Longitudinal axons transmit all signals between the brain and spinal cord. Their axon tracts through the brain stem are established by a simple set of pioneer axons with precise trajectories parallel to the floor plate. To identify longitudinal guidance mechanisms in vivo, the overall role of floor plate tissue and the specific roles of Slit/Robo signals were tested. Ectopic induction or genetic deletion of the floor plate diverted longitudinal axons into abnormal trajectories. The expression patterns of the diffusible cues of the Slit family were altered in the floor plate experiments, suggesting their involvement in longitudinal guidance. Genetic tests of Slit1 and Slit2, and the Slit receptors Robo1 and Robo2 were carried out in mutant mice. Slit1;Slit2 double mutants had severe longitudinal errors, particularly for ventral axons, including midline crossing and wandering longitudinal trajectories. Robo1 and Robo2 were largely genetically redundant, and neither appeared to specify specific tract positions. However, combined Robo1 and Robo2 mutations strongly disrupted each pioneer tract. Thus, pioneer axons depend on long-range floor plate cues, with Slit/Robo signaling required for precise longitudinal trajectories.

The enzyme β-secretase (BACE1) is essentially involved in the production of cerebral amyloidogenic pathology in Alzheimer's disease (AD). The measurement of BACE1 activity in cerebrospinal fluid (CSF) has been reported, which may render CSF measurement of BACE1 a potential biomarker candidate of AD. In order to investigate whether BACE1 protein activity is correlated with regional brain atrophy in AD, we investigated the association between CSF levels of BACE1 and MRI-assessed hippocampus volume in patients with AD (n = 30). An increase in CSF-BACE1 activity was associated with decreased left and right hippocampus volume corrected for global head volume in the AD patients. Boot-strapped regression analysis showed that increased CSF levels of BACE1 activity were associated with increased CSF concentration of total tau but not amyloid-β1-42 in AD. White matter hyperintensities did not influence the results. BACE1 activity and protein levels were significantly increased in AD compared to 19 elderly healthy controls. Thus, the CSF biomarker candidate of BACE1 activity was associated with hippocampus atrophy in AD in a robust manner and may reflect neurotoxic amyloid-β-related processes.

We have characterized a system of early neurons that establish the first two major longitudinal tracts in the embryonic mouse forebrain. Axon tracers and antibody labels were used to map the axon projections in the thalamus from embryonic days 9.0-12, revealing several distinct neuron populations that contributed to the first tracts. Each of the early axon populations first grew independently, pioneering a short segment of new tract. However, each axon population soon merged with other axons to form one of only two shared longitudinal tracts, both descending: the tract of the postoptic commissure (TPOC), and, in parallel, the stria medullaris. Thus, the forebrain longitudinal tracts are pioneered by a relay of axons, with distinct axon populations pioneering successive segments of these pathways. The extensive merging of tracts suggests that axon-axon interactions are a major guidance mechanism for longitudinal axons. Several axon populations express tyrosine hydroxylase, identifying the TPOC as a major pathway for forebrain dopaminergic projections. To start a genetic analysis of pioneer axon guidance, we have identified the transcription factor Pax6 as critical for tract formation. In Pax6 mutants, both longitudinal tracts failed to form due to errors by every population of early longitudinal axons. Taken together, these results have identified potentially important interactions between series of pioneer axons and the Pax6 gene as a general regulator of longitudinal tract formation in the forebrain.

During early vertebrate forebrain development, pioneer axons establish a symmetrical scaffold descending longitudinally through the rostral forebrain, thus forming the tract of the postoptic commissure (TPOC). In mouse embryos, this tract begins to appear at embryonic day 9.5 (E9.5) as a bundle of axons tightly constrained at a specific dorsoventral level. We have characterized the participation of the Slit chemorepellants and their Robo receptors in the control of TPOC axon projection. In E9.5–E11.5 mouse embryos, Robo1 and Robo2 are expressed in the nucleus origin of the TPOC (nTPOC), and Slit expression domains flank the TPOC trajectory. These findings suggested that these proteins are important factors in the dorsoventral positioning of the TPOC axons. Consistently with this role, Slit2 inhibited TPOC axon growth in collagen gel cultures, and interfering with Robo function in cultured embryos induced projection errors in TPOC axons. Moreover, absence of both Slit1 and Slit2 or Robo1 and Robo2 in mutant mouse embryos revealed aberrant TPOC trajectories, resulting in abnormal spreading of the tract and misprojections into both ventral and dorsal tissues. These results reveal that Slit-Robo signaling regulates the dorsoventral position of this pioneer tract in the developing forebrain.

The aspartyl protease BACE1 is the rate limiting enzyme in the synthesis of amyloid beta, which accumulation in the human brain is a hallmark of Alzheimer’s disease (AD). BACE1 has been proposed as a surrogate marker of AD; however, very few BACE1 immunoassays have been reported. In the present study we have screened ten BACE1 antibodies by Western blot and several antibody pairs to develop a new BACE1 sandwich ELISA procedure. We identified one pair that showed little background and good reproducibility. Several dilution buffers and sample denaturation methods were tried to partially unfold BACE1 before capture. We found that dilution in PBS followed by 10 min incubation at 50 °C critically improves the performance of the assay. Finally, we successfully measured BACE1 levels in a few human brain and platelet lysates as well as in plasma and AD CSF. We anticipate that this assay will lay the ground to accurately measure BACE1 levels in human tissues, which could facilitate the molecular diagnosis of AD in the near future.