For a great many reasons, research on mental illness is focused on the frontal cortex.  Its just a small part of the brain, and certainly, many things can go wrong in other places during brain/cognitive development, but, it remains a robust finding, that when the frontal cortex is not working well, individuals have difficulties in regulating thoughts and emotions.  Life is difficult enough to manage, let alone without a well functioning frontal cortex.  So its no surprise that many laboratories look very closely at how this region develops prenatally and during childhood.

One of the more powerful genetic methods is the analysis of gene expression via microarrays (here is a link to a tutorial on this technology).  When this technology is coupled with extremely careful histological analysis and dissection of cortical circuits in the frontal cortex, it begins to become possible to begin to link changes in gene expression with the physiological properties of specific cells and local circuits in the frontal cortex. The reason this is an exciting pursuit is because the mammalian neocortex is organized in a type of layered fashion wherein 6 major layers have different types of connectivity and functionality.  The developmental origins of this functional specificity are thought to lie in a process known as radial migration (here is a video of a neuron as it migrates radially and finds its place in the cortical hierarchy).  As cells are queued out of the ventricular zone, and begin their migration to the cortical surface, they are exposed to all sorts of growth factors and morphogens that help them differentiate and form the proper connectivities.  Thus, the genes that regulate this process are of keen interest to understanding normal and abnormal cognitive development.

Here’s an amazing example of this – 2 papers entitled, “Infragranular gene expression disturbances in the prefrontal cortex in schizophrenia: Signature of altered neural development?” [doi:10.1016/j.nbd.2009.12.013] and “Molecular markers distinguishing supragranular and infragranular layers in the human prefrontal cortex [doi:10.1111/j.1460-9568.2007.05396.x] both by Dominique Arion and colleagues.  In both papers, the authors ask, “what genes are differentially expressed in different layers of the cortex?”.  This is a powerful line of inquiry since the different layers of cortex are functionally different in terms of their connectivity.  For example, layers II-III (the so-called supragranular layers) are known to connect mainly to other cortical neurons – which is different functionally than layers V-VI (the so-called infragranular layers) that connect mainly to the striatum (layer V) and thalamus (layer VI).  Thus, if there are genes whose expression is unique to a layer, then one has a clue as to how that gene might contribute to normal/abnormal information processing.

The authors hail from a laboratory that is well-known for work over many years on fine-scaled histological analysis of the frontal cortex at the University of Pittsburgh and used a method called, laser capture microdissection, where post-mortem sections of human frontal cortex (area 46) were cut to separate the infragraular layer from the supragranular layer.  The mRNA from these tissue sections was then used for DNA microarray hybridization.  Various controls, replicate startegies and in-situ tissue hybridizations were then employed to validate the initial microarray results.

In first paper, the where the authors compare infra vs. supragranular layers, they report that 40 genes were more highly expressed in the supragranular layers (HOP, CUTL2 and MPPE1 were among the most enriched) and 29 genes were highly expressed in the infragranular layers (ZNF312, CHN2, HS3ST2 were among the most enriched).  Other differentially expressed genes included several that have previously been implicated in cortical layer formation such as RLN, TLX-NR2E1, SEMA3E, PCP4, SERPINE2, NR2F2/ARP1, PCDH8, WIF1, JAG1, MBP.  Amazing!! A handful of genes that seem to label subpopulations of projection neurons in the frontal cortex.  Polymorphic markers for these genes would surely be powerful tools for imaging-genetic studies on cognitive development.

In the second paper, the authors compare infra vs. supragranular gene expression in post-mortem brains from patients with schizophrenia and healthy matched controls. Using the same methods, the team reports both supra- and infragranular gene expression changes in schizophrenia (400 & 1200 differences respectively) – more than 70% of the differences appearing to be reductions in gene expression in schizophrenia. Interestingly, the team reports that the genes that were differentially expressed in the infragranular layers provided sufficient information to discriminate between cases and controls, whilst the gene expression differences in the supragranular layers did not.  More to the point, the team finds that 51 genes that were differentially expressed in infra- vs. supragranular expression were also differentially expressed in cases vs. controls  (many of these are also found to be associated in population genetic association studies of schiz vs. control as well!).  Thus, the team has identified layer (function) -specific genes that are associated with schizophrenia.  These genes, the ones enriched in the infragranular layers especially, seem to be at the crux of a poorly functioning frontal cortex.

