Converging lines of research suggest that the hippocampal complex (HC) may have a role in the pathophysiology of major depressive disorder (MDD). Although postmortem studies show little cellular death in the HC of depressed patients, animal studies suggest that elevated glucocorticoid levels associated with MDD may negatively affect neurogenesis, cause excitotoxic damage or be associated with reduced levels of key neurotrophins in the HC.
Antidepressant medications may counter these effects, having been shown to increase HC neurogenesis and levels of brain-derived neurotrophic factor in animal studies. Neuropsychological studies have identified deficits in hippocampus-dependent recollection memory that may not abate with euthymia, and such memory impairment has been the most reliably documented cognitive abnormality in patients with MDD.
Finally, data from imaging studies suggest both structural changes in the volume of the HC and functional alterations in frontotemporal and limbic circuits that may be critical for mood regulation. The extent to which such functional and structural changes determine clinical outcome in MDD remains unknown; a related, but also currently unanswered, question is whether the changes in HC function and structure observed in MDD are preventable or modifiable with effective treatment for the depressive illness.
Brain insulin and insulin receptors in aging and sporadic Alzheimer’s disease.
The search for the causes of neurodegenerative disorders is a major theme in brain research. Acquired disturbances of several aspects of cellular metabolism appear pathologically important in sporadic Alzheimer’s disease (SDAT). Among these brain glucose utilisation is reduced in the early stages of the disease and the regulatory enzymes important for glucose metabolism are reduced.
In the brain, insulin, insulin-like growth factors and their receptors regulate glucose metabolism and promote neuronal growth. To detect changes in the functional activity of the brain insulin neuromodulatory system of SDAT patients, we determined the concentrations of insulin and c-peptide as well as insulin receptor binding and IGF-I receptor binding in several regions of postmortem brain cortex during aging and Alzheimer’s disease.
Additionally, we performed immunohistochemical staining with antibodies against insulin in neocortical brain areas in SDAT and controls. We show for the first time that insulin and c-peptide concentration in the brain are correlated and decrease with aging, as do brain insulin receptor densities.
Weak insulin-immunoreactivity could be demonstrated histochemically in pyramidal neurons of controls, whereas in SDAT a stronger insulin-immunoreactivity was found. On a biochemical level, insulin and c-peptide levels were reduced compared to middle-aged controls, but were unchanged compared to age-matched controls.
Brain insulin receptor densities in SDAT were decreased compared to middle-aged controls, but increased in comparison to age-matched controls. IGF-I receptor densities were unchanged in aging and in SDAT. Tyrosine kinase activity, a signal transduction mechanism common to both receptor systems, was reduced in SDAT in comparison to middle-aged and age-matched control groups.
These data are consistent with a neurotrophic role of insulin in the human brain and a disturbance of insulin signal transduction in SDAT brain and favor the hypothesis that insulin dependent functions may be of pathogenetic relevance in sporadic SDAT.
Divergence of human and mouse brain transcriptome highlights Alzheimer disease pathways.
Because mouse models play a crucial role in biomedical research related to the human nervous system, understanding the similarities and differences between mouse and human brain is of fundamental importance. Studies comparing transcription in human and mouse have come to varied conclusions, in part because of their relatively small sample sizes or underpowered methodologies.
To better characterize gene expression differences between mouse and human, we took a systems-biology approach by using weighted gene coexpression network analysis on more than 1,000 microarrays from brain. We find that global network properties of the brain transcriptome are highly preserved between species.
Furthermore, all modules of highly coexpressed genes identified in mouse were identified in human, with those related to conserved cellular functions showing the strongest between-species preservation. Modules corresponding to glial and neuronal cells were sufficiently preserved between mouse and human to permit identification of cross species cell-class marker genes.
We also identify several robust human-specific modules, including one strongly correlated with measures of Alzheimer disease progression across multiple data sets, whose hubs are poorly-characterized genes likely involved in Alzheimer disease.
We present multiple lines of evidence suggesting links between neurodegenerative disease and glial cell types in human, including human-specific correlation of presenilin-1 with oligodendrocyte markers, and significant enrichment for known neurodegenerative disease genes in microglial modules.
Together, this work identifies convergent and divergent pathways in mouse and human, and provides a systematic framework that will be useful for understanding the applicability of mouse models for human brain disorders.
The nature of selection on the major histocompatibility complex.
Only natural selection can account for the extreme genetic diversity of genes of the major histocompatibility complex (MHC). Although the structure and function of classic MHC genes is well understood at the molecular and cellular levels, there is controversy about how MHC diversity is selectively maintained.
The diversifying selection can be driven by pathogen interactions and inbreeding avoidance mechanisms. Pathogen-driven selection can maintain MHC polymorphism based on heterozygote advantage or frequency-dependent selection due to pathogen evasion of MHC-dependent immune recognition.
Empirical evidence demonstrates that specific MHC haplotypes are resistant to certain infectious agents, while susceptible to others. These data are consistent with both heterozygote advantage and frequency-dependent models. Additional research is needed to discriminate between these mechanisms. Infectious agents can precipitate autoimmunity and can potentially contribute to MHC diversity through molecular mimicry and by favoring immunodominance.
MHC-dependent abortion and mate choice, based on olfaction, can also maintain MHC diversity and probably functions both to avoid genome-wide inbreeding and produce MHC-heterozygous offspring with increased immune responsiveness. Although this diverse set of hypotheses are often treated as competing alternatives, we believe that they all fit into a coherent, internally consistent thesis. It is likely that at least in some species, all of these mechanisms operate, leading to the extreme diversification found in MHC genes.