Jared T. Ahrendsen

Little Exercise, Big Effects: Reversing Aging and Infection-Induced Memory Deficits, and Underlying Processes

We have previously found that healthy aged rats are more likely to suffer profound memory impairments following a severe bacterial infection than are younger adult rats. Such a peripheral challenge is capable of producing a neuroinflammatory response, and in the aged brain this response is exaggerated and prolonged. Normal aging primes, or sensitizes, microglia, and this appears to be the source of this amplified inflammatory response. Among the outcomes of this exaggerated neuroinflammatory response are impairments in synaptic plasticity and reductions of brain-derived neurotrophic factor (BDNF), both of which have been associated with cognitive impairments. Since it has been shown that physical exercise increases BDNF mRNA in the hippocampus, the present study examined voluntary exercise in 24-month-old F344×BN rats as a neuroprotective therapeutic in our bacterial infection model. Although aged rats ran only an average of 0.7 km per week, this small amount of exercise was sufficient to completely reverse infection-induced impairments in hippocampus-dependent long-term memory compared with sedentary animals. Strikingly, exercise prevented the infection-induced exaggerated neuroinflammatory response and the blunted BDNF mRNA induction seen in the hippocampus of sedentary rats. Moreover, voluntary exercise abrogated age-related microglial sensitization, suggesting a possible mechanism for exercise-induced neuroprotection in aging.

Conditional Ablation of Raptor or Rictor Has Differential Impact on Oligodendrocyte Differentiation and CNS Myelination

During CNS development, oligodendrocytes, the myelinating glia of the CNS, progress through multiple transitory stages before terminating into fully mature cells. Oligodendrocyte differentiation and myelination is a tightly regulated process requiring extracellular signals to converge to elicit specific translational and transcriptional changes. Our lab has previously shown that the protein kinases, Akt and mammalian Target of Rapamycin (mTOR), are important regulators of CNS myelination in vivo. mTOR functions through two distinct complexes, mTOR complex 1 (mTORC1) and mTORC2, by binding to either Raptor or Rictor, respectively. To establish whether the impact of mTOR on CNS myelination results from unique functions of mTORC1 or mTORC2 during CNS myelination, we conditionally ablated either Raptor or Rictor in the oligodendrocyte lineage, in vivo. We show that Raptor (mTORC1) is a positive regulator of developmental CNS mouse myelination when mTORC2 is functional, whereas Rictor (mTORC2) ablation has a modest positive effect on oligodendrocyte differentiation, and very little effect on myelination, when mTORC1 is functional. Also, we show that loss of Raptor in oligodendrocytes results in differential dysmyelination in specific areas of the CNS, with the greatest impact on spinal cord myelination.

Aging and infection reduce expression of specific brain-derived neurotrophic factor mRNAs in hippocampus.

Aging increases the likelihood of cognitive decline after negative life events such as infection or injury. We have modeled this increased vulnerability in aged (24-month-old), but otherwise unimpaired F344xBN rats. In these animals, but not in younger (3-month-old) counterparts, a single intraperitoneal injection of E. coli leads to specific deficits in long-term memory and long-lasting synaptic plasticity in hippocampal area CA1-processes strongly dependent on brain-derived neurotrophic factor (BDNF). Here we have investigated the effects of age and infection on basal and fear-conditioning-stimulated expression of Bdnf in hippocampus. We performed in situ hybridization with 6 probes recognizing: total (pan-)BDNF mRNA, the 4 predominant 5 exon-specific transcripts (I, II, IV, and VI), and BDNF mRNAs with a long 3 untranslated region (3 UTR). In CA1, aging reduced basal levels and fear-conditioning-induced expression of total BDNF mRNA, exon IV-specific transcripts, and transcripts with long 3 UTRs; effects of infection were similar and sometimes compounded the effects of aging. In CA3, aging reduced all of the transcripts to some degree; infection had no effect. Effects in dentate were minimal. Northern blot analysis confirmed an aging-associated loss of total BDNF mRNA in areas CA1 and CA3, and revealed a parallel, preferential loss of BDNF mRNA transcripts with long 3 UTRs.

