Reduced laminin immunoreactivity in the blood vessel wall of ageing rats correlates with reduced innervation in vivo and following transplantation

Vascular smooth muscle stiffness and its role in aging

Vascular smooth muscle cells (VSMC) are now considered important contributors to the pathophysiological and biophysical mechanisms underlying arterial stiffening in aging. Here, we review mechanisms whereby VSMC stiffening alters vascular function and contributes to the changes in vascular stiffening observed in aging and cardiovascular disease. Vascular stiffening in arterial aging was historically associated with changes in the extracellular matrix; however, new evidence suggests that endothelial and vascular smooth muscle cell stiffness also contribute to overall blood vessel stiffness. Furthermore, VSMC play an integral role in regulating matrix deposition and vessel wall contractility via interaction between the actomyosin contractile unit and adhesion structures that anchor the cell within the extracellular matrix. Aged-induce phenotypic modulation of VSMC from a contractile to a synthetic phenotype is associated with decreased cellular contractility and increased cell stiffness. Aged VSMC also display reduced mechanosensitivity and adaptation to mechanical signals from their microenvironment due to impaired intracellular signaling. Finally, evidence for decreased contractility in arteries from aged animals demonstrate that changes at the cellular level result in decreased functional properties at the tissue level.

Laminin α4 overexpression in the anterior lens capsule may contribute to the senescence of human lens epithelial cells in age-related cataract

Senescence is a leading cause of age-related cataract (ARC). The current study indicated that the senescence-associated protein, p53, total laminin (LM), LMα4, and transforming growth factor-beta1 (TGF-β1) in the cataractous anterior lens capsules (ALCs) increase with the grades of ARC. In cataractous ALCs, patient age, total LM, LMα4, TGF-β1, were all positively correlated with p53. In lens epithelial cell (HLE B-3) senescence models, matrix metalloproteinase-9 (MMP-9) alleviated senescence by decreasing the expression of total LM and LMα4; TGF-β1 induced senescence by increasing the expression of total LM and LMα4. Furthermore, MMP-9 silencing increased p-p38 and LMα4 expression; anti-LMα4 globular domain antibody alleviated senescence by decreasing the expression of p-p38 and LMα4; pharmacological inhibition of p38 MAPK signaling alleviated senescence by decreasing the expression of LMα4. Finally, in cataractous ALCs, positive correlations were found between LMα4 and total LM, as well as between LMα4 and TGF-β1. Taken together, our results implied that the elevated LMα4, which was possibly caused by the decreased MMP-9, increased TGF-β1 and activated p38 MAPK signaling during senescence, leading to the development of ARC. LMα4 and its regulatory factors show potential as targets for drug development for prevention and treatment of ARC.

Human Dental Pulp Stem Cells Grown in Neurogenic Media Differentiate Into Endothelial Cells and Promote Neovasculogenesis in the Mouse Brain

Dental pulp stem cells (DPSCs) have the capacity to give rise to cells with neuronal-like phenotypes, suggesting their use in brain cell therapies. In the present work, we wanted to address the phenotypic fate of adult genetically unmodified human DPSCs cultured in Neurocult^TM (Stem Cell Technologies), a cell culture medium without serum which can be alternatively supplemented for the expansion and/or differentiation of adult neural stem cells (NSCs). Our results show that non-genetically modified human adult DPSCs cultured with Neurocult NS-A proliferation supplement generated neurosphere-like dentospheres expressing the NSC markers Nestin and glial fibrillary acidic protein (GFAP), but also the vascular endothelial cell marker CD31. Remarkably, 1 month after intracranial graft into athymic nude mice, human CD31+/CD146+ and Nestin+ DPSC-derived cells were found tightly associated with both the endothelial and pericyte layers of brain vasculature, forming full blood vessels of human origin which showed an increased laminin staining. These results are the first demonstration that DPSC-derived cells contributed to the generation of neovasculature within brain tissue, and that Neurocult and other related serum-free cell culture media may constitute a fast and efficient way to obtain endothelial cells from human DPSCs.

