Spontaneous white matter damage, cognitive decline, and neuroinflammation in middle-aged hypertensive rats: An animal model of early stage cerebral small vessel disease

Abstract

Objectives Cerebral small vessel disease (cSVD) is one of the most prevalent neurological disorders. The progressive remodeling of brain microvessels due to arterial hypertension or other vascular risk factors causes subtle but constant cognitive decline and substantially increases the risk for stroke. Preliminary evidence suggests a contribution of the immune system to disease initiation and progression. Since most cSVD animal models are biased towards the hemorrhagic component of the disease1, a more detailed understanding is currently impaired by the unavailability of appropriate animal models. Here, we investigated the spontaneously hypertensive rat (SHR) as a possible model for early onset cSVD. Methods Male SHR and normotensive Wistar Kyoto rats (WKY, n=16 each, 11 weeks at enrolment) were used in this study. Animals were assigned to four experimental groups (Fig. 1). In group 1 (n=3/3), blood brain barrier (BBB) integrity was assessed by FITC-lectin and Evans Blue at 24 weeks. A brain tissue leukocyte profile of was obtained from group 2 (n=3/3) by fluorescence activated cell sorting (FACS) in week 35. In groups 3 and 4 (n=5/5 each), blood pressure was measured biweekly from week 12 to 22. Animals were further subjected to the novel object recognition (week 30) and Morris Water Maze (week 34) tests. Total brain, ventricle and corpus callosum (CC) volumes were determined by cerebral T2 MRI (3T) in week 35. Postmortem analyses included detailed cerebrospinal fluid, peripheral blood analysis by FACS and gene expression studies (laser microdissection and PCR), as well as detailed brain histology (neural cells, white matter density (Luxol Fast Blue), vessels and macro-/microglia). Results Blood pressure in SHR was significantly higher and increased over time (p<0.01 each). In contrast to agematched normotensive WKY, SHR exhibited non-spatial memory deficits (p<0.01). MRI showed brain atrophy (increased ventricle volumes and decreased CC/brain volumes; p<0.01). An increased myelin index, indicating myelin loss in SHR (p<0.01; Fig. 2). Histological analyses confirmed white matter demyelination and unveiled a circumscribed BBB dysfunction in conjunction with micro- and macrogliosis in deep cortical regions (DCR; p<0.05 or below; Fig. 3). FACS and histological analyses further revealed substantial disparities in cerebral CD45high leukocyte counts and distribution patterns between SHR and WKY. SHR showed lower T cells counts in the choroid plexus and meningeal spaces as well as decreased interleukin- 10 levels in the cerebrospinal fluid (p<0.05 or lower). Moreover, both T and NK cells were significantly augmented in the SHR brain microvasculature. Conclusions Our results indicate that SHR share behavioral and neuropathological characteristics with human cSVD patients and further undergird the relevance of immune responses for the initiation and progression of cSVD2.Fig. 2: SHR exhibited increased ventricle (A), decreased CC and brain volumes (B, C), and a higher myelin index (D) indicating white matter loss