Mechanisms of Homeostatic Control of Neuronal Intrinsic Excitability

A neuron’s identity and function are dictated by its electrophysiological signature. The firing pattern of a neuron emerges from the particular combination of ion channels in its membrane. A neuron can “tune” the combination of ionic conductances that it expresses to return back to its target excitability when faced with changing conditions. While this phenomenon of firing rate homeostasis (FRH) is well-established, the mechanisms underlying it have remained mysterious. A prevalent theory proposes that firing rates are maintained through regulatory feedback relying on the detection and stabilization of a single variable, calcium. Within the framework of this theory, all perturbations with equivalent effects on neuronal activity should invoke the same homeostatic response. In a direct test of this hypothesis, we compared two independent experimental manipulations to the Shal potassium ion channel. While we observed FRH following either a conductance-blocking mutation or complete elimination of the Shal protein, the compensating currents and the molecular mechanisms underlying the homeostatic response differed between the two conditions. Neurons lacking the Shal protein enacted transcriptional upregulation of the ion channels Slo, Shab, and Shaker, in part through the transcription factor Krüppel. In contrast, neurons with a non-conducting Shal channel compensated through non-transcriptional modification of a different set of conductances. We propose that neurons have multiple, separable homeostatic signaling systems, including proteostatic and activity-sensitive feedback systems. We then further expand on the mechanisms of FRH to include a role for the Notch signaling system. This canonical pathway for neural development is reactivated following loss of Shal and is necessary for stabilization of firing rates. We propose a model in which the loss of the transcription factor Nerfin-1 de-represses the Notch, and Notch cleavage by presenilin followed by cooperation of NICD with Su(H) results in transcriptional rebalancing of ion channels. These findings have implications for the pathophysiology of human channelopathies and Alzheimer’s disease

Dual separable feedback systems govern firing rate homeostasis.

Firing rate homeostasis (FRH) stabilizes neural activity. A pervasive and intuitive theory argues that a single variable, calcium, is detected and stabilized through regulatory feedback. A prediction is that ion channel gene mutations with equivalent effects on neuronal excitability should invoke the same homeostatic response. In agreement, we demonstrate robust FRH following either elimination of Kv4/Shal protein or elimination of the Kv4/Shal conductance. However, the underlying homeostatic signaling mechanisms are distinct. Eliminating Shal protein invokes Krüppel-dependent rebalancing of ion channel gene expression including enhanced slo, Shab, and Shaker. By contrast, expression of these genes remains unchanged in animals harboring a CRISPR-engineered, Shal pore-blocking mutation where compensation is achieved by enhanced IKDR. These different homeostatic processes have distinct effects on homeostatic synaptic plasticity and animal behavior. We propose that FRH includes mechanisms of proteostatic feedback that act in parallel with activity-driven feedback, with implications for the pathophysiology of human channelopathies

Expression of the Bcl-2 Protein BAD Promotes Prostate Cancer Growth

BAD, a pro-apoptotic protein of the Bcl-2 family, has recently been identified as an integrator of several anti-apoptotic signaling pathways in prostate cancer cells. Thus, activation of EGFR, GPCRs or PI3K pathway leads to BAD phosphorylation and inhibition of apoptosis. Increased levels of BAD in prostate carcinomas have also been reported. It appears contradictory that instead of limiting expression of pro-apoptotic protein, prostate cancer cells choose to increase BAD levels while keeping it under tight phosphorylation control. Analysis of the effect of BAD on prostate cancer xenografts has shown that increased BAD expression enhances tumor growth, while knockdown of BAD expression by shRNA inhibits tumor growth. Tissue culture experiments demonstrated that increased BAD expression stimulates proliferation of prostate cancer cells. These results suggest that increased expression of BAD provides a proliferative advantage to prostate tumors, while BAD dephosphorylation increases sensitivity of prostate cancer cells to apoptosis. Combination of proliferative and apoptotic properties prompts prostate cancer cells to be “addicted” to increased levels of phosphorylated BAD. Thus, kinases that phosphorylate BAD are plausible therapeutic targets; while monitoring BAD phosphorylation could be used to predict tumor response to treatments

β2-adrenoreceptor Signaling Increases Therapy Resistance in Prostate Cancer by Upregulating MCL1

There is accumulating evidence that continuous activation of the sympathetic nervous system due to psychosocial stress increases resistance to therapy and accelerates tumor growth via β2-adrenoreceptor signaling (ADRB2). However, the effector mechanisms appear to be specific to tumor type. Here we show that activation of ADRB2 by epinephrine, increased in response to immobilization stress, delays the loss of MCL1 apoptosis regulator (MCL1) protein expression induced by cytotoxic drugs in prostate cancer cells; and thus, increases resistance of prostate cancer xenografts to cytotoxic therapies. The effect of epinephrine on MCL1 protein depended on protein kinase A (PKA) activity, but was independent from androgen receptor expression. Furthermore, elevated blood epinephrine levels correlated positively with an increased MCL1 protein expression in human prostate biopsies. In summary, we demonstrate that stress triggers an androgen-independent antiapoptotic signaling via the ADRB2/PKA/MCL1 pathway in prostate cancer cells. IMPLICATIONS: Presented results justify clinical studies of ADRB2 blockers as therapeutics and of MCL1 protein expression as potential biomarker predicting efficacy of apoptosis-targeting drugs in prostate cancer

