Cecilia Conde

Temporal lobe epilepsy (TLE) is associated with dysregulation of transforming growth factor β ; (TGFβ) signaling, a key contributor to epileptogenesis. SARA (Smad Anchor for Receptor Activation), a central regulator of this pathway, is controlled by the E3 ubiquitin ligase Smurf2 through ubiquitination. However, the role of the SARA—Smurf2 axis in regulating TGFβ signaling during TLE has not previously been described, and whether this pathway can be therapeutically targeted remains unknown. Using a pilocarpine-induced status epilepticus (SE) model and astrocytes derived from patients with refractory TLE, we identified dysregulation of the SARA—Smurf2 pathway in both experimental systems. In SE rats, SARA and Glial Fibrillary Acidic Protein (GFAP) levels were significantly increased, whereas Smurf2 induction was insufficient to prevent SARA accumulation. In TLE-derived astrocytes, increased SARA and GFAP immunoreactivity was accompanied by reduced Smurf2 immunoreactivity and altered Smurf2 subcellular distribution. Losartan treatment restored SARA and Smurf2 immunoreactivity toward a control-like pattern in both models and reduced seizure frequency and duration in SE animals. These findings point towards a dysregulation of the SARA—Smurf2 axis as a molecular signature of TLE, support SARA as a potential therapeutic target, and provide experimental evidence for the repositioning of Losartan as a potential treatment alternative for drug-resistant epilepsy, warranting further translational and clinical investigation.

Vascular smooth muscle cells (VSMCs) in the tunica media are essential for maintaining the structure and function of the arterial wall. These cells regulate vascular tone and contribute to vasculogenesis and angiogenesis, particularly during development. Proper control of VSMC differentiation ensures the correct size and patterning of vessels. Dysregulation of VSMC behaviour in adulthood, however, is linked to serious cardiovascular diseases, including aortic aneurysm, coronary artery disease, atherosclerosis and pulmonary hypertension. VSMCs are characterised by their phenotypic plasticity, which is the capacity to transition from a contractile to a synthetic, dedifferentiated state in response to environmental cues. This phenotypic switch plays a central role in vascular remodelling, a process that drives the progression of many vascular pathologies. Epigenetic mechanisms, which are defined as heritable but reversible changes in gene expression that do not involve alterations to the DNA sequence, have emerged as key regulators of VSMC identity and behaviour. These mechanisms include DNA methylation, histone modifications, chromatin remodelling, non-coding RNA and RNA modifications. Understanding how these epigenetic processes influence VSMC plasticity is crucial to uncovering the molecular basis of vascular development and disease. This review explores the current understanding of VSMC biology, focusing on epigenetic regulation in health and pathology.

Editorial: Neuronal cytoskeleton and GTPases in health and diseases

Endocytic recycling is an intracellular process that returns internalized molecules back to the plasma membrane and plays crucial roles not only in the reuse of receptor molecules but also in the remodeling of the different components of this membrane. This process is required for a diversity of cellular events, including neuronal morphology acquisition and functional regulation, among others. The recycling endosome (RE) is a key vesicular component involved in endocytic recycling. Recycling back to the cell surface may occur with the participation of several different Rab proteins, which are master regulators of membrane/protein trafficking in nerve cells. The RE consists of a network of interconnected and functionally distinct tubular subdomains that originate from sorting endosomes and transport their cargoes along microtubule tracks, by fast or slow recycling pathways. Different populations of REs, particularly those formed by Rab11, Rab35, and Arf6, are associated with a myriad of signaling proteins. In this review, we discuss the cumulative evidence suggesting the existence of heterogeneous domains of REs, controlling different aspects of neurogenesis, with a particular focus on the commonalities and singularities of these REs and their contribution to nerve development and differentiation in several animal models.