Synaptic Correlates of Increased Cognitive Vulnerability with Aging: Peripheral Immune Challenge and Aging Interact to Disrupt Theta-Burst Late-Phase Long-Term Potentiation in Hippocampal Area CA1

Variability in cognitive functioning increases markedly with age, as does cognitive vulnerability to physiological and psychological challenges. Exploring the basis of this vulnerability may provide important insights into the mechanisms underlying aging-associated cognitive decline. As we have previously reported, the cognitive abilities of aging (24-month-old) F344 × BN rats are generally good, but are more vulnerable to the consequences of a peripheral immune challenge (an intraperitoneal injection of live Escherichia coli) than those of their younger (3-month-old) counterparts. Four days after the injection, the aging, but not the young rats show profound memory deficits, specific to the consolidation of hippocampus-dependent memory processes. Here, we have extended these observations, using hippocampal slices to examine for the first time the combined effects of aging and a recent infection on several forms of synaptic plasticity. We have found that the specific deficit in long-lasting memory observed in the aged animals after infection is mirrored by a specific deficit in a form of long-lasting synaptic plasticity. The late-phase long-term potentiation induced in area CA1 using theta-burst stimulation is particularly compromised by the combined effects of aging and infection—a deficit that can be ameliorated by intra-cisterna magna administration of the naturally occurring antiinflammatory cytokine IL-1Ra (interleukin-1 receptor antagonist). These data support the idea that the combination of aging and a negative life event such as an infection might produce selective, early-stage failures of synaptic plasticity in the hippocampus, with corresponding selective deficits in memory.

Olig1 Function Is Required for Oligodendrocyte Differentiation in the Mouse Brain

Oligodendrocyte differentiation and myelination are tightly regulated processes orchestrated by a complex transcriptional network. Two bHLH transcription factors in this network, Olig1 and Olig2, are expressed exclusively by oligodendrocytes after late embryonic development. Although the role of Olig2 in the lineage is well established, the role of Olig1 is still unclear. The current studies analyzed the function of Olig1 in oligodendrocyte differentiation and developmental myelination in brain. Both oligodendrocyte progenitor cell commitment and oligodendrocyte differentiation were impaired in the corpus callosum of Olig1-null mice, resulting in hypomyelination throughout adulthood in the brain. As seen in previous studies with this mouse line, although there was an early myelination deficit in the spinal cord, essentially full recovery with normal spinal cord myelination was seen. Intriguingly, this regional difference may be partially attributed to compensatory upregulation of Olig2 protein expression in the spinal cord after Olig1 deletion, which is not seen in brain. The current study demonstrates a unique role for Olig1 in promoting oligodendrocyte progenitor cell commitment, differentiation, and subsequent myelination primarily in brain, but not spinal cord.

Signaling mechanisms regulating myelination in the central nervous system.

The precise and coordinated production of myelin is essential for proper development and function of the nervous system. Diseases that disrupt myelin, including multiple sclerosis, cause significant functional disability. Current treatment aims to reduce the inflammatory component of the disease, thereby preventing damage resulting from demyelination. However, therapies are not yet available to improve natural repair processes after damage has already occurred. A thorough understanding of the signaling mechanisms that regulate myelin generation will improve our ability to enhance repair. in this review, we summarize the positive and negative regulators of myelination, focusing primarily on central nervous system myelination. Axon-derived signals, extracellular signals from both diffusible factors and the extracellular matrix, and intracellular signaling pathways within myelinating oligodendrocytes are discussed. Much is known about the positive regulators that drive myelination, while less is known about the negative regulators that shift active myelination to myelin maintenance at the appropriate time. Therefore, we also provide new data on potential negative regulators of CNS myelination.

Juvenile striatal white matter is resistant to ischemia‐induced damage

White matter injury following ischemic stroke is a major cause of functional disability. Injury to both myelinated axons and oligodendrocytes, the myelin producing cells in the central nervous system, occurs in experimental models of ischemic stroke. Age‐related changes in white matter vulnerability to ischemia have been extensively studied and suggest that both the perinatal and the aged periods are times of increased white matter vulnerability. However, sensitivity of white matter following stroke in the juvenile brain has not been evaluated. Interestingly, the late pediatric period is an important developmental stage, as it is the time of maximal myelination. The current study demonstrates that neurons in late pediatric/juvenile striatum are vulnerable to ischemic damage, with neuronal injury being comparable in juvenile and adult mice following ischemia. By contrast, actively myelinating striatal oligodendrocytes in the juvenile brain are resistant to ischemia, whereas adult oligodendrocytes are quite sensitive. As a result, myelin sheaths are remarkably intact and axons survive well in the injured striatum of juvenile mice. In addition to relative resistance of juvenile white matter, other glial responses were very different in juvenile and adult mice following cerebral ischemia, including differences in astrogliosis, fibrosis, NG2‐cell reactivity, and vascular integrity. Together, these responses lead to long‐term preservation of brain parenchyma in juvenile mice, compared to severe tissue loss and scarring in adult mice. Overall, the current study suggests that equivalent ischemic insults may result in less functional deficit in children compared to adults and an environment more conducive to long‐term recovery. GLIA 2016;64:1972–1986

