Keynote Conference - Interevent

Symp#32 Unconventional organelles
Chair Marlene Benchimol
Reductive evolution and the minimal mitochondria of microsporidian parasites
Martin Embley
Institute for Cell and Molecular Biosciences, The Medical School, Newcastle University, UK NE24HH
Microsporidians are important human pathogens causing chronic diarrhoea in children and the elderly, and infecting
immunocompromised patients, including those with HIV/AIDS. In addition to their medical importance, microsporidians have
become models for understanding cellular and genomic reduction in eukaryotes. The adoption of an obligate intracellular lifestyle
has allowed them to lose metabolic pathways and to simplify the structures and functions of cellular organelles. In my talk I will
discuss how such reductive evolution has affected the proteome and functions of their minimal mitochondria – now widely referred
to as mitosomes, and why, despite their reduced nature mitosomes are still essential for parasite viability.
An unconventional organelle: the hydrogenosome
Marlene Benchimol
Universidade Santa Úrsula, Laboratório de Ultraestrutura Celular - Rio de Janeiro - Brazil
Hydrogenosomes are spherical or slightly elongated organelles found in non-mitochondrial organisms. Like mitochondria
hydrogenosomes: (1) are surrounded by two closely apposed membranes and present a granular matrix: (2) divide in three different
ways: segmentation, partition and the heart form; (3) they may divide at any phase of the cell cycle; (4) produce ATP; (5) participate
in the metabolism of pyruvate formed during glycolysis; (6) present a relationship with the endoplasmic reticulum; (7) incorporate
calcium; (8) import proteins post-translationally; (9) present cardiolipin. However, there are differences, such as: (1) absence of
genetic material, at least in trichomonas; (2) lack a respiratory chain and cytochromes; (3) absence of the F0- F1 ATPase; (4) absence of
the tricarboxylic acid cycle; (5) lack of oxidative phosphorylation; (6) presence of peripheral vesicles. Hydrogenosomes are considered
an excellent drug target since their metabolic pathway is distinct from those found in mitochondria and thus medicines directed to
these organelles will probably not affect the host-cell. The main drug used against trichomonads is metronidazole, although other
drugs such as β-Lapachone, colchicine, Taxol, nocodazole, griseofulvin, cytochalasins, hydroxyurea, among others, have been used in
trichomonad studies, showing: (1) flagella internalization forming pseudocyst; (2) dysfunctional hydrogenosomes; (3)
hydrogenosomes with abnormal sizes and shapes and with an electron dense deposit called nucleoid; (4) intense autophagy in which
hydrogenosomes are removed and further digested in lysosomes.
Dynamic control of the contractile vacuole complex and acidocalcisomes and their functional role in the mechanisms of regulatory
volume decrease in Trypanosomatid parasites
Kildare Miranda 1
Wendell Girard-Dias 1 , Wanderley de Souza 1 and Roberto Docampo 2 , 1 Biophysics Institute, Federal University of Rio de Janeiro, 2
University of Georgia
Understanding mechanisms involved in osmoregulation control in protozoan parasites has been a challenge for many research
groups. Among these mechanisms, a cyclic AMP (cAMP) signaling pathway has been shown to play a key role in osmoregulation,
through a mechanism that involves the activation of an unusual organelle named the contractile vacuole complex (CVC). In
Trypanosoma cruzi, the CVC is formed by a central vacuole surrounded by interconnected tubules that undergo dynamic changes
upon osmotic stress and interacts with acidocalcisomes, whose structural organization, chemical properties and physiological activity
may also vary upon events of osmotic stress. Biochemical and molecular data have shown that the sequence of events that take
place in cells submitted to hyposmotic stress leads to an increase in cAMP levels, stimulating the traffic of an aquaporin from
acidocalcisomes to the CVC through a fusion mechanism. Acidocalcisomes contain basic amino acids and high levels of cations and
polyphosphate, a content that once released within the contractile vacuole, leads to an increase in the osmotic pressure towards the
lumen of the organelle, stimulating water transport into the CVC. Functional analysis of mutant parasites that overexpress enzymes
involved in the control of cAMP levels showed alterations in the regulatory volume decrease (RVD), a large and functional CVC and
were more efficient in volume recovery. Taken together, our data show dynamic changes in the osmoregulatory system of T. cruzi,
governed by signaling events that involve a unique mechanism of interaction of the CVC with acidocalcisomal components.
Cell biology of magnetotactic bacteria and their organelles: the magnetosomes
Ulysses Lins
Instituto de Microbiologia, UFRJ, Centro de Ciências da Saúde, Bloco I, Avenida Carlos Chagas Filho, 373 - 21941-902, Rio de Janeiro,
RJ, Brasil
Historically prokaryotes have been described as simple cells with organization principles distantly related to the more complex
eukaryotic cells. Recent advances in understanding the cell biology and molecular mechanisms underlying the ultrastructural
organization of bacterial cells have modified the traditional views used to distinguish eukaryotic from prokaryotic cells. The
complexity of prokaryotic cells and their similarities to eukaryotes is highlighted by the discovery of cytoskeleton elements in bacteria
and further by the description of membranous organelles with specialized functions in a number of prokaryotic species. One of these
organelles, the magnetosome, consists of a nanometer-sized magnetic crystal which is formed in the bacterial cell cytoplasm inside a
bilayer lipid vesicle containing a unique set of proteins. The magnetosomes are organized as chains within the cell and are
surrounded by a distinct cytoskeletal network of filaments. Bacteria that produce magnetosomes are called magnetotactic bacteria
because of their ability to orientate and navigate along magnetic field lines which is a consequence of the magnetic moment
generated by the chains of magnetosomes. The specific proteins expressed by the cell in the magnetosome membrane modulate the
biomineralization of the magnetic crystals within the magnetosome vesicle. Consequently, the controlled biomineralization process
that takes place in magnetosomes produce magnetic crystals with unique morphologies that is dependent on the magnetotactic
bacterial species.
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