Microglia may contribute to cell death in neurodegenerative diseases. We studied the activation of microglia in affected regions of Huntington disease (HD) brain by localizing thymosin beta-4 (Tbeta4), which is increased in reactive microglia. Activated microglia appeared in the neostriatum, cortex, and globus pallidus and the adjoining white matter of the HD brain, but not in control brain. In the striatum and cortex, reactive microglia occurred in all grades of pathology, accumulated with increasing grade, and grew in density in relation to degree of neuronal loss. The predominant morphology of activated microglia differed in the striatum and cortex. Processes of reactive microglia were conspicuous in low-grade HD, suggesting an early microglia response to changes in neuropil and axons and in the grade 2 and grade 3 cortex, were aligned with the apical dendrites of pyramidal neurons. Some reactive microglia contacted pyramidal neurons with huntingtin-positive nuclear inclusions. The early and proximate association of activated microglia with degenerating neurons in the HD brain implicates a role for activated microglia in HD pathogenesis.

The immunohistochemical localization of huntingtin was examined in the Huntington's disease (HD) brain with an antibody that recognizes the wild-type and mutant proteins. Neuronal staining was reduced in areas of the HD striatum depleted of medium-sized neurons; large striatal neurons, which are spared in HD, retained normal levels of huntingtin expression. Neuronal labeling was markedly reduced in both segments of the globus pallidus including in brains with minimal loss of pallidal neurons. In some HD cortical and striatal neurons with normal looking morphology, huntingtin was associated with punctate cytoplasmic granules that at the ultrastructural level resembled the multivesicular body, an organelle involved in retrograde transport and protein degradation. Some immunoreactive processes showed blebbing and segmentation similar to that induced experimentally by hypoxic-ischemic or excitotoxic injury. Huntingtin staining was more concentrated in the perinuclear cytoplasm and reduced or absent in processes of atrophic cortical neurons. Nuclear staining was also evident. Fibers in the subcortical white matter of HD patients had significantly increased huntingtin immunoreactivity compared with those of controls. Results suggest that there may be changes in the neuronal expression and transport of wild-type and/or mutant huntingtin at early and late stages of neuronal degeneration in affected areas of the HD brain.

Prevailing notions of cerebral vascularization imply that blood vessels sprout passively into the brain parenchyma from pial vascular plexuses to meet metabolic needs of growing neuronal populations. Endothelial cells, building blocks of blood vessels, are thought to be homogeneous in the brain with respect to their origins, gene expression patterns and developmental mechanisms. These current notions that cerebral angiogenesis is regulated by local environmental signals contrast with current models of cell-autonomous regulation of neuronal development. Here we demonstrate that telencephalic angiogenesis in mice progresses in an orderly, ventral-to-dorsal gradient regulated by compartment-specific homeobox transcription factors. Our data offer new perspectives on intrinsic regulation of angiogenesis in the embryonic telencephalon, call for a revision of the current models of telencephalic angiogenesis and support novel roles for endothelial cells in brain development.Brain development is supported by concomitant development of brain vasculature. However, blood vessels or endothelial cells are generally considered to lack molecular diversity in the embryonic or mature CNS 1,2 . Currently, it is believed that once the perineural plexuses on the pial surface (pial vessels) surrounding the neural tube are produced, cerebral vasculature develops further by passive vessel sprouting into the brain parenchyma in response to increased tissue mass and oxygen demand 1,2 . Although classical studies identified a ventral to dorsal temporal developmental angiogenesis gradient in the telencephalon 3 , the sequence of angiogenesis was considered to merely shadow neurogenesis and neuronal maturation. Similarly, although shared mechanisms regulating vascular and neuronal development have been recognized 4 , the canonical principles of neuronal development are not seen as applicable to CNS angiogenesis.

After brain injury, neuroblast cells from the subventricular zone (SVZ) expand and migrate toward damaged tissue. The mechanisms that mediate these neurogenic and migratory responses remain to be fully dissected. Here, we show that bromodeoxyuridine-labeled and doublecortin-positive cells from the SVZ colocalize with the extracellular protease matrix metalloproteinase-9 (MMP-9) during the 2 week recovery period after transient focal cerebral ischemia in mice. Treatment with the broad spectrum MMP inhibitor GM6001 significantly decreases the migration of doublecortin-positive cells that extend from the SVZ into the striatum. These data suggest that MMPs are involved in endogenous mechanisms of neurogenic migration as the brain seeks to heal itself after injury.

