Josie Liane Ferreira

A network of plasma membrane invaginations called t-tubules plays an essential role in controlling calcium release from the endoplasmic reticulum at the triads during muscle contraction. Although the importance of t-tubules for muscle physiology is well established, and abnormalities are found in muscle disorders, the mechanisms that mediate t-tubule growth are unknown. We show that the actomyosin cortex beneath the plasma membrane, regulated by Arp2/3 complexes containing Arpc5, acts as a gatekeeper for the membrane availability during t-tubule growth. Enlarged t-tubules are formed upon disruption of Arpc5, impairing the synchronization between plasma membrane depolarization and calcium release. Knockout of Arpc5 in mouse skeletal muscle results in impaired locomotion and posture. Furthermore, we show that human triadopathy patients and Arpc5 knockout mice accumulate enlarged t-tubules. We propose that cortex-dependent membrane availability affects muscle function, offering a potential pathophysiological mechanism for muscle disorders.

Microtubules are ubiquitous yet diverse cytoskeleton filaments. However, tubulin conservation presents challenges in understanding the origins of diverse microtubule architectures. The mechanisms by which microtubule architecture varies through the life cycle of the malaria-causing parasite Plasmodium are not understood and provide a valuable framework for exploring how intrinsic properties of tubulin contribute to architectural variety. Using parasite-purified tubulin, we determine P. falciparum microtubule structures by cryo-electron microscopy. Parasite-specific sequences change the tubulin dimer structure, suggesting how drug susceptibility and polymer properties are modified. Within the P. falciparum microtubule, lateral contacts are smaller but stronger, and the lattice is stiffer than in brain microtubules. Non-canonical microtubule architectures found in parasites are highly similar to those observed in vitro, validating the physiological relevance of these properties. Our findings show how evolutionary adaptation of tubulin modulates the material properties of the microtubule cytoskeleton.

Actin-based motility is required for the transmission of malaria sporozoites. While this has been shown biochemically, filamentous actin has remained elusive and has not been directly visualised inside the parasite. Using focused ion beam milling and electron cryo-tomography, we studied dynamic actin filaments in unperturbed Plasmodium falciparum cells for the first time. This allowed us to dissect the assembly, path and fate of actin filaments during parasite gliding and determine a complete 3D model of F-actin within sporozoites. We observe micrometre long actin filaments, much longer than expected from in vitro studies. After their assembly at the parasite’s apical end, actin filaments continue to grow as they are transported down the cell as part of the glideosome machinery, and are disassembled at the basal end in a rate-limiting step. Large pores in the IMC, constrained to the basal end, may facilitate actin exchange between the pellicular space and cytosol for recycling and maintenance of directional flow. The data also reveal striking actin bundles in the nucleus. Implications for motility and transmission are discussed.

Muscle cells form a network of plasma membrane invaginations called t-tubules that control calcium release from the endoplasmic reticulum at the triads during muscle contraction. Although the importance of t-tubules for muscle physiology is well established, and abnormalities are found in multiple disorders, the mechanisms that mediate t-tubule growth are unknown. We show that the actomyosin cortex beneath the plasma membrane, regulated by Arp2/3 complexes containing Arpc5, acts as a gatekeeper for the membrane availability required for t-tubule growth. Enlarged t-tubules are formed upon disruption of Arpc5, impairing the synchronisation between plasma membrane depolarization and calcium release. Additionally, ablation of Arpc5 postnatally in myofibers results in muscle fatigue and t-tubule abnormalities, as observed in muscle disorders. We propose that the actomyosin cortex impacts muscle function, offering a potential pathophysiological mechanism for muscle disorders.

Actin-based motility is required for the transmission of malaria sporozoites. While this has been shown biochemically, filamentous actin has remained elusive and has to date never been directly visualised inside the parasite. Using focused ion beam milling and electron cryo-tomography, we studied dynamic actin filaments in unperturbed Plasmodium falciparum cells for the first time. This allowed us to dissect the assembly, path and fate of actin filaments during parasite gliding and determine a complete 3D model of F-actin within sporozoites. We show that within the cell, actin assembles into micrometre long filaments, much longer than observed in in vitro studies. After their assembly at the parasite’s apical end, actin filaments continue to grow as they are transported down the cell as part of the glideosome machinery, and are disassembled at the basal end in a rate-limiting step. Large pores in the IMC, constrained to the basal end, may facilitate actin exchange between the pellicular space and the cytosol for its recycling and maintenance of directional actin flow for efficient gliding. The data also reveal striking and extensive actin bundles in the nucleus. Implications of these structures for motility and transmission are discussed.