Development and characterisation of a large diameter decellularised vascular allograft

The aims of this study were to develop a biological large diameter vascular graft by decellularisation of native human aorta to remove the immunogenic cells whilst retaining the essential biomechanical, and biochemical properties for the ultimate benefit of patients with infected synthetic grafts. Donor aortas (n = 6) were subjected to an adaptation of a propriety decellularisation process to remove the cells and acellularity assessed by histological analysis and extraction and quantification of total DNA. The biocompatibility of the acellular aortas was determined using standard contact cytotoxicity tests. Collagen and denatured collagen content of aortas was determined and immunohistochemistry was used to determine the presence of specific extracellular matrix proteins. Donor aortas (n = 6) were divided into two, with one half subject to decellularisation and the other half retained as native tissue. The native and decellularised aorta sections were then subject to uniaxial tensile testing to failure [axial and circumferential directions] and suture retention testing. The data was compared using a paired t-test. Histological evaluation showed an absence of cells in the treated aortas and retention of histoarchitecture including elastin content. The decellularised aortas had less than 15 ng mg-1 total DNA per dry weight (mean 94% reduction) and were biocompatible as determined by in vitro contact cytotoxicity tests. There were no gross changes in the histoarchitecture [elastin and collagen matrix] of the acellular aortas compared to native controls. The decellularisation process also reduced calcium deposits within the tissue. The uniaxial tensile and suture retention testing revealed no significant differences in the material properties (p > 0.05) of decellularised aorta. The decellularisation procedure resulted in minimal changes to the biological and biomechanical properties of the donor aortas. Acellular donor aorta has excellent potential for use as a large diameter vascular graft.

IGF-1 deficiency in a critical period early in life influences the vascular aging phenotype in mice by altering miRNA-mediated post-transcriptional gene regulation: implications for the developmental origins of health and disease hypothesis

Epidemiological findings support the concept of Developmental Origins of Health and Disease, suggesting that early-life hormonal influences during a sensitive period of development have a fundamental impact on vascular health later in life. The endocrine changes that occur during development are highly conserved across mammalian species and include dramatic increases in circulating IGF-1 levels during adolescence. The present study was designed to characterize the effect of developmental IGF-1 deficiency on the vascular aging phenotype. To achieve that goal, early-onset endocrine IGF-1 deficiency was induced in mice by knockdown of IGF-1 in the liver using Cre-lox technology (Igf1 ^f/f mice crossed with mice expressing albumin-driven Cre recombinase). This model exhibits low-circulating IGF-1 levels during the peripubertal phase of development, which is critical for the biology of aging. Due to the emergence of miRNAs as important regulators of the vascular aging phenotype, the effect of early-life IGF-1 deficiency on miRNA expression profile in the aorta was examined in animals at 27 months of age. We found that developmental IGF-1 deficiency elicits persisting late-life changes in miRNA expression in the vasculature, which significantly differed from those in mice with adult-onset IGF-1 deficiency (TBG-Cre-AAV8-mediated knockdown of IGF-1 at 5 month of age in Igf1 ^f/f mice). Using a novel computational approach, we identified miRNA target genes that are co-expressed with IGF-1 and associate with aging and vascular pathophysiology. We found that among the predicted targets, the expression of multiple extracellular matrix-related genes, including collagen-encoding genes, were downregulated in mice with developmental IGF-1 deficiency. Collectively, IGF-1 deficiency during a critical period during early in life results in persistent changes in post-transcriptional miRNA-mediated control of genes critical targets for vascular health, which likely contribute to the deleterious late-life cardiovascular effects known to occur with developmental IGF-1 deficiency.

The profile of the extracellular matrix changes in the aorta after sympathectomy in the hypercholesterolemic rats

