Significance Communication between nerve cells occurs at specialized cellular structures known as synapses. Loss of synaptic function is associated with cognitive decline in Alzheimer’s disease (AD). However, the mechanism of synaptic damage remains incompletely understood. Here we describe a pathway for synaptic damage whereby amyloid-β 1–42 peptide (Aβ 1–42 ) releases, via stimulation of α7 nicotinic receptors, excessive amounts of glutamate from astrocytes, in turn activating extrasynaptic NMDA-type glutamate receptors (eNMDARs) to mediate synaptic damage. The Food and Drug Administration-approved drug memantine offers some beneficial effect, but the improved eNMDAR antagonist NitroMemantine completely ameliorates Aβ-induced synaptic loss, providing hope for disease-modifying intervention in AD.

The activity of Cdk5 and its regulatory subunit p35 is thought to be important in both normal brain function and neurodegenerative disease pathogenesis. Increased Cdk5 activity, via proteolytic cleavage of p35 to a p25 fragment by the calcium-activated protease calpain or by phosphorylation at Cdk5(Tyr15), can contribute to neurotoxicity. Nonetheless, our knowledge of regulation of Cdk5 activity in disease states is still emerging. Here we demonstrate that Cdk5 is activated by S-nitrosylation or reaction of nitric oxide (NO)-related species with the thiol groups of cysteine residues 83 and 157, to form SNO-Cdk5. We then show that S-nitrosylation of Cdk5 contributes to amyloid-β (Aβ) peptide-induced dendritic spine loss. Furthermore, we observed significant levels of SNO-Cdk5 in postmortem Alzheimer's disease (AD) but not in normal human brains. These findings suggest that S-nitrosylation of Cdk5 is an aberrant regulatory mechanism of enzyme activity that may contribute to the pathogenesis of AD.C dk5 is a cyclin-dependent kinase that is activated by proteins p35, p25, and p39 (1-3). As a predominantly neuronalspecific kinase, Cdk5 lacks a role in cell-cycle control but has been implicated in an array of neuronal functions, including cell survival, axon guidance, neuronal migration, and regulation of synaptic spine density (4-6). Dysregulation of Cdk5 activity may play a role in the pathogenesis of stroke and several neurodegenerative disorders, including Alzheimer's disease (AD), amyotrophic lateral sclerosis, Parkinson's disease, and Huntington's disease (7-11). Cdk5 is also hyperactivated in response to oxidative stress, mitochondrial dysfunction, excitotoxicity, amyloid-β (Aβ) exposure, calcium overload, and neuroinflammation, thus contributing to neuronal damage. These neurotoxic stimuli activate calpain, which cleaves the Cdk5 activator p35 (or p39) into p25 (or p29); p25 accumulation thus contributes to Cdk5 activation (12-16). These changes trigger various events associated with neurodegeneration. Although increased Cdk5 activity has been observed in AD brains compared with nondemented control brains, the mechanism remains contentious (17).Similarly, nitric oxide (NO) and related species contribute to a number of neurodegenerative diseases. The major source of NO in neurons is neuronal nitric oxide synthase (NOS1). Excitotoxic stress increases intracellular Ca 2+ , which in turn activates NOS1, thus generating NO. In general, NO can stimulate soluble guanylate cyclase to form cGMP or can S-nitrosylate critical cysteine residues to regulate the activity of multiple target proteins, in some sense akin to phosphorylation. Indeed, S-nitrosylation may explain many cGMP-independent mechanisms of NO action in neurodegenerative diseases (18). Our group has demonstrated that NO contributes to neurodegenerative disorders by redox reaction consisting of S-nitrosylation; in some cases this reaction is followed by further oxidation. Proteins affected in this manner include the gelatinase enzyme matrix metalloprotineas...

We propose a unique mode of TRPM6 regulation in which the Mg(2+) influx is controlled by RACK1 through its interaction with the alpha-kinase and the phosphorylation state of the threonine(1851) residue.