The malaria parasite undergoes a series of extensive morphological changes within its human host and mosquito vector. A scaffold of microtubules beneath a peripheral double membrane establishes and maintains the distinct shapes of all infectious forms, but the underlying structural basis remains unknown. Here we applied in situ electron cryo-tomography after focused ion beam milling to follow changes in the microtubule cytoskeleton throughout the Plasmodium life cycle. This revealed an unexpected level of structural and architectural diversity so far not observed in other organisms. Microtubules in migrating mosquito forms consist of 13 protofilaments reinforced by interrupted luminal helices. Conversely, gametocyte microtubules consist of 13 to 18 protofilaments with doublets, triplets and quadruplets of varying arrangements. We show the microtubule cytoskeleton within the native cellular context, highlighting structurally diverse apical rings which act as microtubule organising centres. This provides a unique view into a relevant human pathogen with an unusual microtubule cytoskeleton.

Malaria parasites live and multiply inside cells. To facilitate their extremely fast intracellular proliferation, they hijack and transform their host cells.

During the symptomatic human blood phase, malaria parasites replicate within red blood cells. Parasite proliferation relies on the uptake of nutrients, such as amino acids, from the host cell and the blood plasma, requiring transport across multiple membranes. Amino acids are delivered to the parasite through the parasite surrounding vacuolar compartment by specialized nutrient-permeable channels of the erythrocyte membrane and the parasitophorous vacuole membrane (PVM). However, further transport of amino acid across the parasite plasma membrane (PPM) is currently not well characterized. In this study, we focused on a family of Apicomplexan amino acid transporters (ApiATs) that comprises five members in Plasmodium falciparum . First, we localized four of the Pf ApiATs at the PPM using endogenous GFP-tagging. Next, we applied reverse genetic approaches to probe into their essentiality during asexual replication and gametocytogenesis. Upon inducible knockdown and targeted gene disruption a reduced asexual parasite proliferation was detected for Pf ApiAT2 and Pf ApiAT4. Functional inactivation of individual Pf ApiATs targeted in this study had no effect on gametocyte development. Our data suggest that individual Pf ApiATs are partially redundant during asexual in vitro proliferation and fully redundant during gametocytogenesis of P. falciparum parasites. IMPORTANCE: Malaria parasites live and multiply inside cells. To facilitate their extremely fast intracellular proliferation they hijack and transform their host cells. This also requires the active uptake of nutrients, such as amino acids, from the host cell and the surrounding environment through various membranes that are the consequence of the parasite’s intracellular lifestyle. In this manuscript we focus on a family of putative amino acid transporters termed ApiAT. We show expression and localization of four transporters in the parasite plasma membrane of Plasmodium falciparum -infected erythrocytes that represent one interface of the pathogen to its host cell. We probed into the impact of functional inactivation of individual transporters on parasite growth in asexual and sexual blood stages of P. falciparum and reveal that only two of them show a modest but significant reduction in parasite proliferation but no impact on gametocytogenesis pointing towards redundancy within this transporter family.

Echoing the repeated convergent evolution of flight and vision in large eukaryotes, propulsive swimming motility has evolved independently in microbes in each of the three domains of life. Filamentous appendages – archaella in Archaea, flagella in Bacteria and cilia in Eukaryotes – wave, whip or rotate to propel microbes, overcoming diffusion and enabling colonization of new environments. The implementations of the three propulsive nanomachines are distinct, however: archaella and flagella rotate, while cilia beat or wave; flagella and cilia assemble at their tips, while archaella assemble at their base; archaella and cilia use ATP for motility, while flagella use ion-motive force. These underlying differences reflect the tinkering required to evolve a molecular machine, in which pre-existing machines in the appropriate contexts were iteratively co-opted for new functions and whose origins are reflected in their resultant mechanisms. Contemporary homologies suggest that archaella evolved from a non-rotary pilus, flagella from a non-rotary appendage or secretion system, and cilia from a passive sensory structure. Here, we review the structure, assembly, mechanism and homologies of the three distinct solutions as a foundation to better understand how propulsive nanomachines evolved three times independently and to highlight principles of molecular evolution.