We previously showed that sympathectomy induces thickened intima and decreases the expression of cytoskeletal proteins associated with a differentiated smooth muscle cell (SMC) phenotype in hypercholesterolemic rats. In the present study, we sought to determine the effect of sympathectomy on various components of the extracellular matrix (ECM) in the aorta from these animals, since the state of SMC differentiation depends on the nature of ECM components. Collagen types I and III, previously reported to be associated with SMC dedifferentiation, and collagen VI, elastin, laminin and elastin-laminin receptor (E/L-R), previously reported to be associated with SMC differentiation, were analyzed by western immunoblot and confocal microscopy in abdominal aortae from sham rats and hypercholesterolemic rats sympathectomized with guanethidine. Both western immunoblot and immunohistological analysis showed an increase in collagens I and III (more for collagen I), with abundant labeling in the media, adventitia and thickened intima in sympathectomized aortae. Collagen IV labeling was decreased in the media and adventitia and was weak in the thickened intima in sympathectomised aortae. The E/L-R increased and was abundantly labeled in the media and weakly in the thickened intima in sympathectomized aortae. Elastin and laminin decreased and appeared less labeled in the media in the sympathectomised aortae. In the thickened intima, laminin was slightly labeled while elastin was not obviously labeled. These data show that sympathectomy favors the ECM features reported in association with a dedifferentiated/immature SMC phenotype and intimal thickening, probably by actions on both SMCs and fibroblasts.

Aging impairs collateral sprouting of nociceptive axons in the rat

Sprouting of uninjured nociceptive axons was examined in young adult, middle aged and aged rats. Axon sprouting from the spared sural nerve, both into adjacent denervated skin and into end-to-side coapted nerve graft, was significantly higher in young rats than in aged rats. Cross-transplantations of the end-to-side coapted nerve grafts between young and aged rats demonstrated that axon sprouting from young recipient nerves into aged donor nerve grafts was significantly deteriorated, whereas the axon sprouting from aged recipient nerves into young donor nerve grafts was not statistically significantly affected. The levels of laminin polypeptides in peripheral nerves were 50-100% higher in young adult than in aged rats. However, the levels of peripherin, NGF isoforms and TrkA in skin, peripheral nerves and DRG, respectively, were not significantly reduced in aged rats. Therefore, impaired sprouting of nociceptive axons in aged rats is due rather to the alterations in peripheral neural pathways, than to the limited sprouting capacity of aged sensory neurons. Decreased levels of extracellular matrix components might be important in this respect.

Chapter 26: Age-related differences in the reinnervation after peripheral nerve injury

Numerous and extensive functional, structural, and biochemical changes characterize intact aged peripheral nervous system. Functional recovery after peripheral nerve injury depends on survival of injured neurons and functional reinnervation of target tissue by regeneration of injured axons and collateral sprouting of uninjured (intact) adjacent axons. The rate of axonal regeneration becomes slower and its extent (density and number of regenerating axons) decreases in aged animals. Aging also impairs terminal sprouting of regenerated axons and collateral sprouting of intact adjacent axons, thus further limiting target reinnervation and its functional recovery. Decreased survival of aged noninjured and injured neurons, limited intrinsic growth potential of neuron, alteration in its responsiveness to stimulatory or inhibitory environmental factors, and changes in the peripheral neural pathways and target tissues are possible reasons for impaired reinnervation after peripheral nerve injury in old age. The review of present data suggests that this impairment is mostly due to the age-related changes in the peripheral neural pathways and target tissues, and not due to the limited intrinsic growth capacity of neurons or their reduced responsiveness to trophic factors. Age-related alterations in the soluble target derived neurotrophic factors, like nerve growth factor, and nonsoluble extracellular matrix components of neural pathways, like laminin, might be important in this respect.

Effects of infantile/prepubertal chronic estrogen treatment and chemical sympathectomy with guanethidine on developing cholinergic nerves of the rat uterus

