Antibodies to PfSEA-1 block parasite egress from RBCs and protect against malaria infection

Novel vaccines are urgently needed to reduce the burden of severe malaria. Using a differential whole-proteome screening method, we identified Plasmodium falciparum schizont egress antigen-1 (PfSEA-1), a 244-kilodalton parasite antigen expressed in schizont-infected red blood cells (RBCs). Antibodies to PfSEA-1 decreased parasite replication by arresting schizont rupture, and conditional disruption of PfSEA-1 resulted in a profound parasite replication defect. Vaccination of mice with recombinant Plasmodium berghei PbSEA-1 significantly reduced parasitemia and delayed mortality after lethal challenge with the Plasmodium berghei strain ANKA. Tanzanian children with antibodies to recombinant PfSEA-1A (rPfSEA-1A) did not experience severe malaria, and Kenyan adolescents and adults with antibodies to rPfSEA-1A had significantly lower parasite densities than individuals without these antibodies. By blocking schizont egress, PfSEA-1 may synergize with other vaccines targeting hepatocyte and RBC invasion.

Methods for analysis of calcium/calmodulin signaling in cilia and flagella

The axonemal microtubules of cilia/flagella act as a scaffold for assembly of the protein complexes that ultimately regulate dynein activity to control the size and shape of ciliary bends. Despite our general understanding of the contribution of microtubule sliding to ciliary and flagellar motility, many questions regarding the regulation of dynein remain unanswered. For example, we know that the second messenger calcium plays an important role in modulating dynein activity in response to extracellular cues, but it remains unclear how calcium-binding proteins anchored to the axoneme contribute to this regulation. Recent work has focused on determining the identity and specific functions of these axonemal calcium-binding proteins. Here, we review our current knowledge of calcium-mediated motility and highlight key experiments that have substantially aided our understanding of calcium signaling within the axoneme.

The Pcdp1 complex coordinates the activity of dynein isoforms to produce wild-type ciliary motility

Generating the complex waveforms characteristic of beating cilia requires the coordinated activity of multiple dynein isoforms anchored to the axoneme. We previously identified a complex associated with the C1d projection of the central apparatus that includes primary ciliary dyskinesia protein 1 (Pcdp1). Reduced expression of complex members results in severe motility defects, indicating that C1d is essential for wild-type ciliary beating. To define a mechanism for Pcdp1/C1d regulation of motility, we took a functional and structural approach combined with mutants lacking C1d and distinct subsets of dynein arms. Unlike mutants completely lacking the central apparatus, dynein-driven microtubule sliding velocities are wild type in C1d- defective mutants. However, coordination of dynein activity among microtubule doublets is severely disrupted. Remarkably, mutations in either outer or inner dynein arm restore motility to mutants lacking C1d, although waveforms and beat frequency differ depending on which isoform is mutated. These results define a unique role for C1d in coordinating the activity of specific dynein isoforms to control ciliary motility.

Pcdp1 is a central apparatus protein that binds Ca(2+)-calmodulin and regulates ciliary motility

A complex that localizes to the C1d central pair projection of cilia controls flagellar waveform and beat frequency in response to calcium.

For all motile eukaryotic cilia and flagella, beating is regulated by changes in intraciliary calcium concentration. Although the mechanism for calcium regulation is not understood, numerous studies have shown that calmodulin (CaM) is a key axonemal calcium sensor. Using anti-CaM antibodies and Chlamydomonas reinhardtii axonemal extracts, we precipitated a complex that includes four polypeptides and that specifically interacts with CaM in high [Ca²⁺]. One of the complex members, FAP221, is an orthologue of mammalian Pcdp1 (primary ciliary dyskinesia protein 1). Both FAP221 and mammalian Pcdp1 specifically bind CaM in high [Ca²⁺]. Reduced expression of Pcdp1 complex members in C. reinhardtii results in failure of the C1d central pair projection to assemble and significant impairment of motility including uncoordinated bends, severely reduced beat frequency, and altered waveforms. These combined results reveal that the central pair Pcdp1 (FAP221) complex is essential for control of ciliary motility.