-transporting distal convoluted tubules (DCT). We demonstrated that REA significantly inhibits TRPM6, but not its closest homologue TRPM7, channel activity. This inhibition occurs in a phosphorylation-dependent manner, since REA has no effect on the TRPM6 phosphotransferase-deficient mutant (K1804R), while it still binds to this mutant. Moreover, activation of protein kinase C by phorbol 12-myristate 13-acetate-PMA potentiated the inhibitory effect of REA on TRPM6 channel activity. Finally, we showed that the interaction between REA and TRPM6 is a dynamic process, as short-term 17␤-estradiol treatment disassociates the binding between these proteins. In agreement with this, 17␤-estradiol treatment significantly stimulates the TRPM6-mediated current in HEK293 cells. These results suggest a rapid pathway for the effect of estrogen on Mg 2؉ homeostasis in addition to its transcriptional effect. Together, these data indicate that REA operates as a negative feedback modulator of TRPM6 in the regulation of active Mg 2؉ (re)absorption and provides new insight into the molecular mechanism of renal transepithelial Mg 2؉ transport.

Transient receptor potential melastatin 6 (TRPM6) plays an essential role in epithelial Mg 2؉ transport. TRPM6 and its closest homologue, TRPM7, both combine a cation channel with an ␣-kinase domain. However, the role of this ␣-kinase domain in TRPM6 channel activity remains elusive. The aim of this study was to investigate the regulation of TRPM6 channel activity by intracellular ATP and the involvement of its ␣-kinase domain. We demonstrated that intracellular Na-and Mg-ATP decreased the TRPM6 current in HEK293 cells heterogeneously expressing the channel, whereas Na-CTP or Na-GTP had no effect on channel activity. Whole cell recordings in TRPM6-expressing HEK293 cells showed that deletion of the ␣-kinase domain prevented the inhibitory effect of intracellular ATP without abrogating channel activity. Mutation of the conserved putative ATP-binding motif GXG(A)XXG (G1955D) in the ␣-kinase domain of TRPM6 inhibited the ATP action, whereas this effect remained preserved in the TRPM6 phosphotransferase-deficient mutant K1804R. Mutation of the TRPM6 autophosphorylation site, Thr 1851 , into either an alanine or an aspartate, resulted in functional channels that could still be inhibited by ATP. In conclusion, intracellular ATP regulates TRPM6 channel activity via its ␣-kinase domain independently of ␣-kinase activity.Within the transient receptor potential family of cation channels, three members from the Melastatin subgroup, TRPM6, 3 TRPM7, and TRPM2, display unique primary structures known as "chanzymes," i.e. fusion of an ion channel poreforming region with an enzymatic domain (1-5). Remarkably, the proteins TRPM6 and TRPM7 share 52% sequence identity and play a crucial role in Mg 2ϩ homeostasis (6, 7). Mutations in the TRPM6 gene are responsible for the autosomal recessive form of familiar hypomagnesemia with secondary hypocalcemia (8, 9), whereas a missense mutation in TRPM7 is responsible for the pathogenesis of Guamanian neurodegenerative disorders (10). In addition, the lethal phenotype caused by inactivation of the TRPM7 gene in lymphocytes cells can be rescued by extracellular Mg 2ϩ supplementation (1, 11). Although the physiological activation mechanism of TRPM6 remains unknown, there is evidence of a constitutive activity in heterologous expression systems. TRPM6 displays a nonselective cation current conducted by divalent ions inwardly and monovalent ions outwardly and exhibits a steep outwardly rectifying current-voltage relation (12). A recent study reported that epidermal growth factor hormone acts as an autocrine/ paracrine magnesiotropic hormone, specifically stimulating TRPM6 current via engagement of its receptor at the basolateral membrane of distal convoluted tubule cells (13). Previous studies showed that both TRPM6 and TRPM7 currents are inhibited by millimolar concentrations of intracellular Mg 2ϩ (1,11,12). In rat basophilic leukemia and Jurkat T cells, a native conductance with properties similar to cells heterogeneously expressing TRPM7 was described and termed magnesium nucleotide-depende...