- Email: [email protected]
Senile plaques as aberrant sprout-stimulating structures
EXPERIMENTAL
NEUROLOGY
(1986)
94,767-776
Senile Plaques as Aberrant Sprout-Stimulating JAMES
W. GEDDES,
KEVIN
J. ANDERSON,
AND CARL
Structures W. COTMAN’
Department of PsychobioIogy, University of California. Irvine, California 92717 Received July 31, I986 In Alzheimer’s disease, the cholinergic septal input to the dentate gyrus molecular layer appears to sprout, presumably in response to the loss of entorhinal input to this region. Neuritic plaques accumulated in regions of septal sprouting and were present in these regions to a much greater degree than in areas of no apparent sprouting. We suggest that the reactive sprouts participate in the pathogenesis of piaque formation. The stimulus for plaque formation may be sprouting induced by a focal accumulation of injury-induced trophic factors. The demonstration of sprouting in Alzheimer’s disease indicates that the appropriate mechanisms are intact. Eventually, however, the fibers succumb to the pathogenic processes in the disorder. o 1986 Academic Press, Inc.
INTRODUCTION Alzheimer’s disease (AD) is a neurodegenerative disorder characterized neuropathologically by the presence of neurofibrillary tangles, neuritic plaques, and neuronal cell loss in specific cortical and subcortical regions. Recently it has become clear that the Alzheimer brain is also capable of sprouting reactions apparently triggered by the lossof specific neuronal populations. In particular, severe neuronal degeneration in layersII and III of the entorhinal cortex has been noted by several investigators (2,8-l 1). In a previous study we reported that in AD patients in whom the loss of cholinergic input to the hippocampus is not severe,cholinergic fibers in the dentate gyrus of Abbreviations: AChE-acetylcholinesterase, AD-Alzheimer’s disease. ’ The authors thank Ron C. Kim, Helena Chang Chui, ha T. Lott, Byung Choi, and Ursula T. Slager for clinical and neuropathological evaluation. We also thank Mark A. Borja and Suzanne M. Cooper for technical assistance. This work was supported by NIA program project AGO0538 and ADRDA Grant B-86-102. K.J.A. is the recipient of a postdoctoral fellowship from the National Institutes of Health (NS07627). 767 0014-4886/86 $3.00 Copyright Q I986 by Academic Press, Inc. All rights of reproduction in any form reserved.
768
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the hippocampal formation sprout within the denervated dentate molecular layer (5). The responseof septohippocampal afferent fibers in AD is similar to that seenin rodents after a lesion of the entorhinal cortex. In addition, a reactive growth of commissural and associational fibers is seenin both AD and in animals with a lesion of the entorhinal cortex. In animal models, sprouting of spared hippocampal afferent fibers results in the complete replacement of synaptic connections within the denervated zone (4) and corresponds with recovery of a hippocampal-dependent behavior (13,2 1). In AD, sprouting may initially compensate for partial denervation and provide an early period of interim stability (3). The sprouting response,however, must remain viable in order not to contribute to the pathogenesisof this disease. In these experiments we have extended our studies on the cholinergic response in the dentate gyrus in AD and examined the disposition of reactive fibers in relation to the accumulation of senile plaques. We demonstrate that senile plaques appear to accumulate in the region of sprouted fibers suggesting that sprouted fibers may ultimately contribute to the makeup and disposition of plaques. In fact, sprouting may be an early phase of plaque formation. METHODOLOGY The distribution of neuritic plaques was examined in hippocampal samples obtained postmortem from five neurologically normal and seven Alzheimer’s type dementia brains. The AD diagnosis was confirmed at autopsy using Bielschowsky’s or Bodian’s silver stains according to the criteria of McKhann et al. (16). Small slices(2 to 3 cm thick) of the hippocampal formation and overlying parahippocampal gyrus were dissected at autopsy and either placed into buffered 4% paraformaldehyde- 1% glutaraldehyde (PH 7.4) or snap frozen in powdered dry ice. Fixed sectionswere left in fixative 24 h at 4’ and transferred to a buffered 30% sucrosesolution and incubated