Pond life is endlessly fascinating – not only because we may discover an unknown species in our bird bath, but also because of the endless, very basic questions about the structure of life that climb up from beneath our lenses.
Two years ago, smelly green sludge took over our local lake. It turned out to be largely the beautiful helical photosynthetic bacterium Anabaena. Like pearls on a necklace, Anabena shows round heterocysts where nitrogen is fixed – a process that is chemically incompatible with the photosynthetic processes taking place in the rest of the chain. Bacteria don’t have internal organelles to isolate chemical processes that don’t get along with each other, so they have to get inventive, and add on another cell in the chain – the bacterial equivalent of building a spare room on the house. We know that the photosynthetic vegetative cells of the chain provide the heterocysts with sugars, and the heterocysts provide the rest of the chain with reduced nitrogen in the form of amino acids. But how does Anabaena get the groceries back and forth along the chain???
Wondering how Anabaena transferred nutrients led to wondering about cyanobacterial cell-cell communication which led to wondering about how cells talk to each other, and then to those amazing cellular structures called “junctions” …
Many of our favorite critters are single-celled, and perform their amazing functions entirely with one cell. However, in every being of more than two cells, those cells need both a way to stick together and a rapid and efficient way to talk to each other. This function is critical for cells ranging from primitive cyanobacteria transferring nutrients, to human cardiac muscle cells coordinating a wave of contraction to keep the other millions of cells alive in a complex body.
These essential functions are performed by the many different kinds of cell junctions, the subject of intensive research for many decades. Some serve a strictly structural function, while others are essential for cell-cell communication. Visible at the level of light microscopy only with specialized labeling techniques, their many complex patterns are fascinating under the electron microscope.
Animal cells have four different types of intercellular junctions: tight junctions, adherens junctions, desmosomes, and gap junctions. The first three are largely structural: tight junctions form tight seals around epithelial cells, preventing leakage and differentiating the functions of the apical and basal portions of the membrane; adherens junctions provide strong mechanical attachment between adjacent cells and anchor intracytoplasmic actin filaments (in cardiac muscle, for example); desmosomes are localized patches that hold cells tightly together and anchor intracytoplasmic keratin intermediate filaments (see Kimball).
A fifth type of junction, the septate junction, occurs primarily in invertebrate cells, though it also occurs in certain vertebrate neurons. By electron microscopy, it appears as pleated sheets separating islands of cell membrane. Its function seems to be similar to that of the tight junction.
The most fascinating are the gap junctions, highly structured and functionally complex intercellular channels permitting rapid movement of ions and small molecules between cells; their role in cellular communication is still perplexing, but has become a prominent question as a result of recent research. We all know how information travels through our bodies via our marvelous network of nerves and synapses, and how our metabolism is regulated by hormones dissolved in our blood – but there is third, more subtle and barely-recognized, communications system directly between cells via these tiny intercellular pores.
Six membrane-spanning connexin proteins surrounding a small channel, the hexagonal structure visible in high-resolution electron micrographs, form each unit of the gap junction. These are then arranged into arrays of varying sizes and shapes, often involving hundreds of individual junctional structures. Transfer of small metabolites between cells is possible through the channels.
Gap junctions appear to play an unexpectedly important and widespread role in cellular coordination, and are present in every solid tissue in the human body. They coordinate the contraction of cardiac muscle cells and play some role in rapid transmission within the nervous system. They are seen in a wide variety of other tissues, such as pancreatic islet cells, granulosa cells of the ovarian follicle, and hepatocytes. In the latter case, only select liver cells react to glucose levels through direct innervation – neighboring cells are coordinated via gap junction signals. Gap junctions also play a role in cell-cell communication in embryonic development. They appear in both vertebrate and invertebrate cells, though the structural proteins are different – innexin replaces connexin in hydras and anenomes, and pannexins are present in higher invertebrates. Certain genetic diseases, such as the peripheral demyelinating disorder Chacot-Marie-Tooth syndrome, appear to result from defective connexins. For an excellent review of gap junction function and dynamics, see Goodenough and Paul.
Cell Junctions and the Watery World:
So what do cell junctions have to do with pond life? A lot, it turns out, though the full picture is just starting to emerge – see the review by Bereiter-Hahn et al. Every invertebrate and multicellular animal with more than a few cells, uses a variety of intercellular junctions to move, digest food, and hold together. Every time you look through the microscope at even a tiny multi-celled creature, you are watching cell junctions in action.