Neural development is a complex process that involves critical events, including cytoskeleton dynamics and selective trafficking of proteins to defined cellular destinations. In this regard, Smad Anchor for Receptor Activation (SARA) is an early endosome resident protein, where perform trafficking- associated functions. In addition, SARA is also involved in cell signaling, including the TGFβ-dependent pathway. Accordingly, SARA, and TGFβ signaling are required for proper axonal specification and migration of cortical neurons, unveiling a critical role for neuronal development. However, the cooperative action between the TGFβ pathway and SARA to this process has remained understudied. In this work, we show novel evidence suggesting a cross-talk between SARA and TGFβ pathway needed for proper polarization, axonal specification, growth and cortical migration of central neurons both in vitro and in vivo. Using microscopy tools and cultured hippocampal neurons, we show a local interaction between SARA and TβRI (TGFβ I receptor) at endosomes. In addition, SARA loss of function, induced by the expression of the dominant-negative SARA-F728A, over-activates the TGFβ pathway, most likely by preserving phosphorylated TβRI. Consequently, SARA-mediated activation of TGFβ pathway impacts on neuronal development, promoting axonal growth and cortical migration of neurons during brain development. Moreover, our data suggests that SARA basally prevents the activation of TβRI through the recruitment of the inhibitory complex PP1c/GADD34 in polarizing neurons. Together, these results propose that SARA is a negative regulator of the TGFβ pathway, being critical for a proper orchestration for neuronal development.

The acquisition of neuronal polarity is a complex molecular process that involves several different cellular mechanisms that need to be finely coordinated to define the somatodendritic and axonal compartments. Amongst such mechanisms, cytoskeleton and membrane dynamics control both the morphological transitions that define neuronal polarity acquisition as well as provide molecular determinants to specific sites in neurons at a defined time point. Small GTPases from the Rab and Rho families are well known molecular determinants of neuronal differentiation. However, during axon specification, a molecular link that couples proteins from these two families has yet to be identified. In this paper, we describe the role of Tuba, a Cdc42-specific guanine nucleotide-exchange factor (GEF), in neuronal polarity through a Rab8a-dependent mechanism. Rab8a or Tuba gain-of-function generates neurons with supernumerary axons whereas Rab8a or Tuba loss-of-function abrogated axon specification, phenocopying the well-established effect of Cdc42 on neuronal polarity. Neuronal polarization associated to Rab8a is also evidencedin vivo, since a dominant negative version of Rab8a severely impaired neuronal migration.Remarkably, Rab8a activates Cdc42 in a Tuba-dependent manner, and dominant negative mutants of both GTPases reciprocally prevent the effect over polarity acquisition in the gain-of-function scenarios. Our results strongly suggest that a positive feedback loop linking Rab8a and Cdc42 activities via Tuba, is a primary event in neuronal polarization. In addition, we identified the GEF responsible for Cdc42 activation that is essential to specify axons in cultured neurons.

The development and maintenance of multicellular organisms require specialized coordination between external cellular signals and the proteins receiving stimuli and regulating responses. A critical role in the proper functioning of these processes is played by endosomal trafficking, which enables the transport of proteins to targeted sites as well as their return to the plasma membrane through its essential components, the endosomes. During this trafficking, signaling pathways controlling functions related to the endosomal system are activated both directly and indirectly. Although there are a considerable number of molecules participating in these processes, some are more known than others for their specific functions. Toward the end of the 90s, SARA protein was described as controlling and facilitating the localization of Smads to TGFβ-receptors during TGFβ signaling activation and, strikingly, SARA was also identified as one of the proteins that bind to early endosomes (EE) participating in membrane trafficking in several cell models. The purpose of this review is to analyze the state of the art of the contribution of SARA in different cell types and cellular contexts, focusing on the biological role of SARA in two main processes: trafficking and cellular signaling, both necessary for intercellular coordination, communication and development.

SARA (Smad Anchor for Receptor Activation) plays a crucial role in Rab5-mediated endocytosis in cell lines localizing to early endosomes where it regulates morphology and function. Here, we analyzed the role of SARA during neuronal development and tested whether it functions as a regulator of endocytic trafficking of selected axonal and membrane proteins. Suppression of SARA perturbs the appearance of juxtanuclear endocytic recycling compartments and the neurons show long axons with large growth cones. Furthermore, surface distribution of the cell adhesion molecule L1 in axons and the fusion of vesicles containing transferring receptor (TfR) in dendrites were increased in neurons where SARA was silenced. Conversely, SARA overexpression generated large early endosomes and reduced neurite outgrowth. Taken together, our findings suggest a significant contribution of SARA to key aspects of neuronal development, including neurite formation.