in the cold for 24 to 48 h. The fixed hippocampal sliceswere then frozen in powdered dry ice and both fixed and previously frozen sliceswere cut on a cryostat at 30 pm. Sectionswere thawed onto subbed slides and unfixed sections were subsequently placed into buffered 4% paraformaldehyde for 1 to 2 h. The sectionswere thoroughly rinsed and then stained for acetylcholinesterase (AChE) activity by two methods, the first a modified version of the Kolle method ( 15), the second a recent AChE stain introduced by Hedreen et al. (6). Adjacent 30-pm sections were stained with cresyl violet or Bielschowsky’ssilver stain. RESULTS In the hippocampal dentate gyrus from control brains, AChE staining was observed in a characteristic laminar pattern. The highest levels of staining
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were seen in the regions subjacent to the stratum granulosum (i.e., supraand subgranular zones). A region of lighter staining was seen in the outer and inner portions ofthe molecular layer (Fig. 1A). In contrast to controls, AChE staining in AD cases most often demonstrated a region of AChE intensification within the outer molecular layer (Fig. 1B). As in controls, AChE staining in AD brains was also visible in the zones adjacent to the granule cell layer. Darkly stained AChE-positive neuritic plaques were often observed in the outer molecular layer corresponding to the zone of AChE intensification. When plaques and intensification were not distributed across the entire extent of the dentate gyrus, plaques were observed only in regions of AChE intensification, with the remaining areas of the dentate molecular layer having a normal pattern of cholinesterase staining (Fig. 4). Neither neuritic plaques nor AChE intensification were apparent in any control cases. Neuritic plaques were also visible in the dentate gyrus molecular layer in all AD cases using Bielschowsky’s silver stain (Fig. 2) and the Hedreen modified AChE stain (Fig. 3). The silver-stained plaques were mainly ofthe immature or mature type, and few end-stage plaques were observed in the outer molecular layer in this study. In the dentate gyrus, plaques were generally distributed in a band from the middle to outer molecular layer corresponding to the region of general AChE intensification. The number of plaques in this region varied greatly, ranging from 5 to more than 100 per 30-lrn-thick section. Bielschowsky’s and AChE staining of alternate sections demonstrated a similar distribution of plaques within the hippocampal formation (Fig. 2). Similar plaque densities were also observed between these sections, but as the two staining techniques could not be used on the same section it was not possible to determine if there was a one to one correspondence between AChE-positive and silver-stained plaques. In cases examined using the Hedreen modification of the AChE method (6), which is selective for fibers, cholinesterase-positive fibers and neuritic plaques were visible in the middle to outer dentate molecular layer. Frequently, several AChE-positive fibers were seen to enter plaques and could often be observed branching within the substance of a plaque. In addition, these fibers often possessed bulbous endings within the plaque. It was also possible to identify several apparent stages of degeneration in AChE-positive fibers including (i) swollen varicosities on focal segments of AChE positive fibers, (ii) bulbous dilations on AChE-positive terminals, and (iii) grape-like clusters of AChE-positive dilations at the periphery of neuritic plaques (Fig. 3). DISCUSSION The results of this study support the hypothesis that plaques may represent an aberrant or aborted sprouting response [see (1 l)]. Neuritic plaques which
FIG. 1. Acetylcholinesterase histochemistry in the dentate gyms molecular layer of the human hippocampal formation. In control cases (A), moderate cholinesterase staining was observed across the molecular layer, with a dense band of suprgranular staining (asterisk). (B) In AD, the overall intensity of the staining was reduced, but a region of cholinesterase intensification was observed in the outer molecular layer. AChE-positive neuritic plaques were also visible in this zone, but not within the inner molecular layer. Abbreviations: SGr-stratum granulosum, OML-outer molecular layer, HiF-hippocampal fissure. Calibration bar = 100 am.