The authors point to 3 such genes (SEMA3E, SEMA6D, SEMA3C) who happen to members of the same gene family – the semaphorin gene family.  This gene family is very important for the neuronal guidance (during radial migration), morphology, pruning and other processes where cell shape and position are regulated.  The authors propose that the semaphorins might act as “integrators” of various forms of wiring during development and in adulthood.  More broadly, the authors provide a framework to understand how the development of connectivity on the frontal cortex is regulated by genetic factors – indeed, many suspected genetic risk factors play a role in the developmental pathways the authors have focused on.

In Robert Sapolsky’s book, “Why Zebras Don’t Get Ulcers“, he details a biological feedback system wherein psychological stress leads to the release of glucocorticoids that have beneficial effects in the near-term but negative effects (e.g. ulcers, depression, etc.) in the long-term.  The key to getting the near-term benefits and avoiding the long-term costs – is to be able to turn OFF the flow of glucocorticoids.  This is normally dependent on circuitry involving the frontal cortex and hippocampus, that allow individuals to reset their expectations and acknowledge that everything is OK again.  Here’s the catch (i.e. mother nature’s ironic sense of humor). These very glucocorticoids can initiate a kind of reorganization or ‘shrinkage’ to the hippocampus  – and this can disable, or undermine the ability of the hippocampus to turn OFF the flow of glucocorticoids.  Yes, that’s right, the very switch that turns OFF glucocorticoid flow is disabled by exposure to glucocorticoids!  Can you imagine what happens when that switch (hippocampus) get progressively more disabled?  Your ability to turn OFF glucocorticoids gets progressively worse and the negative effects of stress become more and more difficult to cope with.

Sounds depressing.  Indeed it is, and there are many findings of reduced hippocampal volume in various depressive illnesses.  The complex problem at hand, then, is how to reverse the runaway-train-like (depression leads to glucocorticoids which leads to smaller hippocampus which leads to more depression) effects of stress and depression?

One new avenue of research has been focused on the ability of the hippocampus to normally produce new cells – neurogenesis – throughout life.  Might such cells be useful in reversing hippocampal remodeling (shrinkage)?  If so, what molecules or genes might be targeted to drive this process in a treatment setting?

The recent paper by Joffe and colleagues, “Brain derived neurotrophic factor Val66Met polymorphism, the five factor model of personality and hippocampal volume: Implications for depressive illness” [doi: 10.1002/hbm.20592] offers some key insights.  They examined 467 healthy participants of the Brain Resource International Database (a personalized medicine company with a focus on brain health) using personality tests, structural brain imaging and genotyping for an A-to-G variation (valine-to-methionine) polymorphism in the BDNF gene.  They report that lower volume of the hippocampus was associated with higher scores of neuroticism (worriers) – but, this negative relationship was not found in all people – just those who carry the A- or methionine-allele.  Thus, those individuals who carry the G/G (valine/valine) genotype of BDNF may be somewhat more protected from the negative (hippocampal remodeling) effects of psychological stress.  Interestingly, the BDNF gene seems to play a role in brain repair!  So perhaps this neuro-biochemical pathway can be explored to further therapeutic benefit.  Exciting!!

By the way, the reason zebras don’t get ulcers, is because their life revolves around a lot of short term stressors (mainly hungry lions) where the glucocorticoid-stress system works wonderfully to keep them alive.  Its only homo sapiens who has enough long-term memory to sit around in front of the TV and incessantly fret about the mortgage, the neighbors, the 401K etc., who have the capacity to bring down all the negative, toxic effects of chronic glucocorticoids exposure upon themselves. My 23andMe profile shows that I am a G/G valine/valine … does this mean I’m free to worry more?  Now I’m worried.  More on BDNF here.

For more than a decade, we’ve known that at least 95% of the human genome is junk – or junque – if you’re offended by the thought that “you” emerged from a single cell whose genome is mostly a vast pile of crap – or crappe – if you insist.  Hmmm, what is this crap?  It turns out to be a lot of random repeating sequences and a massive collection of evolutionary artifacts left over from the evolution of earlier genomes – mainly bits of retroviruses who once inserted themselves irreversibly into our ancestors’ genomes.  One subset of this type of – can we upgrade it from crappe to “relic” now? – is something we’ve labelled “autonomously mobile DNA sequences” or more specifically, “long interspersed nuclear elements (LINEs or L1s)”.  This class of DNA relic comprises more than 15% of the human genome (that’s about 3-5x more than the relevant genomic sequence from which you emerge) and retains the ability to pick itself up out of the genome – via an RNA intermediate – and insert itself into new places in the genome.  This has been observed to happen in the germ line of humans and a few L1 insertions are even responsible for genetic forms of humn disease (for example in the factor VIII gene giving rise to haemophilia).  The mechanism of transposition – or “jumping” as these elements are sometimes called “jumping genes” – involves the assembly of a certain type of transcriptional, transport and reverse-transcription (RNA back to DNA) apparatus that is known to be available in stem cells, but hardly ever  in somatic cells.