Mechanisms of Homeostatic Control of Neuronal Intrinsic Excitability

A neuron’s identity and function are dictated by its electrophysiological signature. The firing pattern of a neuron emerges from the particular combination of ion channels in its membrane. A neuron can “tune” the combination of ionic conductances that it expresses to return back to its target excitability when faced with changing conditions. While this phenomenon of firing rate homeostasis (FRH) is well-established, the mechanisms underlying it have remained mysterious. A prevalent theory proposes that firing rates are maintained through regulatory feedback relying on the detection and stabilization of a single variable, calcium. Within the framework of this theory, all perturbations with equivalent effects on neuronal activity should invoke the same homeostatic response. In a direct test of this hypothesis, we compared two independent experimental manipulations to the Shal potassium ion channel. While we observed FRH following either a conductance-blocking mutation or complete elimination of the Shal protein, the compensating currents and the molecular mechanisms underlying the homeostatic response differed between the two conditions. Neurons lacking the Shal protein enacted transcriptional upregulation of the ion channels Slo, Shab, and Shaker, in part through the transcription factor Krüppel. In contrast, neurons with a non-conducting Shal channel compensated through non-transcriptional modification of a different set of conductances. We propose that neurons have multiple, separable homeostatic signaling systems, including proteostatic and activity-sensitive feedback systems. We then further expand on the mechanisms of FRH to include a role for the Notch signaling system. This canonical pathway for neural development is reactivated following loss of Shal and is necessary for stabilization of firing rates. We propose a model in which the loss of the transcription factor Nerfin-1 de-represses the Notch, and Notch cleavage by presenilin followed by cooperation of NICD with Su(H) results in transcriptional rebalancing of ion channels. These findings have implications for the pathophysiology of human channelopathies and Alzheimer’s disease

Surgical stress delays prostate involution in mice.

Androgens control growth of prostate epithelial cells and androgen deprivation induces apoptosis, leading to prostate involution. We investigated the effects of surgical stress on prostate involution induced by androgen ablation and determined the underlying mechanisms. Androgen ablation in mice was induced by surgical castration and administration of the anti-androgenic drugs bicalutamide and MDV3100. Surgical stress was induced by sham castration under isoflurane anesthesia. Surgical stress delayed apoptosis and prostate involution induced by anti-androgenic drugs. These effects of stress were prevented by administering the selective beta2-adrenoreceptor antagonist ICI118,551 and were also blocked in BAD(3SA/WT) mice expressing phosphorylation-deficient mutant BAD3SA. These results indicate that apoptosis and prostate involution in response to androgen ablation therapy could be delayed by surgical stress via the beta2-adrenoreceptor/BAD signaling pathway. Thus, surgery could interfere with androgen ablation therapy, whereas administration of beta2-adrenoreceptor antagonists may enhance its efficacy

Dual separable feedback systems govern firing rate homeostasis.

Firing rate homeostasis (FRH) stabilizes neural activity. A pervasive and intuitive theory argues that a single variable, calcium, is detected and stabilized through regulatory feedback. A prediction is that ion channel gene mutations with equivalent effects on neuronal excitability should invoke the same homeostatic response. In agreement, we demonstrate robust FRH following either elimination of Kv4/Shal protein or elimination of the Kv4/Shal conductance. However, the underlying homeostatic signaling mechanisms are distinct. Eliminating Shal protein invokes Krüppel-dependent rebalancing of ion channel gene expression including enhanced slo, Shab, and Shaker. By contrast, expression of these genes remains unchanged in animals harboring a CRISPR-engineered, Shal pore-blocking mutation where compensation is achieved by enhanced IKDR. These different homeostatic processes have distinct effects on homeostatic synaptic plasticity and animal behavior. We propose that FRH includes mechanisms of proteostatic feedback that act in parallel with activity-driven feedback, with implications for the pathophysiology of human channelopathies

Synthesis and PI3 Kinase Inhibition Activity of Some Novel Trisubstituted Morpholinopyrimidines

A number of new substituted morpholinopyrimidines were prepared utilizing sequential nucleophilic aromatic substitution and cross-coupling reactions. One of the disubstituted pyrimidines was converted into two trisubstituted compounds which were screened as PI3K inhibitors relative to the well-characterized PI3K inhibitor ZSTK474, and were found to be 1.5–3-times more potent. A leucine linker was attached to the most active inhibitor since it would remain on any peptide-containing prodrug after cleavage by prostate-specific antigen, and it did not prevent inhibition of AKT phosphorylation and hence the inhibition of PI3K by the modified inhibitor

Surgical stress delays prostate involution induced by androgen ablation.

<p>C575BL/6J mice received subcutaneous injections of bicalutamide (50 mg/kg, once daily) and MDV3100 (10 mg/kg, once daily) with or without sham castration. ICI118,551 (ICI) was given 30 minutes before sham castration. 2 days later prostates were excised, dissected, and weighted. At least 3 mice were used for each data point. Error bars represent SD from the average.</p