👤 Ralph J Florijn

The melanocortin-3 receptor (MC3R) and the melanocortin-4 receptor (MC4R), both expressed in hypothalamic nuclei, are key downstream effectors of leptin signaling and play important roles in energy homeostasis. While pathogenic variants in the MC4R gene represent the most common cause of monogenic obesity, the clinical significance of MC3R variants is less clear. MC4R localizes to the primary cilium, a sensory organelle present on nearly all human cells. To better understand the pathophysiological mechanisms of MC3R variants, we investigated whether MC3R localizes to the primary cilium and assessed the impact of rare MC3R variants identified in individuals with obesity on ciliary expression. Using human RPE cells, human NGN2-induced iNeurons, and primary mouse hypothalamic neurons, we found that, in contrast to MC4R, neither wild type MC3R nor rare MC3R variants localized specifically to the primary cilium in vitro in any cell type, including hypothalamic neurons. These findings suggest that MC3R and MC4R may utilize distinct signaling pathways or that additional factors, such as accessory proteins, are required for MC3R targeting to primary cilia in vivo. Further studies are needed to clarify the role of MC3R variants in monogenic obesity and their broader implications for human disease. Show less

Cerebral amyloid angiopathy (CAA) is a common age-related small vessel disease characterized by amyloid-beta (Aβ) accumulation in cortical and leptomeningeal blood vessel walls. Reduced Aβ clearance in the vasculature elevates the risk of CAA, while increasing evidence indicates that enhanced Aβ production in neurons also contributes. The impact of Aβ on the diverse cell types of the neurovascular unit (NVU)-including endothelial cells (ECs), pericytes, neurons, vascular smooth muscle cells (VSMCs), and astrocytes-remains unclear. This narrative review proposes that Aβ accumulation in NVUs during CAA drives a transcriptional response that reduces Aβ clearance while activating a neuron-specific post- transcriptional response that enhances Aβ production. Specifically, Aβ in NVUs was found to initiate a transcriptional cascade that destabilizes endothelial cells, increases blood-brain barrier permeability, and damages pericytes, ultimately inducing inflammatory and dysfunctional changes in VSMCs. These changes cause mitochondrial dysfunction and TGFβ deregulation in neurons, activating profibrotic mechanisms. Post-transcriptional regulation by microRNA networks in neurons affects Aβ processing by controlling the balance between amyloidogenic and non-amyloidogenic pathways through BACE1 and ADAM10 expression respectively. This review improves our understanding of Aβ accumulation in neurovascular units in CAA, potentially leading to better diagnostics, early biomarkers, and tools for prognosis and treatment. Show less