In many if not most species, these junctions have not been studied, hence the lack of awareness of junction function and prevalence among fresh-water enthusiasts. For example, a Google search for “Cell Junctions” and “Nematodes” or “Rotifers” (or most other aquatic critters) brings up little that is informative. A few simple animals, plants, and primitive bacteria have become standard “laboratory rats” and have been the subject of intense scrutiny for years. For example, hydras, the focus of much research, use all of the above junctional types between their surface epitheliomuscular cells (see Chapman). Impulses among the cells of the nerve net of the stalk are coordinated through gap junctions, and blocking these junctions prevents hydras from contracting (see Takaku). Planarian worm regeneration, the subject of intense research since the early 20th century, depends on gap junction communication as one of the main mechanisms coordinating cell organization (see Lobo). These junctions are found in all groups above the coelenterates and, though their role is still poorly understood, are probably essential in coordinating a wide variety of cellular functions, including movement, growth, and cell differentiation.
Plants, Simple and Otherwise:
Because of their thick polysaccharide cell wall, forming intercellular junctions is more of a challenge for plants and algae. Plant and algal cells have only one type of intercellular junction: the plasmodesmata. This structure, though it may fulfill some of the same functions as the animal cell junction, is more complex and quite different in origin.
Plasmodesmata are found in land plants and some species of multicellular algae, including seaweeds. Neighboring plant cells actually appear to have one continuous cell membrane, with cytoplasm connecting through these pores in the cell wall. Within this membrane-lined pore passes a second membranous cylindrical sleeve formed from an extension of the cell’s endoplasmic reticulum. Contractile filaments appear to be present within the plasmodesmata and may faciltate active transport in addition to passive diffusion. The significance of plasodesmata is only beginning to be understood. Recent thinking suggests that land plants should be thought of as supracellular organisms because of the continuity of their cytoplasm and cell membranes throughout the whole plant.
Interestingly, it is thought that some simple colonial organisms get along fine without formal cell junctions. Volvox colonies begin life as an “embryo” with all of the flagella pointing inward; the colony must turn itself inside-out as it matures, a task that it accomplishes with cellular shape changes and robust intercellular cytoplasmic connections (see Hohn).
Rather than cell junctions, much of cellular adhesion in these multicellular green algae depends on the complex fibrillar and polysaccharide matrix of the cell wall. However, it is not entirely clear whether the apparent lack of intercellular communication via junctions in these simple multicellular colonial organisms is real or is due to the fact that we have not yet found them.
Junctions and Evolution:
It is thought that cell junction development reflects increasing evolutionary sophistication with increasingly complex multicellularity (Abedin and King). Sponges appear to possess rudimentary adherens junctions, and possibly septate junctions. Gap junctions based on pannexins as structural molecules appeared with the cnidarians (sea anemones and hydras). Connexin-based gap junctions appeared alongside the development of backbones with the chordates. Interesting, vertebrates appear to have also retained the more primitive pannexin-based gap junction type, and both are used in some tissues. The last type of cell junction to appear was the desmosome, which is seen only in vertebrates and supports epithelial tissues against stress. Later came hemidesmosomes, half-desmosomal structures connecting the bases of epithelial cells to the fibrillar basement membrane that underlies every epithelium.
The Most Primitive Organisms – Cyanobacteria:
So let’s go back to our original question: that little chain of green cells in the mud. Going down toward the roots of the evolutionary tree into the green soup of Anabaena in Somenos Lake, one again finds a type of cell junction playing an essential role. Interestingly, Anabaena is much more than a green, slimy pest in our local lake; it has been the primary experimental organism and poster child for recent research in this
area, with interest focusing on how metabolites shuttle back and forth between the photosynthetic, CO2-fixing vegetative cells and the nonphotosynthetic, nitrogen-fixing heterocysts. Given that different metabolites are manufactured in different cells, intercellular transport, and therefore intercellular junctions, is essential to cyanobacteria. As of today, the function of these junctions is just beginning to be unraveled:
“Principles of intercellular communication are well established for eukaryotes, but the mechanisms and components involved in bacteria are just emerging. Filamentous heterocyst-forming cyanobacteria behave as multicellular organisms and represent an excellent model to study prokaryotic cell-cell communication. A path for intercellular metabolite exchange appears to involve transfer through molecular structures termed septal junctions. They are reminiscent of metazoan gap junctions that directly link adjacent cells (Rudolf et al).”
Although the component proteins of these junctions have been isolated, the structure of septal junctions is still only beginning to be understood. Early electron microscopic studies showed groups of structures, designated “microplasmodesmata”, traversing the peptidogylcan layer between vegetative cells and heterocysts. Recently, complex techniques such as electron tomography have being used to evaluate these structures (see Wilk et al), and the “septal junctions” or “septosomes” found by these advanced methods have proven to be identical to the microplasmodesmata. The most recent research suggests that the septosomes are 5.5-14nm in diameter, and pass through the intercellular peptidogycan layer via “nanopores” 20 nm in diameter. Present knowledge of septal junctions has been comprehensively reviewed by Herrero et al.
So the next time you look through your microscope at a little multicellular creature, wonder at the complex conversation that is going on between its parts. You share a kinship with that tiny creature – for it to move, or for your heart to continue beating, or for your nerves to speak to each other, both of you have cells that talk to each other instantaneously in ways that we are only beginning to understand!
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