Cerebral amyloid angiopathy : new insights from transgenic mice

Cerebral amyloid angiopathy (CAA) is characterized by the deposition of congophilic material \nwithin the walls of small to medium-sized blood vessels of the brain and leptomeninges. The \nincidence of CAA increases with aging, and in its most severe stages, the vascular amyloid \ncauses a breakdown of the blood vessel wall which results in spontaneous, often recurrent, lobar \nintracerebral hemorrhage. CAA is estimated to account for four to twenty percent of all \nnontraumatic intracerebral hemorrhages. Besides this major complication, extensive CAA has \nbeen associated with ischemic white matter damage with progressive dementia, perivascular \ninflammation, and secondary vasculitis. CAA occurs as a sporadic disorder in the elderly and in \nassociation with Alzheimer's disease (AD) with virtually all AD patients showing some degree of \nvascular amyloid in addition to parenchymal plaques. There are also familial forms of CAA such \nas hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D). The vascular \namyloid in these disorders mainly consists of β -amyloid peptide (Aβ ) that is produced by \nproteolytic cleavage from its precursor, which is the β -amyloid precursor protein (APP). The \nmajor Aβ species that is deposited in the vasculature is Aβ 40, while parenchymal amyloid is \nmainly composed of Aβ 42. One major difficulty in studying CAA is that it can be definitely \ndiagnosed only postmortem. Moreover, spontaneous CAA occurs only in old primates and dogs, \nboth of which are not practical models to study the pathogenesis and therapy of CAA. Rodents \ndo not spontaneously develop CAA. \nThe purpose of this thesis was to provide useful model systems to study the pathomechanism of \nvascular amyloid formation and associated pathology. To this end we generated and used mice \nthat are transgenic for human genes bearing mutations that are well known to cause either \nhereditary Aβ -CAA or classical familial AD. In a first study we analyzed CAA and CAA-associated \npathological changes in APP23 transgenic mice. These mice overexpress human APP bearing the \nSwedish K670N/M671L double mutation, a typical early-onset AD-causing mutation, under the \ncontrol of the neuron-specific Thy-1 promoter. In addition to parenchymal amyloid plaques, \nAPP23 mice show consistent amyloid within leptomeningeal, neocortical, hippocampal, and \nthalamic vessel walls. Both CAA frequency and severity significantly increase with aging, \ndemonstrating that not only more vessels are affected, but also that the amyloid burden of \nindividual vessels increases with the progression of amyloid deposition. Cerebrovascular amyloid \ncauses degeneration of vascular smooth muscle cells (SMCs). In severely affected vessels, SMCs \nare completely replaced by the amyloid. Similar to humans, amyloid depositing APP23 mice \ndevelop spontaneous hemorrhages, some of them being recurrent. The bleedings are associated \nwith amyloid-laden vessels and therefore, their anatomical distribution appears very similar to \n \n that of CAA. In aged mice, a quantitative analysis revealed a positive correlation between \nhemorrhages and CAA. Interestingly, no significant relationship between hemorrhages and total \namyloid load was observed. Occasionally, CAA-associated vasculitis is seen in animals with \nextensive vascular amyloid. \nIn a second study, we generated transgenic mice that express human APP E693Q under the \ncontrol of the same neuron-specific Thy-1 promoter (APPDutch mice) that has been used in \nAPP23 mice. In HCHWA-D patients, the APP E693Q Dutch mutation causes severe CAA with \nrecurrent cerebral hemorrhagic strokes often leading to death early in their fifties, or to \ndementia in patients that survive the strokes. In contrast to AD patients that show parenchymal \namyloid plaques, HCHWA-D patients exhibit few parenchymal amyloid deposits. Similar to \nHCHWA-D, aged APPDutch mice show extensive Aβ deposits mainly within the walls of \nleptomeningeal vessels followed by cortical vessels. Parenchymal Aβ deposits are mostly absent. \nIn severely affected vessels, the SMCs are completely displaced by the amyloid. In regions with \nCAA, fresh and old hemorrhages are observed, and activated perivascular microglia and reactive \nastrocytes are found. To examine the mechanism that leads to the almost exclusive vascular \namyloid formation in APPDutch mice, we compared the mice with transgenic mice \noverexpressing wild-type (wt) human APP using the same neuronal promoter (APPwt mice). As \nthey age, APPwt mice develop parenchymal plaques with limited vascular amyloid deposits. A \nbiochemical analysis of Aβ 40 and Aβ 42 levels revealed significant higher Aβ 40:42 ratios in \namyloid depositing and pre-depositing APPDutch mice compared to APPwt mice. To \ndemonstrate that the high Aβ 40:42 ratio in APPDutch mice is linked to the almost exclusive \nvascular amyloid deposition, we crossed APPDutch mice with mice that overexpress human \npresenilin-1 bearing the G384A mutation (PS45 mice) that is known to dramatically increase the \nproduction of Aβ 42. Strikingly, young APPDutch/PS45 double-transgenic mice develop massive \ndiffuse and compact parenchymal amyloid with only very little CAA. Thus, shifting the Aβ 40:42 \nratio towards Aβ 42 is sufficient to redistribute the amyloid pathology from the vasculature to the \nparenchyma. \nA third series of experiments using neurografting techniques was performed to investigate the \nmechanisms involved in the initiation of cerebral amyloidosis in vivo . Cell suspensions of \ntransgenic APP23 and wild-type B6 embryonic brain tissue were injected into the neocortex and \nhippocampus of both APP23 and B6 mice, respectively. In wild-type hosts, APP23 grafts did not \nshow amyloid deposits up to 20 months after grafting. Interestingly, transgenic and wild-type \ngrafts in young APP23 hosts develop amyloid plaques as early as three months after grafting. \nAlthough the majority of the amyloid is of the diffuse type, some compact and congophilic \namyloid plaques are observed in the wild-type grafts. These congophilic amyloid lesions are \n \n surrounded by neuritic changes and gliosis, comparable to the amyloid-associated pathology \nthat has previously been described in APP23 mice. These results support the importance of \nneuronally secreted Aβ for the development of cerebral amyloidosis which can be initiated \ndistant from the site of Aβ production, a finding that supports the observation of the above \nmentioned APPDutch mouse model. \nIn summary, we demonstrate that APP23 and APPDutch mice recapitulate CAA and CAAassociated \npathology observed in humans and thus are valuable models for studying the human \ndisease. Our results stress the importance of neuronally secreted Aβ for the development of CAA \nand emphasize the Aβ 40:42 ratio as an important factor in determining parenchymal versus \nvascular amyloid deposition. The understanding that different Aβ species can drive amyloid \npathology in different cerebral compartments not only provides insights into the \npathomechanism of sporadic and familial CAA but also has implications for current anti-amyloid \ntherapeutic strategies.