The innervation of the uterus is remarkable in that it exhibits physiological changes in response to altered levels in the circulating levels of sex hormones. Previous studies by our group showed that chronic administration of estrogen to rats during the infantile/prepubertal period provoked, at 28 days of age, an almost complete loss of norepinephrine-labeled sympathetic nerves, similar to that observed in late pregnancy. It is not known, however, whether early exposure to estrogen affects uterine cholinergic nerves. Similarly, it is not known to what extent development and estrogen-induced responses in the uterine cholinergic innervation are affected by the absence of sympathetic nerves. To address this question, in this study we analyzed the effects of infantile/prepubertal chronic estrogen treatment, chronic chemical sympathectomy with guanethidine, and combined sympathectomy and chronic estrogen treatment on developing cholinergic nerves of the rat uterus. Cholinergic nerves were visualized using a combination of acetylcholinesterase histochemistry and the immunohistochemical demonstration of the vesicular acetylcholine transporter (VAChT). After chronic estrogen treatment, a well-developed plexus of cholinergic nerves was observed in the uterus. Quantitative studies showed that chronic exposure to estrogen induced contrasting responses in uterine cholinergic nerves, increasing the density of large and medium-sized nerve bundles and reducing the intercept density of fine fibers providing myometrial and perivascular innervation. Estrogen-induced changes in the uterine cholinergic innervation did not appear to result from the absence/impairment of sympathetic nerves, because sympathectomy did not mimic the effects produced by estrogen. Estrogen-induced responses in parasympathetic nerves are discussed, considering the direct effects of estrogen on neurons and on changes in neuron-target interactions.

Aging and neuronal plasticity: lessons from a model

In spite of many well-documented examples of age-related reductions in neuronal plasticity, the causes of such changes remain largely unknown. One example of age-reduced plasticity involves an aberrant sprouting response of mature rat sympathetic neurons into the CNS (hippocampal formation). This phenomenon has proven to be useful for exploring the relative contribution of target aging (extrinsic influences) versus neuronal aging (intrinsic influences) to reduced sprouting. Aged sympathetic neurons mount a robust growth response when confronted with young target tissue or when exposed to exogenous trophic factor in vivo. In contrast, the aged target tissue (the hippocampal formation in this example) exhibits reduced receptivity for sympathetic sprouting. This change in the target does not appear to be due to alterations in baseline levels of trophic or substrate support for axonal growth. Rather, aging appears to dampen the consequences of target denervation so that the aged target elicits less sprouting. Age-related reductions in neuronal sprouting are speculated to reflect increasing commitment to information storage at the expense of neuronal plasticity.

Fiber-type transitions and satellite cell activation in low-frequency-stimulated muscles of young and aging rats

We examined satellite cell content and the activity of satellite cell progeny in tibialis anterior muscles of young (15 weeks) and aging (101 weeks) Brown Norway (BN) rats, after they were exposed for 50 days to a standardized and highly reproducible regime of chronic low-frequency electrical stimulation. Chronic low-frequency electrical stimulation was successful in inducing fast-to-slow fiber-type transformation, characterized by a 2.3-fold increase in the proportion of IIA fibers and fourfold and sevenfold decreases in the proportion of IID/X and IIB fibers in both young and aging BN rats. These changes were accompanied by a twofold increase in the satellite cell content in both the young and aging groups; satellite cell content reached a level that was significantly higher in the young group (p <.04). The total muscle precursor cell content (i.e., satellite cells plus progeny), however, did not differ between groups, because there was a greater number of satellite cell progeny passing through the proliferative and differentiative compartments of the aging group. The resulting 1.5-fold increase in myonuclear content was similar in the young and aging groups. We conclude that satellite cells and satellite cell progeny of aging BN rats possess an unaltered capacity to contribute to the adaptive response.

Innervation of cerebral blood vessels: morphology, plasticity, age-related, and Alzheimer's disease-related neurodegeneration

The light microscopical and ultrastructural morphology of the innervation of the major cerebral arteries and pial vessels is described, including the origins of the different groups of nerve fibres and their characteristic neurotransmitter phenotype. Species and region specific variations are described and novel data regarding the parasympathetic innervation of cerebral vessels are presented. The dynamic nature, or plasticity, of cerebrovascular innervation is emphasized in describing changes affecting particular subpopulations of neurons during normal ageing and in Alzheimer's disease. The molecular controls on plasticity are discussed with particular reference to target-associated factors such as the neurotrophins and their neuronal receptors, as well as extracellular matrix related factors such as laminin. Hypotheses are presented regarding the principal extrinsic and intrinsic influences on plasticity of the cerebrovascular innervation.

