Angelika Hausser

The coordinated regulation of dendritic spine morphology and AMPA receptor (AMPAR) trafficking is fundamental to synaptic plasticity, yet the molecular mechanisms linking these two processes remain incompletely understood. Here we show that RIN1, a Rab5 guanine nucleotide exchange factor (GEF) enriched in hippocampal excitatory neurons, coordinates structural and functional plasticity through its dual ability to activate Rab5 and the tyrosine kinase c-Abl. Loss of RIN1 increases spine density and shifts the spine population towards mushroom morphologies, while decoupling the normal correlation between the size of spine head and the size and architecture of the postsynaptic density area. This structural decoupling is rescued by c-Abl-dependent but not Rab5 GEF-dependent RIN1 activity. Following chemically induced long-term depression (LTD), RIN1-deficient neurons show preferential AMPA receptor recycling over degradation, resulting in impaired LTD expression at the Schaffer collateral - CA1 pathway. Re-expression of wild-type RIN1 but not a GEF-deficient mutant restores receptor degradation and LTD, identifying RIN1-dependent Rab5 activation as the key mechanism directing internalised AMPA receptors towards the endo-lysosomal pathway during LTD. These findings reveal a functional dissociation within RIN1, whereby GEF activity governs AMPA receptor endosomal sorting during LTD expression, while c-Abl activation independently coordinates postsynaptic structural organisation.

How deep into the cell do mechanical signals reach? In this issue, Bhaskar Naidu et al. (https://doi.org/10.1083/jcb.202510026) show that the Golgi responds to cell-spreading force and tunes its secretory output in a loop that feeds back to support spreading.

Protein kinase D3 (PKD3) is an important regulator of triple-negative breast cancer (TNBC) progression by promoting invasion, proliferation, and stem cell maintenance. However, the mechanism underlying these cellular functions has remained unclear. Here, we report that endogenous PKD3 localizes to Rab7/Lamp1-positive vesicles in MDA-MB-231 cells cultured on stiff matrices. Notably, upon PKD3 depletion the size of Rab7-positive vesicles is smaller. This correlates with impaired endosomal acidification, which is associated with dysregulated Wnt signaling and a decline in stemness. Our data thus unveil a previously unrecognized role of PKD3 in regulating endolysosomal dynamics that contributes to the maintenance of the cancer stem cell population.

Protein kinase D (PKD) is a serine/threonine kinase family that controls important cellular functions, most notably playing a key role in the secretory pathway at the trans-Golgi network. Aberrant expression of PKD isoforms has been found mainly in breast cancer, where it promotes various cellular processes such as growth, invasion, survival and stem cell maintenance. In this review, we discuss the isoform-specific functions of PKD in breast cancer progression, with a particular focus on how the PKD controlled cellular processes might be linked to deregulated membrane trafficking and secretion. We further highlight the challenges of a therapeutic approach targeting PKD to prevent breast cancer progression.

Phosphorylation is a ubiquitous mechanism by which signals are transduced in cells. Protein kinases, enzymes that catalyze the phosphotransfer reaction are, themselves, often regulated by phosphorylation. Paradoxically, however, a substantial fraction of more than 500 human protein kinases are capable of catalyzing their own activation loop phosphorylation. Commonly, these kinases perform this autophosphorylation reaction in trans , whereby transient dimerization leads to the mutual phosphorylation of the activation loop of the opposing protomer. In this study, we demonstrate that protein kinase D (PKD) is regulated by the inverse mechanism of dimerization-mediated trans -autoinhibition, followed by activation loop autophosphorylation in cis . We show that PKD forms a stable face-to-face homodimer that is incapable of either autophosphorylation or substrate phosphorylation. Dissociation of this trans -autoinhibited dimer results in activation loop autophosphorylation, which occurs exclusively in cis . Phosphorylation serves to increase PKD activity and prevent trans -autoinhibition, thereby switching PKD on. Our findings not only reveal the mechanism of PKD regulation but also have profound implications for the regulation of many other eukaryotic kinases.

Actin cytoskeleton predominantly regulates the formation and maintenance of synapses by controlling dendritic spine morphology and motility. To visualize actin dynamics, actin molecules can be labelled by genetically fusing fluorescent proteins to actin monomers or using fluorescently tagged actin-binding proteins or single-chain anti-actin antibodies. However, the effects of these labels on the morphology of neurons have not been quantitatively compared yet. In the present study, we analysed Actin-Chromobody-GFP, LifeAct-GFP and EGFP-actin with respect to their effects on actin-related features in mouse cultured hippocampal neurons. The actin-binding probes LifeAct and Actin-Chromobody showed similar affinity to F-actin, and along with EGFP-actin, were enriched in dendritic protrusions. In contrast to EGFP-actin, neither of these constructs was able to detect subtle changes in actin remodelling between mature mushroom shaped spine and less developed filopodia. None of the compared probes altered filopodial motility compared to control EGFP expression, however, within 24 hours expression, minor changes in dendritic spine morphology and density were visible. Furthermore, while EGFP-actin and LifeAct-GFP expression did not alter dendritic arborization, AC-GFP expressing neurons displayed a reduced dendritic arborization. We therefore conclude that careful consideration of cellular consequences is required before performing experiments with a particular actin labelling probe in primary neurons.