Using experimental and computational energy equilibration to understand hierarchical self-assembly of Fmoc-dipeptide amphiphiles

Despite progress, a fundamental understanding of the relationships between the molecular structure and self-assembly configuration of Fmoc-dipeptides is still in its infancy. In this work, we provide a combined experimental and computational approach that makes use of free energy equilibration of a number of related Fmoc-dipeptides to arrive at an atomistic model of Fmoc-threonine-phenylalanine-amide (Fmoc-TF-NH₂) which forms twisted fibres. By using dynamic peptide libraries where closely related dipeptide sequences are dynamically exchanged to eventually favour the formation of the thermodynamically most stable configuration, the relative importance of C-terminus modifications (amide versus methyl ester) and contributions of aliphatic versus aromatic amino acids (phenylalanine F vs. leucine L) is determined (F > L and NH₂ > OMe). The approach enables a comparative interpretation of spectroscopic data, which can then be used to aid the construction of the atomistic model of the most stable structure (Fmoc-TF-NH₂). The comparison of the relative stabilities of the models using molecular dynamic simulations and the correlation with experimental data using dynamic peptide libraries and a range of spectroscopy methods (FTIR, CD, fluorescence) allow for the determination of the nanostructure with atomistic resolution. The final model obtained through this process is able to reproduce the experimentally observed formation of intertwining fibres for Fmoc-TF-NH₂, providing information of the interactions involved in the hierarchical supramolecular self-assembly. The developed methodology and approach should be of general use for the characterization of supramolecular structures.

Repurposing of sodium channel antagonists as potential new anti-myotonic drugs

Myotonia is often a painful and disabling symptom which can interfere with daily motor function resulting in significant morbidity. Since myotonic disorders are rare it has generally proved difficult to obtain class I level evidence for anti-myotonic drug efficacy by performing randomized placebo controlled trials. Current treatment guidance is therefore largely based on anecdotal reports and physician experience. Despite the genetic channel heterogeneity of the myotonic disorders the sodium channel antagonists have become the main focus of pharmacological interest. Mexiletine is currently regarded as the first choice sodium channel blocker based on a recent placebo controlled randomized trial. However, some patients do not respond to mexiletine or have significant side effects limiting its use. There is a clinical need to develop additional antimyotonic agents. The study of Desaphy et al. is therefore important and provides in vitro evidence that a number of existing drugs with sodium channel blocking capability could potentially be repurposed as anti-myotonic drugs. Translation of these potentially important in vitro findings into clinical practice requires carefully designed randomized controlled trials. Here we discuss Desaphy's findings in the wider context of attempts to develop additional therapies for patients with clinically significant myotonia.

A new explanation for recessive myotonia congenita: exon deletions and duplications in CLCN1

OBJECTIVE

To assess whether exon deletions or duplications in CLCN1 are associated with recessive myotonia congenita (MC).

METHODS

We performed detailed clinical and electrophysiologic characterization in 60 patients with phenotypes consistent with MC. DNA sequencing of CLCN1 followed by multiplex ligation-dependent probe amplification to screen for exon copy number variation was undertaken in all patients.

RESULTS

Exon deletions or duplications in CLCN1 were identified in 6% of patients with MC. Half had heterozygous exonic rearrangements. The other 2 patients (50%), with severe disabling infantile onset myotonia, were identified with both a homozygous mutation, Pro744Thr, which functional electrophysiology studies suggested was nonpathogenic, and a triplication/homozygous duplication involving exons 8-14, suggesting an explanation for the severe phenotype.

CONCLUSIONS

These data indicate that copy number variation in CLCN1 may be an important cause of recessive MC. Our observations suggest that it is important to check for exon deletions and duplications as part of the genetic analysis of patients with recessive MC, especially in patients in whom sequencing identifies no mutations or only a single recessive mutation. These results also indicate that additional, as yet unidentified, genetic mechanisms account for cases not currently explained by either CLCN1 point mutations or exonic deletions or duplications.

Acetazolamide efficacy in hypokalemic periodic paralysis and the predictive role of genotype

OBJECTIVES

Acetazolamide has been the most commonly used treatment for hypokalemic periodic paralysis since 1968. However, its mechanism of efficacy is not fully understood, and it is not known whether therapy response relates to genotype. We undertook a clinical and genetic study to evaluate the response rate of patients treated with acetazolamide and to investigate possible correlations between response and genotype.

METHODS

We identified a total of 74 genotyped patients for this study. These included patients who were referred over a 15-year period to the only U.K. referral center or to a Chinese center and who underwent extensive clinical evaluation. For all genotyped patients, the response to acetazolamide therapy in terms of attack frequency and severity was documented. Direct DNA sequencing of CACNA1S and SCN4A was performed.

RESULTS

Only 46% of the total patient cohort (34 of 74) reported benefit from acetazolamide. There was a greater chance of benefit in patients with mutations in CACNA1S (31 responded of 55 total) than in those with mutations in SCN4A (3 responded of 19 total). Patients with mutations that resulted in amino acids being substituted by glycine in either gene were the least likely to report benefit.

CONCLUSIONS

This retrospective study indicates that only approximately 50% of genotyped patients with hypokalemic periodic paralysis respond to acetazolamide. We found evidence supporting a relationship between genotype and treatment response. Prospective randomized controlled trials are required to further evaluate this relationship. Development of alternative therapies is required.

Muscle channelopathies: does the predicted channel gating pore offer new treatment insights for hypokalaemic periodic paralysis?

Hypokalaemic periodic paralysis (hypoPP) is the archetypal skeletal muscle channelopathy caused by dysfunction of one of two sarcolemmal ion channels, either the sodium channel Nav1.4 or the calcium channel Cav1.1. Clinically, hypoPP is characterised by episodes of often severe flaccid muscle paralysis, in which the muscle fibre membrane becomes electrically inexcitable, and which may be precipitated by low serum potassium levels. Initial functional characterisation of hypoPP mutations failed to adequately explain the pathomechanism of the disease. Recently, as more pathogenic mutations involving loss of positive charge have been identified in the S4 segments of either channel, the hypothesis that an abnormal gating pore current may be important has emerged. Such an aberrant gating pore current has been identified in mutant Nav1.4 channels and has prompted potentially significant advances in this area. The carbonic anhydrase inhibitor acetazolamide has been used as a treatment for hypokalaemic periodic paralysis for over 40 years but its precise therapeutic mechanism of action is unclear. In this review we summarise the recent advances in the understanding of the molecular pathophysiology of hypoPP and consider how these may relate to the reported beneficial effects of acetazolamide. We also consider potential areas for future therapeutic development.