Adult neurogenesis and neurodegenerative disease

Enhancement of dentate gyrus neurogenesis, dendritic and synaptic plasticity and memory by a neurotrophic peptide

Pharmacological enhancement of hippocampal neurogenesis is a therapeutic approach for improvement of cognition in learning and memory disorders such as Alzheimer's disease. Here we report the development of an 11-mer peptide that we designed based on a biologically active region of the ciliary neurotrophic factor. This peptide, Peptide 6, induced proliferation and increased survival and maturation of neural progenitor cells into neurons in the dentate gyrus of normal adult C57BL6 mice. Furthermore, Peptide 6 increased the MAP2 and synaptophysin immunoreactivity in the dentate gyrus. Thirty-day treatment of the mice with a slow release bolus of the peptide implanted subcutaneously improved reference memory of the mice in Morris water maze. Peptide 6 has a plasma half life of over 6 h, is blood-brain barrier permeable, and acts by competitively inhibiting the leukemia inhibitory factor signaling. The fact that Peptide 6 is both neurogenic and neurotrophic and that this peptide is effective when given peripherally, demonstrates its potential for prevention and treatment of learning and memory disorders.

The neuronal primary cilium: driver of neurogenesis and memory formation in the hippocampal dentate gyrus?

Considerable attention has recently been focused on the postnatal persistence of neurogenesis in the dentate gyrus of the hippocampus and the roles of signals from the primary cilium in the different functions of an increasing number of tissues and their malfunctions. Here we summarize the evidence that ties sonic hedgehog-triggered proliferogenic signaling from the primary cilia on granule cell progenitors in the adult dentate subgranular zone to maintain a pool of new "blank slate" dentate granule cells. These can be recruited to bundle and encode novel inputs flowing from various regions of the brain into the dentate gyrus via the entorhinal cortex without altering and erasing the synaptic patterns from previous inputs inscribed on older granule cells.

Stem-cell-associated structural and functional plasticity in the aging hippocampus

Aging frequently leads to a functional decline across multiple cognitive domains, often resulting in a severe reduction in life quality and also causing substantial care-related costs. Understanding age-associated structural and functional changes of neural circuitries within the brain is required to improve successful aging. In this review, the authors focus on age-dependent alterations of the hippocampus and the decline of hippocampal function, which are critically involved in processes underlying certain forms of learning and memory. Despite the dramatic reductions in hippocampus-dependent function that accompany advancing age, there is also striking evidence that even the aged brain retains a high level of plasticity. Thus, one promising avenue to reach the goal of successful aging might be to boost and recruit this plasticity, which is the interplay between neural structure, function, and experience, to prevent age-related cognitive decline and age-associated comorbidities.

CXCR4 signaling in the regulation of stem cell migration and development

The regulated migration of stem cells is a feature of the development of all tissues and also of a number of pathologies. In the former situation the migration of stem cells over large distances is required for the correct formation of the embryo. In addition, stem cells are deposited in niche like regions in adult tissues where they can be called upon for tissue regeneration and repair. The migration of cancer stem cells is a feature of the metastatic nature of this disease. In this article we discuss observations that have demonstrated the important role of chemokine signaling in the regulation of stem cell migration in both normal and pathological situations. It has been demonstrated that the chemokine receptor CXCR4 is expressed in numerous types of embryonic and adult stem cells and the chemokine SDF-1/CXCL12 has chemoattractant effects on these cells. Animals in which SDF-1/CXCR4 signaling has been interrupted exhibit numerous phenotypes that can be explained as resulting from inhibition of SDF-1 mediated chemoattraction of stem cells. Hence, CXCR4 signaling is a key element in understanding the functions of stem cells in normal development and in diverse pathological situations.

Impaired adult neurogenesis associated with short-term memory defects in NF-kappaB p50-deficient mice

Neurogenesis proceeds throughout adulthood in the brain of most mammalian species, but the molecular mechanisms underlying the regulation of stem/progenitor cell proliferation, survival, maturation, and differentiation have not been completely unraveled. We have studied hippocampal neurogenesis in NF-kappaB p50-deficient mice. Here we demonstrate that in absence of p50, the net rate of neural precursor proliferation does not change, but some of the steps leading to the final neuron differentiation status are hampered, resulting in approximately 50% reduction in the number of newly born neurons in the adult mutant hippocampus. Additionally, in p50(-/-) mice, we observed a selective defect in short-term spatial memory performance without impairment of hippocampal-dependent spatial long-term memory and learning. Our results highlight the role of NF-kappaB p50 in hippocampal neurogenesis and in short-term spatial memory.