Except, it would seem, for the brain – which as we’ve covered here before – keeps its precious neurons and glia functioning under separate rules.  Let’s face it, if a liver cell dies, you just replace it without notice, but if neurons die, so do your childhood memories.  So its not too surprising, perhaps, that brain cells have special ‘stem-cell-like’ rules for keeping themselves youthful.  This seems to be borne out again in a paper entitled, “L1 retrotransposition in human neural progenitor cells” by Coufal et al., [doi:10.1038/nature08248].  Here the team shows that L1 elements are able to transpose themselves in neural stem cells and that there are more L1 elements (about 80 copies more per cell) in the hippocampus than in liver or heart cells.  So apparently, the hippocampus, which does seem to contain a niche of stem cells, permits the transposition or “jumping” of L1 elements in a way that the liver and heart do not.  Sounds like a fun place to be a gene!

Having blogged here several times on various and sundry roles of BDNF in cognitive function, it was pretty cool to see the recent paper, “Neural stem cells improve cognition via BDNF in a transgenic model of Alzheimer disease” [doi: 10.1073/pnas.0901402106].  It seems that in a transgenic mouse model for Alzheimer’s Disease that injection of neural stem cells into the plaqued/tangled hippocampus can rescue hippocampal-dependent behaviors.  This rescue however, seems to have been dependent on the secretion of BDNF since knock-down of BDNF ablated the rescue, while increasing BDNF improved the rescue.  The stem-cell treatment did not however reduce levels of plaques or tangles but did increase synaptic density – which I’d be happy to have more of – plaques/tangles notwithstanding.  Promising findings!

One of the mental functions many of us take for granted is memory – that is – until we’re at the grocery store.  If you’re like me, you dart out of the house confident that you don’t need a list since you’re just going to “pick up a few things” – only to return home and discover (hours later when you’re comfortably ensconced on the couch) that you forgot the ice cream.  Damn, why can’t I have a more efficient working memory system ?  What’s the matter with my lateral frontal cortex ?  Can I (should I) blame it on my genes ? What genes specifically ?

One group recently reported the use of the so-called BOLD-response (blood oxygen level dependent) as a means to sift through the human genome and identify genes that mediate the level of brain activity in the lateral frontal cortex that occur during a working memory task – somewhat akin to remembering a list of groceries.  Steven Potkin and associates in their paper, “Gene discovery through imaging-genetics: identification of two novel genes associated with schizophrenia” [doi: 10.1038/mp.2008.127] examine the level of brain activity in 28 patients with schizophrenia (a disorder where mental function in the lateral frontal cortex is disrupted) and correlate this brain activity (difference between short and long list) with genetic differences at 100,000 snps spread across the autosomes.

They identify 2 genes (that pass an additional series of statistical hurdles designed to weed-out false positive results) RSRC1 and ARHGAP18, heretofore, never having been connected to mental function.  Although neither protein is neuron or brain-specific in its expression, ARHGAP18 is a member of the Rho/Rac/Cdc42-like GTPase activating (RhoGAP) gene family which are well known regulators of the actin cytoskeleton (perhaps  a role in synaptic plasticity ?) and RSRC1 is reported to bind to actin homologs. Also, RSRC1 may play a role in forebrain development since it is expressed in cdc34+ stem cells that migrate under the control of TGF-alpha (As an aside, yours truly co-published a paper showing that TGF-alpha is regulated by early maternal care – possible connection ? Hmm).  A last possibility is a role in RNA splicing which many SR-proteins like RSRC1 function in – which also could be important for synaptic function as many mRNA’s are stored in synaptic terminals.

The authors’ method is completely novel and they seem to have discovered 2 new points from which to further explore the genetic basis of mental disability.  It will be of great interest to see where the research leads next.