The secretory mechanisms in equine platelets are independent of cytoskeletal polymerization and occur through membrane fusion.

Studies in animal models are useful to understand the basic mechanisms involved in hemostasis and the functional differences among species. Ultrastructural observations led us to predict differences in the activation and secretion mechanisms between equine and human platelets. The potential mechanisms involved have been comparatively explored in the present study. Equine and human platelets were activated with thrombin (0.5 U/ml) and collagen (20 µg/ml), for 90 seconds, and samples processed to evaluate: i) ultrastructural changes, by electron microscopy, ii) actin polymerization and cytoskeletal assembly, by polyacrylamide gel electrophoresis, and iii) specific molecules involved in activation and secretion, by western blot. In activated human platelets, centralization of granules, cytoskeletal assembly and fusion of granules with the open canalicular system were observed. In activated equine platelets, granules fused together forming an organelle chain that fused with the surface membrane and released its content directly outside the platelets. Human platelets responded to activation with actin polymerization and the assembly of other contractile proteins to the cytoskeleton. These events were almost undetectable in equine platelets. When exploring the involvement of the synaptosomal-associated protein-23 (SNAP-23), a known regulator of secretory granule/plasma membrane fusion events, it was present in both human and equine platelets. SNAP-23 was shown to be more activated in equine platelets than human platelets in response to activation, especially with collagen. Thus, there are significant differences in the secretion mechanisms between human and equine platelets. While in human platelets, activation and secretion of granules depend on mechanisms of internal contraction and membrane fusion, in equine platelets the fusion mechanisms seem to be predominant.

Molybdenum cofactor deficiency presenting as neonatal hyperekplexia: a clinical, biochemical and genetic study.

We report a newborn with exaggerated startle reactions and stiffness of neonatal onset, the prototypical signs of hyperekplexia. Startle and flexor spasms, leading to apnoea, did not respond to treatment with clonazepam but did partially to sodium valproate. Molecular analysis of GLRA1 revealed no mutations. The incidental finding of hypouricemia led to a work-up for molybdenum cofactor (MoCo) deficiency; the diagnosis was confirmed by the altered urine chemistries, including elevated urine S-sulphocysteine. Despite persistence of the spasms, clinical or electrographic seizures were never detected before the infant died at age 1 month. In this patient, the concurrence of hyperekplexia and MoCo deficiency was suggestive of impaired gephyrin function. GPH mutational analysis, however, showed no abnormalities. The patient was eventually found to harbour a novel c.1064T > C mutation in exon 8 of the MOCS1 gene. Despite extensive sequence analysis of the gene, the second causative mutation of this recessive trait still awaits identification. MoCo deficiency should be considered in the differential diagnosis of neonatal hyperekplexia, particularly in the instances of refractoriness to clonazepam, an early demise in infancy or the evidence of no mutations in the GLRA1 gene.