👤 Niculin J Herz

The melanocortin receptor accessory protein 2 (MRAP2), which is abundantly expressed in the brain including the hypothalamus, has emerged as a key regulator of melanocortin-4 receptor (MC4R) activity. We sought to delineate the physiological significance of MRAP2 in MC4R neurons, with a particular focus on metabolic, autonomic and cardiovascular functions. Selective deletion of MRAP2 in MC4R neurons causes obesity that was associated with hyperphagia and impairment in glucose homeostasis and insulin sensitivity. MC4R agonist Melatonan II (MTII)-induced anorectic effects were blunted in mice lacking MRAP2 in MC4R neurons, whereas Celastrol retained its efficacy in reducing food intake and body weight. MRAP2 deletion also reduced baseline sympathetic nerve activity (SNA), particularly the SNA subserving the kidney. This was associated with reduced innervation of the kidney. In addition, MTII-induced increases in renal and brown adipose tissue (BAT) SNA as well as hepatic vagal nerve activity were significantly attenuated in MC4R neuron MRAP2-deficient mice. Transynaptic tracing revealed that MC4R neurons projecting to BAT and kidneys were localized to specific brain nuclei including the paraventricular nucleus of the hypothalamus, providing anatomical substrate for MRAP2 regulation of sympathetic outflow. Although the loss of MRAP2 in MC4R neurons did not affect arterial pressure, it caused a significant decrease in heart rate and baroreflex sensitivity. Finally, MRAP2 deficiency in MC4R neurons attenuated MTII-induced increase in arterial pressure and heart rate. These findings demonstrate that in addition to its role in energy balance and glucose homeostasis MRAP2 in MC4R neurons is crucial for cardiovascular autonomic regulation and is required for the development of obesity-associated hypertension and autonomic dysfunction. Show less

Studies of amyloid-β (Aβ) in Alzheimer's disease pathology have revealed the peptide's complex roles in synaptic function. The study by Siddu et al. in this issue clarifies the contexts in which Aβ peptides may be synaptogenic or synaptotoxic. This commentary integrates the study's major findings with the salient findings of others that, over recent years, have redefined Aβ from a troublesome waste product into a physiological agent of the innate immune response and a modulator of synaptic homeostasis. Convergent evidence demonstrates how free, nonaggregated Aβ supports synaptic structure and activity, whereas oligomeric assemblies enact an adaptive brake on excitatory drive that can become maladaptive with age and inflammation. This redefined perspective on Aβ function emphasizes an evolutionarily conserved feedback loop linking neuronal activity, amyloid generation, and synaptic tuning that protects energy balance under stress but, when dysregulated, promotes proteostatic failure, persistent neuroinflammation, and network dysfunction characteristic of Alzheimer's disease. Show less

Batten disease is characterized by early-onset blindness, juvenile dementia and death within the second decade of life. The most common genetic cause are mutations in CLN3, encoding a lysosomal protein. Currently, no therapies targeting disease progression are available, largely because its molecular mechanisms remain poorly understood. To understand how CLN3 loss affects cellular signaling, we generated human CLN3 knock-out cells (CLN3-KO) and performed RNA-seq analysis. Our multi-dimensional analysis reveals the transcriptional regulator YAP1 as a key factor in remodeling the transcriptome in CLN3-KO cells. YAP1-mediated pro-apoptotic signaling is also increased as a consequence of CLN3 functional loss in retinal pigment epithelia cells, and in the hippocampus and thalamus of Cln3 Show less

Batten disease, one of the most devastating types of neurodegenerative lysosomal storage disorders, is caused by mutations in CLN3. Here, we show that CLN3 is a vesicular trafficking hub connecting the Golgi and lysosome compartments. Proteomic analysis reveals that CLN3 interacts with several endo-lysosomal trafficking proteins, including the cation-independent mannose 6 phosphate receptor (CI-M6PR), which coordinates the targeting of lysosomal enzymes to lysosomes. CLN3 depletion results in mis-trafficking of CI-M6PR, mis-sorting of lysosomal enzymes, and defective autophagic lysosomal reformation. Conversely, CLN3 overexpression promotes the formation of multiple lysosomal tubules, which are autophagy and CI-M6PR-dependent, generating newly formed proto-lysosomes. Together, our findings reveal that CLN3 functions as a link between the M6P-dependent trafficking of lysosomal enzymes and lysosomal reformation pathway, explaining the global impairment of lysosomal function in Batten disease. Show less

Epigenetic modifications of chromatin play an important role in the regulation of gene expression. KMT4/Dot1 is a conserved histone methyltransferase capable of methylating chromatin on Lys79 of histone H3 (H3K79). Here we report the identification of a multisubunit Dot1 complex (DotCom), which includes several of the mixed lineage leukemia (MLL) partners in leukemia such as ENL, AF9/MLLT3, AF17/MLLT6, and AF10/MLLT10, as well as the known Wnt pathway modifiers TRRAP, Skp1, and beta-catenin. We demonstrated that the human DotCom is indeed capable of trimethylating H3K79 and, given the association of beta-catenin, Skp1, and TRRAP, we investigated, and found, a role for Dot1 in Wnt/Wingless signaling in an in vivo model system. Knockdown of Dot1 in Drosophila results in decreased expression of a subset of Wingless target genes. Furthermore, the loss of expression for the Drosophila homologs of the Dot1-associated proteins involved in the regulation of H3K79 shows a similar reduction in expression of these Wingless targets. From yeast to human, specific trimethylation of H3K79 by Dot1 requires the monoubiquitination of histone H2B by the Rad6/Bre1 complex. Here, we demonstrate that depletion of Bre1, the E3 ligase required for H2B monoubiquitination, leads specifically to reduced bulk H3K79 trimethylation levels and a reduction in expression of many Wingless targets. Overall, our study describes for the first time the components of DotCom and links the specific regulation of H3K79 trimethylation by Dot1 and its associated factors to the Wnt/Wingless signaling pathway. Show less

A functionally important, interface domain between transmembrane segments (TMSs) IV and XI of the NhaA Na+/H+ antiporter of Escherichia coli has been unraveled. Scanning by single Cys replacements identified new mutations (F136C, G125C, and A137C) that cluster in one face of TMS IV and increase dramatically the Km of the antiporter. Whereas G125C, in addition, causes a drastic alkaline shift to the pH dependence of the antiporter, G338C alleviates the pH control of NhaA. Scanning by double Cys replacements (21 pairs of one replacement per TMS) identified genetically eight pairs of residues that showed very strong negative complementation. Cross-linking of the double mutants identified six double mutants (T132C/G338C, D133C/G338C, F136C/S342C, T132C/S342C, A137C/S342C, and A137C/G338C) of which pronounced intramolecular cross-linking defined an interface domain between the two TMSs. Remarkably, cross-linking by a short and rigid reagent (N,N'-o-phenylenedimaleimide) revived the Li+/H+ antiport activity, whereas a shorter reagent (1,2-ethanediyl bismethanethiosulfonate) revived both Na+/H+ and Li+/H+ antiporter activities and even the pH response of the dead mutant T132C/G338C. Hence, cross-linking at this position restores an active conformation of NhaA. Show less