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were intensely cholinesterase-positive were localized in the region of cholinesterase intensification. Within the plaque itself, there was sprouting of neurites as if attracted to the forming plaque. Based on the results of this and previous studies, a sequence of plaque formation is proposed in the following model. The progressive neurodegeneration associated with AD leads to the enhanced production of neurite-promoting factors and generalized sprouting in the terminal zone of the affected neurons. The demonstration of sprouting in AD suggests that the appropriate neurite-promoting factors are present. In the region of the sprouting response, there is a focal accumulation of trophic activity. The source of the growth factor activity is unknown, but may be reactive astrocytes or microglia responding to the neuronal degeneration (25). In addition to trophic factors, these nonneuronal cells may possess a favorable surface such as laminin or fibronectin ( 14,20), inducing an exhuberant local sprouting response of the adjacent neurites. Cholinesterase-positive neurites could be seen sprouting toward the plaque, similar to observations by Probst and colleagues in a Golgi preparation ( 18). The next event in the proposed sequence of neuritic plaque formation is that some of the sprouted neurites themselves degenerate. Numerous cholinesterase-positive swollen neurites, similar to those described in aged monkeys (22), were apparent within the plaque. Ultrastructural studies suggest that the dystrophic neurites are mainly axonal (23,24). This neuritic dystrophy may result from an absent or inappropriate target (19); inadequate neurotrophic activity (in contrast to neurite-promoting activity); impairment of axonal transport; or an antigenic response to the presence of tau in the sprouted neurites. Tau, a fetal microtubule-associated protein, is not normally found in the adult brain and is the major antigenic determinant in paired helical filaments (12). The sprouting and dystrophic neurites, surrounding the original growth center, may thus represent the early or immature plaque suggested by Wisnewski and Terry (25). This proposed sequence of events, beginning with an enhanced sprouting response induced by a focal accumulation of growth factors, is in marked
FIG. 3. Acetylcholinesterase-containing axons in the human hippocampus, visualized using the technique of Hedreen et al. (6). A-a normal cholinesterase-positive fiber. B and C-degenerating fibers, with swollen varicosities on focal segments visible in B and a bulbous dilation of a terminal apparent in C. D, E, and F-cholinesterase-positive neuritic plaques. In D, cholinesterase-positive fibers surround an unidentified target. Sprouting of fibers toward a source of trophic activity may represent the initial stage in plaque formation. Cholinesterase-positive fibers were seen entering the neuritic plaques in E and F, and grape-like clusters ofAChE-positive dilations, indicative of dystrophic neurities, were apparent in the periphery of the plaque. Calibration bars = 30 pm.
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contrast to the lack of trophic factors proposed by Appel(1) and by Hefti (7). The above model suggeststhat sprouting is an early phase of plaque formation and that sprouted fibers contribute to the makeup and disposition of plaques. Accordingly, the model predicts that those neuronal systemscapable of sprouting may be recruited into neuritic plaques. To date, with the exception of somatostatin, all fiber systemswhich have been colocalizedwith plaques (17, 19,22) also participate in neural plasticity. Theseinclude acetylcholine-, catecholamine-, and substanceP-containing neurons. It is conceivable that all fiber systemsmay be capable of a sprouting responseand contribute to the make-up of plaques, but that some respond more readily than others. The determinants of the fibers contributing to an individual plaque would therefore be the specificity of the trophic factor(s) and the nature of the adjacent neurites. REFERENCES 1. APPEL, S. H. 1981. A unifying parkinsonism, and Alzheimer 2. BALL, M. J. 1978. Topographic degeneration in hippocampal
hypothesis for the cause of amyotrophic lateral sclerosis, disease. Ann. Neural. 10: 499-505. distribution of neurofibrillary tangles and granulovacuolar cortex of aging and demented patients. Acta Neuroputhol.