Endogenous Neuronal Replacement in the Juvenile Brain Following Cerebral Ischemia

Replacement of dead neurons following ischemia, either via enhanced endogenous neurogenesis or stem cell therapy, has long been sought. Unfortunately, while various therapies that enhance neurogenesis or stem cell therapies have proven beneficial in animal models, they have all uniformly failed to truly replace dead neurons in the ischemic core to facilitate long-term recovery. Remarkably, we observe robust repopulation of medium-spiny neurons within the ischemic core of juvenile mice following experimental stroke. Despite extensive neuronal cell death in the injured striatum of both juveniles and adults at acute time points after ischemia (24u202fh and 7u202fd), mature newborn neurons replaced lost striatal neurons at 30u202fd post-ischemia. This neuronal repopulation was found only in juveniles, not adults, and importantly, was accompanied by enhanced post-ischemic behavioral recovery at 30u202fd. Ablation of neurogenesis using irradiation prevented neuronal replacement and functional recovery in MCAo-injured juvenile mice. In contrast, findings in adults were consistent with previous reports, that newborn neurons failed to mature and died, offering little therapeutic potential. These data provide support for neuronal replacement and consequent functional recovery following ischemic stroke and new targets in the development of novel therapies to treat stroke.

Gestational Diabetes Prevalence at Moderate and High Altitude

BACKGROUNDnHigh-altitude (HA) pregnancies have been associated with decreased glucose levels and increased insulin sensitivity versus sea level. Our objective was to determine if the prevalence of gestational diabetes mellitus (GDM) and the impact of demographic characteristics on GDM diagnosis differed at moderate altitude (MA) versus HA.nnnMETHODSnUsing a retrospective cohort design, we compared women living at HA (>8250 ft) and MA (4000-7000 ft) during pregnancy. Exclusion criteria were as follows: multiple gestation, preexisting diabetes, unavailable GDM results, or relocation from a different altitude during pregnancy. GDM diagnosis was determined using Carpenter and Coustan criteria. Data were compared by t-test (continuous variables) or chi-squared tests (categorical variables). Univariate, multivariate, and stepwise regression models were used to assess the impact of various factors on GDM prevalence.nnnRESULTSnThere was no difference in GDM prevalence between altitudes in these populations; the relationship between altitude and GDM was nonsignificant in all regression analyses. At MA, maternal age, Hispanic ethnicity, body mass index (BMI), and gestational age (GA) at testing increased GDM incidence in univariate analyses. At HA, maternal age, Hispanic ethnicity, and multiparity increased GDM incidence in univariate analyses.nnnCONCLUSIONnWhile GDM prevalence did not differ between MA and HA, the impact of maternal demographic characteristics on GDM risk varied by altitude group. Higher BMI and greater GA at testing increased the incidence of GDM at MA, but not at HA. Multiparity had an effect at HA, but not MA. These differences may represent subtle differences in glucose metabolism at HA.

The protein tyrosine phosphatase Shp2 regulates oligodendrocyte differentiation and early myelination and contributes to timely remyelination.

Shp2 is a nonreceptor protein tyrosine phosphatase that has been shown to influence neurogenesis, oligodendrogenesis, and oligodendrocyte differentiation. Furthermore, Shp2 is a known regulator of the Akt/mammalian target of rapamycin and ERK signaling pathways in multiple cellular contexts, including oligodendrocytes. Its role during later postnatal CNS development or in response to demyelination injury has not been examined. Based on the current studies, we hypothesize that Shp2 is a negative regulator of CNS myelination. Using transgenic mouse technology, we show that Shp2 is involved in oligodendrocyte differentiation and early myelination, but is not necessary for myelin maintenance. We also show that Shp2 regulates the timely differentiation of oligodendrocytes following lysolecithin-induced demyelination, although apparently normal remyelination occurs at a delayed time point. These data suggest that Shp2 is a relevant therapeutic target in demyelinating diseases such as multiple sclerosis. SIGNIFICANCE STATEMENT In the present study, we show that the protein phosphatase Shp2 is an important mediator of oligodendrocyte differentiation and myelination, both during developmental myelination as well as during myelin regeneration. We provide important insight into the signaling mechanisms regulating myelination and propose that Shp2 acts as a transient brake to the developmental myelination process. Furthermore, we show that Shp2 regulates oligodendrocyte differentiation following demyelination and therefore has important therapeutic implications in diseases such as multiple sclerosis.