The contribution of failing adult hippocampal neurogenesis to psychiatric disorders

PURPOSE OF REVIEW

Failing adult neurogenesis is increasingly considered a factor in the pathogenesis and course of psychiatric disorders. The level of evidence in favor of such hypotheses varies, but disturbed cellular plasticity in the hippocampus may be a common aspect of several neuropsychiatric diseases.

RECENT FINDINGS

This review covers the literature from mid-2006 to the end of 2007. We discuss studies and theoretical papers dealing with the contribution of adult neurogenesis to dementias and neurodegeneration, major depression, schizophrenia, and alcohol and drug abuse. Of these disorders, most progress has recently been made with schizophrenia for which, in contrast to the other conditions, suggestive genetic evidence exists (e.g. Disc1, Npas3).

SUMMARY

Failing adult hippocampal neurogenesis may not explain major depression, addiction or schizophrenia, but contributes to the hippocampal aspects of the disease. We propose that the key to a more thorough understanding of this contribution will come from increased knowledge on the functional relevance of new neurons in the hippocampus and better clinical data relating to symptoms possibly related to such function. Research on the molecular basis of adult hippocampal neurogenesis may help to explain how hippocampal aspects of these disorders develop.

Potential therapeutic interest of adenosine A2A receptors in psychiatric disorders

The interest on targeting adenosine A(2A) receptors in the realm of psychiatric diseases first arose based on their tight physical and functional interaction with dopamine D(2) receptors. However, the role of central A(2A) receptors is now viewed as much broader than just controlling D(2) receptor function. Thus, there is currently a major interest in the ability of A(2A) receptors to control synaptic plasticity at glutamatergic synapses. This is due to a combined ability of A(2A) receptors to facilitate the release of glutamate and the activation of NMDA receptors. Therefore, A(2A) receptors are now conceived as a normalizing device promoting adequate adaptive responses in neuronal circuits, a role similar to that fulfilled, in essence, by dopamine. This makes A(2A) receptors particularly attractive targets to manage psychiatric disorders since adenosine may act as go-between glutamate and dopamine, two of the key players in mood processing. Furthermore, A(2A) receptors also control glia function and brain metabolic adaptation, two other emerging mechanisms to understand abnormal processing of mood, and A(2A) receptors are important players in controlling the demise of neurodegeneration, considered an amplificatory loop in psychiatric disorders. Current data only provide an indirect confirmation of this putative role of A(2A) receptors, based on the effects of caffeine (an antagonist of both A(1) and A(2A) receptors) in psychiatric disorders. However, the introduction of A(2A) receptors antagonists in clinics as anti-parkinsonian agents is hoped to bolster our knowledge on the role of A(2A) receptors in mood disorders in the near future.

Adult hippocampal neurogenesis and aging

The demographic changes in the foreseeable future stress the need for research on successful cognitive aging. Advancing age constitutes a primary risk factor for disease of the central nervous system most notably neurodegenerative disorders. The hippocampus is one of the brain regions that is prominently affected by neurodegeneration and functional decline even in what is still considered "normal aging". Plasticity is the basis for how the brain adapts to changes over time. The discovery of adult hippocampal neurogenesis has added a whole new dimension to research on structural plasticity in the adult and aging hippocampus. In this article, we briefly summarize and discuss recent findings on the regulation of adult neurogenesis with relevance to aging. Aging is an important co-variable for many regulatory mechanisms affecting adult neurogenesis but so far, only few studies have specifically addressed this interaction. We hypothesize that adult neurogenesis contributes to a neural reserve, i.e. the maintained potential for structural plasticity that allows compensation in situations of functional losses with aging. As such we propose that adult neurogenesis might contribute to the structural correlates of successful aging.

Two genetic rat models of arterial hypertension show different mechanisms by which adult hippocampal neurogenesis is increased

To investigate strain differences and genetic effects on different aspects of neurogenesis, we compared young adult spontaneously hypertensive/hyperactive rats (SHR) and stroke-prone SHR (SHRSP) with the genetic control WKY strain. In both hypertensive/hyperactive strains, the number of newly generated neurons and the number of lineage-determined cells as detected by doublecortin (DCX) immunoreactivity were significantly increased. SHRSP had significantly more DCX-positive cells than the other groups. Whereas cell proliferation as measured by Ki67 expression was increased in SHR, we found no difference between SHRSP and WKY. In summary, we found increased net neurogenesis in both hypertensive/hyperactive strains. However, this phenotype was based on different mechanisms in the course of neuronal development: cell proliferation in SHR and cell survival in SHRSP. In addition, we found that within strains the number of DCX-positive cells was not predictive of the net number of new neurons and that the increase in neurogenesis was not significantly correlated with blood pressure in SHR and WKY. However, in both SHR and SHRSP, cell proliferation showed an association with blood pressure recordings.