42: 73-80.
COTMAN, C. W., AND K. J. ANDERSON. 1986. Synaptic plasticity and functional stabilization in the hippocampal formation: possible role in Alzheimer’s disease. In S. WAXMAN, Ed., PhysioIogic Basis for Functional Recovery in Neurological Disease. Raven Press, New York, in press. 4. COTMAN, C. W., AND NIETO-SAMPEDRO, M. 1984. Cell biology of synaptic plasticity. Science225: 1287-1294. 5. GEDDES, J. W., MONAGHAN, D. T., COTMAN, C. W., Lore, I. T., KIM, R. C., AND CHUI, H. C. 1985. Plasticity of hippocampal circuitry in Alzheimer’s disease. Science 230: 1179-l 181. 6. HEDREEN, J. H., BACON, S. J., AND PRICE, D. L. 1985. A modified histochemical technique to visualize acetylcholinesterase-containing axons. J. Histochem. Cytochem. 33: 1343.
140. 7.
8. 9. 10. 11.
HEF~I, F. 1983. Is Alzheimer disease caused by lack of nerve growth factor? Ann. Neural. 13: 109-l 10. HOOPER, M. W., AND VOGEL, F. S. 1976. The limbic system in Alzheimer’s disease. Am. .I. Pathol. 85: l-13. HYMAN, B. T., VAN HOESEN, G. W., DAMASIO, A. R., AND BARNES, C. L. 1984. Alzheimer’s disease: cell specific pathology isolates the hippocampal formation. Science 225: 1168-l 170. HYMAN, B. T., VAN HOESEN, G. W., KROMER, L. J., AND DAMASIO, A. R. 1986. Perforant pathway changes and the memory impairment of Alzheimer’s disease. Ann. Neural., in press. KEMPER, T. L. 1978. Senile dementia: a focal disease in the temporal lobe. Pages 105-l 13 in K. NANDY, Ed., Senile Dementia, a Biomedical Approach. Elsevier North-Holland Biomedical Press. Amsterdam.
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12. KOSIK, K. S., JOACHIM, C. L., AND SELKOE, D. J. 1986. Microtubule-associated protein tau (7) is a major antigenic component of paired helical filaments in Alzheimer disease. Proc. Natl. Acad. Sci. USA 83: 4044-4048. 13. LOESCHE, J., AND STEWARD, 0.1977. Behavioral correlates ofdenervation and reinnervation of the hippocampal formation of the rat: recovery of alteration performance following unilateral entorhinal cortex lesions. Brain Rex Bull. 2: 3 l-39. 14. MANTHORPE, M., ENGVALL, E., RUOSLAHTI, E., LONGO, F. M., DAVIS, G. E., AND VARON, S. 1983. Laminin promotes neuritic regeneration from cultured peripheral and central neurons. J. Cell Biol. 97: 1882-l 890. 15. MATTHEWS, D. A., NADLER, J. V., LYNCH, G., AND COTMAN, C. W. 1974. Development of cholinergic innervation in the hippocampal formation of the rat. I. Histochemical demonstration of acetylcholinesterase activity. Dev. Biol. 36: 130- 141. 16. MCKHANN, G., DRACHMAN, D., FOLSTEIN, M., KATZMAN, R., PRICE, D., AND STADLAN, E. M. 1984. Clinical diagnosis of Alzheimer’s disease: report of the NINCDS-ADRDA work group under the auspices of the Department of Health and Human Services task force on Alzheimer’s disease. Neurology 34: 939-944. 17. MORRISON, J. H., ROGERS,J., SCHERR,S., BENOIT, R., AND BLOOM, F. E. 1985. Neuritic plaques of Alzheimer’s patients contain somatostatin immunoreactivity. Nature (London) 314: 90-92. 18. PROBST, A., BASLER, V., BRON, B., AND ULRICH, J. 1983. Neuritic plaques in senile dementia of Alzheimer type: a Golgi analysis in the hippocampal region. Brain Rex 268: 249254. 19. ROGERS, J., AND MORRISON, J. H. 1985. Quantitative morphology and regional and laminar distributions of senile plaques in Alzheimer’s disease. J. Neurosci. 5: 280 l-2808. 