G protein-gated IKACh channels as therapeutic targets for treatment of sick sinus syndrome and heart block

Dysfunction of pacemaker activity in the sinoatrial node (SAN) underlies “sick sinus” syndrome (SSS), a common clinical condition characterized by abnormally low heart rate (bradycardia). If untreated, SSS carries potentially life-threatening symptoms, such as syncope and end-stage organ hypoperfusion. The only currently available therapy for SSS consists of electronic pacemaker implantation. Mice lacking L-type Ca_v1.3 Ca²⁺ channels (Ca_v1.3^−/−) recapitulate several symptoms of SSS in humans, including bradycardia and atrioventricular (AV) dysfunction (heart block). Here, we tested whether genetic ablation or pharmacological inhibition of the muscarinic-gated K⁺ channel (I_KACh) could rescue SSS and heart block in Ca_v1.3^−/− mice. We found that genetic inactivation of I_KACh abolished SSS symptoms in Ca_v1.3^−/− mice without reducing the relative degree of heart rate regulation. Rescuing of SAN and AV dysfunction could be obtained also by pharmacological inhibition of I_KACh either in Ca_v1.3^−/− mice or following selective inhibition of Ca_v1.3-mediated L-type Ca²⁺ (I_Ca,L) current in vivo. Ablation of I_KACh prevented dysfunction of SAN pacemaker activity by allowing net inward current to flow during the diastolic depolarization phase under cholinergic activation. Our data suggest that patients affected by SSS and heart block may benefit from I_KACh suppression achieved by gene therapy or selective pharmacological inhibition.Pacemaker activity of the sinoatrial node (SAN) controls heart rate under physiological conditions. Abnormal generation of SAN automaticity underlies “sick sinus” syndrome (SSS), a pathological condition manifested when heart rate is not sufficient to meet the physiological requirements of the organism (1). Typical hallmarks of SSS include SAN bradycardia, chronotropic incompetence, SAN arrest, and/or exit block (1–3). SSS carries incapacitating symptoms, such as fatigue and syncope (1–3). A significant percentage of patients with SSS present also with tachycardia-bradycardia syndrome (3). SSS can also be associated with atrioventricular (AV) conduction block (heart block) (1–3). Although aging is a known intrinsic cause of SSS (4), this disease appears also in the absence of any associated cardiac pathology and displays a genetic legacy (1, 2). Heart disease or drug intake can induce acquired SSS (2). Symptomatic SSS requires the implantation of an electronic pacemaker. SSS accounts for about half of all pacemaker implantations in the United States (5, 6). The incidence of SSS has been forecasted to increase during the next 50 y, particularly in the elder population (7). Furthermore, it has been estimated that at least half of SSS patients will need to be electronically paced (7). Although pacemakers are continuously ameliorated, they remain costly and require lifelong follow-up. Moreover, the implantation of an electronic pacemaker remains difficult in pediatric patients (8). Development of alternative and complementary pharmacological or molecular therapies for SSS management could improve quality of life and limit the need for implantation of electronic pacemakers.Recently, the genetic bases of some inherited forms of SSS have been elucidated (recently reviewed in 1, 9) with the discovery of mutations in genes encoding for ion channels involved in cardiac automaticity (4, 9, 10). Notably, loss of function of L-type Ca_v1.3 Ca²⁺ channels is central in some inherited forms of SSS. For instance, loss of function in Ca_v1.3-mediated L-type Ca²⁺ (I_Ca,L) current causes the sinoatrial node dysfunction and deafness syndrome (SANDD) (10). Affected individuals with SANDD present with profound deafness, bradycardia, and dysfunction of AV conduction (10). Mutation in ankyrin-B causes SSS by reduced membrane targeting of Ca_v1.3 channels (11). The relevance of Ca_v1.3 channels to SSS is demonstrated also by work on the pathophysiology of congenital heart block, where down-regulation of Ca_v1.3 channels by maternal Abs causes heart block in infants (12). Additionally, recent data show that chronic iron overload induces acquired SSS via a reduction in Ca_v1.3-mediated I_Ca,L (13).In mice and humans, Ca_v1.3 channels are expressed in the SAN, atria, and the AV node but are absent in adult ventricular tissue (14, 15). Ca_v1.3-mediated I_Ca,L plays a major role in the generation of the diastolic depolarization in SAN and AV myocytes, thereby constituting important determinants of heart rate and AV conduction velocity (14, 16). The heart rate of mice lacking Ca_v1.3 channels (Ca_v1.3^−/− mice) fairly recapitulates the hallmarks of SSS and associated symptoms, including bradycardia and tachycardia-bradycardia syndrome (17, 18). In addition, severe AV dysfunction is recorded in Ca_v1.3^−/− mice to variable degrees. Typically, these mice show first- and second-degree AV block (16, 17, 19). Complete AV block with dissociated atrial and ventricular rhythms can also be observed in these animals. The phenotype of Ca_v1.3^−/− mice thus constitutes a unique model for developing new therapeutic strategies against SSS (10).The muscarinic-gated K⁺ channel (I_KACh) is involved in the negative chronotropic effect of the parasympathetic nervous system on heart rate (20, 21). Two subunits of the G-protein activated inwardly rectifying K⁺ channels (GIRK1 and GIRK4) of the GIRK/Kir3 subfamily assemble as heterotetramers to form cardiac I_KACh channels (22). Indeed, both Girk1^−/− and Girk4^−/− mice lack cardiac I_KACh (20, 21, 23). We recently showed that silencing of the hyperpolarization-activated current “funny” (I_f) channel in mice induces a complex arrhythmic profile that can be rescued by concurrent genetic ablation of Girk4 (24). In this study, we tested the effects of genetic ablation and pharmacological inhibition of I_KACh on the Ca_v1.3^−/− mouse model of SSS. We found that Girk4 ablation or pharmacological inhibition of I_KACh rescues SSS and AV dysfunction in Ca_v1.3^−/−. Thus, our study shows that I_KACh targeting may be pursued as a therapeutic strategy for treatment of SSS and heart block.     

