Embryonic MAP2 lacks the cross-linking sidearm sequences and dendritic targeting signal of adult MAP2

The most prominent microtubule-associated protein of the neuronal cytoskeleton is MAP2. In the brain it exists as a pair of high-molecular weight proteins, MAP2a and MAP2b, and a smaller form, MAP2c, which is particularly abundant in the developing brain. High-molecular weight MAP2 is expressed in dendrites, where its messenger RNA is also located, but is not found in axons; it has been shown to be present in fine filaments that crosslink dendritic microtubules. This correlates with the primary structure of high-molecular weight MAP2, which consists of a short carboxy-terminal tubulin-binding domain and a long amino-terminal arm, which forms a filamentous sidearm on reconstituted microtubules. Here we report that the high- and low-molecular weight forms of MAP2 are generated by alternative splicing and share the entire C-terminal tubulin-binding domain as well as a short N-terminal sequence. In contrast to high molecular weight MAP2, embryonic brain MAP2c lacks 1,342 amino acids from the filamentous sidearm domain. Furthermore, the mRNA for low molecular weight MAP2c is not present in dendrites, indicating that the dendritic targeting signal is specific for the high-molecular weight form.     

Broad phylogenomic sampling improves resolution of the animal tree of life

Long-held ideas regarding the evolutionary relationships among animals have recently been upended by sometimes controversial hypotheses based largely on insights from molecular data. These new hypotheses include a clade of moulting animals (Ecdysozoa) and the close relationship of the lophophorates to molluscs and annelids (Lophotrochozoa). Many relationships remain disputed, including those that are required to polarize key features of character evolution, and support for deep nodes is often low. Phylogenomic approaches, which use data from many genes, have shown promise for resolving deep animal relationships, but are hindered by a lack of data from many important groups. Here we report a total of 39.9 Mb of expressed sequence tags from 29 animals belonging to 21 phyla, including 11 phyla previously lacking genomic or expressed-sequence-tag data. Analysed in combination with existing sequences, our data reinforce several previously identified clades that split deeply in the animal tree (including Protostomia, Ecdysozoa and Lophotrochozoa), unambiguously resolve multiple long-standing issues for which there was strong conflicting support in earlier studies with less data (such as velvet worms rather than tardigrades as the sister group of arthropods), and provide molecular support for the monophyly of molluscs, a group long recognized by morphologists. In addition, we find strong support for several new hypotheses. These include a clade that unites annelids (including sipunculans and echiurans) with nemerteans, phoronids and brachiopods, molluscs as sister to that assemblage, and the placement of ctenophores as the earliest diverging extant multicellular animals. A single origin of spiral cleavage (with subsequent losses) is inferred from well-supported nodes. Many relationships between a stable subset of taxa find strong support, and a diminishing number of lineages remain recalcitrant to placement on the tree.     

Selective localization of messenger RNA for cytoskeletal protein MAP2 in dendrites

For nerve cells to develop their highly polarized form, appropriate structural molecules must be targeted to either axons or dendrites. This could be achieved by the synthesis of structural proteins in the cell body and their sorting to either axons or dendrites by specific transport mechanisms. For dendrites, an alternative possibility is that proteins could be synthesized locally in the dendritic cytoplasm. This is an attractive idea because it would allow regulation of the production of structural molecules in response to local demand during dendritic development. The feasibility of dendritic protein synthesis is suggested both by the existence of dendritic polyribosomes and by the recent demonstration that newly synthesized RNA is transported into the dendrites of neurons differentiating in culture. However, to date there has been no demonstration of the selective synthesis of an identified dendrite-specific protein in the dendritic cytoplasm. Here, we use in situ hybridization with specific complementary DNA probes to show that messenger RNA for the dendrite-specific microtubule-associated protein MAP2 (refs 3-5) is present in dendrites in the developing brain. By contrast the mRNA for tubulin, a protein present in both axons and dendrites is located exclusively in neuronal cell bodies.