20. ROGERS, S. L., LETOIJRNEAU, P. C., PALM, S. L., MCCARTHY, J., AND FURCHT, L. T. 1983. Neurite extension by peripheral and central nervous system neurons in response to substratum-bound fibronectin and laminin. Dev. Biol. 98: 2 12-220. 21. SCHEFF,S. W., AND COTMAN, C. W. 1977. Recovery of spontaneous alternation following lesions of the entorhinal cortex in adult rats: possible correlation to axon sprouting. Behav. Biol. 21: 286-293. 22. STRUBLE, R. G., CORK, L. C., WHITEHOUSE, P. J., AND PRICE, D. L. 1982. Choline& innervation in neuritic plaques. Science 216: 4 13-4 15. 23. TERRY, R. D., GONATAS, N. K., AND WEISS, M. 1964. Ultrastructural studies in Alzheimer’s presenile dementia. Am. J. Pathol. 44: 269-283. 24. TERRY, R. D., AND WISNEWSKI, H. M. 1970. The ultrastructure ofthe neurofibrillarytangle and the senile plaque. Pages 145-155 in G. E. W. WOLSTENHOLME AND M. O’CONNOR, Eds. CIBA Foundation Symposium on Alzheimer’s Disease and Related Conditions. J & A Churchill, London. 25. WISNEWSKI, H. M., AND TERRY, R. D. 1973. Reexamination of the pathogenesis of the senile plaque. Pages 1-26 in H. M. ZIMMERMAN, Ed., Progress in Neuropathology, Vol. 2. Grune & Stratton, New York.
NEUROLOGY
(1986)
94,767-776
Senile Plaques as Aberrant Sprout-Stimulating JAMES
W. GEDDES,
KEVIN
J. ANDERSON,
AND CARL
Structures W. COTMAN’
Department of PsychobioIogy, University of California. Irvine, California 92717 Received July 31, I986 In Alzheimer’s disease, the cholinergic septal input to the dentate gyrus molecular layer appears to sprout, presumably in response to the loss of entorhinal input to this region. Neuritic plaques accumulated in regions of septal sprouting and were present in these regions to a much greater degree than in areas of no apparent sprouting. We suggest that the reactive sprouts participate in the pathogenesis of piaque formation. The stimulus for plaque formation may be sprouting induced by a focal accumulation of injury-induced trophic factors. The demonstration of sprouting in Alzheimer’s disease indicates that the appropriate mechanisms are intact. Eventually, however, the fibers succumb to the pathogenic processes in the disorder. o 1986 Academic Press, Inc.
INTRODUCTION Alzheimer’s disease (AD) is a neurodegenerative disorder characterized neuropathologically by the presence of neurofibrillary tangles, neuritic plaques, and neuronal cell loss in specific cortical and subcortical regions. Recently it has become clear that the Alzheimer brain is also capable of sprouting reactions apparently triggered by the lossof specific neuronal populations. In particular, severe neuronal degeneration in layersII and III of the entorhinal cortex has been noted by several investigators (2,8-l 1). In a previous study we reported that in AD patients in whom the loss of cholinergic input to the hippocampus is not severe,cholinergic fibers in the dentate gyrus of Abbreviations: AChE-acetylcholinesterase, AD-Alzheimer’s disease. ’ The authors thank Ron C. Kim, Helena Chang Chui, ha T. Lott, Byung Choi, and Ursula T. Slager for clinical and neuropathological evaluation. We also thank Mark A. Borja and Suzanne M. Cooper for technical assistance. This work was supported by NIA program project AGO0538 and ADRDA Grant B-86-102. K.J.A. is the recipient of a postdoctoral fellowship from the National Institutes of Health (NS07627). 767 0014-4886/86 $3.00 Copyright Q I986 by Academic Press, Inc. All rights of reproduction in any form reserved.
