin-modulating protein synaptopodin (SP-positive), which is present in large, strong and stable spines and those lacking it (SP-negative). Our analysis revealed that neither TNF-deficiency nor the absence of TNF-R1, TNF-R2 or TNF-R 1 and 2 (TNF-R1/R2) degrades the general lognormal-like, skewed distribution of spine head sizes (all spines, SP-positive spines, SP-negative spines). However, TNF, TNF-R1 and TNF-R2-deficiency affected the width of the lognormal distribution, and TNF-R1/2-deficiency shifted the distribution to the left. Our findings demonstrate the robustness of the lognormal-like, skewed distribution, which is maintained even in the face of genetic manipulations that alter the distribution of spine head sizes. Our observations are in line with homeostatic adaptation mechanisms of neurons regulating the distribution of spines and their head sizes., (© 2024 The Author(s). The Journal of Comparative Neurology published by Wiley Periodicals LLC.)

The cytokine tumor necrosis factor (TNF) is involved in the regulation of physiological and pathophysiological processes in the central nervous system. In previous work, we showed that mice lacking constitutive levels of TNF exhibit a reduction in spine density and changes in spine head size distribution of dentate granule cells. Here, we investigated which TNF-receptor pathway is responsible for this phenotype and analyzed granule cell spine morphology in TNF-R1-, TNF-R2-, and TNF-R1/R2-deficient mice. Single granule cells were filled with Alexa568 in fixed hippocampal brain slices and immunostained for the actin-modulating protein synaptopodin (SP), a marker for strong and stable spines. An investigator blind to genotype investigated dendritic spines using deconvolved confocal image stacks. Similar to TNF-deficient mice, TNF-R1 and TNF-R2 mutants showed a decrease in the size of small spines (SP-negative) with TNF-R1/R2-KO mice exhibiting an additive effect. TNF-R1 mutants also showed an increase in the size of large spines (SP-positive), mirroring the situation in TNF-deficient mice. Unlike the TNF-deficient mouse, none of the TNF-R mutants exhibited a reduction in their granule cell spine densities. Since TNF tunes the excitability of networks, lack of constitutive TNF reduces network excitation. This may explain why we observed alterations in spine head size distributions in TNF- and TNF-R-deficient granule cells. The changes in spine density observed in the TNF-deficient mouse could not be linked to canonical TNF-R-signaling. Instead, noncanonical pathways or unknown developmental functions of TNF may cause this phenomenon., (© 2022 The Authors. The Journal of Comparative Neurology published by Wiley Periodicals LLC.)

The majority of excitatory synapses terminating on cortical neurons are found on dendritic spines. The geometry of spines, in particular the size of the spine head, tightly correlates with the strength of the excitatory synapse formed with the spine. Under conditions of synaptic plasticity, spine geometry may change, reflecting functional adaptations. Since the cytokine tumor necrosis factor (TNF) has been shown to influence synaptic transmission as well as Hebbian and homeostatic forms of synaptic plasticity, we speculated that TNF-deficiency may cause concomitant structural changes at the level of dendritic spines. To address this question, we analyzed spine density and spine head area of Alexa568-filled granule cells in the dentate gyrus of adult C57BL/6J and TNF-deficient (TNF-KO) mice. Tissue sections were double-stained for the actin-modulating and plasticity-related protein synaptopodin (SP), a molecular marker for strong and stable spines. Dendritic segments of TNF-deficient granule cells exhibited ∼20% fewer spines in the outer molecular layer of the dentate gyrus compared to controls, indicating a reduced afferent innervation. Of note, these segments also had larger spines containing larger SP-clusters. This pattern of changes is strikingly similar to the one seen after denervation-associated spine loss following experimental entorhinal denervation of granule cells: Denervated granule cells increase the SP-content and strength of their remaining spines to homeostatically compensate for those that were lost. Our data suggest a similar compensatory mechanism in TNF-deficient granule cells in response to a reduction in their afferent innervation., (© 2021 The Authors. The Journal of Comparative Neurology published by Wiley Periodicals LLC.)

Extracellular matrix molecules are involved in the cellular functions of proliferation, migration, morphological differentiation, and synaptic plasticity. One candidate molecule of the extracellular matrix is the chondroitin sulfate proteoglycan neurocan. To determine whether neurocan expression is regulated by neuronal activity in the adult rat brain, we studied changes in hippocampal neurocan mRNA and protein expression following electrical stimulation of the perforant pathway in urethane-anesthetized rats. After 24 h of intermittent, unilateral 20 Hz stimulation, in situ hybridization revealed increased neurocan mRNA in glial fibrillary acidic protein (GFAP)-positive astrocytes bilaterally in all hippocampal subfields. These changes were quantified in the dentate molecular layer, the termination zone of the perforant pathway, using laser microdissection in combination with quantitative reverse transcription-polymerase chain reaction (RT-PCR). Immediately after 24 h stimulation, a six-fold upregulation was detected, which returned to control levels by 3 days post-stimulation. Neurocan immunoreactivity was similarly upregulated bilaterally. Immunostaining intensity reached a maximum by 4 days and returned to control levels by 14 days. The pattern of neurocan expression in the hippocampus depended on the intensity and duration of electrical stimulation. Under conditions of less intense afferent stimulation (4-24 h of 2.0 Hz paired-pulse stimulation, interpulse interval 40 ms), increases in neurocan mRNA and immunoreactivity were restricted to the ipsilateral termination zone of the stimulated perforant pathway. This layer-specific neurocan upregulation was not affected by intraperitoneal application of the NMDA-receptor antagonist MK-801. In conclusion, our data indicate that synaptic activity regulates the astrocytic expression of neurocan in a graded manner., (Copyright 2006 Wiley-Liss, Inc.)