In response to stress, the heart undergoes extensive cardiac remodeling that results in cardiac fibrosis and pathological growth of cardiomyocytes (hypertrophy), which contribute to heart failure. Alterations in microRNA (miRNA) levels are associated with dysfunctional gene expression profiles associated with many cardiovascular disease conditions; however, miRNAs have emerged recently as paracrine signaling mediators. Thus, we investigated a potential paracrine miRNA crosstalk between cardiac fibroblasts and cardiomyocytes and found that cardiac fibroblasts secrete miRNA-enriched exosomes. Surprisingly, evaluation of the miRNA content of cardiac fibroblast–derived exosomes revealed a relatively high abundance of many miRNA passenger strands (“star” miRNAs), which normally undergo intracellular degradation. Using confocal imaging and coculture assays, we identified fibroblast exosomal–derived miR-21_3p (miR-21*) as a potent paracrine-acting RNA molecule that induces cardiomyocyte hypertrophy. Proteome profiling identified sorbin and SH3 domain-containing protein 2 (SORBS2) and PDZ and LIM domain 5 (PDLIM5) as miR-21* targets, and silencing SORBS2 or PDLIM5 in cardiomyocytes induced hypertrophy. Pharmacological inhibition of miR-21* in a mouse model of Ang II–induced cardiac hypertrophy attenuated pathology. These findings demonstrate that cardiac fibroblasts secrete star miRNA–enriched exosomes and identify fibroblast-derived miR-21* as a paracrine signaling mediator of cardiomyocyte hypertrophy that has potential as a therapeutic target.

Astrocytes have long been regarded as essentially unexcitable cells that do not contribute to active signaling and information processing in the brain. Contrary to this classical view, it is now firmly established that astrocytes can specifically respond to glutamate released from neurons. Astrocyte glutamate signaling is initiated upon binding of glutamate to ionotropic and/or metabotropic receptors, which can result in calcium signaling, a major form of glial excitability. Release of so-called gliotransmitters like glutamate, ATP and D-serine from astrocytes in response to activation of glutamate receptors has been demonstrated to modulate various aspects of neuronal function in the hippocampus. In addition to receptors, glutamate binds to high-affinity, sodium-dependent transporters, which results in rapid buffering of synaptically-released glutamate, followed by its removal from the synaptic cleft through uptake into astrocytes. The degree to which astrocytes modulate and control extracellular glutamate levels through glutamate transporters depends on their expression levels and on the ionic driving forces that decrease with ongoing activity. Another major determinant of astrocytic control of glutamate levels could be the precise morphological arrangement of fine perisynaptic processes close to synapses, defining the diffusional distance for glutamate, and the spatial proximity of transporters in relation to the synaptic cleft. In this review, we will present an overview of the mechanisms and physiological role of glutamate-induced ion signaling in astrocytes in the hippocampus as mediated by receptors and transporters. Moreover, we will discuss the relevance of astroglial glutamate uptake for extracellular glutamate homeostasis, focusing on how activity-induced dynamic changes of perisynaptic processes could shape synaptic transmission at glutamatergic synapses.

Serotonin receptors 5-HT1A and 5-HT7 are highly co-expressed in brain regions implicated in depression. However, their functional interaction has not been established. In the present study we show that 5-HT1A and 5-HT7 receptors form heterodimers both in vitro and in vivo. Foerster resonance energy transfer-based assays revealed that, in addition to heterodimers, homodimers composed either by 5-HT1A or 5-HT7 receptors together with monomers co-exist in cells. The highest affinity to form the complex was obtained for the 5-HT7-5-HT7 homodimers, followed by the 5-HT7-5-HT1A heterodimers and 5-HT1A-5-HT1A homodimers. Functionally, heterodimerization decreases 5-HT1A receptor-mediated activation of Gi-protein without affecting 5-HT7 receptor-mediated signalling. Moreover, heterodimerization markedly decreases the ability of the 5-HT1A receptor to activate G-protein gated inwardly rectifying potassium channels in a heterologous system. The inhibitory effect on such channels was also preserved in hippocampal neurons, demonstrating a physiological relevance of heteromerization in vivo. In addition, heterodimerization is critically involved in initiation of the serotonin-mediated 5-HT1A receptor internalization and also enhances the ability of the 5-HT1A receptor to activate the mitogen-activated protein kinases. Finally, we found that production of 5-HT7 receptors in hippocampus continuously decreases during postnatal development, indicating that the relative concentration of 5-HT1A-5-HT7 heterodimers and, consequently, their functional importance undergoes pronounced developmental changes.

Spinal muscular atrophy (SMA), a frequent neurodegenerative disease, is caused by reduced levels of functional survival of motoneuron (SMN) protein. SMN is involved in multiple pathways, including RNA metabolism and splicing as well as motoneuron development and function. Here we provide evidence for a major contribution of the Rho-kinase (ROCK) pathway in SMA pathogenesis. Using an in vivo protein interaction system based on SUMOylation of proteins, we found that SMN is directly interacting with profilin2a. Profilin2a binds to a stretch of proline residues in SMN, which is heavily impaired by a novel SMN2 missense mutation (S230L) derived from a SMA patient. In different SMA models, we identified differential phosphorylation of the ROCK-downstream targets cofilin, myosin-light chain phosphatase and profilin2a. We suggest that hyper-phosphorylation of profilin2a is the molecular link between SMN and the ROCK pathway repressing neurite outgrowth in neuronal cells. Finally, we found a neuron-specific increase in the F-/G-actin ratio that further support the role of actin dynamics in SMA pathogenesis.

The serotonergic system and in particular serotonin 1A receptor (5-HT1AR) are implicated in major depressive disorder (MDD). Here we demonstrated that 5-HT1AR is palmitoylated in human and rodent brains, and identified ZDHHC21 as a major palmitoyl acyltransferase, whose depletion reduced palmitoylation and consequently signaling functions of 5-HT1AR. Two rodent models for depression-like behavior show reduced brain ZDHHC21 expression and attenuated 5-HT1AR palmitoylation. Moreover, selective knock-down of ZDHHC21 in the murine forebrain induced depression-like behavior. We also identified the microRNA miR-30e as a negative regulator of Zdhhc21 expression. Through analysis of the post-mortem brain samples in individuals with MDD that died by suicide we find that miR-30e expression is increased, while ZDHHC21 expression, as well as palmitoylation of 5-HT1AR, are reduced within the prefrontal cortex. Our study suggests that downregulation of 5-HT1AR palmitoylation is a mechanism involved in depression, making the restoration of 5-HT1AR palmitoylation a promising clinical strategy for the treatment of MDD.