Effects of chronic oestrogen treatment are not selective for uterine noradrenaline-containing sympathetic nerves: a transplantation study

Previous studies have shown that chronic administration of oestrogen during postnatal rat development dramatically reduces the total content of noradrenaline in the uterine horn, abolishes myometrial noradrenergic innervation and reduces noradrenaline-fluorescence intensity of intrauterine perivascular nerve fibres. In the present study we analysed if this response is due to a direct and selective effect of oestrogen on the uterine noradrenaline-containing sympathetic nerves, using the in oculo transplantation method. Small pieces of myometrium from prepubertal rats were transplanted into the anterior eye chamber of adult ovariectomised host rats. The effect of systemic chronic oestrogen treatment on the reinnervation of the transplants by noradrenaline-containing sympathetic fibres from the superior cervical ganglion was analysed on cryostat tissue sections processed by the glyoxylic acid technique. In addition, the innervation of the host iris was assessed histochemically and biochemically. The histology of the transplants and irises was examined in toluidine blue-stained semithin sections. These studies showed that after 5 wk in oculo, the overall size of the oestrogen-treated transplants was substantially larger than controls, and histology showed that this change was related to an increase in the size and number of smooth muscle cells within the transplant. Chronic oestrogen treatment did not provoke trophic changes in the irideal muscle. Histochemistry showed that control transplants had a rich noradrenergic innervation, associated with both myometrium and blood vessels. Conversely, in oestrogen-treated transplants only occasional fibres were recognised, showing a reduced NA fluorescence intensity. No changes in the pattern and density of innervation or in the total content of noradrenaline of the host irises were detected after chronic exposure to oestrogen. We interpreted these results to indicate that the effects of oestrogen on uterine noradrenaline-containing sympathetic nerves are neither selective or direct, but result from an interaction between sympathetic nerve fibres with the oestradiol-primed uterine tissue. A potential effect of oestrogen on the neurotrophic capacity of the uterus is discussed.

The role of NGF in pregnancy-induced degeneration and regeneration of sympathetic nerves in the guinea pig uterus

In the guinea pig, pregnancy is associated with a generalised depletion of noradrenaline in uterine sympathetic nerves and, in the areas of the uterus surrounding the foetus, by a complete degeneration of sympathetic nerve fibres. These pregnancy-induced changes have been interpreted as a selective effect of placental hormones on the system of short sympathetic fibres arising from the paracervical ganglia. An alternative explanation is that pregnancy affects the neurotrophic capacity of the uterus. We measured NGF-protein levels in the guinea pig uterine horn, tubal end and cervix at early pregnancy, late pregnancy and early postpartum, using a two-site enzyme-linked immunosorbent assay. For comparative purposes the distribution and relative density of noradrenaline-containing sympathetic nerve fibres were assessed histochemically, and tissue levels of noradrenaline were measured biochemically, using high-performance liquid chromatography with electrochemical detection. In all the uterine regions analysed, NGF-protein levels showed a decline at term pregnancy, but in no case was this change statistically significant. After delivery, NGF-protein levels showed a marked increase in the cervix as well as in both the fertile and empty horns. These results suggest that alterations in NGF-protein do not account for the impairment of uterine sympathetic innervation during pregnancy, but may contribute to their recovery after delivery.

NGF expression in the aged rat pineal gland does not correlate with loss of sympathetic axonal branches and varicosities

The factors that determine the ability of some, but not all neurons, to sustain their axonal projections during aging remain largely unknown. Because sympathetic neurons remain responsive to nerve growth factor (NGF) in old age, it has been proposed that the selective decrease observed in the sympathetic innervation to some targets in aged rats may be the result of a deficit in target-derived NGF. In this study we utilized two different techniques to demonstrate decreased target innervation by sympathetic fibers in the aged rat pineal gland, which is an appropriate and relevant model for examining mechanisms of neuron-target interactions in aging. Tyrosine hydroxylase immunoreactive profiles were quantified in pineal glands of young and aged male Sprague-Dawley rats. The density of tyrosine hydroxylase-immunoreactive fibers was 30% lower in aged pineals, although the remaining fibers contained 20% more tyrosine hydroxylase-immunoreactivity. Othograde tracing of the pineal sympathetic innervation using biotinylated dextran revealed that average axon length, varicosity numbers, branch point numbers, and numbers of terminations were all decreased by approximately 50% in aged tissues, indicating possible functional deficits. These findings suggest that whole branches, along with their associated varicosities were lost in old age. A sensitive quantitative ribonuclease protection assay and a two-site ELISA assay were used to examine whether reduced NGF availability might correlate with sympathetic nerve atrophy. No significant differences were detected in either NGF mRNA or NGF protein levels when comparing young and aged pineal glands, suggesting that atrophy in aged sympathetic neurons is not causally related to reduced availability of NGF at the target. Our results indicate that mechanisms other than NGF expression need to be explored in order to explain the age-related axonal regression observed in this target.