Cigarette smoking, nicotine replacement therapy and smokeless tobacco use during pregnancy are associated with cognitive disabilities later in life in children exposed prenatally to nicotine. The disabilities include attention deficit hyperactivity disorder (ADHD) and conduct disorder. However, the structural and neurochemical bases of these cognitive deficits remain unclear. Using a mouse model we show that prenatal nicotine exposure produces hyperactivity, selective decreases in cingulate cortical volume and radial thickness as well as decreased dopamine turnover in the frontal cortex. The hyperactivity occurs in both male and female offspring and peaks during the “active” or dark phase of the light-dark cycle. These features of the mouse model closely parallel the human ADHD phenotype, whether or not the ADHD is associated with prenatal nicotine exposure. A single oral, but not intraperitoneal, administration of a therapeutic equivalent dose (0.75 mg/kg) of methylphenidate decreases the hyperactivity and increases the dopamine turnover in the frontal cortex of the prenatally nicotine exposed mice, once again paralleling the therapeutic effects of this compound in ADHD subjects. Collectively, our data suggest that the prenatal nicotine exposure mouse model has striking parallels to the ADHD phenotype not only in behavioral, neuroanatomical and neurochemical features but also with respect to responsiveness of the behavioral phenotype to methylphenidate treatment. The behavioral, neurochemical and anatomical biomarkers in the mouse model could be valuable for evaluating new therapies for ADHD and mechanistic investigations into its etiology.

The outcome of dopaminergic signaling and effectiveness of dopaminergic drugs depend on the relative preponderance of each of the five dopamine receptors in a given brain region. The separate contribution of each receptor to overall dopaminergic tone is difficult to establish at a functional level due to lack of receptor subtype specific pharmacological agents. A surrogate for receptor function is receptor protein or mRNA expression. We examined dopamine receptor mRNA expression by quantitative reverse transcription real-time PCR in the striatum, globus pallidus, frontal cortex and cingulate cortex of embryonic and postnatal mice. Samples of each region were collected by laser capture microdissection. D1-and D2-receptor mRNAs were the most abundant in all the regions of the mature brain. The D1-receptor was predominant over the D2-receptor in the frontal and cingulate cortices whereas the situation was reversed in the striatum and globus pallidus. In the proliferative domains of the embryonic forebrain, D3-, D4-and D5-receptors were predominant. In the corpus striatum and cerebral cortex, the D3-and D4-receptors were the only receptors that showed marked developmental regulation. By analyzing D1 receptor protein expression, we show that developmental changes in mRNA expression reliably translate into changes in protein levels, at least for the D1-receptor.

We estimated the proportion of cells eliminated by histogenetic cell death during the first 2 postnatal weeks in areas 1, 3 and 40 of the mouse parietal neocortex. For each layer and for the subcortical white matter in each neocortical area, the number of dying cells per mm² was calculated and the proportionate cell death for each day of the 2-week interval was estimated. The data show that cell death proceeds essentially uniformly across the neocortical areas and layers and that it does not follow either the spatiotemporal gradient of cell cycle progression in the pseudostratified ventricular epithelium of the cerebral wall, the source of neocortical neurons, or the ‘inside-out’ neocortical neuronogenetic sequence. Therefore, we infer that the control mechanisms of neocortical histogenetic cell death are independent of mechanisms controlling neuronogenesis or neuronal migration but may be associated with the ingrowth, expansion and a system-wide matching of neuronal connectivity.

We describe a mouse model in which p27 Kip1 transgene expression is spatially restricted to the central nervous system neuroepithelium and temporally controlled with doxycycline. Transgenespecific transcripts are detectable within 6 h of doxycycline administration, and maximum nonlethal expression is approached within 12 h. After 18 -26 h of transgene expression, the G1 phase of the cell cycle is estimated to increase from 9 to 13 h in the neocortical neuroepithelium, the maximum G 1 phase length attainable in this proliferative population in normal mice. Thus our data establish a direct link between p27 Kip1 and control of G1 phase length in the mammalian central nervous system and unveil intrinsic mechanisms that constrain the G 1 phase length to a putative physiological maximum despite ongoing p27 Kip1 transgene expression.