768
GEDDES ET AL.
the hippocampal formation sprout within the denervated dentate molecular layer (5). The responseof septohippocampal afferent fibers in AD is similar to that seenin rodents after a lesion of the entorhinal cortex. In addition, a reactive growth of commissural and associational fibers is seenin both AD and in animals with a lesion of the entorhinal cortex. In animal models, sprouting of spared hippocampal afferent fibers results in the complete replacement of synaptic connections within the denervated zone (4) and corresponds with recovery of a hippocampal-dependent behavior (13,2 1). In AD, sprouting may initially compensate for partial denervation and provide an early period of interim stability (3). The sprouting response,however, must remain viable in order not to contribute to the pathogenesisof this disease. In these experiments we have extended our studies on the cholinergic response in the dentate gyrus in AD and examined the disposition of reactive fibers in relation to the accumulation of senile plaques. We demonstrate that senile plaques appear to accumulate in the region of sprouted fibers suggesting that sprouted fibers may ultimately contribute to the makeup and disposition of plaques. In fact, sprouting may be an early phase of plaque formation. METHODOLOGY The distribution of neuritic plaques was examined in hippocampal samples obtained postmortem from five neurologically normal and seven Alzheimer’s type dementia brains. The AD diagnosis was confirmed at autopsy using Bielschowsky’s or Bodian’s silver stains according to the criteria of McKhann et al. (16). Small slices(2 to 3 cm thick) of the hippocampal formation and overlying parahippocampal gyrus were dissected at autopsy and either placed into buffered 4% paraformaldehyde- 1% glutaraldehyde (PH 7.4) or snap frozen in powdered dry ice. Fixed sectionswere left in fixative 24 h at 4’ and transferred to a buffered 30% sucrosesolution and incubated in the cold for 24 to 48 h. The fixed hippocampal sliceswere then frozen in powdered dry ice and both fixed and previously frozen sliceswere cut on a cryostat at 30 pm. Sectionswere thawed onto subbed slides and unfixed sections were subsequently placed into buffered 4% paraformaldehyde for 1 to 2 h. The sectionswere thoroughly rinsed and then stained for acetylcholinesterase (AChE) activity by two methods, the first a modified version of the Kolle method ( 15), the second a recent AChE stain introduced by Hedreen et al. (6). Adjacent 30-pm sections were stained with cresyl violet or Bielschowsky’ssilver stain. RESULTS In the hippocampal dentate gyrus from control brains, AChE staining was observed in a characteristic laminar pattern. The highest levels of staining
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were seen in the regions subjacent to the stratum granulosum (i.e., supraand subgranular zones). A region of lighter staining was seen in the outer and inner portions ofthe molecular layer (Fig. 1A). In contrast to controls, AChE staining in AD cases most often demonstrated a region of AChE intensification within the outer molecular layer (Fig. 1B). As in controls, AChE staining in AD brains was also visible in the zones adjacent to the granule cell layer. Darkly stained AChE-positive neuritic plaques were often observed in the outer molecular layer corresponding to the zone of AChE intensification. When plaques and intensification were not distributed across the entire extent of the dentate gyrus, plaques were observed only in regions of AChE intensification, with the remaining areas of the dentate molecular layer having a normal pattern of cholinesterase staining (Fig. 4). Neither neuritic plaques nor AChE intensification were apparent in any control cases. Neuritic plaques were also visible in the dentate gyrus molecular layer in all AD cases using Bielschowsky’s silver stain (Fig. 2) and the Hedreen modified AChE stain (Fig. 3). The silver-stained plaques were mainly ofthe immature or mature type, and few end-stage plaques were observed in the outer molecular layer in this study. In the dentate gyrus, plaques were generally distributed in a band from the middle to outer molecular layer corresponding to the region of general AChE intensification. The number of plaques in this region varied greatly, ranging from 5 to more than 100 per 30-lrn-thick section. Bielschowsky’s and AChE staining of alternate sections demonstrated a similar distribution of plaques within the hippocampal formation (Fig. 2). Similar plaque densities were also observed between these sections, but as the two staining techniques could not be used on the same section it was not possible to determine if there was a one to one correspondence