In oculo transplants of myometrium from postpartum guinea pigs fail to support sympathetic reinnervation

Sympathetic nerves to the enlarged fetus-containing region of the uterus undergo degenerative changes during late pregnancy and show slow regrowth after parturition. It is not known whether this unusual response of sympathetic nerves to smooth muscle hypertrophy is due to the sensitivity of short adrenergic neurons to hormonal changes, or whether the nerves respond to changes in the neurotrophic capacity of the target. We have investigated this question using in oculo transplantation. Small pieces of myometrium from the uterine horn of virgin guinea pigs, or from the region previously occupied by the placenta and fetus in postpartum guinea pigs, were transplanted into the anterior eye chamber. After 3 wk in oculo, the pattern of reinnervation of the transplants was assessed on whole mount stretch preparations stained for tyrosine hydroxylase. The histology of the transplants was examined in toluidine blue-stained semithin sections. Myometrial transplants from virgin donors and uterine artery transplants from both virgin and postpartum donors became organotypically reinnervated by sympathetic fibres from the host iris. In contrast, sympathetic nerves did not reinnervate myometrial transplants from postpartum donors, although they approached the transplants and became distributed in the surrounding connective tissue. All transplanted tissues showed a normal histological appearance. Both the myometrium and uterine artery from postpartum donors retained a hypertrophic appearance after 3 wk in oculo. We interpret these results to indicate that the degeneration of sympathetic nerves in late pregnancy, as well as their slow regrowth to the uterus after delivery, may be due to changes in uterine smooth muscle rather than a particular sensitivity of short adrenergic neurons to hormonal changes.

Uninjured aged sympathetic neurons sprout in response to exogenous NGF in vivo

The extent to which the loss of plasticity by aged neurons is due to changes in the neuronal environment or to a loss of growth potential of the neurons has not been determined. In previous studies we observed that young adult cerebrovascular axons undergo a sprouting response following a 2-week intracerebroventricular infusion of nerve growth factor (15 microg; NGF). The present study used electron microscopy to examine the innervation of the intradural segment of the internal carotid artery of the aged rat and to determine whether aged sympathetic perivascular axons would respond to in vivo infusion of NGF. Young adult and aged Fischer 344 female rats received a 2-week intracranial infusion of NGF (15 microg) or vehicle (VEH) and were perfused for electron microscopy. Although there was no change in the total number of perivascular axons associated in aged VEH when compared with young adult VEH, a significant reduction was observed in aged VEH when total axons and sympathetic axons were expressed per microm2 vascular wall, reflecting an age-related increase in blood vessel size. Following NGF infusion, aged sympathetic axons were significantly increased by 192% compared with aged VEH cases. These results suggest that there is a proportional reduction in sympathetic cerebrovascular neurons with aging but that they exhibit robust sprouting in response to an exogenous neurotrophin.