Effect of antituberculosis drug resistance on response to treatment and outcome in adults with tuberculous meningitis

BACKGROUND: Tuberculous meningitis (TBM) caused by Mycobacterium tuberculosis resistant to 1 or more antituberculosis drugs is an increasingly common clinical problem, although the impact on outcome is uncertain. METHODS: We performed a prospective study of 180 Vietnamese adults admitted consecutively for TBM. M. tuberculosis was cultured from the cerebrospinal fluid (CSF) of all patients and was tested for susceptibility to first-line antituberculosis drugs. Presenting clinical features, time to CSF bacterial clearance, clinical response to treatment, and 9-month morbidity and mortality were compared between adults infected with susceptible and those infected with drug-resistant organisms. RESULTS: Of 180 isolates, 72 (40.0%) were resistant to at least 1 antituberculosis drug, and 10 (5.6%) were resistant to at least isoniazid and rifampicin. Isoniazid and/or streptomycin resistance was associated with slower CSF bacterial clearance but not with any differences in clinical response or outcome. Combined isoniazid and rifampicin resistance was strongly predictive of death (relative risk of death, 11.63 [95% confidence interval, 5.21-26.32]) and was independently associated with human immunodeficiency virus infection. CONCLUSIONS: Isoniazid and/or streptomycin resistance probably has no detrimental effect on the outcome of TBM when patients are treated with first-line antituberculosis drugs, but combined isoniazid and rifampicin resistance is strongly predictive of death.     

In vivo induction of massive proliferation, directed migration, and differentiation of neural cells in the adult mammalian brain

The development of an in vivo procedure for the induction of massive proliferation, directed migration, and neurodifferentiation (PMD) in the damaged adult central nervous system would hold promise for the treatment of human neurodegenerative disorders such as Parkinson's disease. We investigated the in vivo induction of PMD in the forebrain of the adult rat by using a combination of 6-hydroxydopamine lesion of the substantia nigra dopaminergic neurons and infusions of transforming growth factor alpha (TGFalpha) into forebrain structures. Only in animals with both lesion and infusion of TGFalpha was there a rapid proliferation of forebrain stem cells followed by a timed migration of a ridge of neuronal and glial progenitors directed toward the region of the TGFalpha infusion site. Subsequently, increasing numbers of differentiated neurons were observed in the striatum. In behavioral experiments, there was a significant reduction of apomorphine-induced rotations in animals receiving the TGFalpha infusions. These results show that the brain contains stem cells capable of PMD in response to an exogenously administered growth factor. This finding has significant implications with respect to the development of treatments for both acute neural trauma and neurodegenerative diseases.