Expression of the gene for the neuronal intermediate filament protein alpha-internexin coincides with the onset of neuronal differentiation in the developing rat nervous system.

While neurofilaments have long been considered early markers of neuronal differentiation, they cannot be detected in most newly postmitotic neurons of the developing central nervous system (CNS). Here we show that these neurons already express the neuronal intermediate filament protein alpha-internexin at high levels. alpha-internexin is expressed by most, if not all, neurons as they begin differentiation and shows no overlap with vimentin, whose expression in the CNS is restricted to mitotic neuronal precursors. In the adult, alpha-internexin is the only intermediate filament gene expressed by the cerebellar granule cells, the source of the thin-caliber parallel fibers; conversely, neurofilament proteins are highly expressed in large neurons, which express alpha-internexin at low levels. These data suggest that neuronal intermediate filaments may regulate axonal stability and/or diameter through changes not only in their number, but also in their subunit composition.

Alpha-internexin, a novel neuronal intermediate filament protein, precedes the low molecular weight neurofilament protein (NF-L) in the developing rat brain.

alpha-Internexin is a 66 kDa protein that copurifies with intermediate filaments (IF) from rat spinal cord and optic nerve. This protein is axonally transported in rat optic nerve along with the neurofilament triplet proteins in slow component a. Polymerization in vitro and distribution in vivo confirm that alpha-internexin is a neuronal IF. We raised 2 highly specific monoclonal antibodies to alpha-internexin which were applied to frozen rat brain sections and Western blots of cytoskeletal extracts. These results indicate that alpha-internexin is primarily an axonal protein found in most, if not all, neurons of the CNS. Immunoreactive proteins of similar molecular weight were found in cytoskeletal extracts of CNS tissue from several additional species, including mouse and cow. While the distribution of alpha-internexin as given by immunocytochemical methods is similar to that of low molecular weight neurofilament protein (NF-L) in the adult, its distribution in the embryo is far more extensive. At embryonic day 16, when the expression of NF-L is still limited to a relatively small number of cells and levels of expression are low, alpha-internexin is already found at much higher levels and in cells not yet expressing NF-L in detectable quantities. Similar results are found at embryonic day 12. These data suggest that neuronal IF in the developing nervous system contain a higher proportion of alpha-internexin than their adult counterparts, and that expression of alpha-internexin precedes that of NF-L in many or most neurons of the developing brain.

The predicted amino acid sequence of alpha-internexin is that of a novel neuronal intermediate filament protein.

Our laboratory recently isolated and began to characterize a 66 kd rat brain cytoskeletal protein, dubbed alpha-internexin for its interactions in vitro with several other cytoskeletal proteins. Although alpha-internexin bore several of the characteristics of intermediate filament (IF) proteins, including the recognition by an antibody reactive with all IF proteins, it did not polymerize into 10 nm filaments under the conditions tested. Here we show that the predicted amino acid sequence of a cDNA encoding alpha-internexin shows the latter to be an IF protein, probably most closely related to the neurofilament proteins. Northern blotting shows that alpha-internexin expression is brain specific, and that rat brain alpha-internexin mRNA levels are maximal prior to birth and decline into adulthood, while the converse is seen for NF-L, the low molecular weight neurofilament subunit, suggesting that these two proteins play different roles in the developing brain.

Ingestive behaviour and physiology of the medicinal leech.

Ingestion lasts 25 min in Hirudo medicinalis and is characterized by pharyngeal peristalsis which fills the crop. This peristalsis has an initial rate of 2.4 Hz which decays smoothly to 1.2 Hz at termination of ingestion. During ingestion, the leech body wall undergoes peristalsis which appears to aid in filling the crop diverticula. Body peristalsis begins at a rate of 10 min-1 and decreases linearly to 2 min-1 at termination. The body also undergoes dorsoventral flexions when blood flow is occluded. Blood meal size increases slightly with leech size: 8.4 g for 1-g leeches and 9.7 g for 2-g leeches. However, relative meal size decreases markedly with increasing animal size; from 8.15 times body mass for 1-g to 4.80 times for 2-g leeches. When intact leeches were exposed to micromolar concentrations of serotonin, there was an increase in the rate of pharyngeal peristalsis and the size of the blood meals. Leeches excrete the plasma from their ingested blood meals. Excretion is activated during ingestion, which increases feeding efficiency by increasing the proportion of blood cells in the ingestate. Excretion continues for 4-6 days following ingestion, removing all the remaining plasma from the ingestate. Leech ingestion comprises stereotyped muscular movements, secretion of saliva and excretion of plasma. A strikingly similar feeding physiology is seen in the blood-sucking insect Rhodnius, and we suggest that efficient sanguivory may require the convergent evolution of similar ingestive mechanisms.