Plasticity in adult and ageing sympathetic neurons

The nature of neural plasticity and the factors that influence it vary throughout life. Adult neurons undergo extensive and continual adaptation in response to demands that are quite different from those of early development. We review the main influences on the survival, growth and neurotransmitter expression in adult and ageing sympathetic neurons, comparing these influences to those at work in early development. This "developmental" approach is proposed because, despite the contrasting needs of different phases of development, each phase has a profound influence on the mechanisms of plasticity available to its successors. Interactions between neurons and their targets, whether effector cells or other neurons, are vital to all of these aspects of neural plasticity. Sympathetic neurons require access to target-derived diffusible neurotrophic factors such as NGF, NT3 and GDNF, as well as to bound elements of the extracellular matrix such as laminin. These factors probably influence plasticity throughout life. In adult life, and even in old age, sympathetic neurons are relatively resistant to cell death. However, they continue to require target-derived diffusible and bound factors for their maintenance, growth and neurotransmitter expression. Failure to maintain appropriate neuronal function in old age, for example in the breakdown of homeostasis, may result partly from a disturbance of the dynamic, trophic relationship between neurons and their targets. However, there is no clear evidence that this is due to a failure of targets to synthesize neurotrophic factors. On the neural side of the equation, altered responsiveness of sympathetic neurons to neurotrophic factors suggests that expression of the trk and p75 neurotrophin receptors contributes to neuronal survival, maintenance and growth in adulthood and old age. Altered receptor expression may therefore underlie the selective vulnerability of some sympathetic neurons in old age. The role of neural connectivity and activity in the regulation of synthesis of target-derived factors, as well as in neurotransmitter dynamics, is reviewed.

Responses of mature and aged sympathetic neurons to laminin and NGF: an in vitro study

Whilst the potent effects of NGF and laminin on developing neurons are well documented, relatively little is known about the effects of, or altered availability of or altered responsiveness to, these substances on the growth of adult neurons. We have therefore examined this question using explant cultures of sympathetic neurons from the superior cervical ganglion (SCG) of mature and aged rats. Explants were grown on substrata containing different doses of laminin, either with or without added NGF in culture medium containing FCS. Individually, laminin and NGF had relatively small effects on neurite outgrowth and length, which tended to be reduced in old neurons. In contrast, laminin in the presence of exogenous NGF exerted a powerful effect on nerve growth which was substantially greater than the sum of the effects of the individual factors. This synergy was evident in all experimental groups and was greatest in old explants at high doses of laminin, where growth was comparable to that of mature neurons. The dose-response curve of old neurons to laminin in the presence of added NGF indicated reduced responsiveness. These results suggest that variations in the availability of laminin and/or exogenous NGF, together with altered patterns of neuronal responsiveness, may contribute to impaired neuronal plasticity in old age.

Decreased receptivity of pathway connective tissue to sympathetic nerve ingrowth in the developing rat

Sympathetic axons can form atypical pathways to denervated orbital targets in neonatal rats but not in rats aged 30 or more days. The objective of this study was to determine if connective tissue pathways that carry sympathetic nerves lose their ability to sustain axonal sprouting during the early postnatal period. Regions of periorbital sheath known to contain large numbers of sympathetic axons that travel to distal orbital targets were excised from rats (sympathectomized 3 days previously) on postnatal days 6-7, 14-15, 30-31, and 48-49 and placed in anterior chambers of adult host rats. Tissues were removed 3, 6, or 10 days post-transplant and sympathetic ingrowth was analyzed by catecholamine histofluorescence in whole-mount or cryosectioned specimens. Connective tissue transplants from 6-15-day-old donors showed significant fiber ingrowth by 3 days in oculo, and innervation was maximal by 6 days. In contrast, sprouting into 30-49-day-old tissue was significantly slower, with most transplants lacking fibers at 3 days, and with small numbers of short fibers present at 6 days. We conclude that maturational changes occur in periorbital connective tissue pathways in the early postnatal period which make them less receptive to ingrowth by sympathetic nerves. The findings that connective tissue pathways are better substrates for sympathetic sprouting in the neonatal rat supports the view that developmental changes in these tissues are likely to contribute to the impaired reinnervation of orbital targets by contralateral neurons in juvenile and adult rats.