between AChE-positive and silver-stained plaques. In cases examined using the Hedreen modification of the AChE method (6), which is selective for fibers, cholinesterase-positive fibers and neuritic plaques were visible in the middle to outer dentate molecular layer. Frequently, several AChE-positive fibers were seen to enter plaques and could often be observed branching within the substance of a plaque. In addition, these fibers often possessed bulbous endings within the plaque. It was also possible to identify several apparent stages of degeneration in AChE-positive fibers including (i) swollen varicosities on focal segments of AChE positive fibers, (ii) bulbous dilations on AChE-positive terminals, and (iii) grape-like clusters of AChE-positive dilations at the periphery of neuritic plaques (Fig. 3). DISCUSSION The results of this study support the hypothesis that plaques may represent an aberrant or aborted sprouting response [see (1 l)]. Neuritic plaques which
FIG. 1. Acetylcholinesterase histochemistry in the dentate gyms molecular layer of the human hippocampal formation. In control cases (A), moderate cholinesterase staining was observed across the molecular layer, with a dense band of suprgranular staining (asterisk). (B) In AD, the overall intensity of the staining was reduced, but a region of cholinesterase intensification was observed in the outer molecular layer. AChE-positive neuritic plaques were also visible in this zone, but not within the inner molecular layer. Abbreviations: SGr-stratum granulosum, OML-outer molecular layer, HiF-hippocampal fissure. Calibration bar = 100 am.
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were intensely cholinesterase-positive were localized in the region of cholinesterase intensification. Within the plaque itself, there was sprouting of neurites as if attracted to the forming plaque. Based on the results of this and previous studies, a sequence of plaque formation is proposed in the following model. The progressive neurodegeneration associated with AD leads to the enhanced production of neurite-promoting factors and generalized sprouting in the terminal zone of the affected neurons. The demonstration of sprouting in AD suggests that the appropriate neurite-promoting factors are present. In the region of the sprouting response, there is a focal accumulation of trophic activity. The source of the growth factor activity is unknown, but may be reactive astrocytes or microglia responding to the neuronal degeneration (25). In addition to trophic factors, these nonneuronal cells may possess a favorable surface such as laminin or fibronectin ( 14,20), inducing an exhuberant local sprouting response of the adjacent neurites. Cholinesterase-positive neurites could be seen sprouting toward the plaque, similar to observations by Probst and colleagues in a Golgi preparation ( 18). The next event in the proposed sequence of neuritic plaque formation is that some of the sprouted neurites themselves degenerate. Numerous cholinesterase-positive swollen neurites, similar to those described in aged monkeys (22), were apparent within the plaque. Ultrastructural studies suggest that the dystrophic neurites are mainly axonal (23,24). This neuritic dystrophy may result from an absent or inappropriate target (19); inadequate neurotrophic activity (in contrast to neurite-promoting activity); impairment of axonal transport; or an antigenic response to the presence of tau in the sprouted neurites. Tau, a fetal microtubule-associated protein, is not normally found in the adult brain and is the major antigenic determinant in paired helical filaments (12). The sprouting and dystrophic neurites, surrounding the original growth center, may thus represent the early or immature plaque suggested by Wisnewski and Terry (25). This proposed sequence of events, beginning with an enhanced sprouting response induced by a focal accumulation of growth factors, is in marked
FIG. 3. Acetylcholinesterase-containing axons in the human hippocampus, visualized using the technique of Hedreen et al. (6). A-a normal cholinesterase-positive fiber. B and C-degenerating fibers, with swollen varicosities on focal segments visible in B and a bulbous dilation of a terminal apparent in C. D, E, and F-cholinesterase-positive neuritic plaques. In D, cholinesterase-positive fibers surround an unidentified target. Sprouting of fibers toward a source of trophic activity may represent the initial stage in plaque formation. Cholinesterase-positive fibers were seen entering the neuritic plaques in E and F, and grape-like clusters ofAChE-positive dilations, indicative of dystrophic neurities, were apparent in the periphery of the plaque. Calibration bars = 30 pm.