The hypogastric and thirteenth thoracic ganglia of the rat: effects of age on the neurons and their extracellular environment

Morphometric analyses of the neurons and microvessels of perfusion-fixed hypogastric (HG) and 13th thoracic (T13) ganglia have been performed in male Wistar rats aged 4, 24 and 30 mo. Estimations of HG volume employing the Cavalieri principle have also been performed and showed that the size of the aged HG is increased by 42%. Routine histological staining of the ganglia with Masson's trichrome indicated that this may be due to the increased amount of interstitial connective tissue which was apparent in the aged animals. The number of neurons per unit area progressively decreased by 38% between ages 4 and 24 mo and by 16% between ages 24 and 30 mo in the HG and by 25% (4 and 24 mo) and 2% (24 and 30 mo) in the T13 ganglion. The total number of neurons in the HG however, estimated by a physical disector analysis, was constant with age. The number of microvessels per unit area, microvessel diameter, neuronal and nuclear areas did not differ significantly between the 3 age groups studied. This observed increase in ganglionic volume and decrease in neuronal packing density may be associated with changes in the extracellular matrix, in particular in glycosaminoglycans whose presence was indicated by metachromasia of the ganglia with toluidine blue. The extracellular matrix was therefore characterised using a panel of monoclonal antibodies against glycosaminoglycans and laminin. Chondroitin-6 sulphate and chondroitin-4 sulphate were present in the interstitial connective tissue, and there was an increase in the expression of both these epitopes at 24 mo, noteably surrounding neuron cell bodies. The expression of chondroitin-4 sulphate/dermatan sulphate was unchanged, thus implying a decreased expression of dermatan sulphate with age. Keratan sulphate and the native chondroitin sulphate epitopes were absent from the ganglia at both ages. Laminin expression was increased in the aged ganglia. It is therefore clear that the constituents of the extracellular matrix are not constant throughout the adult lifespan and that the extracellular matrix may influence neuronal survival in old age. This is the first report characterising age-related changes in the extracellular matrix of autonomic ganglia.

Autonomic nervous system as a model of neuronal aging: the role of target tissues and neurotrophic factors

The aim of this study was to determine the role of target tissues and neurotrophic factors in the growth and atrophy of autonomic neurons during development and aging. Using quantitative neuroanatomical techniques, it is shown that, although axonal and dendritic growth is apparent throughout postnatal development, different patterns of growth are found in autonomic neurons innervating different target tissues. For example, sympathetic neurons innervating the submandibular gland continue to grow well into maturity, but those innervating the iris cease net growth early in postnatal development. Similarly, although neuronal atrophy was observed in aged autonomic ganglia, this was not a general phenomenon but was specific to neurons innervating particular target tissues. Sympathetic neurons innervating the middle cerebral artery showed significant axonal and dendritic atrophy in old age, whereas neurons innervating the iris were morphologically unchanged. The trophic influence of peripheral target tissues on their innervating neurons has been shown to decline in old age possibly as a result of decreased availability of target-derived neurotrophic factors such as nerve growth factor (NGF) [Gavazzi et al. (1992) Neuroreport, 3:717-720]. Therefore, in an attempt to reverse neuronal atrophy where it occurred, NGF was infused via miniosmotic pumps over the peripheral axons of aged neurons. NGF induced increases in soma size, dendritic length and axonal arborization. However, in contrast to young adult neurons, no increase in the number of dendritic branch points or primary dendrites was observed, suggesting that some aspects of neuronal plasticity are impaired in old age. In sum, these results show a range of age- and target-specific differences in the axonal and dendritic morphology of autonomic neurons that may result from differing trophic interactions with their target tissues.

Can the neurotrophic hypothesis explain degeneration and loss of plasticity in mature and ageing autonomic nerves?

The causes of age-related degeneration in the peripheral nervous system remain unclear. The search for clues has focused on developmental mechanisms and particularly on the neurotrophic hypothesis and its principal player, nerve growth factor, reduced levels of which are thought to cause degeneration of some autonomic and central neurons in old age. Nerve growth factor may well be important in the mature and ageing nervous system, but recent experiments on sympathetic nerves in ageing rats suggest that lack of NGF is not the only limiting factor in neuronal growth and survival. Other candidates include laminin, which is bound in the extracellular matrix and may act in synergy with NGF to regulate neuronal maintenance and growth in maturity. Reduced, region-specific patterns of availability of one or both of these substances may underlie age-related degeneration in autonomic nerves. Different combinations of these factors may influence particular aspects of neuronal plasticity, such as collateral sprouting and regeneration. In addition to extrinsic factors, it appears increasingly likely that altered neuronal responsiveness to neurotrophic factors in old age contributes to structural and functional deficits in autonomic nerves.