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contrast to the lack of trophic factors proposed by Appel(1) and by Hefti (7). The above model suggeststhat sprouting is an early phase of plaque formation and that sprouted fibers contribute to the makeup and disposition of plaques. Accordingly, the model predicts that those neuronal systemscapable of sprouting may be recruited into neuritic plaques. To date, with the exception of somatostatin, all fiber systemswhich have been colocalizedwith plaques (17, 19,22) also participate in neural plasticity. Theseinclude acetylcholine-, catecholamine-, and substanceP-containing neurons. It is conceivable that all fiber systemsmay be capable of a sprouting responseand contribute to the make-up of plaques, but that some respond more readily than others. The determinants of the fibers contributing to an individual plaque would therefore be the specificity of the trophic factor(s) and the nature of the adjacent neurites. REFERENCES 1. APPEL, S. H. 1981. A unifying parkinsonism, and Alzheimer 2. BALL, M. J. 1978. Topographic degeneration in hippocampal
hypothesis for the cause of amyotrophic lateral sclerosis, disease. Ann. Neural. 10: 499-505. distribution of neurofibrillary tangles and granulovacuolar cortex of aging and demented patients. Acta Neuroputhol.
42: 73-80.
COTMAN, C. W., AND K. J. ANDERSON. 1986. Synaptic plasticity and functional stabilization in the hippocampal formation: possible role in Alzheimer’s disease. In S. WAXMAN, Ed., PhysioIogic Basis for Functional Recovery in Neurological Disease. Raven Press, New York, in press. 4. COTMAN, C. W., AND NIETO-SAMPEDRO, M. 1984. Cell biology of synaptic plasticity. Science225: 1287-1294. 5. GEDDES, J. W., MONAGHAN, D. T., COTMAN, C. W., Lore, I. T., KIM, R. C., AND CHUI, H. C. 1985. Plasticity of hippocampal circuitry in Alzheimer’s disease. Science 230: 1179-l 181. 6. HEDREEN, J. H., BACON, S. J., AND PRICE, D. L. 1985. A modified histochemical technique to visualize acetylcholinesterase-containing axons. J. Histochem. Cytochem. 33: 1343.
140. 7.
8. 9. 10. 11.
HEF~I, F. 1983. Is Alzheimer disease caused by lack of nerve growth factor? Ann. Neural. 13: 109-l 10. HOOPER, M. W., AND VOGEL, F. S. 1976. The limbic system in Alzheimer’s disease. Am. .I. Pathol. 85: l-13. HYMAN, B. T., VAN HOESEN, G. W., DAMASIO, A. R., AND BARNES, C. L. 1984. Alzheimer’s disease: cell specific pathology isolates the hippocampal formation. Science 225: 1168-l 170. HYMAN, B. T., VAN HOESEN, G. W., KROMER, L. J., AND DAMASIO, A. R. 1986. Perforant pathway changes and the memory impairment of Alzheimer’s disease. Ann. Neural., in press. KEMPER, T. L. 1978. Senile dementia: a focal disease in the temporal lobe. Pages 105-l 13 in K. NANDY, Ed., Senile Dementia, a Biomedical Approach. Elsevier North-Holland Biomedical Press. Amsterdam.
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